ASCE Eq. 13.3-1 for equipment on its own foundation

[chris3eb]

Hello all,

I am designing a foundation for a generator. I am using equation 13.3-1 from ASCE7 05:

F_p= (0.4a_pS_DSW_p)(1+2z/h) / (R_p / I_p)

Where:
z = height in structure of point of attachment of component
with respect to the base.
h = average roof height of structure with respect to the base

I am considering the tank and the foundation separately. The tank has a height of 2.7' and the generator (which sits on top of the tank) has a height of 6.1'. I believe z for the tank would be 0' and for the generator, 2.7'. What would h be since there is really no roof height? Should I use the top of the generator (6.1' + 2.7'= 8.8') for h for both the tank and the generator?

Thanks,
Chris
(By the way, this is my first post, but I plan on frequenting these boards!)

 

[msquared48]

The overturning forces for the generator is the required % of the weight lumped at the CG of the generator times the distance from the CG of the generator to the mounting bolts to the tank below.

The overturning for the tank, since the generator is mounted above it, is the sum of:

1. the % weight of the tank times the distance from it's CG to the platform where it is mounted, plus
2. the % weight of the generator times the distance from it's CG to the platform where the tank is mounted.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[chris3eb]

msquared48,

What you are wrote is how to calculate the overturning moment once the horizontal seismic force has been calculated. What I am looking for, though, is how to calculate the horizontal seismic force since the equation seems to be set up such that the equipment is actually enclosed in a building (Which in my case it is not).

Thanks,
Chris

 

[msquared48]

So you are concerned with the overturning forces generated by the generator and tank on the building structure itself then?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JedClampett]

I always use 3 for the number in the parentheses (1 + 2z/h). It can't get any larger than that and it avoids me thinking about this very question.
I think it's just a way to vary the accelerations based on relative height.

 

[nutte]

Jed, that's fine if you're in a low seismic area and the forces are small. In areas with large seismic forces, this may be prohibitivey conservative. This is a question I've wondered before, but I don't have a good answer, either.

 

[chris3eb]

msquared48,

I should have been more clear before. There is no building. The generator (and tank) is being installed outside on its own independent foundation.

I am referring to calculating F_p on page 144 of ASCE 7-05 (Equation13.3-1). I know how to check all of the failure mechanisms once I get F_p.

JedClampett,

Yes that is always an option as the code specifies that z/h need not exceed 1.0. But you have the possibility of tripling your seismic force.

Thanks,
Chris

 

[SteveGregory]

Agree with Jed, use z/h = 1 for rooftop equipment (or below) and z/h = 0 for equipment at grade.

 

[ChipB]

Chris,

You are using the equation(Equation 13.3-1)to design the force delivered to the foundation. Code is really not very clear in this area. If you look at the title of Section 13.3, it's for the design of the equipment itself (i.e. tanks shell thickness, bolts of the generator to the tank, etc.) This is the responsibility of the equipment vendor, not the foundation engineer. This force is always going to be high. The equipment vendor isn't applying a redundancy factor or an over strength factor. If you use this force in your foundation design, you are double dipping when you apply the redundancy and over strength factors in your load combinations.

You need to go to Chapter 15 for your foundation design. Given that it is a tank, I would venture to make an educated guess, you are going to fall under Section 15.4.2

Hope this helps,
Chip

 

[chris3eb]

Chip,

I think you have have it right about using chapter 15.

An article in Structure Magazine (

http://www.structuremag.org/article.aspx?articleID=1405)

gives the following rule of thumb: if an item is self supporting (i.e. sits directly on its foundation) it is a nonbuilding structure, whereas most other items in industrial facilities are components.

Section 15.4.2 (Rigid Nonbuilding Structures gives the equation V=0.30S_DSWI which is essentially the same as the lower bound in Section 13.3.1 F_p=0.3S_DSI_pW_p

Section 13.3.1 says the redundancy factor is 1.0 and ignores the overstrength factor. It is my understanding that I would also use a redundancy of 1.0 (12.3.4.1 condition 4) and ignore the overstrength factor (R didn't help me so Ω can't hurt me) using Section 15.4.2.

Section 15.4.2 gives me V, but I believe that is the same as F_p applied to the center of gravity of my generator.

The generator has a 400 gallon tank underneath it. I am assuming that I don't need to worry about section 15.7 Tanks and Vessels because it seems like that is for large, field fabricated tanks, not small prefab tanks.

 

[JStephen]

I would assume that a 2' high tank under a 6' generator was just part of the generator assembly.

 

[Michel60]

Good afternoon Chris3eb,

Often the cut sheets for generator sets and other equipment will have the CM dimension provided considering the wet weight of the unit, so there should be no reason to separate the generator section from the belly tank. Check and see if you can obtain some of the manufacture's literature. And if you're unit is ground based z=0 as others have suggested.

regards,
Michel