INTERNAL FORCES 79 



fiber stress, is used in computations for beams of wood, steel, iron, 

 and reinforced concrete. 



The unit moment of resistance is a number which contains all 

 the known quantities in an expression, leaving only the unknowns 

 to be found. For example, the moment of resistance of a wooden 

 beam in which we can use a maximum fiber stress of 1200 Ibs. per 

 square inch is 12 QO &fc 



M r g , 



and by dividing the fiber stress by 6 the unit moment of resistance 

 equals 200, from which we get 



M r - 200 6/i 2 = RbW. 



Some men use R for wooden beams, but where the divisor is 

 so small the only advantage is some slight simplification of the 

 work, provided a table of values of R has been previously com- 

 puted for the woods used. In reinforced-concrete work a number 

 of factors enter into the formula for the resisting moment and the 

 use of a table, or of a diagram which is really a graphical table, 

 for all possible values of R is almost indispensable for the designer. 

 Where a number of factors enter into a computation it is easy to 

 forget to use some. 



Deflection 



The amount of deflection when a beam is loaded is measured 

 on the bottom or top of the beam for convenience. The difference 

 in elevation between the end of the beam and the middle is the 

 deflection. The deflection actually used in computations is the de- 

 flection at the neutral axis, but the deflection measured on the 

 bottom or top, which for obvious reasons is more readily obtained 

 than the deflection of the neutral plane, is close enough for all 

 practical purposes. 



Deflection in beams and girders used in buildings is important 

 only when the lower side carries a plastered ceiling. The deflec- 

 tion is limited- to a .maximum of one-three-hundre<l-:ui<l-sixtirth 

 of the span to prevent cracks in the plaster. A greater deflection 

 is not unsightly and is permissible when constant. Wood and steel 

 beams straighten when the load is relieved and deflect when the 

 load is increased. It is the movement that causes plaster to crack, 

 so this must be limited. For beams and trusses under moving 

 loads the deflection must be limited to an amount which will not 

 set up dangerous vibrations, but with this the ordinary structural 



