# Concrete Torsion



## McEngr (Jan 2, 2012)

Happy New Year Structurals:

I have come across a torsion problem in the SERM that is confusing me. On page 1-26 of the SERM, the problem states, "Because the required value of At/s=0.00466 in^2/in per arm is less than 25bw/fyt=0.0050 in^2/in, the minimum permissible area of longitudinal reinforcement is given by ACI eq. 11-24 as..."

Even though Alan Williams solves equation 11-23 and then 11-24, shouldn't the At/s&lt;=25bw/fyt also be satisfied?

I think it all hinges on my understanding of what A/s is of page 1-25. Is A/s = (Av+2At)/s ?


----------



## Phalanx (Jan 5, 2012)

The value of At/s used in equation 11-24 is the greater of 25bw/fy and Tu/(2*.85*Aoh/fy) (Eq 11-21). 

Do not use (Av + 2At)/s because the AL requirements are independent of shear.


----------



## McEngr (Jan 6, 2012)

phalanx,

It sounds to me like you know how to perform the torsion design, but did you actually reference my question from the SERM? Thanks.


----------



## Amry69 (Jan 6, 2012)

He is not subtracting At*Ph*Fyt/Fysfrom [ACI11-24 SERM pg 1-24] which would reduce the steel because he needs more steel to staticfy 25bw/fyt

After all is said and don't you should recheck but he was pretty close so didn't bother.


----------



## ipswitch (Jan 6, 2012)

We use hyperbolic cosine to calc torsion in steel beams. Never really have seen it before in concrete. Interesting discusion.


----------



## McEngr (Jan 6, 2012)

Amry69 said:


> He is not subtracting At*Ph*Fyt/Fysfrom [ACI11-24 SERM pg 1-24] which would reduce the steel because he needs more steel to staticfy 25bw/fyt
> 
> After all is said and don't you should recheck but he was pretty close so didn't bother.


Thanks Amry. That's what I figured, but I think it's odd that Alan Williams didn't go the further step and actually show the calc and say something like, "negligible difference, therefore ok."

Appreciate the discussion. I'm hoping others will be studying and collaborating here as hard as I am. I'm stupid ambitious enough to try for both days and pass the first time.


----------



## Amry69 (Jan 6, 2012)

McEngr said:


> Amry69 said:
> 
> 
> > He is not subtracting At*Ph*Fyt/Fysfrom [ACI11-24 SERM pg 1-24] which would reduce the steel because he needs more steel to staticfy 25bw/fyt
> ...


Good luck. This test has kicked me in the balls. Failed Vert and Lat in April 2011 and Vert (by a c-hair) in Oct 2011. I'm going to slow down and take Lat in Oct 2012. I can tell by the posts that you're way smarter than me (an almost as anal). You'll do fine.


----------



## McEngr (Jan 6, 2012)

Wow Amry. That's a big compliment. If it's any consolation, I'm a terrible speller and probably misspelled at least 5 words in this post.  If you have any study questions, I'll collaborate with you as it also benefits me.


----------



## ipswitch (Jan 7, 2012)

I mean "discussion." I knew that thing was wrong....

Like I said before. If anyone needs help with bridges I'll be happy to chime in.


----------

