Large Generator Pad 4

[steelbeam]

I am designing a generator pad on ground with an allowable soil bearing of 1500 to 2000 psf. The mat will be 52 ft X 16 ft. The genset operating weight including the enclosure and ancillary equipment
will be about 138 kips. My question is what is the rule of thumb for the mass of the pad vs the equipment weight; 3,4 or 5 times the weight? The generator has a diesel engine. Thanks to anyone who
can help me with this.

 

[SlideRuleEra]

Look at this another way. To be an effective inertia block, the mat needs to be rigid. To make a 52 foot long block rigid, mat thickness needs to be about 1/10 the mat's greatest dimension. Say the mat is 5 feet thick.

The concrete mat weights 52' x 16' x 5' x 150 lb/ft³ = 624 kips. That makes the mat 4.5 times heavier than the machine... which should work very nicely.

The machine will have to be anchored very securely for the mat to act as an inertia block. What size and number anchor bolts will be used?

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[steelbeam]

There will be 18 spring isolators (9 on each side) with two 5/8" diam anchors bolts on each per the Caterpillar drawings.

 

[Archie264]

To make a ... block rigid, mat thickness needs to be about 1/10 the mat's greatest dimension.

Mr. SlideRuleEra, where does this rule of thumb come from? (In case it's not clear, I'm not doubting it or challenging you; I'm asking to learn.)

 

[SlideRuleEra]

steelbeam - Since the machine is supported by spring isolators, it will not benefit from a true inertia block. The isolators will keep the machine and the foundation from vibrating as a single unit. This in not a problem, just another (less expensive) solution. Caterpillar knows what they are doing. Just design a robust mat foundation, it will likely be somewhat less than 5' thick, but still fairly massive.

If the machine needed to be anchored to an inertia block, I would expect to see 20, or so, anchor bolts with a diameter of, say 2".

Archie - I don't have a specific source for that 1/10 ratio, I just sort of picked it up from working in the heavy industrial field. The fact that this particular generator has a diesel prime mover (a reciprocating machine) is one clue that an inertia block should be conservatively rigid.

For general information see this paper on my website: Principles of Design for Engines & Compressors

The 1/10 ratio has other (rule of thumb) applications, too. When the span / depth ratio of a steel beam is lowered to about 10, the chance increases that shear governs rather than moment. Also, deflection becomes almost trivial.

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[WARose]

I typically use 4-5 times the weight of a reciprocating machine (or centrifugal machine) as my minimum weight of the block. If that is not practical, I will consider the weight of the moving parts as the 5 times multiplier.

The full rule of thickness is: 2' minimum, 1/5 th of least foundation dimensions, or 1/10 th of largest foundation dimensions, whichever is greater. (Someone asked where this comes from. My source is 'Design of structures and foundations for vibrating machines', by S. Arya, M. O'Neill and G. Pincus. An oldie but goodie.)

A word to the wise though: these rules are good starting points. But (and I say this without knowing how much unbalanced force or the soil type you are dealing with, or some of the other parameters), I have seen numerous problems from people who just simply followed “rules of thumb” and didn't do a full blown dynamic analysis. Your case may not require it (indeed, many don't), but be aware that these rules alone won't cover you in some circumstances.

 

[JedClampett]

WARose, we all go into projects with the best of intentions, but then reality gets in the way. We either can't get the rotating mass weight or the Geotechnical Engineers haven't given us the applicable soil properties or when we ask for the machine unbalanced forces, the manufacturer says, "This equipment is isolated, there can't be any vibration..." and they hang up the phone.
So we continue to do the best we can with rules of thumb.

 

[WARose]

WARose, we all go into projects with the best of intentions, but then reality gets in the way. We either can't get the rotating mass weight or the Geotechnical Engineers haven't given us the applicable soil properties or when we ask for the machine unbalanced forces, the manufacturer says, "This equipment is isolated, there can't be any vibration..." and they hang up the phone.

