EXTERNAL FORCES 



51 



to take care of this amount of bending moment, the moment of 

 inertia is thereby altered. Changing the moment of inertia has 

 the effect of greatly increasing the tension in the steel over the 

 supports. The posi- 

 tive and negative 

 moments should total 

 \WL and not \WL, 

 this in effect making 

 the positive and nega- 

 tive moments for in- 

 terior spans and sup- 

 ports =T^WL. 



When one panel is 

 loaded and an adja- 

 cent panel is not 

 loaded there will be 

 an uplift in the un- 

 loaded panel, for it opposes only the dead load to the combined 

 dead and live load on the loaded panel. To make the positive 

 and negative moment coefficients in each panel equal gives the 

 necessary stiffness and increased weight. 



Assuming spans equal in length and loaded uniformly, the 



negative bending moment 

 coefficients to use over 

 supports are shown in Fig. 

 45. Each square repre- 

 sents a support and the 

 coefficients are given as 

 decimal instead of com- 

 mon fractions. The mo- 

 Fig. 46 Coefficients for Maximum Positive mpnts __ rormtant at the> 



n ,. m( , TT / T J I11CUIO <U U lUIlMtHIL lit LUC 



Bending Moments for Uniform Loads on 



Equal Spans of Continuous Beams. edges and across the tops. 

 M - Cwf, in which C - coefficient. 



Fig. 45 Coefficients for Maximum Negative 



Bending Moments for Uniform Loads over 



the Supports of Continuous Beams with 



Equal Spans. M - CwP, in which 



C coefficient. 



wP 

 Example: M = - 0.125 trP. 



8 



wP 



Examples: M - - 0.125 wP. 



o 



Assuming spans equal 

 in length and loaded uni- 

 formly, the positive bend- 

 ing moment coefficients to use for the spans are shown in Fig. 46. 

 The coefficients, however, are the theoretical coefficients for beams 

 with constant moment of inertia. They should not be used for 

 (lit- reasons given above, for it is best to have the positive and 

 negative moments equal. 



