14 STEEL ROOF TRUSSES AND MILL BUILDINGS. CHAP. I. 



and (g) are used for shops or buildings where the main part of the building is required to be covered 

 by a crane and side sheds are used for lighter work. 



Roof Arches. Roof arches are used where a large clear floor space is required as in coliseums, 

 exposition buildings and train sheds, Fig. 8. The arches are braced in pairs and carry the roof 

 covering. Arches may have one, two or three hinges, or may be made without hinges. Three- 

 hinged arches are statically determinate structures, while the stresses in all other arches are 

 statically indeterminate. Arches without hinges are used for domes. Three-hinged roof arches 

 have been commonly used in America, although the two-hinged roof arch is more economical 

 and has many advantages. Arches may have a horizontal tie as in the Chicago Stock Pavilion 

 and the Government Building, or the horizontal reactions may be carried by the foundations 

 as in the St. Louis Coliseum, Fig. 8. For the calculation of the stresses in three-hinged and two- 

 hinged roof arches, see the author's "The Design of Steel Mill Buildings." 



Pitch of Roof. The pitch of a roof is given in terms of the center height divided by the span; 

 for example a 6o-ft. span truss with | pitch will have a center height of 15 ft. The minimum 

 pitch allowable in a roof will depend upon the character of the roof covering, and upon the kind 

 of sheathing used. For corrugated steel laid directly on. purlins, the pitch should preferably be 

 not less than j (6 in. in 12 in.), and the minimum pitch, unless the joints are cemented, not less 

 than . Slate and tile should not be used on a less slope than J and preferably not less than |. 

 The lap of the slate and tile should be greater for the less pitch. Gravel should never be used 

 on a roof with a greater pitch than about , and even then the composition is very liable to run. 

 Asphalt is inclined to run and should not be used on a roof with a pitch of more than, say, 2 in. 

 to the foot. If the laps are carefully made and cemented a gravel and tar or asphalt roof may be 

 practically flat; a pitch of f to I in. to the foot is, however, usually preferred. Tin may be used 

 on a roof of any slope if the joints are properly soldered. Most of the patent composition roofings 

 give better satisfaction if laid on a roof with a pitch of 5 to j. Shingles should not be used on a 

 roof with a pitch less than J, and preferably the pitch should be | to f. 



Pitch of Truss. There is very little difference in the weight of Fink trusses with horizontal 

 bottom chords, in which the top chord has a pitch of i, |, or . The difference in weight is quite 

 noticeable, however, when the lower chord is cambered; the truss with the ^ pitch being then 

 more economical than either the i or the j pitch. Cambering the lower chord of a truss more 

 than, say, 1-40 of the span adds considerable to the weight. For example the computed weights 

 of a 6o-ft. Fink truss with a horizontal lower chord, and a 6o-ft. Fink truss with a camber of 3 ft. 

 in the lower chord, showed that the cambered truss weighed 40 per cent more for the j pitch and 



15 per cent more for the | pitch, than the truss having the same pitch with horizontal lower 

 chord. It is, however, desirable for appearance sake to put a slight camber in the bottom chords 

 of roof trusses, for the reason that to the eye a horizontal lower chord will appear to sag if viewed 

 from one side. 



In deciding on the proper pitch, it should be noted that while the f pitch gives a better slope 

 and has a less snow load than a roof with i or -5 pitch, it has a greater wind load and more roof 

 surface. Taking all things into consideration \ pitch is probably the most economical pitch for a 

 roof. A roof with \ pitch is, however, very nearly as economical, and should preferably be used 

 where corrugated steel roofing is used without sheathing, and where the snow load is large. 



Spacing of Trusses and Transverse Bents. The weight of trusses and columns per square 

 foot of area decreases as the spacing increases, while the weight of the purlins and girts per square 

 foot of area increases as the spacing increases. The economic spacing of the trusses is a function 

 of the weight per square foot of floor area of the truss, the purlins, the side girts and the columns, 

 and also of the relative cost of each kind of material. For any given conditions the spacing 

 which makes the sum of these quantities a minimum will be the economic spacing. It is desirable 

 to use simple rolled sections for purlins and girts, and under these conditions the economic spacing 

 will usually be between 16 and 25 ft. The smaller value being about right for spans up to, say, 

 60 ft., designed for moderate loads, while the greater value is about right for long spans, designed 

 for heavy loads. 



