equipment pad 1

[newdnbea]

for seismic design ASCE requires components to be securely fasten to the supporting structure. sometimes we design concrete pads to support an HVAC units outside buildings. it seems funny we anchor the unit to the pad but the pad itself is not securely attached to something else. since the pad is embeded very shallow to the soil to me the pad and the equipment can be thrown the air together during an earthquake. is there a min embedment requirement for shallow foundations? thanks.

 

[JedClampett]

If you were to calculate the weight of the pad, you'd find that the mass alone is enough to "anchor" the pad. If you were to follow the loads, the frictional resistance alone is enough to overcome the seismic.
Beyond that, you have passive soil pressure resisting any movement.
In northern areas, minimum embedment is governed by frost depth. Here in the southwest, it's more of a common sense issue, and one or two feet deep feels right.

 

[newdnbea]

that is because you only calc the seismic load using the weight of the equipment. if the pad just sit on top of the soil i would think the seismic load calc should consider the selfweight of the pad also although this may not be addressed in any code probably.

 

[newdnbea]

i just feel it is funny we want to use the pad to secure the unit. but who secure the pad?

 

[newdnbea]

well the real seismic load the unit gets is from soil through the pad although code assume a fake load from the air. probably it is better not to secure the pad to the soil to get less seismic load. i dont know.

 

[photoengineer]

I believe the pad is heavy enough to prevent the generator from tipping over or moving an excessive amount. It also ensures that the generator will move as a unit and not be stressed by the ground motion. Because it's not possible to isolate the generator from a seismic load, you will get some movement (and should design flexible fuel lines and power lines with slack) regardless of how it is secured.

Cedar Bluff Engineering

http://cedarbluffengineering.webs.com

 

[cntw1953]

During an earthquake event, the earth moves both vertically and horizontally, and the direction of excitation is reversable. However, the vertical excitation coefficient is such a small fraction of gravity, it would not lift anything up to the air, but without fastening, the equipment could bounce on the pad (it's mass and frequency differ from the concrete pad), subsequently damage the pad, or the equipment.

Watch out potential for shear failure (one & two way) if the pad seemd thin. Also, can the equipment tollerate uneven settlement caused by frost heave? Throw some thoughts on these.

 

[msquared48]

newdnbea:

You could take that line of reasoning with any structure. The result would be ridiculous. The buck has to stop somewhere and it is with the foundation.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[newdnbea]

thanks everyone. i guess i get some kind of answer for myself. methink the footing needs to be anchored to the soil to resist the seismic load is because the corrent code method assumes a pseudo force applied to the structure from the air. but the reality is upper structure gets seismic load is because its footing anchored in soil, the real load is from the soil. how can a floating slab get seismic load and how much is it? code use different equation to get another pseudo force... is it better not to anchor the footing?

 

[cntw1953]

During a seismic event, the lateral excitation will cause things slide side ways, and depending on center of mass, overturning, or tilt, could be the result, if the footing wasn't well anchored, and/or there was weak base below.

Earthquake is the movement of earth plates, structural damages are due largely to non-synchronized/random motion/displacement between earth and structures.

The phenomenon of dynamic earth-structure interface alone is complicate enough by itself, I sincerely hope the AIR wouldn't get into this mix. But, what a theory, and what I know! Maybe given another 100 years the Air Dynamic (not related to wind, but earthquake), Soil Dynamic and Structural Dynamic would be linked together in the scientific textbook. Glad that I won't be there to see it.

 

[newdnbea]

the unit i am dealing with is very short. i mean it's almost impossible to tilt it unless soil moves vertically in 2012. without anchoring the floating slab could slide and clide with something else? i think the code should address the floating structure differently probably. an flying airplane will not get any seismic load unless you anchor it to the earth.

 

[cntw1953]

You need to refresh your understanding on seismic forces. The knowledge might not be important for a small, non-essential equipment/structure, one day the building by you may fall to ground because the believe in - no vertical soil/structure movement at such event. Good luck.

 

[newdnbea]

i do think we need to anchor the foundation as i mentioned above even for a small one or it may clide with something else which is why i post it here. code maker may need to add something for anchoring the floating slab to make the code more reasonable and less funnier at least be consistent. if you think the unit needs to be anchor to the floating slab for some reason. what if i treat the floating slab and the unit on it as one bigger "unit" or skid. does it need to be anchored on its support which is the soil? it just easily makes me feel logically uncomfortable even before i realize what it is.

 

[steve1]

I always thought that the factor's of safety against overturning and sliding ensured that the foundation would act as an anchor.

 

[sandman21]

You have passive forces along each face, friction along the bottom, and soil bearing, the soil provides support.

 

[newdnbea]

ASCE says that all the components shall be bolted, welded or positively fastened to their supports without considering friction. i dont want to debate what is a nonstructural component or what is a nonbuilding structure. i believe a skid, a hvac unit or a transformer supported on a floating slab shall be considered a component. so the unit shall be anchored to the slab without considering friction. once fasten together i think i have the freedom to think the unit and the floating slab is one big unit you can disagree if you want. following the same rule it should be fastened to its support without considering friction. a lot of project specs suggest to ignore the top 3 ft of soil for passive resistance. then if you have a pad embebed less than 3 ft then it is not anchored. no friction no lateral soil resistance.

that is why i think we may need a min embedment for the pad.

 

[cntw1953]

Foundation or no foundation. Answer to this question should be weighted through the function of of this equipment. Is it an essential element that requires non-interrupt operation at any event? Does it cost more to replace than the cost of foundation? What are the worst consequences if it fails....

Judgement is more important than code.

 

[sandman21]

If a geotech. report states you cant use the top feet of soil, clearly you need to find a different way to anchor the unit.

Every geotech. report I have read allows the use of both passive and friction to resist loads, unless the soil are so bad or there is undocumented fill. The IBC also allows the use of friction. There is no reason, when soil are not an issue that, friction and passive pressure can not be used to resist loads.

 

[StructTaco]

I design in California, for seismic loads mainly, and have done many equipment pads for large pieces of equipment.

The equipment must be anchored to the slab through mechanical means, not friction, per the code.

The seismic load is applied at the C.G. of the equipment to calculate the overturning moment. The foundation/mat slab will provide resistance to the overturning moment through the weight of the slab itself...so the anchorage or connection to the slab must be able to resist these overturning forces (these will be tension loads).

The seismic load must also have shear or lateral resistance, this is accomplished through the foundation interacting with the soil, usually friction will be enough in equipment pads such as this.

So the equipment anchors have to be designed for the tension due to the uplift due to overturning and the shear due to the lateral load.

Usually the pads are about 12" thick, sometimes with a turned down edge to get the minimum 12" below lowest adjacent grade. The soil needs to be sufficient to support the anticipated soil pressures due to dead, live and seismic loads, and is usually far less than the code minimum of 1500 psf.

The code also says that any structure must be separated so that adjacent structures do not pound against each other. But equipment pads are usually on the exterior, ground mounted and away from the structure. This is usually not a concern at all in equipment pads.

The weight of the equipment and pad are what keeps everything in place, further anchorage is not required such as soil anchors, piles, deepened foundations etc...

That's my 2 cents, although I usually charge 3 cents for all that...

 

[newdnbea]

thanks taco. that is i usually do also. if the equipment need to be anchored to pad without considering friction i just think the pad to soil should not consider friction also for the same reason. just try to be consistent. anyway. it seems like nobody really think that way. thanks a lot.