Give me light: Light mast/pole 25 metre high

[STATICPH]

Hi,

Some “person” force me to quote on this pole – for some reason he don’t want to go to any other ENG, I haven’t done this kind of work before so please help me with a few tips. (My experience is more with concrete, timber and isolated steel beams and cols, but I’ll like to do this thou, with your help I might consider doing it)

This pole is already constructed, now they need a certificate but got no engineer, don’t worry my terms will be that the calcs must show it work (or we will make a plan (hopefully)).

Some specs:
26 metre pole that exist out of 4 tube sections welded on top of each other.
Sections are 6m high with the last one 8m (top one), with reducers between each section.
Sections used from bottom to top (all in mm): 300x6 / 250x6 / 200 x6 / 150 x 7.11 (7.11 funny size, but that is what stand on the hand drawing this is supposed to be the metal worker's shop drawing - maybe 6mm as well)

At the top they have a “Crows nest” with all the light fittings and board walk (3m wide – but don’t have any other dims yet)

The base plate is 25mm thick x 500x500 plate with 8x M32 bolts casted 700mm into footing. (This sound tiny)

The base was designed by others (I do not understand why they don’t want to do the pole – will call them thou)

Wind:
regional basic wind speed 40 m/s , altitude +/- 0 to 200m , Terrain: Sports field/Town area , Cape Town Suburbs

My Q’s
How many hours should I allow for this design?

Do you have some tips on design? (Sorry a bit clumsy Q this, but as I noted, I haven’t done this before, I will design for full wind load as a static force, and vibration, I havn't done that vibration calcs before.)

Any other comments will be appreciated.

tx

 

[JedClampett]

In the states, AASHTO and ASCE 7 give wind loads for structures. AASHTO is more conservative. It's not horribly complicated, but it helps if you have a template to work off of. There are companies that design poles exclusively. Maybe if you ask one of their engineers, he/she could guide you through it. Or go to the Florida DOT website and download their HighMast Program.

http://www.dot.state.fl.us/structures/proglib.shtm

As far as an estimate, you're on your own. Depends how hungry you are for work. I'd say 20 hours is plenty to learn the problem and design it, but that's your call.

 

[rb1957]

if somebody is "forcing" you to quote ... do you have pictures ??

seriously, if you don't want the job, quote a really high number of hours (maybe this is what they really want, to show that someone else's quote is reasonable ??).

to be very serious, if you're inexperienced and show it good and somethng happens, who are they going to come for ??

 

[JStephen]

With a statically-loaded beam with well-defined loads, it is fairly simple to analyze it and confirm adequacy.

The pole problem sounds like something where wind vibration could be a major effect. In dealing with some similar problems, I found there was a wide range of values used for some of the design factors. Ideally, you want to either have a lot of experience with very similar projects, or have some sort of codified design method that is applicable to your exact geometery. Without one or the other, you're liable to have to fall back on some very conservative assumptions in order to be reasonably sure of your design.

 

[bmagdalena]

I just had guy come to my office and asked to do structural analysis on existing antenna that was slightly modified.
Our answer was that on top on engineering time we would add additional $20,000 (deductible on our insurance) just in case. Since we would stamp it, it would be our liability. The guy decided to go with new antenna.

 

[STATICPH]

tx guys,

Interesting answers, I’m going to source an expert to guide me, and I should quote high.

 

[STATICPH]

Update for the interested ones out there

I eventually called the original engineer that designed the pole for quoting purposes ect, it happen to be that the contractor have included his preliminary drawings in the quote and the client have "stolen" the design, however, his design should have been O'k',

The client gave this drawings to another contractor who have manufactured the pole to one additional instruction from the client - use a pole with thinner sides,
The pole deflect too much (1.8 metre or something) in strong wind and the client need a certificate.

So I have never quoted on the job in the end......