Foundation for substation equipment

[RobTrans]

I am looking for some literature references which can assist with the design of large block foundations for various heavy equipment in substations (eg 11000 kg Transformer). As the loads on the foundation are static and the base area is large, bearing pressures under the base are not normally an issue. What I am looking for is, information on specifically is how deep should one make the concrete block and what should this be based on. Many existing drawing is have looked at show concrete blocks 0.8m-1.0m deep, this appears to be very conservative.
Thanks

 

[Ron]

Almost all electrical transmission equipment emits vibration (usually consistent with the current frequency of 60Hz). This should be considered for the large equipment.

 

[RobTrans]

Yes I have the current frequency of 50Hz. But I am not sure what effect this would have on a large concrete block foundation almost a meter thick. What effect does this frequency have on the concrete base?

 

[dik]

I've used 30" deep 'pilecaps' for transformers weighing 450K and they work, if designed correctly... pile load increases slightly due to continuity. The Canadian Foundation Design Manual recommends 4' thick to provide a more uniform rigid mass. Also brings in Mass concrete due to the thickness.

Dik

 

[RobTrans]

Thanks Dik, we use the metric system, so i am not certain what 450K is. We are busy with a 11000 kilogram (kg) transformer.

 

[jgailla]

450K is 450,000 pounds, I assume. This is a little over 200,000 kilograms, so about 20 times heavier than yours.

 

[Mccoy]

Yes I have the current frequency of 50Hz. But I am not sure what effect this would have on a large concrete block foundation almost a meter thick. What effect does this frequency have on the concrete base?

I believe the vibrations may have some effects on the soil, that is to decrease the static subgrade modulus, usually not too much but in some conditions (soil layer resonance) by a significant amount.

 

[dik]

Normal 60 cycle 'hum' has little effect on foundation loading, in addition, many conductors/connectors can take a little movement (not a lot). Big forces come into play from electrical 'faults' that can significantly load the components...

Dik

 

[Mccoy]

I take 50 Hz is the European frequency. In reference to my previous reminder about soil resonance, a layer resonating to such a frequence would be superficial and pretty thin, probably bypassed by a 1 m thick foundation block.
The degradation of the subgrade modulus is higher for higher angular frequencies such as those involved in alternate power generation. According to the models, degradation would be in the range of about 75% to 95% of the original value, for some ideal and realistic conditions of foundation geometry and soil rigidity. Intensity of the vibrations does not appear in the equations, a linear stress-strain relationship is assumed.
Of course, a layer of loose sands may be subject to significant densification and settlements but that is so obvious that probably nobody would build vibrating machinery on it, I hope.

 

[paddingtongreen]

The ones that I've designed were all surrounded by built in oil catchment pits that forced a greater than normal depth to get enough volume.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[dik]

Done a few with concrete containment, but most in the last few years, we use a material called Sorbweb. This allows water to pass, but jells into a solid to prevent transformer from soaking into the groundwater... Piles are designed as columns in these areas and have greater vertical reinf and ties, in the event the stone and Sorbweb has to be removed. Sorbweb, once it has gelled, cannot be re-used.

Dik

 

[paddingtongreen]

That sounds like a great improvement.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin