174 HOMOGENEOUS BEAMS 



As the amount of load a beam must carry, particularly the 

 live load, is very uncertain at best, the addition of a slight 

 weight due to the weight of the beam itself is seldom con- 

 sidered to be a factor of great importance. When beams 

 carry floors, it is customary to find the weight of the floor 

 per square foot and then multiply this value by the distance 

 between beams and by the span to get the total load on the 

 beam. Many engineers assume that the weight of the 

 beams themselves add a certain weight per square foot to 

 the weight of the floor. This added weight is assumed to 

 be 8 Ib. for wooden beams and 6 Ib. for steel beams. Thus, 

 if all the materials composing a floor, exclusive of beams, 

 were calculated to weigh 12 Ib. per sq. ft., a weight of 20 Ib. 

 per sq. ft. would be taken to constitute the total load of a 

 floor supported by wooden beams, while 18 Ib. per sq. ft. 

 would be taken for the total load of one supported by steel 

 beams. This method, while not absolutely accurate, is 

 one way of estimating the weight of the beams in a floor. 

 In concrete beams and stone beams, however, the weight of the 

 beam itself must almost always be considered. 



DEFLECTION 



Deflection is the name applied to the distortion or bend- 

 ing produced in a beam when subjected to bending stresses. 

 The measure of the deflection at any point on a beam is the 

 vertical displacement of the point from its original position. 



Stiffness is a measure of the ability of a body to resist 

 bending; this property is very different from the strength 

 of the material or its power to resist rupture. 



The stiffness of a beam does not depend so much on the 

 elasticity of the material of which it is composed as on its 

 length of span. This property of stiffness is as important 

 in building construction as mere strength, and the two 

 should be considered together; thus, the floor joists of a 

 building may be strong enough to resist breaking, but they 

 may also be so long as to lack stiffness, in which case the floor 

 will be springy and will vibrate from persons walking on it. 

 If there is a plastered ceiling on the under side of the joists 

 of such a floor, the deflection of the joists may cause the 



