COLUMNS FIXED AT THE: BASE 9 1 



The maximum fibre stress occurs at the foot of the knee brace, 

 and is given by the formula 



(3 0a) 





E 



The nomenclature being the same as for (30) except h, which 

 is the distance in inches from the point of contra-flexure to the top of 

 the column. 



(2) Columns fixed at the base and top. In this case it can be 

 seen by inspection that the point of inflection is at a point y = - 

 and we have for this case 



B = H> + C ( 3 la) 



M^ = I il = M^ (32a) 



It is difficult to realize the exact conditions in either (l) or (2), 

 in Case II, and it is probable that when an attempt is made to fix 

 columns at the base, the actual conditions lie some place between (i) 

 and (2). It would therefore seem reasonable to assume the minimum 

 value, y = - as the best value to use in practice. This assumption is 

 commonly made and will be made in the problems which follow. 



Having the external forces H 1 , B, C and F 1 the stresses K, U and 

 L are computed by formulas (27), (28) and (29). The remaining 

 stresses 'in the truss can then be computed by the ordinary algebraic or 

 graphic methods. 



For a simple graphic solution of this problem, where the ex- 

 ternal forces B and C are not computed, see Fig. 55 and Fig. 57. 



Maximum Stresses. It is not probable that the maximum 

 snow and wind loads will ever come on the building at the same time, 

 and it is therefore not necessary to design the structure for the sum of the 

 maximum stresses due to dead load, snow load and wind load. A 



