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.
The pressure exerted by wind on roofs is in every case normal to the plane of the roof surface.
The amount of wind pressure usually assumed in proportioning framed structures is 30 lbs. per square foot on a vertical surface, which corresponds to a velocity of from 70 to 80 miles per hour. This velocity includes all storms except tornadoes, which cannot be provided for. Table III. gives the normal pressures on roof surfaces of different slopes for a pressure of 30 lbs. per square foot on a vertical surface.
Snow loads of from 10 lbs. to 20 lbs. per square foot of horizontal projection of the roof should be provided for. There are records of snow and ice deposits weighing 40 lbs. per square foot having formed on roofs in northern latitudes, but this is a very exceptional occurrence. When the roof has a pitch of 450 or more, snow load need not be considered. In New England latitudes, for roofs of ordinary pitch, it will be sufficient to assume 30 lbs. per square foot of roof surface for snow and wind loads combined. The maximum strains from wind and jib crane loads will so seldom occur together in the horizontal bracing that a combination need not be provided for. If they should occur at the same time, once in a year or so, the factor of safety will enable the metal to withstand the strain without injury.
The overturning effect of wind acting on the building as a whole and tending to revolve it about the bases of the leeward columns need be considered only in the case of tall narrow buildings. Wind acting on the sides of a building will necessitate the use of knee braces running from the columns to the bottom chords of the roof trusses, and the strains in these braces will be considerable. These strains will produce bending strains in the columns which must be provided for.