Reinforced Masonry - High Wind Areas

[therattler]

I am designing a building with reinforced masonry walls (9' high) in a high wind (120+ mph) area. I am coming up with very high reinforcing steel requirements,like a #6 every cell, or a No. 8 every other cell. Could I be missing something, this seems awful high. What are the usual (economical) approaches? TIA

 

[haynewp]

What size cmu are you using?

Is the wall cantilevered?

 

[whyun]

Also, what is the design uniform lateral force you are using? It would help to see how it is derived to see if it is the correct force or not...

 

[Lutfi]

WOW!!!

I live in Florida design for 130 MPH winds and I find this reinforcement a bit much for a wall that is 9 ft high.

You need to design the wall for wind pressure in zone 4 “corner zone”. I use components and cladding pressures.

I have several questions for you:

1. How did you compute the wind load? What code did you use? I recommend ASCE 7-98 or 02.
2. Which code are you using for masonry design?
3. What size CMU are using
4. What strength CMU are you using?
5. What boundary conditions are you assuming? Normally I assume pin-pin for the one foot wide strip. Argument can be made that the base has some fixity. However, I am conservative and pin-pin would yield conservative results.
6. Are you using hand calculations or some software? Be careful using software because if you are not familiar with the theory and use, you could very easily leave a variable checked or unchecked which will give bad results.
7. What grade steel are you using?
8. What load combinations are you using? Be careful here if you use ASCE 7 and not use the ASCE 7 combinations, your calculations needs to be adjusted accordingly. Normally you would get higher pressures if you do not use ASCE 7 load combinations.

Check you procedure again for errors.

Regards,


Lutfi

http://www.cdeco.com

 

[JAE]

Lutfi - just curious since you're used to these high winds (and this applies to all wind speeds of course) but in the corner area, if you have substantial masonry perpendicular to the wall, and the corner distance isn't usually a real large dimension, can you ignore the corner wind forces by assuming that at the corners, the perpendicular wall sort of braces the other wall? Just wondering if you ever considered this...or if anyone else out there does too.

 

[bylar]

Seems to me too much reinfocing.
I get a pressure of app 35 psf in the end zone.
If the wall is supported top and bottom (normal exterior wall condition) with 8" block with rebar in the center I get a #6 @ 16' o/c (grade 60)

 

[therattler]

Thanks to all who replied, I reviewed my calcs very carefully, and found an error. Once corrected, reinforcement came back into line with what I believe are reasonable values. Thanks again, I will try to return the favor to the forum.

the rattler

 

[pcronin]

JAE,
Be carefuly relying on the perpendiculr wall at corners. This is often where the architect will place their expansion and control joints. If you know there is not a control joint there, you can ignore the edge wind load, if not, detail a few more bars in the edge zone.

That's usually better than overdesigning the entire length of wall, but if the difference isn't that much, a little fluff never hurt. The main cost is the placing of the bars and grouting, a slightly larger bar that required does not increas the cost that much.