Free Standing Masonry Wall

[kdg8159]

I am designing a free standing masonry firewall. I have read several related threads about the loading but my question is about the height. Is there a height limitation (h/t) if I am designing the wall for the imposed loads? Also, I have noticed references to a 'slender wall' but have not been able to find what the additional design requirements are for this. Thanks!

 

[ishvaaag]

Reliance upon the long term tensile strength of masonry is adventurous. I worked 75 days with an architect charged for decades with the maintenance of the main temple in our town, did some calculations on the status of the oldest tower on it (1730, near 100 m tall finished) and in the process the man made clear his opinion on that the practical tensile strength of masonry was ... nil. Imagine that with such concept you should limit yourself to stability with weight, leading to stubby walls.

http://eurocodes.jrc.ec.europa.eu/doc/WS2009/3_pm_Roberts.pdf

BS 5628 Pt.1 Slenderness Limits

Free standing wall
Height ? 12tef

There may be more, I found this in google without going to my files/books/docs.

http://cig.bre.co.uk/nsdl/docs/fsw_doc.htm

http://www.iitk.ac.in/nicee/IITK-GSDMA/EQ12b.pdf

This last reference without further checks states that for a wind load of 1.15 kN/m2 (that almost any exposed wall will see some time in its life) limit slenderness is just 4. Stubby.

Hence either good determination of tensile strength (nil?) or better reinforce for a reliable behaviour. Surely there are more limit slendernesses out there.

 

[KootK]

As isgvaag rightly points out, if you're using unreinforced masonry,you'll want the thing to look like a darn gravity dam Hopefully, you'll be reinforcing your wall.

In the absence of explicit h/t guidance in your jurisdiction, I might use 1/2 the ratio for simple span walls utilizing the assumption that the deflected shape of a cantilever looks like the def shape of 1/2 a simple span wall of

You might be able to justify a less conservative approach when it comes to slenderness. Under the cant load case, you probably don't have any axial load on the wall other than self weight. This is a pretty favorable situation for stability.

Timoshenko's stability book has a neat example studying this condition. I think that, using a distributed load rather than a concentrated load at the top, you can carry 4 times as much load before becoming elastically unstable. You'll wanna check that number though...

I suspect that your wall will be pretty stocky just to meet strength requirements.

 

[asixth]

You are right, there are limitations to the slenderness of masonry walls and you will need to check the robustness clause of your masonry walls. I can only quote the Australian masonry code because that is all that I have designed masonry with, but for 8" block cantilever masonry wall the height to satisfy robustness is 7.5' unless you have engaged piers that allow the wall to span horizontally onto the piers.

But the slenderness limits should be under robustness or something similar depending on what code you are using.

 

[jike]

I have always designed free standing firewalls as reinforced, never unreinforced as the previous posts have discussed. Another way is to post-tension them (no experience here but have considered doing it).

I have always felt that loads are somewhat unpredictable with thermal shock, variation in pressures during collapse of structure on one side, etc. Although, many engineers design for the 5 psf interior pressure plus wind load on exterior portions (parapet or lateral extensions beyond the building face) and/or seismic (whichever is greater), the Canadian Code requires about 10 psf (ultimate). Whatever you do, be conservative!

I do not know of any limiting height/thickness ratio for engineered reinforced walls. You might also look at deflection and include the P-delta effect.

Make sure the architectural details (roofing, flashing, drywall, ceiling, floor, etc.) allow the structure on either side to freely fall away from the firewall without imposing large forces that cannot be resisted.

 

[BAretired]

If the wall is several stories high, you may wish to consider using a tied firewall instead. This requires that the construction on each side of the firewall be designed to resist the forces of a fire on the opposite side.



BA

 

[JoshPlumSE]

In the 1997 UBC there were two explicit methods given based on the h/t ratio being greater than or less than 30.

In the 2008 version of ACI 530, section 3.3.5.3 also gives an h/t limit of 30. But, the design procedure is the same except for a more stringent limit on the axial stress. Essentially, the code is now telling us to use the slender wall design procedures for any walls that are designed with LRFD loads.

One gaping hole in the code is that if you are designed to the ASD procedure then there doesn't seem to be a maximum limit to the H/t or H/r ratio.

Josh