chapter5 trusses

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CHAPTER 5 TRUSSES

5.1 Introduction A truss is a triangulated framework of members in which loads are primarily resisted by axial forces in the individual members. The most commonly used truss is single span, simply supported and statically determinate with joints assumed to act as pins. Trusses can be pitched with sloping rafters as shown in Figure 5.1 or can have parallel top and bottom chords. Trusses with parallel chords are often referred to as lattice girders.

Roof covering

RafterPurlins

Internal bracingmembers

Main tie

Figure 5.1 Typical roof structure

5.2 Typical uses A common application of pitched trusses is for roofs. Lattice girders have a wider variety of uses including support of roofs and floors particularly with longer spans or heavier loads.

The support of long span flat roofs is generally accomplished by using trusses with parallel chords. Pitched roofs are normally supported by pitched trusses, even for modest spans, the exception being the specialised area of pitched roof portal frames. Portal frames are beyond the scope of this publication and will not be considered further.

One advantage of trusses is that they can be delivered to site as one complete unit, as several smaller units or even as individual elements. The choice will depend upon the size of the truss, the ease of transport between the fabrication shop and the site and the availability of space on site.

Handbook of Structural Steelwork 3rd Edition - Chapter 5 Trusses

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5.2.1 Spans The most efficient form of truss to be employed in any given situation is usually controlled by the span to be covered. Figure 5.2 shows a variety of pitched roof trusses together with the spans over which they are customarily used. For spans in excess of these values, lattice girders may be more practical. However, lattice girders are used for a whole range of spans (greater than approximately 7 m).

Figure 5.3 shows two types of lattice girder – the N-girder or Pratt truss and the Warren girder. These trusses have depth to span ratios typically in the range of 1:10 to 1:14.

<7 m 7 - 11 m

11 - 17 m

11 - 17 m

17 - 25 m

17 - 25 m

25 - 31 m

25 - 31 m

Figure 5.2 Typical roof trusses and associated spans

a) N-girder or Pratt truss

b) Warren girder

Compression chord

Tension chord

Compression chord

Tension chord

Figure 5.3 Lattice girders


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5.3 Design concept Typical roof trusses are plane frames consisting of sloping rafters which meet at the apex or ridge of the frame (see Figure 5.1). The lower ends of the rafters are prevented from spreading by a horizontal main tie, whilst internal bracing members triangulate the truss and carry primarily axial forces. The internal members also reduce the segment lengths of the chords which enables lighter weight and therefore more efficient chords to be used.

5.3.1 Roof arrangement The roof coverings may be made from a variety of materials ranging from traditional slates or tiles, profiled steel sheeting or more exotic materials. These coverings are supported on purlins (members running between the trusses), which are supported by the rafters and therefore apply loads to the rafters. The purlins also provide out of plane stability-to-the truss. Stability to the truss must be provided at all times, including during erection, when temporary bracing may be used.

The spacing of the purlins (which can range from as little as 900 mm to over 3.5 m) is normally dictated by the roofing material. If the purlins are only located at points where internal members meet, (the panel points) then the truss members will be subjected primarily to axial forces. However, if the spacing is such that the purlins are supported between the panel points, then rafters will need to be designed for combined axial load and bending. Figure 5.4 shows the two possible options.

5.3.2 Pre-cambering Deflections of nominally flat trusses (Pratt trusses or Warren trusses) must be considered if ponding and therefore overloading are to be avoided. Two possible solutions are to either pre-camber the truss or to have a shallow slope in the top chord. The concept of pre-cambering is often extended to longer span pitched roof trusses where the nominally horizontal bottom chord may in

a) Purlins at panel points

Local bending action

b) Purlins between panel points

Figure 5.4 Purlins at or between panel points


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fact slope upwards slightly from the supports. This is carried out so that under loading, the bottom chord does not deflect below the horizontal.

5.3.3 Typical sections The sections used for the members of a typical roof truss may be single angles, double angles (single angles fastened back to back), single channels, double channels or single T sections. For members with more than one component (double angles or double channels), the elements may be connected directly to each other. Alternatively a gusset plate may be inserted between them which enables a connection to be made to other members so that eccentricities at the connections are minimised. For single component members this is not possible and a lapped joint with its consequent eccentricity is unavoidable.

If, as is normally the case, the members consist of angles, channels and T sections then the axial loads should be determined assuming that the joints are pinned. The moments caused by eccentricities at the ends need not be considered explicitly and the individual members may be checked using Clauses 4.7.10 to 4.7.13 of BS 5950-1. These clauses give values for the effective lengths to be taken for buckling about the various axes. Care must be taken to ensure that all possible axes of buckling are recognised and this will often involve consideration of buckling about the a-a, b-b, u-u and v-v axes. The assumption implied in this approach is that the members may be represented by lines meeting at a point located at the nodes. If the frame is welded, it is customary to detail the frame so that the centroidal axes of the members lie on these lines. If the frame is bolted, then it is usual to ensure that the lines of the bolt holes meet at the nodes. Any moments arising from minor eccentricities are allowed for in the choice of effective lengths. Figure 5.5 shows some typical details from an example of bolted roof truss, using back to back angles for the members, with gusset plates at the connections.

Figure 5.6 shows a welded truss using T sections, detail 2 show that the members node without any eccentricity. Figures 5.5 and 5.6 are only examples of a number of typical details from a wide variety of solutions which may be adopted.


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Detail 1Detail 2

2

1

Figure 5.5 Bolted roof truss and typical details

1

2

3

Detail 2 Detail 3Detail 1

Figure 5.6 Welded roof truss and typical details


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5.3.4 Joint Capacities The detailing of the joints is a vital part of truss design. The capacity of the truss may be controlled by the capacity of the joints as much as by the capacity of the members If members are selected so that their capacity is almost fully utilised, the resulting joint details required to transmit the applied forces, can be very impractical. The joints should therefore be considered at an early stage in the design, in conjunction with the selection of the members. As mentioned above, the joint eccentricities will effect design of the truss and its members. The joints adopted in practice must not invalidate the assumptions made at the design stage.


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