10 PRACTICAL STRUCTURAL DESIGN 



the face of the supports and this length is L, which 

 is always greater than S. 



I = any portion of L or S used to obtain a distance from 



the support to the center of gravity of a load. When 



distances are required to several loads, subscripts cor- 



.-. responding to those under the respective 



(j?J loads are used. Thus to load P, we use I,', 



\ to load P,, we use 1,, etc. 



p 



5. _! >j The use of the letters is illustrated in Fig. 



Lj .^ j ^ fa w hi c h a cantilever beam is shown; and 



in Fig. 2, in which is shown a beam on two 

 supports. To illustrate a uniformly distri- 

 buted load the weight of the beam is used. 

 Sometimes the letters a, 6, c, etc., are used to designate portions 

 of a span or lengths less than the whole, but these will be dealt 

 with as they may arise. The letters x, y, z, etc., are used simi- 

 larly and will be dealt with when required. 



DEAD LOAD. The weight of the structure and permanent loads. 

 LIVE LOAD. The load the structure is designed to carry in ad- 

 dition to the dead load. The live load consists of machinery, 

 merchandise, people, etc. 



IMPACT. The effect of a live load in motion. It is added to the 

 li ve load and varies from twenty-five per cent for a slowly moving 



load on a rigid structure to one uj., .j_ 



hundred per cent for a live load 

 suddenly applied. 



WIND. This force acts perpendi- 

 cularly to the pressed surface, so is J, 



horizontal on the sides of a building w* -S 



and is a diagonal load when it acts Fig 2 _ Beam ^ siing on Two 

 on a sloping roof. Supports 



Moments 



A cantilever beam is supported at one end and may be loaded 

 at any point with a concentrated load. The load tends to bend 

 the beam down. The uniformly distributed weight of the beam 

 tends to bend the beam also, so on all loaded beams there must be 

 considered the actual weight of the beam plus added loads it may 

 carry. The name " cantilever " gives a hint that the bending action 

 is similar to that of a lever. 



