Quick and Dirty Seismic Analysis for Environmental Structures?

[CivilPipe]

Does it exist?

I design environmental structures (sewage pumping stations, water booster stations, tanks, basins, etc.) that fall into seismic design category B and C. Some of these structures have small masonry electrical rooms. I am looking for a quick and dirty approach to checking seismic on these structures. Any suggestions?

 

[JedClampett]

It's not that hard to do a Base Shear analysis. And I think there's even a simplified analysis allowed for regions of low seismic accelerations.
But watch out, sewage structures fall into Design Class III, so your seismic accelerations need to be very low to be in Category B or C. And if a booster pump station is necessary for fire suppression, you're in Design Class IV.

 

[CivilPipe]

Jed,
SPS I am working on now is:
Occupancy Category III
Site Class B
Ss = .322 g
S1 - .125 g
Fa = 1.0
Fv = 1.0
Sms = .322 g
Sm1 = .125 g
SDs = .215g
SD1= .089
Which gives me SDC B.
Does that seem right to you?

At what location on a deep buried structure do you check the base shear. It would seem to me that the greatest shear would be near the top of the ground and that you would only consider the mass above that point.

What about the change in earth pressure due to seismic? Most of the buried structures I design are located in the flood plain and I usually consider the ground water to be at the same elevation as the top of the structure. It is very unlikely that this saturated condition and an earthquake would occur at the same time so I think I am pretty safe neglecting the increase in earth pressure.

 

[JedClampett]

I'm away from the office, but I'll check on Friday.

 

[CivilPipe]

Jed,

Have a chance to look at this yet?

Thanks!

 

[JedClampett]

It looks like you were right. I'm just not used to many water or wastewater facilites falling into Design Category B. But you're still going to have to use the Equivalent Lateral Load method per ASCE 7-05, Section 12.8. See Table 12.6-1. It's really not that tough.

 

[CivilPipe]

Thanks Jed,

Where in a deep buried structure do you usually check the base shear?

It would seem that the base for seismic purposes would not be the base of the structure but some point above the base near the ground surface.

What about lateral earth pressure do to seismic?

Thanks!

 

[JedClampett]

Seismic base shear is calculated at the ground surface. This was discussed in the Structural Engineering Other Topics thread a while ago.
Lateral pressure from seismic is given in the Geotechnical Report. The ones I've seen are maximum at the ground surface and vary to zero at the base. The maximum is usually only 15 or 30 psf. It's usually not significant. I add a vehicle surcharge and ignore the lateral seismic soil, figuring it's very unlikely they would be simultaneous.