Justin,

An arch is about the strongest form you can have, as far as compression loading is concerned, and your depiction shows that it would be in compression.  Thus, I would say you don't need tremendously heavy stock to support the letters. You don't say what the letters will be made from, so I can't venture to suggest an actual size without that knowledge.  Also, for side (wind) loading purposes, you will need to calculate the total surface area of all the letters (approximately) and add them up. The total square footage will determine how much wind loading there will be on the sign.  You should probably figure on a design load of 35 pounds per square foot for general purposes.   Knowing the square footage of the exposure you can calculate the necessary section modulus of the member resisting the loading, that is, the arched piece they're attached to.

Just offhand, I'd say that if you used 1-1/2" x 3" 11 gauge rectangular tubing you'd be pretty well covered, unless the letters are bigger/heavier than they look. If you want to be extra sure go to 2x4 tubing.  You might even be able to get away with using a piece of 3/4"x4" alloy 5160 spring steel such as is used for semi-trailer springs.  Even annealed that should give your bender some challenge. :-)

Note: the arch configuration is inherently strong so you want to bend the rectangular tubing the "easy" way so that the wider section is resisting the side (wind) loading.

Disclaimer: I am not an engineer, nor have I done any calculations on this and you are solely responsible for any consequences of relying on my guesswork. Sorry, the lawyers make this necessary!

If in doubt, take your finished, accurate drawing to a civil engineer and have him/her do the necessary calculations to determine the size of support members required.  It shouldn't cost more than a couple hundred bucks and would be damn good insurance in my opinion.  We live in too litigious a society these days to take unnecessary chances.

Rich

P.S.  My biggest concern, in looking at your drawing, is not the arch but rather the design of those 14-16 foot wide gate leaves.  I see no diagonal member or other element in your design  to resist sagging.  That's a really wide leaf and will exert tremendous force on the hinges and the horizontal members.  If nothing is done to brace it against sagging it will end up drooping in the middle and causing problems.  If these gates will be operated by an automatic opener it is even more critical that they be rigid and properly hinged (i.e., lower thrust bearings and upper radial thrust bearings).  I hope that the upper hinge points will be thoroughly secured into poured, reinforced concrete, too.   Iron gates are heavy and the forces involved are considerable!  If you're making this of hollow thin tubing that mitigates some of the issues but then you have the issue of early failure due to rusting of thin members.  Tanstaafl.

rsw

Wow..great feedback! More than I can digest at first glance :-)

Planning on using 3/16 plate for the lettering. The gate is actually pretty light material 1/2" tubing and 2" channel to negate sagging as much as possible and I will be spraying it with galvanizer first to ward off the rust for as long as possible. The gates will remain open throughout the day due to high traffic volume and will be closed manually at dark. I am still going to make provisions for the future installment of a gate opener system should they choose to go that route down the road.

I will sleep on this stuff and check back tomorrow

From the abundance of the heart, the mouth speaks. Thanks for sharing!

I've taken into consideration the great points that have been made.

My line of thinking is to use the collar tie and mount the arch to the top of that and eliminate a lot of the uncertainties involved with just a lone arch span.

My question now is this...what material should I use..let me rephrase that: in your opinion, what stock of material would be sufficient to use as a collar tie in this instance that would not be subject to sagging?

Thanks again,

Justin

I think you are wrong- Exactly what you need is an engineer.

This is what they are for.

If you wing it, and the thing blows down in a windstorm, and lands on somebody, you can pretty much kiss everything you own goodbye.

A few hundred bucks spent on a structural engineer would be a very cheap price to pay to know that liability was shared, and the likelihood of collapse was actually calculated by somebody.

If you were doing this for yourself, that would be one thing- but on a paying job, I think you are crazy not to get this engineered.

And nobody, engineer or not, is going to be able to make very accurate calcs without knowing how big and what material the vertical posts are, what kind of footings they have, what your local wind loads and seismic factors are, and if snow is a problem, and so on.

What works perfectly well in one location might be seriously overbuilt in another, and crazy wimpy in a third.

If the job doesnt have a few hundred extra to pay an engineer, you underbid it anyway...

36 feet of ANYTHING is one big honking span. And, since steel comes in 20 footers, there are connections- which are additional failure points.

Thanks for the insights, Reis. I'm located in central Ohio where we don't have much to worry about in the way of weather and earthquakes..besides snow, I can't imagine a whole lot knocking this thing over besides a tornado. I have been considering having an engineer look into this all along, maybe I should just bite the bullet and do that.

Very Informative and interesting discussion!