AISC LRFD and Deflection Calculation
AISC LRFD and Deflection Calculation
(OP)
Reference Thread:
thread507-214648: AISC LRFD and Deflection Calculation
Hey guys so I am designing a monopole which is 104' tall. I am using stainless steel as my material and am tapering it to avoid any vortex shedding which may occur. I'm not too worried about the pole being overstressed but I am worried about the deflection. I have the pole modeled in RISA-3D and I am currently using 2010 ASCE LRFD load combinations to check the design of the pole and then switching to 2010 ASCE ASD load combinations when I check my deflection limit which is L/75. I would like a second opinion on whether or not I am approaching this right. let me know thanks!
thread507-214648: AISC LRFD and Deflection Calculation
Hey guys so I am designing a monopole which is 104' tall. I am using stainless steel as my material and am tapering it to avoid any vortex shedding which may occur. I'm not too worried about the pole being overstressed but I am worried about the deflection. I have the pole modeled in RISA-3D and I am currently using 2010 ASCE LRFD load combinations to check the design of the pole and then switching to 2010 ASCE ASD load combinations when I check my deflection limit which is L/75. I would like a second opinion on whether or not I am approaching this right. let me know thanks!






RE: AISC LRFD and Deflection Calculation
RISA-3D is an appropriate analytic tool to check defletions (when used correctly).
I presume you have modeled the taper in a reasonable fashion -- if you're concerned about that, you'll need to provide more detail.
(Side note, it's been a while since I looked at vortex shedding, but I don't recall that taper alone would prevent it).
One major challenge for a monopole will be modeling the boundary conditions at the base. Your foundation (presumably a drilled shaft?) is not a fixed end.
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The name is a long story -- just call me Lo.
RE: AISC LRFD and Deflection Calculation
I have the boundary condition as fixed but I am providing a 30' square x 4' deep footing to counter overturning and big anchors embed deep into the footing.
RE: AISC LRFD and Deflection Calculation
(For simplicity, I'm assuming ASCE 7 as the governing code. For a monopole, it probably isn't. The underlying principles are the same)
About the footing -- unless it bears on rock, it will rotate under wind load. Perhaps not much, but even a small rotation amplifies to large deflections at the tip of your monopole. You either need to accept that rotation, or find a way to quantify it.
What is the intent of your deflection criteria. Is it to prevent impinging on some clearance to another structure or a power line (etc)? Or is it for the integrity and slenderness of your monopole itself?
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The name is a long story -- just call me Lo.
RE: AISC LRFD and Deflection Calculation
RE: AISC LRFD and Deflection Calculation
I'm not a RISA user and am not aware if they have a tapered beam element ( I believe ANSYS has one ). I do use GTSTRUDL for frames with tapered tubular legs. If you break up the shaft into many prismatic finite elements about 1 foot long, you can model the shaft as a step wise tapered column and calculate the wind pressure on each segment and get the total tip deflection.
I was just curious what are you sticking up 104'-0 in the wind on a SS pole? I'm on a committee with a guy that does flag poles that are like 400' tall.
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