Is there a deflection limit for a sign pole? 1

[structure567]

Trying to see if there are any specific deflection limit for a 10' cantilever steel column supporting a 6'x 6' solid sign. I am considering L/120 since this is an exterior member without any finishes. What are your thoughts? Thanks in advance.

 

[phamENG]

There are a few things to consider here:

1) What is your solid sign made of? If it's a hollow box with lots of parts, the inner connections may not handle being whipped around too well. Same goes for electronic signage.
2) Related to (1), how heavy is your sign? If you have an aluminum sheet with the occasional stiffener, overall stability won't be affected too much. If you have a 3" steel plate, a little movement might create a secondary moment in your column that leads to unacceptable stresses in the column or instability in the foundation.
3) Where is this on a micro scale? Parking lot? Highway? Will people be close to it? If it's close to people and bouncing around like one of those crazy tube people at car dealerships in a light breeze, it'll probably get some complaints.
4) Where is this on a macro scale? Do you have to worry about earthquakes? Hurricanes? In these cases, so long as your not creating a secondary stability problem, deflection isn't much of a concern for signage structures.

 

[dauwerda]

AASHTO's Specification for Structural Supports for Highway Signs, Luminaires, and Traffic Signals may be a good resource for you.

 

[dreber]

If it's an AASHTO project, there may be limits. If it is a IBC project, there are no code prescribed limits.

If I were you, I would apply the ASD level wind load, calc deflection at the top of the sign (including some assumed deflection in the foundation) and ask myself it that much wobbling in a wind storm would be un-nerving.

 

[structure567]

Thank you for your response.

1) What is your solid sign made of? If it's a hollow box with lots of parts, the inner connections may not handle being whipped around too well. Same goes for electronic signage.it will be made of aluminum material. It will not be hollow
2) Related to (1), how heavy is your sign? If you have an aluminum sheet with the occasional stiffener, overall stability won't be affected too much. If you have a 3" steel plate, a little movement might create a secondary moment in your column that leads to unacceptable stresses in the column or instability in the foundation.The sign will weight at maximum 120 lbs, the thickness will be around 1"
3) Where is this on a micro scale? Parking lot? Highway? Will people be close to it? If it's close to people and bouncing around like one of those crazy tube people at car dealerships in a light breeze, it'll probably get some complaints.This will be on a very open area with minimal human and vehicle traffic. There is a single road going by it but it is placed at least 7 feet away.
4) Where is this on a macro scale? Do you have to worry about earthquakes? Hurricanes? In these cases, so long as your not creating a secondary stability problem, deflection isn't much of a concern for signage structures.It is located in California so earthquakes can be a factor but the wind is the governing load at this location.

As of now the deflection of the column is around 2 inches.

 

[BridgeSmith]

If it's a roadside sign, you may have to consider how much movement will make it unreadable. If it's bouncing around alot, can drivers still read it? Is it a retroreflective sign? Those only work within a fairly narrow range of view/reflection angles.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[phamENG]

1" thick aluminum plate? Interesting. That and the dimensions make it sound like a sign for an establishment and not a directional or informational highway sign.

If that's the case, I don't think I'd worry too much about it. L/120 at a 10 year MRI wind is probably sufficient.

 

[BridgeSmith]

Per the AASHTO LRFD sign spec. mentioned by dauwerda above, deflection under the Service II load combination for signs is 2.5% of the structure height, and the maximum slope at the top of the vertical supports of .35 inches per ft.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[skeletron]

L/120 is reasonable and below a practical limit of maybe 1-1/2" to 2". I think other problems start to unfold once you creep beyond those limits.

Also check the rotation as @BridgeSmith suggests. Seems like 2 degrees is probably a good practical limit.

 

[oldrunner]

Certainly want to check fatigue limits, particurly if the pole section is welded to a base plate. Most of your questions can be probably answered by the AASHTO code mentioned above. Even though the height is short, add in the P-delta moment just the same. How many years do you expect the sign post to be in this location? The sign will also be subjected to twisting or torsional loads also. (Assuming one pole with large sign above). What is the maximum wind velocity that you are designing for?

 

[rb1957]

6' square, 1" thick, Al weighs 500 lbs ! Maybe you meant a "thin" sheet with 1" stiffeners on it ? (this'd be < 0.2" thick for 120 lbs weight)

maybe mounting a sign on a unipole in a windy area isn't a great idea ?

be careful of Al and Steel together (corrosion).


another day in paradise, or is paradise one day closer ?

 

[phamENG]

rb- signs on single poles in windy areas is very common. Sure, it'll deflect and twist and do all sorts of other things that make it want to fall down, but then again - don't airplanes? That's why we're here!

 

[BridgeSmith]

We do aluminum sign panels and backing angles on (galvanized) steel posts fairly commonly and haven't had any corrosion issues. The biggest thing we have to watch is the bolted connections to the aluminum backing angles. Although the yield strength of the aluminum alloys used for structural shapes (35ksi) approaches that of steel, the modulus of elasticity is barely more than 1/3 of steel's (10,100 ksi vs. 29,000ksi), so it deforms much easier. Usually, that just means larger, heavier washers are required where a bolt goes through an aluminum angle so that the bolt doesn't pull through the angle. Occasionally, you have to upsize the bolt to get more bearing area at the perimeter of the hole in the aluminum.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[chicopee]

If this 10' tall sign post is self standing, for a tip deflection ,I would suggest +or-1 degree. Also double check the stresses from wind vibration.

 

[r13]

I remember an old rule of thumb for human perception on plumb-ness - 1%.

 

[structure567]

Thank you everyone for your replies!
To clear some other questions, I am only designing the steel column and foundation. The sign and the connector will be done by another engineer. This will be a deep foundation so there will be no base connection. Will I still need to consider wind fatigue and wind stress due to vibration? I have also designed the deflection using the 50 year MRI instead of the 10 year so I believe it is conservative.

L/120 is reasonable and below a practical limit of maybe 1-1/2" to 2". I think other problems start to unfold once you creep beyond those limits.

what other problems arises once the deflection goes over 2"?

 

[BridgeSmith]

You should check fatigue, but it's unlikely to control unless there's a weld near the base. The fatigue stress limits for plain sections are fairly high.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[chicopee]

treat the pole as a cantilever beam and analyze the stresses at the base. You will have bending, compressive, torsional(perhaps) and shear stresses.

 

[skeletron]

"What other problems arise once the deflection goes over 2"?" @structurure567:

Deflection is a serviceability limit state. 2" represents an arbitrary visual threshold limit for myself to start considering stiffening the element or calculating the additional variables (fatigue, vibration, etc).