CHAPTER II 

 Internal Forces 



THE term " bending moment " is a description of the 

 breaking effect of external forces on a beam. The term 

 " resisting moment " is a description of the effect of internal 

 forces in a beam set up to resist breaking. Beams not stayed 

 laterally will bend to one side, in which case the full value of the 

 resisting moment is not obtained. 



The action of the resisting forces may be illustrated by simple 

 framework, for a frame is merely a light beam containing little 

 or no superfluous material. The ideal frame contains no superflu- 

 ous material, but if this is obtained by an increase in cost of fabri- 

 cation the frame is not ideal from the standpoint of the user, 

 regardless of the mathematically ideal condition. 



The designer soon finds that mathematical analysis of stresses 

 treats frames as lines through the center of gravity of pieces. It 

 is with pieces that the designer has to deal. In a mathematical 

 design of a frame all forces act at points, and when a line of infini- 

 tesimal thickness, dealt with by the mathematician, is replaced by 

 a piece of wood or steel or concrete, certain stresses are set up 

 around the points. Then joints are constructed and an analysis 

 must be made of the forces around the joints, this involving the 

 design of rivets and fastenings to keep the joints from moving. 



A simple beam contains some superfluous material, but much 

 of this material acts to transmit stresses in various directions and 

 thus takes care of internal stresses which otherwise would inter- 

 fere with the direct action assumed to take place along the lines 

 connecting the points around which the forces act. 



The strongest frame is a triangle, for a triangle with sides of 

 fixed length cannot distort under direct stresses acting along the 

 center lines of the pieces of which it is composed. 



The capital letters A, B, and C are used to designate the three 

 angles of a triangle, and the corresponding small letters are used 



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