How is a bridge designed?

Data for the design

A substantial amount of data is needed at the beginning of the design work for a bridge:

1. A plan of the site showing all obstacles to be bridged such as rivers, streets, roads or railroads, the contour lines of valleys and the desired alignment of the new traffic route.
2. Longitudinal section of the ground along the axis of the planned bridge with the conditions for clearances or required flood widths. Desired vertical alignment of the new route.
3. Required width of the bridge, width of lanes, median, walkways, safety rails etc.
4. Soil conditions for foundations, results of borings with a report on the geological situation and soil mechanics data. The degree of difficulty of foundation work has a considerable influence on the choice of the structural system and on the economical span length.
5. Local conditions like accessibility for the transport of equipment, materials and structural elements. Which materials are available and economical in that part of the country? Is water or electric power at hand? Can a high standard of technology be used or must the bridge be built with primitive methods and a small number of skilled labourers?
6. Weather and environmental conditions, floods, high and low tide levels, periods of drought, range of temperatures, length of frost periods.
7. Topography of the environment - open land, flat or mountainous land, scenic country. Town with small old houses or city with high rise buildings. The scale of the environment has an influence on the design.
8. Environmental requirements regarding aesthetic quality. Bridges in towns that affect the urban environment and that are frequently seen at close range - especially pedestrian bridges - need more delicate shaping and treatment than bridges in open country. Is protection of pedestrians against spray and noise needed? Is noise protection necessary for houses close to the bridge?

The designer should have seen the bridge site and its environment.

The creative process in designing large bridges

he data described above must be fully assimilated and remembered. The bridge must then take its initial shape in the imagination of the designer. For this process to take place, the designer should have first consciously seen and studied many bridges in the course of a long learning process. He should know, when a beam bridge, an arch or a suspension bridge will be suitable, what influence foundation conditions have on the choice of spans and structural systems and which depths of girder he will need for a certain span etc.

This means that in order to design a feasible bridge, the designer must have an extensive knowledge of bridges that can be readily called to mind. At auspicious moments an intuitive flash may provide a new solution, which fulfills the task better than known conventional solutions (intuition, creativity leading to innovations).

As soon as a design idea has taken shape in the mind, then the first sketches can be drawn - best on tracing paper over the plan of the longitudinal section along the axis of the planned bridge, freehand with a soft pencil, but with the scale in hand. For this one should have learned sketching at school! For a beam bridge (simplest type) one begins with the probable line of the roadway taking into account a suitable beam depth, then the position of piers and abutments are assumed and the bottom edge of the beam is drawn. A high slenderness ratio is chosen, if required by technical constraints or for aesthetic reasons; a low slenderness ratio is chosen if low costs are decisive in competition.

The first sketch will then be critically regarded and questioned: are the proportions between spans and clearance under the beam good? Do the piers relate well to the surroundings? What are the soil conditions at the site of the piers and abutments? Is the curvature of the vertical alignment good? If a haunched girder is chosen, does the curved line of its bottom edge fit in with a possible curvature in plan?

A second - or even a third sketch may follow including the cross sections of the superstructure and concepts for the piers. How will the proportions of the piers be, height to width? Would two or three columns be better? The bridge is now considered and drawn 3-dimensionally. The sketches are hung up on a wall at eye-level, so that they may be looked at from a greater distance and at different angles, including at a slant; this encourages the criticism of colleagues, and in particular the comments of those who have to build it. Suitable construction methods may be discussed and considered.

The designer should now shut himself away with these first results, meditate over them, thoroughly think over his concept and concentrate on it with closed eyes. Has every requirement been met, will it be well-built, would not this or that be better looking or better for later detailing? (I call this process that of being pregnant with a design.) One then begins to draw again, to listen to the opinions of colleagues, of art advisors and also of laymen. If the engineer is not gifted with a good artistic sense or not trained in this respect, then he should consult now - better at an earlier stage - an architect who has experience with bridges. Misguided ambition should not cause an engineer to inflict a badly designed bridge on the world, which will remain a lasting reproach over the years.

For larger bridges, one should work through one or two alternatives trying out other spans and other structural systems and make comparisons, in order to arrive at the best solution.

After several such correction phases, a fair copy of the chosen solution will now be drawn. Only now should calculations begin, and in the first place with simple and rough approximations to check whether the assumed dimensions will be sufficient and whether the necessary sectional areas of reinforcing steel or of prestressing tendons will leave sufficient space, to allow the concrete to be placed and compacted without difficulty.

Then some runs with computer programs can be made, using different depths or other variables in order to find the most economical dimensions; these should, however, only be chosen if no other essential requirements, such as aesthetics, length of approaches, grades etc. are affected.

Once the designer or the design team have made their choice, then the principle design drawings with all dimensions and explanations can be drawn up for approval of the authorities.

Since drawings alone are not sufficient to judge the appearance and impact of the bridge on its environment, one should get a model made with parts of the surrounding scenery or use good photos from different view points showing perspectives of the bridge that are drawn exactly to scale. Such models and photos are also helpful for the information of citizens concerned or representatives of agencies for environmental control and they are above all important for the client.

The final design for realization

fter the approval of the design, the final design work can begin with rigorous calculations of forces, stresses etc. for all kinds of loads or attacks and then the structural detailing has to be done.

The scaffolding and equipment, which will be needed for the construction of the particular type of bridge, also has to be worked out. Numerous drawings and tables with thousands of numbers and figures for all dimensions, sizes and levels must be made with specifications for the required type and quality of the building materials. This phase entails the greatest amount of work for the bridge engineer, and calls for considerable knowledge and skill. The phase of conceptual and aesthetic design needs a comparatively small amount of time, but is decisive for the expressive quality of the work.