Verifying the stiffness of a pipe, Al vs. Steel

[HSWSac]

First off, I'm not a Structural Engineer, but I'm an electrical engineer and we install equipment on poles. We recently changed our pole design from Steel to Aluminum and found a drastic difference in stiffness. After researching the numbers, I believe I've found the info I'm looking for. If I can get verification that I did this correctly, that'd be a HUGE help.

Original pipe: 12' long, Sch 40 3" Nominal

Youngs Modulus of Elasticity (E):
- 29.5 million PSI for Steel
- 10 million PSI for Aluminum

This immediately tells me the Aluminum pole of the same dimension is going to be 3 times less stiff. (Right?)

I found the Moment of Inertia (I) values for various size pipe. By calculating I*E, I found that I'd need a 4" Nominal Pipe at Sch 80 to get the same I*E as a 3" Sch 40 Steel pipe. 4" Sch 40 would be a lot better than the 3", but still short of the stiffness of the steel.

I also found a deflection formula on Wikipedia for Cantilever Deflection, which I think is applicable.

Cantilever Deflection = (Force * Length^3) / (3 * E * I)

This seemed to be helpful for comparing the different results.

Attached is a screen cap of a table I created in excel. If I'm on the right track, that would be a huge help. Basically, we need to verify what we need to do in Aluminum to match the performance of the steel.

 

[Grahammachin]

Drawoh,

I have just checked through the British Standard as I couldn't remember off the top of my head but the 0.5 factor was for 6061 alloy, there is no reference to any factors for the non-heat treatable 6*** series only heat treatable. There are lower factors for 1***, 3*** and 5*** series alloys which are non-heat treatable.

The factors are for a welded joints carried out in-accordance with BS8118 part 2 where it states "Pre-heat shall not be used for materials of 25mm thickness or less, except that heating to remove moisture" and that if heated to remove water "welding is not commenced at a metal temperature in excess of 50C (40C for 7*** series alloys)."

Any other questions please let me know, or if you have any further info on non-heat treated 6*** series alloys, as I will be the first to admit I don't know everything to do with aluminium and am always willing to learn more.

Graham

 

[csd72]

One thing that has not been mentioned - how is this pole connected to the base?

If it is a bolted connection with a base plate then much of your deflection may be occuring at the base due to the bending of the base plate. The same EI comparison would be applicable for this meaning that the aluminum base plate would need to be 1.45 times the thickness of the steel one to achieve the same stiffness.

 

[civilperson]

Aluminum is superior due to lack of need for zinc coating or paint.

 

[apsix]

But aluminium in contact with concrete suffers from accelerated corrosion when in the presence of chloride salts.

 

[Tmoose]

On a similar track as CD72 - the description of your "bracing" sounds like it is compensating for the base plate flexibility.