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

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      CHAPTER II.

      HOW TO DETERMINE THE STRENGTH OR SAFE LOAD OF WOODEN FLOORS.

      The strength of a floor evidently depends upon the strength of the joists, headers, trimmers and girders of which it is composed, and more especially on the weakest of these, in the same way that the strength of a chain is determined by the strength of its weakest link. The joists, headers, trimmers, etc., taken as single pieces, are simply wooden beams, and their strength may be computed by the rules given in Chapter I.

      The application of these rules to floors, however, may not be readily apparent to every one, and in some cases it is possible to give special rules which are more convenient to use for floors, so that a few examples, showing the method of determining the strength of floors, may prove of interest to the readers of this volume.

      In dealing with “the strength of floors,” we have two different problems to consider: (1) to determine the strength of a floor already built, or planned, and (2) to determine the size of beams to support a given load, with a given span. In this chapter we will consider the first of these problems—that is, to determine the strength or safe load of a given floor.

      DISTINCTION BETWEEN “STRENGTH” AND “SAFE LOAD.”

      When we speak of the strength of a beam, we generally mean the load required to break it, and in which is included the weight of the beam itself. Now in the case of a wooden beam, its own weight, compared with the weight it will support, is usually so small that it need not generally be taken into account, but the weight of a wooden floor, meaning all of the material contained between its under and upper surface, is usually a considerable item, so that a distinction must be made between “safe strength” and “safe load.” In this chapter the term “safe strength” will be used to designate the maximum weight that the floor can support with safety, including the weight of all the materials used in its construction, and for forming the ceiling below.

      The safe load of a floor is the maximum load which can be placed on top of the finished floor, or hung beneath the floor with safety, and is found by subtracting the weight of the material from the safe strength. Thus if the safe strength of a given floor is equal to 80 pounds per square foot of floor, and the materials used in the construction of the floor (and ceiling) weigh 20 pounds per square foot of floor, then the safe load will be 60 pounds per square foot. The load on a floor usually consists either of people, furniture, machinery or merchandise. The strength, or the safe load, of a floor is usually spoken of as so many pounds per square foot, as that is the only practicable unit of measurement.

      WEIGHT OF WOODEN FLOOR CONSTRUCTION.

      Wooden floors usually consist of beams, commonly called “joists” or “floor joists,” one or two thicknesses of flooring boards, and in a finished building of a ceiling underneath the beams. In figuring the weight of 7/8-inch flooring boards it will be sufficiently accurate to estimate the weight of a single thickness at 3 pounds per square foot. The joists may also be figured at 3 pounds per foot, board measure, with the exception of hard pine and oak joists, which should be figured at 4 pounds per foot, board measure. The weight of the joists must also be reduced to their equivalent weight per square foot of floor. Thus the weight of a 2 × 12 inch joist is about 6 pounds per lineal foot. If the joists are spaced 12 inches on centers, this will be equal to 6 pounds per square foot, but if the joists are 16 inches on centers, there will be but 1 lineal foot of joist to every 1 1-3 square feet, which will be equivalent to 4 7/8 pounds per square foot, and if they are 20 inches on centers, the weight will be equal to 3 3-5 pounds per square foot; spaced 24 inches on centers, the weight will be 3 pounds per square foot.

      The weight of a lath and plaster ceiling should be taken at 10 pounds per square foot, and of a 3/4-inch wood ceiling at 2 1/2 pounds per square foot. Corrugated iron ceiling weighs about 1 pound per square foot.

      Table V will be found convenient in figuring the weight of floor joists.

Table V.—Weight of Floor Joists per Square Foot of Floor.

      We will now show how the strength and safe load of a few simple forms of floor construction may be computed.

      The simplest floor is that which consists of a series of parallel joists of equal size, spaced a uniform distance apart, and supported at each end either by walls or partitions. Such a floor is shown in Fig. 14.

      Fig. 14.—Plan of Simple Form of Floor Construction

      The strength of such a floor is measured by the strength of a single beam, but the strength per square foot may be obtained directly by:

      Rule 1.—To find the safe strength per square foot in pounds, multiply twice the breadth of a single joist by the square of the depth, and the product by the value of A; divide by S times the square of L; S being the distance between the centers of the joists in feet, and L the span in feet. A represents the strength of the wood, as given in Table I, page 5.

      Placed in the shape of a formula the above rule will read: Safe strength per square foot in pounds =

in which × denotes multiplication, B the breadth of a single joist, D the depth of the joist, both in inches; S the distance on centers, in feet, and L the span in feet.

      Having determined the safe strength, subtract СКАЧАТЬ