Light Pole Splice Design 6

[OzEng80]

Hi

I am designing a street light pole that will consist of a tapering octagonal tube with a wind turbine (mounted on the top - 9m), solar panel arrangement and a light pole (about 3/4 way up the pole). The client wants to introduce a splice connection about half way up the pole. I am struggling to determine an appropriate methodology (and detail) for this arrangement. I have observed that pole splices appear to take the following forms (see attached pics):

Telescoping - two pieces are wedged together with the applied force providing a 'clamping' force (through the tapering section). Ingal provides a methodology (and applied force of 2t) in their 'Pole Assembly & Installation For Street Light Poles' pdf at

http://www.ingaleps.com.au/shop/detail/street-lighting-poles/

Bolting - two pieces are telescoped as per above, except two sets (of two) bolts are provided at two (only) of the quarter points. It appears that a nut is welded to the outer (female section) and the bolt is holding the inner (male) section in place by the clamping force of the bolts (the bolts are unlikely to penetrate the male section due to wiring and adjustment issues). I have spent a lot of time looking for a detail/drawing of this to determine if this is a ring or something on the male section that the bolts align with - but i have failed.

I was really hoping for a design methodology for the telescoping detail that allows the determination of a clamping force (and overlap distance) for an applied moment & axial force. I find the bolted arrangement even harder to get my head around (i don't know to justify shear flow through the ends of a few bolts...) - so would welcome any clarification of this at all.

Any guidance or advice would be much appreciated.

Thank you.

 

[Sparweb]

OzEng,

I have some drawings from Rohn squirreled away, here. I had asked them for a 50' tower at the time, and it is assembled in two sections. Being a tapering tower, the splice mid-way is just a friction-fit of the two sections, wedged together. For your reference, the 18-sided shape has a 30.25" round mounting flange (bolt circle 26.5" x 8 places). The cross-section of the "octo-decagon" is about 22.5" across flats at the bottom. The top of the lower section is 15.903" across flats. The top of the splice is 29' high. The splice overlap is 26" long. The top cross section goes from 16.763" to 11.5" at the very top of the total 50' tower. The wall thickness is 0.25" bottom section, and 0.1875" in the top section.

This is a very beefy tower, and intended for a rather large wind turbine - yours may be smaller, and you've already said your tower is shorter, too. Hope it helps "scale" your ideas. The splice L/D ratio is about 1.6.

Forgive another tangent, please, but may I ask who is supplying the design loads? The turbine manufacturer? There is some speculation (in the wind power grapevine) that this information is drying up. I doubt it's true (considering how the new certification rules help better define these very loads) but the rumour persists. Other concerns are that tower manufacturers are no longer certifying towers for wind turbine use (giving out the drawings that I have in my very hand) - would that be leading your client to seek your help?


STF

 

[OzEng80]

Hi SparWeb - thanks for the info.

I take it there was no 'clamping force' or member preparation specified on the drawings?

FYI - My client is a small 'green' company looking to develop a pole and footing system comprised of 'of the shelf' wind turbines and solar equipment. I have not been able to obtain turbine loadings. My (cyclonic) design loads actually exceed the rated capacity of the turbine so I have adopted a projected area analysis (of unit and blades) with a wind coefficient of 1.0 on the basis that the unit has 'locked up'. I have seen several approaches to this (including taking the full swept area of the blades) and would be interested to hear your thoughts. After inspecting the unit (it has flexible, aluminium blades) I am confident that the blades will either snap off or be bent back during the design event.

Transmissiontowers - I am still struggling with the splice lock device detail ... ASCE 48 states that 'supplemental locking devices shall be used if relative movement of the joint is critical or if the joint might be subjected to uplift forces. In resisting uplift forces, locking devices shall be designed to resist 100 % of the maximum uplift load.' Since I have the potential for uplift across the joint I am intending to design the device for this load.

I have attached a sketch of my understanding on the arrangement (I am not confident I interpreted correctly)
Is the locking device just a 'J' bolt maneuverer inside through the lap splice and tightened as shown?
I have also shown a proposed detail on the opposite side of the sketch for you comment (would have to drill through the nut on site)....

It was my intention not to have any field drilling of the hot dip galvanised poles to avoid having to touch up the steel.

Thanks for the assistance.

 

[Sparweb]

I take it there was no 'clamping force' or member preparation specified on the drawings?

FYI - My client is a small 'green' company looking to develop a pole and footing system comprised of 'of the shelf' wind turbines and solar equipment. I have not been able to obtain turbine loadings. My (cyclonic) design loads actually exceed the rated capacity of the turbine so I have adopted a projected area analysis (of unit and blades) with a wind coefficient of 1.0 on the basis that the unit has 'locked up'. I have seen several approaches to this (including taking the full swept area of the blades) and would be interested to hear your thoughts. After inspecting the unit (it has flexible, aluminium blades) I am confident that the blades will either snap off or be bent back during the design event.

