Bonding strap and dissimilar metals?

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Contrivers

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Hello all, my first post here. I was asked a question today and really didn't know the right answer, not unusual since I'm basically fresh out of school still.

We have a set of aluminum stairs that were attached to a steel structure, isolators were used so there is no mechanical connection as to prevent a galvanic reaction.(I was told this and have not walked it down to verify). I know these aluminum stairs need to be attached to facility ground, I just don't know how. I am assuming a bonding strap will be used but will it be exothermically welded, have drilled and tapped ends, and what kind of material is the strap to be made of?

I have read our specs at work and they mention, avoid bonding dissimilar metals, if you must make sure you remain at or under a galvanic reaction of 1 and they give you a chart outlining this. I have also been looking for a copy of MIL-B-5087 thinking it might hold the answer but can not find a copy.

I googled it and found a bunch of great info on adhesives and explosion welding which I don't think is right for this application. Is there a spec or a code somewhere outlining how to do this or am I looking to deep into this?

Thanks for any help.
 
Re: Bonding strap and dissimilar metals?

This same discussion comes up from time to time regarding bonding of an aluminum pool cage. 680.26(C) mandates the connections to be of stainless steel, brass, copper, or copper alloy. Some contrators instinctively use a an aluminum lug.
 
Re: Bonding strap and dissimilar metals?

Most compression type electrical connections are tin plated. Tim and aluminum are very close to each other in the galvanic series and there will be no problem.
Don
 
Re: Bonding strap and dissimilar metals?

Am I the only one who is wondering why you need to bond an aluminum staircase? It is not the sort of thing that I would suspect would be "likely to become energized." Generally, the nearest wire is about four feet above the steps, inside the outlet box that contains a light switch. What am I missing here?
 
Re: Bonding strap and dissimilar metals?

Pierre, when the stairs are in use they provide access from one platform level to another, when not in use they sort of fold/reel up the stairs. To keep the weight down they are using aluminum.

Charlie b, is it likely to become energized, I haven?t asked our AHJ's opinion to get his take on it but our design standard does mention that all structural members be bonded to provide a low impedance path through the structure to an earth ground should the structure sustain a lighting stroke. Which is a daily event during the summer months.

I didn?t find anything in our design standard that addresses the likelihood of a conductive material to become energized by the facility power system and what precautions should be taken if it was likely or unlikely. It also didn?t mention what kind of strap is to be used when bonding dissimilar metals.

Lightning aside, I would hate to say no, don?t worry about bonding it and then the unlikely event occurs and hurts someone. I imagine these straps are reasonable priced.

Btw, if it wasn?t bonded and left isolated, isn?t it possible the assembly could build up a static charge?
 
Re: Bonding strap and dissimilar metals?

Originally posted by Contrivers:
. . . our design standard does mention that all structural members be bonded to provide a low impedance path through the structure to an earth ground . . .
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I am doubtfuk that these stairs would be considered a "structural member", so I'd ask the structural engineer, and I'd ask that a drilled tab for a lug be part of the next set of stairs if it is required.
 
Re: Bonding strap and dissimilar metals?

Originally posted by Contrivers:
Btw, if it wasn?t bonded and left isolated, isn?t it possible the assembly could build up a static charge?
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Unlikely unless close to a strong source of R.F., like a TV or radio transmitter. If that occurs, a simple braided strap bolted to the stairs and to any nearby grounded metal should suffice.
 
Re: Bonding strap and dissimilar metals?

I am with you Charlie B. I see no reason to bond the stairs, unless I am missing something.
 
Re: Bonding strap and dissimilar metals?

Originally posted by Contrivers:
our design standard does mention that all structural members be bonded to provide a low impedance path through the structure
Click to expand...
Stairs, particularly those that "sort of fold/reel" are not structural members of the building.

That would be like saying a metal filing cabinet is a structural member of a building.

In my opinion Structural members are those that add support to the structure.
 
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