CMU Shear Wall Base Connection

[Ryanemma]

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.

 

[hokie66]

Introducing a "plastic hinge" into a CMU wall is an indication that someone has too much time on their hands. There is too much preoccupation with having building elements perform in accordance with the way they are analyzed. Just assume it is pinned, but build it in accordance with standard practice.

 

[JAE]

I agree with hokie66. And I think shear walls should have a direct connection to their supporting elements (foundations, floors, etc.

 

[Ryanemma]

@ JAE What would be a direct connection? 4" of rebar sticking out of the foundation wall into the first course bond beam. Anything longer would induce a moment at the bottom which may or may not be a bad thing. I just don't want cracking in the wall.
Has anybody read TR149A?

 

[JAE]

I don't consider 4" of rebar developed into CMU grout as a direct connection.

Secondly, how do you develop shear across the CMU-Foundation interface if not with developed rebar? Using friction isn't a good idea.

 

[Ryanemma]

@ JAE

http://www.ce.udel.edu/courses/CIEG407/Class_12/TEK 05-05A.pdf

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.

 

[hokie66]

Ryanemma,
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.

 

[Ryanemma]

The shear I referred to was out of plane. Shear from wind force pushing on the wall perpendicular to wall. That would be the shear the dowels would have to resist.

 

[JAE]

OK - so this is simply CMU cladding and not a shear wall.

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:

Just assume it is pinned, but build it in accordance with standard practice.

 

[Ryanemma]

I've specified to the steel manufacturer that I want an h/240 max deflection in the columns. This yields a 2.18" deflection at the top of a 45' wall. Upland on stiffening the columns with fully grouted reinforced pilasters.

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.

 

[SteveGregory]

There is an updated version [NMCA Tek 5-5B (2011)]. It states, "When the masonry is designed with a base hinge, it is important to properly detail the building corners to accommodate the movements. A vertical isolation joint should be placed near the building corner and proper consideration should be given to the masonry and steel connections at corner columns. Flexible anchors and/or slotted connections should be used." This may be a good idea with any moment-frame structure.

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.