Other Applications of Trusses - Steel Structures.

Trusses are often used as secondary structures in buildings and bridges in the form of triangulated bracing. Bracing is generally required to resist horizontal loading in buildings or bridges or to prevent deformations and provide torsional rigidity to stiffening girders or box girders.

In buildings, bracing is often required for stability and to transmit horizontal wind loads or crane surges down to foundation level. To avoid the use of heavy com- pression bracing members, the members are usually arranged so that they always act in tension. Although this requires a high degree of redundancy it is normally more economical than providing compression members. Some examples of wind bracing to single-storey building are illustrated in Fig. 19.7(a). For multi-storey buildings with ‘simple’ connections, vertical bracing is required on all elevations to stabilize the building.Normally the floor slabs act as horizontal bracing which transmits the lateral wind loads to the vertical bracing. If the steel frame to the building is erected before the floors are constructed, temporary horizontal bracing must be supplied which can be removed once the floors are in place. Bracing at floor levels may be required if the slabs are discontinuous.Although horizontal bracing in buildings can often be hidden within the depth of the floor, vertical bracing can be obtrusive and undesirable, particularly if the building is clad in glass. In such an instance rigidly-jointed frames are adopted in which the wind loading is resisted by bending in the beams and stanchions. However, if the joints to such frames are made with friction-grip bolts, then temporary bracing may be required for stability prior to the joints being completed. Figure 19.7(b) illustrates some typical bracing systems to multi-storey buildings.

In bridges, secondary truss or bracing frames are often required for stability  and to resist lateral loads due to wind in addition to loads due to road and railway loading such as centrifugal or braking forces. Depending on the  type of structure the bracing may be temporary or permanent, usually placed in both horizontal  and vertical planes. For trusses, permanent plan bracing is provided at both  chord levels for underslung bridges and at deck level for through or semi-through bridges. Where headroom permits, plan bracing is also provided at the top chord level for through trusses, thus conveniently providing restraint to the top com- pression chord and avoiding the need for stiff U-frames. In addition, vertical bracing is also provided between trusses to reduce differential loading and therefore  distortion between trusses and to provide added restraint to the compression chord.

In composite steel plate girder and concrete slab decks temporary vertical bracing may be required when the concrete is poured to provide lateral restraint to the plate girder compression flanges. It may be removed once the concrete has gained sufficient strength to act compositely with the steelwork. In stiffening girders or box girders bracing is often provided in place of plated diaphragms to avoid torsional distortion and to maintain the shape of the cross section under service loads. Figure 19.7(c) illustrates some uses of trusses in bridgeworks. Other uses of trusses in bridgeworks include trestling, i.e. triangulated temporary support frames normally used to support medium- to large-span bridges over land during  erection: see  Fig. 19.7(d).

Other applications of trusses. (a) Bracing to single-storey building; (b) bracing to multi-storey building; (c) typical bracing to bridges; (d) other uses
Fig. 19.7 Other applications of trusses. (a) Bracing to single-storey building; (b) bracing to
multi-storey building; (c) typical bracing to bridges; (d) other uses

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