Behavior of wall stud and reinforced screed/mortar

[Pixy]

Screed or mortal mix made up of cement and sand are very hard to chip off from floor especially if there is a wire mesh underneath the screed/mortar.

Which begs the questions. What would happen if say 4" of mortar (made of cement and sand like used in screed without any gravel) with wire mesh at the middle layer were welded to non bearing wall made of thick stubs that are anchored well below and above (see illustration above).

I just want to know what is the behavior of this thick mortar wall. The wire mesh or even very small dowels can have dowel action effect. Is it not? Without any gravel. How big can pieces of mortar just drop off between the wire mesh or small rebar mesh if they would be distance let's say 6 inches apart?

Or in other words, how many inches can dowel action work on mortar mix only? (I'm aware aggregate interlock in normal reinforced concrete are strong because of the aggregate interlock) but it is non-bearing wall only with fully anchored vertical stub support distant 23 inches apart (between thick vertical C-purlin stubs 2x4")).

Thank you.

 

[jayrod12]

If you're trying to account for composite action between your studs and the cementitious cladding, I wouldn't do that. What about when the bending is in the opposite direction? You still need your studs to calc out for that and the cladding will have no effect in that instance.

 

[Pixy]

I wasn't describing cementitious cladding (or outside cover of the studs) but 4" mortar/screed with wire mesh welded right inside the 2x4" C-purlin studs.

What you mean about bending in the opposite direction? The studs bend? So if it's mortar inside the stubs, still no effect during bending of the stubs? Kindly elaborate.

 

[phamENG]

What kind of behavior are you considering? Diaphragm/shear wall? Flexural? Between studs probably wouldn't do that much for you. The NDS and, I believe, the AISI documents for light gauge studs have shear wall capacities for plaster on lathe wall assemblies - which is essentially what you're describing but with a different cementious material.

I think you'll end up with significant ductility issues. Small drifts will cause this assembly to crumble and cyclical loading would result in rapidly increasing deflections and failure.

 

[jayrod12]

Maybe I'm not understanding what you are trying to do. You show a metal stud wall, and then you show a floor slab and an arrow connecting them. I was under the impression you were thinking you could install some form of dowels from the wall studs to the mortar/screed. I also assumed the mortar/screed was being installed on the face of the wall in the final configuration and you were asking if that improves the wall stud capacities.

You further talk about dowel action. What are you trying to build? How do the dowels relate to the wall studs? Are you filling the cavity between the wall studs with mortar/screed? Why?

Remember, none of us have the information you do regarding the project in front of us. We don't know what you're trying to build, or why you're trying to do what you are. Explain the end goal, explain why you are trying to do what you are. Include some hand-sketches or something of sections so we can understand what you want.

 

[Pixy]

Yes. I want to fill the cavity between the wall studs with mortar/screed. This is to make it lighter (no gravel). I won't use any cladding (only formwork).

Now if you put wire mesh inside the mortar/screed inside the studs. The mortar with no gravel won't just fall out. The dowel action question is regarding the wire mesh inside the mortar that can make it very strong like floor screed with wire mesh.

 

[jayrod12]

Strong to resist what? Why do you feel that filling the stud cavity with mortar is going to benefit your wall performance? What load are you trying to resist.

 

[Pixy]

Concrete composed of cement, sand and gravels are heavy.

Mortar composed of cement and sand are light. This is only used in floors, but do you know how hard it is to remove old screen with wire mesh? You can't use hammer or pull up the wire mesh. The mortar is so strong it takes a chipping hammer to break them.

So If you fill up the wall studs (thick studs) with mortar and the wire mesh welded to the studs inside. The mortar inside won't just fall down because of the wire mesh (like dowel action). This can create a lightweight wall with much better fire protection than the gypsum x cladding.

 

[phamENG]

So you don't care about the wall's ability to resist any sort of load, just the fire rating?

It won't work. You have incredibly efficient thermal bridges spaced at 24" o/c or less. That's why stud wall fire wall systems have something outside of the studs - you want a continuous barrier with little to no thermal break in it.

And "light" is relative. A solid cement/sand wall is still going to weigh a whole lot more than a hollow wall with a couple layers of type X gyp on it.

