648 PKOCEEDINGS OF SECTION H. 



If the compression side is tlie weaker — 



M, =-\f, {Jed) i{d-\ l<cl^ = \f, hi- b (^i - ^^ 



Now, for any given beam of known reinforcement k is constant, and 

 ■|/'j, is also constant, so that we may write — 



Avhere R^, depends upon — and/',, 



If the tension side is the weaker — 



M„ = « x/, (^d - \^ci^ = of, ./ (i I). 



Now if p represents the ratio. -- M^ = /» bd"- f ^ I 1 1. 



ha \ 3 / 



and we mav write — 



M =-E,,M^ 



In Figure 2 the values of E,. and E^. are plotted as curves for 

 variable percentages of steel. 

 The data assumed are — 



y, = 17,000 lbs. 2)er stjuare inch. 



/; = ooo „ „ 



and n = 15. 



At the present time engineers have a wide choice of values for 

 these three constants, and the result is that different designers will 

 arrive at different results for beams to carry the same load. 



It would be very convenient if the engineers in Australia would 

 meet in conference and decide upon standard data of calculation. 

 In the meantime I should advise any one engineer to make up his 

 mind to adopt always the same data, and keep all his designs con- 

 sistent. ^ 



The working stress (17,000 lb.) gives a factor of 4 on an ultimate 

 strength of about 30 tons per square inch denoting a mild steel pro- 

 curable under the British Standard Specification for Structural Steel; 

 600 lb. per square inch is suitable for 1.2.4. concrete. The ratio 



E, 



could, of course, be determined by careful experiments on the actual 

 concrete to be used, but 15 is the value adopted by the R.I.B.A., the 

 Prussian Government Regulations, and the Austrian Government. 



It will be seen that the cun^es for R,. and E, intersect at a 

 common value of 91*5, which is obtained with 0-61 per cent, of steel. 

 This signifies that the working stresses are reached simultaneously in 

 the concrete and the steel. It does not obviously follow that this is 

 the most economical reinforcement. To test this it will be necessary 

 to assume costs of steel and concrete respectively. Taking steel laid 



