Roof jumping upwards with no apparent reason
Roof jumping upwards with no apparent reason
(OP)
We have a large flat metal deck roof on 100 ft long steel joist @ 6 ft OC. The mechanical contractor has recently replaced 3 roof top units (Air Handling Units) with 3 new and lighter ones (approx 8 kip each). After the old units were removed and the new units installed several people working on the roof felt that the roof is somehow bouncing upwards (pop upwards) with no apparent reason. It is not vibrating. Just a small bounce and then nothing for an hour or so. We have no snow yet and the only load present is the dead weight plus the unit weight approx 8 kips shared by 5 joists about 15 ft from the edge.
Any thoughts?
Thank you
Any thoughts?
Thank you






RE: Roof jumping upwards with no apparent reason
If it had developed a "memory" through frictional forces due to rust, galvanic bonding of the decking material, etc., there could be some localized resistance to that movement. These localized areas contributing to the resistance to move are probably failiing over time, resulting in the popping upward felt, and stimulated to move by the work on the roof.
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Roof jumping upwards with no apparent reason
RE: Roof jumping upwards with no apparent reason
RE: Roof jumping upwards with no apparent reason
But that should only happen once, when the change took place about two months ago. They claim that it is happening daily if not hourly. Even though concentrated the RTU dead load change is very small as compared to the dead weight alone? Anyway, I will report back after I visit the site tomorrow and see if I can experience it first hand.
BTW - This roof was designed for 40 psf of live load and 20 psf of dead load about 20 years ago.
Thanks everyone
RE: Roof jumping upwards with no apparent reason
In college years ago I built a steel structure with a thin metal grating decking supported on all edges. The structure was sound, but I remember that when people walked on it, it would 'pop' in and out as the panel flipped between two different equilibrium shapes. I think to avoid this, you can either be very careful in your connections to not create any sort of out of plumbness/local distortion in the screws/puddle welds or you can possible create some sort of crimp action the keep the decking in one definite shape.
M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Roof jumping upwards with no apparent reason
I suspect the unit replacement is incidental and not related to this issue. It's probab;y been going on as long as the building has been in place, but nobody noticed it before.
RE: Roof jumping upwards with no apparent reason
M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)
RE: Roof jumping upwards with no apparent reason
RE: Roof jumping upwards with no apparent reason
According to the field guys this happens all the time with or without wind. As far temperature is concerned we have a range between 45-50 F at day time and 30 to 40 F at night. The roof in insulated and the building in heated.
Thank you everyone.
RE: Roof jumping upwards with no apparent reason
RE: Roof jumping upwards with no apparent reason
RE: Roof jumping upwards with no apparent reason
Your response makes sense. When I visit the site tomorrow I will check and see if this is happening when a door opens and when all the doors are open for a while.
FYI - Flat roof deck, on exposed steel joists with insulation and roofing membrane. There is no ceiling. It is heated and cooled by the RTU's.
Thanks
RE: Roof jumping upwards with no apparent reason
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Roof jumping upwards with no apparent reason
Dik
RE: Roof jumping upwards with no apparent reason
The only moving parts are the fans but they are isolated (resting on springs).
RE: Roof jumping upwards with no apparent reason
RE: Roof jumping upwards with no apparent reason
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Roof jumping upwards with no apparent reason
Dik