Right. Sometimes dynamic analysis is kind of a shot in the dark. Interestingly enough, over the years, the trend has been towards better equipment info and lousier Geotech info. Use to be, I was taking guesses at the unbalanced reactions (through calculations of my own).......now, a lot of times, you get a full set of reactions from the vendor. If the geotech end would improve.....I'd be in heaven!

There is nothing wrong with rules of thumb......but be aware of the risks (and make everyone aware). Otherwise you will make me some money. (As I have had several projects over the years fixing such problems.)

 

[StrP88]

SlideRuleEra (Structural),

Talking about you working in heavy industrial field, I had a question but I didn't know how to get a hold of you till seeing in forum you showed up.

I don't want to sound rude and use somebody's else ( steelbeam (Structural))forum to ask my question, may I ask how I can get a hold of you regarding a problem that sounds you have dealt with that in past.

Please refer to below forum from me:
Protection Frame for trucks falling on side
thread507-404525

http://www.eng-tips.com/viewthread.cfm?qid=404525

If you had designed, have any information please let me know. I am desperately looking for some guide on this

Thank you

P.S. steelbeam (Structural) I sincerely apologize for communicating with Sidrule through ur forum.

 

[Archie264]

SlideRuleEra,

Thanks. If a picture is worth a thousand words then one rule of thumb -- borne of experience and passed down over the years -- is worth a thousand Greek letters and subscripts.

 

[boo1]

see 'Design of structures and foundations for vibrating machines', at:

http://www.slideshare.net/nguyenhoainam5/design-of-structures-foundations-for-vibrating-machines

 

[SlideRuleEra]

Archie - Thank you. I try to mention only the old ways that may still be useful.

StrP88 - You have received good advice on that thread already. Suggest that you return to it and post your preliminary sketches and calcs, as suggested in a recent comment on that thread. You know the most about what is needed. It's best to start with comments on your ideas and go forward from there.

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[StrP88]

SlideRuleEra (Structural)

The solutions were not helping to give me a start to design such a structure but telling me that is an impractical structure.

I just wanted to ask you in the industry you worked have u have across to this issue and structure.

No sketches and design been done by me to post. I was trying to get the jump start.

 

[delagina]

@sliderulera, Is thermal differences normally checked for compressor foundation?
I've seen a number of compressor foundation, I've even design a few myself but none have any notes re thermal differences for mass concrete.

http://www.cement.org/for-concrete-...logy/concrete-design-production/mass-concrete

 

[SlideRuleEra]

StrP88 - This is steelbeam's thread. Return to your existing thread and post the following information:

1. Provide a complete description of the trucks, with approximate dimensions and preferably a photo of one.

2. Tell us what product is being carried and dumped by the trucks (crushed stone? coal? wood chips? etc.)

3. Describe the spot where the product is being dumped (On a flat slab? In a pit? From an elevated trestle? Etc.)

That is how you jump start a project... furnish full information on what is known.

delagina - The link you gave has the basic steps needed for mass concrete placement. The "catch" is to make them happen. Foundations for our electric generating stations require large amounts of mass concrete, sometimes up to 15 feet thick. This is in South Carolina, much of it in the summer (hot & humid). Concrete of the thickness steelbeam is considering in not a big deal.

1. Have thorough, well written technical specs that address the issue (not general "boiler plate") as part of the bid package.

2. The Contractor prepares a detailed plan for each mass concrete placement and reviews it with the Engineer a couple of days before the placement. Coordinate with the concrete batch plant for timely delivery.

3. Use fly ash concrete to slow heat release.

4. Replace as much water with flaked ice as needed, sometime that may be all of it.

5. Start the placement early in the day during hot weather, say at dawn on even at night under extreme conditions.

6. Contactor has plenty of skilled workers so that work is not slowed by employee fatigue.

7. Backup concrete placement equipment available on short notice.

8. Start true wet curing as soon as possible and continue for 7 days.

Take these steps and thermal issues can be managed successfully on many projects.

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[Tmoose]

I believe the "width" of the foundation (perpendicular to the crankshaft centerline) will need to be greater than height of the equipment CG to provide stability. For some soil and loading conditions much wider.

The concrete-to-equipment weight ratio would not prevent me from designing a canoe.