Seismic Load for Underground structure

[andy1213]

This sounds silly but Is there a seismic load for structure below the ground level? I.e. a 3m x 3m x 2m block foundation and embedded 1.5m below the soil. My colleagues calculate the seismic force as a factor (0.1~0.4) of the foundation weight using Table 18-I-C of UBC 94; I think this is wrong. In my opinion, think there is no seismic load for portion of structure below ground level. Please correct me.

 

[Qshake]

If the object has enough mass and can move out of phase with the surronding soil, a seismic load should be applied. Also, a deflection should be investigated for the foundation depending on what is founded on it. If there is sensitive equipment which is intolerant to displacment a stiffer foundation may be required etc. In short there are many aspects to be concerned with in this case.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[cvg]

seismic forces are transmitted through the ground, so obviously the loads are still there. In fact, arguably, they might be higher since you have intimate contact with the ground through which the shear and shock waves are traveling.

The lateral support for the structure however is guite a bit better than for one above ground.

 

[DTJ]

IBC 2003 does not require you to include these forces unless you are in a SDC D or higher. On a recent project we ran into this, and it increased the forces on the retaining wall my almost double.

 

[andy1213]

Thank you for the response guy but I want to make my question clearer. I have that block foundation supporting a vertical vessel; I have the loads including seismic loads provided by the equipment group, which I only have to load on the top of foundation. My question is, if I still have to calculate the seismic load due to the weight of foundation alone and how can I compute that? Sorry but I cannot find any reference to do that. As I said earlier, my colleagues just compute by assuming 0.1 to 0.4 of the foundation weight yet they cannot show any references to justify it. DTJ, plant is located in seismic zone 4.

 

[kslee2006]

The seismic force has negligible effect on the concrete block foundation itself, however, the increased pressure on the soil below and the overall structural stability need to be investigated.

 

[Qshake]

While I agree with the overall structural stability comment by kslee2006, I disagree that the seismic force has a negligible effect on the foundation.

You must transfer lateral and vertical load via a path into the ground. Unless the concrete block is keyed into rock, how does it transfer lateral load to the surrounding soil? In my opinion - in one or two means or a combination thereof. first is the passive pressure of the earth on the footing and the second is the friction at the base of the footing. If the ground moves independently of the foundation you can't use the passive pressure and you're left with friction. Since the frition is acting at the base of the concrete block why isn't the inertial force of the block considered? It should be.

If the friction is found to be insufficient then you should use anchors or something similar to provide a positive connection into the soil.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[haynewp]

I have often wondered about seismic loads on underground structures and retaining walls. I know AASHTO addresses seismic on retaining walls. It seems to be generally disregarded in my area which has always kind of bothered me. I am not sure about how closely it is addressed in areas like California. I wish I knew of another good reference regarding this.

 

[Taro]

I have worked on many projects on the U.S. west coast where the geotechnical engineer has specified a dynamic load to be applied for earthquake effects. It has typically been a uniform pressure distribution or a concentrated load applied at 0.6 times the depth of the structure below grade. I believe they use some variation of the Monobe-Okabe equation to determine the dynamic load. One reference that discusses this is Chapter 7 of the 2003 NEHRP Commentary (

http://www.bssconline.org/NEHRP2003/comments/).

 

[NaderShokoufi]

There area many different ways of considering the earthquake effect on an underground structure. The most common one is the Monobe-ocabe method. but there are more accurate methods like displacement method : in this method, a displacement is applied to the underground structure, this displacement will be calculated as a factor of the soil type and the depth of the bedrock. the method is complex and is not convinient for small projects but would work perfectly when designing big underground structures like Subway stations.
You can also model the interaction behavior of soil and structure but the results would be much dependent of some unknown soil factors !
in your case, if the soil around the foundation are adequately compacted, I guess that there would be no need to worry about the earthquake load on the foundation.