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Ground Continuity of Supporting BOLTED Steel Structures

Ground Continuity of Supporting BOLTED Steel Structures

Ground Continuity of Supporting BOLTED Steel Structures

(OP)
Hi all,

WE have an installation involving providing additional structural steel bracing to an existing steel building (administration building)

This EXISTING steel building, prior to installation of supporting bracing structures, is already bonded to the existing underground grounding grid at multiple points.

The new additional structural supports are connected to the existing building columns at around 1.5~2.0 METERS away along the perimeter at multiple points along the building perimeter and are attached to the existing structural columns, and are also supported by 10 meter concrete pile foundations. These structural bracings are bolted to the building columns, painted with anti-corrosive paint and are interconnected to one another as well.

On my technical assessment, we have considered these structural bracings AS PART OF THE EXISTING BUILDING structural framing and are effectively bonded to the existing building frame by mechanical bolted connections at multiple points (around 12 points). As these bracing structures are also interconnected to each other via structural members (forming like a small "piperack-like" configuration), we consider them as multiple paralleled resistance path. Their piles are also around 1.2-2.0 meters away from the existing building piles (10 meter pile) which would be within the existing building's zone of influence.

In this regard, I would believe that there is no need to provide extra supplementary ground connections to the existing grid as they already are:

a. Forming part of the existing structural framing of the existing building which in turn is already bonded to the grid on multiple points around the building (at all corners)
b. The new structural braces are bolted to the existing building columns are multiple points and are structurally interconnected to one another by bolted connections
c. The structural bracings are around 1.2 to 2 meters away from the building

I would appreciate any actual encounter of such and any reference to it would be highly appreciated. Sorry for the long story.

RE: Ground Continuity of Supporting BOLTED Steel Structures

As I understand, the problem it is at the plate PL13 connected by bolts.
If the dimensions are in mm the contact surface it is about 140*140=19600 mm^2.
If we consider NFPA 780/2011 recommendations:
4.16.3 Connections to Framework. Conductors shall be connected to areas of the structural metal framework that have been cleaned to base metal, by use of bonding plates having a surface contact area of not less than 5200 mm2 (8 in^2) or by welding or brazing.
So this it will be a very good contact-in my opinion.
NEC Art.250.8 Connection of Grounding and Bonding Equipment.
(A) Permitted Methods.
(5) Machine screw-type fasteners that engage not less than two threads or are secured with a nut.

RE: Ground Continuity of Supporting BOLTED Steel Structures

(OP)
Thanks Zanoter4,

What about the bolt threads' contacts? if there engaged more than 2 threads, will they suffice and be equivalent to NEC 250.8 (5)?

My practical logical understanding is that, even with the existing building framework, I am pretty sure most members within the building are also bolted together as a framework. I am considering these new additional structural "braces" as already part of the building framework and are overall mechanically bonded (or electrically) to the already grounded building framework.

They were only applied with anti-corrosive paint and I would think the painted surfaces are not considered electrically insulated.

I would like to get feedbacks as it seems, it is a norm to bond every new structure. However, I am taking these building braces as an exemption. Having these new (and already installed) brace structures, installed with underground stabs would require a lot of work at site at this time.

RE: Ground Continuity of Supporting BOLTED Steel Structures

If the concrete pilings are not yet poured, consider dropping a bare cable down each hole and uffer ground the new construction.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ground Continuity of Supporting BOLTED Steel Structures

(OP)
Thanks Waross,

Unfortunately, the piles have been poured and installed already. I am looking into justifying that bonding these extra structures separately to U/G grid is generally not required because of the way they are built around the existing building.

RE: Ground Continuity of Supporting BOLTED Steel Structures

(OP)
Does anyone know what is the Electrical Conductivity of an anti-corrosive paint primer?

I am looking into this on a different perspective. If there is sufficient conductivity of painted steel structures interconnected into numerous joints, I would think the combined parallel resistances will be very low to comply with the bonding continuity.

Looking into existing structures, I have seen, like pipe racks, they are bonded to the grid on alternate segments (not all segments are bonded to grid). The same applies that their joints and members are painted and interconnected by bolts. Hence, they are not ideal still.

For the application I posted, these bracings are all connected to the building structure by numerous bolted connections that I would consider them already part of the building grounded structure.

RE: Ground Continuity of Supporting BOLTED Steel Structures

I haven't seen painted pipe racks for some time now.
The racks that I see are constructed of a steel similar to COR-TEN steel and are not painted.

Quote (WIKI)

Weathering steel, often referred to by the genericized trademark COR-TEN steel and sometimes written without the hyphen as corten steel, is a group of steel alloys which were developed to eliminate the need for painting, and form a stable rust-like appearance after several years exposure to weather.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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