ASCE 7 Air Permeable Cladding Reduction for MWFRS Pressure

[trou33]

Does anyone have a design approach for a reduction to the MWFRS wind pressure if a cladding is used with a given percent openness that involves more than just reducing the pressure based on the percent open?

Here is the situation: We're designing a facility to be hardened for an extreme wind event (165mph wind speed, exposure C, +/-0.55). A large portion of this facility has rooftop equipment (that cannot be placed inside) that is to be protected from debris in the design event. Protection from rain or other common elements is not necessary - only things like large hail and other flying debris. To provide this we're designing trusses to span over this equipment with intermediate framing as a canopy type structure and using press-locked bar grating for protection.

The bar grating itself and the connections to the framing is selected using C&C loading, which from ASCE 7-10 30.1.5 does not permit a reduction to be used for air-permeable cladding unless test data or literature is provided. However, a reduction to the MWFRS pressure on the structure is what I'm curious about. The design wind criteria is resulting in significant pressures (specifically roof uplift) that can certainly be designed for, but if I can reduce these at all, there would be a cost savings.

Recent studies on the matter would also be appreciated.

Thank you.

Make no little plans.

 

[phamENG]

Not without testing of some kind. Bar grating is pretty dense, so my guess is that it will act more like a solid surface than not. You'll have wind pressure directly on the bars, but then you'll also have drag on the vertical faces. Lattice frames can get a good reduction because the ratio of open area to exposed surface for drag is really high (so drag becomes a minor factor). Bar grating is the opposite, so even though you have 75% reduction in area, you may only have a 20% reduction in wind loads. Hard to pin down without either testing it or building a model to determine airflow and drag (which is outside the abilities of most of us structural folks).

 

[JAE]

Agree with phamENG,
The percent-open ratio doesn't correspond to the actual pressure very well. Depends on spacing and the shape of the bars, wires, or whatever you have....thus the testing requirement.

This also suggests why FEMA 361 and ICC 500 commentary suggests that parapets and other stuff on or around the roof is a bad idea for high wind storms.