Tapered Octagon/ Dodecagon A-frame Column Design

[nmoore21]

Within the ASCE 48 there are provisions to check the local buckling against an allowable compressive stress based on w/t. There is also a section based on truss members, that per the definition is a member that is designed to only withstand axial forces, that takes into account the unbraced length of the member. In an A-frame situation I can run into quite high compressive forces with moments. What would be the appropriate way to check the global buckling capacity of the tapered column section? There does not seem to be any provisions for checking the column section as a whole for allowable compression based on unbraced length when moments and shears are present. Any help/ thoughts would be much appreciated

 

[transmissiontowers]

What computer program are you using to do the analysis? Most of the firms that design these A-Frame dead ends have custom software that checks the members. PLS-POLE is often used to model the dead end structures and I believe they detect buckling from their non-linear analysis and the results are close to the manufacturer's custom software.

Many of the members of ASCE 48 will be in Branson, Mo for the SEI conference later this year, if you want to talk to the real experts in person.

You can also look at ASCE 113, which we are currently revising. It covers dead end A-Frames but refers the design of the members back to ASCE 48.

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I have been called "A storehouse of worthless information" many times.

 

[nmoore21]

I am using RISA 3D to generate the member forces. From there we have a spreadsheet created to check the various sections based upon the ASCE 48. The main purpose of the question is that we are trying to determine how closely our results would match that of PLS-POLE and if we need to further investigate buying the program for future designs.

Thanks for the head up on the SEI conference, might have to make that trip and try talking to some of them.

 

[transmissiontowers]

I checked L/r for one of our tapered columns and at the midpoint it is under 50 so I doubt overall buckling will be a problem. In your RISA program, do you generate properties for the tapered tube and do a step-wise taper every foot or so, or does RISA have a tapered tube element? Years ago we had ANSYS and it has a general tapered beam element that we used to use to model the tapered polygons. We now use PLS-POLE (which checks the pole by ASCE 48) and GTSTRUDL for general frames. GTStrudl has a code check for ASCE 48-2005 but you have to generate your properties as step-wise beams connected together. They are working on a property generator so it is not so cumbersome but for now you can do a spreadsheet to generate the member properties. GTSTRUDL was recently sold to Intergraph if you want to check prices, or you can request a student version limited to a few joints and members.

I don't know if the PLS boys have a demo version of PLS-POLE but it is widely used. They are considering putting in a deflection load case check for substation structures but it is not at the top of their list.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[transmissiontowers]

The Branson ETS conference starts Sept 27, 2015 and it is always well attended by people in our industry.

http://www.etsconference.org/

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I have been called "A storehouse of worthless information" many times.