Wind loads on vinyl fabric and rectangular steel frame

[mikesg]

Dear colleagues,

I have to design a rectangular frame 10m/3m (33ft/10ft) that will have a vinyl advertisement on it, it will be vertical and the big dimension is the height. I am not allowed to connect (e.g. tie) the fabric in intermediate lines of the frame, only on the edges. It would be easy if this was not fabric but something with flextural rigidity.

With fabric I am concerned about the tensile forces trying to pull together (especially the long) sides of the frame because of catenary action of the fabric.

I have calculated the expected tension based on the E modulus and the thicnkess of the vinyl by taking iteratively the "deflection" due to elongation, and comparing catenary reactions to the tension causing the elongation until I get a match between these.

I would like to recheck my calculations in order to be sure I have not missed something. Therefore I would highly appreciate if you could provide me with pointers where I can find relevant information for the calculation of such a frame.

Thanks a lot for your responsiveness!
Mike

 

[ishvaaag]

There is specialized software for fabric, tensile etc. Perhaps the more popular is ForTen 2000, 3000, adn 4000.

http://www.forten32.com/index.php?option=com_content&task=view&id=18&Itemid=40

Since I don't use it I don't know if it cares for aerodynamic effects; the slenderness of your advertisement may make it susceptible to dynamics effects so the loads will be higher than mere static from some equilibrium shape found the way you describe. It might turn that ForTen neither cares for these effects, I just don't know. For the fabric, follow the advice of the sellers; for the frame, go conservative, even if not overly so.

 

[MiketheEngineer]

Yes - these forces can be somewhat high. Often, the fabric or connections are designed to break away if the winds become too high.

 

[rb1957]

"catenary" = self weight, no? ... it'd be negligible, no?

won't you tension the cr@p out of the vinyl sheet, to make it taunt ?

wind load on a 30m2 "barn door" ... ideally you'd want the vinyl sheet to tear before the frame work fails. the vinyl sheet loads the two sides of the frame it attaches to.

 

[mikesg]

Thank you for your valuable responses!

@ishvaaag - I might give it a try, although at this point I would like to better understand the behaviour of this structure. But I will keep the link for future reference.

@MiketheEngineer - I was thinking about that, but I am not quite sure I have enough control over the guys who will install the fabric and the connection. The tensile forces (let them be T) I get are about 10 times the reaction normal to the fabric (V).

@rb1957 - I mentioned "catenary" not because of self weight (it's the wind I want to deal with), but rather to illustrate the fact that it is not rigid and IMO works more like a cable than a strut. I agree that it would be better to make sure the vinyl breaks before the frame, as mentioned.

Many thanks,
Mike

 

[rb1957]

that's possible (to get 10x in-plane load) 'cause the useful component is small. don't forget that the frame sees both components, and also the original tension in the vinyl (if fact, double the vinyl tension, no?, depending on how you tie it off)

 

[MiketheEngineer]

The vinyl will work as a cable - so the tension forces can skyrocket esp if they install "taut"

The basic formula is : H = wL^2 / 8d
H = tensile force at center
w = loading - plf
L = length of cable
d = sag

Watch your units.

So if you have little initial sag - you will have very high tensile forces very quickly!!

 

[mikesg]

As far as I have been informed they intend to glue it and there will be no pretension. I will include some pretension in my calculations in case they decide to go for tightening rope.

Thanks again, your responses were valuable and important to me!
Mike

 

[rb1957]

"glue" ?? well that'll fail first then !

i'd've used a hoop stress analogy ... stress = pR/t, load/in = pR
load/in is usefull for your glue,
R is the shape you think it'll deflect to

 

[MiketheEngineer]

Generally - these things are like big boxy rectangular signs. If it is glued, there may be no pre-stress - but there is little or no initial sag either!! When (if) they fail - they generally "pull-in" the longer sides.

And don't count on the glue to fail - some of the newer stuff is incredibly strong. I have seen wood, metal and cloth fail well before the glue did. I buy some stuff known as Hobby-Poxy a thickened "super-glue" with a catalyst drier. You put two pieces of ANYTHING together with that stuff and hit it with a little catalyst and ten seconds later you will NOT get it apart. Not your fingers either!!.

 

[SAIL3]

I can not see you getting there from here.Usually there is sufficient pretension in non-temp fabric structures to prevent flutter/fatique of the fabric and more importantly the supporting structure is specially configured from the beginning to efficiently handle the large tension loads required.
Your set-up of just a flat rectangle is not a very efficient
configuration.
Worse still, if there is no pretension at all, I can see the fabric just beating itself to death from wind parallel to the sign.
If the client is prepared to keep replacing the fabric when it fails, then that is another matter.
At a minimum, if the project goes ahead and you are still involved, I would search for an existing similar installation and find out how it performed in real life.

 

[azcats]

What is your frame mounting to? Are there any other options out there? Can you put a solid backing behind your frame?

Check the billboard industry. They have panel free or hurricane frames that are designed and pre-fabricated for just this purpose.

If it's going onto a wall, there are brackets designed to stretch vinyl against walls.

http://www.formetco.com/billboards_hframe.html

http://www.tiffinoutdoor.com/PanelFree/panelfree.aspx

http://www.formetco.com/saa_vinyalhardware.html

 

[csd72]

mikesg,

there have been a few good threads on almost this exact thing in the past with some very good advice given. Use the google search at the top of the page to look for these.

As far as checking what you have done, if you have a copy of roarks then there is a catenary formula at the back of the book that gives you a way of calculating deflection and then fabric tension. It does not allow for pretension but in my experience, the effect of this is small.

I would ignore the long direction and just treat this as a one way catenary across the 3m span.