Wind load on Open structure v.s. Sign

[SKJ25POL]

Is it conservative for a quick calculation, that wind load on a steel truss (an open structure)be treated as solid plate (like long rectangle plate/sign)?

My boss tells me rather than calculating wind load on an open structures (on a face of truss) assume it is like a free standing wall or a sign, this sounds alright unless there is something we are missing?

I appreciate your comments.

Thank you

 

[jayrod12]

It is conservative. But definitely makes your analysis much easier. My argument always has to do with the next owner that dislikes the open structure look and sheaths the whole thing completely.

 

[SAIL3]

yes, it usually is conservative, except when one has more than two frames in that direction, then it may not be......

 

[BadgerPE]

"yes, it usually is conservative, except when one has more than two frames in that direction, then it may not be......"

Can you explain this? I'm not following.

 

[SKJ25POL]

SAIL3 (Structural,

I am not following too. I am kind of freshman. Can you please explain it more in detail?

Thank you very much.

 

[JStephen]

I think the meaning- see the attached sketch- a single frame/truss with wind loading normal to it, if treated as a solid sign, that is conservative. If there are multiple frames/members stacked behind each other, the projected area is the same, but the wind loading is increased, and it may not be conservative to treat that as a solid surface.

 

[SAIL3]

JS is correct....

 

[SKJ25POL]

JStephen and SAIL3,
Thank u for both of you but the statement of "If there are multiple frames/members stacked behind each other, the projected area is the same, but the wind loading is increased, and it may not be conservative to treat that as a solid surface." doesn't click for me can you possibly explain it more in detail how it won't be conservative for trusses behind the first one?
Why I should I worry for trusses behind? I use the first truss load and apply it to them

Thank you

 

[KootK]

Their point is that one truss should not be assumed to shield its neighbour from wind pressure. You've made the same assumption so all is well.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jimstructures]

The MBMA Building Systems Manual from 1986 with the 1989 supplement had an example with design methodology to do design of multiple bare frames standing up with no wall panel (and maybe no roof panel) with each catching a portion of the wind. If you worked it all the way through you ended up with very large Main Frame Wind Resisting Systems, i.e. large brace rods in multiple bays. I think it was done by some Western Canadian engineers at some college. I don't have my manual at home with me.

I also don't think the newer MBMA Building Systems Manual use the same exact method but again I don't have the manual in front of me. The method may not apply to trussed towers.

Hope this Helps.

Jim H