Roof without walls- How to design for wind?

Garwood · Mar 22, 2021

I want to make a flat roof covered parking area between two buildings. I have 3 heavy steel trusses 50' long and would like to attach 30-40 foot long sections of 18 or 20 gauge pan decking to create the simple 40ft x 50ft roof.

I got to thinking about how this would be a giant sail. Could get quite the lift in a wind storm. I was planning on splitting a 12 yard truck to do the footings, so 20,000+ lbs of concrete at each end of this tied into the monolithic building foundations at either end.

How do I design for the wind's potential to "lift" this roof?

Trboatworks · Mar 23, 2021

There is engineering out there which addresses it directly.

I started to design a similar size pavilion roof and got scared- the tie downs and connections from post to beam/beam to truss are going to see some load while as you mention the footers need to be right with correct attachment to post and able to withstand lateral load as well as lift.
I am in a 120 mph coastal zone but anywhere has to account for thunder storms.
If it were me I would find a buddy who does this sort of design work and run it past him/her.

We dropped a large steel building with huge hanger doors on this property- I was surprised at the wind load case which varies for open door vs closed etc.
It has been several years since I took a swing at it so I don't have the engineering stuff in front of me but you got it- wind lift can launch the roof and that roof can kill people.
I would be careful if this has any chance of seeing Joe civilian anywhere near it..

That said where we camp thermals run through the campsite (as they run through every spot of land on the planet..).
It can be wild- you don't see anything but as the vortex crosses the campground all the easy-ups etc take off.
One year a guy had a 40' long steel frame car port type tent with cinder blocks as weights to hold it down- the whole mess went airborne and sailed 100 yards till it crashed down- luck was there as no one was hit by anything.

Oops- I just became 'that guy' grumbling about hiring a damn engineer...

I know anyone of us can apply some common sense and probably get the thing overbuilt enough to do the trick but..
For some reason of late I have become more aware of and cautious about liability.

moonlight machine · Mar 23, 2021

Trboatworks said:
There is engineering out there which addresses it directly.

I started to design a similar size pavilion roof and got scared- the tie downs and connections from post to beam/beam to truss are going to see some load while as you mention the footers need to be right with correct attachment to post and able to withstand lateral load as well as lift.
I am in a 120 mph coastal zone but anywhere has to account for thunder storms.
If it were me I would find a buddy who does this sort of design work and run it past him/her.

We dropped a large steel building with huge hanger doors on this property- I was surprised at the wind load case which varies for open door vs closed etc.
It has been several years since I took a swing at it so I don't have the engineering stuff in front of me but you got it- wind lift can launch the roof and that roof can kill people.
I would be careful if this has any chance of seeing Joe civilian anywhere near it..

That said where we camp thermals run through the campsite (as they run through every spot of land on the planet..).
It can be wild- you don't see anything but as the vortex crosses the campground all the easy-ups etc take off.
One year a guy had a 40' long steel frame car port type tent with cinder blocks as weights to hold it down- the whole mess went airborne and sailed 100 yards till it crashed down- luck was there as no one was hit by anything.

Oops- I just became 'that guy' grumbling about hiring a damn engineer...

I know anyone of us can apply some common sense and probably get the thing overbuilt enough to do the trick but..
For some reason of late I have become more aware of and cautious about liability.

Race teams many times have a tent full length of the semi rig and 25' or so wide. Some teams weight the tent down with "T" size nitrogen bottles, I guess about 180 lbs each. I have seen wind take the tent and 6 nitrogen bottles and flip the whole works, bottles and all over the trailer. Our engineer calculated how much wind it would take to do it. I don't remember the speed but it was surprisingly low.....

gustafson · Mar 23, 2021

YOu would think there would be some clever trick aside from making it weigh more than the lift generated by 'x' speed wind to keep it from flying away. Like those flaps on nascar roofs that kill lift when you are suddenly going 180 mph backwards.

Scruffy887 · Mar 23, 2021

I tried to get a special truss made for my back porch and truss company said the 10' x 20' long space was beyond what they could calculate for lift. I built my own trusses and turned it into a sun room.

DDoug · Mar 23, 2021

I hate to sound like a broken record, but Eng-Tips Engineering Forums has a few discussions on this.

kustomizer · Mar 23, 2021

gustafson said:
YOu would think there would be some clever trick aside from making it weigh more than the lift generated by 'x' speed wind to keep it from flying away. Like those flaps on nascar roofs that kill lift when you are suddenly going 180 mph backwards.

