Chapter OneStructural Design Process
1.1 Nature of the Process
This book is primarily intended as a textbook for students enrolled in professionally accredited architecture programs. A secondary audience includes interns preparing for the architectural registration exams. There may also be limited markets for professional architects, structural engineers desiring to better understand the architect's approach to structures in the context of the larger design problem, and persons interested in pursuing a career in either architecture or structural engineering.
Architects have a huge array of issues to address in architectural practice. Among these are the following: keeping rain out of a building, getting water off a site, thermal comfort, visual comfort, space planning, fire egress, fire resistance, corrosion and rot resistance, vermin resistance, marketing, client relations, the law, contracts, construction administration, the functional purposes of architecture, the role of the building in the larger cultural context, security, economy, resource management, codes and standards, and how to make a building withstand all the forces to which it will likely be subjected during its lifetime. This last subject area is referred to as architectural structures.
Because of the extraordinary range of demands on an architect's time and skills and the extraordinary number of subjects that architecture students must master, architectural structures are typically addressed in only two or three lecture courses in an accredited architectural curriculum in the United States. These two or three lecture courses must be contrasted with the ten or twelve courses that will normally be taken by a graduate of an accredited structural engineering curriculum. This contrast in level of focus makes it clear why a good structural engineering consultant is a very valuable asset to an architect. However, having a good structural consultant does not relieve the architect of serious responsibility in the structural domain. All architects must be well versed in matters related to structures. The architect has the primary responsibility for establishing the structural concept for a building, as part of the overall design concept, and must be able to speak the language of the structural consultant with sufficient skill and understanding to take full advantage of the consultant's capabilities.
Most books on structures are written by structural engineers for an audience of structural engineers. This focus is not appropriate in nature to the needs of the architect, who must understand how structure fits into the larger design context. Furthermore, it is not appropriate in scale, inasmuch as the texts required by an engineering student over the course of that student's education will fill an entire cabinet. Given the wide range of other learning responsibilities of an architecture student, there is not enough money to acquire, or time to use, an entire cabinet full of books on structures. To support the learning needs of architecture students, a text is needed that is different in both scope and approach from the reference material provided for engineers.
It is the goal of this text to supply the architecture student with a comprehensive set of learning and reference materials to help prepare that student to enter the workforce as a serious professional, competent to deal with structural issues at the level, and in the manner, appropriate to architects. This book can also serve as a valuable reference for architectural interns preparing for the architectural registration examinations.
1.2 General Comments Regarding Architectural Education
Structural design is one of the more rigorous aspects of architectural design. Much knowledge has been generated and codified over the centuries that human beings have been practicing in and developing this field. This book gives primary attention to those things that are known, quantified, and codified.
However, very few things in the realm of architecture yield a single solution. To any given design problem, there are many possible solutions, and picking the best solution is often the subject of intense debate. Therefore, no one should come to this subject matter assuming that this text, or any text, is going to serve up a single, optimized solution to any design problem, unless that design problem has been so narrowly defined as to be artificial.
In design, there is always a great deal of latitude for personal expression. Design is purposeful action. The designer must have an attitude to act. Architecture students develop an attitude through a chaotic learning process involving a lot of trial and error. In going through this process, an architecture student must remain aware of a fundamental premise: the process is more important than the product; that is, the student's learning and development are more important than the output. The student has a license to make mistakes. It is actually more efficient to plow forward and make mistakes than to spend too much time trying to figure out how to do it perfectly the first time. To paraphrase the immortal words of Thomas Edison: To have good ideas, you should have many ideas and then throw out the bad ones. Of course, throwing out the bad ones requires a lot of rigorous and critical thinking. No one should ever fall in love with any idea that has not been subjected to intense and prolonged critical evaluation and withstood the test with flying colors. Furthermore, important ideas should be subjected to periodic reevaluation. Times and conditions change. Ideas that once seemed unassailable may outlive their usefulness or, at the very least, need updating in the light of new knowledge and insights.
This text focuses primarily on exploring the known, quantified, and codified, but it also honors the chaotic learning process described here. On some projects, students will be given fairly wide latitude to generate concepts and to explore. Optimally, the educational experience will be stronger if the student explores this subject matter in the context of a design process, such as would occur in a studio environment, where feedback is provided by enlightened people with a wide range of experience and philosophical points of view.
In pursuing this subject matter, it is valuable to have a frame of reference regarding the roles of the architect, as the leader of the design team, and the structural engineer, as a crucial contributor of expertise and hard work needed to execute the project safely and effectively. The diagram in Figure 1.1 will help provide that frame of reference.
In contemplating the diagram in Figure 1.1, keep in mind that design and analysis are two sides of the same coin and that the skills and points of view of architects and engineers, although distinctive, also overlap and sometimes blur together. The most effective design teams consist of individuals with strong foci who can play their respective roles while having enough overlap in understanding and purpose that they can see each other's point of view and cooperate in working toward mutually understood and shared goals. The most harmful poison to a design team is to have such a separation in points of view and understanding that a rift develops between the members of the team. Cooperation is the watchword in this process, as in all other team efforts.
1.3 Background of the Reader
The prerequisites of a student for optimum utilization of this text include the following:
A working knowledge of plane and solid geometry (This is absolutely fundamental to the design of architecture and should be a part of any architect's basic repertoire.)
A working knowledge of arithmetic (This is part of the basic repertoire of any educated, thinking person. No architect can make good judgments without the arithmetic that reinforces a sense of scale, proportions, and economy.)
