Height To Thickness Ratio Of A Reinforced Masonry Wall

[gshrewspe]

I am designing a cantileavered reinforced masonary wall. What is the maximum height to thickness ratio allowed by the code for this type of wall? Where may I find a code reference to this issue?

 

[whyun]

1997 UBC Sec 2107.2.14.2 indicate h'/t = 30 as a dividing line between slender and non-slender wall. I am presuming your situation is an exterior masonry wall with a cantilevered parapet above the roof level.

I am not aware of an absolute maximum ratio but statically, you can't avoid having a very thin wall and make the wall work out as height increases.

Out-of-plane slender wall design at strength level is in Sec 2108.2.4.

 

[gshrewspe]

whyun
Thanks for the reply. I will give you a little more detail of the wall I am designing. The will is a free standing cantileavered firewall exposed to wind load only. In this case for the h/t value. Would the "h" value have a multiplier of "2"?

 

[DaveAtkins]

If the wall is resisting wind load only, the h/t ratio is not an issue. The h/t ratio affects the allowable axial stress, Fa, but not the capacity of the wall in bending.

 

[whyun]

I am in partial agreement with Dave. h/t does affect the wall when subject to vertical loads and out-of-plane lateral loads. Moment on the wall will have contribution from flexure, eccentricity of applied vertical load, bending due to maximum flexural displacement of the wall. High h/t will result in making the third term very large.

I agree with Dave that h/t is not an issue with a freestanding wall. Disgrgarding the h/t, design the wall first and see that h/t comes out to. Regardless of the height, you would use a 6" wall at a minimum, probably a 8". In the end h/t wont look so bad.

 

[m8589]

I think you need to check: fa/Fa+fb/Fb < 1.00 (or 1.33 ? )where fa is axial compression due to dead load and fb is flexure compression due to wind.

I think code (ASCE 7-98) does not allow 30% stress increase in wind load combination I am not sure about ACI-530. Any comments ?

for a cantileavered wall effective height h'=2h

 

[jheidt2543]

Check the website for the National Concrete Masonry Association (NCMA). They have a series of TEK Notes that address all aspects of concrete masonry design and constrution. You can order their publications or if time is short, you can find a member firm near you and get copies. I found a NCMA member firm that had a link to the TEK Notes.