DESIGN OF STEEL MILL BUILDINGS. 7 



by using formula (7). If the gables are closed a deep cup is formed, and the normal pressure 

 should be increased 30 to 70 per cent. 



That the uplifting force of the wind is often considerable in exposed localities is made evident 

 1>\ the fact that highway bridges are occasionally wrecked by the wind. 



The wind pressure is not a steady pressure, but varies in intensity, thus producing excessive 

 vilir.it ions which cause the structure to rock if the bracing is not rigid. The bracing in mill 

 buildings should be designed for initial tension, so that the building will be rigid. Rigidity is 

 of more importance than strength in mill buildings. 



Miscellaneous Loads. Data on the weights of materials are given in Chapter II. The 

 weights and other data for hand cranes are given in Table 133 and of electric cranes are given 

 in Tabli- 130, Part II. 



Minimum Loads. For minimum loads to be calculated on roofs see 27, "Specifications for 

 Stri-1 Frame Buildings" in the last part of this chapter. 



STRESSES IN ROOF TRUSSES AND MILL BUILDINGS. For the calculation of the 

 stresses in roof trusses and in the framework of steel frame mill buildings, see the author's " The 

 Design of Steel Mill Buildings." 



DESIGN OF STEEL MILL BUILDINGS. 



General Principles of Design. The general dimensions and the outline of a mill building 

 will be governed by local conditions and requirements. The questions of light, heat, venti- 

 lation, foundations for machinery, handling of materials, future extensions, first cost and cost 

 of maintenance should receive proper attention in designing the different classes of structures. 

 One or two of the above items often determines the type and general design of the structure. 

 Where real estate is high, the first cost, including the cost of both land and structure, causes 

 the adoption in many cases of a multiple story building, while on the other hand where the site 

 is not too expensive the single story shop or mill is usually preferred. In coal tipples and shaft 

 houses the handling of materials is the prime object; in railway shops and factories turning out 

 heavy machinery or a similar product, foundations for the machinery required, and convenience 

 in handling materials are most important; while in many other classes of structures such as weaving 

 sheds, textile mills, and factories which turn out a less bulky product with light machinery, and 

 which employ a large number of men, the principal items to be considered in designing are light, 

 heat, ventilation and ease of superintendence. 



Shops and factories are preferably located where transportation facilities are good, land is 

 cheap and labor plentiful. Too much care cannot be used in the design of shops and factories 

 for the reason that defects in design that cause inconvenience in handling materials and workmen, 

 increased cost of operation and maintenance are permanent and cannot be removed. 



The best modern practice inclines toward single floor shops with as few dividing walls and 

 partitions as possible. The advantages of this type over multiple story buildings are (i) the 

 light is better, (2) ventilation is better, (3) buildings are more easily heated, (4) foundations for 

 machinery are cheaper, (5) machinery being set directly on the ground causes no vibrations in 

 the building, (6) floors are cheaper, (7) workmen are more directly under the eye of the superin- 

 tendent, (8) materials are more easily and cheaply handled, (9) buildings admit of indefinite 

 extension in any direction, (10) the cost of construction is less, and (n) there is less danger from 

 damage due to fire. 



The walls of shops and factories are made (i) of brick, stone, or concrete; (2) of brick, hollow 

 tile or concrete curtain walls between steel columns; (3) of expanded metal and plaster curtain 

 walls and glass; (4) of concrete slabs fastened to the steel frame; and (5) of corrugated steel fastened 

 to the steel frame. 



The roof is commonly supported by steel trusses and framework, and the roofing may be 

 slate, tile, tar and gravel or other composition, tin or sheet steel, laid on board sheathing or on 

 concrete slabs, tile or slate supported directly on the purlins, or corrugated steel supported on 

 board sheathing or directly on the purlins. Where the slope of the roof is flat a first grade tar 



