In-Plane Tilt Wall Shear Reinforcing? 1

[AggieYank]

I have a question regarding minimum in-plane shear reinforcement in tilt-up concrete walls. My reference here is ACI 318-03.

>>My comments are denoted by (>>). For design of concrete walls, I refer to Chapter 14.

14.2.3: Walls Design for shear shall be in accordance with 11.10.

>>So off we go to 11.10

11.10.1: Design for shear forces perpendicular to face of wall shall be in accordance with provisions for slabs in 11.12. Design for horizontal in-plane shear forces in a wall shall be in accordance with 11.10.2 through 11.10.9. Alternatively, it shall be permitted to design walls with a height not exceeding two times the length of the wall for horizontal shear forces in accordance with Appendix A and 11.10.9.2 through 11.10.9.5.

>>Design for shear perpendicular to the face is no problem. It seems to me that for design of horizontal in-plane shear forces, we have two choices: 1) Design per 11.10.2 through 11.10.9 OR 2) Design per Appendix A and 11.10.9.2 through 11.10.9.5. Appendix A references strut-and-tie models, which I’m going to take a pass on. So, we design per 11.10.2 through 11.10.9.

11.10.2: Design of horizontal section for shear in plane of wall shall be based on Eq. (11-1) and (11-2), where shear strength Vc shall be in accordance with 11.10.5 or 11.10.6 and shear strength Vs shall be in accordance with 11.10.9.

>>Simple enough it seems. Eq. (11-1) is simply PHI Vn > Vu. Eq. (11-2) is Vn = Vc + Vs. Vc and Vs are given in later sections of 11.10, no problem.

>>11.10.3 limits the maximum nominal shear strength of the wall, ok.

>>11.10.4 says to use 0.8 times the length of the wall for the variable “d”, unless you have too much time on your hands, no problem.

>>11.10.5 gives a formula for Vc, which is 2 sqrt(f’c) b d for walls subject to axial compression, and a different formula for Vc when wall is in axial tension. No problem here.

>>11.10.6 gives an alternate, more detailed calculation of Vc. I’m not using this, no problem here.

>>11.10.7 says to not design for extra shear at the base of the wall, depending on the length of wall and wall height, no problem here.

11.10.8: When factored shear force Vu is less than PHI Vc / 2, reinforcement shall be provided in accordance with 11.10.9 or in accordance with Chapter 14. When Vu exceeds PHI Vc / 2, wall reinforcement for resisting shear shall be provided in accordance with 11.10.9.

>>So, when the factored shear is less than half the concrete shear strength, you can refer to 11.10.9 OR go back to Chapter 14. Simple enough. Going back to Chapter 14 makes sense, as there are minimums given there (which are less than the minimums given in 11.10.9). If the factored shear is higher than half the concrete shear strength, proceed on to 11.10.9. Ok, simple enough right?

>>11.10.9 and it’s subsections give equations for strength of horizontal shear reinforcing steel, minimum horizontal reinforcing steel (0.0025 x section), minimum vertical reinforcing steel (0.0025 x section), and spacing requirements for both horizontal and vertical reinforcing steel.

>> Now, here is my question. Back in 11.10.1, it says to design per 11.10.2 through 11.10.9, which means we have to conform to 11.10.9 as well. Why does section 11.10.8 say you have the choice to proceed to chapter 14 OR to 11.10.9, when 11.10.1 requires you to follow 11.10.9 as well? It seems like the solution to my question is that 11.10.1 should say “Design for horizontal in-plane shear forces in a wall shall be in accordance with 11.10.2 through 11.10.8. Alternatively……” Has this been addressed anywhere? Or am I misinterpreting something?

Thanks everybody.

 

[JAE]

I think you are interpretating the "11.10.2 through 11.10.9" comment a bit too tightly.

I would take it that 2 through 9 is the general applicable paragraphs that apply to shearwall design. Within 2 through 9, you still follow the instructions, so to speak, and if the flow of logic ALLOWS you to bypass 9, then you do, despite the supposed "mandate" earlier that you follow 2 through 9.

There are all sorts of sections within codes where you are told that "such and such" shall be designed per sections x through y...and then within x through y there are paragraphs that don't apply to particular design efforts.

Skipping a paragraph within x-y doesn't mean you've violated the code, you've simply follow the logic within.

 

[JAE]

..I might add an example - in your post you suggest that 11.10.6 would be skipped as its a more complicated formula for Vc. Does this also mean you've violated the "mandate" to design per "11.10.2 through 11.10.9"? I would say no. The code outlines the area that is applicable but you still follows its logic, allowances, exceptions, etc.

 

[AggieYank]

Thanks JAE. Your insight makes sense. I'm going to follow the logical process, which is to skip 11.10.9 when it's not required per 11.10.8, but it seems like ACI could have made it much clearer by denoting the right sections in 11.10.1.

 

[JAE]

Hey AggieYank - I agree - the codes can be quite convoluted at times.