Concrete Shear (ACI)- Historical (1950) 2

[shepherd]

In looking at many older concrete structures, it seems to be very common to see stirrup spacing on the order of the member depth. I've generally ignored any contribution of these stirrups since they would not intercept the assumed 45 degree failure plane. A couple of questions to anyone who has evaluated similar structures, or is familiar with the ACI code from that era:

1) Was there a different design approach for shear in/around 1950 that allowed these to be included in shear strength calculations?

2) Was there a requirement similar to today's code that required minimum stirrups at d/2 spacing when actual shear exceeds 1/2 of the member shear strength?

Thanks

 

[larsacious]

Concrete structures during the mid-1900's were designed unconservatively for shear due to a code issue which allowed the designer to use the full phi*Vc for members with no shear reinforcing. Today those same members without shear reinforcing would only have a value of 1/2*phi*Vc.

This is the major challenge with renovation projects from this era since it is more difficult to reinforce member for shear than flexure.

 

[shepherd]

larsacious,

I recall seeing an article that had mentioned the unconservative shear values, but wasn't able to put my finger on it.

Thanks,

 

[BAretired]

"Reinforced Concrete Design" by Sutherland and Reese, Copyright 1943 - Example 7-20 is a design of a beam with f'c = 3000 psi. A maximum shear stress of 0.06f'c = 180 psi was permitted with web reinforcement. The concrete was considered adequate for a shear stress of 0.02f'c or 60 psi and shear reinforcement carried the excess. Wherever shear reinforcement was required, a limiting spacing of d/2 was used.

BA

 

[shepherd]

BA,
Was there a minimum steel area stipulated in order to use the increased allowable stress?

In looking through "Structural Renovation of Buildings" by Newman, he mentions the 1947 code allowed a stress .02f'c and allowed a stirrup spacing of d. He goes on to reference a Concrete International Article (Elstner "Concrete Beam Shear Design" Sept 1992 p.70) that states beams with stirrups spaced at greater than d/2 are quite safe; however, there is no mention of how to quantify. I'll try to get my hands on that article.

 

[BAretired]

shepherd,

I could find no mention of a minimum steel area in Sutherland and Reese. If I recall correctly, the method outlined in that book was precisely the same procedure as I used when I began structural design in 1955.

BA

 

[msquared48]

1955?!?!

Oops! Guess I better keep my mouth shut...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

Mike, I forgot to tell you that I graduated when I was only ten.

BA