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  • 1.  Trussed Tower Loading vs Lattice Frameworks - How they are applied

    Posted 05-07-2018 04:53 PM

    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:

     

    1. 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?

     

    1. 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?

     

    1. 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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  • 2.  RE: Trussed Tower Loading vs Lattice Frameworks - How they are applied

    Posted 05-08-2018 09:59 AM
    Edited by Tirza Austin 05-08-2018 09:59 AM
    Thanks for inquiring,
    Have you used any 3D modelling Software?
    Perhaps 3D load was not considered in your design so that Auto Lateral Load will be great impact on your tower although you can not describe any location of Building and Environment. Although it seems to stable accordance to your consideration.


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    Md Mohymenul Islam S.M.ASCE
    Meherpur
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  • 3.  RE: Trussed Tower Loading vs Lattice Frameworks - How they are applied

    Posted 05-09-2018 10:19 AM
    Edited by Tirza Austin 05-09-2018 10:19 AM
    I use 3Ds Max, Revit 3D modeling Soft. It works very well.

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    MD. Hossain S.M.ASCE
    Student
    Dhaka
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    Original Message


  • 4.  RE: Trussed Tower Loading vs Lattice Frameworks - How they are applied

    Posted 05-08-2018 09:59 AM
    Ryan - For trussed towers refer to the TIA (Telecommunications Industry Association) design standard.  TIA-222-G.  Wind forces are calculated based on the solidity ratio of one face and applied on one face.  Using an open structure area load is extremely conservative.  Shape factors can be complicated with trussed structures like this so the solidity ratio considering each member's shape and size is used instead.  Let me know if you have additional questions.

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    Joseph Johnston P.E., M.ASCE
    Infinigy Engineering PLLC
    Albany NY
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  • 5.  RE: Trussed Tower Loading vs Lattice Frameworks - How they are applied

    Posted 05-08-2018 10:02 AM
    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 remove preview
    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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  • 6.  RE: Trussed Tower Loading vs Lattice Frameworks - How they are applied

    Posted 05-08-2018 10:03 AM
    Edited by Tirza Austin 05-08-2018 10:02 AM
    ​I've always interpreted it the way you suggest in your question 1 - that the trussed tower loading is intended to only be applied to the exposed face and that its pressure also accounts for loading on the shielded members on the back face.

    I believe this is borne out by the fact that the trussed tower load (73 psf) is not double the lattice frame loading (84 psf, if doubled), and that it is less than double because the second face's members see a lightly lower pressure because the front face breaks up the wind.

    Regarding question 2, your simplified assumption is reasonable.  It might be 15% conservative (84/73), but I personally have no objections with a slight bit of conservatism in designs.  Loadings are not an exact science.  You could always fine-tune it once your member sizes are nearly locked in.

    Regarding question 3 - also sounds reasonable.  I think it's correct for the overall net pressure/load in the direction of the wind (confirmed by the fact you have 1.4x the projected area).  I think there may also be some loadings resolved in the direction perpendicular to the wind that cancel each other out for net overturning, but might affect local member and connection designs - I'd have to think about that some more.....


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    Greg Thein, PE
    Cleveland, OH
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  • 7.  RE: Trussed Tower Loading vs Lattice Frameworks - How they are applied

    Posted 05-08-2018 10:04 AM
    Ryan,

    You didn't mention the function of your tower.  If it is a transmission tower, I would refer you to ASCE 10 - Design of Latticed Steel Transmission Structures (http://www.asce.org/templates/publications-book-detail.aspx?id=12069) and ASCE 74 - Guidelines for Electrical Transmission Line Structural Loading (http://www.asce.org/templates/publications-book-detail.aspx?id=8072).  Even if it isn't for a transmission tower, I would highly recommend both of these documents as they cover very well how to design latticed towers and how to blow wind on them.  There is even a section in ASCE 74 (Section 2.1.6.1.3) that specifically discusses how to apply yawed wind on towers.  (In the full interest of disclosure, I am the current Vice-Chair of ASCE 10.)

    If this is a communication tower, I would recommend TIA 222 (https://global.ihs.com/doc_detail.cfm?&csf=TIA&item_s_key=00122271&item_key_date=821231&input_doc_number=TIA%2D222&input_doc_title=) as this describes how to blow wind and apply ice to communication towers.  I would still reference ASCE 10 though as, in my opinion, it is a much better document when it comes to actual tower design and analysis.  But of course I am biased.

    I hope this helps you out.


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    Otto Lynch P.E., F.SEI, F.ASCE
    Vice President
    Power Line Systems, Inc.
    otto@...
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  • 8.  RE: Trussed Tower Loading vs Lattice Frameworks - How they are applied

    Posted 05-09-2018 01:56 PM
    Otto:

    Thanks for the information and ASCE Documents.  I will look into these.  

    As for the purpose of the tower, this system is for a grain handling and storage facility.  Within the tower will be two (2) 72" x 42" bucket elevators with various catwalk/conveyor trusses framing into the sides of the tower at differing elevations.  The tower will also host cantilevered platforms along with internal platforms at various elevations.

    Thanks for the information everyone.  Truly helpful.  And an FYI, we are utilizing RISA 3D for the design.

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    Ryan Heyer P.E.
    Fargo ND
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    Original Message


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