Transformer foundation

[emanc]

Is it ok not to have a mass concrete foundation for a transformer about 5 tons heavy. The contractor proposed to just place the transformer above 2 parallel I-beams embedded into the RC flooring of the elect room.

 

[msquared48]

I have seen them placed on metal skids before, but always resting on a concrete mat, outside, as in local substations.

With what you mention, placement on an elevated concrete floor slab, I would be concerned with seismic restraint of the unit, and, if the unit is oil cooled, the location of a sump of sufficient size to catch a leak or rupture. The embedded metal beams would also have to be grounded as it sounds like at least the top flange would be exposed.

Mike McCann
McCann Engineering

 

[emanc]

Thanks. The slab is on fill, not elevated. I've seen calculations before of equipment foundations wherein the foundation is made heavier than the weight of the equipment about 3 or 5 times to resist overturning during earthquake, is this correct?

 

[msquared48]

Depends on the size of the slab, thickness and reinforcing. The bigger the footing L & W, the less the thickness, but the more the reinforcing. Everthing is related here and works together. There is no one answer.

You are designing a spread footing with overturning and a vertical P load. You cannot exceed your maximum allowable soil stress.

Start with some L, W, and T and see what your soil stresses are. Adjust from there. When the soil stresses are acceptable, calculate the steel. Check for beam and punching shear too. The footing must be thick enough to handle any embedded rods to anchor the transformer too. Special plates can be added to the ends of the rods (or other mechanisms) to increase the pullout without increasing the slab depth.

As an alternative to the hand method, RISA FOOT would handle this problem easily, except for the anchor bolts required...

Hope this helps.

Mike McCann
McCann Engineering

 

[emanc]

The slab is only 150mm thick with 12mm bars @ 300mm oc. The overall dimension of slab is 3.8m x 4.5m. The slab may not be able to resist the bending moment and maybe too weak against punching shear. Moreover, the slab sits only on earth fill.

 

[msquared48]

The slab is already poured?

Mike McCann
McCann Engineering

 

[emanc]

no the pouring permit is now on hold

 

[twinnell]

emanc,

The foundations you speak of having mass of 3 to 5 times the weight of the equipment is usually for rotating equipment. You increase the mass to ensure that the natural frequencies of the equipment and foundation do not match.

As far as for your transformer, as long as the floor slab can handle the load, you are okay.

I don't know the purpose of the beams embeded in the floor for a transformer. Transformers usually come on a skid with bolt holes for anchors. It could be that the contractor wants to weld it down, but why the extra expense of embeding the beams to weld down the transformer when it can be bolted.

Now, if the equipment is switchgear and not a transformer, then that is a different story. For switchgear, steel channels or other steel members are leveled and embedded in the concrete to insure that the switchgear is level. It is critical for the individual pieces of the switchgear to line up along the length of the embed.

 

[emanc]

The contractors wants to weld the slab rebar to the embedded steel beam but the rebar is few @ 300 mm o.c. only. My other concern is that the bottom flange of beam is embedded 100mm into the 150mm slab meaning there will only be 50mm thick concrete left @ bottom of beam. If the rebars will not hold the beam from sliding down the concrete, it will punch through the slab. The beam is 200mm deep so the top flange protrudes on slab by 100mm and will not help resisting the punching shear.

 

[msquared48]

Scrap the beams and just design the slab as twinnell said, providing enough reinforcing and not exceeding the allowable soil stress for the P + M condition.

Mike McCann
McCann Engineering

 

[emanc]

mike-
thanks, right now we are waiting for the contractor's designer to present his calculation since this is a design and build contract. We suspect the designer used the wrong weight 3 tons only instead of 5.5 tons. If the design is not readily available, we intend to modify on site by making the slab under the transformer a bit thicker and add more slab reinf welded around the beam location, say make it 150 mm oc. We were not able to identify this issue earlier so other works will be dragged due to this delay if we don't act quickly. If the contractor's struc engr can prove his design is ok, then we will go ahead.