Any way to design a 9 m (30 ft) high cantilever masonry fire wall? 2

[nivoo_boss]

Hey everyone!

So I have to solve a problem with a firewall between two buildings. I cannot tie it to the buildings on both sides so it has to be free-standing. I'm thinking about 240 mm CMU filled and reinforced blocks but even just with these the slenderness is about 2,8 times over the allowed slenderness by EC6.

What might be the solution here? Piers? How to design the piers - as RC columns perhaps? The support moment from wind is not too much, design moment is around 14 kNm/m at the base.

 

[WinelandV]

Wouldn't you need to design the brace points for the full wind load then, and making sure that the wall can span between the brace points? I suppose you could use a forest of bracing, but color me skeptical of that much bracing being used.

Please note that is a "v" (as in Violin) not a "y".

 

[JAE]

Isn't there a big difference between:

A) a code mandated "FIRE DEMISING WALL" and
B) an exterior wall which requires a minimum fire resistance due to property lines?

The code definitely requires wall type A) to be standing after the fire.
I'm not so sure about a simply exterior wall with a set fire resistance rating.

 

[OldDawgNewTricks]

In this case with the wall at the property line, I don't think it needs to remain standing and can be tied to your structure.

For walls that do need to remain standing, the 5 psf horizontal live load is a minimum. The wind loads also need to be considered (although you might be able to reduce the return period in order to reduce the pressure). For more information, see NFPA 221 screenshots below:

 

[nivoo_boss]

We use Eurocodes here by the way. EN 1991-1-4 has a paragraph called "Free-standing walls and parapets" (7.4.1), which works well for my case.

For that height, pilasters won't do it... you need some really deep counterforts built into the wall or, more likely, some real structural steel framing.

So you're basically saying that a sway frame for example, with 9 m high RC columns cannot be done? Because that's what the piers pretty much are - they are cast-in-place RC columns.

 

[DaveAtkins]

Does an interior firewall need to be designed as an exterior wall, for the rare instance where it becomes an exterior wall after a fire?

I believe this is open to interpretation. IBC requires the firewall to remain structurally stable for the number of hours it is rated, with the lateral load on the firewall being the interior partition load (5 psf service load or 8 psf ultimate load). So a 3 hour firewall only needs to be structurally stable for 3 hours during a fire.

There are several other threads which discuss this.

All that being said, the Code prescribes minimum loads. I have no problem with engineers choosing to design them as exterior walls.

DaveAtkins

 

[OldDawgNewTricks]

DaveAtkins, I say the answer to your question is yes. If the wall becomes an exterior wall, it should be designed as such. The wind doesn't know it is not supposed to blow on the wall. However, the exterior wind pressures for the temporary condition can be reduced quite a bit to account for the reduced likelihood of strong winds occurring during the short time frame. But depending on the project (location, exposure classification, etc.), the reduced wind pressure could still exceed the minimum 5 psf if you use ASCE 37 procedures.

 

[phamENG]

If it's on a property line with your building on one side of the wall and somebody else's building's exterior wall on the other side:

1) it should just be an exterior wall
2) it should be tied into your building
3) it should be designed for wind pressure assuming the other building is gone as all codes I'm familiar with don't allow you to consider shielding from other structures (unless you're willing to guarantee that building will stand there forever shielding your building, even if you building stands for some multiple of its design service life...I wouldn't)

 

[BAretired]

I agree with phamENG's comments 1), 2) and 3), but we don't have to assume the other building will be gone; it is not there in the first place and may never be there.

I don't see why your fire wall would need to remain standing if your building burnt down.

If the wall does not remain standing after a fire, it represents a hazard to the neighboring property. That is not reasonable; the neighbor has a right to enjoy the use of his property without the possibility of a high wall collapsing on it, so the wall must remain standing if the building burns down. The wall should be tied back to the existing building in such a way that its required fire resistance rating is maintained. This may require counter-forts, fire resistant bracing or other means of stabilization.

 

[jayrod12]

Sorry BA, I disagree.

We do not design building with that in mind. Or at least the people I know don't.

Why would a wall not connected to, or not owned by, the adjacent structure need to remain standing if it's building burnt down? My building, my wall, my building comes down, my wall comes down.

 

[dik]

You shouldn't rely on the presence of an adjacent building for your design, unless for snow loading considerations. I'd never consider this for wind loading. I did a project outside of Lindsay where the developer knew that for unobstructed sites, the snow loading could be reduced, and he wanted me to design it for this loading.

