CMU allowable unbraced heights 1

[steeledan]

I have a 30' tall unbraced interior load-bearing wall (not a shear wall, just picking up .2klf DL + .2klf LL roof loads, in Seismic Cat 'A'). Per Enercalc design, can use 8" CMU w/ #5 @ 32" o.c. But I see in ACI 530 Table 5.5.1 that max allowable height/thickness ratio = 18. To meet this criteria would mean that I couldn't even use 12" CMU here.

Similarly, I have a 24' tall exterior load-bearing shear wall on same project - still not very heavily loaded & I can easily reinforce it to meet allowable capacity requirements. But 12" CMU wouldn't meet the above h/t ratio here either.

Are there any exceptions to this h/t ratio criteria?

 

[SteelPE]

Try reading the title of Chapter 5.

 

[ishvaaag]

Traditional codes for masonry derived from traditional practices that used sturdy bearing walls, the sole ones thought (and seen) to stand the challenges of time. For example, the DIN code of the sixties that I remember only allowed single wythe bearing inner walls 12 cm thick (maybe 15) for two floors loading, the exterior walls started at no less of 25 cm (maybe 30) thick as a minimum, and all were to be prudently engaged at quite short counterfort orthogonal walls at say 3 to 5 meters. This for story heights say of under 3 m.

So if such were the practices of old, it is quite unlikely that more modern codes, that have added to the lack of confidence in masonries, even when reinforced, may be more lenient on slenderness/stability issues.

For these cases where some degree of slenderness is wanted an embedded steel structure may be the solution.

 

[steve1]

http://www.structuremag.org/article.aspx?articleID=663

 

[steeledan]

Thanks, steve1. Not so much, SteelPE.

 

[ishvaaag]

I may add that here (Spain) it is also the case that engineered walls both now and recent decades, reinforced and unreinforced may have been designed slender when proven correct, and now exist also software that usually does the task, since when reasonably laid, bearing masonry can show very reasonable stresses. With the general enforcement of reinforced masonry (if well built to good engineered practice) this ***should*** not cause particular problem; I contrarily would be quite wary of non-stabilized tall unreinforced walls in almost any situation; and unconfortable with the reinforced cases if bemusing chunks big enough could go unattached.

 

[SteelPE]

Just trying to point out the fact that the title of chapter 5 was "Empirical Design of Masonry". Meaning not engineered. If you engineer the wall you will use either Chapter 2 or Chapter 3. Chapter 5 does not apply in those instances where you engineer the masonry.