Tilt-Up Wall Nominal Moment Capacity vs Cracked Moment Capacity

[Stenbrook]

In ACI 318-11 Section 14.8.2.4, it gives a requirement that the nominal moment capacity of a concrete wall shall be greater than the cracking moment of that wall. My question is that if all of my factored loads (Pu, Mu, Vu) are all lower than the allowable nominal values (Pn, Mn, Vn) and my combined axial and flexural calculations are sufficient, and the only thing that doesn't check out in my calculations is the Mc<Mn, is it unreasonable to say that the panel is sufficient? I'm trying to understand the theory behind the Mc<Mn equation and why it requires me to put more reinforcement into my panel when I am at a Mu/Mn ratio of less than 0.15 for all cases. Worst case scenario is that my nominal moment is 15% larger than my cracking moment. Can someone explain to me the importance of this check??

Thanks in advance!

 

[KootK]

If the flexural strength of the panel is less than the cracking moment, the panel could fail suddenly without warning as soon as the cracking moment is exceeded. We consider this a problem with gravity loaded members and would prefer gradual member failure/sagging that would hopefully alert occupants of an impending failure.

I consider this less important, and might be willing to waive the requirement, for a wall panel where the bending loads are mainly the result of transient environmental effects (Wind/EQ). Another option, for complete code compliance, might be to treat the panel as plain concrete.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[spats]

Also consider that it's not unusual for the bending moments during panel lifting to exceed those for the in-place loads. You definitely don't want a sudden failure while lifting the panel.

 

[Stenbrook]

Well for my case, the panel has already been installed but they did not put the openings in the panel correctly so I'm not worried about lifting loads.

 

[KootK]

In an existing application, I'd be even more willing to let it slide. Bending forces result mostly from wind and earthquake rather than eccentrically applied gravity load, right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Stenbrook]

I would say so. Considering our panel is only exposed to the wind above the 4th floor there really isn't a lot of wind on the panel to begin with. The location is very low seismic and is not even required by local jurisdiction to be checked. I believe you have answered my question and confirmed my thoughts that there is some wiggle room with this check.