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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СКАЧАТЬ ÷ 400, or 54 square inches. As the breadth of the beam is 10 inches, the bearing should equal 54 ÷ 10, or 5.4 inches. For floor joists a bearing of 4 inches is usually ample, and for girders a bearing of six inches is usually sufficient; 4 inches, however, should be considered as the minimum bearing, unless the beams are securely tied in place.

      TABLES FOR THE STRENGTH AND STIFFNESS OF WOODEN BEAMS.

      Tables III and IV will be found very convenient in figuring the safe loads for beams supported at both ends, and loaded either with a distributed load or a concentrated load applied at the center.

      By following the directions given, the results obtained by the use of the tables should be the same as by using the corresponding rule, and with less figuring.

      When the bending or deflection of the beam is of no importance use Table III, and when excessive bending must be avoided use Table IV.

      Table of safe quiescent loads for horizontal rectangular beams of Georgia yellow pine, one inch broad, supported at both ends, load uniformly distributed. For concentrated load at center divide by two. For permanent loads (such as masonry) reduce by 10 per cent.

      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 9-10 of tabular loads; for spruce beams, 7-10; for common white pine, 3-5, and for white oak, 3-4.

      To use Table III for beams that run less than the nominal dimensions. In many localities floor joists as carried in stock are more or less scant of the nominal dimensions, and for such joists a reduction in the safe load must be made to correspond to the reduction in size. For beams having the full depth multiply the load in table by the actual breadth, as 1 5/8, 1 3/4, 2 7/8, or whatever it may be. For beams 1/4 inch scant in both dimensions the safe load may be obtained by multiplying the safe load as given in the table by the following factors:

For beams 1 3/4″ × 5 3/4″ by 1.6 For beams 1 3/4″ × 11 3/4″ by 1.67
2 3/4″ × 5 3/4″ by 2.52 2 3/4″ × 11 3/4″ by 2.63
1 3/4″ × 6 3/4″ by 1 5/8 1 3/4″ × 13 3/4″ by 1.68
2 3/4″ × 6 3/4″ by 2.55 2 3/4″ × 13 3/4″ by 2.65
1 3/4″ × 7 3/4″ by 1.64 1 3/44″ × 14 3/4″ by 1.69
2 3/4″ × 7 3/4″ by 2.58 2 3/4″ × 14 3/4″ by 2.66
1 3/4″ × 9 3/4″ by 1.66 1 3/4″ × 15 3/4″ by 1.7
2 3/4″ × 9 3/4″ by 2.61 2 3/4″ × 15 3/4″ by 2.66

      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 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.