59. A properly designed truss will be perfectly rigid in the direction of its length under any stress likely to come upon it, but of course it depends upon the roof framing, or upon diagonal or lateral braces to keep it from tipping sideways, and if the truss is supported by posts, some provision must be made to prevent the building racking, as shown by Fig. 131. (A lateral brace is one placed at right angles to the plane of the truss; as a rule it is made in the form of a strut, but capable of resisting tension also. Sway braces are the diagonal rods between the laterals).
In the case of open buildings such as train sheds, drill halls, exposition buildings, etc., the bracing of the roof and walls to resist the wind pressure against the side or end requires nearly as much engineering skill as for designing the trusses. Buildings with masonry walls are more rigid than those with frame walls, or in which the trusses are supported by posts, but even with brick buildings the effect of the wind blowing against the side or end of the building and roof should be carefully considered.
A simple truss roof, such as is shown by Fig. 121, supported by brick walls of moderate height will usually be sufficiently braced by the purlins and ceiling joists, especially if the sheathing is laid diagonally. If the roof is nipped, the hip trusses in connection with the ceiling beams, rafters and purlins will brace the roof endways so as to make a very rigid construction. If the roof terminates against gable walls, the roof should be braced in the end panels by diagonal braces extending from the purlins to the ceiling joists close to the gable. The ceiling joists and purlins should be well anchored to the gables.
Fig. 130-Fastening of Z-Bars.
60. When a roof, such as is shown by Fig. 121, is placed over a long room, there is danger of the building being sprung in the centre of the long sides by wind pressure. This may be provided for by building horizontal trusses on top of the tie-beams of the main transverse trusses, as shown by Fig. 132, utilizing the wall as one chord of the truss and the main tie-beams for the vertical members. With the wind from the direction indicated by the arrows the bracing on that side forms a Warren truss with the wall in compression and the diagonals in tension, while the bracing against the opposite wall forms a Howe truss with the braces in compression. The bracing should therefore be well bolted to the tie-beams, or secured with lag screws, so as to resist either tension or compression. The end braces should also be well bolted to the wall. By means of this bracing or trussing, the horizontal wind pressure will be transferred from the centre of the building to the ends.
Fig. 132. - Plan Showing Horizontal Bracing.
The author recently roofed a building 70 feet wide and 125 feet long with walls 30 feet high, the interior being one large room, with Howe trusses braced in this way, and it has been found to answer very satisfactorily. The bracing of long and narrow churches is considered in Chapter IV (Outside Finish, Gutters, Shingle Roofs).
When trusses are supported by posts there should be a rigid connection between the post and truss, Figs. 67, 72 and 81, show the usual method of bracing steel trusses from the posts and for wooden trusses and posts, examples of bracing are shown in Chapter VI (Coliseums, Armories, Train Sheds, Exposition Buildings, Etc).
61. The method of longitudinal bracing employed in the roof shown by Fig. 75 is partially shown by Fig. 133, the lines a, a and b, b representing angles fastened to the tie-beams of the trusses, T, and acting as both struts and ties. Corresponding braces were used between the rafters of the trusses.
The method of bracing the roof over the Kansas City Auditorium is shown by the lower diagram of Fig. 92. This building is about 314x198 feet, and is virtually one story in height with an auditorium 72 feet in maximum height occupying most of the interior. The roof is of wood covered with tar and gravel and supported by 10 steel trusses, arranged in pairs, each pair of trusses being braced together by lateral struts and diagonal rods in the planes of the top chords and by 5 panels of vertical transverse bracing between each pair of trusses, with five lateral struts between each pair of tie-beams.
Fig. 133. - Plan of Roof Shown in Fig. 75.
Details of these trusses with a plan and interior view of the building were published in the Engineering Record of July 22, 1899. The walls of this building are of brick and stone.
Examples of the bracing of large armory roofs are given in Chapter VI (Coliseums, Armories, Train Sheds, Exposition Buildings, Etc).