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truss, roof, conditions, type, purlins and bracing

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For spans under 35 feet, a riveted or beam girder is ordinarily more economical than a truss, unless the conditions of loading are peculiar.

Selection of Type.

The type of truss selected depends gen erally upon (1) span, (2) pitch of roof, (3) covering of roof, (4) available depth, (5) load to be carried.

All the above considerations affect jointly the choice of type; no single type would be used under certain lengths of span, for instance, with different combinations of the other conditions. A short span and flat roof might lead to a lattice truss, but if the roof had a steep pitch another type would be used.

The covering of the roof affects the position and number of panel points, and therefore the type. If the planks rest directly on the top chord of trusses, then the panels can be arranged as may be most economical. If the roof is of corrugated iron, the size of sheets will limit the spacing of purlins, and, as these should come at the panel points, this will determine the number of panels.

The position of a monitor or skylight would also largely deter mine the number of panels.

If the depth is limited, then certain types cannot economi cally be used. If there is a ceiling or shafting to be carried, or any other conditions making a horizontal bottom chord essential, then this must be provided.

In almost all cases, therefore, there are certain conditions that determine arbitrarily certain features of the truss, and these indi rectly fix the type that should be used.

On pages 109 and 110 are given types in general use, and a consideration of the points noted above will illustrate their appli cation to these types.


An important feature in all trussed roofs is the bracing. Trusses cannot be economically designed without sup porting at intervals the top chord against lateral deflection. As was noted in the case of beams, the allowable fibre stress must be reduced with the ratio of length to radius of gyration.

This support is given by the plank if directly attached to the truss, or by purlins. Such purlins should be efficiently connected

to the truss. If the conditions of framing are such that the regu lar construction does not hold the truss, then special steel bracing must be used. In the case of very large roofs, special steel brac ing should always be used, as there would not be sufficient stiff ness in the connections of purlins to properly brace the trusses.

Such bracing is generally of the kind known as X bracing, alternate panels of adjacent trusses being connected by angles or rods. Not every bay is braced, Vitt every other bay, or a less number, depending on conditions.

Considerations Affecting Design of Trusses. Light trusses are subject to distortion in shipping, handling and erection. To guard against such distortion it is sometimes important, therefore, to provide more than the strength calculated for vertical loads when the truss is in position.

In designing a roof, certain features that affect the weight of a truss can often readily be avoided. Some of these are indicated as follows : Long web members should be arranged so that the stress will be ten sion, not compression. • It is not economical to use a double system of web members, such as a lattice truss, except in the case of light loads and shallow depth.

No web members should be provided that do not take direct load or are not needed for support of the chords.

Concentrated loads, such as purl ins, or hangers, etc., should, if possi ble, come at panel points, as otherwise the bending stress in the chords increases materially the weight of truss.

The roof plank resting directly on the top chord of truss increases the weight of truss, but the saving in purlins sometimes offsets this.

The spacing of trusses should, if possible, be such as will develop the full strength of the members of the truss. In some cases the conditions are such that the lightest sections which it is practicable to use are not strained nearly to their capacity.

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