Hollow masonry walls

[ktulu]

I am reworking through the morning problems from the NCEES Sample Exam.

The question I have pertains to masonry walls. The question asks for the total axial load, P, at the midheight of a 12 foot high wall.

Given:

Roof dead load = 15 psf

Non-reducible roof snow load = 40 psf

Average wall dead load = 54 psf

Design wind (pressure or suction) = 20 psf

Seismic forces do not govern...

Unfortunately, I have looked at the answer, and am confused as to where the constants come from. Specifically, the solution has:

P = 6*(average wall dead load) + 12*(roof dead load + non-reducible roof snow load) = 980 pounds per linear foot.

So, as I was writing this, I figured out that the 6 & 12 are wall heights, but now, how did they come up with the combination?

I know, it sounds like jumble, but I am confused as to how they solved this question....

ktulu

 

[bigray76]

I am reworking through the morning problems from the NCEES Sample Exam.
The question I have pertains to masonry walls. The question asks for the total axial load, P, at the midheight of a 12 foot high wall.

Given:

Roof dead load = 15 psf

Non-reducible roof snow load = 40 psf

Average wall dead load = 54 psf

Design wind (pressure or suction) = 20 psf

Seismic forces do not govern...

Unfortunately, I have looked at the answer, and am confused as to where the constants come from. Specifically, the solution has:

P = 6*(average wall dead load) + 12*(roof dead load + non-reducible roof snow load) = 980 pounds per linear foot.

So, as I was writing this, I figured out that the 6 & 12 are wall heights, but now, how did they come up with the combination?

I know, it sounds like jumble, but I am confused as to how they solved this question....

ktulu

ktulu,

The question asks for the total axial load at midheight, therefore the 6 is applied to the average wall dead load per square foot - since the wall is 12 foot high, the load would only be based on the upper 6 feet for the dead load of the wall.

I do not have the sample exam in front of me, but I seem to remember the 12 feet being the tributary area of the roof that would contribute load to the wall (I will double check when I get home, but I believe the span for the roof was 24' - therefore you would take half of that width as the tributary area).

-Ray

 

[McEngr]

ktulu, this problem can be answered straight from the cerm masonry chapter. If you don't have the CERM, I'd order it and get it next-day delivery!!! No joke!

bigray has hopefully made it clear for you. The maximum wall load will reside at the midheight of the wall. Therefore, you must add the lineal foot weight of half the wall to the roof loads.

Good luck. If you have ANYTHING, and I mean ANYTHING, I'd love to help. It's our duty to pay it forward...

 

[ktulu]

Thanks, guys.

I think I see where you are coming from.