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
Example XIV.—What is the safe load for an 8 × 12 inch Georgia pine girder, of 14 feet span?
Answer.—Safe load for 1 × 12, from table = 2056 lbs. Multiplying by the breadth, 8 inches, we have 16,448 lbs. as the safe load for the girder.
Example XV.—What is the safe load for a 3 × 12 inch white pine beam of 16 feet span 1/4 inch scant in both dimensions?
Answer.—The safe load for a hard pine beam 1 × 12, 16 feet span, is given in the table as 1800 lbs. To find safe load for 2 3/4 × 11 3/4 beam, multiply by 2.63 = 4734 lbs., which is the safe load for a 2 3/4 × 11 3/4 inch hard pine beam of 16 feet span.
The strength of a white pine beam will be 3-5ths of this, or 2840 lbs.
To use Table IV for beams that are scant of the nominal dimensions:
The loads given in Table IV apply only to beams having the full depth indicated. To obtain the load for any thickness of beam, multiply the load in the table by the exact thickness of the beam, as 1 5/8, 1 3/4, 2 3/8, or whatever it may be.
For beams scant in both dimensions the correct load may be obtained by multiplying the load given in the tables by the following factors:
| For 1 3/4 × 5 3/4 by 1.5 | For 1 3/4 × 9 3/4 by 1.6 |
| 2 3/4 × 5 3/4 by 2.5 | 2 3/4 × 9 3/4 by 2.55 |
| 1 3/4 × 7 3/4 by 1.6 | 1 3/4 × 11 3/4 by 1.64 |
| 2 3/4 × 7 3/4 by 2.5 | 2 3/4 × 11 3/4 by 2.6 |
| 1 3/4 × 13 3/4 by 1 2/3 | |
| 2 3/4 × 13 3/4 by 2.6 |
TABLE IV.—MAXIMUM LOADS FOR HARD-PINE BEAMS CONSISTENT WITH STIFFNESS.
Table of maximum distributed loads which can be supported by horizontal rectangular beams of Georgia yellow pine one inch broad, and supported at both ends, with safety and without deflecting more than one-thirtieth of an inch per foot of span.
For beams of any width greater than 1 inch, multiply the load in table by the width of the beam in inches.
For beams of Oregon pine, use 4-5 of tabular load; for spruce beams, 5-7, and for white pine beams, 3-5.
Example XVI.—What is the maximum load, consistent with stiffness, for a Georgia pine beam 3 × 14 inches, 24 feet span?
Answer.—The load in Table IV for a 1 × 14 beam is 1042 lbs. For 3 × 14 inch it will be 3 × 1042, or 3126 lbs.
Example XVII.—What is the maximum load, consistent with stiffness, for a white pine beam measuring 2 3/4 × 11 3/4 inches, having a span of 18 feet?
Answer.—For a 1 × 12 hard pine beam the load is 1168 lbs. For 2 3/4 × 11 3/4 inch hard pine beam multiply by 2.6 = 3037 lbs. For white pine, use 3-5ths of this, or 1822 lbs.
*In all of these rules the breadth and depth of the beam are to be measured in inches and the span in feet, the final result being in pounds.
†See Rule 4.
*In all of these rules the breadth and depth of the beam are to be measured in inches and the span in feet, the final result being in pounds.
*For the beam in Fig. 8 the distance L should always be measured from the support to a line passing through the center of the load, and in case of the beam in Fig. 9 the product of W and L must equal the product of W’ and L’ or if W equals W’ then L must equal L’.
*The beam shown in Fig. 11 has the same strength as that shown in Fig. 10, provided that the distance L is equal in the two cases; i.e., it makes no difference whether the cantilever end is supported by being fixed in a wall or by being balanced by an equal load on the other end of the beam.
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