Earthquake Design
Earthquake Design
(OP)
I am designing a RC square shaped tunnel. This structure will set on grade and have a conical pile of rocks piled on top of it. I have questions pertaining to EQ design of this structure maybe someone can help:
The code I am using gives the "minimum required analysis procedures" that I am allowed to use. Are they referring to the minimum as being the most conservative? Or are they referring to being the least accurate? Which method is more conservative and or least accurate? Response Spectrum Analysis, Equivalent Lateral Force? Response History Analysis?
Since I am familiar with structural dynamics and computer modeling, I'd rather use RSA or RHA, but is this OK if the code say that the "minimum allowable precudure" is equivalent lateral force?
Lastly, does anyone have suggestion on how to account for the load that the pile of rocks will induce on the tunnel when an earthquake strikes?
Thanks. Please help if you can!!!
The code I am using gives the "minimum required analysis procedures" that I am allowed to use. Are they referring to the minimum as being the most conservative? Or are they referring to being the least accurate? Which method is more conservative and or least accurate? Response Spectrum Analysis, Equivalent Lateral Force? Response History Analysis?
Since I am familiar with structural dynamics and computer modeling, I'd rather use RSA or RHA, but is this OK if the code say that the "minimum allowable precudure" is equivalent lateral force?
Lastly, does anyone have suggestion on how to account for the load that the pile of rocks will induce on the tunnel when an earthquake strikes?
Thanks. Please help if you can!!!






RE: Earthquake Design
How the pile of rocks acts depends. I would assume they're "fixed" to the top/roof, basically from friction. Determine the weight of the rocks, its fairly simple from there. It seems like it would get messy if they aren't fixed somehow to the top, unless its a place where the rocks won't be able to move off the top (high walls on each side or some similar situation).
Are the rocks going to just sit on top of the tunnel? Or are they going to be attached somehow?
RE: Earthquake Design
A more refined (RSA or Time History) analysis will be more accurate for most structures but I'm not certain with a tunnel and a pile of rocks. Using this analysis it is more likely that you can design the structure so that it meets a owner specified critieria of being servicable following a large earthquake.
Another consideration is that particular to underground structures. Most will not see a large force from inertia like above ground structures do. Most concern is over the differential ground motion and the usual lack of movement joints in a tunnel to go along with the movement.
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Earthquake Design
If I chose to do a RSA or THA, what would be the best way to include the pile of rocks in determining the mass matrix for the eigensolution? Or similiarly, for the equivalent lateral force procedure, what mass should I use in coming up with the equivalent laterl loads?
Thanks for the help!!
RE: Earthquake Design
Lastly, I'm not sure that the person or entity that derived the design criteria for this project is correct in asking for the lateral force method. There is a great deal of soil structure interaction that must be accounted for which will not be in the crude lateral force method.
I suggest a discussion with a geotechnical engineer or geophysicist on the matter and check out ASCE's Journal of Geotechnical Engineering for related information.
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Earthquake Design
RE: Earthquake Design
You've mentioned it in your reply..."essentially acting like their attached"...they are not, so why include them. They could slide across the top in or out of phase with the ground motion. I would include them for vertical dead load though. This will keep the compressive force in the walls of the tunnel and help in the beam-column action of the walls.
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Earthquake Design
This seems similiar to the analysis of a suspended pool. In the case of a pool, I believe you are required to use a multiplier of 2.2, or something along those lines for the seismic lateral force of the water. There is a thread on this site about it.
RE: Earthquake Design
I would not be opposed to keeping the load in the lateral analysis given the right conditions.
But an enclosed pool offers a boundary condition that is at least consistent the a positive load path. that is the inertia of the water acting against the sides of the pool or in the case of a tank, the water mass acting on the wall of the tank.
In the case of rocks, it is just friction. However, I do agree that not all of the rocks may fall away from the tunnel.
Regards,

Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Earthquake Design
My two cents.