Faculty of Engineering
Department of Civil Engineering
“Conceptual Design of Bridges”
Graduation Project 1
Mentor: Candidate:
Assist. Prof. Dr. Jelena Ristic Emre Gök
Email: emregok59@yandex.com
Skopje, 2019
1
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Conceptual Design of Bridges: A Comprehensive Guide for Civil Engineering Students, Thesis of Advanced Calculus
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Faculty of Engineering Department of Civil Engineering
“Conceptual Design of Bridges”
Graduation Project 1 Mentor: Candidate: Assist. Prof. Dr. Jelena Ristic Emre Gök Email: emregok59@yandex.com Skopje, 2019
Table of Contents
Abstract 3
Introduction 4
Nature of The Structural Design Process 5
Description and Justification 6
Public and Personal Knowledge 7
Regulation 8
Design Process 9
Aesthetics in Bridge Design 10
Definition of Aesthetics 11
Qualities of Aesthetic Design 12
Guidelines for Medium- and Short-Span Bridges 23
References 35
Abstract The design process of a bridge can be divided into four basic stages: conceptual design, preliminary design, detailed design and construction design. The purpose of the conceptual design is to come up with various feasible bridge schemes and to decide on one or more final concepts for further consideration. The purpose of the preliminary design is to select the best scheme from these proposed concepts and then to ascertain the feasibility of the selected concept and finally to refine its cost estimates. The engineer follows the logical steps based on mathematics and physics after the general
1. Introduction Often engineers deceive themselves into believing that if they have gathered enough information about a bridge site and the traffic loads, the selection of a bridge type for that situation will be automatic. Engineers seem to subscribe to the belief that once the function of a structure is properly defined, the correct form will follow. Furthermore, that form will be efficient and aesthetically pleasing. Perhaps we believe some great differential equation exists, and, if we could only describe the relationships and the gradients between the different parameters, apply the correct boundary conditions, and set the proper limits of integration, a solution of the equation will give us the best possible bridge configuration. Unfortunately, or perhaps fortunately, no such equation exists that will define the optimal path. If we have no equation to follow, how is a conceptual design formulated? (In this context, the word design is meant in its earliest and broadest sense; it is the configuration one has before any calculations are made.) Without an equation and without calculations, this process will be presented below.
2. Nature of The Structural Design Process The structural design process itself is probably different for every engineer because it is dependent on personal experience. However, certain characteristics about the process are common and serve as a basis for discussion. For example, we know (1) that when a design is completed in our minds, we must then be able to describe it to others; (2) that we have different backgrounds and bring different knowledge into the design process; and (3) that the design is not completely open ended, constraints exist that define an acceptable solution(s). These characteristics are part of the nature of structural design and influence how the process takes place. A model of the design process incorporating these characteristics has been presented by Addis (1990) and includes the following components: output, input, regulation, and the design procedure. A schematic of this model is shown in Figure 1. Fig 1. Model of structural design process (Addis, 1990)
reinforced concrete frame. Early on, screens will be displayed on the monitor asking the analyst to supply coordinates of joints, connectivity of the members, and boundary conditions. From this information, the computer program generates a mathematical model of stick members that have no depth, joints that have no thickness, and supports modeled as rollers, hinges, or are completely restrained. Often the mathematical model inductively assumes plane sections remain plane, distributed force values to be concentrated at nodes, and idealized boundary conditions at the supports. Next, the user is asked to supply constants or parameters describing material behavior, all of which have been determined inductively from experimental observations. Finally, the values of forces at the nodes determined by the equation solvers in the program must be interpreted as to their acceptance in the real world. This acceptance is based on inductively determined safety factors, load and resistance factors, or serviceability criteria. In short, what appears to be infallible deductive justification of a proposed design is, in fact, based on inductive concepts and is subject to possible error and, therefore, is fallible. Oftentimes engineers select designs on the basis that they are easy to justify. If an engineer feels comfortable with the analysis of a particular bridge type, that bridge configuration will be used again and again. For example, statically determinate bridge structures of alternating cantilever spans and suspended spans were popular in the 1950s before the widespread use of computers because they were easier to analyze. The same could be said of the earlier railroad truss bridgeswhose analysiswasmade simple by graphical statics. One advantage of choosing designs that are easily justified is that those responsible for checking the design have no difficulty visualizing the flow of forces from one component to another.Now, with sophisticated computer software, an engineer must understand how forces are distributed throughout themembers of more complex systems to obtain a completed design. The advantage of simple analysis of statically determinate structures is easily offset by their lack of redundancy or multiple load paths. Therefore, it