What you aren't taking into account is that object on your bridge IS accelerating - it could not possibly be applying a force to your bridge if it weren't. The pressure you are describing is a force, and the Statics equation for a force is F=ma. If your object's acceleration = 0, the force being applied to your bridge would also = 0. In this case, the object's acceleration = gravity = -9.83 m/s^2.
So yes, the speed of the wind in itself is nothing - it's the acceleration of the air molecules (and whatever else is being carried by the wind) as a result of the wind times the mass of the air molecules (and whatever else is being carried by the wind) that applies the force to the scoreboard - not the wind.
agrabes is saying the wind is being decelerated from 90mph to 0mph in such a small amount of time (when it hits that board) that this acceleration component of a 90mph wind will be so much greater than any other factor, assuming the board's shape is constant, that you don't have to consider whether the wind is accelerating or decelerating before it hits the board's surface.
Essentially a gust of wind going from 0mph to 90mph doesn't exert a greater force on the board than a gust going from 50mph to 90mph over the same duration. It's the 90mph to 0mph that matters.
ASCE Code Windspeed is based on 90 mph 3 sec gust for much of the US (including tornado alley). Only on the coastlines does that speed go up, which is due to the sustained winds of a hurricane. This is based on a 50 year return.
some other factors:
Not sure what importance factor is on the scoreboard, but there are four categories of buildings (ag/storage the lowest, hospitals and essential facilities the highest). I would list this as a Category III building. That would mean the wind load is multiplied by 1.15 the normal load of an average building.
If the steel is designed to code, it probably has at least 50% additional strength until it would reach a yield stress. That is just a random number and varies depending if we are talking bending stress, axial tension stress, axial compression stress, etc.
I think the biggest challenge would meeting deflection criteria. I am sure the scoreboard manufacturer doesn't want that truss to deflect much either from gravity loads or wind loads. My opinion is that they will have to beef it up so much that it never fail in sense of falling down - it would likely fail because the thing deflects too much and you have issues w/ the scoreboard not working. This whole frame would have to be designed much different than if it was just holding a metal panel sign.
oh and the wind load on the board would most likely be static, they would probably not consider acceleration. I think if the period of the structure is more than 1 sec, then you look at the dynamics (probably a building 20+ stories, depending on lateral system type).
Very insightful. Yes, deflection controlled most of the design aspects of the steel frame. Corner effect from ASCE 6.5.14 & Figure 6-20 controlled most of it.
All content owned by CycloneFanatic.com - All rights reserved 2005-09. By viewing this website you agree to the Terms of Service, Site Rules and Legal Disclaimer. The words, views, images and opinions expressed or provided by users do not reflect the opinions or views of CycloneFanatic.com or Iowa State University. The names, words, symbols, and graphics representing Iowa State University are trademarks and copyrights of the University protected by the trademark and copyright laws of the United States of America and other countries and are used on this web site under license from the University. Original site design, premise & construction by Jeremy Lind.