Trusses - Selection of elements and connections.

1 Elements
For light roof trusses in buildings the individual members are normally chosen from rolled sections for economy; these are illustrated in Fig. 19.5(a). Structural hollow sections are becoming more popular due to their efficiency in compression and their neat and pleasing appearance in the case of exposed trusses. Structural hollow sections, however, have higher fabrication costs and are only suited to welded construction. For larger-span heavily-loaded roof trusses and small-span bridge trusses it often becomes necessary to use heavier sections such as rolled universal beams and columns and multiples of the smaller rolled sections such as back-to-back angles and channels, Fig. 19.5(b).

Typical element cross sections. (a) Light building trusses; (b) heavy building trusses and light, small-span bridges; (c) road and railway bridges
Fig. 19.5 Typical element cross sections. (a) Light building trusses; (b) heavy building trusses
and light, small-span bridges; (c) road and railway bridges

For large-span bridge trusses, compound or fabricated sections are normally necessary, particularly for the chords and the compression web members. Figure 19.5(c) illustrates some typical arrangements often used, although the choice available to the designer is very wide. For heavily-loaded bridge trusses, fabricated box sections offer economy in material due to their high efficiency in compression.The open sections shown assist the connections to the web members of the truss but require lacing or battening and are prone to distortion during fabrication.

Providing suitable access to all members and surfaces for inspection, cleaning and painting should be a primary consideration in deciding on the sections and details to be incorporated in the design. Laced sections are disadvantageous in this respect as access can be severely restricted. In highly corrosive environments welded closed box or circular hollow sections with welded connections are usually used in order to reduce maintenance costs as all exposed surfaces are readily accessible.

2 Connections
There are basically three types of connections used for connecting truss elements to each other, that is, welding, bolting and riveting.Riveting is rarely used in the UK due to the very high labour costs involved, although it is still widely used in developing countries where labour costs are low. Small-span trusses which can be trans- ported whole from the fabrication shop to the site can be entirely welded. In the case of large-span roof trusses which cannot be transported whole, welded sub- components are delivered to site and are either bolted or welded together on site.

Generally in steelwork construction bolted site splices are much preferred to welded splices for economy and speed of erection. In light-building roof trusses entirely bolted connections are less favoured than welded connections due to the increased fabrication costs, and usually bolted connections require cumbersome and obtru- sive gusset plates. However, bolted connections are more widely used in bridge trusses, particularly medium- to large-span road bridges and railway bridges, due to their improved performance under fatigue loading. In addition, bolted connections may sometimes permit site erection of the individual elements without the need for expensive heavy craneage. Gusset plates are often associated with bolted bridge trusses, their size being dependent on the size of the incoming members and the space available for bolting.

Gusset plates also enable the incoming members to be positioned in such a way that their centroidal axes meet at a single point, thus avoiding load eccentricities.

Ideally for all types of trusses the connections should be arranged so that the centres of gravity of all incoming members meeting at the joint coincide. If this is not possible the out-of-balance moments caused by the eccentricities must be taken into account in the design.

Some typical joint details are illustrated in Fig. 19.6.

Typical joints in trusses. (a) Welded RHS building roof truss; (b) bolted bridge truss
Fig. 19.6 Typical joints in trusses. (a) Welded RHS building roof truss; (b) bolted bridge
truss

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