PROBLEMS IN DESIGN OF BEAMS 95 



the top for lag screws or spikes will take care of unequal loading 

 on beams. 



When a hah" stirrup is used it must be investigated for bending 

 at the bearing edge. Assume a bearing length of 4 ins. 



M - 4x t f 35 - 10,700 in. Ibs., half on each leg. 



A 



This computation considers the legs as cantilever beams uniformly 

 loaded. 



The thickness of the metal is f in. (0.375 in.) and the width is 

 to be found. For wrought iron with a fiber stress of 10,000 Ibs. per 



10 000 

 square inch, R = = 1667. For steel with a fiber stress of 



14 

 14,000 Ibs. per square inch, R = = 2333. 



M 5350 



" Rh* ~ 1667 x 0.375 2 " 

 of which each leg will be one-half, or 11.4 ins. for wrought iron. 



5350 

 For steel, 6 = ^5^5 no _- 2 = 16.3 ins., of which each leg will be 



X U.Oiu 



one-half, or 8.15 ins. 



The reason for the low stresses used is due to the blacksmith 

 work required to bend the metal to the required shape, the heating 

 annealing the metal and restoring disturbed molecules to a normal 

 condition. Cold working has a contrary effect within limits 

 but may crack the metal, thus nullifying the effect of the strain 

 which sets up internal stresses that apparently cause an increase 

 in strength. 



The effect of increasing the thickness of the metal is to make 

 a considerable reduction in width on the supporting girder. Try 

 a J-in. steel strap. 



^ * OMQ 7T 9.2 ins., of which each leg will be one-half, 



Zooo X U.O 



or 4.6 ins. By increasing the thickness \ in. the width of the strap 

 has been reduced nearly one-half. The wide strap will weigh 

 10.4 Ibs. per lineal foot. The narrow strap of thicker metal will 

 weigh 7.8 Ibs. per lineal foot, so will be the cheaper strap to use. 



If a stirrup is not designed to be safe according to calculations 

 such as those illustrated it should not be used. A lack of strength 

 in bending is sometimes claimed to l>e taken care of by using a 

 longer support and holding it down with lag screws or spikes. 



