Note that BS EN gives a slightly different long-term modulus of elasticity of concrete for shrinkage loading, so theoretically there should be a fourth model for analysing shrinkage effects. Again, cracked section properties may be needed where the slab is in tension. Section properties are calculated in the same way as for the long-term model but using the short-term modulus of elasticity. A ‘short term’ composite model: The transient actions (mainly the vertical loads due to traffic) are applied to a short-term composite model.Where the slab is in tension, cracked section properties may be needed. The section properties of the longitudinal composite members and the transverse members representing the slab are calculated using long-term concrete modulus of elasticity of the concrete. A ‘long term’ composite model: The permanent actions applied to the completed structure (chiefly the superimposed dead loads such as surfacing, and the restraint of curvature due to shrinkage) are applied to a long-term composite model.Longitudinal members represent the steel girders only, while transverse members are not usually necessary (they may be set as ‘dummy’ members to keep the same model arrangement as composite models). A ‘steel-only’ model: The self weight of the steel beams and the weight of the wet concrete during construction are applied to a steel-only grillage model.Grillage analysis: application of loading in stagesĪt least three different grillage models will be required to model the response of the structure to the range of permanent and variable actions: Typical grillage layout for 2-span multi-girder steel composite bridge Also, the Eurocode requirements for checking lateral torsional buckling may make a finite element buckling analysis essential for checking the wet-concrete construction load case. While grillage analysis is widely used, and is still considered to be most appropriate for most bridge decks, it is recognised that finite element analysis programs are becoming more widely available and easier to use. Use of a finite element model will give more detailed results, especially for non-uniform beams. Use of a grillage model is suitable in many situations.
It would not be recommended for detail design, but is a useful tool for preliminary design. slab or bracing) and does not consider skew effects.
It does not take account of transverse distribution, it gives no output for transverse design (e.g. There are three modelling options for a typical multi-girder steel composite bridge:Ī line beam is a fairly crude tool.
A typical multi-girder steel composite bridge
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