Equipment Foundation Pad with Eccentricities 2

[RDR89]

Hi all, I have an equipment foundation pad as shown in the attached. Wind load is pretty high (V=170mph) and the equipment itself is slender in the short direction (B as shown). The equipment itself is offset from the center of the foundation pad (shown by e1) but due to the lateral load, there is also a “moment” (I put moment in quotes because the equipment is bolted at the base so really there is a force-couple acting). I tried to find what my additional eccentricity would be based on the moment and applied load (e2) but turns out that e2 is larger than the desired width of the pad. There are constraints preventing them from making the pad wider. I am not sure if I am looking at this correctly. If I am, I don’t know what to do aside from making the pad super deep. Any thoughts/advice?

Edit: I will also add that the equipment itself is only ~540 lb so it does not provide much resistance to OT.

 

[jayrod12]

You could use helical piles which would provide uplift resistance better than just self weight of pad.

I assume since you said "desired" width of pad, that you could make it larger, but someone (likely owner or contractor) has had their mind set on a specific size. It would likely be far cheaper to just widen the pad.

 

[RDR89]

For sure. I guess I’m also looking to see if I am analyzing this correctly before I tell them it can’t work.

 

[jayrod12]

Honestly, it sounds like you are. If they're giving you a moment from operation of the equipment, then that plus the wind load, would produce overturning. And having it off-centre of the pad makes it less efficient from an overturning stability perspective.

So yeah, my recommendation to the client would be it needs one of the following:
- Wider pad
- Extremely thick
- Tension anchors like helical piles or the like.

They can have a contractor price the three options and when they tell them the wider pad is cheapest, then they look like the hero.

 

[SlideRuleEra]

There are constraints preventing them from making the pad wider.

...equipment itself is only ~540 lb so...

...I don’t know what to do aside from making the pad super deep.

Must not be a very large pad, which may make a difference. To get the best answer, numbers need to be evaluated... not just concepts.

What is the pad's planned thickness now?
What thickness is "super deep"?

Reason for question: Engineer's idea of "super deep" may not be the same as the Owner's.​

What is the pad's concrete volume now?
What is the concrete volume of the "super deep" pad?

Reason for question: Going from, say, 2 yd[sup]3[/sup] to 4 yd[sup]3[/sup] in not a big a deal as 6yd[sup]3[/sup] to 12 yd[sup]3[/sup].
​

 

[RDR89]

Good points. They want the slab to be 5’x3’x12” deep. In this case B=3. When I said super deep, I was hypothesizing, sorry. Hadn’t actually crunched the numbers. So yea the pad is pretty small

 

[RiverBeav]

Just a trouble shooting comment, but just making sure that you are including the weight of the footing when you are determining the total eccentricity on the system (i.e. Total Resisting Moment = weight of footing*B/2 + weight of equipment * B/2-e1). The equation on your drawing is just talking about the weight of the equipment having too big of an eccentricity, but the equipment can be anchored to the concrete pad for tension to stabilize the equipment itself.

 

[RiverBeav]

The P/A + M/S equation is slightly different when you are dealing with soils, since soils can't take tension, and is usually written something like Smax = P/bd (1+6(e/d) for e/d<=1/6.

It applies just the same if the column/equipment is not concentric on the footing. Mathematically, the eccentric DL adds a moment to the footing. I usually add this DL moment to the moment from the wind overturning, and divide the total moment by the total system weight to get the total eccentricity, that goes into those soil P/A+M/S equations.

 

[SlideRuleEra]

RDR89 - Follow up on RiverBeav's suggestion. Equipment weighing 540 lb. on concrete weighing 2250 lb (the 12" thickness) will have a pretty low center of gravity to resist overturning from high wind... unless the equipment is really "tall".

Three feet thick... I'll have to agree with you, that is "super thick"... but still may be the best solution if wider is not practical and 3' thick works.

 

[RDR89]

Thank you all! This has been very helpful discussion. @RiverBeav I did neglect the concrete self-weight in the calculation. Treating this like a footing, I've seen where the footing weight is neglected and the eccentricities determined solely by the loads, so I did not include it here.

 

[BAretired]

Footing weight must be included; the combined eccentricity will be reduced substantially by including footing weight.
Assuming F[sub]wind[/sub] can act in either direction, F[sub]wind[/sub](H+T) + 540*e[sub]1[/sub] is the total moment at bottom of footing.
T = ftg. thickness.
The resultant force should fall within the kern of the footing.

 

[RiverBeav]

The only time that you should/can neglect the weight of the footing for footing design is on gravity/axially loaded footing designs, or potentially axial loaded footing designs where the column isn't centered on the footings. There is a whole discussion about net bearing pressures and gross bearing pressures, but on those types of footings, a lot of times you can design for net bearing pressures.

As soon as you are using the footing for lateral stability (shear wall footings, cantilevered columns, equipment pads), you are using the weight of the footing for stability and can't neglect it.

 

[SlideRuleEra]

Weight of the footing would be neglected if the anchorage of the equipment to the footing is not adequate... which happens. That anchorage is something else to evaluate.

 

[RDR89]

You all have been immensely helpful. Thank you so much!