Strength Of Beams, Floor And Roofs - Including Directions For Designing And Detailing Roof Trusses, With Criticism Of Various Forms Of Timber Construction. Frank E. Kidder

      Fig. 17.—Another Example of Floor Construction.

      One-half of the area supported by one of the headers is 27 square feet. Dividing 2427 by 27, we have 90 pounds as the strength per square foot of floor.

      To determine the strength of the trimmer B we must first determine how thick a beam it will require to support the 1800 pounds stair load, which is practically concentrated at the center. Rule 4, page 6, should be used. By this rule we find that it will require a 14-inch beam, 2 7/8 inches thick. Hence we must deduct 2 7/8 + 1 1/4, or 4 1/8 inches, from the breadth of our trimmer, to see how much we have left to support the headers. Deducting 4 1/8 from 10 we have 5 7/8 inches for thickness left to support the headers. This is a little less than we had in trimmer A, hence the strength will not be quite as great, but as it is not likely that the full amount of all these three loads will come on the beam at the same time, we may safely rate its strength the same as that of trimmer A, or 90 pounds per square foot of floor. Comparing the strength of the different portions of the floor, we find the common joists have a strength of 106 pounds, the trimmers a strength of 90 pounds, and the headers a strength of 127 pounds per square foot, showing that the trimmers are the weakest part of the construction.

      The weight of the floor itself will be 6 1/2 pounds for the joist, 6 pounds for flooring, and 1 pound for corrugated iron ceiling, or a total of 13 1/2 pounds, making the safe load for the floor 76 1/2 pounds each side of the stair well, and 92 1/2 pounds elsewhere.

      These four examples will serve to show the method of determining the strength and bearing capacity of a floor already constructed or planned. When the floor supports partitions, these must also be taken into account. If the partition is parallel with the beams then only the beams under the partition are affected. When the partition runs across the beams all the beams are affected and the weight of the partition, reduced to pounds per square foot of the floor area, must be added to the weight of the floor before subtracting from the safe strength to obtain the safe load.

      Thus if in Example I there were a lath and plaster partition 10 feet high running across the joists at the center of the span, we first find the weight of the partition per lineal foot of the floor. A lath and plaster partition with 2 × 4 studding may be figured at 20 pounds per square foot, and as it is ten feet high the weight per lineal foot will be 200 pounds. As the load is concentrated at the center of the span it will be equivalent to a distributed load of twice that amount, or 400 pounds. As this is distributed over a span of 16 feet, dividing 400 by 16, we have 25 pounds per square foot of floor to be subtracted from the safe load already found, making the final safe load 37 1/4 pounds.

      If the partition were 4 feet from one wall, or one-fourth of the span, the effect on the beams would be one and one-half times what it would be if the load were distributed, or 18 3/4 pounds per square foot. If the partition comes at a distance of one-third of the span from one support the load should be multiplied by 1.78 to obtain the equivalent distributed load. (See page 11.)

      If the partition supports a floor or ceiling above, the weight on the partition should be added to the weight of the partition itself.

      It will be seen from all that has gone before that the strength of a floor often depends more upon the way it is framed, the size of the headers and trimmers and the position of the partitions, than upon the strength of the common joists.

      It may be well to add that while every floor should be “bridged,” the bridging does not increase the bearing capacity of the floor, as a whole, but merely helps to distribute a concentrated load to three or four joists.

*As a general rule, if the girders have a safe strength equal to 90 per cent. of that of the floor joists it will be sufficient, as not more than 6/10 of the floor area is likely to be loaded at one time.

*Theoretically, this method of considering a beam loaded in different ways as made up of a number of single beams or slices is not strictly correct, but the error lies on the safe side, and as the method is very much simpler than that of determining the size of beam by the bending moments, the writer feels justified in recommending it.