Wouldn't that be fun to listen to in a windstorm.

We were at the big antique motorcycle swap and show in Davenport Iowa back when hurricane Iisiac was doing its thing, the wind took 100's of easy up tents, concrete blocks and all up in the air 40-50 feet, tied them in knots, shreaded them and shoved all up against the chainlink fence at the end of the fairgrounds all in about 10 seconds, prettty cool to watch.

I would think with as many free standing pole barns and carports as there are scattered around it can;t be majic but I would think if you took a nice dimentioned drawing with your ideas to an engineer it wouldn't cost more that a few hunfread bucks to have it right and mitigate the liability,

gbent · Mar 23, 2021

Open sided roof isn't to bad. When you put a wall on one side is when it starts to get sporty. 20 MPH wind is only 1 pound/ft2. But the pressure is squared as the wind speed increases linearly. That V2 gets you pretty quickly.

boslab · Mar 23, 2021

Ridge venting, roofs get sucked off due to pressure drop over them usually I used to think they got lifted, bit counter intuitive but that Bernoulli guy must be right long distance over short distance under whala an airfoil or wing or pile of wreckage in next doors garden
Mark

henrya · Mar 23, 2021

There are many, many carports built all over the country everyday. Some blow away in storms, some don't. I have to think that this information is available from a structural engineer. Who no one wants to hire unless they need to. This is a case where you need to. I would love to hear what you find out.

Illinoyance · Mar 23, 2021

Get a copy of ASCE 7
Minimum design loads for buildings and other structures.
Check all the connections for resistance to uplift especially the roof panels to the rest of the structure.

Almost forgot: You better check for snow load.

Rudd · Mar 23, 2021

Just as anecdata:
Down heah we design for say 137 mph - depending on where you are, could be a little more, could be a little less.
1 1/2 deep 22 ga. B deck is good for a 5' span on an enclosed building.

Scruffy887 · Mar 23, 2021

Just another 2 cents. I would not tie it to existing buildings with a lot of strength. If its going to go, let it go by itself and not take existing structures with it.

Garwood · Mar 23, 2021

Scruffy887 said:
Just another 2 cents. I would not tie it to existing buildings with a lot of strength. If its going to go, let it go by itself and not take existing structures with it.

The roof trusses could tear off and the rest of the buildings wouldn't notice.

I like lots of heavy steel and thick concrete.

I could build this roof with 70' long W30x116 beams instead, but that's an awful waste of 25,000 lbs of steel and I'd have to rent a machine to set them in place.

EmGo · Mar 23, 2021

Maybe you could design it to create downforce instead of up-load. Then you'd just need some beefy columns to support it .... and don't go out there in high winds

Phil in Montana · Mar 23, 2021

Might be better to add walls and doors...Phil

boslab · Mar 24, 2021

EmanuelGoldstein said:
Maybe you could design it to create downforce instead of up-load. Then you'd just need some beefy columns to support it .... and don't go out there in high winds

I suppose downforce could be generated by having a flat top roof and a curved under face so the air has further to go under it than over it, like an airfoil or upside down wing, I might have got that completely wrong btw as I don’t know a great deal about aerodynamics, I’m sort of applying fluid flow from college years ago, I still find venturimeters fascinating ( big pipe, transition small pipe reverse transition big pipe, then we got to fabricate one and weld it, I enjoyed that)
Would an upside down wing push down or suck down or whatever the heck they do, do wings get sucked up or pushed up, is it lift or suck that gets planes up in the air, still undecided, are the falling if they fly upside down, or it could be magic
Mark

dian · Mar 24, 2021

i dont think this would work well. winds will not always be steady and in one direction.

boslab · Mar 24, 2021

Cannot fault your logic, making an upside down flying saucer would definately make its way into the architecture magazines where everything is completely impractical.
Mark

Street · Mar 24, 2021

usually wind will have a dominate direction that it is strongest in.

Yes it will have some uplift but not as much as a 3 sided building as there is no containment of forces, as to how it acts like a sail i have not gone into that area.

definitely some more reading required on how to do this safely but a design engineer may just design it like a normal roof without internal pressure differential ( like a closed semi closed building ) but i suppose the underside could have airfoil issues.
Just get a designer to have a look at it 50 foot is quite a span.

Roof without walls- How to design for wind?

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