is better to choose continuous beams with multiple redundancies even though the justification process requires more effort to ensure that it has been done properly. Not only is there an interrelationship between deductive and inductive reasoning, there is also an interrelationship between description and justification. The configuration described for a bridge structure determines its behavior. Triangles in trusses, continuous beams, arches, and suspension systems have distinctly different spatial characteristics and, therefore, behave differently. Description and justification are linked together, and it is important that a bridge engineer be proficient in both areas with an understanding of the interactions among them. 2.2 Public and Personal Knowledge Engineer brings both public and personal knowledge to the design process. Public knowledge is accumulated in books, databases, software, and libraries and can be passed on from generation to generation. Public knowledge includes handbooks of material properties, descriptions of successful designs, standard specifications, theoretical mechanics, construction techniques, computer programs,
cost data, and other information too voluminous to describe here. The link between judgment and experience has been explained this way: Good judgment comes from experience and experience comes from bad judgment. Sometimes experience can be a tough teacher, but it is always increasing our knowledge base. 2.3 Regulation Our bridge designs are not open ended. There are many constraints that define the boundaries of an acceptable design. These constraints the factors that should be considered in bridge design are the demands of the customers, the architect's drawing, the codes used, the engineer's knowledge, materials, economic factors, environmental problems, legal problems, and finally the least influential political factors. For example, if a bridge is to traverse coastal wetlands, the restrictions on how it can be built will often dictate the selection of the bridge type. If contractors in a particular region are not experienced in the construction method proposed by an engineer, then that may not be the proper design for that locality. A bridge designer should pay attention to environmental factors. For example, if a bridge is to traverse coastal wetlands, the restrictions on how it can be built will often dictate the selection of the bridge type. If contractors in a particular region are not experienced in the construction method proposed by an engineer, then that may not be the proper design for that locality. Geometric constraints on alignment are quite different for a rural interstate overcrossing than for a densely populated urban interchange. Somehow a bridge designer must be able to satisfy all these restrictions and still have a bridge with pleasing appearance that remains personally and publicly satisfying. 2.4 Design Process The process of design is what occurs within the rectangular box of Figure 3.1. An engineer knows what the output has to be and what regulations govern the design, but because each person has accumulated different knowledge and experience, it is difficult to describe a procedure for design that will work in all cases. As Addis (1990) says: “Precisely how and why a structural engineer chooses or conceives a particular structure for a particular purpose is a process so nebulous and individual that I doubt if it is possible to study it at all.” It may not be possible to definitively outline a procedure for the design process, but it is possible to identify its general stages. The first is the data gathering stage, followed by the conceptual, rhetorical, and schematic stages. In the data gathering stage, one amasses as much information as one can find about the bridge site, topography, functional requirements, soil
design procedure outlined by Leonhardt (1982), he encourages a designer to seek criticism by posting sketches of the proposed design on the walls around the office so others can comment on them. It is surprising what additional pairs of trained eyes can see when they look at the sketches. Well, maybe it is no surprise because behind every pair of eyes is a whole different set of experiences and knowledge, which brings to mind what deMiranda (1991) says about the three mentalities that must be brought to the design process: One should be creative and aesthetic, the second analytical, and the third technical and practical, able to give a realistic evaluation of the possibilities of the construction technique envisagedand the costs involved. If these threementalities do not coexist in a single mind, they must always be present on terms of absolute equality in the group or team responsible for the design. In short, make the sketches, talk about them, make revisions, let others critique them, defend the design, be willing to make adjustments, and keep interacting until the best possible design results. It can be a stimulating, challenging, and intellectually rewarding process. The function of the design process is to produce a bridge configuration that can be justified and described to others. Now is the time to apply the equations for justification of the design and to prepare its description on plans and in specifications. Computers can help with the analysis and the drawings, but there are still plenty of tasks to keep engineers busy. The computer software packages will do thousands of calculations, but they must be checked. Computer-driven hardware can plot full-size plan sheets, but hundreds of details must be coordinated. Model specifications may be stored in a word processor file, but every project is different and has a unique description.A lot of labor follows the selection of the bridge configuration so it must be done right. As Leonhardt (1982) says: The phase of conceptual and aesthetic design needs a comparatively small amount of time, but is decisive for the expressive quality of the work.
