Concrete Shear Walls - Flexural Capacity

[preynolds]

I am looking at three different resources on the design of concrete shear walls. The shear portion isn't an issue as it is fairly cut and dry. All three resources figure the flexural capacity in different ways. The NCEES SEII example 260 seems almost too simple. The IBC Structural/Seismic Design Manual, Vol 3 solves for the flexural capacity by summing forces and using bars in compression or tension depending on the location relative to an assumed compression block. The Macgregor text and the PCA design notes use a plug and chug formula.

What do people generally use to figure the capacity in flexure?

thanks
pat

 

[OCI]

The second method you mentioned is the one I would use. It's best done with a spreadsheet because you need to do a strain compatibility analysis that includes the effect of axial loads as well. It can be done by hand but pretty cumbersome because you have to use trial and error to arrive at a solution for each case of axial and flexural loading.

 

[abusementpark]

I know of the one from the MacGregor text, which is taken from the work done by A.E. Cardenas and his colleagues in the 1970s when they were developing the ACI shear wall provisions. It is limited to a uniformly distributed wall.

Their equations are based on the following assumptions: all steel in the tension zone yields in tension, all steel in the compression zone yields in compression, the tension force acts at the middepth of the tension zone, the total compression force (sum of steel and concrete compression contributions) acts at middepth of the compression zone. I believe that testing has shown that it satisfactorily will predict the moment strength.

So, I'm not sure why it would not be a valid approach. I am curious how the results would compare with the other methods you have mentioned.

 

[ishvaaag]

Attached go a number of Mathcad worksheets that deal with sectional analysis in compatibility of deformations for rectangular sections of reinforced concrete, for cases

status of the section
design of sections
capacity of sections

 

[JoshPlumSE]

I don't do concrete shear walls. But, is it so difficult to go back to first principals?

Assume the crushing strain is 0.0033.... Assume a depth to neutral axis. Then you know the stress and strain in every bar.

Use assumption #1 to solve for depth of compression block needed to balance statics. This gives you a revised depth of neutral axis. Re-solve for te stress and strain in every bar.

Since I've never really do concrete shear walls I may be missing something. But, it doesn't seem all that difficult. At least not conceptually....

 

[OCI]

Josh,

That's the Idea. You have to assume a neutral axis depth. Then the sum of forces is your axial load and sum of forces about the centroid is your moment. It's just that each assumption is only one point on the interaction envelope and you might need a few to check the adequacy of the section.