List of Design Standards for Preliminary Design: [Show]
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.
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.
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.
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.
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.
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. The Eurocode Standard (EN 1991-1-7 clause 4.3.2(1)
quotes clearances from roadway surfacing to the underside of the deck to
avoid impact damage.
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. EN 1997-2: 'Ground investigation
and testing' covers the requirements for the Soil Survey. Other topics which need to be considered are:
Existing services (Gas, Electricity, Water, etc)
Rivers and streams (liability to flood)
Existing property and rights of way
Access to site for construction traffic
The following table is intended to be a rough guide to the useful span ranges
of various types of deck.
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 Y and U 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 300m
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 1000m
Cable stayed bridges.
350m to ?
Suspension bridges.
A span to depth ratio of 20 will give a starting point for estimating deck construction depths.
Continuity over supports
Reduces number of expansion joints.
Reduces maximum bending moments and hence construction depth or the material used.
Increases sensitivity to differential settlement.
Factory made units
Reduces the need for soffit shuttering or scaffolding; useful when headroom is restricted or access is difficult.
Reduces site work which is weather dependent.
Dependent on delivery dates by specialist manufactures.
Specials tend to be expensive.
Special permission needed to transport units of more than 29m long on the highway.
Length of structure
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.
Substructure
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%.
The preliminary design process will produce several apparently viable schemes. The procedure from this point is to:
Estimate the major quantities.
Apply unit price rates - they need not be up to date but should reflect any differential variations.
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.
Bridge Components | Choice of Deck