USEFUL DATA 



two, or three panels, and also for slabs having panels of markedly different sizes, an 

 analysis should be made of the moments developed in both slab and columns, and the 

 values given herein modified accordingly. Slabs with paneled ceiling or with depressed 

 paneling in the floor are to be considered as coming under the recommendations 

 herein given. 



(r) Bending Moments in Columns. — Provision should be made in both wall columns 

 and interior columns for the bending moment which will be developed by unequally 

 loaded panels, eccentric loading, or uneven spacing of columns. The amount of mo- 

 ment to be taken by a column will depend upon the relative stiffness of columns and 

 slab, and computations may be made by rational methods, such as the principal of 

 least work, or of slope and deflection. Generally, the larger part of the unequalized 

 negative moment will be transmitted to the columns, and the column should be 

 designed to resist this bending moment. Especial attention should be given to wall 

 columns and corner columns. 



WORKING STRESSES 



General Assumptions. The following working stresses are reconmiended for 

 static loads. Proper allowances for vibration and impact are to be added to live 

 loads where necessary to produce an equivalent static load before applying the unit 

 stresses in proportioning parts. 



In selecting the permissible working stress on concrete, the designer should be 

 guided by the working stresses usually allowed for other materials of construction, 

 so that all structures of the same class composed of different materials may have 

 approximately the same degree of safety. 



The following recommendations as to allowable stresses are given in the form of 

 percentages of the ultimate strength of the particular concrete which is to be used; 

 this ultimate strength is that developed at an age of twenty-eight days, in cylinders 8 

 inches in diameter and 16 inches long, of proper consistency f made and stored under 

 laboratory conditions. In the absence of definite knowledge in advance of construc- 

 tion as to just what strength may be expected, the committee submits the following 

 values as those which should be obtained with materials and workmanship in accord- 

 ance with the recommendations of this report. 



Although occasional tests may show higher results than those here given, the Com- 

 mittee recommends that these values should be the maximum used in design. 



Table of Compressive Strengths of Different Mixtures of Concrete 

 (In Pounds per Square Inch) 



Aggregate 



Granite, trap rock 



Gravel, hard limestone and hard sandstone 



Soft limestone and sandstone 



Cinders 



Note. — For variations in the moduli of elasticity see section on "Working Stresses." 



Bearing. When compression is applied to a surface of concrete of at least twice 

 the loaded area, a stress of 35 per cent of the compressive strength may be allowed 

 in the area actually under load. 



tSee Consistency, page 191. 



209 



