- A UML sequence diagram or SD is similar to an MSC but written with a different notation. Presumably the same semantic issues arise, but possibly not since UML semantics are not well-defined. An example is shown in Figure 5. The timelines are dotted rather than solid, and the name of the component is inside a box at the head of each timeline.
- UML Sequence Diagrams display an interaction as a two-dimensional chart. Sequence Diagrams are used to design, document and validate the architecture, interfaces and logic of the system by describing the sequence of actions that need to be performed to complete a task. Why Creating Sequence Diagrams is Faster with Creately.
Sequence diagrams are easier to maintain. Sequence diagrams are easier to generate. Sequence diagrams can be easily updated according to the changes within a system. Sequence diagram allows reverse as well as forward engineering. Drawbacks of a sequence diagram. Sequence diagrams can become complex when too many lifelines are involved in the. UML Sequence Diagrams display an interaction as a two-dimensional chart. Sequence Diagrams are used to design, document and validate the architecture, interfaces and logic of the system by describing the sequence of actions that need to be performed to complete a task. Why Creating Sequence Diagrams is Faster with Creately.
3.1 Design and Construction as an Integrated System
In the planning of facilities, it is important to recognize the close relationship between design and construction. These processes can best be viewed as an integrated system. Broadly speaking, design is a process of creating the description of a new facility, usually represented by detailed plans and specifications; construction planning is a process of identifying activities and resources required to make the design a physical reality. Hence, construction is the implementation of a design envisioned by architects and engineers. In both design and construction, numerous operational tasks must be performed with a variety of precedence and other relationships among the different tasks.
Several characteristics are unique to the planning of constructed facilities and should be kept in mind even at the very early stage of the project life cycle. These include the following:
- Nearly every facility is custom designed and constructed, and often requires a long time to complete.
- Both the design and construction of a facility must satisfy the conditions peculiar to a specific site.
- Because each project is site specific, its execution is influenced by natural, social and other locational conditions such as weather, labor supply, local building codes, etc.
- Since the service life of a facility is long, the anticipation of future requirements is inherently difficult.
- Because of technological complexity and market demands, changes of design plans during construction are not uncommon.
In an integrated system, the planning for both design and construction can proceed almost simultaneously, examining various alternatives which are desirable from both viewpoints and thus eliminating the necessity of extensive revisions under the guise of value engineering. Furthermore, the review of designs with regard to their constructibility can be carried out as the project progresses from planning to design. For example, if the sequence of assembly of a structure and the critical loadings on the partially assembled structure during construction are carefully considered as a part of the overall structural design, the impacts of the design on construction falsework and on assembly details can be anticipated. However, if the design professionals are expected to assume such responsibilities, they must be rewarded for sharing the risks as well as for undertaking these additional tasks. Similarly, when construction contractors are expected to take over the responsibilities of engineers, such as devising a very elaborate scheme to erect an unconventional structure, they too must be rewarded accordingly. As long as the owner does not assume the responsibility for resolving this risk-reward dilemma, the concept of a truly integrated system for design and construction cannot be realized.
It is interesting to note that European owners are generally more open to new technologies and to share risks with designers and contractors. In particular, they are more willing to accept responsibilities for the unforeseen subsurface conditions in geotechnical engineering. Consequently, the designers and contractors are also more willing to introduce new techniques in order to reduce the time and cost of construction. In European practice, owners typically present contractors with a conceptual design, and contractors prepare detailed designs, which are checked by the owner's engineers. Those detailed designs may be alternate designs, and specialty contractors may also prepare detailed alternate designs.
Example 3-1:Responsibility for Shop Drawings
The willingness to assume responsibilities does not come easily from any party in the current litigious climate of the construction industry in the United States. On the other hand, if owner, architect, engineer, contractor and other groups that represent parts of the industry do not jointly fix the responsibilities of various tasks to appropriate parties, the standards of practice will eventually be set by court decisions. In an attempt to provide a guide to the entire spectrum of participants in a construction project, the American Society of Civil Engineers issued a Manual of Professional Practice entitled Quality in the Constructed Project in 1990. This manual is intended to help bring a turn around of the fragmentation of activities in the design and construction process.
Shop drawings represent the assembly details for erecting a structure which should reflect the intent and rationale of the original structural design. They are prepared by the construction contractor and reviewed by the design professional. However, since the responsibility for preparing shop drawings was traditionally assigned to construction contractors, design professionals took the view that the review process was advisory and assumed no responsibility for their accuracy. This justification was ruled unacceptable by a court in connection with the walkway failure at the Hyatt Hotel in Kansas City in 1985. In preparing the ASCE Manual of Professional Practice for Quality in the Constructed Project, the responsibilities for preparation of shop drawings proved to be the most difficult to develop.  The reason for this situation is not difficult to fathom since the responsibilities for the task are diffused, and all parties must agree to the new responsibilities assigned to each in the recommended risk-reward relations shown in Table 3-1.
Traditionally, the owner is not involved in the preparation and review of shop drawings, and perhaps is even unaware of any potential problems. In the recommended practice, the owner is required to take responsibility for providing adequate time and funding, including approval of scheduling, in order to allow the design professionals and construction contractors to perform satisfactorily.
|Owner||Design Professional||Construction Contractor|
|Provide adequate time and funding for shop drawing preparation and review||Prime|
|Arrange for structural design||Prime|
|Provide structural design||Prime|
|Establish overall responsibility for connection design||Prime|
|Accomplish connection design (by design professional)||Prime|
|Alternatively, provide loading requirement and other information necessary for shop drawing preparation||Prime|
|Alternatively, accomplish some or all of connection design (by constuctor with a licensed P.E.)||Prime|
|Specify shop drawing requirements and procedures||Review||Prime|
|Approve proper scheduling||Prime||Assisting||Assisting|
|Provide shop drawing and submit the drawing on schedule||Prime|
|Make timely reviews and approvals||Prime|
|Provide erection procedures, construction bracing, shoring, means, methods and techniques of construction, and construction safety||Prime|
Example 3-2:Model Metro Project in Milan, Italy 
Under Italian law, unforeseen subsurface conditions are the owner's responsibility, not the contractor's. This is a striking difference from U.S. construction practice where changed conditions clauses and claims and the adequacy of prebid site investigations are points of contention. In effect, the Italian law means that the owner assumes those risks. But under the same law, a contractor may elect to assume the risks in order to lower the bid price and thereby beat the competition.
According to the Technical Director of Rodio, the Milan-based contractor which is heavily involved in the grouting job for tunneling in the Model Metro project in Milan, Italy, there are two typical contractual arrangements for specialized subcontractor firms such as theirs. One is to work on a unit price basis with no responsibility for the design. The other is what he calls the 'nominated subcontractor' or turnkey method: prequalified subcontractors offer their own designs and guarantee the price, quality, quantities, and, if they wish, the risks of unforeseen conditions.
At the beginning of the Milan metro project, the Rodio contract ratio was 50/50 unit price and turnkey. The firm convinced the metro owners that they would save money with the turnkey approach, and the ratio became 80% turnkey. What's more, in the work packages where Rodio worked with other grouting specialists, those subcontractors paid Rodio a fee to assume all risks for unforeseen conditions.
Under these circumstances, it was critical that the firm should know the subsurface conditions as precisely as possible, which was a major reason why the firm developed a computerized electronic sensing program to predict stratigraphy and thus control grout mixes, pressures and, most important, quantities.
3.2 Innovation and Technological Feasibility
The planning for a construction project begins with the generation of concepts for a facility which will meet market demands and owner needs. Innovative concepts in design are highly valued not for their own sake but for their contributions to reducing costs and to the improvement of aesthetics, comfort or convenience as embodied in a well-designed facility. However, the constructor as well as the design professionals must have an appreciation and full understanding of the technological complexities often associated with innovative designs in order to provide a safe and sound facility. Since these concepts are often preliminary or tentative, screening studies are carried out to determine the overall technological viability and economic attractiveness without pursuing these concepts in great detail. Because of the ambiguity of the objectives and the uncertainty of external events, screening studies call for uninhibited innovation in creating new concepts and judicious judgment in selecting the appropriate ones for further consideration.
