Ryan,
Check out the following Youtube video from an Engineer for RISA 3d. It was very helpful for Non-Building Structures and Wind loading.
I use ASCE 7-10 ASD and I use the trussed towers section to determine the wind loads. I calculate the wind pressure every 10 feet along the height of the tower (or whatever the tower section heights are). The bottom section is seeing the wind pressure calculated at 10' height and then the section from 10' to 20' is seeing the wind pressure calculated at 20' height etc. rather than calculating the wind pressure at 150' and applying it to the entire 150' of the tower. This is more work for sure but results in an overall smaller load.
I use the 4 load cases in Figure 27.4-8.
Case 1 is full wind pressure acting on all projected surfaces normal to each axis.
Case 2 is 75% of the wind pressure acting on each axis with a torsional load.
Case 3 is 75% of the wind pressure acting on all axis simultaneously
Case 4 is same 75% of case 2 acting simultaneously on all axis
Use the load combinations in 2.4 if you are designing with ASD. You will see that the wind load has a 0.6 multiplier.
You may also want to look at Ice loading as well in chapter 10
Wind Loading on Non-Building Structures in RISA
YouTube |
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Wind Loading on Non-Building Structures in RISA |
This recorded webinar shows you how to calculate and apply wind loading for a non-building or industrial type structure in RISA-3D. Examples like towers, tanks and open structures are shown. This is part 2 of a 3 part series of webinars. |
View this on YouTube > |
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Randall Lehman P.E., M.ASCE
Honeyville Metal Inc.
Topeka IN
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Original Message:
Sent: 05-07-2018 16:20
From: Ryan Heyer
Subject: Trussed Tower Loading vs Lattice Frameworks - How they are applied
I am designing a 150' tall, 14' square trussed tower. We loaded our tower using the trussed tower loading. Our ultimate trussed tower loading turned out to be about 73psf. After running the same criteria through lattice frame loading, that load turned out to be around 42psf. We loaded our tower using the 73psf, and applied that loading to all faces of our tower, since we felt like all members would easily have a wind shadow and be able to take wind. The numbers we're getting are extremely high.
After looking over everything I'm starting to think we put too much load on this tower. I'm hesitant to go with the lighter load, however these reactions I'm getting are unbelievably high.
So my questions are as follows:
- After reading online it seems like lattice Frameworks is for a 2d structure and must be loaded on all faces. Trussed Tower loading accounts for the extra wind shadows behind the front face and in my case that 73psf wind loading, only needs to be applied to the front face. Is that correct?
- When modelling this tower, I had used an open structure load distribution that applies loading to every member under that area based on the member's size. This is very useful as member sizes change during design and having the program take care of loading the members based on their size is very helpful rather than me manually changing it each time. My question is, if my above assumptions are correct, would loading a truss tower, using the lattice frameworks wind load on every face, be comparable to using the trussed tower loading on just the front face? I assume it would be slightly on the conservative side, however still applicable?
- Can you help further explain the difference behind the "wind loading along tower diagonal" vs "wind normal to face" under Trussed Towers? For loading 45 degrees to the faces of this square tower, I had planned on taking the "wind normal to face" loading, x .707 and using the loading on both faces. Can this "wind along tower diagonal" somehow take the place of doing that?
I have talked over this with multiple engineers in our office and we have not been able to prove that we can just apply the trussed tower loading to only the front face, so we've just put that load on all faces, however with this big project I want to make sure I properly load structure as it will have a big impact on my design.
I appreciate any light you can shed on this topic to clear things up. I didn't find much information regarding these topics in the ASCE7-10 besides a few figures so any information you can provide would be greatly appreciated.
Thank you in advance!
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Ryan Heyer, PE
Structural Engineer
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