An introduction to trigonometry and vectors
Basic skills in sketching
Basic skills in fashioning scale models out of cardboard, wood, plastic, and/or metals
A basic knowledge of computers, including word processing, spreadsheet analysis, and computer-aided design (CAD)
An understanding of calculus is helpful, but this book is crafted in such a manner that calculus is not crucial to grasping the concepts.
A computer with appropriate software is such a powerful tool for learning and exploration that any course of study that does not take advantage of that tool is far from ideal in preparing an architecture student for the future workplace. Therefore, students intending to use this text to maximum advantage in a full assault on the subject should have access to a computer with word processing, a spreadsheet program, and a structural analysis program. Examples of the latter are Multiframe, Strudl, SAP, RISA, STAAD.Pro, Tekla Xsteel, S-Frame, ETABS, MIDAS, ProSteel 3D, and RamSteel. This book will provide examples of the principles of analysis on which these programs are based. It will also take the student through many of these examples in the form of assignments designed to reinforce the concepts.
The computer analysis programs are important for several reasons:
1. They eliminate much of the tedious math, allowing the student to focus on concepts and to explore the behavior and attributes of many more structural forms than would be possible if the student were straddled with the responsibility of carrying out all of the math longhand.
2. The computer facilitates the analysis of very complex three-dimensional structures that simply could not be done reliably by longhand analysis.
3. The programs provide visualization tools that are invaluable for exploring both geometry and structural behavior.
1.4 Vehicles for Delivering the Concepts
1. Freebody diagrams. These are at the absolute heart of structural design. Understanding how freebodies are constructed and interpreted is vital to the most basic concepts in structures.
2. Math (primarily geometry and arithmetic). These give scale and rigor to everything the architect does in structural design.
3. Spreadsheet programs for computers. These programs are powerful aids in organizing and carrying out computations. They provide:
Sophisticated and rapid computational tools
Ease of use
A record of the inputs that can be used in checking and troubleshooting
A record of the equations that can be used in checking and troubleshooting
Graphic output for visualization and presentations
These programs are already commonplace tools for architects to use in generating budgets and doing value analysis. Applying them in a structures course to generate computational templates is an obvious match.
4. Computer simulations showing axial forces, axial stresses, moments, bending stresses, shear forces, shear stresses, and deformation under various loading conditions. These programs are a requirement in any serious course in structures. The ease they provide in visualizing and exploring structural behavior is simply unprecedented. The use of these programs is featured heavily in the examples and assignments in this book.
5. Physical testing and physical models demonstrating the structural behavior of elements and/or systems of elements. The tactile feedback provided by physical experiments and models is a powerful aid to a student's comprehension. They are not easy to make in a manner that truly simulates the behavior of a full-sized structure, but they are worth the effort. Some phenomena, such as buckling, are better understood in physical models than in any other learning media. Moreover, models teach students about statistical variations in performance that are not apparent in purely computational processes. There is nothing like testing a series of models that were intended to be identical to help students understand why safety factors are important.
6. Design solutions embodied in actual building structures. There are vast insights to be gathered from the successful designs born of great minds that have grappled with this subject over the centuries. These should be revisited often, each time with a fresh eye to see things that may have been overlooked before. They should include examples where the integration of structure with the other building systems has been addressed in at least a competent, if not inspired, manner.
7. Practical examples in value engineering-that is, demonstrating efficient ways to determine the structural costs of providing greater architectural amenities-such as the following:
The structural cost of increasing span to reduce the number of columns interfering with efficient space planning The structural cost of using rigid frames, as opposed to shear walls or triangulating struts, as a way to promote freer movement of people and equipment through a structure The structural cost of introducing openings for admitting natural light to illuminate the interior of a building
8. Data on properties of materials.
9. Data on dimensions and section properties for common structural elements, such as standard rolled and formed steel sections.
10. Load tables for columns, beams, and trusses. These are particularly helpful for quick sizing and for doing cost-benefit analysis for common building types. Introducing students to the great compendia that are the source of this information is also an important goal of this text.
11. The written word. Words alone are a poor means of understanding and communicating structural behavior. However, words provide a indispensable tool in organizing our ideas about the subject.
12. Assignments and projects. Exercise is the primary road to learning.
1.5 Expectations Regarding the Outcome of the Learning Process
Learning goals for a student working with this book are expressed in terms of three levels of achievement in design activity.
The first level of structural design activity is primarily qualitative, including the following:
Concept generation-that is, understanding what kinds of elements need to be included in the structural system to deal with the entire array of vertical and lateral loads on a structure; understanding how to make the structural system mesh with the spatial and functional requirements of the architectural design.
Applying simple rules of thumb to establish the proportions of structural elements-for example, the depth of a parallel-chord truss will typically be in the range of 0.042 to 0.062 times the span of the parallel-chord truss; the final depth will depend on a variety of structural, economic, and architectural factors to be worked out in later stages of the design process.
Architects should be able to perform these design activities competently and should do them routinely in practice.
The second level of structural design activity is semi-quantitative, including the following:
Geometric definition of a structure (This can be fairly straightforward, such as in the case of a system of beams and columns laid out on a regular grid, to quite challenging, such as in the case of a hyperbolic paraboloid network, a geodesic dome, or a free-form structure like architect Frank Gehry's art museum in Bilbao.)
Quick, approximate sizing of elements, such as beams and open-web joists, using tables of standard elements
Architects should be able to perform these activities competently enough to have a sense of what an engineer might be doing in support of the architect on a given project. A significant goal of this text is to provide students with a sufficient understanding of the structural issues and engineering design processes to confidently engage an engineer in the overall design process. Some architects will choose to perform these functions in practice; others may choose to have their engineering consultants perform such functions.