tornado pics- masonry wall failure

[a2mfk]

With all due respect to the victims of this tragedy, there are also lessons to be learned from a forensic engineering standpoint.

I have done a lot of forensic investigations of wind damage, but this is one of the few lateral pressure failures of CMU that I have ever seen (at least IMO it is). Well, I should say the connection to the roof diaphragm may have been the failure mechanism also. But there is no rebar sticking out of the wall which looks to be at least 15 feet tall! I would like to see how the wall would look with #5s at 48" oc or less.

Picture 60

http://www.weather.com/news/photos-tornado-outbreak-midwest-deep-south-20140427

 

[EngineeringEric]

To help out, a direct link. Looks like it was a failure at the roof as well. Also looks like no reinforcement, bond beams or ladder/truss. Sad on design and destruction levels

 

[JedClampett]

It looks like the Zone 5 (corner pressures) are a real thing.
It's weird, it doesn't even look like there is mortar between the blocks.

 

[Archie264]

Not disagreeing with any of that but I thought that designing to tornado loads was beyond our scope and purview.

 

[TehMightyEngineer]

Right but tornadoes do a great job of simulating code level wind loads away from the center of the tornado.

As for this one the bricks on the ground mind indicate the direction of the wind pressures. Possibly the front wall pushed in opening up the building to positive pressure and blowing out that side wall (note the bricks on the ground). If the front wall pushed in then it was probably a combination of no reinforcement and minimal/poor/nonexistant attachment to the roof diaphragm. Either that or they cleared the bricks away from the front of the store.



Maine EIT, Civil/Structural.

 

[a2mfk]

Like TME said, not all wind pressures experienced by a structure due to a tornado will exceed the code minimum wind pressures. That wall was good for very little lateral load with no reinforced grouted cells. I don't know the building codes in that area of Mississippi, but due to their location near the Gulf I would expect at least 110+mph wind design (old wind speeds).

I also agree with you TME, positive pressure failed that front wall and negative (maybe combined with high internal pressures) failed the side wall.

 

[azcats]

Were any cells even grouted? I'd bet the wall is still standing with #5 @48" and a bond beam at the top.

First link is the building. Street view will give you the elevation.

Building

This link shows the billboard over the guys shoulder that's still standing in the parking lot of the same building.

Billboard in Lot

 

[azcats]

That building is near the top of MS and would have been 90 MPH in the old code.

 

[XR250]

Probably just attached to a 16ga roof edge angle - if they were lucky

 

[3DDave]

There's a high gradient near a tornado. While it's a surprise the wall looks like it fell over almost unaided, the standing billboard nearby is not.

 

[a2mfk]

If you ran calcs on an unreinforced masonry wall that is about 15 ft tall with no reinforced, solid grouted cells, and no dead load from the roof, I would be surprised if it was good past 50mph. Since you are completely depending on the tensile bond strength of masonry or perhaps more of a shear flow mechanism, then all you need is a little hairline shrinkage or settlement crack in the mortar joint and you have no bending resistance beyond self weight.

My surprise is that this does not happen more often during actual wind events. I have done a LOT of forensic investigations following wind events, and I think all of the CMU failures I have witnessed or seen photographs of are likely initiated by a lack of support at the top of the wall (roof or floor blown away) and the CMU fails by tipping over, sometimes intact. Actual bending stress failures are rare in my experience, though in theory it should be a huge problem, other things happen to fail first (ex. windows and doors failing, letting in wind, roof fails).

I attached an example of what I mean from another tornado where it appears the CMU failure is a loss of roof support.