Civil Engineering Reference
In-Depth Information
3.2 DEAD LOADS OF STEEL AND STEEL-CONCRETE
COMPOSITE BRIDGES
The dead loads acting on railway and highway bridges consist of the weight
of all its structural parts, fittings, finishing, curbs, lighting and signing
devices, gas and water mains, electricity and telephone cables, etc. These
loads are permanent and remain constant in position and magnitude. To cal-
culate the straining actions on the bridge components, the weight of the
structural parts has to be initially assumed. The assumed weights have to
be assessed after designing and predicting cross sections of all structural parts.
When there is a considerable difference between the assumed and predicted
weights, the calculation of the loads and design has to be repeated until close
agreement is achieved between assumed and predicted weights. It should be
noted that most current codes of practice provide guidance for the unit
weights of commonly used materials in steel and steel-concrete composite
bridges, which can also be used to estimate the dead loads acting on the brid-
ges. Furthermore, the dead loads of previously designed existing bridges can
be used to provide guidance to dead loads expected on similar bridges under
construction.
3.2.1 Dead Loads of Railway Steel Bridges
As an example, let us estimate the dead loads acting on different components
of the traditional double-track open-timber floor plate girder railway steel
bridge shown in Figure 1.20. Starting with the dead loads acting on a
stringer, these loads are half the weight of the track loads (train, sleepers,
and rails), own weight of the stringer, and weight of stringer bracing.
The track load varies from country to country and can be found in the
national code of practice of the country of construction. A commonly
assumed track load is 6 kN/m acting along the stringer length, which is
the spacing between two adjacent cross girders. The own weight of a stringer
depends on its length and the type of steel used. The weight of a stringer can
be reasonably assumed to be from 1 to 1.5 kN/m all over the stringer length.
Finally, the weight of stringer bracing can be reasonably assumed to be 0.2-
0.3 kN/m acting along the stringer length. By knowing the assumed total
dead load acting on the stringer, the straining actions resulting from dead
loads comprising bending moment and shear force can be calculated.
The dead loads acting on an intermediate cross girder (see Figure 1.20)
are the concentrated dead loads coming from the stringers, which are sup-
ported by the cross girders and own weight of cross girder. Once again, the
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