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.

 

[HTURKAK]

- EC-6 suggests the slenderness ratio of the members should not be greater than 27. ( But for vertical loading)

- If the slenderness is about 2,8 times over the allowed , is the free ht around 18.0 meters ?

- I would go with confined masonry wall ( with vertical and horizontal RC or steel profile confining elements and plastering both sides)



my opinion..





I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[nivoo_boss]

If it is free-standing then the effective height would be 2x 9 m=18 m and slenderness would be 18/0,24=75 and 75/27=2,777. That's how I derived the 2,8 times more than allowed.

 

[HTURKAK]

I got your point..

My approach would be confined masonry with horizontal and vertical elements at every 3.0 m and use the rules of
EN 1996-3..









I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[DaveAtkins]

What is the purpose in the Code of the slenderness ratio? Is it to prevent buckling due to vertical gravity loads? Your wall has no gravity loads (other than selfweight).

DaveAtkins

 

[manstrom]

I would say no. Not really possible without counterforts or big piers.

In cases like this, the wall is usually tied to both buildings with ties that would melt away in a fire.

 

[Aesur]

I would say this is possible, but impractical. If you can design a masonry retaining wall that high, you should be able to do a fire wall, just may need thicker CMU to meet all the requirements. Can you shed some light as to why you cannot attach it to the structures? There may be some kind of breakaway clips as often used in wood for similar conditions. Another option could be concrete? The footing however for such a wall would be ridiculous as a cantilever as if one side came down then the wind is applied directly to the wall.

 

[DaveAtkins]

The IBC allows you to design a fire wall for internal pressure (5 PSF), not full wind load, so a tall fire wall is theoretically possible.

DaveAtkins

 

[IceNine]

The IBC allows you to design a fire wall for internal pressure (5 PSF), not full wind load, so a tall fire wall is theoretically possible.

Dave,
What section of the IBC code is this noted in?

 

[DaveAtkins]

Paragraph 1607.15.2 in the 2018 IBC.

DaveAtkins

 

[Aesur]

Paragraph 1607.15.2 in the 2018 IBC.

Interesting, this appears to be new in 2018 IBC. I would be curious if a building official would interpret "minimum horizontal allowable stress load of 5 psf" as design for this, not, this is the absolute minimum. Are these fire walls supposed to be removed if one side collapses, because if this occurs, this is now an exterior wall, not an interior wall. keep in mind when the code says "minimum" it means "minimum", not design for only this.

 

[nivoo_boss]

Can you shed some light as to why you cannot attach it to the structures?

I'm sorry, if I use wrong words in English, I'm an Estonian and do not know all the vocabulary in English correctly. Anyway, to answer your question, the building I'm designing is right next to the property line and the neighbouring property does not have a building there. I could tie it to the building that I'm designing, but if that would burn down, then the wall would fall over as well, theoretically. The firewall between the properties is required because if it is not built, the neighbouring property owner cannot build anything near the property line because of fire safety - in here, a free space of 8 m is required between buildings.

 

[jayrod12]

I don't see why your fire wall would need to remain standing if your building burnt down. So I'd be tying that into my building and stopping worrying about it. I'm certain that's how all of the other buildings adjacent property lines are constructed.

Usually we only worry about breakaway connections for firewalls, when both structures on either side are connected to the same wall. A 30 foot tall freestanding firewall isn't really possible without some pretty hefty pilasters at a regular spacing.

 

[IceNine]

The whole point of the firewall is that is has to remain standing if one side collapses, in order to protect the remaining side from fire.
I believe a 30ft cantilevered wall is possible with heavily reinforced 12" or 16" CMU with a large footing. Not as economical as two walls, but sometimes double walls aren't feasible.

 

[DaveAtkins]

Are these fire walls supposed to be removed if one side collapses, because if this occurs, this is now an exterior wall, not an interior wall.

I have always thought that after a fire, the fire wall can be braced before the building sees a design wind load.

DaveAtkins

 

[IceNine]

Paragraph 1607.15.2 in the 2018 IBC.

I'm reading this the same as Aesur...the 5 psf is a minimum, not the only design load. I don't have access to the 2018 IBC Commentary, perhaps there is clarification?

 

[nivoo_boss]

I'm going with piers, spaced at 3 m. Piers are basically cast in place cantilever RC columns with high moment and only self-weight working against it. Even the foundations did not get too large. It basically looks like this figure below I found from the internet:

 

[dik]

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

-Dik

 

[robyengIT]

http://www.bekaert.com

> downloads > Masonry reinforcement > Murfor (a lot of tech doc)

 

[Aesur]

I have always thought that after a fire, the fire wall can be braced before the building sees a design wind load.

I was curious if something like that was the norm, I could see that allowing for the lower 5 psf ASD live load; not sure how to note and require bracing where it actually happens as I have rarely seen bracing added due to forensics taking weeks or months to determine the cause for insurance.