CAMBRIA STEEL. 101 



amount of this reduction can de determined by reference to the 

 explanations and tables therefor on pages 82 and 83. 



The thrust of floor arches, which is considerable, particularly in 

 the case of long spans or distances between tie-rods, should be 

 taken into account where it tends to produce lateral flexure of the 

 floor beams. 



Explanations of this and a formula for reducing the unit stresses 

 from vertical loading, on account of the additional stresses caused 

 by horizontal forces, are given on pages 78 to 81 inclusive. 



In some instances the allowable deflection will govern the 

 design rather than the transverse strength, as in the case of beams 

 carrying plastered ceilings, in which the deflection should be 

 limited to ^ inch per foot of span, or ^ of the distance between 

 supports in order to avoid cracking the plaster. 



This limit of deflection is indicated in the tables by full hori- 

 zontal lines, the figures below which correspond to loads or 

 spacings for the given spans that will produce greater deflections 

 than the allowable limit for plastered ceilings. 



The deflection limits of the Tables of Safe Loads have been 

 calculated for the total loads, including the weight of the section 

 used as a beam. The superimposed live load will not produce all 

 of this deflection, and therefore the deflection limit of the tables 

 includes an element of safety for the reason that the beams will 

 be deflected, after being put in place, by their own weight and that 

 of the floor materials before the plastering is applied. 



In cases where the deflection limits the use of the beam for the 

 safe loads corresponding to the fibre stresses of the tables, the 

 beam may be used with a less load such as to produce only the 

 allowable deflection. The lesser load corresponding to the limit 

 of deflection may be obtained for any span from the Table of 

 Safe Loads as follows: 



w 

 W = 



