This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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 determine the number of panels.
If the depth is limited, then certain types cannot economically 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 indirectly 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 application to these types.
Bracing. An important feature in all trussed roofs is the bracing. Trusses cannot be economically designed without supporting 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 regular construction does not hold the truss, then special steel bracing must be used. In the case of very large roofs, special steel bracing should always be used, as there would not be sufficient stiffness 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, but 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 tension, 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 purlins, or hangers, etc., should, if possible, 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.
Practical Considerations. Trusses are generally riveted up complete in shop and shipped whole, unless it is impracticable to do so. Not only is riveting in the field expensive, but the rivets are not so strong, being generally hand-driven instead of power-driven.
In some cases it is not practicable to rivet the trusses complete, on account of their size. If they are to be shipped by railroad, it is always necessary to be sure that they do not exceed the limits of clearance necessary along the route they have to traverse.
These limits have to be obtained in each special case, as the clearances of bridges and heights of cars vary. This consideration sometimes makes it necessary to ship all the parts separately and to rivet in the field, or to make one or more splices of the truss as a whole. The weight of trusses, with regard to the rigging available for handling and transporting them, has also to be considered.
During the process of erection it should be remembered that in the design of the truss the lateral bracing of the completed structure is generally figured on, and until the structure is complete, ample temporary bracing should be provided. Many failures of roofs are due to neglect of this precaution.
Determination of Loads. The loads for which a roof truss should be figured are: the dead weight of all materials; an assumed snow load, varying with the latitude and slope of roof; a wind load, varying with the slope of roof; a ceiling load, if there is to be any; and such other special loads as may occur in particular cases.
Snow varies from 12 to 50 pounds per square foot of roof, according to the degree of moisture or ice in it. On a flat roof an average allowance for snow is 30 lbs. per square foot of roof. A roof sloping at an angle of 60° to the horizontal would not generally need to be figured for snow, unless there were snow guards to keep the snow from sliding off.
The wind is assumed to blow horizontally, and the resulting horizontal pressure is generally taken at 40 lbs. per square foot. The normal pressure with different slopes on this basis is indicated in the following table:
In pounds per square foot, for an assumed horizontal wind pressure of 40 lbs. per square foot.
Angle of Roof with Horizontal
Pressure Normal to Surface of Roof
Pressure on Horizontal Plane
Pressure on Vertical Plane
In the calculation of the maximum strain, the combinations of dead load, snow load, and live load should be considered. It is not necessary, however, to consider the wind and snow acting on the same side at the same time as a wind giving the assumed pressure would blow all the snow off this side. Wind on one side and snow on the other side, or snow on both sides, generally give the maximum live-load strains.
2Panel Truss. Fig.116
4 PANEL TRUSS Fig.118
Flat Pitch Roof Truss. Fig.119.
Parallel Chord Roof Truss. Fig.120.
The total dead and live loads should not be taken as less than 60 lbs. per square foot, and, in general, the conditions render allowance for a greater total load necessary.
The design of trusses will be taken up in the course on Theory and Design.