Interior non-load bearing CMU wall 2

[dcredskins]

I have an interior non load bearing 8" CMU wall between concrete floors (11'-0" floor to floor height). It is connected at base with dowels, however, not connected to the underside of the concrete slab at the top. I would like to connect at the top as well to make sure wall is laterally braced. However, the GC is asking me why do I need to brace it as there is no lateral load except 5 psf internal pressure. How do you deal with this situation? I would appreciate your suggestions. Are there any code recommendations to cover such conditions?

 

[civilperson]

Seismic load may apply to the CMU weight. I like the idea of a pair of angles confining the top edges of wall to the overhead slab.

 

[dcredskins]

Civilperson, thanks for the quick response.

Seismic load is not an issue in this case. Wind load governs.

 

[jike]

I would brace it at the top unless the wall can span horizontally. Do you need to allow the slab to deflect? If so, you can allow a gap between the top of the wall and the slab. Provide angles fastened to the deck and tight to each side of the wall. An alternate is to provide one angle fastened to the deck and bolted to the wall in vertical slotted holes.

If you are in an earthquake zone, your lateral forces on the wall have to be evaluated also. It may exceed the 5 psf.

 

[civilperson]

Wind load DOES NOT GOVERN if there is no wind in the interior. The main framing may have wind loading as greater than seismic, but once again seismic loading still exists on EVERY component, even those with light or no wind.

 

[JLNJ]

A pair of angles works nicely. If the wall has a lot of corners then restraint at the top might not be needed.

One blanket statement about restraints in partition walls can add a lot of $$ to a job. Make sure you spec them only where you need them.

 

[dougantholz]

It is fairly common practice to pinch the top of the wall with angles at about 6 ft on center along the top of the wall. Connect the angles to floor but not the wall. leave enough room for the floor to deflect and be done with it.
Civilperson - you are wrong. There are areas of this country (and outside of that) that have very little to no ground acceleration. The Florida Building Code does not even have seismic design in it. There is however in every code an internal wind/live load of 5 psf to handle differential pressures that can exist inside the building.

 

[dcredskins]

My detail shows (connection of top of the wall and underside of the slab) angle brace. However, the contractor forgot to put it, and already finished the ceiling and walls. To put those angle braces per my detail, he has to tear apart his finishing and redo it. And obviously, he does not want to do it. And he is asking me why he needs angle braces. Apparently, he is asking how much lateral forces we are talking about. I hope you guys understand where I am coming from.

 

[JKStruct]

I think civilperson is making the point that some level of seismicity exists everywhere, regardless of special building code provisions that may allow exceptions in certain areas. It is fairly common among some engineers to overlook seismic forces on some components because wind may govern the base shear or the design of the LLRS.

5 psf is in most codes I've seen, but the seismic component force may be higher depending on where you are in the country. I don't know much about the florida building code, but if there are no seismic provisions, that does not mean that there is NOT seismicity, it just means that the powers-that-be feel comfortable codifying 5psf as the controlling force in this situation.

Back to the original question, I usually sandwich the top of the walls with angles every so often, connected in such a way to allow for deflection -- i guess i agree with the others. You can also span the wall horizontally between cross walls, columns, pilasters, etc.

It does potentially add significant cost, but that's part of the choice in partition wall construction, which is often determined by architectural considerations (sound attenuation, durability, etc.) Assuming a bracing detail was on the contract documents, your contractor should have included it in his price. You shouldn't have to justify your engineering to him. But say that in a nice way.

 

[JKStruct]

oh, that's a predicament. It's a long shot, but if you do have dowels into the foundation, can the masonry wall cantilever with the 5 psf? Obviously, your base connection now has to transfer moment that wasn't there in the original design, and you have to look at the footing. It might not work, but it's worth investigating if you have a good relationship with the contractor. Or check if the wall can span horizontally.

 

[cvg]

if the CMU is un-reinforced, than the angle at the top may not prevent a failure during a seismic event anyway. Un-reinforced masonry walls don't perform very well in earthquakes.

 

[hokie66]

I think JKStruct's post summed it up.

Just another, perhaps obscure, warning about the top of non-bearing walls. If the top is required to be sealed, and if the lower floor is likely to deflect more than the upper, be sure that a low modulus sealant is used. Had an instance in an underground carpark where the podium transfer slab deflected little, the carparking floor below deflected as expected, and the top blocks hung from the podium slab because the sealant was so strong and inflexible.

 

[apsix]

Does the ceiling lining provide enough restraint to avoid the need for the restraint angles?

 

[dcredskins]

apsix, I do not think we can consider ceiling lining as bracing system. It should be angle braces, to my knowledge.

 

[apsix]

Normally no, but your case is not typical, unless you want to play hard-ball with the contractor.
Gyp board can be use for structure bracing (with limitations) so I wouldn't rule it out yet.
Of course the ceiling construction details may mean it is not suitable.

 

[COEngineeer]

well, I always check both wind and earthquake. Does not matter whether it is interior or not. If you are counting on your shear diaphragm to be taken by that wall, then you need to connect it. Check your shear diaphragm (seismic or wind) and see how much that wall is taking. Is it flexible diaphragm?

Never, but never question engineer's judgement

 

[Lion06]

There is an exception of designing architectural components for seismic forces in ASCE 7, but you still have to support the interior 5psf load. If you do provide angles, I would slot them in the direction of the wall length to prevent the wall from wanting to act like a shearwall (unless you want it to, but it didn't sound like it).

 

[tedamiao]

In my opinion, you have to ask contractor to open ceiling to add angles. You may use shot pins for angle to deck attachment.