One of the most important aspects of design innovation is the necessity of communication in the design/construction partnership. In the case of bridge design, it can be illustrated by the following quotation from Lin and Gerwick concerning bridge construction: 
The great pioneering steel bridges of the United States were built by an open or covert alliance between designers and constructors. The turnkey approach of designer-constructor has developed and built our chemical plants, refineries, steel plants, and nuclear power plants. It is time to ask, seriously, whether we may not have adopted a restrictive approach by divorcing engineering and construction in the field of bridge construction.
If a contractor-engineer, by some stroke of genius, were to present to design engineers today a wonderful new scheme for long span prestressed concrete bridges that made them far cheaper, he would have to make these ideas available to all other constructors, even limiting or watering them down so as to 'get a group of truly competitive bidders.' The engineer would have to make sure that he found other contractors to bid against the ingenious innovator.
If an engineer should, by a similar stroke of genius, hit on such a unique and brilliant scheme, he would have to worry, wondering if the low bidder would be one who had any concept of what he was trying to accomplish or was in any way qualified for high class technical work.
Innovative design concepts must be tested for technological feasibility. Three levels of technology are of special concern: technological requirements for operation or production, design resources and construction technology. The first refers to the new technologies that may be introduced in a facility which is used for a certain type of production such as chemical processing or nuclear power generation. The second refers to the design capabilities that are available to the designers, such as new computational methods or new materials. The third refers to new technologies which can be adopted to construct the facility, such as new equipment or new construction methods.
A new facility may involve complex new technology for operation in hostile environments such as severe climate or restricted accessibility. Large projects with unprecedented demands for resources such as labor supply, material and infrastructure may also call for careful technological feasibility studies. Major elements in a feasibility study on production technology should include, but are not limited to, the following:
- Project type as characterized by the technology required, such as synthetic fuels, petrochemicals, nuclear power plants, etc.
- Project size in dollars, design engineer's hours, construction labor hours, etc.
- Design, including sources of any special technology which require licensing agreements.
- Project location which may pose problems in environmental protection, labor productivity and special risks.
An example of innovative design for operation and production is the use of entropy concepts for the design of integrated chemical processes. Simple calculations can be used to indicate the minimum energy requirements and the least number of heat exchange units to achieve desired objectives. The result is a new incentive and criterion for designers to achieve more effective designs. Numerous applications of the new methodology has shown its efficacy in reducing both energy costs and construction expenditures.  This is a case in which innovative design is not a matter of trading-off operating and capital costs, but better designs can simultaneously achieve improvements in both objectives.
The choice of construction technology and method involves both strategic and tactical decisions about appropriate technologies and the best sequencing of operations. For example, the extent to which prefabricated facility components will be used represents a strategic construction decision. In turn, prefabrication of components might be accomplished off-site in existing manufacturing facilities or a temporary, on-site fabrication plant might be used. Another example of a strategic decision is whether to install mechanical equipment in place early in the construction process or at an intermediate stage. Strategic decisions of this sort should be integrated with the process of facility design in many cases. At the tactical level, detailed decisions about how to accomplish particular tasks are required, and such decisions can often be made in the field.
Construction planning should be a major concern in the development of facility designs, in the preparation of cost estimates, and in forming bids by contractors. Unfortunately, planning for the construction of a facility is often treated as an after thought by design professionals. This contrasts with manufacturing practices in which the assembly of devices is a major concern in design. Design to insure ease of assembly or construction should be a major concern of engineers and architects. As the Business Roundtable noted, 'All too often chances to cut schedule time and costs are lost because construction operates as a production process separated by a chasm from financial planning, scheduling, and engineering or architectural design. Too many engineers, separated from field experience, are not up to date about how to build what they design, or how to design so structures and equipment can be erected most efficiently.' 
Example 3-3: Innovative use of structural frames for buildings
The structural design of skyscrapers offers an example of innovation in overcoming the barrier of high costs for tall buildings by making use of new design capabilities. A revolutionary concept in skyscraper design was introduced in the 1960's by Fazlur Khan who argued that, for a building of a given height, there is an appropriate structural system which would produce the most efficient use of the material.
Before 1965, most skyscrapers were steel rigid frames. However, Fazlur Khan believed that it was uneconomical to construct all office buildings of rigid frames, and proposed an array of appropriate structural systems for steel buildings of specified heights as shown in Figure 3-1. By choosing an appropriate structural system, an engineer can use structural materials more efficiently. For example, the 60-story Chase Manhattan Building in New York used about 60 pounds per square foot of steel in its rigid frame structure, while the 100-story John Hancock Center in Chicago used only 30 pounds per square foot for a trusted tube system. At the time the Chase Manhattan Building was constructed, no bracing was used to stiffen the core of a rigid frame building because design engineers did not have the computing tools to do the complex mathematical analysis associated with core bracing.
Figure 3-1: Proposed Structural System fir Steel Buildings
(Reprinted with permission from Civil Engineering, May 1983)
3.3 Innovation and Economic Feasibility
Innovation is often regarded as the engine which can introduce construction economies and advance labor productivity. This is obviously true for certain types of innovations in industrial production technologies, design capabilities, and construction equipment and methods. However, there are also limitations due to the economic infeasibility of such innovations, particularly in the segments of construction industry which are more fragmented and permit ease of entry, as in the construction of residential housing.
Market demand and firm size play an important role in this regard. If a builder is to construct a larger number of similar units of buildings, the cost per unit may be reduced. This relationship between the market demand and the total cost of production may be illustrated schematically as in Figure 3-2. An initial threshold or fixed cost F is incurred to allow any production. Beyond this threshold cost, total cost increases faster than the units of output but at a decreasing rate. At each point on this total cost curve, the average cost is represented by the slope of a line from the origin to the point on the curve. At a point H, the average cost per unit is at a minimum. Beyond H to the right, the total cost again increases faster than the units of output and at an increasing rate. When the rate of change of the average cost slope is decreasing or constant as between 0 and H on the curve, the range between 0 and H is said to be increasing return to scale; when the rate of change of the average cost slope is increasing as beyond H to the right, the region is said to be decreasing return to scale. Thus, if fewer than h units are constructed, the unit price will be higher than that of exactly h units. On the other hand, the unit price will increase again if more than h units are constructed.
Figure 3-2: Market Demand and Total Cost Relationship
Nowhere is the effect of market demand and total cost more evident than in residential housing.  The housing segment in the last few decades accepted many innovative technical improvements in building materials which were promoted by material suppliers. Since material suppliers provide products to a large number of homebuilders and others, they are in a better position to exploit production economies of scale and to support new product development. However, homebuilders themselves have not been as successful in making the most fundamental form of innovation which encompasses changes in the technological process of homebuilding by shifting the mixture of labor and material inputs, such as substituting large scale off-site prefabrication for on-site assembly.
There are several major barriers to innovation in the technological process of homebuilding, including demand instability, industrial fragmentation, and building codes. Since market demand for new homes follows demographic trends and other socio-economic conditions, the variation in home building has been anything but regular. The profitability of the homebuilding industry has closely matched aggregate output levels. Since entry and exist from the industry are relatively easy, it is not uncommon during periods of slack demand to find builders leaving the market or suspending their operations until better times. The inconsistent levels of retained earnings over a period of years, even among the more established builders, are likely to discourage support for research and development efforts which are required to nurture innovation. Furthermore, because the homebuilding industry is fragmented with a vast majority of homebuilders active only in local regions, the typical homebuilder finds it excessively expensive to experiment with new designs. The potential costs of a failure or even a moderately successful innovation would outweigh the expected benefits of all but the most successful innovations. Variation in local building codes has also caused inefficiencies although repeated attempts have been made to standardize building codes.
