Design Large Sign for Wind Loads

[HauntedMines]

Howdy -

We're a small nonprofit that hosts a single annual event. For advertising this event we construct a large sign on a hilltop that's visible from the nearby highway. In years past this sign would last a few weeks before it would be destroyed by wind. Obviously our guess on how strong it needs to be were inadequate; thus now it's time for the math...

We'd like to know (1) how to estimate the wind loads on the sign, and (2) any comments on our proposed new design.

Site characteristics: Exposure C or D - a grassy hilltop, continuous winds of 60mph are common, gusts of 100mph are common. The prevailing wind is onto the back of the sign.

History: The http://hauntedmines.org/pics/h2006-2007-build-sjr-0884.jpg was block letters cutout from OSB (like plywood), each letter 8ft tall. So it's not a solid billboard. These were attached to 4" round wood posts, with the letter bottoms about 4ft off the ground. The failure mode was that the letters themselves snapped (we saw that one coming!), and on the bigger letters the 4x4 posts snapped just above ground.

Second year's sign was build from 4x4 posts, using 2x4 diagonal braces on the back. The letters were attached to a big wood grid approx 96ft wide, with the bottoms 6-7 ft off the ground. See http://hauntedmines.org/pics/h2008-build-080930-143904.jpg. Failure was that the braces pulled up & out from the back and the 4x4 posts snapped like toothpicks. The letters themselves survived OK.

Third sign we built from 6x6 posts, using 2x4 and 4x4 diagonal braces on the back. It looked like the prior year just a bit bigger. We were feeling (over)confident so we made 10ft hi letters, and the top of the sign was about 18ft off the ground. We also started adding 3/16" steel cable guy wires to front and back of each post going 45-degrees down to the ground. We only got three sets installed before a huge winstorm came. Failure mode was the un-guyed posts snapped just above ground, splintering the 4x4 diagonal braces in tension! The three guy-wired posts fractured near the centers but remained standing, like one might see under a buckling load.

Wind Load Guess: Now for some math - and the needed help. Looking around the web for wind load formulas, I'm estimating a 100mph wind would put about 30psi onto the back of the sign, as a uniform load. I figured the cutout letters would be about 2/3rds of a flat plate. Is that about right?

New Design Idea: We're poor, so it has to be done cheap. OUr new design will increase it to 12ft letters on a frame 120ft wide, 20ft high at the top. We found that steel beams aren't too costly. So we plan to use seven 20ft 6W9 steel I-beams set on concrete piers, 20ft apart. The grid for the letters will be four steel cables stretched taught, running horizontally spaced 4ft apart, and we'll clamp the letters to the the cables. Each post will be guy-wired 45-degrees down to ground anchors front and back.
And the ends, posts will also have guy wires off to each side so the whole frame doesn't rack left or right. See http://hauntedmines.org/pics/hm2012-Giant-Hill-Sign.png.

Comments and ideas welcome. Thanx!

- Steve

 

[HauntedMines]

Sorry, that's 30psf (lb/ft^2), not psi, for my guess of the wind pressure.

 

[Studiot]

Hello HauntedMines and welcome to Physics Forums.

This is really an engineering issue and would do better in one of those forums. It may be worth clicking on the 'report button' and asking a Mentor to move it!
(report can be used for good as well as bad)

Some comment on your design.

I have doubts that you will be able to successfully tension your for cables sufficiently along the spine of your uprights.
Your stays are really in the wrong planes, being set up to resist the wind.
If you have seen or done any wire fencing you will have noted that substantial shoring is required at the end posts, in the direction of the strained wires.
Further the fewer uprights you have the larger they will be and the greater the windage they will offer and the greater the leverage on the foundation blocks.
The problem with wind is that it is fickle so that one instant it blows strongly and the next it drops, and the next it blows again. This has the effect of cranking the uprights back and fore against the founds with great leverage as you have discovered.
Are the bottom hinges for ease of removal (they are difficult to erect) or to help offset this working of the stanchion by the wind?

