This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
2. In I-beams, and in others with a thin vertical web, the greatest unit-shearing stress in a section practically equals Ss, as given by equation 7, if the area of the web is substituted for A.
Examples. 1. What is the greatest value of the unit-shearing stress in a wooden beam 12 feet long and 6 X 12 inches in cross-section when resting on end supports and sustaining a uniform load of 0,400 pounds ? (This is the safe load as determined by working fibre stress; see example 1, Art. 65.)
Since A = 6 X 12 = 72 square inches;
Ss = 3,200/72 = 44 pounds per square inch, and the greatest unit-shearing stress equals
3/2 Ss = 3/2 44 = 66 pounds per square inch.
Apparently this is very insignificant; but it is not negligible, as is explained in the next article.
2. A steel I-beam resting on end supports 15 feet apart sustains a load of 8,000 pounds 5 feet from one end. The weight of the beam is 375 pounds, and the area of its web section is 3.2 square inches. (This is the beam and load described in examples 2 and 3, Art. 65.) What is the greatest unit-shearing stress ?
The maximum external shear occurs near the support where the reaction is the greater, and its value equals that reaction. Calling that reaction R, and taking moments about the other end of the beam, we have
R x 15 - 375 X 7½ - 8,000 X 10 = 0; therefore 15 R = 80,000 + 2,812.5 = 82,812.5; or, R = 5,520.8 pounds.
Hence Ss = 5,520.8/3.2 = 1,725 pounds per square inch.
1. A wooden beam 10 feet long and 2 X 10 inches in cross-section sustains a middle load of 1,000 pounds. Neglecting the weight of the beam, compute the value of the greatest unit-shearing stress.
Ans. 37.5 pounds per square inch.
2. Solve the preceding example taking into account the weight of the beam, 00 pounds.
Ans. 40 pounds per square inch.
3. A wooden beam 12 feet long and 4 X 12 inches in cross-section sustains a load of 3,000 pounds 4 feet from one end. Neglecting the weight of the beam, compute the value of the greatest shearing unit-stress.
Ans. 02.5 pounds per square inch. 71. Horizontal Shear. It can be proved that there is a shearing stress on every horizontal section of a loaded beam. An experimental explanation will have to suffice here. Imagine a pile of'six boards of equal length supported so that they do not bend. If the intermediate supports are removed, they will bend and their ends will not be flush but somewhat as represented in Fig. 41. This indicates that the boards slid over each other during the bending, and hence there was a rubbing; and a frictional re-sistance exerted by the boards upon each other. Now, when a solid beam is being bent, there is an exactly similar tendency for the horizontal layers to slide over each other; and, instead of a frictional resistance, there exists shearing stress on all horizontal sections of the beam.
In the pile of boards the amount of slipping is different at different places between any two boards, being greatest near the supports and zero midway between them. Also, in any cross-section the slippage is least between the upper two and lower two boards, and is greatest between the middle two. These facts indicate that the shearino- unit-stress on horizontal sections of a solid beam is greatest in the neutral surface at the supports.
It can be proved that at any place in a beam the shearing unit-stresses on a horizontal and on a vertical section are equal.
It follows that the horizontal shearing unit-stress is greatest at the neutral axis of the section for which the external shear (V) is a maximum. Wood being very weak in shear along the grain, timber beams sometimes fail under shear, the "rupture" being two horizontal cracks along the neutral surface somewhat as represented in Fig. 42. It is therefore necessary, when dealing with timber beams, to give due attention to their strength as determined by the working strength of the material in shear along the grain.
Example. A wooden beam 3 X 10 inches in cross-section rests on end supports and sustains a uniform load of 4,000 pounds Compute the greatest horizontal unit-stress in the beam.
The maximum shear equals one-half the load (see Table B, page 55), or 2,000 pounds. Hence, by equation 7, since A = 3 X 10 = 30 square inches,
SS = 2,000/30 = 66 2/3 pounds per square inch.
This is the average shearing uni -stress on the cross-sections near the supports; and the greatest value equals
3/2 X 66 2/3 = 100 pounds per square inch.
According to the foregoing, this is also the value of the greatest horizontal shearing unit-stress. (If of white pine, for example, the beam would not be regarded as safe, since the ultimate shearing strength along the grain of selected pine is only about 400 pounds per square inch.)
72. Design of Timber Beams. In any case we may proceed as follows:-(1) Determine the dimensions of the cross-section of the beam from a consideration of the fibre stresses as explained in Art. 66. (2) With dimensions thus determined, compute the value of the greatest shearing unit-stress from the formula,
Greatest shearing unit-stress = 3/2 V÷ ab, where V denotes the maximum external shear in the beam, and b and a the breadth and depth of the cross-section.
If the value of the greatest shearing unit-stress so computed does not exceed the working strength in shear along the grain, then the dimensions are large enough; but if it exceeds that value, then a or b, or both, should be increased until 3/2 V ÷ ab is less than the working strength. Because timber beams are very often "season checked" (cracked) along the neutral surface, it is advisable to take the working strength of wooden beams, in shear along the grain, quite low. One-twentieth of the working fibre strength has been recommended* for all pine beams.