Mill buildings differ so greatly in character and purpose that it is impossible to formulate tables of dead weights which will suit all cases. The use to which the building is to be put, its location, the character of the roof covering, the presence or absence of cranes, etc., all affect the dead weight, and generally each case must be considered individually. For most purposes of design the loads may be divided into: (1) roof loads; (2) floor loads; (3) crane loads; (4) snow and wind loads, and (5) miscellaneous loads.

For making rough estimates the diagram of weights of roof trusses given in Fig. I will prove useful. These weights have been figured separately and do not quite agree with any of the published formulas. From this diagram, the table (Table I.) giving the weights of roof coverings and the table (Table III.) of wind and snow loads, the total weight to be carried is found. Were it possible to realize in actual practice the small sections required, the weight of trusses would be directly proportional to the load carried. Iron purlins weigh from 2 lbs. to 4 lbs. per square foot of ground covered, according to the spacing of the trusses. Good practice in the United States requires that roofs in northern latitudes shall be figured for at least 40 lbs. per square foot of roof surface.

The Building Law of New York City requires that floors shall be proportioned to carry the following minimum loads per square foot: Office buildings, 100 lbs.; public halls, 120 lbs.; stores, factories, warehouses, etc., 150 lbs.; floors carrying heavy machinery, 250 lbs. to 400 lbs. In every case the floor must be strong enough to carry its maximum load. Mr. C. J. H. Woodbury, in his book on "The Fire Protection of Mills," gives a table of weights per square foot of floor of various kinds of merchandise, which is reprinted herewith (Table I.) and which will be found valuable in determining loads on floors.

For small traveling cranes of one or two tons capacity it is safe to consider the total weight of one end of the crane and its load as twice the capacity of the crane. For cranes of larger capacities Table II. gives the maximum weight which will come on two carrying wheels at one end of the crane when the fully loaded trolley is at that end. The corresponding figures for the other end would be somewhat smaller, but not enough so to affect materially the construction of the building. From the figures in Table II. the strength of traveling crane runway girders and columns may be calculated.

The strains due to the pres-ence of jib cranes vary so greatly in, number, character and intensity in different cases, that they do not admit of any general tabular statement. They must, however, be carefully figured in each case and fully provided for in the design. The principal strains produced will be in the lower chord bracing of the roof trusses, and the bending strains in the supporting columns.

Weight be per sq ft.

Weight of the Roof trusses per sq ft. of Area Covered.

Total Weight of Roof Trusses, Capacity 40 lbs per sq. ft. Units 12,000, 15,000 Pitch 6" per ft.

Fig. 1. Diagrams Showing Weights of Roof Trusses.