Question on Wind Loads 1

[AggieYank]

I posted this in the Civil Code Issues, but apparently no one frequents that, so I'll post it here:

This is a quite common situation, but one I'm not familiar with. How do you calculate the wind load on an irregular structure such as a frame with a sign in it (ex: a sign over a road). I would think the analysis would, according to ASCE 7-02, be in line with 6.5.13. My boss isn't comfortable with that since he isn't familiar with this also, and wants me to use the entire area of the frame and the wind load from a simplified analysis (method 1).

 

[MotorCity]

AggieYank,

I do not have a copy of ASCE 7-02 in front of me, but at the very least I would apply the wind load to any portion of the frame which a sign could be mounted to. It would also be prudent to increase this tributary area if the sign could extend beyond the frame.

You may want to have a look at your DOT standards or AASHTO also.

Good luck.

 

[broekie]

The reference you are probably looking for is the "Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals".

Wind load is basically applied to the cross sectional area of the frame + the signs. In addition, there are fatigue criteria for the frame/supports that need to be evaluated due to galloping, vortex shedding, natural wind gusts, and truck induced gusts.

NCHRP Reports 469 and 494 may also be helpful. Some of the NCHRP reports can be downloaded for free, if I remember correctly, at

http://www4.trb.org/

 

[chichuck]

I think you are correct to use Sec. 6.5.13. Then use Figure 6-21 to figure the correct Cf value. Since the flat signboards cover some of the exposed area of the lattice support structure, there is some uncertainty about what value of epsilon to use iln Figure 6-21. I would use only the parts of the support frame that is not covered by the signs to figure epsilon, figure the Cf value for the frame. Then figure the Cf value for the signs only by using Figure 6-20. Note that for most signs, the Cf values will probably be about 1.2 or 1.3.

As an alternate approach, you could include the frame and sign together to figure epsilon, and probably get a slightly higher epsilon and thus a slightly lower Cf value for the combination signs/frame. You'd have to ignore the fact that the flat signs are quite different from the flat-sided or round frame members. With this approach, I'd still suggest using Figure 6-20 to figure Cf for the signs themselves, and apply wind loads to them separately.


Regards,

chichuck

Its hard to know which approach would give the higher wind loads, but I suspect they would not differ by a lot.