Can 4" CMU be designed as a structural interior partition wall?

[StructuralJoe]

I have an Architect and G.C. insisting that 4" CMU can be used as a partition wall (and resist the code minimum 10psf interior design pressures).

I personally have seen another engineer cave-in and put this on their drawings but IMHO it is garbage and non-structural.

Has anyone attempt (and suceeded) designing a 4" CMU wall 8'-8" in height to resist a nominal pressure of 10psf?

Can anyone provide me with sample calculations?

 

[jike]

I think code interior pressure is 5 psf or seismic which is greater.

 

[StructuralJoe]

Jike,

You are correct, 5psf...

What are your thoughts on this?

 

[KBVT]

I am not sure you can even get reinforcing within the cavity of 4" CMU block.

 

[StructuralJoe]

A #3 rebar will fit, the hole is a minimum 1" wide (as much as 1-5/8") the problem I have seen in the past is the mortar interferes with grouting, so it must be grouted every 3rd course.

In addition I believe code does not permit it for exterior and bearing use but doesn't comment on partition wall use.

 

[jike]

4" cmu might be typically used for small closets or small chases where it is braced by other CMU walls at close intervals. I would be inclined to go to 6" CMU for other applications.

The Architect might be thinking of the old empirical method where you could go up to 24 times the thickness = 8 feet max. height. The wall needs to be braced top & bottom, of course. I believe the IBC now allows 36 times = 12 feet which seems way too much to me.

 

[structuresguy]

We use 4" CMU fairly regularly for small partition and chase walls. I do advise that you use #3 bars, even though #4 bars technically fit in the cell. THere is very little tolerance on bar placement due to the narrow opening.

I have been hearing recently from masonry subs that 6" CMU is not available (here in central florida), except by special order. So we do not spec 6" CMU much anymore.

 

[concretemasonry]

Rethink the use of 4" masonry for a wall since a 4" unit requires more labor than a 6" or 8"unit.

The 4" typically does not have a great enough MINIMUM core space for rebar and grout even though the average indicates that. There is no minimum ASTM requirement for the core dimensions and only minimums for the face shells and webs. The cores of the CMU are tapered and usually very small at one end. Material is cheap so they always contain more material to reduce the manufacturing and handling cost. It is possible to make a minimum unit, but it is just too expensive.

A 4" is costly to lay because the height/thickness ratio (tippy) requires more labor even if is does not contain reinforcement. Some contractors will lay a 12" block at the same labor cost as a 4" block.

Dick

 

[StructuralJoe]

Structuresguy,

Have you ever run calc's on it?

I have 2 concerns:

#1. I believe I have read that code doesn't permit grouting 4" CMU, is this true, does anyone know of the code location?

#2. Does the concrete really flow into the cell? Would you recommend grouting at 2' intervals?

 

[StructuralJoe]

Dick,

I agree fully, my father and grandfather are masons.

Typically they can only lay 16" to 24" height at a time.

The problem is the G.C. is the lead in this design build venture and they are insisting this is the way to go...

I truely believe it will be V.E. out at a later date.

My major problem is that more than one strutural engineer / architect has specified the use of this system for the dog shelter I am designing.

I believe I can hand calc the design to provide enough reinforcing but in reality will the force properly tranfer/develop into the rebar?

 

[concretemasonry]

It can work if you can get enough grout (not mortar) into the cores to bond the rebar to the masonry units. Especially if you are going more than one course.

That is one of the reasons you should not put anything into a "structural" wall that is not really necessary or you cannot see or verify. - This one of the pit-falls trying to engineer everything. For centuries, 3" or 4" hollow clay tile have been used for interior walls (2.5 - 2.7 meters high)in construction. A few days, after the walls are laid, an electrician attacks the walls with a hammer and routes out a chase near an opening to install the wiring. After that, the wall is plaster and is functional for 100+ years. This is not an isolated situation, but a proven performance and an accepted standard of practice.

 

[StructuralJoe]

Concretemasonry,

I agree, to an extent...

I have run some basic calculations on it:

5psf LL lateral pressure by code
8'-8" CMU Wall Height

Fb = 25 psi for hollow un-grouted CMU using M or S portland cement

M = wl^2/8 = 46.9 LB*FT/FT = 563.3 LB*IN/FT

S = 21 IN^3 hollow -or- 26 IN^3 solid grouted

fb = M/S = 563.3/21 = 26.8 psi

fb > Fb ==>> does not work ungrouted at 8'-8" but it would work for an 8'-0" tall wall (by code)

It will work "solid grouted" full height un-reinforced at 8'-8" ==>> fb = 21.7psi << 25.0 psi = Fb

As strange as it may sound the numbers work...

I still do recall something in code about not being allowed to grout CMU thinner than 6"...

But if un-reinforced works, reinforcing it will only add to it's capacity...

The G.C. stated that their pre-liminary bidding was for 4" fully-grouted CMU partitions with #3 at 48" o/c...

I offered 6" ungrouted (unreinforced) CMU which also works easily to code... but they didn't bite so 4" it is...

I still don't "LIKE IT" but code is code and if it works... and no one knows of any code reference not allowing it (and I don't know of any) then that's what it is...

Thanks to everyone for your posts...

 

[concretemasonry]

That is the problem when the GC has his mind made up and you try to engineer something that should not be engineered.

The 6" wall is an obvious(and cheaper) solution but it may incur some changes in archaic or provincial architectural details.

The code is only the worst way someone can legally build until someone is willing to accept the responsibility.

In a foreign country where they do not make anything larger than a 6" block, they use our standards (basically ACI 530 and earlier versions going back to the ancestral standards) on 20 story loadbearing buildings because they just "use the codes them better" according to the engineers there.

Have you considered any 3 dimensional analysis, partial fixity, etc., but that may be out of the question for such a minor wall.

Dick