3. Aesthetics in Bridge Design If we recognize that the conceptual design of a bridge begins in the mind, we only need now to convince ourselves that the design we conceive in our mind is inherently beautiful. It is our nature to desire things that are lovely and appeal to our senses. We enjoy good music and soft lights.We furnish our homes with fine furniture and select paintings and colors that please our eyes. We may say that we know nothing about aesthetics, yet our actions betray us.We do know what is tasteful, delights the eye, and is in harmony with its surroundings. Perhaps we have not been willing to express it. We need to realize that it is all right to have an opinion and put confidence in what has been placed within us.We simply need to carry over the love of beauty in our daily lives to our engineering projects. When an engineer is comparing the merits of alternative designs, some factors are more important than others. The conventional order of priorities in bridge design is safety, economy, serviceability, constructability, and so on. Somewhere down this list is aesthetics. Little doubt exists that aesthetics needs a priority boost and that it can be done without significantly infringing upon the other factors.
In recent years, engineers have come to realize that improved appearance does not necessarily increase the cost. Oftentimes the most aesthetically pleasing bridge is also the least expensive. Sometimes a modest increase in construction cost is required to improve the appearance of a bridge. Menn (1991) states that the additional construction costs are about 2% for short spans and only about 5% for long spans. Roberts (1992) seconds this conclusion in his article on case histories of California bridges. Public expenditures on improved appearance are generally supported and appreciated. Given a choice, even with a modest increase in initial cost, the public prefers the bridge that has the nicer appearance. Unfortunately, an engineer may realize this after it is too late. Gottemoeller (1991) tells of the dedication of a pedestrian bridge over a railroad track in the heart of a community in which speaker after speaker decried the ugliness of the bridge and how it had inflicted a scar on the city. Function or costs were not primary concerns of the public, only its appearance. Needless to say, they rejected a proposal for constructing a similar bridge nearby. It is unfortunate that an engineer has to build an ugly bridge that will remain long after its cost is forgotten to learn the lesson that the public is concerned about appearance. It is not possible in this short chapter to completely discuss the topic of bridge aesthetics. Fortunately, good references are dedicated to the subject, which summarize the thoughts and give examples of successful bridge designers throughout the world. Two of these resources are Esthetics in Concrete Bridge Design, edited byWatson and Hurd (1990), and Bridge Aesthetics Around theWorld , edited by Burke (1991). A third reference of note is Bridgescape: The Art of Designing Bridges by Gottemoeller (2004). By drawing on the expertise in these references, we will attempt to identify those qualities that most designers agree influence bridge aesthetics and to give practical guidelines for incorporating them into medium- and short-span bridges. 3.1 Definition of Aesthetics The definition of the word aesthetics may vary according to the dictionary one uses. But usually it includes the words beauty, philosophy, and effect on the senses. A simple definition could be: Aesthetics is the study of qualities of beauty of an object and of their perception through our senses. 3.2. Qualities of Aesthetic Design
The function of a bridge must be defined and understood by the designer, client, and public. How that functionis satisfied can take many forms, but it must always be kept in mind as the basis for all that follows. Implied with the successful completion of a bridge that fulfills its function is the notion that it does so safely. If a bridge disappears in a flood or other calamity, one does not take much comfort in the fact that it previously performed its function. A bridge must safely perform its function with an acceptably small probability of failure. Proportion Artists, musicians, and Mathematicians realize that fora painting a composition or a geometric pattern to be pleasing it must be in proper proportion. Consider the simple case of dividing a line into two segments. Dividing the line into unequal segments generates more interest than division into equal segments. Around 300 BC, Euclid proposed that a pleasing division of the line would be when the ratio of the shorter segment to the longer segment was the same as the ratio of the longer segment to the whole. Stating Euclid’s proposition mathematically, if the total length of the line is x and the longer segment is unity, then the shorter segment is x – 1 and the equality of ratios gives (x − 1)/1 = 1/x. The positive root of the resulting quadratic equation is (√ 5 +1)/2 = 1.618. This ratio of the total length of the line to the longer segment has been called the golden ratio , the golden proportion, the golden section, and the golden number. This particular proportion between two values is not limited to mathematics but is found in biology, sculpture, painting, music, astronomy, and architecture (Livio, 2002). Throughout history, the ratio for length to width of rectangles of 1.618 has been considered the most pleasing to the eye. There still are advocates (Lee, 1990) of geometric controls on bridge design and an illustration of the procedure is given in Fig 3. Fig 3. Proportioning of Mancunian Way cross section (Lee, 1990)
