downforce/windspeed when approaching rooftop pad

[wtownrfr]

I should start off by giving the background to my question. I work in the construction industry as a roofing system specifier, and I am currently working on an existing roof that has failed. The roof in question is in the +- 150 ft radius of a rooftop helipad on a hospital structure. Obviously I'm investigating why the roof failed and how to remidy the issue, my question would be.... would anyone have any source (specification/guidance) of what the windspeed would be @ roof level when the helicopter is landing/taking-off. Any idea of the presure from the wash?


Thanks

Andy

[jetflite]

150ft works out to be about 45m, In Australia we have requirements for rooftop helicopter landing sites (HLS), safety requirements, weight limits etc. i can track them down. But i think good information know is...
1. How high is the building (re: wind velocity)
2. Location (Sea side,inland,moutain,altitude etc.)
With regard to down wash, in my opinion i would say that would't be a factor...prob more static loads from the helicopters skids/landing gear. . on the HLS sureface, and the support systems that supports the load of 45m radius HLS, with helicopters from 1-2000KG coming and going...
And also by Failed do you mean - Cracking ? Callapsed etc ?

just my 2 cents.

JF

[wtownrfr]

1. the building is only 40 ft above grade, no extrordinary wind (inland)or altitude issues(>=500'). The roof is rigid insulation glued with a foam adhesive to the roof deck, then a rubber(singleply-epdm) glued to the insulation with a contact adhesive. The issues is that in certain sections the insulation is popping up from the roof deck, my concern was that with roof top HVAC units in the area, the down wash from the rotors would create area of negative pressure that could have effected the bond between the insulation and the roof deck.

[Torquetalk]

Stand to be corrected, but I think the pressure value of the helicopter downwash should broadly reflect it's weight, assuming no wind and a low in-ground-effect hover. Once the helicopter is above several feet, some of the downwash (and thus weight) will begin to escape or be distributed over a broader area.

Maximum pressure experienced by the roof should be as the helicopter lands: This would be equivelant to it's weight, plus the momentum of the helicopter as it is set down; but hopefully this force will not be great as it would imply a hard landing.

TT

[jetflite]

i agree with TT, as the rotorwash would become relitively insignifigant after about 1 rotor diameter, As it states in the Australian HLS criteria. . roof top HLS must be able to with stand the static loads of the helicopter...so if you have say around 2000kg of helicopter coming and going on a hospital HLS. . this would have more of an imapact on the surface.

If i understand you correctly parts of the roof have debonded enough to warrant a rebuild. . is it concrete ? (i'm not a construction wizz) or is it a synthetic surface ?

JF

[[email protected]]

It sounds like you just need stronger glue to stop the debonding.

[helmet fire]

You dont say which country you are calling from....

Most will have speciific experts/consultants that can answer these questions in a jiffy, and for something as serious as your project, I strongly advise you to get such advice. The last thing you need is liability associated with a piece of roofing going up through the rotor system of a helo full of sick kids, or pieces fallling down on the theatre patients!



But let me up jetflite's weights a little: the typical light EMS helicopter is about 3000kg through to the larger machines up to 6000kg.

[NickLappos]

Having provided such advice in the past, I can say with authority that TT has it precisely correct. The load the hovering helo imparts on the roof is its weight, which is spread across the entire rotor disk area so that the pressure loading is very low, typically from 3 to 7 pounds per square foot. Most roofs are designed for rain and snow loads, so thay bear from 30 to 50 lbs/ft sq. Therefore a helo has no impact on a roof's safety, since a helo is equal to about 2 inches of standing water, or less.

A friend in Illinois designed a rooftop pad structure that spread the skid/wheel load out across the disk area so that a helo could be landed on any rooftop in the state at a cost of about $10,000 or less for the wooden pad structure.

[cl12pv2s]

You might find some info in the FAA advisory circular on Heliport design...I don't know
AC 150/5390-2B Heliport Design (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/412DE833D6BAA71286256F340060C072?OpenDocument&Highlight=heliport)