MWFRS Windward Wall Pressure Calculation 2

[Jerehmy]

I was talking with colleagues to see what elevation they use for calculating MWFRS windward wall pressures:

1) Top of wall;
2) Average wall height;
3) Take qz evaluated and top and bottom of wall and take average;
4) Some other variation like qz evaluated at 2/3 the top of wall height.

I can’t find anything that says what the correct way is. Is it stated anywhere how it should be done? What do you guys do. For what its worth, the 1998 wind losd guide by Mehta and Perry uses top of wall.

 

[jayrod12]

The NBCC indicates reference height is mid-height of the roof, or 6m, whichever is greater. Where the roof slope is less than 7 degrees, the eave height can be substituted for the mean height of the roof.

 

[Jerehmy]

Talking about windward wall heights for multi-story. Why would roof height be used for windward walls?

 

[jayrod12]

I'm just regurgitating what's included in the NBCC for low-rise structures with H/W ratios less than 0.5 and total height less than 20m. For taller buildings h is defined as the actual point above ground.

 

[JAE]

Jerehmy - this is really an international site so providing the applicable building code, standards, etc. can be helpful in answering your question.



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[Jerehmy]

Apologies, I was referencing IBC and ASCE-7

 

[wannabeSE]

I assume this is a freestanding wall in ASCE 7-10 section 29.4. It is the top of the wall. See definition of q_h in 29.4.1.

For the windward walls on a building, the wind force is uniformly increasing above 15'. Below 15' the force is uniform with z=15'.

 

[Jerehmy]

Not a freestanding wall, a multi-story building.

When calculating the wind pressures for the windward wall on a multi-story building, you can step the wind loads as you increase height. This is usually done per story level.

So say you have a story in 15 foot increments. For exposure C, Kz for the first 15ft is 0.85.

Story level 2 is from 15ft to 30ft. Kz @ 30ft = 0.98. But that is for calculating the velocity pressure at 30 feet. The average wall height for story level 2 is 22.5 feet. Thus, using 0.98 is conservative, but it is what most people do. I was wondering if other engineers use a more precise calculation of Kz.

Kz at the average wall height of 22.5 ft is 0.92, which is significantly less than 0.98. Even taking Kz as 2/3 the story height, or at 25ft, is 0.94. Which is still conservative, yet significantly less, than 0.98.

This is what I'm talking about.

 

[MotorCity]

For a short building, I would consider using stepped increments as you describe. But for a tall, multistory building, I would apply the actual pressure diagram and not step it in 1 story increments. You will probably induce too much conservatism into your loads if you use the pressure at the top of each story and apply it to each story.

 

[P1ENG]

I usually apply the average from midpoint of wall to midpoint of wall at the floor line at each story. The average is a simple function of an integration:

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant