Overturning check on a free standing wall??? 2

[cenee]

I AM HAVING THIS DISCUSSION WITH OTHER ENGINEERS AND I THOUGHT IN ASK HERE TOO. I AM DESIGNING THIS 15ft FREE-STANDING CMU WALL THAT DOESN’T EVEN HAVE A SMALL ROOF ON TOP.
IT TURNED OUT A 12” CMU WALL WITH A CONTINUES FOOTING WIDTH OF 5ft. I CHECKED THE SOIL PRESSURE (for e=M/P), IT WAS OK.
THEN OVERTURNING????, DO I HAVE TO CHECK THAT FOR A FREE-STANDING WALL???
I AM TOLD THAT CHECK IS ONLY FOR RETAINING WALLS (RM / M > 1.5).
I CHECKED IT ANYWAYS AND IT DIDN’T WORK. TO MEET THE 1.5 RATIO, THE CONTINIUS FOOTING NEEDS TO BE AT LEAST 6.5ft IN WIDTH.
IS THERE SUCH A CHECK FOR A FREE-STANDING (that of <1.5)???
Thanks.

 

[Qshake]

Why wouldn't you check the overturning or overall stability? Do you want the wall to fall down in a light wind?! I certainly hope there are no intentional purposes to allow people near a wall that isn't checked for overturning, how disasterous!

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[JStephen]

I would think you'd want to check it for wind, seismic, maybe for tilt due to settlement. Not sure about your moment ratios.

 

[Ussuri]

Having designed freestanding masonry walls in the past I am surprised you can get the wall itself to work in flexure. I have always had to go for reinforced masonry as opposed to making the wall very thick. The maximum moment due to wind pressure always exceeds the flexural resistance of the masonry.

You really must check for overall stability because I wouldn't want to lean my bike on it if you didn't

 

[SperlingPE]

Overturning should be checked.
Would a deeper footing (add weight) help with yoru overturning?
What type of load (wind) and what magnitude are you using for overturning?

 

[DaveAtkins]

I'm surprised the soil pressure is acceptable, but the wall doesn't work for overturning. I find the opposite is generally true. Then again, I usually use an allowable soil bearing pressure of 2000 psf.
Also, is the footing deep enough to be below the frost penetration level? If so, are you counting on the soil overburden when you do your overturning check?

DaveAtkins

 

[haynewp]

Assuming wind controls, what is your wind load and allowable soil pressure you are using?


The ideal engineer is a composite ... He is not a scientist, he is not a mathematician, he is not a sociologist or a writer; but he may use the knowledge and techniques of any or all of these disciplines in solving engineering problems.
-N. W. Dougherty

 

[sundale]

A structural wall like this needs to withstand wind forces and seismic forces in accordance with the IBC 1605 load combinations. These load combinations will prescribe overturning safety factors larger than 1.5...

I would also consider designing the wall to cantilever for a 280 plf (ultimate) lateral force applied at the wall top per the intent of IBC 1604.8.2.

 

[Taro]

sundale, 1604.8.2 is not intended for freestanding walls. It is a minimum connection force for walls that rely on floor/roof diaphragms for stability.

 

[sundale]

I agree with you Taro. That's why I said the intent of, not the letter of, this section.

"shall be anchored to ... or other structural elements" could, however, be considered to be the footing in this case, since this is the ONLY connection pertaining to the wall's overturning stability.

If the cmu wall is a simply span (typically) then the 280 plf represents 1/2 the tributary seismic force that would go into a supporting upper diaphragm. Designing the cmu wall, its connection to the footing and the footing for this same force, without any support from a supporting upper diaphragm (i.e. freestanding), is admittedly a debatable calculation.

 

[Taro]

I don't think designing the wall for the prescriptive diaphragm anchorage force addresses either the letter OR the intent of the code. It certainly couldn't hurt anything to perform additional meaningless calculations as long as the required design checks were done properly. But to recommend such additional checks might just confuse someone who isn't certain what the required checks are.

 

[Ron]

Yes, you need to check the wall for overturning and yes, your ratio is correct. Don't forget that in overturning your bearing pressure increases on the downward side, so you have to check the bearing pressure under that condition, not just the vertical static condition.

Your footing width doesn't look to be out of range for the wall height, considering you have reasonable wind loads. If you are in a high wind area, I would expect the footing to perhaps be even wider. One thing to consider is to put the footing a bit lower in the ground to take advantage of the soil overburden to resist overturning. Soil is cheaper than concrete.

 

[pcronin]

The IBC Combination that should control overturning is 0.6Dead+Wind. Basically it will give you a safety factor of 1/0.6= 1.6667.

Make sure you develop your wind pressures with ASCE 7 under "Other Structures" and take your Cf value from the sign geometry and Solid Sign at Grade criteria.

 

[DaveVikingPE]

What about handball, tennis ball or Jai-Lai ball impacts?