102. Buildings of this class are usually large, one-story buildings consisting of floor, walls and roof, and with one or more level galleries around the outside walls. Posts in such buildings are not as objectionable as in the other class of buildings considered in this chapter, but it is generally desirable to keep the center clear of columns. At the Columbian Exposition, 1893, nearly all of the main buildings were roofed by means of steel trusses and purlins. The roof construction of many of the buildings is described and illustrated in Volumes XXVI. and XXVII. of the Engineering Record. Figs. 101, 102 and III, Chapter II (Foundations On Compressible Soils)., show the types of trusses used in three of the buildings.

The buildings at the Expositions held at Omaha in 1898-9, and at Atlanta, Ga., in 1895, were entirely roofed with timber trusses. Most of the buildings of the Exposition held at Buffalo in 1901 were constructed with wooden trusses, and the main buildings of the Louisiana Purchase Exposition, held in 1904 at St. Louis, had wooden trusses.

The type of truss most commonly used where the buildings have been roofed with wooden trusses is that of the Howe Truss, as this is the most economical and practical truss for such buildings. Details of the roof construction of the Horticultural and Forestry Building, the Agricultural Building and of the Music Hall at Buffalo, all wooden structures, are published in the Engineering Record of Aug. 24, Jan. 12, and Feb. 9, 1901.

In roofing large empty buildings of this character, the principal consideration, aside from the strength of the trusses, is to secure transverse stiffness to resist racking under wind pressure. With a wooden roof, this is best secured by an arrangement of posts and trusses, such as is shown in Fig. 246. By using double rows of posts on the outside, they can be braced so as to give great transverse stiffness, and they also reduce the span of the main truss, while the inner rows of posts may be utilized for the support of galleries. As an example of an economical and desirable construction for large exposition buildings with a wooden roof, the author has chosen the Agricultural Building at Buffalo (1901), the description being taken from the Engineering Record of Jan. 12, 1901.