BUILDING LAWS AND SPECIFICATIONS.
The requirements of the Building Departments of different cities vary considerably as regards detail matters, but are in quite close agreement on points affecting the strength of structures.
The following table shows the requirements of different cities as regards live loads : Table XI gives, in pounds per square inch, the transverse strength of various stone constructions, brick and concrete: Where walls are carried by the steel framing at• each story, they are generally made 12 inches thick.
The question of height also affects the requirements of fire resistance and prevention. There is considerable variation on these points. Table XII gives the requirements of some cities whose laws are explicit as to the thickness of walls and the pro portion of loads on columns and foundations.
The above applies to exterior and bearing walls of business, manufacturing and public buildings, 75 feet to 125 feet long and 26 feet or less clear span. Hotels and tenements may have the 3 upper stories 13 inches and following down from that in the sequence given above.
the upper 75 feet of wall, and 4 inches thicker for each 60 feet below. The New York law allows curtain walls to be built be tween piers or steel columns and not supported on steel girders, provided the thickness is 12 inches for the upper 60 feet and 4 inches thicker for each 60 feet below.
Wind Pressure. The Philadelphia law requires 30 pounds per square foot to be calculated on exposed surfaces of isolated buildings ; on office buildings 25 pounds per square foot at the 10th floors, and 21 pounds less for each story below and 21 pounds more for each story above, up to a maximum of 35 pounds.
The combined stress in columns resulting from direct ver tical loads and the bending due to the above wind pressures is allowed to be 30 per cent above that for simply direct loading by the Philadelphia law, and 50 per cent by the New York law.
Reduction in Live Load on Columns, Girders and Foun dations. The Philadelphia law allows the live loads used in calcu lation of columns, girders and foundations for all but manufacturing and storage buildings, to be reduced by the following formula : ___ = 100 4 V A ; and for light manufacturing buildings, by x = 100 — VA , where x = the percentage of live load to be used, and A = the area supported.
The New York law requires the full live load of roof and top floor, but allows a reduction in each succeeding lower floor of 5 per cent until this reduction amounts to 50 per cent of the live load ; not less than 50 per cent of the live load may be used in the cal culations. For foundations not less than 60 per cent of the live
load may be used.
Where the laws limit the height to about 125 feet, the requirements as regards fire protection and prevention are in general that the floors and roofs shall be constructed of steel beams and girders, between which shall be sprung arches of tile or terra cotta or brick, or approved systems of concrete and con crete-steel. All weight-bearing metal of every description shall be covered with non-combustible materials, generally terra cotta or wire lath and cement.
In buildings of this height the use of wood for top floors laid in wood screeds imbedded in concrete, and of wood for all interior finish, is allowed.
Under the New York City law, buildings above sixteen stories are required to have their upper stories constructed entirely with out wood, except that the so-called fireproof wood may be used for interior finish. The floors, hOwever, are required to be of tile or mosaic or other non-combustible material, the wood top floor not being allowed.
Factor of Safety. The foregoing values represent the work ing values of unit-stresses. They are in all cases a certain percent age of the strains under which rupture would occur. This percentage varies with the different classes of material and the different classes of structure. The quotient of the breaking strain divided by the allowable or safe working strain is called the " factor of safety." Steel and wrought iron used in ordinary building construction have generally a factor of safety of 4 ; timber, generally from 6 to 8; cast iron, from 6 to 10 ; stone from 10 to 15.
One reason for this variation in factors of safety for different materials is that certain materials vary more than others in their internal and accordingly in some cases there is a greater likelihood than in others, of an individual piece being below the average strength. Other reasons are found in the varying effects of time. Changes in internal structure are likely to occur in the lapse of years; and there is the further liability that through ignorance or carelessness the structure may be put to uses for which it was never designed.
All these conditions make it unwise from the standpoint of safety to use working stresses very near the breaking strains.
Steel is less subject to variation than other materials. Timber has knots, shakes, dry rot, and other defects not readily discerned, which may greatly reduce its strength below the average. Cast iron has blow-holes, cracks, flaws, internal strains, and unequally distributed metal, which are of frequent occurrence and very Fig 40 likely to escape detection. Stone has seams, crack , flaws, and a structure not uniform, all causing uncertainty and variations in the strength of individual pieces.