School Roof Failure from Snow - Shoring?

[CBSE]

In the attached picture is a local school that has a massive drift on it's low roof. The school was built in the 50's. The roof beams are deflecting approx. 4-6", the wall is bowing outwards...assuming the columns are buckling as well.

Question, is there any "safe" way to shore this sucker up? We have discussed bringing in a track hoe and pulling snow off the roof, but are concerned that it will add additional weight and maybe cause the entire thing to collapse. We talked about a boom lift and a guy with a shovel, but if he gets close to the gym roof, and the low roof fails, then there is potential that the gym wall could collapse onto him.

I don't know how you would begin to design a shoring system to keep this post and beam building from caving in. I haven't seen it done, I'm sure it has been done, but I just have no experience with it. Is it possible? Trying to keep the low roof from collapsing and then pulling the gym wall with it.

 

[SJBombero]

I would think that a man on a boom lift would incur less risk than anyone working below the roof to shore it.

 

[TehMightyEngineer]

Looks like a similar setup to the roof failure I looked at a few years back.

First job is getting that snow load off of there without overload or injury. Definitely wouldn't put any equipment or people on or in the structure while doing this. I'd say snow removal from the ground would be safe enough, though. I'd get some of those long roof rakes and pull it off of the worst areas. Then you can see about a manlift getting in close or getting someone up there with shovels safely.

Once you get into the repair that's where I'd really worry. My job I only had one snapped joist and the rest were fine. Sistered on a new joist and we were good to go. For yours that might be the end of that roof.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[CBSE]

I like the idea of the rake. We have another big storm coming tomorrow so I'm having them use either a rake and a lift or a track-hoe (gently) with a spotter.

 

[TehMightyEngineer]

If you have another one coming have them install and strap down a foam & wood structure that will fill the drift space and prevent a drift from forming there. It wont look pretty but will keep the structure from collapsing.

After that storm blows through you'll probably be looking at demoing the finishes around the structure and performing a detailed inspection. I'd suspect you can do this safely without shoring but I'd not let the structure be re-occupied by non-essential personnel until you've come up with a shoring or repair plan.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[oldestguy]

I've used the snow rake and it works wonders. However, I'd mount the user on a dump truck so he has a place to put the long handle when the "rake" is at the eaves. Also standing well above the ground, he would be safer if something suddenly heads towards him. Maybe put two or three guys and trucks to work to even out the unloading. Aim he trucks at right angle to the building, because they can be tipped over also. Lacking a truck only hold the extreme end of the handle and walk back and forth. Safety can't be too extreme either.

 

[Compositepro]

The buckling columns make the situation look bad, but those cannot be structural columns. They appear to be posts for carrying wires from the ceiling to desks in the middle of the room.

 

[CBSE]

The bent posts are just conduit for wire, but it gives perspective to the amount of deflection in the beams. In any event, there's going to be some pretty good repairs on this one.

 

[dik]

I'd use a mobile working platform and snow rake to get the snow off the roof and on to the ground with no one below... deal with the snow on the ground later. I'd deal with getting the load off the roof before the storm. Can even work during the upcoming snow storm as long as winds are not an issue.

Dik

 

[CBSE]

That is the route we are going. They have a crew starting work here in about a hour or so. And people complain about engineers being conservative!

 

[KootK]

To play Devil's advocate, how sure are you about that 6" of deflection? My quick estimate indicates that, at 6" deflection, those electrical riser posts should be buckled out about 20". They look to be buckled out about 6" which would represent something more like 0.6" deflection. Or was there some slip capacity built into the posts that was exhausted enroute to the current situation? How long and deep are the roof beams?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Also, I bet those beams had 30-40 years to creep under dead load before those poor man's data center electrical risers showed up. The delaminatins are a little suspect too as they aren't manifested at the highest shear locations as one might expect if they were induced by excessive load.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

Your sketch show the high snow load exactly opposite of where you photo shows the drift. Electrical conduit chases have no load resistance so not uncommon for them to buckle. You photo shows no wall distress, though this appears to be some variant of a PEMB, so thrust is likely.

Is the snow wet or dry? If dry, you might be able to vacuum from roof. Sewer vacuum boom truck could handle that.

 

[kipfoot]

Although you've possibly resolved this issue by now, I'll add a couple thoughts:

-the photos don't look like 4"-6" deflection, so I'm interested if that's a number you measured yourself.
- if the wall is bowing outward, it may be an indication that there's a moment connection creating a frame over these classrooms.
- with about a 40' span (my guess), you might expect a 2" deflection and this deflection would probably be what limited the design. meaning that there could be additional capacity beyond the deflection limit.
-wind will keep depositing snow from the upper roof onto the lower one, but it will also blow snow off the lower roof. It looks like the roof has its max drift and therefore the wind is your friend.
-posts in the classroom will keep it until spring if you determine that it was really overstressed.
-rain in the forecast, and/or a couple slightly above freezing days, are/is a bigger problem than another snow storm.

 

[CBSE]

Update: A contractor removed most of the snow off of the roof. The contractor had stated 4-6" of deflection. I didn't personally go inside the building to measure because I felt it was a life safety issue and I did not want to put myself in that situation. I did a visual down the wall line from the outside, and yes, the wall had a slight outward bow in it. My best guess is that the bow was from the columns buckling under stress, no other explanation for it. I have not been in the building yet, but will be tomorrow, I'm guessing most of the deflection will be gone, but we will see.