Roof jumping upwards with no apparent reason 7

[ellas1]

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

 

[msquared48]

Well, with less load, the roof structure will want to go up.

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

 

[ishvaaag]

Look also for temperature effects ... some steel structures move and make noise on the daily thermal change.

 

[MiketheEngineer]

Tell your guys to stay off the roof?? Problem solved??

 

[ellas1]

msquoared48

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

 

[IsaacStructural]

The entire roof is jumping? or just some local spots?

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)

 

[hawkaz]

If this is occuring daily, I'm on board with Ishvaaag. Probably thermal. Calc out the thermal expansion in 100' long joists- it will be significant. They should have provided relief for the expansion somewhere.
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.

 

[IsaacStructural]

I agree Hawkaz, this has probably be ongoing. I'm think as the panels heat/cool, or possibly just from the workers walking on them, they pop in and out from convex and concave shapes.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[SteelPE]

What about air pressure changes? Maybe either internal or external. A slight gust of wind comes lowering the air pressure on the outside causing the roof to lift slightly?

 

[ellas1]

SteelPE

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.

 

[SteelPE]

What about changes in pressure on the inside of a building? I assume it's a large open area that is under the roof? What is someone was opening and closing a door every once in a while creating pressure changes in the space below? Just a thought.

 

[hokie66]

I think SteelPE is onto something. Pressurization of the interior could do this...after all, the air handling units have just been changed. When you say this is a "flat metal deck roof", is the deck the roofing or is there roofing above? What kind of roofing? Is there a ceiling?

 

[ellas1]

hokie66

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

 

[paddingtongreen]

We had a similar situation once. I suggested that it only seemed to be new behavior because they weren't monitoring the roof before the modification. They checked with some of the workers who confirmed that it was the same as before.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[dik]

Any chance this can be a dynamic problem with the new equipment causing harmonic issues? Are there any moving parts? and can you modify it to change any fundamental periods of vibration?

Dik

 

[ellas1]

dik

The only moving parts are the fans but they are isolated (resting on springs).

 

[azcats]

Could the HVAC units starting provide a pressure drop/raise in the space below? Perhaps the airflow isn't balanced between rooms? Anecdotally, I've seen residential doors close when A/C units start up.

 

[msquared48]

Did some Mexican Jumping Beans escape?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

Could it be the air?... was called in on a problem once where a vibrating screen was in harmonic with a pre-eng bldg and the entire roof was shaking... solution was just to change the vibration of the screen... it was the air that was moving and forcing the roof vibration.

Dik