In addition to the scale economies visible within a sector of the construction market, there are also possibilities for scale economies in individual facility. For example, the relationship between the size of a building (expressed in square feet) and the input labor (expressed in laborhours per square foot) varies for different types and sizes of buildings. As shown in Figure 3-3, these relationships for several types of buildings exhibit different characteristics.  The labor hours per square foot decline as the size of facility increases for houses, public housing and public buildings. However, the labor hours per square foot almost remains constant for all sizes of school buildings and increases as the size of a hospital facility increases.
Figure 3-3: Illustrative Relationships between Building Size and Input Labor by Types of Building
(Reprinted with permission from P.J. Cassimatis, Economics of the Construction Industry,
The National Industry Conference Board, SEB, No. 111, 1969, p.53)
Example 3-4: Use of new materials
In recent years, an almost entirely new set of materials is emerging for construction, largely from the aerospace and electronics industries. These materials were developed from new knowledge about the structure and properties of materials as well as new techniques for altering existing materials. Additives to traditional materials such as concrete and steel are particularly prominent. For example, it has been known for some time that polymers would increase concrete strength, water resistance and ability to insulate when they are added to the cement. However, their use has been limited by their costs since they have had to replace as much as 10 percent of the cement to be effective. However, Swedish researchers have helped reduce costs by using polymer microspheres 8 millionths of an inch across, which occupy less than 1 percent of the cement. Concretes made with these microspheres meet even the strict standards for offshore structures in the North Sea. Research on micro-additives will probably produce useful concretes for repairing road and bridges as well.
Example 3-5: Green Buildings
The Leadership in Energy and Environmental Design (LEED) Green Building Rating System is intended to promote voluntary improvements in design and construction practices. In the rating system, buildings receive points for a variety of aspects, including reduced energy use, greater use of daylight rather than artificial lights, recycling construction waste, rainfall runoff reduction, availability of public transit access, etc. If a building accumulates a sufficient number of points, it may be certified by the Green Building Alliance as a 'green building.' While some of these aspects may increase construction costs, many reduce operating costs or make buildings more attractive. Green building approaches are spreading to industrial plants and other types of construction.
3.4 Design Methodology
While the conceptual design process may be formal or informal, it can be characterized by a series of actions: formulation, analysis, search, decision, specification, and modification. However, at the early stage in the development of a new project, these actions are highly interactive as illustrated in Figure 3-4.  Many iterations of redesign are expected to refine the functional requirements, design concepts and financial constraints, even though the analytic tools applied to the solution of the problem at this stage may be very crude.
Figure 3-4: Conceptual Design Process
(Adapted with permission from R.W. Jensen and C.C. Tonies, Software Engineering,
Prentice Hall, Englewood Cliffs, NJ, 1979, p.22)
The series of actions taken in the conceptual design process may be described as follows:
- Formulation refers to the definition or description of a design problem in broad terms through the synthesis of ideas describing alternative facilities.
- Analysis refines the problem definition or description by separating important from peripheral information and by pulling together the essential detail. Interpretation and prediction are usually required as part of the analysis.
- Search involves gathering a set of potential solutions for performing the specified functions and satisfying the user requirements.
- Decision means that each of the potential solutions is evaluated and compared to the alternatives until the best solution is obtained.
- Specification is to describe the chosen solution in a form which contains enough detail for implementation.
- Modification refers to the change in the solution or re-design if the solution is found to be wanting or if new information is discovered in the process of design.
As the project moves from conceptual planning to detailed design, the design process becomes more formal. In general, the actions of formulation, analysis, search, decision, specification and modification still hold, but they represent specific steps with less random interactions in detailed design. The design methodology thus formalized can be applied to a variety of design problems. For example, the analogy of the schematic diagrams of the structural design process and of the computer program development process is shown in Figure 3-5 .
Figure 3-5: An Analogy Between Structural Design and Computer Program Development Process
(Reprinted with permission from E.H. Gaylord and C. N. Gaylord, eds., Structural Engineering Handbook,
2nd Ed., McGraw-Hill Book Company, New York, 1979.)
The basic approach to design relies on decomposition and integration. Since design problems are large and complex, they have to be decomposed to yield subproblems that are small enough to solve. There are numerous alternative ways to decompose design problems, such as decomposition by functions of the facility, by spatial locations of its parts, or by links of various functions or parts. Solutions to subproblems must be integrated into an overall solution. The integration often creates conceptual conflicts which must be identified and corrected. A hierarchical structure with an appropriate number of levels may be used for the decomposition of a design problem to subproblems. For example, in the structural design of a multistory building, the building may be decomposed into floors, and each floor may in turn be decomposed into separate areas. Thus, a hierarchy representing the levels of building, floor and area is formed.
Different design styles may be used. The adoption of a particular style often depends on factors such as time pressure or available design tools, as well as the nature of the design problem. Examples of different styles are:
- Top-down design. Begin with a behavior description of the facility and work towards descriptions of its components and their interconnections.
- Bottom-up design. Begin with a set of components, and see if they can be arranged to meet the behavior description of the facility.
The design of a new facility often begins with the search of the files for a design that comes as close as possible to the one needed. The design process is guided by accumulated experience and intuition in the form of heuristic rules to find acceptable solutions. As more experience is gained for this particular type of facility, it often becomes evident that parts of the design problem are amenable to rigorous definition and algorithmic solution. Even formal optimization methods may be applied to some parts of the problem.
3.5 Functional Design
The objective of functional design for a proposed facility is to treat the facility as a complex system of interrelated spaces which are organized systematically according to the functions to be performed in these spaces in order to serve a collection of needs. The arrangement of physical spaces can be viewed as an iterative design process to find a suitable floor plan to facilitate the movement of people and goods associated with the operations intended.
A designer often relies on a heuristic approach, i.e., applying selected rules or strategies serving to stimulate the investigation in search for a solution. The heuristic approach used in arranging spatial layouts for facilities is based generally on the following considerations:
- identification of the goals and constraints for specified tasks,
- determination of the current state of each task in the iterative design process,
- evaluation of the differences between the current state and the goals,
- means of directing the efforts of search towards the goals on the basis of past experience.
Hence, the procedure for seeking the goals can be recycled iteratively in order to make tradeoffs and thus improve the solution of spatial layouts.
Consider, for example, an integrated functional design for a proposed hospital.  Since the responsibilities for satisfying various needs in a hospital are divided among different groups of personnel within the hospital administrative structure, a hierarchy of functions corresponding to different levels of responsibilities is proposed in the systematic organization of hospital functions. In this model, the functions of a hospital system are decomposed into a hierarchy of several levels:
- Hospital--conglomerate of all hospital services resulting from top policy decisions,
- Division--broadly related activities assigned to the same general area by administrative decisions,
- Department--combination of services delivered by a service or treatment group,
- Suite--specific style of common services or treatments performed in the same suite of rooms,
- Room--all activities that can be carried out in the same internal environment surrounded by physical barriers,
- Zone--several closely related activities that are undertaken by individuals,
- Object--a single activity associated with an individual.
In the integrated functional design of hospitals, the connection between physical spaces and functions is most easily made at the lowest level of the hierarchy, and then extended upward to the next higher level. For example, a bed is a physical object immediately related to the activity of a patient. A set of furniture consisting of a bed, a night table and an armchair arranged comfortably in a zone indicates the sphere of private activities for a patient in a room with multiple occupancy. Thus, the spatial representation of a hospital can be organized in stages starting from the lowest level and moving to the top. In each step of the organization process, an element (space or function) under consideration can be related directly to the elements at the levels above it, to those at the levels below it, and to those within the same level.
