150 UNIVERSITY OF COLORADO STUDIES 



S2=tbe maximum intensities of tension and compression in the 

 steel of the rib, in pounds per square inch. 



T =total normal thrust in pounds per inch width of rib. 



A,=the area of concrete section in square inches per inch width 

 of rib; it is essentially equal numerically to d because the steel area 

 in most cases is comparatively small. 



A2=area in square inches of the average amount of steel flange 

 section per inch width of rib = area in square inches of a steel rib's 

 flange sections divided by the spacing of steel ribs in inches. 



(7j= twice the distance in inches from the common centroid of 

 section to the most remote fibre of concrete=for symmetrical sec- 

 tions, d. 



<:/2= twice the distance in inches from the common centroid to 

 the outermost fibre of steel. 



M=the bending moment in foot pounds per inch width of rib. 



I, = moment of inertia of Aj with respect to the common neutral 



axis. 



I2= moment of inertia of K^ about the same axis. 



The stresses in the extreme fibres of concrete ribs with no steel 

 are found by making A, and \ each zero in equation (21), thus: — 



S,=^^?^. (23) 



A,- I, 



In these equations the coeflicient of elasticity for steel is assumed 

 to be twenty times as great as that for concrete, and for each material 

 the coefficient is considered the same in tension and compression. 

 For steel E= 30,000,000 pounds per square inch. 



For an unsymmetrical section the equations for S, and S^ are 

 identical in form with those given above, only since the common 

 centroid of the materials of the section is no longer at the mid depth, 

 d^ and d^ must respectively be replaced by twice the distances to the 

 most remote fibres of steel and concrete on each side of the neutral 

 axis. 



By comparing formulas (21) and (22), it is at once seen that 

 the greatest steel stresses are about twenty times the greatest concrete 



