Does a high percentage of steel prevent lightning damage? 1

[Barbicane]

The Electrical Engineers want to spend less on lightning protection by using the legs of self supporting towers, 40 to 80 feet high as the down conductors. This has been allowed by NFPA-780 for years. The National Electric Code has permitted the use of rebar as a grounding electrode for decades. Doesn't ACI permit foundations without rebar under limited conditions? Part of the transition to the new system is greater reliance on exothermic welds. I don't like heating high strength steel to cherry red with this process because it weakens high strength steel. Pipe sections are especially vulnerable to this process. Guyed towers do not seem to be as vulnerable to lightning strikes as self supporting towers probably because the charge is divided as it travels down the metal pathways. One of the reasons we have "overdone" the lightning protection was a singular event that caused a 40 foot self-supporting tower to nearly topple because a #6 wire in a 3/4 inch steel conduit caused a high resistance connection that generated so much steam so fast that it blew out a sizable chunk of a 5 x 5 x 5 foot tower foundation. Does extra steel in a foundation have benificial effects on overall conductivity? Does carbonization reduce conductivity?

 

[VoyageofDiscovery]

Hi Barbicane,

I am surprized that the electrical engineers want to go this route, spend less on electrical wiring at the detriment of the structure. I doubt the increase in steel within the foundation will prevent the destruction of the foundation. Conductivity is more a function of site specifics like fractured rock or the presence of certain soils than the amount of steel in the foundation. Typically guyed and self supporting structures are designed with lightning protection in a crow foot configuration and tested for ground conductivity for insurance purposes as well.

HTH

VOD

 

[BigH]

Let me tell you - I have lost more electronics since moving to Asia (with no lightning protection) than I want to see - and this even when pulling out wires when we see a thunderstorm coming (two tvs, two stereos, a computer motherboard, a satellite dish box) - put in the proper grounding!!!!!!

 

[flsigns]

Since most AJH codes do not allow rebar to contact soil on exterior of footers, how can extra rebar improve any grounding aspects of the structure. Anybody done any studies on "bottle brush" lightning disapators?

 

[Barbicane]

The standard rebar cage is usually 6 inches less in every direction than the nominal dimensions of the foundation. This looks a lot like a Faraday Cage. Could this steel tend to exclude lightning from the core of the foundation? Can lightning even penetrate to the steel since this would violate our current understanding of the skin effect?
This contract is very unusual in that it requires "randomly spaced vertical bars" throughout the middle of the foundation to hold up the top mat. It really does say that. How do you inspect such a thing? The theory behind letting the contractor drive bars into the ground to support the upper mat is that there will be too little oxygen at a depth of six feet to do much damage to the steel. Freeze thaw is not a problem.

FAA has done experiments with bottle brush systems in Orlando Florida and replaced them with a standard Franklin System (air tips) after a couple of years. FAA also did experiments in Tampa, Florida with umbrella type protection, which is a spin-off from NASA research to protect the space shuttle. They took them out too after a couple of years and replaced them with a standard Franklin system

 

[MikeDB]

Concrete foundations can be used as grounding electrodes. IEEE 80 has a chapter that deals with that. Lightning traveling thru the concrete (via steel structure to anchor bolts to rebar) can vaporize moisture in the concrete causing the concrete to crack or exploded. If the rebar is all bonded together, then more steel should spread the current out and reduce the flow on any one bar.

Our company uses two different methods. One is to attach ground wires to the steel above the foundation and attach them to a ground grid of buried wire and perimeter rods. This provides an alternate path around the concrete and if lower resistance, will carry most of the current.

The other method includes the above as well as bonding the anchor bolts to the rebar and extending the wire out of the foundation to the same ground grid as above. Not being an EE, I can't say which is best, but I like the first method because it is less expensive and I would think that the current flow thru the concrete would be less.