Longitudinal Torsion Steel Woes

[KootK]

I'm developing a really great, all encompassing, concrete beam design spreadsheet. A mathcad sheet to be specific. At least it was gonna be all encompassing until I got to the issue of longitudinal torsion steel.

Since I am using a spreadsheet, it will take the form of an analysis rather than a design. Since ACI treats long. torsion steel as additive to flexural steel, I need to determine how much of the steel is "dedicated" to torsion before the program determines the flexural capacity.

Here's where I struggle:

1. I get the amount needed for torsion from ACI 11.6.3.7. and 11.6.2.2. for compatibility torsion. However, I don't know where to take the steel from. If I have more than one row of tension steel for example, do I take out the required torsion steel from the the top row / bottom row / or perhaps the centroid? I've been taking from the bottom just because that seems most conservative. But it this necessary? Since the torsion steel is just resisting axial load, there's no reason for it's placement at the extreme locations is there?

2. ACI says the steel must be "evenly distributed". I would read that as meaning evenly distributed about the perimeter of the beam. In two references I have, however, they divide the Area of steel evenly into the number of rows of steel provided (ie. 3 for a beam with one row of mid-height bars).

3. Although I subtract out the torsion steel for determining flexural strength, do I need to keep it in for other checks:

(a) Deflection - I kept the torsion steel in because I thought that it would still contribute to flexural stiffness, especially under service loads. I also kept it in for the calc. of p' for the same reason.

(b) Max Steel - I kept the torsion steel in because I'm paranoid about non-ductile failure.

(c) Min Steel - I kept it in because I figured that the torsion steel wouldn't even be utilized until after the section cracks anyhow.

I think that all of these issues must have been dealt with in the development of the CRSI beam tables but they don't say how in the commentary sections. Can anyone help me with this?

Alternately, I would prefer to simply add the required Axial force to my analysis rather than compute the required torsion steel area. It seems simpler and more rational to do it this way (it's also what is prescribed for prestressed concrete in 11.6.3.10). Can I do this?

The only drawback that I can think of is that the additional tension force could actually increase the calcualted moment capacity of the section if the neutral axis falls above the centroid of the gross section. But, if that is the case, then why is it OK for prestressed concrete??

 

[JAE]

AdamP
We've always simply added the required longitudinal torsion steel to what was separately determined for the flexure.

We don't ever add axial force to the section as the longitudinal force resisted is not really an axial tension across the whole section, but a tensile stress developed from the torsional shear.

We've also developed a "RISA" spreadsheet for beams coming out of a frame analysis that calculates the flexural, shear, and torsion reinforcing required. We rarely go back to our analysis and re-analyze the deflections due to the generally small amount of torsional rebar as a percent of the flexural rebar. Your point about maximum steel checks including the long. steel is probably valid, but consider this - most of your beams, in meeting 7.13 will have a doubly reinforced condition anyway, so the small longitudinal torsion steel shouldn't usually affect the 75%Ab value all that much.

Also, we rarely have conditions where torsion is actually considered in a concrete frame due to re-distribution of forces (see ACI section 11.6.2.2) In perimeter beams, we usually close the stirrups and (in older days) ignored the torsion.

Hope the above ramblings are of some small help.