The proportioning of the Mancunian Way Bridge cross section in Manchester, England, was carried out by making a layout of golden section rectangles in four columns and five rows. The three apexes of the triangles represent the eye-level position of drivers in the three lanes of traffic. The profile of the cross section was then determined by intersections of these triangles and the golden sections. It may be that proportioning by golden sections is pleasing to the eye, but the usual procedure employed by successful designers hasmore freedomand arriving at a solution is often by trial and error. It is generally agreed that when a bridge is placed across a relatively shallow valley , as shown in Figure 4, the most pleasing appearance occurs when there are an odd number of spans with span lengths that decrease going up the side of the valley (Leonhardt, 1991). Fig 4. Bridge in shallow valley: flat with varying spans; harmonious (Leonhardt, 1991) The bridge over a deep valley in Figure 5 (Leonhardt, 1991) again has an odd number of spans, but they are of equal length. In this case, the negative spaces provide a transition of pleasing rectangular shapes from vertical to horizontal. Fig. 5 Bridge in deep V-shaped valley: large spans and tapered piers (Leonhardt, 1991). Adding to the drama of the bridge is the slender continuous girder and the tall, tapered piers. An example of such a bridge is the Magnan Viaduct, near Nizza on the French Riviera, shown in Figure 6 (Muller, 1991).
Fig. 8 Example of superstructure continuity on single hammerhead piers. Fig. 3.9 Example of poor depth transitions and awkward configurations due to lack of superstructure continuity It shows what can happen when the least effort by a designer drives a project. It does not have to be that way. Consider this quotation from Gloyd (1990): “When push comes to shove, the future generation of viewers should have preference over the present generation of penny pinchers.” A designer should also realize that the proportions of a bridge change when viewed from an oblique angle as seen in Figure 10 (Menn, 1991).
Fig. 10 Single columns increase the transparency of tall bridge (Menn, 1991) To keep the piers from appearing as a wall blocking the valley, Leonhardt (1991) recommends limiting the width of piers to about one-eighth of the span length (Fig. 11). He further recommends that if groups of columns are used as piers, their total width should be limited to about one-third of the span length (Fig. 12). Fig. 11 Proportion for pier width not to exceed one-eighth of the span (Leonhardt, 1991). Fig. 12 Proportion for total width of groups of columns not to be larger than one-third of the span (Leonhardt, 1991)
Harmony between the whole structure and its surroundings depends on the scale or size of the structure relative to its environment. A long bridge crossing a wide valley (Fig. 13 ) can be large because the landscape is large. But when a bridge is placed in an urban setting or used as an interstate overpass, the size must be reduced. Illustrations of bridges that are in harmony with their environment are the overpass in Figure 15 and the Blue Ridge Parkway (Linn Cove) Viaduct, Figure 16. A bridge that is in harmony derives its size and scale from its surroundings. Fig. 15 Well-proportioned concrete arch, West Lilac Road overpass, I- Fig. 16 Blue Ridge Parkway (Linn Cove) Viaduct, Grandfather Mountain, North Carolina Order and Rhythm When discussing order and rhythm in bridge structures, the same words and examples are often used to describe both. For example, the bridge in Figure 17 illustratesboth good order and rhythm.The eye probably first sees the repeating arches flowing across the valley with the
regularity of a heartbeat. But also one perceives that all of the members are tied together in an orderly manner in an uninterrupted flow of beauty with a minimum change of lines and edges. If a girder were to replace one of the arch spans, the rhythm would be lost. Rhythm can bring about order, and good order can bring about a wholeness and unity of the structure. Fig. 17 TunkhannockViaduct nearNicholson, Pennsylvania,designed by A. Burton Cohen. Contrast and Texture Contrast, as well as harmony, is necessary in bridge aesthetics. As often present in music and in paintings, bright sounds and bright colors are contrasted with soft and subtle tones—all in the same composition. Incorporation of these into our bridges keeps them from becoming boring and monotonous. All bridges do not have to blend in with their surroundings. Fernandez-Ordóñez (1991) quotes the following from Eduardo Torroja: When a bridge is built in the middle of the country, it should blend in with the countryside, but very often, because of its proportions and dynamism, the bridge stands out and dominates the landscape. This dominance seems to be especially true of cable-stayed and suspension bridges, such as seen in Figures 18 and 19. This dominance of the landscape does not subtract from their beauty.