PROBLEMS IN DESIGN OF BEAMS 93 



is to be divided by the stress given in this column in order to obtain 

 the number of square inches bearing surface. The student should 

 pay attention to this column, for it explains the reason why steel 

 and iron post caps are used instead of the old-fashioned wooden 

 bolsters. If the load on a column is carried straight down on the 

 ends of fibers, the full bearing capacity of the wood can be utilized. 

 When a bolster is set between the foot of a post on one floor 

 and the top of the post on the floor below, the compression across 

 the grain of the wood in the bolster governs the carrying capacity 

 of the post, or the bolster will crush. 



The fifth column gives the allowable shearing stress with the 

 grain, the use of this column having been explained in the examples 

 when a test was made of the weight-carrying capacity of a beam 

 BO it would not fail in shear. 



There is a shear parallel with the grain and if through some 

 unavoidable circumstance it ever becomes necessary to design so 

 a wide beam overhangs the sides of a support this shear will act. 

 It should not exceed the safe shear with the grain. 



There is a shear across the grain, that is, a tendency for the 

 beam to be cut at the edge of the support. Provided the allow- 

 able compression across the grain is not exceeded, i.e., sufficient 

 bearing surface is provided, the effect of this shear is negligible. 



The use of hangers and stirrups is common to-day. They save 

 head room but increase the insurance rate, for the reason that metal 

 is affected by intense heat. A large piece of timber will char on 

 the surface and must be exposed to an intense flame for a long 

 time before it begins to burn. The heat that will merely char a 

 timber and do it little harm will heat wrought iron and steel to 

 such an extent that the stirrup will be weakened and permit the 

 suspended beam to drop. A study of a bending moment curve 

 shows that at the bearing end of a beam there is practically no 

 moment, so the area of a beam may be reduced nearly one-half 

 at the supports without impairing the bearing capacity. If the 

 strength of a stirrup is reduced one-half by fire the beam may drop. 



Many types of stirrups are on the market, and before adopting 

 anything other than a plain Ixjnt strap of steel or wrought iron 

 the designer should require the manufacturers to furnish records 

 of tests on the stirrups they propose to supply. 



To design a stirrup: First obtain the area required for bearing, 

 then the thickness to prevent straightening at the edge of the sup- 



