152 REINFORCED CONCRETE CONSTRUCTION 



From this expression the best depths for various assumed widths 

 may readily be determined and the desirable proportions finally 

 selected. 



In the design of beams, and especially T-beams, it is important 

 for the student to note that liberal spacing favors large rods and 

 few in number, while good bond strength without waste of 

 material favors small rods. Also, if bent rods are to be used 

 for web reinforcement, then numerous small rods are also 

 advantageous. 



61. Conditions Met With in Design of T-beams. In practice 

 the design of T-beams will take one of the following forms: (1) 

 The dimensions may be given, to find the safe resisting moment 

 of the beam or the stresses in the steel and concrete under a given 

 load; (2) the flange of the T-beam may form a portion of a floor 

 slab which is already designed, in which case the dimensions of 

 the flange are given also the loading and specified working 

 stresses and the design comprises the determination of suitable 

 web dimensions and steel area; (3) the loading and working 

 stresses may be given, to determine suitable proportions for the 

 entire beam that is, the flange does not form a part of a floor 

 system already determined. 



(1) The values of k and j may be found from Eqs. (3) and 

 (6), or from Eqs. (2), (4) and (5), of Art. 59, and then the values 

 of the moment of resistance from Eqs. (7), or the fiber stresses 



from Eqs. (8) . If the value of k is found to be less than ^ , then 



the problem falls under Case I and the formulas for rectangular 

 beams apply. 



(2) Depth and width of beam should be selected with reference 

 to shearing strength, space for necessary rods, and other con- 

 siderations. The depth having been selected, the amount of 

 steel may be approximately determined by either Eq. (a) or Eq. 

 (b). The amount of steel being known, the value of / may be 

 determined by Eq. (7) . The value of k should also be found from 

 Eqs. (2) or (3) in order to ascertain if the beam falls under Case 

 I or Case II. The stress in the concrete, corresponding to the 

 allowable working stress in the steel, is then found from Eq. (8). 

 (This method has been explained by illustrative problems 

 following Art. 59.) 



(3) First, select suitable proportions for the web. A flange 

 thickness is then assumed such as to give satisfactory proportions 



