Civil Engineering Reference
In-Depth Information
Reinforcement details of the type shown in Figure 9.30 (a) are suitable for decks
up to about 13 m wide, where the bending moments from side cantilever and internal
slab are reasonably well balanced. The advantage of this layout is that it reduces the
congestion of reinforcement over the web, facilitating the concrete casting. Where
the moments turning down into the web from the slab are more substantial, a full
portal detail is required, Figure 9.30 (b). Bending moments from the cantilever may
be shared between the top slab and the web, while bending moments from the top
slab can only turn down into the web. Consequently, portal type corner reinforcement
should in general be designed to favour the moment from the slab rather than from
the cantilever.
9.5.5 Loads applied at the bottom of webs
For through bridges, and for other special cases, some, or all, the live load may be
applied to the web via the bottom slab. The web is thus in tension, and requires hanging
steel that is in addition to the shear steel. The bending moments in the web will be
combined with a direct tension.
9.5.6 Truss analogy
Truss analogy is described in Chapter 3. However, it is worth mentioning it again in
the context of this section on webs, as it is the perfect tool for resolving many diffi cult
design problems; when for instance some loads are applied half way down the web,
where holes need to be made in a web or where the web changes in depth close to the
support.
9.5.7 Trussed webs
Reducing the weight of the deck and reducing the quantity of expensive prestressing
steel are two of the principal aims of the concrete bridge designer. In order to decrease
prestress the deck must be made deep, but that in turn increases the weight of the
webs. One solution to this dilemma is to use trussed webs.
In the early days of reinforced concrete, when labour was still relatively cheap
compared with materials, reinforced concrete trussed bridges were quite frequently
built. However, in addition to their high labour content, these bridges suffered from a
technical problem that contributed to their eclipse for many decades. As all the nodes
of these trusses are inevitably monolithic rather than pinned, the defl ection of the
truss gives rise to secondary moments in the members. These secondary moments are
the greater, the stockier the proportions of the truss. Thus the designer tends to chase
after his tail, having to increase the size of members to carry the bending moments, and
thereby increasing those moments.
Three developments have combined to make concrete trusses viable structures once
again. First, if the deck is prestressed, the defl ection of the truss under dead loads is
greatly reduced and may even be cancelled. Consequently the secondary moments in
truss members are greatly reduced at source. Second, member prestressing may be
used effectively to counteract the secondary moments in members. This technology is
only viable on deep trusses where the members are long enough to allow prestressing
