wind load on bridge mounted signs

huangyhg

wind load on bridge mounted signs
i am trying to calculate the shear and tension forces caused by the dead load and wind load acting on a bridge mounted sign.
i already calculated the wind force acting into the face of the sign, where tributary area is the total area of the face, and point of application located middle point of the sign height.
now, when i calculate the wind force acting behind the sign, i'm not sure what tributary area to use. only part of the sign area is exposed above the bridge deck, and the traffic barrier railing on the bridge edge is blocking some of the wind load too. also, the bottom part is being shielded from the wind by the exterior beam adjacent to the sign.
please let me know how to calculate the wind load behind the sign, and where should i apply that load in order to get the greatest tension and shear forces.
thanks.
i don't know of any aashto spec that directly addresses wind loading on signs partially shielded.  the wind loads in the "aashto signs, luminaires, etc. guide" does incorporate wind gusts, but doesn't go to the extent that is required for buildings (opened, closed, pitched roof, etc.)  i would use the same wind pressure as you used on the fully exposed face and apply it to the exposed area on the shielded side - and apply some factor for conservatism.
apply the load at the centroid of the exposed area (as seen in elevation).
use your judgement.  chances are that the full wind load applied to the fully exposed face will control the design of the connections and   
thanks for your response.
i had been doing basically what you explained, but i wasn't sure if there was a more accurate way to design the signs.
when you solve for the reactions at the supports of the sign beams, the wind pressure behind the sign case governs over the other one. that's why i am so concern about it. also, i am designing a large number of signs.

if you are designing the support(s) or attachment component i would use the gross area of the sign. as i am sure you are aware, asce 7-98 specifically states to avoid using shielding (reducing exposure area) except to be concerned about the "tunneling effect" (increases pressure). if aashto criteria is worse than asce, use it. i have also seen (while serving as a county bridge engineer), maintenance personnel install taller signs on the same supports (with contacting engineering), so i also vote widmis.
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