Civil Engineering Reference
In-Depth Information
6.22 Modifi cation of bending stresses due to creep following change
of cross section
When the cross section of a bridge deck is changed during the construction sequence,
the bending stresses due to self weight and prestress are affected by creep. For instance,
post-tensioned statically determinate beams of the type described in Chapter 10 are
precast, prestressed, and launched into place. The beams are then joined together
by a reinforced concrete slab, Figure 6.27. The weight of this slab, in the form of
fl uid concrete, is applied to the beams as a superimposed dead load. However, once
this second phase concrete has hardened, the section properties of the beams change
substantially, due to the much increased area of top fl ange. Initially, the second phase
concrete slab will be unstressed. However, as the beams continue to shorten and to
defl ect under the effect of self weight and prestress, this new concrete will gradually
pick up stress and the bending stresses in the fi rst phase concrete will be modifi ed. The
stresses in the beam will tend towards the monolithic situation, where the full section
of the beam including the additional top slab, had been cast in one phase.
The stresses in the beam due to the initial self weight, the prestress and the weight
of the wet second phase slab are the 'as-built stresses'. The stresses in the beam that
would have arisen if the complete cross section had been built and stressed in one
phase are the 'monolithic stresses'. The 'design stresses' lie between the two extremes
and depend on the creep factor in the same way as the bending moments described
above.
The creep affects all the section properties of the beam. Thus the as-built stresses
would be
Figure 6.27
Creep due to change in cross section