 

[jayrod12]

Agreed with Pham. I don't feel that is the most optimum way to provide the fire protection. There's a reason it isn't a common construction technique.

Even with only mortar (which honestly isn't substantially lighter than standard concrete) your wall will still weigh close to 2-2.5 kN/m^2. A steel stud wall with double type x on each side only weighs 0.7 kN/m^2 conservatively, closer to 0.5. That's substantially lighter than the screeded wall.

 

[Pixy]

The stub wall will be well anchored at the top and bottom. It is non-load bearing. The problem with gypsum x board is it takes only 30 minutes to burn it. While the 4" mortar with wire mesh will take 1.5 hours to burn through. 3 times better fire barrier if the 4" mortar is used as sandwich.

So with the wire meshed inside and welded to the center of the stubs. The mortar won't break out even if there is no gravel, no?

 

[jayrod12]

What level of protection is required? You can get 1 hour out of 16mm type X gypsum, 2 hours out of 2 layers.

The steel studs will provide direct heat transfer from one side of the wall to the other with your scenario. That to me is less protective than the drywall.

 

[Pixy]

For basic concrete, dowel action works because of aggregate interlock.

For mortar, with no aggregate interlock, how do you compute or model how dowel action work here (so can know the minimum distances of mortars before they are crossed by mesh or bar to avoid the middle segments just coming out?

Ultimately, the studs with cavity filled by mortar will be covered by gypsum x. So you have superior fire barrier.

 

[Pixy]

What level of protection is required? You can get 1 hour out of 16mm type X gypsum, 2 hours out of 2 layers.

The steel studs will provide direct heat transfer from one side of the wall to the other with your scenario. That to me is less protective than the drywall.

But even if the steel studs melt. The mortar with wire mesh is still there to give barrier to fire. It won't just fall off. Unless you mean when the steel studs melt, the wire mesh melts as well?

For a steel studs whose cavity is filled with mortars. Are their actual experiments where the steel studs just melted inside and ouside? Pls. show the tests. This will dissuade me from thinking of it.

 

[jayrod12]

If you're going to cover the wall in gypsum anyway, why bother filling the cavity with mortar? You're now increasing weight for no benefit.

Where I practice, the wall construction from a fire rating standpoint typically has to meet some codified wall type. So in your case, they would only consider the gypsum board and studs, the mortar fill is just additional dead weight.

So not only are you making that wall much more complicated to construct, but also making it heavier meaning the structure below may need to be beefed up. For what? In my mind, no benefit.

I can see that there really is no dissuading you. And I don't have any sort of testing like you're requesting, because I've never seen a wall construction like that proposed for the reasons we've already indicated. So unfortunately I'm going to bow out of further responses.

Maybe the contractor on the project will have better luck changing your mind once you see the additional price tag associated with it.

 

[Pixy]

Here is the logic.

In fire assemblies which include both the studs and gypsum x. You get performance by taking all their parts together. That is. If the studs melt, the gypsum x falls apart.

But with the Studs with mortar inside. If the stud melts, the mortar won't fall down because it acts like mini column by anchoring it below and above. Do you see why this is more superior?

The wall will still stand. I used to do concrete walls with about 10 kN per meter in the beam and this is heavy. If you remove the gravel. It can make the weight only 7 to 8 kN. Gravel being stones are obviously heavier than sands. Is it not?

My question is how do you characterize or model the dowel action of mortar (since it has no aggregate interlock)?

 

[dold]

That is. If the studs melt, the gypsum x falls apart.

The whole point is to not let the studs melt as they are inside the gyp board. There are rated/tested/approved assemblies of metal studs and gyp board that will provide up to 3 hour protection. [URL unfurl="true"]https://www.usg.com/content/usgcom/en/design-studio/wall-assemblies.html[/url]

You're not planning on specifying this mortar wall thing on a project are you? Like others have said, it's just a heavy wall with no benefit. It seems like your mortar wall would just be a hazard to firefighters in case of a fire, on account of having large panels of mortar just waiting to collapse.

Just use a rated wall assembly. By the time you build a metal stud wall and fill it with mortar somehow, you could have just built a concrete or masonry wall.

Not to mention you'll never get a building permit if you specify this mortar wall and call it a 2 hour firewall.