I would recommend considering using a greater number of smaller uprights, more closely spaced.
Instead of wires I would recommend weldmesh which would provide easy fixing points for your letters and offer minimal wind resistance.
Cast the mesh into a continuous strip of concrete you could do away with the stanchions alltogether.

Standard scaffolding would also offer low wind resistance and be suitable.

Finally you could drill some holes through the letters, reducing their windage.

An interesting project.

 

[HauntedMines]

Hi Studiot -

Thanx for the advice to move this to another forum. I don't find a "Report" button, maybe because I'm the original poster of this thread (or just tired!)? Moderator, please move this thread to a more appropriate forum (General Engineering?). Or someone report this for me if you could please.

Re: The hinges - that's from a non-technical requirement. Our contract with the event site requires that we take the sign down soon after the event, so it's only erected for a few weeks a year. Our idea was to winch it up when needed, and lower it when done.

Re: horizontal cable tension - yeah, that was a definite concern. Plus you made me realize that the wind will bow (deflect) those cables as it pushes on the letters, tending to draw the posts together adding tension to the ends. I didn't consider that force. A quick remedy does come to mind, and that's to add a rigid steel beam across the top.

But then, I like you idea of the weldmesh. I'll look into that. Tho I don't understand how it would stand on it's own without posts (stanchions) and cables to hold it upright.

Thanx!
- Steve

 

[Danger]

I roger that welcome.
While your scale is somewhat beyond my experience, I worked in a sign shop for several years. I have a recommendation that might not fit your intent or your budget.
That is to use a banner rather than a rigid sign. They're made of a thread-impregnated polyvinyl material that is incredibly strong. Adhesive vinyl lettering is then applied (5 year outdoor lifespan).
The trick with that is in the handling of wind. I live in the Chinook corridor, so I know wind. We cut U-shaped slits in the material, including through the letters. In reasonable conditions, the sign appears absolutely normal. When the wind kicks up, it blows the flaps open and passes through rather than huff and puff and blow the whole sign down.
It might not be appropriate for your circumstance, but I hope that it helps a bit.

edit: It just occurred to me that the same basic ideas could be applied to a sign made of something like plywood. In place of the slits, you could cut flaps and reinstall them with hinges.
Or... maybe make a rigid sign, but mount the uprights on honkin' huge springs, such as from a railway locomotive truck system. Or... hmmm... I'll get back to you.

 

[Studiot]

By the way, everyone here knows Danger is obsessed with flaps.

 

[Danger]

:tongue:

 

[Studiot]

Given that you want a demountable structure I would suggest an open framework structure, scaffold is ideal and there are lots of specialist components available.
A 3-dimensional arrangement of some sort will take care of the wind. Either change your 'I' beams for scaffold lattices or erect square scaffold 'towers' that are self bracing. Either can easily be clamped/unclamped to hook bolts partly buried in a concrete foundation.
The horizontals could be weldmesh or scaffold lattice or formed from scaffold tube slung across the towers.

In the past I have erected similar structures on motorway overbridges, which are slightly longer than your distance.
The company I have used in the UK specialises in temporary rigging/stands etc for open air events -pop concerts sports meetings and so forth.
I would estimate that you should be able to get a scaffold solution erected/demounted in about 6 hours, no crane being required.

Presumably you can leave the concrete bases ready for next year.

Don't forget to grease the cast in bolts against the weather when you leave.

go well

 

[HauntedMines]

Thanx all for the good ideas.

I'll bring these up at the next board meeting which is this week. Using scaffolding seems like a simple and inexpensive answer, but I'm uncertain if our host site will allow it; they're picky about aesthetics. But I do know they'll allow us to leave concrete footings/anchors up on the hill year round, since they won't be visible from the main museum site.

I'll also check into a 12fx x 120ft vinyl sign with *flaps* ;-) And hinges are cheap so we could also hinge & spring our giant letters to let them swing open in the wind. Hmmm... I wonder if that counts as a moving sign? If would get more attention!

I'm still wondering - is my guess of 30 lb/ft^2 about right for the 100mph wind load?

- Steve

 

[256bits]

You do realize you are dealing with a substantial force here with the wind.

30 lb/ft^2 sounds about right in the ballpark.

