Truss and Stanchion - Structural Forms.

The truss and stanchion system is essentially an extension of the beam-and-column solution, providing an economic means of increasing the useful span.

Typical truss shapes are shown in Fig. 1.11.

Truss configurations
Fig. 1.11 Truss configurations

Members of lightly-loaded trusses are generally hot-rolled angles as the web  elements, and either angles or structural tees as the boom and rafter members, the latter facilitating ease of connection without the use of gusset plates. More heavily loaded trusses comprise universal beam and column sections and hot-rolled channels, with connections invariably employing the use of heavy gusset plates.

In some instances there may be a requirement for alternate columns to be omitted for planning requirements. In this instance load transmission to the foundations is effected by the use of long-span eaves beams carrying the gravity loads of the inter- mediate truss to the columns: lateral loading from the intermediate truss is trans- mitted to points of vertical bracing, or indeed vertical cantilevers by means of longitudinal bracing as detailed in Fig. 1.12. The adjacent frames must be designed for the additional loads.

Additional framing where edge column is omitted
Fig. 1.12 Additional framing where edge column is omitted

Considering the truss and stanchion frame shown in Fig. 1.13, the initial assumption is that all joints are pinned, i.e. they have no capacity to resist bending moment.

Truss with purlins offset from nodes
Fig. 1.13 Truss with purlins offset from nodes

The frame is modelled in a structural analysis package or by hand calculation, and, for the load cases considered, applied loads are assumed to act at the node points.

It is clear from Fig. 1.13 that the purlin positions and nodes are not coincident; consequently, due account must be taken of the bending moment induced in the rafter section.The rafter section is analysed as a continuous member from eaves to apex, the node points being assumed as the supports, and the purlin positions as the points of load application (Fig. 1.14).

Rafter analysed for secondary bending
Fig. 1.14 Rafter analysed for secondary bending

The rafter is sized by accounting for bending moment and axial  loads, the web members and bottom chord of the truss being initially sized on  the basis of axial load alone.

Use of structural analysis packages allows the engineer to rapidly analyse any number of load combinations.Typically, dead load, live load and wind load cases are analysed separately, and their factored combinations are then investigated to deter- mine the worst loading case for each individual member. Most software packages provide an envelope of forces on the truss for all load combinations, giving maximum tensile and compressive forces in each individual member, thus facilitating rapid member design.

Under gravity loading the bottom chord of the truss will be in  tension and the rafter chords in compression. In order to reduce the slenderness of the compression members, lateral restraint must be provided along their length, which in the present case is provided by the purlins which support the roof cladding. In the case of load reversal, the bottom chord is subject to compression and must be restrained. A typical example of restraint to the bottom chord is the use of  ties, which run the length of the building at a spacing governed by the slenderness limits of the compression member; they are restrained by a suitable end bracing system. Another solution is to provide a compression strut from the chord member to the roof purlin, in a similar manner to that used to restrain compression flanges of rolled  sections used in portal frames. The sizing of all restraints is directly related to the compressive force in the primary member, usually expressed as a  percentage of  the compressive force in the chord. Care must be taken in this instance to ensure that, should the strut be attached to a thin-gauge purlin, bearing problems in thin- gauge material are accounted for. Examples of restraints are shown in Fig. 1.15.

Restraints to bottom chord members
Fig. 1.15 Restraints to bottom chord members

Connections are initially assumed as pins, thereby implying that the centroidal axes of all members intersecting at a node point are coincident. Practical considerations invariably dictate otherwise, and it is quite common for member axes to be eccentric to the assumed node for reasons of fit-up and the physical constraints that are inherent in the truss structure. Such eccentricities induce  secondary bending stresses of the node points, which must be accounted for not only by local bending and axial load checks at the ends of all constituent members, but also in connection design. Typical truss joints are detailed in Fig. 1.16.

Typical joints in trusses
Fig. 1.16 Typical joints in trusses

It is customary to calculate the net bending moment at each node point due to any eccentricities, and proportion this moment to each member connected to the node in relation to member stiffness.

In heavily-loaded members secondary effects may be of such magnitude as to require member sizes to be increased quite markedly above those required when considering axial load effects alone. In such instances, consideration should be given to the use of gusset plates, which can be used to ensure that member centroids are coincident at node points, as shown in Fig. 1.17. Types of truss connections are very much dependent on member size and loadings. For lightly-loaded members,welding is most commonly used with bolted connections in the chords if  the truss is to be transported to site in pieces and then erected. In heavily-loaded members, using gusset plates, either bolting, welding or a combination of the two may be used.

Ideal joint with all member centroids coincident
Fig. 1.17 Ideal joint with all member centroids coincident
However, the type of connection is generally based on the fabricator’s own reference.

Where the roof truss has a small depth at the eaves, lateral loading is resisted either by longitudinal wind girders in the plane of the bottom boom and/or rafter, or by designing the columns as vertical fixed-base cantilevers, as shown in Fig. 1.18.

Sway resistance for truss roofs
Fig. 1.18 Sway resistance for truss roofs

Where the truss has a finite depth at the eaves, benefit can be obtained by developing a moment connection at this position with the booms designed for appropriate additional axial loads. This latter detail may allow the column base to be designed as a pin, rather than fixed, depending on the magnitude of the applied loading, and the serviceability requirements for deflection.

Longitudinal stability is provided by a wind girder in the plane of the truss boom and/or rafter at the gable wall, the load from the gable being transmitted to the foundations by vertical bracing as shown in Fig. 1.19.

Gable-end bracing systems
Fig. 1.19 Gable-end bracing systems

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