Since the primary factor relating spaces is the movement of people and supplies, the objective of arranging spaces is the minimization of movement within the hospital. On the other hand, the internal environmental factors such as atmospheric conditions (pressure, temperature, relative humidity, odor and particle pollution), sound, light and fire protection produce constraining effects on the arrangement of spaces since certain spaces cannot be placed adjacent to other spaces because of different requirements in environmental conditions. The consideration of logistics is important at all levels of the hospital system. For example, the travel patterns between objects in a zone or those between zones in a room are frequently equally important for devising an effective design. On the other hand, the adjacency desirability matrix based upon environmental conditions will not be important for organization of functional elements below the room level since a room is the lowest level that can provide a physical barrier to contain desirable environmental conditions. Hence, the organization of functions for a new hospital can be carried out through an interactive process, starting from the functional elements at the lowest level that is regarded as stable by the designer, and moving step by step up to the top level of the hierarchy. Due to the strong correlation between functions and the physical spaces in which they are performed, the arrangement of physical spaces for accommodating the functions will also follow the same iterative process. Once a satisfactory spatial arrangement is achieved, the hospital design is completed by the selection of suitable building components which complement the spatial arrangement.
Example 3-6: Top-down design style
In the functional design of a hospital, the designer may begin with a 'reference model', i.e. the spatial layouts of existing hospitals of similar size and service requirements. On the basis of past experience, spaces are allocated to various divisions as shown schematically in Figure 3-6. The space in each division is then divided further for various departments in the division, and all the way down the line of the hierarchy. In every step along the way, the pertinent information of the elements immediately below the level under consideration will be assessed in order to provide input for making necessary adjustments at the current level if necessary. The major drawback of the top-down design style is that the connection between physical spaces and functions at lower levels cannot be easily anticipated. Consequently, the new design is essentially based on the intuition and experience of the designer rather than an objective analysis of the functions and space needs of the facility. Its greatest attraction is its simplicity which keeps the time and cost of design relatively low.
Figure 3-6: A Model for Top-Down Design of a Hospital
Example 3-7: Bottom-up design style
A multi-purpose examination suite in a hospital is used as an illustration of bottom-up design style. In Figure 3-7, the most basic elements (furniture) are first organized into zones which make up the room. Thus the size of the room is determined by spatial layout required to perform the desired services. Finally, the suite is defined by the rooms which are parts of the multi-purpose examination suite.
Figure 3-7: A Model for Bottom-up design of an Examination Suite
3.6 Physical Structures
The structural design of complex engineering systems generally involves both synthesis and analysis. Synthesis is an inductive process while analysis is a deductive process. The activities in synthesis are often described as an art rather than a science, and are regarded more akin to creativity than to knowledge. The conception of a new structural system is by and large a matter of subjective decision since there is no established procedure for generating innovative and highly successful alternatives. The initial selection of a workable system from numerous possible alternatives relies heavily on the judicious judgment of the designer. Once a structural system is selected, it must be subjected to vigorous analysis to insure that it can sustain the demands in its environment. In addition, compatibility of the structural system with mechanical equipment and piping must be assured.
For traditional types of structures such as office buildings, there are standard systems derived from the past experience of many designers. However, in many situations, special systems must be developed to meet the specified requirements. The choice of materials for a structure depends not only on the suitability of materials and their influence on the form of the structure. For example, in the design of an airplane hangar, a steel skeleton frame may be selected because a similar frame in reinforced concrete will limit the span of the structure owing to its unfavorable ratio or resistance to weight. However, if a thin-shelled roof is adopted, reinforced concrete may prove to be more suitable than steel. Thus, the interplay of the structural forms and materials affects the selection of a structural system, which in turn may influence the method of construction including the use of falsework.
Example 3-8: Steel frame supporting a turbo-blower 
The design of a structural frame supporting a turbo-blower supplying pressurized air to a blast furnace in a steel mill can be used to illustrate the structural design process. As shown in Figure 3-8, the turbo-blower consists of a turbine and a blower linked to an air inlet stack. Since the vibration of the turbo-blower is a major concern to its operation, a preliminary investigation calls for a supporting frame which is separated from the structural frame of the building. An analysis of the vibration characteristics of the turbo-blower indicates that the lowest mode of vibration consists of independent vibration of the turbine shaft and the blower shaft, with higher modes for the coupled turbo-blower system when both shafts vibrate either in-phase or out-of-phase. Consequently, a steel frame with separate units for the blower side and the turbine side is selected. The columns of the steel frame are mounted on pile foundation and all joints of the steel frame are welded to reduce the vibration levels.
Since the structural steel frame also supports a condenser, an air inlet and exhaust, and a steam inlet and exhaust in addition to the turbo-blower, a static analysis is made to size its members to support all applied loads. Then, a dynamic analysis is conducted to determine the vibration characteristics of the system incorporating the structural steel frame and the turbo-blower. When the limiting conditions for static loads and natural frequencies of vibration are met, the design is accepted as satisfactory.
Figure 3-8: Steel Frame Supporting a Turbo-Blower
Example 3-9: Multiple hierarchy descriptions of projects
In the previous section, a hierarchy of functional spaces was suggested for describing a facility. This description is appropriate for functional design of spaces and processes within a building, but may be inadequate as a view of the facility's structural systems. A hierarchy suitable for this purpose might divide elements into structural functions such as slabs, walls, frames, footings, piles or mats. Lower levels of the hierarchy would describe individual design elements. For example, frames would be made up of column, beam and diagonal groups which, in turn, are composed of individual structural elements. These individual structural elements comprise the limits on functional spaces such as rooms in a different hierarchical perspective. Designers typically will initiate a view appropriate for their own concerns, and these different hierarchical views must be synthesized to insure consistency and adequacy of the overall design.
3.7 Geotechnical Engineering Investigation
Since construction is site specific, it is very important to investigate the subsurface conditions which often influence the design of a facility as well as its foundation. The uncertainty in the design is particularly acute in geotechnical engineering so that the assignment of risks in this area should be a major concern. Since the degree of uncertainty in a project is perceived differently by different parties involved in a project, the assignment of unquantifiable risks arising from numerous unknowns to the owner, engineer and contractor is inherently difficult. It is no wonder that courts or arbitrators are often asked to distribute equitably a risk to parties who do not perceive the same risks and do not want to assume a disproportionate share of such risks.
Example 3-10: Design of a tie-back retaining wall
This example describes the use of a tie-back retaining wall built in the 1960's when such construction was uncommon and posed a considerable risk. The engineer designing it and the owner were aware of the risk because of potentially extreme financial losses from both remedial and litigation costs in the event that the retaining wall failed and permitted a failure of the slope. But the benefits were perceived as being worth the risk--benefits to the owner in terms of both lower cost and shorter schedule, and benefits to the engineer in terms of professional satisfaction in meeting the owner's needs and solving what appeared to be an insurmountable technical problem.
The tie-back retaining wall was designed to permit a cut in a hillside to provide additional space for the expansion of a steel-making facility. Figure 3-9 shows a cross section of the original hillside located in an urban area. Numerous residential dwellings were located on top of the hill which would have been prohibitively costly or perhaps impossible to remove to permit regrading of the hillside to push back the toe of the slope. The only realistic way of accomplishing the desired goal was to attempt to remove the toe of the existing slope and use a tie-back retaining wall to stabilize the slope as shown in Figure 3-10.
Figure 3-9: Typical Cross Section of Hillside Adjoining Site
Figure 3-10: Schematic Section of Anchored Steel Sheet Pile Retaining Wall
A commitment was made by both the owner and the engineer to accomplish what was a common goal. The engineer made a commitment to design and construct the wall in a manner which permitted a real-time evaluation of problems and the ability to take mitigating measures throughout the construction of the wall. The owner made a commitment to give the engineer both the professional latitude and resources required to perform his work. A design-construct contract was negotiated whereby the design could be modified as actual conditions were encountered during construction. But even with all of the planning, investigation and design efforts, there still remained a sizable risk of failure.