Your I-beam weighs about 20feet x 9pounds/foot = 180 pounds.

area = 20 x 120 = 2400 ft^2.
Force = 2400 x 30 = 72,000 pounds of force on the whole sign.
Yes, I am using the complete area as you do not jnow what could be blown from the wind and get hung up on the structure, and it gives somewhat of a 'safety' factor for local gust on a member and wind swirl -

With seven I-beams that it about 10,000 pounds to each I-beam.
With a W6x9 beam I get a deflection of 3.5 inches and a stress of 83000 psi ( this stress may be over the limit for you I-beam -assuming 60000 psi ) maybe some one can check those figures to verify - beam simply supported at both ends, uniform load.

Also, some design criteria you may want to consider, as the design relies solely on taught cables as support for the members being upright
- if any hinge at the bottom fails, and as the i-beam cannot support itself, and load is transferred to adjacent members and the whole structure could fail.
- if anyone of the cables fail, the load is transferred to adjacent members and the whole structure could fail.

it would be best practice to secure each I-beam securely to the concrete pier with some sort of flange, ( without a hinge ) so it stands on its own. Or have a hole in the concrete about 3 foot deep to imbed the i-beam into when assembling. It is not really ease of assembly that you should be most concerned about but rather if your structure can withstand the wind force.

You may also want to reconsider the 1/2 inch j-bolts and beef that up to inch and increase the length into the concrete so that each bolt on its own could take the load there in case anyone or all three others fail. I have not done a calculation but half inch does seem somewhat inadequate, so this is just a suggestion.

You may want to consider adding cables third the way down and/or two thrirds the way down from the top as that is where more of the stress/strain is in the i-beam. The actual top is rather stress free and what you are doing with the one cable is constaining the top but not intermediate locations.

Edit - In any event you may want to inquire with structural engineer in your local area about the design of the structure - and you may be lucky to have advice for free since you are a non-profit organization.

 

[HauntedMines]

Thanx 256bits et al,

I appreciate the design load advice and the deflections, etc. It is all sensible.

I'm researching design changes along several parallel lines:

• Increasing from 7 posts (stanchions) at 20ft spacing to 13 posts at 10ft spacing (significant cost impact for the added concrete piers, but maybe still viable if we can get donated concrete)
• Using a steel beam across the top of all posts to maintain tension on the support lattice - the cables or weldmesh (relatively inexpensive)
• Using weldmesh instead of cables to support the letters (having difficulty finding a local supplier for the size we need, but still looking)
• Using a large vinyl banner with wind flaps instead of plywood cutout letters. Best cost we got so far was about $1000 (half the budget).
• Using construction scaffolding as the support structure (still investigating costs)
• Cable supports at 1/3, 2/3, tops of each post (cheap and easy)
• Increased hinge size and mounting bolts if we stay with hinging (cheap)
• Add short safety chains/cables as a fail-safe for the posts if the hinges fail, to keep the posts from swinging violently in the wind should a hinge fail (cheap and easy)

I've rejected the idea of springing the support posts or using elastic cables, because having large heavy steel beams swinging around in the wind is a bit too risky for my taste! Sorry Danger! But it would look cool...

I'll post a revised design once I get more costs in, feedback from the rest of my group of volunteers, and secure possible suppliers. And we'll tap a local structural engineer for his opinions before we actually build this.

Thanx again!
- Steve

 

[Studiot]

One further thought.

Instead of erecting one long rack for the letters why not individual frames one for each letter?

That would significanly reduce windage and make stabilisation much easier.

A (say) 6 foot cube of scaffolding clamped down or weighted down with kentledge to a concrete strip for each letter?

 

[256bits]

And another further thought...

If you make your signs out of something such as expanded metal mesh,or similar, the wind load should reduce considerably. From afar the letters should be deciferable.

Or have the leters painted onto a square/rectangulat mesh. Close up you would not see a letter, but again from afar a good contrast between the unpainted ( or a background paint ) versus the letter paint should again bring the letters out, depending on the size of the mesh.
I guess that you would have to test out with a letter on the hill before a full commitment for a whole sign. The eye-brain would see a solid object when in fact the object is perforated.