CMU Shear Wall Base Connection
CMU Shear Wall Base Connection
(OP)
I am designing a 45ft high 8" partially grouted cmu block wall (#4 @ 48"). I will have spandrels every 10ft to transfer wind load from CMU wall to steel columns.
I specified a max deflection of the columns at h/240 and a max deflection of the spandrels at l/600.
The steel columns are moment frames.
My question is am I better of tying the CMU block wall into the foundation wall / footer which will be poured concrete or only extending rebar up out of the poured concrete wall a couple inches into the 8" CMU wall.
Is a shear only connection best for the CMU to concrete foundation wall, I.e. let the connection at the base of the wall "hinge" and let the steel columns carry all the moments?
There are details online of a plastic hinge to allow the base of a cmu wall to hinge which I came across and was interested in.
I specified a max deflection of the columns at h/240 and a max deflection of the spandrels at l/600.
The steel columns are moment frames.
My question is am I better of tying the CMU block wall into the foundation wall / footer which will be poured concrete or only extending rebar up out of the poured concrete wall a couple inches into the 8" CMU wall.
Is a shear only connection best for the CMU to concrete foundation wall, I.e. let the connection at the base of the wall "hinge" and let the steel columns carry all the moments?
There are details online of a plastic hinge to allow the base of a cmu wall to hinge which I came across and was interested in.






RE: CMU Shear Wall Base Connection
RE: CMU Shear Wall Base Connection
RE: CMU Shear Wall Base Connection
Has anybody read TR149A?
RE: CMU Shear Wall Base Connection
Secondly, how do you develop shear across the CMU-Foundation interface if not with developed rebar? Using friction isn't a good idea.
RE: CMU Shear Wall Base Connection
http://www.ce.udel.edu/courses/CIEG407/Class_12/TE...
This PDF talks about allowing a plastic hinge to develop with two details shown. If you could, check it out. I'm trying to reconcile in my mind what the advantage / disadvantage would be. Allowing the wall to hinge seems to make sense in that no moment in the wall base would be developed. Then again, what is so bad about letting a moment develop?
But with respect to the 4" of rebar, the idea is that the rebar would handle the shear, not friction of the wall. With a spandrel 10ft up, the shear would be low at the low at the base.
Have you seen TR149?
Thanks for responding.
RE: CMU Shear Wall Base Connection
You seem to be concentrating your attention on the out of plane behavior of the wall, but your thread title says this is a shear wall. A shear wall is primarily for in plane forces, and as JAE pointed out, you don't want to depend on 4" long dowels for that.
The details you referenced build in a control joint...a straight crack...at the base. Never seen it done.
RE: CMU Shear Wall Base Connection
RE: CMU Shear Wall Base Connection
I read through the NCMA Tek that you linked to. I've never done that nor seen it done in any building. I think it is an attempt by the masonry folks to reconcile their product with the fact that pre-manufactured metal buildings are many times designed for a drift of L/100 (3" for a 25 ft. tall wall) and typically masonry "likes" L/360 to L/600.
I suppose in a non-seismic heavy area details like this might work. I have a hard time visualizing how a block wall can hinge across a 3/8" joint with mortar. Seems like the compression side would crush the mortar or something while the tension side would open up the sealed joint and tear the sealant.....maybe that is better than a crack through mortar.
Also - at the corners there would be really weird behavior with hinges oriented at 90 degrees to each other.
If an 8" wall rocks sideways at 3" in 300" of height (L/100) then the crack in the mortar on the outside face at the base would be - at most assuming a rigid wall - 0.08 inches wide...so I'm not sure what is gained by the hinge.
What hokie66 says above is my preference:
RE: CMU Shear Wall Base Connection
Having said that I anticipate a much lower deflection at the top, possibly h/300.
I don't see the advantage of a hinge other than to eliminate a moment at the base of the CMU cladding which I was worried might cause cracking.
RE: CMU Shear Wall Base Connection
Just to it make the thread murky again, a hinge doesn't mean that you shouldn't fully develope the rebar coming out of the concrete foundation. Fig. 3 is for a shear wall and it shows a lap splice above the "hinge". The joint will crack and the steel will stretch.
Fig. 2 is for a non-shear wall and it shows 2 inches of embedment of the foundation rebar above the "hinge". I am not sure that I could use the detail in Fig. 2.
If the drift is limited to h/300 based on 70% of the full design wind load, then I don't think a hinge would be needed. This number comes from IBC Table 1604.3. The commentary mentions 0.7 as a conversion factor from a 50-year event to a 10-year event. You may want to be a little more conservative than these limits.