Deflection of Sign Structure

[CTW]

I'm working on a sign structure which consists of a pole and a cantilevered arm (both HSS round members), and I've confused myself with the deflection. The deflection of the pole is understood, but should deflection of the arm be considered? I would think the structure would deflect as a whole since it's freestanding, thus making the deflection of the pole critical and sizing the arm for strength not deflection.

 

[epr]

Most definitely.
Depending on the size, the deflecton could be unsightly. For instance, a single-pole, full-flag billboard, with a face of 16' by 60' would look visibly distorted with only a couple of degrees of rotation of the cantilevered arm. If the arm is extended even further, the deflection could be considerably larger. I remember seeing an extended cantilevered arm on a billboard at the Walt Whitman Bridge in Philadelphia. The deflection was so large that another post was added (after-the-fact)to support the cantilevered end of the sign. (No I did not design that one!)
However, if the sign is small, a couple of degrees of rotation may not even be noticed.

 

[CTW]

The signs are relatively small so the dead load deflection due to the signs is not significant. What I was wondering is should the deflection of the arm due to wind be considered? Maybe I'm making this more complicated than it is, but my thoughts are that when the wind hits the sign, the pole and arm deflect together. The arm would not deflect very much on it's own without the pole beginning to rotate. Therefore, a higher rotation can be tolerated on the arm than on the pole.

 

[ChipB]

You probably already know this but....
Make sure you check stresses due to torsion as well.

 

[epr]

There are two different actions to be concerned about with a cantilevered sign under wind loading.
The first is lateral deflection of the vertical post, and the second is torsional rotation of the vertical post combined with lateral deflection of the cantilevered arm.
And also note that the center of the wind force is not at the center of the sign face in most building codes, it is usually located at 1/3 from the edge of the sign face.

 

[bvi]

Don't forget the basics! Just calculate the deflection using virtual work. Two elements, about a five minute hand calculation, if that. No need to guess. If you are very uncomforable with this, use a computer program like STAAD.