Sorry, no assembly details, it's more of a spec sheet. I have listed all dimensions that I can, short of e-mailing you the file, though I didn't point out some strength details before. They specify 65 ksi steel.

Loads, that's a whole different story. You may be correct about the blades: I've seen some made from bent aluminum sheet that make me want to stand a mile away. If that's what you're talking about, then they probably won't survive the extreme wind event for some locations... leading to a more fundamental question about the turbine! Thankfully you're not asked to address that (but be wary of it anyway). Be that as it may, there are certified types out there now that will take whatever mother nature's got coming.

The NREL have published reports on dynamic load tests from instrumented wind turbines. You can look them up on the NREL website. (try TP 500-38550) I've taken a stab at reducing their numbers a little (long time ago) and found that they agree with theory well enough that you can use simple estimations of the blades' mass-moment of inertia, spin them at the critical max RPM, and then use the gyroscopic effect to determine the pitch moment that the effect will apply. I have worked so far with free-yawing WT's, so I use a slew rate of 180 degrees per second as the worst case, and get significant pitch moments as a result. You can find similar gyro conditions in propellor-powered aircraft design standards.

Thrust is proportional to the lift on each blade, but it takes too much hard slogging to find that. I found equations on Wikipedia that are NOT conservative when compared to a detailed blade-element analysis of the same rotor. Using the blades alone on a "locked up" rotor as your flat-plate area is not conservative either. Using the flat-plate drag on the whole rotor's swept disk is very conservative, not excessively conservative.

Weight is also an obvious load. Wind turbines tend to be heavier than antennas, so it will have a greater effect on the tower, exacerbating over-turning moments at the base.

If the blades do bend back in extreme winds, consider what happens if they strike the tower. This can raise the turbine unit off of its mount, depending on how that's been designed.


STF

 

[transmissiontowers]

Your sketch is a little off. The Thomas & Betts splice lock that we use has an "L-Bolt" about 1/2" diameter in the center of a 1.5" steel plug with a groove in the back and a sholder on the front. After you put the 2 shafts together, you drill a 1.5" hole through both shafts and insert the splice lock. The "L-Bolt" is loose and slides past the shafts. When the sholder bottoms out, the L-Bolt is tightened and clamps the SPLOCK in place. The shear force does not go through the L-Bolt, but it is transferred through the 1.5" plug.

If you have uplift and vibration from the wind turbine, I would probably stay away from the slip joint and use flange plates. FWIR, the huge wind turbine columns use an internal flange and an internal staircase to climb the structure.

I found my SPLOCK and have taken a picture of it.

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

 

[OzEng80]

Thanks for your comments Sparweb - i will chase the manufacturer a bit harder. The unit is tiny as far as turbines go (Nheowind 3D 04) and is purported to have minimal gyroscopic effects due to the unusual blade arrangement (try googling it).

I went back and checked the calcs (i didn't do the prelims and have taken over the job) and realized that i was miles off with my 'projected area analysis (of unit and blades) with a wind coefficient of 1.0 on the basis that the unit has 'locked up'' statement. The current basis of design is for the full swept area of the blades with a drag coefficient of 1.3. So at this point i will be revisiting the design with same area with a drag coefficient of 1.0 (which still seems very conservative to me - but should address some of the issues that could arise from the design simplifications).

transmissiontowers - thank you. A picture tells a thousand words! I take it the steel exposed by the drilled holes is treated/painted?

 

[transmissiontowers]

Yes, you paint after drilling the hole. If you fab the pole sections knowing that the Splock will be used, you put holes in the upper (outer) section and galvanize the poles, then you press the 2 sections together, and then use the hole as a template to drill the inner hole and paint everything. Since it is a bearing connection to transfer the shear between the 2 sections, you cannot use slots. And because of the fab tolerances in the pole sections, you cannot count on pressing the 2 together to line up 2 holes.

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

 

[BAretired]

Sounds reasonable to me. Why would horizontal slots be even considered? There seems to be no possible validation for such an idea...or am I missing something?

BA

 

[transmissiontowers]

I guess I should have specified Vertical slots are not used because the Splock is a bearing connection. Now if the pole is big enough to put a man inside and the tolerances are close enough to use a friction connection, you could use a vertical slot and have the guy inside hold the bolt head.

I would still recommend a flange connection because of the slip fit irregularities with tapered shaft fabrication and the vibration and uplift possibilities with the wind turbine.

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