Seismic Calc - Underground Structures

[ReverenceEng]

Easy question (I hope).

I have some equipment that is being mounted to the floor, walls, and ceiling of a concrete tunnel (all four sides) that goes under a tarmac. Think: typical walkway that connects terminals underneath the tarmac.

I would normally use the SEISMIC DEMANDS ON NONSTRUCTURAL COMPONENTS PER CHAPTER 13 of ASCE 7-10, of which the ratio of attachment height to height of structure plays a role in the overall resultant seismic force using ASCE 7-10 Eqn 13.3-1.

However, I'm not sure the Z and h values are entirely representative of the situation here. Maybe I can't or shouldn't even really use this calculation.


Should I use this section?

If so, how should I resolve the z and h ratio? z/h=1.0 for ceiling mounted equipment seems a bit much considering this entire thing is below grade.

 

[WARose]

You should check it because the tunnel itself will be set into motion during a seismic event. I'd probably use the z/h=1 if that what it comes out to (that is also the limit IIRC).

There are some other ways to do it....but probably not justified for figuring the anchorage of a base on some minor piece of equipment.

 

[azcats]

Barring some odd circumstance, for an item below grade (or seismic base), I'd use z/h = 0 in that equation.

Check out the expanded seismic commentary:

https://ascelibrary.org/doi/pdf/10.1061/9780784412916.chc11c22

(hopefully that link works)

 

[EDub24]

I would also use z/h=0. That ratio is meant to account for additional seismic forces due to the building displacement relative to ground displacement. A tunnel moves with the ground so there is no additional displacement.

 

[WARose]

A tunnel moves with the ground so there is no additional displacement.

I'm not sure if I buy that because the tunnel has (theoretically) differential movement relative to different points in the tunnel. (Especially if it is large.)

 

[miningman]

I know miners who were underground during a local earthquake and didn't know anything had happened until they came to surface.........extrapolate from that what you will..

 

[ReverenceEng]

I was thinking that Z=0 would perhaps make sense, since, well, I would use z=0 if this were a building and the attachment is at grade. I wasn't confident since I've never worked in a situation like this, but I concur with z=0. It's not saying no seismic force, it's just not adding in effects for being higher than grade.

 

[IDS]

I haven't looked at analysis of non-structural components in underground structures, but I have done many seismic analyses of underground structures, and I agree with WA Rose. The void inside a tunnel does result in amplification of the ground movement. I'd suggest starting with the most conservative interpretation of the rules for a simplified analysis, and do a more refined analysis if it might give significant savings.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[bones206]

I generally use z/h = 0 for below-grade components. z/h = 1 is really conservative. If you want to account for soil-structure interaction effects that could amplify the movement of the tunnel relative to the surrounding soil, then you could use a z/h factor between 0 and 1, but it would be a judgement call. To get a more accurate estimate of that factor, you could generate a floor response spectrum for the tunnel that incorporates the effects of soil-structure interaction. But I've only seen that done for nuclear plants.

 

[BUGGAR]

If the tunnel is critical for a worker's safe exit, I suggest you go conservative. No steam lines down there, I trust.

 

[bones206]

You could always use a higher importance factor if the component is critical for safety.