Civil Engineering Reference
In-Depth Information
diaphragm web. However, it may not be economical or convenient to carry the
whole shear in prestress, and it is possible to superimpose the two truss models, each
carrying a part of the load. With the prestress arrangement shown in Figure 9.37 (e)
the prestress tendons pick up the web shear at the bottom of the web, eliminating the
need for hanging steel.
The best design for a heavily loaded diaphragm is offered by partial prestressing,
where the prestress maintains the top fi bre of the diaphragm in compression under
permanent loads, and reduces the requirement for hanging steel and for shear
reinforcement. Passive reinforcement is provided to control the width of bending
cracks at the SLS, and to make up the shortfall in bending and shear at the ULS.
This approach reduces the congestion which is the principal problem of reinforced
diaphragms, and avoids the very intense prestress required for an entirely prestressed
solution. Unfortunately, some codes of practice do not allow partial prestressing.
The prestressed diaphragm of the River Lea Viaduct on the Stanstead Abbotts
Bypass (
15.3.4
) is shown in Figure 9.38. The arrangement of prestressing shown only
lifts part of the shear, the remainder being carried by hanging steel. The prestress
doubles up over the top tension tie; the diaphragm is fully prestressed for bending.
The equivalent truss shows how the prestress and reinforcement work together, and
how the access hole in the diaphragm may be rationally sized.
Figure 9.39 shows the situation where a box is carried by twin bearings that are not
placed directly beneath the webs. Here, a proportion of the shear which is applied by
the shear fi eld from the two spans either side of the support shown in Figure 9.37 (b)
may be transmitted directly to the bearing. Only that portion falling below point A on
Figure 9.38
River Lea Viaduct, Stanstead Abbotts Bypass: prestressed diaphragm
