When the engineer is sure that a design idea has emerged in his mind, he should pick up a pencil and a scale and by the help of sketching, learned at school, he should start from sketching the probable road direction, beam depth(For beam bridge) the piers, the abutments and the bottom edge of the beam is drawn.
For a heavily funded project, high slenderness ratio is preferable otherwise if the decisive factor is the cost then slenderness ratio can be reduced. Analyze the sketch critically for the proportion between the spans, the clearance under the beam, soil conditions around the piers and the abutments, the adaptiveness of the piers to the surroundings, no. of piers and for the curvature of the vertical alignment. More than one sketches may follow after the critical analysis with changes in the design and supporting conditions. Work out the alternatives, discuss with colleagues, architects advisors and the client to draw out a final sketch.
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 pre-stressing 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 has made its choice, then the principle design drawings with all dimensions and explanations can be drawn up for approval of the authorities. As the map alone is not sufficient to clearly show the locality and impact on the environment and appearance so a model or some well shot photos can help the citizens, client and critics to realize the existence of bridge.
After 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.
Some interesting bridge designs
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