AISC LRFD and Deflection Calculation 2

[Chengineer4]

Reference Thread:
thread507-214648

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!

 

[Lomarandil]

Well, you're correct to use ASD load combinations to check deflection.
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.

 

[Chengineer4]

the pole is located near the coast so there is significant wind load the ASD load combinations I'm using to check deflection is 1DL + 0.6WL. should I create a seperate load combination which will be 1DL + 1WL for only service loads?

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.

 

[Lomarandil]

It depends how your wind load is defined -- it's changed over the last few editions of the codes. If your winds are at an ultimate level (say, from ASCE 7-10 or later), the 0.6 load factor is correct for ASD. If the winds are at a service level (e.g. from 7-05 or before), then you will want to use a 1.0 load factor for ASD (and 1.6 for your LRFD strength design)

(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.

 

[JoshPlumSE]

When checking deflections be sure to go to the Codes tab of the Global parameters and turn off the "STiffness Adjustments" per AISC's Direct Analysis Method. These are intended to soften the structure so that the elastic P-Delta effects approximately capture the INELASTIC buckling that usually occurs with steel structures. But, this effect is not intended to apply to deflection calculations.

 

[transmissiontowers]

I do Transmission Line tapered poles in galvanized steel and PLS-CADD (PLS-POLE) is a specialized program that handles the shafts. We use slip joints where the sections slide over about 1.5 diameters at the joints. Is your pole 8, 12, or 16 sides? It could be round but they are not used much in our industry. Welded flange plates are sometimes used to join sections. The vortex shedding is not usually a problem for tapered poles that support T-Line conductors due to the damping of the wires, but they could happen in a free standing pole. They sometimes hang a heavy chain inside the pole to stop the vibrations on lightning masts in substations. They are ugly, but spiral strakes are welded on the outside of the shaft to stop the vibrations. ASCE 48 is a good reference for tapered poles. Some utilities have had arms fall off during construction when no weight is supported on the horizontal arms, so they sometimes hang weights on them until the wire is strung.

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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