Slab on grade Foundation for substation switchgear...where do I start? 1

[BG29]

***I have searched the forum thorughly but none of the threads have led to anything significant***

Hello! I am an engineer for a consulting firm in the power delivery sector. I am working on a small substation for a client and I have been tasked with designing a slab on gade foundation to support the outdoor switchgear. I am a civil/structural EIT, since graduating in December 2018 I have not used my "structural skills" and I am a bit rusty.

In school I took reinforced and prestressed concrete but never had to design a slab on grade. I have been looking through aCI-318 and ACI-360 but I am not quite sure that they cover my situation.

The slab will sit outside in the substation yard and support the medium voltage switchgear. The proposed dimensions are 30'x12'x3' for the slab, the equipment dimensions are 27.5'x9' and weights roughly 30k.

We are still waiting on soil data but to begin my calcs I will assume average soil, 4ksi concrete, location is in Texas so low chance of freezing or upheave.... past this I am not quite sure where to start.

I want to solve this, any places on where to start would be very helpful!

 

[RPMG]

Start with the Texas Building Code Chapter 18. Not to be short, but I'm out of time.

 

[atrizzy]

Treat it like a footing supporting any other piece of equipment:

Calculate the center of gravity of the equipment, from which you can derive the vertical loads and any eccentricities.
Calculate overturning forces due to wind and seismic effects.
Ensure that your footing doesn't overturn, and that all applied load combinations result in acceptable soil pressures under your footing.
Design your reinforcing to suit the forces in your footing resulting from the soil pressures underneath, or more likely, to suite temperature/shrinkage reinforcing requirement (if you have a large concrete mass).

Switchgear don't produce much vibration or dynamic loading, so you're safe not checking that IMHO.

 

[BG29]

@RPMG this substation is on private property and its an outdoor substation, I never though to check Texas building code, but I will just to be sure!

@atrizzy, thank you! I follow, let me run this down and see if I confuse myself further. My brain is a flurry of possibly design approaches and I am also lookign for some sort of code that I can reference to ensure I have performed all design checks.

 

[atrizzy]

Your building code's load combinations and soil bearing capacity, combined with concrete design fundamentals should be all you need.

 

[WARose]

I'd never take a slab on grade approach for a piece of power equipment foundation. I would do it like a footing/mat (as a poster above suggested).

You always want to be conservative for such (vital) equipment.

 

[Teguci]

Soils Report - I've designed a Transformer and switchgear foundation that had to be on a deep foundation (piles).

 

[JAE]

For Texas in particular, but substation design in general, you probably are looking at a stiffened slab-on-grade approach.
This is basically a slab that is stiffened with turned-down concrete beams that form a large-grid waffle slab type system. The beams, or ribs, are usually spaced between 8 and 12 feet on center in both directions. The idea is to create a fairly stiff mat of concrete and use the ribs as stiffening elements to help the overall slab system work as one to spread the load out, resist uplift or overturning and, in Texas specifically, resists shrink-swell movements caused by expansive clays.

This link: Link provides some general information on it.

The WRI document: WRI Link

 

[SlideRuleEra]

The proposed dimensions are 30'x12'x3' for the slab, the equipment dimensions are 27.5'x9' and weights roughly 30k.

Of course, go through the appropriate design steps... and don't be surprised when you discover that all that is needed is to determine the quantity of rebar and how to position it in the concrete. One simple concrete placement of only 40 yd³ is going to be a cost effective solution that is hard to beat... considering costs for design, subsurface preparation, forming, rebar placing, concrete pouring, finishing, etc.

Do take into consideration that those dimensions (3' thick, in particular) border on being mass concrete. If placement is to be conducted in "hot" weather, some (straightforward) specification attention to placement requirements will be a good idea.

http://www.SlideRuleEra.net

 

[BG29]

Thank you all! I am loving this by the way, I miss doing real engineering, we dont get to do much in power delivery consulting.

Another contractor is performing the site prep and grading, we are waiting on our soil bores to come back but I am using bearing capacity of 1,500 psf to be conservative for design. We will also put some select fill under this foundation.

I am not familiar with the waffle pattern foundation @JAE, thank you for the links. I will look them over!

 

[BG29]

@SlideRuleEra 3' was a quick and dirty first approximation. I am designing now to hopefully make it a bit shallower.

Since this is not designed to have piles or footings, i cannot design it as a two way or one way slab...to calculate the reinforcement I assume I will compare the bearing pressure of the soil to the weight of the switchgear in X and Y directions to determine lateral and longitudnal reinforcement?

 

[SlideRuleEra]

BG29 - Understand. Suggest using whatever (constant) thickness that your calcs indicate as a baseline to compare against more complex designs. Heed WARose's recommendation about making the footing/mat to "inexpensive"... heavy industry construction need to be robust.

A good learning experience for you, make the most of it.

http://www.SlideRuleEra.net

 

[TheDirtPounder]

I’ve designed many foundations (and supporting steel platforms where required) for switchgear and switchgear enclosures and I would stress that you specify proper subgrade compaction. Many of these switchgear assemblies have very tight deflection criteria to allow the gear to operator properly. Depending on how the skid (if the gear is on a skid) is anchored, will determine how big of an issue settlement will be on the gear.

 

[BG29]

@ihereth, the switch gear is an ABB in 2 parts, the main section with CPT and PT , then the throat and 6 feeder sections. The drawings show the main section on channels and the feeder sectiosn on channels.
ABB drawings show anchor bolts with steel plates holding down the channel.

I will also state I am new to substations, my experience is largely distribution and transmission but the learning curve is steep.

 

[Bowsers]

Where are you getting your loads from?

I've seen these designed with mat slabs, and sometimes with piles.

Consult IEEE693 if you have it available. It's a great resource in the design of power systems.

Also, you need to use an F1554 bolt, I can't remember if its Gr 36 or 55, depends on the loading.