The wall was successfully built--not according to a pre-devised plan which went smoothly, and not without numerous problems to be resolved as unexpected groundwater and geological conditions were encountered. Estimated costs were exceeded as each unexpected condition was addressed. But there were no construction delays and their attendant costs as disputes over changed conditions and contract terms were reconciled. There were no costs for legal fees arising from litigation nor increased interest costs as construction stopped while disputes were litigated. The owner paid more than was estimated, but not more than was necessary and not as much as if he had to acquire the property at the top of the hill to regrade the slope. In addition, the owner was able to attain the desired facility expansion in far less time than by any other method.
As a result of the success of this experience and others, the use of tie-back retaining walls has become a routine practice.
3.8 Construction Site Environment
While the general information about the construction site is usually available at the planning stage of a project, it is important for the design professionals and construction manager as well as the contractor to visit the site. Each group will be benefited by first-hand knowledge acquired in the field.
For design professionals, an examination of the topography may focus their attention to the layout of a facility on the site for maximum use of space in compliance with various regulatory restrictions. In the case of industrial plants, the production or processing design and operation often dictate the site layout. A poor layout can cause construction problems such as inadequate space for staging, limited access for materials and personnel, and restrictions on the use of certain construction methods. Thus, design and construction inputs are important in the layout of a facility.
The construction manager and the contractor must visit the site to gain some insight in preparing or evaluating the bid package for the project. They can verify access roads and water, electrical and other service utilities in the immediate vicinity, with the view of finding suitable locations for erecting temporary facilities and the field office. They can also observe any interferences of existing facilities with construction and develop a plan for site security during construction.
In examining site conditions, particular attention must be paid to environmental factors such as drainage, groundwater and the possibility of floods. Of particular concern is the possible presence of hazardous waste materials from previous uses. Cleaning up or controlling hazardous wastes can be extremely expensive.
Example 3-11:Groundwater Pollution from a Landfill
The presence of waste deposits on a potential construction site can have substantial impacts on the surrounding area. Under existing environmental regulations in the United States, the responsibility for cleaning up or otherwise controlling wastes generally resides with the owner of a facility in conjunction with any outstanding insurance coverage.
A typical example of a waste problem is illustrated in Figure 3-11. In this figure, a small pushover burning dump was located in a depression on a slope. The landfill consisted of general refuse and was covered by a very sandy material. The inevitable infiltration of water from the surface or from the groundwater into the landfill will result in vertical or horizontal percolation of leachable ions and organic contamination. This leachate would be odorous and potentially hazardous in water. The pollutant would show up as seepage downhill, as pollution in surface streams, or as pollution entering the regional groundwater.
Figure 3-11: Cross-Section Illustration of a Landfill
Before new construction could proceed, this landfill site would have to be controlled or removed. Typical control methods might involve:
- Surface water control measures, such as contour grading or surface sealing.
- Passive groundwater control techniques such as underground barriers between the groundwater and the landfill.
- Plume management procedures such as pumping water from surrounding wells.
- Chemical immobilization techniques such as applying surface seals or chemical injections.
- Excavation and reburial of the landfill requiring the availability of an engineered and environmentally sound landfill.
The excavation and reburial of even a small landfill site can be very expensive. For example, the estimated reburial cost for a landfill like that shown in Figure 3-11 was in excess of $ 4 million in 1978.
3.9 Value Engineering
Value engineering may be broadly defined as an organized approach in identifying unnecessary costs in design and construction and in soliciting or proposing alternative design or construction technology to reduce costs without sacrificing quality or performance requirements. It usually involves the steps of gathering pertinent information, searching for creative ideas, evaluating the promising alternatives, and proposing a more cost effective alternative. This approach is usually applied at the beginning of the construction phase of the project life cycle.
The use of value engineering in the public sector of construction has been fostered by legislation and government regulation, but the approach has not been widely adopted in the private sector of construction. One explanation may lie in the difference in practice of engineering design services in the public and private sectors. In the public sector, the fee for design services is tightly monitored against the 'market price,' or may even be based on the lowest bid for service. Such a practice in setting professional fees encourages the design professionals to adopt known and tried designs and construction technologies without giving much thought to alternatives that are innovative but risky. Contractors are willing to examine such alternatives when offered incentives for sharing the savings by owners. In the private sector, the owner has the freedom to offer such incentives to design professionals as well as the contractors without being concerned about the appearance of favoritism in engaging professional services.
Another source of cost savings from value engineering is the ability of contractors to take advantage of proprietary or unusual techniques and knowledge specific to the contractor's firm. For example, a contractor may have much more experience with a particular method of tunneling that is not specified in the original design and, because of this experience, the alternative method may be less expensive. In advance of a bidding competition, a design professional does not know which contractor will undertake the construction of a facility. Once a particular contractor is chosen, then modifications to the construction technology or design may take advantage of peculiar advantages of the contractor's organization.
As a final source of savings in value engineering, the contractor may offer genuine new design or construction insights which have escaped the attention of the design professional even if the latter is not restrained by the fee structure to explore more alternatives. If the expertise of the contractor can be utilized, of course, the best time to employ it is during the planning and design phase of the project life cycle. That is why professional construction management or integrated design/construction are often preferred by private owners.
3.10 Construction Planning
The development of a construction plan is very much analogous to the development of a good facility design. The planner must weigh the costs and reliability of different options while at the same time insuring technical feasibility. Construction planning is more difficult in some ways since the building process is dynamic as the site and the physical facility change over time as construction proceeds. On the other hand, construction operations tend to be fairly standard from one project to another, whereas structural or foundation details might differ considerably from one facility to another.
Forming a good construction plan is an exceptionally challenging problem. There are numerous possible plans available for any given project. While past experience is a good guide to construction planning, each project is likely to have special problems or opportunities that may require considerable ingenuity and creativity to overcome or exploit. Unfortunately, it is quite difficult to provide direct guidance concerning general procedures or strategies to form good plans in all circumstances. There are some recommendations or issues that can be addressed to describe the characteristics of good plans, but this does not necessarily tell a planner how to discover a good plan. However, as in the design process, strategies of decomposition in which planning is divided into subproblems and hierarchical planning in which general activities are repeatably subdivided into more specific tasks can be readily adopted in many cases.
From the standpoint of construction contractors or the construction divisions of large firms, the planning process for construction projects consists of three stages that take place between the moment in which a planner starts the plan for the construction of a facility to the moment in which the evaluation of the final output of the construction process is finished.
The estimate stage involves the development of a cost and duration estimate for the construction of a facility as part of the proposal of a contractor to an owner. It is the stage in which assumptions of resource commitment to the necessary activities to build the facility are made by a planner. A careful and thorough analysis of different conditions imposed by the construction project design and by site characteristics are taken into consideration to determine the best estimate. The success of a contractor depends upon this estimate, not only to obtain a job but also to construct the facility with the highest profit. The planner has to look for the time-cost combination that will allow the contractor to be successful in his commitment. The result of a high estimate would be to lose the job, and the result of a low estimate could be to win the job, but to lose money in the construction process. When changes are done, they should improve the estimate, taking into account not only present effects, but also future outcomes of succeeding activities. It is very seldom the case in which the output of the construction process exactly echoes the estimate offered to the owner.
In the monitoring and control stage of the construction process, the construction manager has to keep constant track of both activities' durations and ongoing costs. It is misleading to think that if the construction of the facility is on schedule or ahead of schedule, the cost will also be on the estimate or below the estimate, especially if several changes are made. Constant evaluation is necessary until the construction of the facility is complete. When work is finished in the construction process, and information about it is provided to the planner, the third stage of the planning process can begin.
