Wind uplift on prestress

[barebear]

I am getting more involved with the installation of solar water and electric panels on existing structures. Normally it is a simple matter of transfering the wind loads to the ground with proper selection of connectors. I have been asked to design a installation on an existing prestressed hollow core roof system located on the coast with uplifts in excess of 50 psf. I am concerned that the uplift will put the bottom strands in compression during a hurricane. I can not however find any information on such failures in the past.

 

[ron9876]

I don't have any experience with hollowcore failing in uplift but I did see some Tee's that failed in uplift during Hurricane Andrew.

Are you sure that you are adding a net uplift to the planks?

 

[csd72]

I would check your sums as this appears unlikely.

4 inches of concrete is approx 50psf.

Have you included the weight of the screed over, have you reduced the wind pressure for the roof area?

That said, I have had similar winds uplift a 36' x 7" wall panel in a shopping centre - I had to tie the thing down!

 

[JedClampett]

It's one thing to have to tie a panel down, but another to worry about a structural overstress due to an uplift wind. In Florida, they use precast double tees and hollow core all the time for roof members. I think that the normal reinforcing they put in for handling is enough to resist the small moments from wind uplift.
If the roof is existing, is it OK by your numbers without your additional solar panels? If it's not, maybe you're missing something the designer considered.

 

[ron9876]

Like I said above I saw rooftop double tees that clearly failed in uplift. It was amazing. They were fairly long span and untopped. The prestressing applies an uplift which is additive with the wind uplift. If there isn't anything on top of them (like 2" topping) which has some weight you could end up with significant uplift and I don't think that they put reinf/strand in the top.

However if they are normal solar panels then you are probably not adding much if any net uplift.

 

[rapt]

It is not putting the bottom strands in compression that is the problem, it is the level of tension in the top and whether there is reinforcement to control this tension. Just because the concrete around the bottom strands is in compression does not mean the strands are. They will still be in tension but they will be in the compression zone and not resisting the uplift moments and stresses.

IT is a simple calculation, basically your transfer calculation with a wind load uplift effect added. If there is tension in the top, there must be reinforcemnt to control the tension and cracking.

 

[Ron]

ron9876...yes, those were amazing, particularly the Macy's store in Coral Gables. I have a picture of one double tee sitting on top of another. I also got an amazing example of internal pressure at the Sears store automotive section there as well. The uplift deflected the tie beam upward and the internal pressure pushed the masonry walls outward almost two inches before the tie beam came back down to lock it in place...displaced outward.

Hollow core panels have symmetric strands so they have good resistance to uplift as well as live loads. Their keying at the ends from the topping serves to hold them down as well as the relatively high dead load.