ENERCALC V6 - slender masonry walls - maximum reinforcement ratio

Ever wonder how much steel you can stuff in slender masonry walls these days. Did you think that ENERCALC V6 would perform an internal check? Think again. See my question and ENERCALC's responce (for your enjoyment).


MY QUESTION:

Dear Technical Support:
video and users guide is silent relating to the max % of rho balanced entry under the material properties of the general tab.
Please see attachment (ACI-530-08)Commentary section 3.3.3.5. How is the program determining rho balanced? I assume that the equations in the attached commentary section are used with alpha = 1 (1.0*yield strain). I also assume that P/db in the numerator of the equations is calculated according to the service level load combination P=D+.75L+.525Qe. Please comment on the value assumed for the third term (ie .525Qe). Thanks


Benjamin J. McCann P.E.
bmccann@ctengineering.com

CT ENGINEERING Inc.
Structural Engineers
▪ 180 Nickerson Street ▪ Suite 302 ▪ ▪ Seattle, Washington 98109 ▪ ▪ Phone: 206-285-4512 ext: 312 ▪ Fax: 206-285-0618 ▪


ENERCALC REPLY:

Good Morning Ben,

Sorry for the delay in getting back to you on this one, but it is an excellent question and one that has required some research on my part and some conversations with the developers.

The answer is that the Masonry Slender Wall module is currently using reinforced concrete concepts (and 1.0 x steel yield strain) to determine rho balanced. It then respects the user-specified "max % of rho balanced" as the upper limit on reinforcing. This is workable, provided that the user observes the calculated value of rho balanced and enters a meaningful factor as the "max % of rho balanced" to arrive at the desired upper limit.

I'm glad you asked the question, though, because it has given me reason to propose to the developers that we should implement the provisions in ACI 530, which are more specific to concrete masonry. So I would expect that we will soon be seeing the reference to rho balanced disappear from masonry modules, as that term is not used in ACI 530. I would also expect that we will soon be determining the upper limit on reinforcing in masonry sections by using 1.5 x steel yield strain, along with usable masonry strains as defined by ACI 530 rather than as defined for concrete sections in ACI 318.

At present, none of the masonry modules are incorporating the axial load from the specified load combination into the determination of rho max.

· The Masonry Lintel module does not perform a check on the reinforcing limits. I’m going to provide the developers with some recommendations on this. But even if a reinforcing limit check gets implemented in this module, it will not consider the axial load from the specified load combination, because the beam modules in the Structural Engineering Library do not incorporate axial load in any way.
· The Masonry Column module currently bases its maximum reinforcing limit check on 0.04An. ACI 530 specifies that value as an absolute upper limit on the area of reinforcing in a column, but it also says that designs should still respect the value of the upper limit as determined in section 3.3.3.5. I’m going to provide the developers with some recommendations on this, including the incorporation of the axial load effects from the specified load combination.
· As mentioned above, the Masonry Slender Wall module is basing its maximum reinforcing limit check on the user-specified "max % of rho balanced" multiplied by the calculated value of rho balanced. I’m going to provide the developers with some recommendations on this, including the incorporation of the axial load effects from the specified load combination.

Finally, you asked for comment on the value assumed for the 0.525Qe term in the load combination specified for determining axial load on masonry elements. The factor of 0.525 occurs in Basic Combination #6 in section 2.4.1 of ASCE 7-05, which is (Equation 16-13) of IBC 06 and IBC 09. However, the combination specified for determining axial load on masonry elements differs slightly from the equation in ASCE 7 and IBC, because it omits the roof live load or snow term. So I will need to provide the developers with some guidance on how to handle this requirement, and at the present time, I think I am going to point them to the expanded form of this equation as presented in section 12.4.2.3 of ASCE 7-05. I think I will eliminate the roof live load or snow load term, and ask that they calculate a value of axial load based on all of the remaining terms, including the vertical component of the seismic load.

Thanks for raising great question, and keep your eyes peeled for some nice enhancements in the masonry modules as a result.

Cheers,
Chris

Chris Conrad, P.E.
Director of Development
ENERCALC, Inc.
cconrad@enercalc.com
Technical Services: 949-645-0441
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