The evaluation stage is the one in which results of the construction process are matched against the estimate. A planner deals with this uncertainty during the estimate stage. Only when the outcome of the construction process is known is he/she able to evaluate the validity of the estimate. It is in this last stage of the planning process that he or she determines if the assumptions were correct. If they were not or if new constraints emerge, he/she should introduce corresponding adjustments in future planning.
3.11 Industrialized Construction and Pre-fabrication
Another approach to construction innovation is to apply the principles and organizational solutions adopted for manufacturing. Industrialized construction and pre-fabrication would involve transferring a significant portion of construction operations from the construction site to more or less remote sites where individual components of buildings and structures are produced. Elements of facilities could be prefabricated off the erection site and assembled by cranes and other lifting machinery.
There are a wide variety and degrees of introducing greater industrialization to the construction process. Many components of constructed facilities have always been manufactured, such as air conditioning units. Lumber, piping and other individual components are manufactured to standard sizes. Even temporary items such as forms for concrete can be assembled off-site and transported for use. Reinforcing bars for concrete can also be pre-cut and shaped to the desired configuration in a manufacturing plant or in an automated plant located proximate to a construction site.
A major problem in extending the use of pre-fabricated units is the lack of standardization for systems and building regulations. While designers have long adopted standard sizes for individual components in designs, the adoption of standardized sub-assemblies is rarer. Without standardization, the achievement of a large market and scale economies of production in manufacturing may be impossible. An innovative and more thorough industrialization of the entire building process may be a primary source of construction cost savings in the future.
Example 3-12:Planning of pre-fabrication
When might pre-fabricated components be used in preference to components assembled on a construction site? A straightforward answer is to use pre-fabricated components whenever their cost, including transportation, is less than the cost of assembly on site. As an example, forms for concrete panels might be transported to a construction site with reinforcing bars already built in, necessary coatings applied to the forms, and even special features such as electrical conduit already installed in the form. In some cases, it might be less expensive to pre-fabricate and transport the entire concrete panel to a manufacturing site. In contrast, traditional construction practice would be to assemble all the different features of the panel on-site. The relevant costs of these alternatives could be assessed during construction planning to determine the lowest cost alternative.
In addition to the consideration of direct costs, a construction planner should also consider some other aspects of this technology choice. First, the planner must insure that pre-fabricated components will satisfy the relevant building codes and regulations. Second, the relative quality of traditional versus pre-fabricated components as experienced in the final facility should be considered. Finally, the availability of components at the required time during the construction process should also be considered.
Example 3-13:Impacts of building codes
Building codes originated as a part of the building regulatory process for the safety and general welfare of the public. The source of all authority to enact building codes is based on the police power of the state which may be delegated by the state legislature to local government units. Consequently, about 8,000 localities having their own building codes, either by following a national model code or developing a local code. The lack of uniformity of building codes may be attributed to a variety of reasons:
- Neighboring municipalities may adopt different national models as the basis for local regulation.
- Periodic revisions of national codes may not be adopted by local authorities before the lapse of several years.
- Municipalities may explicitly decline to adopt specific provisions of national model codes or may use their own variants of key provisions.
- Local authorities may differ in interpretation of the same language in national model codes.
The lack of uniformity in building codes has serious impact on design and construction as well as the regulatory process for buildings. Among the significant factors are:
- Delay in the diffusion of new building innovations which may take a long time to find their ways to be incorporated in building codes.
- Discouragement to new production organizations, such as industrialized construction and prefabrication.
- Duplication of administrative cost of public agencies and compliance cost incurred by private firms.
3.12 Computer-Aided Engineering
In the past twenty years, the computer has become an essential tool in engineering, design, and accounting. The innovative designs of complicated facilities cited in the previous sections would be impossible without the aid of computer based analysis tools. By using general purpose analysis programs to test alternative designs of complex structures such as petrochemical plants, engineers are able to greatly improve initial designs. General purpose accounting systems are also available and adopted in organizations to perform routine bookkeeping and financial accounting chores. These applications exploit the capability for computers to perform numerical calculations in a pre-programmed fashion rapidly, inexpensively and accurately.
Despite these advances, the computer is often used as only an incidental tool in the design, construction and project management processes. However, new capabilities, systems and application programs are rapidly being adopted. These are motivated in part by the remarkable improvement in computer hardware capability, the introduction of the Internet, and an extraordinary decline in cost. New concepts in computer design and in software are also contributing. For example, the introduction of personal computers using microcircuitry has encouraged the adoption of interactive programs because of the low cost and considerable capability of the computer hardware. Personal computers available for a thousand dollars in 1995 have essentially the same capability as expensive mainframe computer systems of fifteen years earlier.
Computer graphics provide another pertinent example of a potentially revolutionary mechanism for design and communication. Graphical representations of both the physical and work activities on projects have been essential tools in the construction industry for decades. However, manual drafting of blueprints, plans and other diagrams is laborious and expensive. Stand alone, computer aided drafting equipment has proved to be less expensive and fully capable of producing the requiring drawings. More significantly, the geometric information required for producing desired drawings might also be used as a database for computer aided design and computer integrated construction. Components of facilities can be represented as three dimensional computer based solid models for this purpose. Geometric information forms only one component of integrated design databases in which the computer can assure consistency, completeness and compliance with relevant specifications and constraints. Several approaches to integrated computer aided engineering environments of this type have already been attempted. 
Computers are also being applied more and more extensively to non-analytical and non-numerical tasks. For example, computer based specification writing assistants are used to rapidly assemble sets of standard specifications or to insert special clauses in the documentation of facility designs. As another example, computerized transfer of information provides a means to avoid laborious and error-prone transcription of project information. While most of the traditional applications and research in computer aids have emphasized numerical calculations, the use of computers will rapidly shift towards the more prevalent and difficult problems of planning, communication, design and management.
Knowledge based systems represent a prominent example of new software approaches applicable to project management. These systems originally emerged from research in artificial intelligence in which human cognitive processes were modeled. In limited problem domains such as equipment configuration or process control, knowledge based systems have been demonstrated to approach or surpass the performance of human experts. The programs are marked by a separation between the reasoning or 'inference' engine program and the representation of domain specific knowledge. As a result, system developers need not specify complete problem solving strategies (or algorithms) for particular problems. This characteristic of knowledge based systems make them particularly useful in the ill-structured domains of design and project management. Chapter 15 will discuss knowledge based systems in greater detail.
Computer program assistants will soon become ubiquitous in virtually all project management organizations. The challenge for managers is to use the new tools in an effective fashion. Computer intensive work environments should be structured to aid and to amplify the capabilities of managers rather than to divert attention from real problems such as worker motivation.
3.13 Pre-Project Planning
Even before design and construction processes begin, there is a stage of 'pre-project planning' that can be critical for project success. In this process, the project scope is established. Since construction and design professionals are often not involved in this project scope stage, the terminology of describing this as a 'pre-project' process has arisen. From the owner's perspective, defining the project scope is just another phase in the process of acquiring a constructed facility.
The definition of a project scope typically involves developing project alternatives at a conceptual level, analyzing project risks and economic payoff, developing a financial plan, making a decision to proceed (or not), and deciding upon the project organization and control plan. The next few chapters will examine these different problems at some length.
The danger of poor project definition comes from escalating costs (as new items are added) or, in the extreme, project failure. A good definition of scope allows all the parties in the project to understand what is needed and to work towards meeting those needs.
Example 3-14: The Project Definition Rating Index (PDRI) for Building Projects The Construction Industry Institute has developed rating indexes for different types of projects to assess the adequacy of project scope definitions. These are intended to reflect best practices in the building industry and provides a checklist for recommended activities and milestones to define a project scope. The rating index is a weighted sum of scores received for a variety of items on the scope definition checklist. Each item in the checklist is rated as 'not applicable' (0), 'complete definition' (1), 'minor deficiencies' (2), 'some deficiencies' (3), 'major deficiencies' (4) or 'incomplete or poor definition' (5). Lower scores in these categories are preferable. Some items in the checklist include:
- Business Strategy for building use, justification, plan, economic analysis, facility requirements, expansion/alteration consideration, site selection issues and project objectives.
