Tornadoes and Concrete

[Greg Higgins]

Let me preface this by saying that I have not witnessed this, but one thing that comes to mind is that rebar is "tied" together with wire. IF, and I say IF, the concrete wall, be it solid concrete or a block wall, was either poorly constructed (i.e., the concrete mix was inadequate, not cured properly, etc) or damaged by flying debris, then I think it would be possible for the rebar to appear that it had snapped when in reality, the wire tie simply did not maintain its integrity and "released" the 2 pieces of rebar, thus appearing that the rebar had failed.

As for steel buildings, I have seen these fail due to not enough connections (nuts and bolts) holding the pieces together. This is a builder failure and not a construction failure. In other words, if the assembly had been constructed properly, it would not have failed.

Greg Higgins

 

[Jason Foster]

picture anaylsis

I did find this image of what looks like a warehouse in Memphis Tennessee that was destroyed by the tornado this year

http://www.flickr.com/photos/mckeephotographer/2705536226/sizes/l/

This photo illustrates some interesting things. While it obviously demonstrates that a simple concrete wall does not make for a safe structure. In evaluating the photo, it looks to be a warehouse type building that had truck docks. The building was like an exposed steel structure, with concrete walls. This was NOT a concrete reinforced building. The concrete wall seem very thin in comparison to it's height...perhaps 6", yet 25' tall or more. I suspect the rebar shown was only to maintain the structure of the wall itself and was not weight bearing. I suspect there was internal steel wide flange columns that acted as the building skeleton structure. That failed as shown in the photo with the amount of debris located within the building. I also suspect that there were some rather large spans to the roof and could not withstand the wind load during the tornadic event.

Overall, more study as to building structures and severe weather events is warranted. As an architect, I'm paying more and more attention to this every chase.

 

[Rob Herman]

Let me preface this by saying that I have not witnessed this, but one thing that comes to mind is that rebar is "tied" together with wire. IF, and I say IF, the concrete wall, be it solid concrete or a block wall, was either poorly constructed (i.e., the concrete mix was inadequate, not cured properly, etc) or damaged by flying debris, then I think it would be possible for the rebar to appear that it had snapped when in reality, the wire tie simply did not maintain its integrity and "released" the 2 pieces of rebar, thus appearing that the rebar had failed.

I believe the wire ties are simply there to hold the reinforcing steel in position until the concrete is placed, not for structural integrity.

 

[Greg Higgins]

Rob you are exactly right. I was not referring to the wire ties providing structural integrity, but rather holding the rebar in place. Once the concrete has cured, the wire ties serves no purpose. However, if the concrete wall was compromised by flying debris (check out the Texas Tech Wind Damage Assessment tests) then the concrete could break away exposing the rebar (I've seen this numerous times during demolition and at high explosives testing). If this occurred near where 2 pieces had been held together with the wire ties, some folks may think that the storm damage "snapped" the rebar.

Jason, what you are referring to is "tilt wall construction", a pain in the back side of the fire service as this type of building tends to fail very quickly in a fire situation. My comments regarding "lack of" or "poor" connection points are readily seen in these buildings. You can research damage due to high winds and find many examples of this type of construction falling / failing under moderate wind conditions. You might also check out "Building Construction for the Fire Service" by Frank Brannigan; an excellent resource of how buildings are constructed and fail.

 

[Simon Timm]

Can regular poured concrete (excluding concrete block and base foundations) be severely damaged in strong tornadoes?

 

[craig buchanan]

Can regular poured concrete (excluding concrete block and base foundations) be severely damaged in strong tornadoes?

The answer to this really depends on your question. The simple answer is YES. The more complex answer comes down to what a tornado actually is in the real world. The rapidly moving wind even at the top end probably would not tend to harm concrete. Tornado's are very seldom just wind. They are more closely related to a blender with stuff thrown in. The wind is OK. All the little bits is what can "erode" the concrete or puncture it with something.

 

[Heidi Farrar]

For what it's worth, here is a photo of damage at Parkersburg that shows snapped rebar. I had originally posted this in the 5/25/08 DISC thread. This of course still doesn't answer any questions regarding whether this was really caused by just wind or by a wind-debris combination... but it's obviously something that caught the survey photographer's eye. (Larger storm damage catalog found here.) Also, if I am not mistaken, this appears to be asphalt plastered to a power pole (again, for what it's worth).

 

[david diehl]

Heidi,
from what I can see of that Re-bar is that it was snapped off before the tornado damaged the building, it appears to me that there is rust on the segment that should be shiny due to the snap-off.

 

[Rob Herman]

Also, if I am not mistaken, this appears to be asphalt plastered to a power pole (again, for what it's worth).

Does look like asphalt, but I think its just mud.

 

[Greg Higgins]

The pictures of the hadite block wall shows that the wall was not constructed to any type of specification. A kink in the EF scale ratings is that the construction was built to code.

 

[Heidi Farrar]

Heidi,
from what I can see of that Re-bar is that it was snapped off before the tornado damaged the building, it appears to me that there is rust on the segment that should be shiny due to the snap-off.

Yeah, I noticed both the rust and how that smaller piece is just laying there beside the part that's still in the block. The presence of that photo in the larger catalog does seem to imply that the survey team placed some level of significance on that example, but now I wonder "of what significance" did they intend it to mean? Did they want to show that there was a structural defect? or did they just see snapped rebar and become captivated by it? We'll probably never know, but it is pretty intriguing.

On closer inspection, that does look like mud, now that you mention it, Rob. I must have still had Greg's asphalt-peeling story in my mind when I saw that photo a while ago; I had noticed it back when the photos were released and it never occurred to me that it was asphalt then. I guess it just goes to show how things can seem when your mind is in a certain place (which may also explain what happened with the survey team and the rebar...)