Tilt-panel reinforcing over an opening 2

[UcfSE]

The panel section over an opening in a tilt wall is typically just like a deep beam, except that it is deep for architectural reasons rather than structural. We don't normally have to have a beam 5 feet deep for a span of, say, 10 feet for typical load-bearing panels (15-20 feet of tributary width, 50-60 psf dead load and 100 psf live load). Given that, are we required to follow the deep beam requirements in the ACI 318-02 code, namely 10.6.7, 10.7, 11.8 and appendix A? The strength-based requirements are easy enough given relatively light loads for the depth we usually see, but the minimum steel requirements seem above and beyond over conservative for such cases.

Do we need to provide all of that steel even though the beam depth isn't required for strength? Can I base my steel on what would beam size would be required?

 

[JAE]

The last sentence in 10.6.7 suggests that the skin reinf. doesn't have to exceed 1/2 the required flexural reinforcing, wich for a very deep beam may be quite small.

Going over 10.7, it does seem to dump you into App. A. I can't find any specific commentary that suggests there is an upper limit to beam depth where it becomes a "wall" vs. a deep beam...other than the commentary for 10.6.7 that states: "Where the provisions for deep beams, walls, or precast panels require more steel, those provisions (along with their spacing requirements) will govern."

That seems to imply even greater steel, not an upper limit. The behavior or very deep beams is more a shear problem than a flexural one. I would think then that Appendix A or a finite element model would be necessary. I'm sure other engineers have traditional and approximate methods used over the years. Intuitively I think you are right that loads of reinforcing doesn't make sense...but I'm also wondering if Appendix A or FEM wouldn't simply prove that true.

 

[UcfSE]

Designing by Appendix A, strut-and-tie method, I do need only a small amount of steel. With appendix A, I'm looking at two #5 bars, for one instance, versus half a dozen bars or more for a minimum flexural steel ratio of 0.0033. Am I still required to meet all of the minimums for horizontal and vertical steel ratios in chapters 10 and 11 and appendix A?

Appendix A has some minimums also in A3.3 if you are using bottle-shaped struts. Since I am not using bottle-shaped struts I don't have to meet A3.3 until you get to chapter 11.8 where you have a choice between meeting 11.8 or A3.3 (11.8.6). The problem isn't the flexural steel itself but rather the extra reinforcing for crack control and the limits are more on spacing than an actual area of steel required. When your maximum spacings are on the order of 6 to 8 inches for horizontal and vertical bars, then you're back to a lot of steel again, all for a beam that isn't required by analysis to be so deep.

I do not have access to any FEM software at work except my pencil, and that is not really the route I would like to take.

 

[WillisV]

UcfSE,

Per ACI-02 10.7.3 minimum flexural tension reinforcing of deap beams must conform to section 10.5. Per 10.7.4 minimum horizontal and vertical reinforcing in the side faces of deep beams shall satisfy either A3.3.3 or 11.8.4 and 11.8.5.

So for the main tensile reinforcing you must meet 10.5.1 which requires 0.0033 unless if per 10.5.3 you provide 1/3 more tensile reinforcing than required for analysis. So use 3#5's and your fine for this provision.

For the side reinforcing etc. per 10.7.4 just meet the wall minimums.

 

[WillisV]

Also as a follow up to JAE's comment, section 11.10.1 of ACI-02 in talking about the design of shearwalls says "alternatively it shall be perimitted to design walls with a height not exceeding two times the length of the wall for horizontal shear forces in accordance with Appendix A"...thus effectively providing a maximum deep beam depth of 2L before being considered a shearwall.

 

[kslee2006]

This is only a personal opinion, if a shallow section can carry the loads above the opening without shear concerns, then the entire (deeper) section needs not to be classified
as a beam (flexural) element, it can be reinforced as walls with additional reinforcing around the opening and the corners.

 

[UcfSE]

Thank you guys. Your information helped me to understand my situation better, and save quite a bit of steel.