- Owner Philosophy with regard to reliability, maintenance, operation and design.
- Project Requirements for value engineering, design, existing facility, scope of work review, schedule and budget.
- Site Information including applicable regulatory reporting and permits requirements.
- Building Programming including room by room definitions for use, finishes, interior requirements and hvac (heating, ventilating and air conditioning).
- Design Parameters including all components and a constructability analysis.
- Equipment including inventory, locations and utility requirements.
- Au, T. and P. Christiano, Structural Analysis, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1987.
- Building Research Advisory Board, Exploratory Study on Responsibility, Liability and Accountability for Risks in Construction, National Academy of Sciences, Washington, D.C., 1978.
- Drucker, P.F., Innovation and Entrepreneurship: Practice and Principles, Harper and Row, New York, 1985.
- Gaylord, E., and C. Gaylord (Editors), Structural Engineering Handbook, McGraw-Hill Book Co., New York, 1979.
- Levitt, R.E., R.D. Logcher and N.H. Quaddumi, 'Impact of Owner-Engineer Risk Sharing on Design Conservatism,' ASCE Journal of Professional Issues in Engineering, Vol. 110, 1984, pp. 157-167.
- Simon, H.A., The Science of the Artificial, Second Edition, MIT Press, Cambridge, MA, 1981.
- Tatum, C.B., 'Innovation on the Construction Project: A Process View,' Project Management Journal, Vol. 18, No. 5, 1987, pp. 57-67.
- Pre-Project Planning Research Team, Pre-Project Planning Handbook Construction Industry Institute, Publication 39-2, April 1995.
- See 'ASCE Unveils Quality Manual', ENR, November 5, 1987, p. 14) Back
- See V. Fairweather, 'Milan's Model Metro', Civil Engineering, December 1987, pp. 40-43.Back
- See T.Y. Lin and B.G. Gerwick, Jr. 'Design of Long Span Concrete Bridges with Special References to Prestressing, Precasting, Structural Behavior and Economics,' ACI Publication SP-23, First International Symposium, 1969, pp. 693-704 Back
- See Linnhoff, B., D.W. Townsend, D. Boland, G.F. Hewitt, B.E.A. Thomas, A.R. Guy, and R.H. Marsland, User Guide on Process Integration for the Efficient Use of Energy, Institution of Chemical Engineers, Rugby, Warks., England, 1982. Back
- 'More Construction for the Money,' Summary Report of the Construction Industry Cost Effectiveness Project, The Business Roundtable, New York, 1983, pg. 30. Back
- See 'The Quiet Revolution in Skyscraper Design, ' Civil Engineering, May 1983, pp. 54-59. Back
- See J. Landis, 'Why Homebuilders Don't Innovate,' Built Environment, Vol. 8, No. 1, 1982, pp. 46-53. Back
- See P.J. Cassimates, Economics of the Construction Industry, National Industry Conference Board (SBE No. 111), 1969. Back
- See F. Moavenzadeh, 'Construction's High Technology Revolution,' Technology Review, October, 1985, pp. 32-39. Back
- For more information on Green Buildings see the LEED website: http://www.usgbc.org/LEED/LEED_main.asp Back
- See R.W. Jensen and C.C. Tonies (Editors), Software Engineering, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1979, p. 22. Back
- See S.J. Fenves, 'Computer Applications,' in Structural Engineering Handbook, (Gaylord, E. and C. Gaylord, Editors), McGraw-Hill Book Co., New York, NY, 1979. Back
- See T. Au, E.W. Parti and A.K.C. Wong, 'Computer Applications for Health Care Facility Design,' Computers in Biology and Medicine, Vol. 1, No. 4, 1971, pp. 299-316. Back
- The authors are indebted to E. D'Appolonia for suggesting this example. Back
- See E. D'Appolonia, R. Alperstein and D.J. D'Appolonia, 'Behavior of Colluvial Slope', ASCE Journal of Soil Mechanics and Foundations Division, Vol. 93, No. SM4, 1967, pp. 447-473. Back
- The material in this example is adapted from A.L. Tolman, A. P. Ballestero, W.W. Beck, G.H. Emrich, 'Guidance Manual for Minimizing Pollution from Waste Disposal Sites,' Report to the Municipal Environmental Research Laboratory, U.S. Environmental Protection Agency, EPA-600/2-78-142, August 1978. Back
- For discussions of industrialized building, see Bender, Richard, A Crack in the Rear View Mirror - A View of Industrialized Building, Von Nostrand Reinhold Co., 1983; Nutt-Powell, Thomas, E., Manufactured Homes: Making Sense of a Housing Opportunity, Auburn House, 1982; or Warzawski, A., M. Avraham, and D. Carmel, 'Utilization of Precast Concrete Elements in Building,' ASCE Journal of Construction Engineering and Management, Vol. 110, No. CO4, 1984, pp. 476-485. Back
- See C.G. Field and S.R. Rivkin, The Building Code Burden, Lexington Books, D.C. Heath and Co., Lexington, MA, 1975. Back
- See Rehak, Daniel R. and L.A. Lopez, Computer Aided Engineering Problems and Prospects, Dept. of Civil Engineering, University of Illinois, 1981. Back
- See PDRI for Building Projects Research Team, PDRI: Project Definition Rating Index for Building Projects, Construction Industry Institute, Resource 155-2, July 1999.Back
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This UML sequence diagram tutorial introduces the commonly used elements of UML sequence diagrams and explains how to use them.
All diagrams in this guide were created with Trace Modeler,an easy-to-use and smart editor for UML sequence diagrams developed by the author. Check out the 30 sec demo to see how easy it really is..
UML sequence diagrams are used to show how objects interact in a given situation. An important characteristic of a sequence diagram is that time passes from top to bottom : the interaction starts near the top of the diagram and ends at the bottom (i.e. Lower equals Later).
A popular use for them is to document the dynamics in an object-oriented system. For each key collaboration, diagrams are created that show how objects interact in various representative scenarios for that collaboration.
The UML sequence diagram gallery contains many examples, but here's a typical sequence diagram based on a system use case scenario :
The diagram above shows how objects interact in the 'rent item' collaboration when the item is not available during the requested period.
To clarify how execution switches from one object to another, a blue highlight was added to represent the flow of control. Note that this highlight is not part of the diagram itself.
As with all UML diagrams, comments are shown in a rectangle with a folded-over corner :
To relate the comment to whatever diagram elements it is about, connect them with dashed lines.
Objects as well as classes can be targets on a sequence diagram, which means that messages can be sent to them. A target is displayed as a rectangle with some text in it. Below the target, its lifeline extends for as long as the target exists. The lifeline is displayed as a vertical dashed line.
The basic notation for an object is
Where 'name' is the name of the object in the context of the diagram and 'Type' indicates the type of which the object is an instance. Note that the object doesn't have to be a direct instance of Type, a type of which it is an indirect instance is possible too. So 'Type' can be an abstract type as well.
Both name and type are optional, but at least one of them should be present. Some example :
As with any UML-element, you can add a stereotype to a target. Some often used stereotypes for objects are «actor», «boundary», «control», «entity» and «database». They can be displayed with icons as well :
An object should be named only if at least one of the following applies
- You want to refer to it during the interaction as a message parameter or return value
- You don't mention its type
- There are other anonymous objects of the same type and giving them names is the only way to differentiate them
Try to avoid long but non-descriptive names when you're also specifying the type of the object (e.g. don't use 'aStudent' for an instance of type Student). A shorter name carries the same amount of information and doesn't clutter the diagram (e.g. use 's' instead).
