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Preliminary Design

In selecting the correct bridge type it is necessary to find a structure that will perform its required function and present an acceptable appearance at the least cost.
Decisions taken at preliminary design stage will influence the extent to which the actual structure approximates to the ideal, but so will decisions taken at detailed design stage. Consideration of each of the ideal characteristics in turn will give some indication of the importance of preliminary bridge design.

  1. Safety.
    The ideal structure must not collapse in use. It must be capable of carrying the loading required of it with the appropriate factor of safety. This is more significant at detailed design stage as generally any sort of preliminary design can be made safe.
  2. Serviceability.
    The ideal structure must not suffer from local deterioration/failure, from excessive deflection or vibration, and it must not interfere with sight lines on roads above or below it. Detailed design cannot correct faults induced by bad preliminary design.
  3. Economy.
    The structure must make minimal demands on labour and capital; it must cost as little as possible to build and maintain. At preliminary design stage it means choosing the right types of material for the major elements of the structure, and arranging these in the right form.
  4. Appearance.
    The structure must be pleasing to look at. Decisions about form and materials are made at preliminary design stage; the sizes of individual members are finalised at detailed design stage. The preliminary design usually settles the appearance of the bridge.

Constraints

The construction depth available should be evaluated. The economic implications of raising or lowering any approach embankments should then be considered. By lowering the embankments the cost of the earthworks may be reduced, but the resulting reduction in the construction depth may cause the deck to be more expensive.
Headroom requirements have to be maintained below the deck; the minimum standards for UK Highway bridges are given in TD 27 of the Design Manual for Roads and Bridges.
If the bridge is to cross a road that is on a curve, then the width of the opening may have to be increased to provide an adequate site line for vehicles on the curved road.
It is important to determine the condition of the bridge site by carrying out a comprehensive site investigation. BS 5930: Code of Practice for Site Investigations includes such topics as:

  1. Soil survey
  2. Existing services (Gas, Electricity, Water, etc)
  3. Rivers and streams (liability to flood)
  4. Existing property and rights of way
  5. Access to site for construction traffic

Selection of Bridge Type

The following table is intended to be a rough guide to the useful span ranges of various types of deck.

Span

Deck Type

 
Up to 20m
 

Insitu reinforced concrete.
Insitu prestressed post-tensioned concrete.
Prestressed pre-tensioned inverted T beams with insitu fill.

 
 
16m to 30m
 
 
 

Insitu reinforced concrete voided slab.
Insitu prestressed post-tensioned concrete voided slab.
Prestressed pre-tensioned M and I beams with insitu slab.
Prestressed pre-tensioned box beams with insitu topping.
Prestressed post-tensioned beams with insitu slab.
Steel beams with insitu slab.

 
30m to 40m
 
 

Prestressed pre-tensioned SY beams with insitu slab.
Prestressed pre-tensioned box beams with insitu topping.
Prestressed post-tensioned beams with insitu slab.
Steel beams with insitu slab.

 
30m to 250m
 
 

Box girder bridges - As the span increases the construction tends to go from 'all concrete' to 'steel box / concrete deck' to 'all steel'.
Truss bridges - for spans up to 50m they are generally less economic than plate girders.

150m to 350m

Cable stayed bridges.

350m to ?

Suspension bridges.


Preliminary Design Considerations

  1. A span to depth ratio of 20 will give a starting point for estimating construction depths.
  2. Continuity over supports
    1. Reduces number of expansion joints.
    2. Reduces maximum bending moments and hence construction depth or the material used.
    3. Increases sensitivity to differential settlement.
  3. Factory made units
    1. Reduces the need for soffit shuttering or scaffolding; useful when headroom is restricted or access is difficult.
    2. Reduces site work which is weather dependent.
    3. Dependent on delivery dates by specialist manufactures.
    4. Specials tend to be expensive.
    5. Special permission needed to transport units of more than 29m long on the highway.
  4. Length of structure
    1. The shortest structure is not always the cheapest. By increasing the length of the structure the embankment, retaining wall and abutment costs may be reduced, but the deck costs will increase.
  5. Substructure
    1. The structure should be considered as a whole, including appraisal of piers, abutments and foundations. Alternative designs for piled foundations should be investigated; piling can increase the cost of a structure by up to 20%.

Costing and Final Selection

The preliminary design process will produce several apparently viable schemes. The procedure from this point is to:

  1. Estimate the major quantities.
  2. Apply unit price rates - they need not be up to date but should reflect any differential variations.
  3. Obtain prices for the schemes.

The final selection will be based on cost and aesthetics. This method of costing assumes that the scheme with the minimum volume will be the cheapest, and will be true if the structure is not particularly unusual


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