Pad Mount Transformer Slab on Grade

[WickedJester]

Hey,

I am working on designing a slab on grade for three pad mount transformers. The desire is to have all three on the same pad. Waiting on manufacture drawing to determine loading. Also requested any soil info we have.

I ordered an ACI 360 book to start. Should there be any other manuals I should consider?

I am thinking the pad won't be ro extensive. Possibly 4-6" with possibly WWF for crack control.

Any guidance would be appreciated.

 

[ScottyUK]

You might want to consider a fire wall or blast wall between the transformers to prevent mutual detrimental effects if one of them has a bad day. A bund is possibly worth considering too. Both those things might even be mandatory, but if you want code-specfic answers it's usually a good idea to say which country you're in.

 

[WickedJester]

Ah, thanks. I'm a young engineer new to the utility field.

The design is being done in the United States.

Thanks

 

[SlideRuleEra]

WickedJester - Don't skimp on concrete or reinforcing - they are cheap compared to the cost of either transformers or customer's loss of service from failure of the pad.

Also, transformers are heavy, for their size. The three transformers will not be set in place simultaneously - all it takes is a second or two of differential (structural) loading as they are put in place to crack an unreinforced (with WWF) slab.

I'm not saying to overdesign the pad, just take into account that the loading is not a predictable as the manufacturer's data may indicate. Properly compact the soil, make the pad a little thicker (say an additional 2" to 4", or so) than calculations indicate, and put an appropriate quantity of rebar in the pad.

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[WickedJester]

All thank you for your replys.

SRE as an alternative I was thinking of designing as three separate pads with joints, but no reinforcing steel to join the pads. I believe this would avoid issues of unequal loading.

 

[SlideRuleEra]

WJ - Be sure to review your alternate proposal with the user - before you make the design. Recall your original post: "The desire is to have all three on the same pad."

Three transformers are probably not just three independent transformers but may be integrated components of one, 3-phase electrical circuit. There may, or may not, be a reason to have all three on one foundation (perhaps, ganged electrical switching).

Keep in mind that civil / structural is providing support (pun intended) to the electrical system. Civil designs that may be seem best in their own right, are actually not appropriate.

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[WickedJester]

SRE - Haha yeah I am definitely in a support role here. I am going to discuss with the user first before proceeding. I was just curious if there would be any unforseen issues. I believe it's mostly for weed control, but would still have all pads adjoining each other. Just not connected structurally. So poured with a joint but not rebar to tie them together.

 

[SlideRuleEra]

I assume these are relatively small distribution transformers, not part of a substation or switchyard. Be sure that the pad(s) are large enough to resist overturning. If you get the ok to go with 3 pads, make sure there is a small gap (with expansion material in it) between them so any differential settlement does not affect the other pads. As mentioned earlier, design with "industrial" use in mind, i.e., with rebar and a little more concrete than "code" would require.

IMHO, over time you will find that most of the projects we deal with need to exceed any (civil/structural) "code" in general use.

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[WickedJester]

SRE - Do you have any suggestions on design references I should look at?

Currently I have a copy of ACI 360 on order and have a copy of Designing Floor Slabs On Grade, Ringo & Anderson.

 

[SlideRuleEra]

WJ - Consider whether the pad(s) are a "floor" or an at-grade "foundation".

IMHO, a floor is large enough horizontally (compared to thickness) that it needs joints. Joints are where much of design work is concentrated. A modest sized foundation is typically monolithic. What is the approximate size of the pad(s) and weight of each transformer?

For floors, the Ringo - Anderson book is considered a classic reference - can't do better than that. Still, don't think of either this book or ACI 360 as "cookbooks". You will have to determine what criteria is important for your project to make a meaningful start.

I realize this may seem like trying to "lift yourself by your own bootstraps"... but this transformer pad is a great place to start. Has your boss assigned you a mentor, or is he/she able to help with technical advice?

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[Padlock01]

Hi Wicked Jester I would agree with ScottyUK, put firewalls between Transformers and a Palisade fence and gates if there away from a substation also if they are MV transformers and there are NER's required then you might want to firewall them too.

 

[WickedJester]

SRE - I would consider it more of a foundation than a floor. Had planned on no joints in the slabs. With just have expansion joints between the slabs with no rebar to tie the pads together.

My plan so far was to use Ringo - Anderson and ACI 360 as a reference for determining the slab thickness and reinforcement. Also we have some standards for smaller transformer foundations.

I have not been assigned a mentor, might be a little while with the current work we have. I have been in touch with our station/generation side and have asked them for any suggestions. Definitely have the feeling of "lift yourself by your own bootstraps". Ha. But I do enjoy the challenge.

 

[SlideRuleEra]

WJ - Using the Ringo-Anderson book and ACI 360 the way you intend is a good way to start. Since you already have the book, here are links to magazine articles by Boyd C. Ringo, that may help narrow down the amount of material you have sift through:

Basics of Subgrade Preparation

Choosing Design Methods For Industrial Floor Slabs

Effect of Design Variables On Floor Thickness Requirements

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[ScottyUK]

This document might be of interest, even though it's based on UK practice.

http://files.engineering.com/getfil...bstation+Design+Guide+Rev+D+(EDS+07-0105).pdf