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Supporting a 30' sign

Supporting a 30' sign

Supporting a 30' sign

(OP)
I am a mechanical engineer and have been given some sturctural work to do.  
We are erecting a 30 ft. tall sign that is 8 ft. wide and about 15 in thick. It's one of those large sign that you see outside of a plaza/stripmall.  

It's supported by two square posts that are about 12 in X 12 in. I need to know what calculations I need to do and if possible where to find the equations.  

Unfortunately I haven't done any sturctural calculations since second year of university so I'm a little rusty.

Any help is greatly appreciated as always

Thanks

Marc

RE: Supporting a 30' sign

You need to calculate your wind loads, which would be described in your local or state building code, then design the columns and footings appropriately.
Ethically, if you're not sure what you're doing, shouldn't you find someone who is?  Like architects "designing" structures.....

RE: Supporting a 30' sign

It is basically a cantilever fixed on the ground. If the gravity loads are small, it will be nearly entirely governed by wind loads that you may take from the applicable code, say IBC, UBC or ASCE 7. If you want an outright upperbound in non-tornado areas take 300 kgf/m2 or 60 psf.

If you want an exact evaluation you better make a structural model contemplating everything at hand.

For the simplified thing

you will determine moment on every of the stems by

M=q·L^2/2

where M is the governing moment at the service level
q is the tributary wind load per unit of length (here height), factored or not depending which code you are thinking to use.

L is the whole length (total height of the cantilever)

Then you need to check your box shape in accord with ASD
equations for flexure, or chapter F in LRFD.

If you contemplate the vertical loads you will use chapter H in LRFD or the equations in ASD for flexocompression.

Then you will have to deal with te foundation. Bolt forces can (for a moment only thing) be derived by dividing the moment at the base by the arm. If the base plate is engaged in the footing the bolts only will suffer tension and compression, that is, if bearing on the concrete byu the plate is enough. Proper engagement/embedment of the bolts is required for the proper functionality of your structure. As an alternative an encasement of your tubes or a structural stub able to pass the stresses to the foundation may be provided. This can be a shear tab or a shear and moment inclusion.

Then the footing will need be checked against slippage and overturning. Use a safety factor of 1.5 against slippage and 2 for overturning.

If your cladding is very sensitive to breakup, you will need as well make a proper check on deflection and distortion along the beam in order to ensure that the cladding won't crack.

RE: Supporting a 30' sign

pylko

you must refer to your country.

In Spain the architects not only design the structures, they are liable of not doing if failing to so do. We (architects) are to all effects the structural designers and engineers of record for everything that is residential, high rise included. Of course, we are trained to so do. And we are doing it efficiently according to our codes, I don't see our buildings falling.

RE: Supporting a 30' sign

See ASCE-7, 6.5.13 Design Wind Loads on Open Buildings and Other Structure.
The Equation I would use is:
F = qz*G*Cf*Af (lb)(N)

where
qz = velocity pressure evaluated at height z of the centroid area Af using exposure defined in Section 6.5.6.3;

G = gust factor from Section 6.5.8;

Cf = net force coefficients from Figure 6-18 through 6-22; and

Af = projected area normal to the wind except where Cf is specified for the actual surface area, ft2 (m2)

RE: Supporting a 30' sign

Wind loads will almost certainly control unless the sign is unusually heavy.

Where is the site?  (city, state only)



Please see FAQ731-376  by VPL for tips on how to make the best use of Eng-Tips Fora.

RE: Supporting a 30' sign

(OP)
Thanks for the help

I have already begun looking for an architect or structural engineer who is local and has designed similar signs before.  Obviously this will be necessary to get the permit.  

I wanted to get a head start and already have the simple calculations done to move the project along quicker.

Thanks again....now at least I know most of the calculations I need to do.  

RE: Supporting a 30' sign

If this sign is located above or adjacent to a roadway it should probably be designed in accordance with the "AASHTO design guide for highway signs and luminares". This design guide is fairly thorough and covers topics from calculating loads to designing the supporting members.

A geotechnical analysis is a good idea to ensure that adequate post embedment is provided to prevent overturning of the sign structure.

Good Luck.

RE: Supporting a 30' sign

Hi Mustangs,
   As you can see the problem is not so easy.  Most large sign fabrication and erection companies such as Plastiline, etc. keep a P.E. on staff or retainer to handle the public safety considerations.  There are also some tricks in design used such as fusing the sign panel structure so it blows out at a lower critical dynamic pressure before overloading the primary structure or footing.  You should get an experienced P.E. involved sooner than later, because many regulatory authorites do not allow their registered P.E.'s to step in and cover a junior engineer's work.

RE: Supporting a 30' sign

Okay, guys.... I have designed many signs that varied in height from few feet to about 100 foot high with bill board that is 30 wide and 60 foot long.

Design of signs is tricky. You must know and use the proper wind loads and wind code provisions.

Then you must be familiar with thee sign industry construction practices. They do things a little different from a structural steel building.

Then you have good knowledge with soil conditions ant the site. From there you must determine the type of foundations (spread footing that will resist over turning moment or us a drilled pile type).

I suggest that you hire a competent engineer to help you with this task. Public safety is number one concern and sign failure can be disastrous.

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