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 com plete, on account of their size. If they are to be shipped by rail road, 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 clear ances 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 avail able 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 com plete, ample temporary bracing should be provided. Many fail ures 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 particu lar 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 gener ally 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 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 pres sure 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.
The total dead and live loads should not be taken as less than 60 lbs. per square foot, and, in general, the conditions render allow ance for a greater total load necessary.
The design of trusses will be taken up in the course on Theory and Design.