Wind Load on Lattice Deadend Tower

[Silasf]

Hello,

I'm analyzing a lattice deadend structure and had a question about how wind load was applied to the lattice bracing. In RISA you can create an "open structure" load and it applies the load on the projected area of the member. However, I've also seen the load applied to the member ends an the actual member sees no wind load. This makes a huge difference with long bracing members. I've looked though ASCE 113 and found this in section 3.1.5.7.

The wind forces calculated on a complete lattice truss system can be distributed to the panel points of the structure without further consideration.

This seems to justify only putting the load at the panel points of the structure and not along the member itself. That being said it seems odd to me that the load can get to the member ends isn't placed along the structure itself. Does anyone have any further information justifying this? Has there been wind tunnel testing that proves the bracing members will not fail in bending from wind load along the member itself.

Thanks for the help.

Silas

 

[Bagman2524]

Just putting load at the member ends sounds like a bad idea to me.

 

[dhengr]

You only have to apply the actual loadings (real world loadings) to those bracing members which might be overloaded, over stressed, or fail due to the true loadings. All the rest you can just calc. the end reactions, using the real loads hopefully, to be used as loads on their supporting members, which might be critical. This saves you the trouble of all that messin around with members that aren’t critical. The same thinking applies to shop drawings, if you first check all the dimensions and details which are wrong, and need some red lining, you save yourself a lot of time. You don’t have to mess around with all the stuff that’s o.k., that’s just a waste of time.

 

[msquared48]

You also have to consider icing too, and this is along the length of the bracing, just like the wind.

Use the real world loads, not a figment of someone's imagination just to simplify the analysis.

Mike McCann
MMC Engineering

 

[Silasf]

It's not "figments of someone's imagination." It is in the code. I just wondered if someone had a justification.

 

[msquared48]

Sorry Silsaf, the comment was not directed at you. It is directed at the code writers allowing the practice. I would never do that for a real world situation, particularly a tower. Personally, I try to emulate the real world conditions as much as possible, especially with the computer tools we have nowadays.

Mike McCann
MMC Engineering

 

[hokie66]

A lattice tower is just a truss, oriented vertically. A standard way of analyzing trusses is to collect the loads at the panel points, find the axial forces, then combine these axial forces and the bending. Computer programs can do this all together, but that doesn't mean the old way is incorrect.

 

[Silasf]

Thanks hokie66. That sounds like where it came from. Thanks again.