CHAPTER I. 

 STEEL ROOF TRUSSES AND MILL BUILDINGS. 



Definitions. The following definitions will assist the reader in a study of roof trusses and 

 steel frame buildings. 



Truss. A truss is a framed structure in which the members are so arranged and fastened 

 at their ends that external loads applied at the joints of the truss will cause only direct utresses 

 in the members. In its simplest form a truss is a triangle or a combination of triangles. In thu 

 chapter it will be assumed (i) that the structure is not constrained by the reactions, (2) that the 

 axes of the members meet in a common point at the joints, and (3) that the joints have friction- 

 less hinges. 



Transverse Bent. A transverse bent consists of a truss supported at the ends on column* 

 and braced against longitudinal movement by knee braces attached to the lower chord of the 

 truss and to the columns. 



Purlin. A beam that rests on the top chords of roof trusses and supports the hca thing 

 that carries the roof covering, or supports the roof covering directly, or supports rafters. 



Rafter. A beam that rests on the purlins and supports the sheathing, or may support sub- 

 purlins. Rafters are not commonly used in mill buildings. 



Sub-purlin. A secondary system of purlins that rest on the rafters and arc spaced so a* to 

 support the tile or slate covering directly without the use of sheathing. 



Sheathing. A covering of boards or reinforced concrete that is carried on the purlins or 

 rafters to furnish a support for the roof covering. 



Girt. A beam that is fastened to the columns to support the side covering either directly 

 or to support the side sheathing. 



Monitor Ventilator. A framework at the top of the roof that carries fixed or movable louvres. 

 or sash in the clerestory. 



Clerestory. The clear opening in the side framework of a monitor ventilator of a building. 

 also the clear opening on the side of a building. 



Louvres. Slats made of metal or wood which are placed in the clerestory of a monitor 

 ventilator to keep out the storm. Louvres may be fixed or movable. The opening of a monitor 

 ventilator is also called a louvre. 



Panel. The distance between two joints in a roof truss or the distance between purlins. 



Bay. The distance between two trusses or transverse bents. 



Pitch. The pitch of a truss is the center heighc of the truss divided by the span where the 

 truss is symmetrical about the center line. 



Other terms are defined when they are first used. 



DATA FOR THE DESIGN OF ROOF TRUSSES AND STBKL FRAME BUILDING*. 

 Weight of Roof Trusses. The weight of roof trusses varies with the span, the distance 

 between trusses, the load carried or capacity of the truss, and the pitch. 

 The empirical formula 



where 



