grounding building steel

[lile001]

In a grounding class sponsored by the NFPA, the instructor insisted that the building steel frame could not be used as a ground for a separately derived system unless it was

A. Bonded to the main service entrance ground bus (obviously)

and

B. "Intentionally grounded" at several points i.e. there was a separate ground connection from the building steel to a ground rod, to a ufer ground, or a ground ring in addition to being bonded to the main ground bus.

I can't find any support in the code for B.

Assuming the main service entrance ground bus is properly connected to the water pipe, a ground ring, and a ufer ground, are these ADDITIONAL connections between the building steel frame and one of the grounding methods required?

What are the advantages or disadvantages of adding, say, a ground rod at each corner of a steel frame building bonded to the steel? Or bringing out a bare copper cable from some rebar and bonding it to the steel frame at 4 corners of the building? Do the steel bolts that hold the frame to the concrete bond it to the the rebar system, or are they not connected to rebar? Would these extra ground connections provide some protection against lightning, providing a ground path for lightning that hits the steel frame?

 

[sandsnow]

Re: grounding building steel

Up to the 2002 NEC, there was no direction as to what was "effectively grounded" (except for the definition in Art. 100) when talking about building steel.

In the 2005 NEC, conditions are stated as to when the building steel is considered an electrode and the words "effectively grounded" has been removed.

See section 250.52 (A)(2)1 (2005 NEC). Ten feet or more of structural member in contact with earth or concrete in direct contact with earth.
The anchor bolts could give you the ten feet.

Its kind of a catch 22 here as well. Even if you don't have 10 feet in contact with earth or concrete, you have to bond the bldg steel under 250.104 (C) (2005 NEC). Once you do that it then qualifies as an electrode under 250.52(A)(2) 2 or 3 (2005 NEC).

As far as adding ground rods at each corner of the building you will get differing opinions from engineers on this topic. I'll let someone else comment here.

 

[lile001]

Re: grounding building steel

Originally posted by sandsnow:

See section 250.52 (A)(2)1 (2005 NEC). Ten feet or more of structural member in contact with earth or concrete in direct contact with earth.
The anchor bolts could give you the ten feet.

...If they aren't painted....

Originally posted by sandsnow:

Its kind of a catch 22 here as well. Even if you don't have 10 feet in contact with earth or concrete, you have to bond the bldg steel under 250.104 (C) (2005 NEC). Once you do that it then qualifies as an electrode under 250.52(A)(2) 2 or 3 (2005 NEC).

I am thinking about calling for the building steel to be bonded to the rebar at 4 points (or more for a very large building) with a bare copper wire. This probably exceeds the minimum requirements in the code, however I think it is good insurance that the entire building is really tied together. It seems to me that bonding to rebar is a lower impedance and less likely to be damaged than a ground rod.

Unless of course there is epoxy coated rebar...

--Lawrence

 

[lile001]

Re: grounding building steel

Originally posted by lile001:


Unless of course there is epoxy coated rebar...

--Lawrence [/QB]

I emailed an architect, and he says he has specified epoxy coated rebar only once in 30 years. This may not be the case in salt spray areas or areas with odd soil conditions, but here in the midwest epoxy coated rebar is probably not very common.

 

[Riponite]

metal building frame

metal building frame

Don't forget that the metal frame scetion also states that it is a "single" member. Not quite sure anchor bolts fall into the single member category.

 

[iwire]

Re: grounding building steel

I am thinking about calling for the building steel to be bonded to the rebar at 4 points (or more for a very large building) with a bare copper wire.

This would not be an unusual request.

Many large buildings we wire include a 4/0 bare encircling the building with cad welded taps to rebars, building columns, metallic utilities etc.

The ring will usually have a couple of test wells where a bolted connection is used so that it can be broken for testing.

 

[don_resqcapt19]

Lawrence,

I am thinking about calling for the building steel to be bonded to the rebar at 4 points (or more for a very large building) with a bare copper wire. This probably exceeds the minimum requirements in the code, however I think it is good insurance that the entire building is really tied together.

What function will that serve?
Don