CONCRETE DESIGN 301 



method of placing these stirrups are still matters of discus- 

 sion that depend much on the experience of the designer. 

 One important thing to be re- JTT?: 



membered is that they are com- 



paratively short and that J| 



there is danger of their pulling FIG. 2 



out of the concrete. For these reasons they are ffequently 



securely fastened to the slab reinforcement or bent over at 



the ends. Sometimes deformed bars are used, as they give 



a more secure grip on the concrete. 



When inclined stirrups are used, they should be fastened to 

 the main reinforcing bars in such a way that they will not slip. 



An approximate formula for shear in a rectangular beam 



reinforced at the bottom is v = , where v = unit shearing 

 bjd 



stress in pounds per square inch at any point, V = total 

 vertical shear at that point in pounds, and the other terms 

 have the same meaning as before. The Joint Committee 

 recommends for a grade of concrete mentioned above a unit 

 safe shear of 40 pounds. If V is greater than this, stirrups 

 must be used. The same formula may be used for T beams 

 with b' inserted instead of b and jd referring to T beams. Even 

 with web reinforcement, v should never exceed 120 pounds. 



No absolute rules can be given for the size and placement 

 of stirrups. In many instances J in. square or A" X l" flats 

 are used, but not in all cases. The spacing of stirrups is 

 greater at the center of the span than at the supports. The 

 Joint Committee recommends a maximum limit of spacing 

 of three-fourths the depth of the beam. The spacing at the 

 supports is often one-fifth or one-sixth of the depth of the 

 beam. Great care is required in the design of stirrups of 

 T beams to insure that the slab and the beam proper are 

 securely tied together. 



An approximate formula for ordinary rectangular beams 



Vs 

 with vertical stirrups is P = , where P = stress in one stirrup, 



id 



s = spacing of stirrups, V = pro portion ot shear supposed to 

 be carried by reinforcement, usually J, and / and d have the 

 values already given. 



