62 REINFORCED CONCRETE CONSTRUCTION 



usually considered a safe stress. Beams designed by the 

 ultimate load formulas will generally be of smaller cross-sectional 

 dimensions than when the straight-line formulas are employed, 

 but, on the other hand, a larger amount of steel is required. 

 Experienced designers, however, will arrive at satisfactory 

 results by either the ''factor of safety" (referred to ultimate 

 strengths) or the "working stress" methods, but there seems 

 to be no good reason why the simple formulas based on the 

 straight line stress variation should not be used for purposes of 

 design, safe working stresses being employed. 



It may be well to state here, that the straight line theory of 

 stress distribution will be assumed in all the discussions which 

 follow. 



PROBLEMS 



20. Solve Problem 16 for the ultimate resisting moment, assuming a 2000-lb. 

 concrete and assuming the elastic limit of the steel equal to 40,000 Ib. 

 per square inch. Consider the given value of E c to be the initial 

 modulus. 



21 Solve Problem 18 by the ultimate load formula, assuming a 2400 Ib. con- 

 crete and assuming the elastic limit of the steel equal to 35,000 Ib. per 

 square inch. Use a factor of safety of 4. Consider the given value of 

 E c to be the initial modulus. 



[In the preceding paragraphs have been shown the two usual 

 methods of calculating the maximum fiber stresses in the concrete 

 and steel of a reinforced concrete beam. The method of pro- 

 cedure is to determine the vertical section of the beam where the 

 moment is a maximum and apply the formulas at that section. 

 The formula for p, containing the values of f c and/ s , determines 

 the amount of steel reinforcement which is needed to cause the 

 beam to be of equal strength in tension and compression. The 

 formulas for resisting moment determine the bending moment 

 which a beam will safely withstand (for an existing structure) or 

 the size of the beam needed to resist a given bending moment 

 (for a proposed structure). 



In steel I-beams the above mentioned calculations, which for 

 convenience we shall call "moment calculations," are the only 

 ones needed except in the case of short beams heavily loaded, 

 when the matter of diagonal compression must be investigated. 

 Steel beams are strong in tension but the thinness of the web 



