"74 BEPORT — 1901. 



TUESDAY, SEPTEMBER 17. 

 The following Eeport and Papers were read : — 



1. Report an the Small Screw Gauge. — See Reports, p. 407. 



2, A Portable folding Banc/e- Finder, for Use ivlth Infantry. 

 By G. Forbes, ^.^aS'. 



3. Machinery for Engraving. By Mark Barr. 



4. Recent Detieloiynients of Chain Driving. By C. E.. Garrard. 



5i Measure77ient of the Hardness of Materials by Indentcttion by a 

 Steel Sphere. By T. A. Heabson. . 



6. On the Critical Point in Rolled Steel Joists. By E. J. Edwards. 



In selecting rolled steel joists for floors there are two elements which determine 

 the section to be used with a given load per square foot of floor area. 



First, the stress per square inch produced by the load. 



Second, the deflection produced by the .same load. 



At first, particularly with small spans, it is the stress per square inch which is 

 the governing element : this stress must not exceed safe working limits. As the 

 lioor span is increased the deflection becomes the ruling element, the stress per 

 square inch falling into the background. 



The deflection must not be sufficient to crack the ceiling where there is one, 

 nor sufficient to be unsightly where there is none. 



In the diagrams exhibited two curves ave shown, one in black and the other in 

 red. The former is the curve of a given maximum stress, and shows the loads a 

 steel joist will carry for various spans. The red curve gives the loads which pro- 

 duces a deflection which is a constant given fraction of the span, viz., 5 J^. 



The curves cross each other, and the point of crossing the author calls the 

 critical point. At this point the distributed load produces the given stress and 

 given deflection. Before the critical point is reached the load produces the specified 

 stress, but is insufficient to produce the limiting deflection ; after the critical point 

 is passed the distributed load produces the specified deflection, but is insufficient 

 to produce the specified stress ; in other words, the limit of deflection is reached 

 before the limit of stress. Examples are given of various sizes of steel joists with 

 the limiting stresses and deflections. 



Generalising, up to the critical point the stress curve is the more important ; 

 beyond this the deflection curve is more important. The two important parts of 

 the curves taken together are called the curve of loads, which is a curve with a 

 kink in it. 



The first part of curve is drawn from the formula W = — and the second 



9LtZ 



QQT 



part from tjie formula W=i,i_. Explanations showing how the equations are 



arrived at are given in thfe paper. - - 



With a factor of safety of 3 and a breaking stress of 32 tons per square inch, 

 and a deflection oi -iixs spaii, the cri'tfcar point is' ftrtf span of twenty-seven times 



