Determination of Loads, The first step in the calculatior of a beam or girder is to determine the exact amount of load to be carried, and its distribution. Loads may be uniformly distributed or concentrated, or both in combination. The case of a simple floor or roof beam usually involves only the calculation of the area carried and the load per square foot. The load per square foot is made up of two parts - namely, dead load, or the weight of the construction; and live load, the superimposed load. The latter is generally specified by law, as noted previously under "Building Laws and Specifications."

The calculation of the dead load has to be made in detail to fit each case. In the case of a floor beam this would consist of the arch between the beams, the steel beams and girders, the filling on top of the arch, the wood or other top flooring, the ceiling, and the partitions. These weights cannot be accurately determined until the spacing and size of beams are fixed; so their features have to be assumed at first. The process in general is illustrated by the following case:

Assume a terra cotta arch 8 inches deep, beams spaced about 5 feet center to center, 3 inches of filling and screeds on top of the arch, a 7/8-inch hemlock under floor, and a l 1/8-inch oak top floor. The weights then are as follows:

 8-in. arch 30 lbs. Steel = 18/5 = 3.6, or say = 4 ,, Filling = 3 x 5 = 15 ,, 7/8-in. floor = 7/8 x 2, say = 2 ,, 1 1/8 -in. top = 1.125 X 3.67, say = 4 ,, Ceiling (no furring) = 7 ,, Partition = 32 x 10/5 = 64 ,, Total Dead Load 126 lbs.

The calculation of the dead weight per square foot of partitions is made up of the weight of blocks, if of terra cotta, and of the plastering on both sides. If the structure is of wire lath, the weight is that of the framing and plastering. These weights per square foot have already been given in the chapter on Fire-proofing.

Only the height of the story is used, as the partition stops at the ceiling. In the above case it is assumed that the partition may go anywhere, and therefore, in some cases, may come directly over a beam, thus being entirely carried by it. If the partitions are in general located so as to come between beams, and no provision is desired for other possible locations, the above partition load might be reduced one-half, as a partition would then be carried by two beams. Or if the partitions came only over girders, the load might be omitted entirely in the calculation of the beams.

In the above total dead load, it should be noted that the allowance for steel does not include the weight of girders. This of course should not be included for the beams. In the calculation of the girders the weight of the girder itself should be added.

The calculation of dead load cannot be absolutely exact, any more than can the determination of the exact amount of live load that will have to be carried. It should always, however, be worked out in detail as above, so that as close an approximation as possible shall be made.

Tables XIV and XVI, of Part I, and Table XVII, Part II, give the weights of different materials and forms of construction, for use in determination of dead loads under different conditions.

The floor arch is assumed to carry all its load vertically to the beams, and the load therefore is the product of the area and the load carried per square foot. This neglect of thrust from the arch is on the safe side as regards the determination of amount of load on the beam.

Distribution of Loads. The load on a girder is generally concentrated at one or more points, and involves the calculation of the reactions from the beams. Girders therefore, as a general thing, are not calculated until after the beams. A girder may also have a uniform load from one side, or from a partition or wall,

## Table XVII. Weights Of Various Substances And Materials Of Construction

 SUBSTANCE. AVERAGE WEIGHT IN POUNDS PER CUBIC FOOT. Aluminum 162 Ash 38 to 47 Asphaltum 62 to 112 Brass (cast) 490 to 525 Brick 100 to 150 Brickwork 100 to 140 Cement, Portland 80 to 110 Cement, Rosendale 55 to 65 Cherry 42 Chestnut 41 Clay, Potter's, dry 112 to 143 Clay, in dry lumps 65 Coal - Anthracite 52 to 60 Coal - Bituminous 47 to 52 Coke 23 to 32 140 Concrete - Cinders and Portland cement 96 Copper, cast 542 Copper, rolled 555 Cypress 64 Earth - Common loam, dry and loose 72 to 80 Earth - Common loam, dry and rammed 90 to 100 Earth - Common loam, soft-flowing mud 110 to 120 Elm 35 Gneiss, common 168 Gneiss, in loose piles 96 Gold, cast pure or 24 karat 1,204 Gold, pure-hammered 1,217 Granite 160 to 178 Gravel 90 to 130 Hemlock 25
 SUBSTANCE. AVERAGE WEIGHT IN POUNDS PER CUBIC FOOT. Hickory 53 Iron,cast 450 Iron, wrought 480 Lead, commercial 710 Limestone 153 to 178 Lime, quick 95 Mahogany 35 to 53 Marble 158 to 180 Masonry, granite or limestone, dressed 165 Masonry, granite or limestone, rubble 154 Masonry, granite or limestone, dry rubble 138 Masonry, sandstone 1/8 less than above Mortar, hardened 87 to 112 Oak, live 60 Oak, white 47 Oak, red 32 to 45 Pine, white 25 Pine, yellow Northern 34 Pine, yellow Southern 45 Poplar 29 Platinum 1,342 Quartz 165 Sand 90 to 130 Snow, freshly fallen 5 to 12 Snow, moist compacted 15 to 50 Slate 175 Spruce 25 Steel 490 Sycamore 37 Tar 62 Terra Cotta 106 Terra Cotta masonry 112 Tin, cast 459

Note. Where weights of wood are given above they are for perfectly dry wood. Green timbers weigh from one-fifth to one-half more than dry, ordinary building timbers, one-sixth more than dry.