When you want to show how a client interacts with the elements of a collection, you can use a multiobject. Its basic notation is
Again, a name and/or type can be specified. Note however that the 'Type' part designates the type of the elements and not the type of the collection itself.
The basic notation for a class is
Only class messages (e.g. shared or static methods in some programming languages) can be sent to a class. Note that the text of a class is not underlined, which is how you can distinguish it from an object.
When a target sends a message to another target, it is shown as an arrow between their lifelines. The arrow originates at the sender and ends at the receiver. Near the arrow, the name and parameters of the message are shown.
A synchronous message is used when the sender waits until the receiver has finished processing the message, only then does the caller continue (i.e. a blocking call). Most method calls in object-oriented programming languages are synchronous. A closed and filled arrowhead signifies that the message is sent synchronously.
The white rectangles on a lifeline are called activations and indicate that an object is responding to a message. It starts when the message is received and ends when the object is done handling the message.
When a messages are used to represent method calls, each activation corresponds to the period during which an activation record for its call is present on the call stack.
If you want to show that the receiver has finished processing the message and returns control to the sender, draw a dashed arrow from receiver to sender. Optionally, a value that the receiver returns to the sender can be placed near the return arrow.
If you want your diagrams to be easy to read, only show the return arrow if a value is returned. Otherwise, hide it.
Messages are often considered to be instantaneous, i.e. the time it takes to arrive at the receiver is negligible. For example, an in-process method call. Such messages are drawn as a horizontal arrow.
Sometimes however, it takes a considerable amount of time to reach the receiver (relatively speaking of course) . For example, a message across a network. Such a non-instantaneous message is drawn as a slanted arrow.
You should only use a slanted arrow if you really want to emphasize that a message travels over a relatively slow communication channel (and perhaps want to make a statement about the possible delay). Otherwise, stick with a horizontal arrow.
A found message is a message of which the caller is not shown. Depending on the context, this could mean that either the sender is not known, or that it is not important who the sender was. The arrow of a found message originates from a filled circle.
With an asynchronous message, the sender does not wait for the receiver to finish processing the message, it continues immediately. Messages sent to a receiver in another process or calls that start a new thread are examples of asynchronous messages. An open arrowhead is used to indicate that a message is sent asynchrously.
A small note on the use of asynchronous messages : once the message is received, both sender and receiver are working simultaneously. However, showing two simultaneous flows of control on one diagram is difficult. Usually authors only show one of them, or show one after the other.
Message to self
A message that an object sends itself can be shown as follows :
Keep in mind that the purpose of a sequence diagram is to show the interaction between objects, so think twice about every self message you put on a diagram.
Creation and destruction
Targets that exist at the start of an interaction are placed at the top of the diagram. Any targets that are created during the interaction are placed further down the diagram, at their time of creation.
A target's lifeline extends as long as the target exists. If the target is destroyed during the interaction, the lifeline ends at that point in time with a big cross.
A message can include a guard, which signifies that the message is only sent if a certain condition is met. The guard is simply that condition between brackets.
If you want to show that several messages are conditionally sent under the same guard, you'll have to use an 'opt' combined fragment. The combined fragment is shown as a large rectangle with an 'opt' operator plus a guard, and contains all the conditional messages under that guard.
A guarded message or 'opt' combined fragment is somewhat similar to the if-construct in a programming language.
If you want to show several alternative interactions, use an 'alt' combined fragment. The combined fragment contains an operand for each alternative. Each alternative has a guard and contains the interaction that occurs when the condition for that guard is met.
At most one of the operands can occur. An 'alt' combined fragment is similar to nested if-then-else and switch/case constructs in programming languages.
When a message is prefixed with an asterisk (the '*'-symbol), it means that the message is sent repeatedly. A guard indicates the condition that determines whether or not the message should be sent (again). As long as the condition holds, the message is repeated.
The above interaction of repeatedly sending the same message to the same object is not very useful, unless you need to document some kind of polling scenario.
A more common use of repetition is sending the same message to different elements in a collection. In such a scenario, the receiver of the repeated message is a multiobject and the guard indicates the condition that controls the repetition.
This corresponds to an iteration over the elements in the collection, where each element receives the message. For each element, the condition is evaluated before the message is sent. Usually though, the condition is used as a filter that selects elements from the collection (e.g. 'all', 'adults', 'new customers' as filters for a collection of Person objects). Only elements selected by the filter will receive the message.
If you want to show that multiple messages are sent in the same iteration, a 'loop' combined fragment can be used. The operator of the combined fragment is 'loop' and the guard represents the condition to control the repetition.
Again, if the receiver of a repeated message is a collection, the condition is generally used to specify a filter for the elements.
For example, to show that the bounds of a drawing are based on those of its visible figures we could draw the following sequence diagram :
Several things are worth noting in this example
- a local variable 'r' was introduced to clarify that it is the result of getBounds that is added.
- naming the resulting Rectangle 'bounds' avoids the introduction of an extra local variable.
- the loop condition is used as a filter on the elements of the figures collection.
Of all the UML diagram types, the sequence diagram type is the one where it matters most to choose the right tool for the job. The reason being that you have very little freedom when it comes to positioning elements on a sequence diagram :
- some elements must be placed in a certain region
- some elements must surround others
- many elements are interconnected
- most elements have a fixed orientation
- the grid-like structure practically demands a uniform spacing
- there are plenty of opportunities for elements to overlap in a bad way
You really need a tool that was designed with sequence diagrams in mind. Don't even think about using a general-purpose drawing tool, you'll waste hours connecting, resizing and laying out shapes.
With that many constraints you would think that current tools take care of the layout for you, right? Think again.
Most UML-based CASE tools offer only basic support for sequence diagrams and have low usability. Although they're an improvement over general-purpose editors, they offer little assistance when it comes to layout issues and you'll still waste a lot of time moving elements around.
When you evaluate a tool, find out how it reacts when you change an existing diagram. Add stuff, move elements around and look at the resulting diagram. Is it still a visually pleasing diagram, or do you have to step in and manually redo the layout?
|A checklist of things to try when you're evaluating a tool for sequence diagrams|
|Add a new message.|
Did you have to connect arrows and activations by hand?
|Move a target all the way to the left or right.|
Are the message arrows still connected to the correct side of their activations?
|Insert a new message in the middle of the diagram.|
Were the existing elements below it automatically moved down to make space for it?
|Change the receiver of a message.|
Did the activations and arrows adjust themselves accordingly? Even when you made it into a message to self?
|Move a message or an activation up or down.|
Were they and the elements around them adjusted accordingly or did you have to do that yourself?
|Move a message in and out of a combined fragment.|
Was the fragment automatically resized and moved in correspondence to its contents?
|Scroll down so that the targets are no longer visible.|
Does the tool provide any hints as to which lifeline belongs to which target?
|Change the name of a message.|
Did you edit it in-place on the diagram itself?
Pick the tool with the best automatic layout features, it will save you an enormous amount of time.
Construction Sequence Diagram Maker
I never quite found what I was looking for and ended up creating my own. Trace Modeler supports all of the above and has some really neat layout features and benefits that I haven't seen anywhere else, I invite you to watch the demo and then try it yourself!
This quick introduction discussed only a few of the possible constructs you may encounter on a UML sequence diagram. A forthcoming article will discuss when an how to use sequence diagrams in projects. Look for it in the articles section or subscribe to the news feed to receive notification when new articles and content is published.
As you set off to find out all there is to know about UML sequence diagrams, bear in mind that it is more important to know when and how to use them, than to know every possible construct in the UML specification.
Construction Sequence Diagram Definition
For practical tips on how to improve your own diagrams, have a look at the Pimp my diagram series of articles. Each episode discusses and improves a real-world sequence diagram example found on the web.
Be sure to take a peek in the gallery section, it contains many examples of sequence diagrams.