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RE: concrete shear walls

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Title: concrete shear walls
See below.  These references assume seismic design, if its wind some code sections may be incorrect.
-----Original Message-----
From: brian stanley [mailto:bstanley(--nospam--at)morabitoconsultants.com]
Sent: Friday, June 06, 2003 6:02 AM
To: seaint(--nospam--at)seaint.org
Subject: concrete shear walls

I am in the process of designing a mid-height concrete building resisted by concrete shear walls.  The shear walls are concentrated around the elevator and stairs and unfortunately there is plenty of torsion.  Due to the door openings, The shear walls are in several shapes C's, T's and Z's.  
 Is there any kind of limiting factor for the flange returns?  Yes, ACI 318 chapter 21 covers this.  ACI limits flange dimensions to 25% of the wall height or 1/2 the distance between adjacent walls (section 21.7.5.2 in ACI 318-02)  When I have done these complicated shapes in the past, FEA is usually warranted.  You will be amazed at the shear stresses at the joints.  This can get complicated when considering the flange dimensions in different directions.  I asked Gosh this question in an ACI seminar once, he said to trust the FEA approach.  The 25% rule is for people that don't want to use the software.  It can be done by hand if you want.  Do a shearflow calc for a quick answer.  If the core is tall enough, you can treat it like a beam/column section to determine internal stresses.   

Many of the shear wall look similar to the ones shown in PCI Design Handbook, page 3-37 Table 3.7.3.  How exactly do you go about designing an abnormally shaped shear wall with axial, shear and moment?  PCI doesn't address the topic.  Can you just enter the shape into PCA Column program with the axial force and moment (check shear by hand) and analyze it with different reinforcing schemes?  
If you have software that will account for every bar and its distance from the neutral axis, that is a more economical approach.  I have written spreadsheets that do this.  It is much more economical than placing all the steel at the end of the wall.  I haven't use PCA column for a while, so I don't know the limits of the software.  You will still need to check strains in the concrete per ACI section 21.7.6.2.

 It would seem that putting large quantities of reinforcing at the ends would help best will the flexural design, but is it common practice to designing using distributed layers of bars?  Aren't there stress concentrations at the intersections of the C's, T's and Z's?  How do you check this?
One last thought, if you use steel to transfer forces through corners, remember you have to develop the bar on both sides of the joint.  Most people miss this and only develop it into the long section. 

Brian Stanley, P.E.
Baltimore, Md

Hope this helps,

Jake Watson, P.E.

Salt Lake City, UT