Structural Effects of a Lightning Strike 2

[shesanEIT]

Does a lightning strike apply any additional forces to a structure?

I have done some research on this and have come across: "Any conductor carrying lightning current will be subject to an electromagnetic force F=Bli in Newtons."
B = the component of the magnetic field at right angles to the conductor
l = length of the conductor (meter)
i = the current (ampere)

Would this be the only force applicable? Is there another known way to quantify this? We are designing a tall structure and want to ensure there are no additional loads from a lightning strike that would affect the structure, even though we can't imagine it would be enough to even consider.

Thank you!

 

[TehMightyEngineer]

I'd imagine some thermal stresses at the location of the strike, but otherwise the effects should dissipate rapidly as you move away from the strike location.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[shesanEIT]

Thanks @TehMightyEngineer

 

[Lomarandil]

Exactly, thermal forces. There's probably an analogy to the short circuit force that is checked in substation bus (electrical bus, not automotive bus) structures.

 

[TehMightyEngineer]

Stumbled across this: [URL unfurl="true"]http://www.hitachi-infra.com.sg/services/_social_infrastructure_systems/paravane/thunder/index.html[/url]

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[shesanEIT]

Thank you @Lomarandil,that sounds pretty close. I found an equation for the short circuit force in substation bus structures and will apply it to approximate.

 

[shesanEIT]

Thanks @TehMightyEngineer. "When lightning strikes a lightning rod, a large lightning current flows into the ground through the lightning rod conduction wires and the steel framework of the building."...wonder what would happen to the steel structure if there was no lightning rod?

 

[EdStainless]

I have seen a building have to be rewired and replumbed after a lightning strike.
They pulled out long lengths of wire, and 30' down the wall was where it found an easier path and blew the insulation off, and arced a hole in the water line.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[thaidavid40]

In addition to the electrical motive force, there will also be a thermally-induced air-pressure shock wave (a big "boom", for those of you in Rio Linda!). Depending on where the strike occurs, and what materials surround the strike site, you could very well see shock-displaced structural (and non-structural) members and elements.
Dave

Thaidavid

 

[shesanEIT]

Does load factoring take this as an extreme event like seismic?

 

[thaidavid40]

I don't know that I have ever seen "lightning strikes" listed as a specific IBC or ASCE load. I do think that you are probably on the right track in accounting for it as an "extreme event", to be evaluated as an ultimate load, to be compared to ultimate tensile strength limits.
Dave

Thaidavid

 

[TehMightyEngineer]

I would imagine a bigger concern than the load would be the risk of fire.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[shesanEIT]

Thanks for the help folks

 

[thaidavid40]

I forgot to mention that, for the aspects of this design related to the structural effects, you might want to approach this from the same standpoint as weapons blast effects on structures. These is a wealth of that information from the military codes and guides on the WBDG website.

Thaidavid

 

[shesanEIT]

Interesting approach, I will take a look at those codes. Thanks for the reference!

 

[Ron]

Properly constructed steel framed buildings conduct lightning to ground without affect; however, poorly constructed steel framed buildings where gaps exist at connections and there is "looseness" in the connection have a potential for arcs and welding at those locations. That is particulary problematic for high-strength bolts.