Generator Foundation

[Sweever]

I am analyzing whether a new generator pad is required for a 26,000lb (wet weight) generator. I have read the other post on this subject, however mine situation is somewhat different. The existing slab is 30' x 20' x min. 9" thick, and is founded directly on rock. This pad currently houses 3 generators and one is going to be replaced. Going through the manufacturer design data, they are recommending a concrete mass equal to 2 times the wet weight of the unit and based on the manufacturers foundation thickness equation (foundation depth = Weight/ (density of concrete*width* length of foundation), I need a min. 3 foot thick slab. The existing 9" slab is reinforced with 2 layers (T&B) of #4 bars each way and is anchored to the existing rock with 15-#4 vertical dowels grouted into the rock 9" and extending into the slab 6".

I don't know the capacity of the of the rock, but would assume 21,000psf at a minimum so bearing capacity is not a concern. The new generator is on vibration dampers (8)

My question would be how to know if the existing dowels are suitable to mobilize the mass of the rock below?

Thanks

 

[WARose]

I would say you would mobilize the mass of the rock included in the dowel failure zone.

But I think that is neither here nor there....that mass recommendation is (typically) based on a elastic base. Here it will be much more rigid. So what I am getting at is: it's great to engage this mass.....but it will likely not tell you the story as far as the response goes. Likely that response will be governed by some sort of flexural displacement (of the slab; in uplift) between the dowels (or perhaps another mode of displacement). I would check this carefully (perhaps with a time-history analysis).

 

[spieng89]

Sort of agree with WArose on utilizing the rock foundation mass in evaluating vibratory concerns, but I'd differ in the fact that you would be able to utilize more than just the failure cone of the doweled anchorage. Vibration would engage the entire mass of foundation rock. A failure cone would only be important in an uplift/shear failure scenario which vibration doesn't qualify in my opinion in this scenario.