Cold - Formed Steel Driven Piles & Potential Corrosion Issues

[cruzinbear]

I am designing ground mount support system for some solar panels. The client is suggesting to extend the columns of the structure to be used as foundation (basically flagpole, IBC 1807.3.2.1). If we went this route, we are going to use galvanized cold-formed steel members however I want to know if I should account for some level of corrosion decay over a period of 25 years (service life). Does anyone have experience in this?

According to the geotechnical report, no water was found when they drilled 15 ft into the ground which is more than what the future foundation would be. However, given the corrosive nature of soil, and the fact that this is near a water basin, I am thinking shouldn't I take something in consideration since this is not a concrete caisson pile but rather a bare steel member?

Any thoughts or advise would be much appreciated. Thanks you.

 

[WARose]

Most (good) soils reports will give some idea of how corrosive the soils are. In the case of steel, I've typically seen them estimate how corrosive a soil is based on chloride ion content, electrical resistivity, and pH. From that they will typically say how corrosive the site may be.

Usually, the allowable pile stress (as per code) are low enough to where you are covered for structural steel. However, I typically include a bit of a corrosion allowance anyway. In your case, it sounds like you are using light gauge steel......I would recommend such members for that type of application (even if galvanized) because they are so thin.

 

[WARose]

Pardon me, above I meant to say: I [red]wouldn't[/red] recommend such members for that type of application (even if galvanized) because they are so thin.

 

[TLHS]

Driven?

I can't see a cold formed section liking that...

 

[cruzinbear]

WARose, thanks for the post. In this situation, what limits would you consider before you conclude that it is not a viable option? Keep in mind, I'm using a 12 Gage member. At most it will be embedded 6 ft into the soil and sticking above ground like 3 ft.


TLHS, please elaborate. Thanks.

 

[Ron]

12 gauge is a heavy cold formed section; however, it is a very light member with regard to your application. I agree with WARose...I would not recommend a light gauge steel section for such application. The galvanizing for cold formed light gauge sections is corrosion inhibitive; but is not intended for below grade service.

 

[PittEng88]

I agree with WARose, most good geotech reports will tell you if there is a potential for corrosion. If it's not included I think it would be worth a call to the geotech to find out. I know this is not exactly applicable to your situation, but in Section 10.7.5 of AASHTO's LRFD Bridge Design Specifications, they give some parameters as to what defines a corrosive environment as well as ways in mitigating it's effects. Also, I am sure if you would talk to the geotech he could provide you with some recommendations on what you should consider in your design.

 

[wannabeSE]

Technically, HSS (ASTM A500) members are cold-formed steel and are available in fairly thick sections that can be hot-dipped galvanized. Perhaps the OP can provide more information on the proposed members.

 

[WARose]

If I was using a structural steel member (based on the research I've seen and what the soils report said), I'd have a corrosion allowance somewhere around 1/16 or 1/8 th of an inch. Even for 12 gauge, that would eat up the whole thing. Galvanizing is great, but galvanizing gets knocked off (especially during a driving process if you are not using an auger).

 

[cruzinbear]

Thanks for all the responses everybody.

WARose, regarding the 1/16", over what period of time? I know that depends on the corrosion properties of the soil for a given location and the type of metal, but can you give me a reference where I can find a conservative estimate or just get a general idea. Thanks.

 

[PittEng88]

I think what WARose means is that when you go to calculate your section properties of the pile, you would subtract a 1/16" of thickness off the pile and then determine your capacities based off of that reduced section. This is typically what we do for bridge foundations in PA when using Steel HP Piles in a potentially corrosive environment.

Are planning on driving these piles?

 

[WARose]

WARose, regarding the 1/16", over what period of time? I know that depends on the corrosion properties of the soil for a given location and the type of metal, but can you give me a reference where I can find a conservative estimate or just get a general idea. Thanks.

I actually looked at a reference I have on this last night (after I read your post). It is: "Corrosion of Steel Pilings in Soils", Monograph 58, National Bureau of Standards, US Department of Commerce (1962).

Basically they pulled a bunch of piling and sheet piling that had been in the ground for between 7 to 40 years (in soil that had a pH range from 2.3 to 8.6). The observed degradation (given in percentages) varied greatly and I don't have an average for it but I'd say most of the time it was around 10%. (With a range of values from virtually none to as high as 30%.) From that, I arrived at the values I've typically used.