He was quite unhappy when, at the end of the project, that I sent a note to him and the AHJ that a caveat should be placed on the property to make sure the adjacent construction could not be undertaken.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[wth]

For a similar project, where the existing building was already there we simply designed the whole structure for the required fire resistance and tied the wall into the new building. We only increased it from R90 to R120 though, probably would've done it differently if the gap was bigger.

 

[BAretired]

Sorry BA, I disagree.

We do not design building with that in mind. Or at least the people I know don't.

Why would a wall not connected to, or not owned by, the adjacent structure need to remain standing if it's building burnt down? My building, my wall, my building comes down, my wall comes down.

You can disagree all you like, but if your 30' high wall collapses toward my property and kills my grandchild playing 20' inside my property line, you will be liable; and the jurisdiction which allowed such a hazard to exist would be liable as well.

 

[phamENG]

Not to split hairs, but....(hold on while I get my razor and magnifying glass)

Who in their right mind would allow their grandchild to play 20' from a burning mid/high rise building? There's gotta be some culpability of the guardian in that case...

 

[BAretired]

You are splitting hairs, phamENG. Fire can spread quickly and supervision of the child may not have been perfect. But if you don't like that example, if my pigs were crushed in a sty located within my property, liability would still exist for both my neighbor and the jurisdiction permitting the hazard to exist.

The wall cannot be permitted to fall onto a neighboring property under any foreseeable circumstances, including fire. That should be obvious, so why isn't it?

 

[phamENG]

It was a joke, BA. (I'd hoped the parenthetical would convey that.) I get your meaning, and I agree.

I don't see the need to make it freestanding, though. I admit that I don't do mid and high rise designs in dense urban areas (apart from the occasional historical AR or restoration), but I find it hard to imagine that every building in New York City, Chicago, or any other metro area is designed to stand independently of its exterior walls.

 

[BAretired]

Sorry I missed the joke,phamENG; perhaps my sense of humor needs work.

I do not suggest that a building should be able to stand independently of its exterior walls; shear walls are required for the stability of many, if not most buildings. I do believe that some thought should be given to preventing concrete or masonry walls from collapsing across property lines in the event of fire.

 

[jayrod12]

I do believe that some thought should be given to preventing concrete or masonry walls from collapsing across property lines in the event of fire.

Believe that all you want, but that's not the case for the designers I know. But maybe we're drastically different than other areas. My point was, and will continue to be, the wall in this thread does not need to be a freestanding wall.

 

[BAretired]

Believe that all you want, but that's not the case for the designers I know. But maybe we're drastically different than other areas. My point was, and will continue to be, the wall in this thread does not need to be a freestanding wall.

You keep reiterating that, but you don't give any reasons for your opinion. Maybe you and the designers you know are mistaken, ever think of that?

You are erecting a wall thirty feet high, constructed of heavy masonry materials, on the property line. What reason can you (or the designers you know) give for allowing the wall to collapse into the neighbor's property in the event of a fire?

There are of course, solutions other than creating a freestanding wall. I would expect the authority having jurisdiction to require a reasonable clearance between your hazardous wall and the property line. Alternatively, he could rule that material other than masonry be used if you are unable to guarantee its stability after a fire. Perhaps a wood stud wall, with suitable fire separation from the property line would be a better choice. It is frequently done in Canada and the USA. That way, the wall will be tied to the building and will burn right along with it. Brick veneer, if present, will fall straight down and not present a danger to other properties.

 

[Aesur]

Isn't the point of a fire wall to stop a fire from spreading only? Could not the same argument be made by the new construction owners, that the existing adjacent property built on property line without said firewall that could collapse onto their property? I see both sides of this argument, I wouldn't want something falling into the neighbors yard, nor into my yard, but I don't recall seeing these always being done as freestanding walls either. I need to go back and re-read these chapters.

 

[Aesur]

Reading through 706.2 Structural Stability, it states "fire walls shall be designed and constructed to allow collapse of the structure on either side without collapse of the wall under fire conditions." I therefore read this to say in this case since it could only be possibly attached to one side of the building, ie can't have breakaway clips on other sides, then the wall should be designed as freestanding and not attached to the new construction, or attached with breakaway. So then the question becomes how long does said wall have to remain in place, can lower wind loads be used to design said wall, etc.. I argue here that you have to apply full wind loading to the wall because it's exterior and shielding by other buildings is not permitted per code.