Generator Pad Design Questions
Generator Pad Design Questions
(OP)
Dear readers,
I've been asked to provide a price for designing a foundation for a 5,000 lb emergency generator set which is to be installed on grade next to a apartment building with a 13.5' deep cellar. When I check the 12" thick concrete basement wall for the soil and surcharge (generator & pad) loads I'm finding that the wall may not work in bending. Any thoughts about how to bring the slab loads down to the bottom of the wall? Sonotubes? Piers with square footings? Has anyone had any luck with either of these approaches? The generator is rotating type and I would size the slab for 3x the weight as I've seen suggested elsewhere on this site.
Lastly, I'm assuming that the soil here is uncontrolled backfill and would be specifiying 12" lifts of controlled fill over an area a few feet wider that the footing while using the code minimum 1.5 tsf allowable bearing and 60 pcf lateral soil load on wall. After reading the code it seems I can do this without requiring a geotech for this job but would appreciate if anyone can advise of anything Im missing?
Chad Serman
www.sermanengineering.com





RE: Generator Pad Design Questions
In addition, can you hang the inboard edge of slab on the wall, as by SS angle and SS anchor bolts?
These two alone may be sufficient for that loading, along with support adjustments in case of outboard end movement through the years.
Assuming the length of time for generator operation will not bother residents if vibrations get transferred into the building. Even so, better than no power.
RE: Generator Pad Design Questions
The top of the wall is actually braced by a concrete slab at the ground level so there are no floor joists. So, checking the wall simply supported with a trapezoidal lateral load with surcharge from the generator and pad will not work if the wall is minimally reinforced. I am planning to try to chip away the concrete cover to see the bar size and spacing but, assuming that there is not enough reinforcement, it seems my only option is to support the generator pad on piers at the corners at least that bring the load down to the level of the adjacent footing to avoid adding horizontal load to the wall. Just hoping to get feedback from anyone who has seen this before?
Chad Serman
www.sermanengineering.com
RE: Generator Pad Design Questions
RE: Generator Pad Design Questions
Likely a local utility contractor can do the job. Lengths will depend on what is encountered as they go in.
http://www.foundationperformance.org/pastpresentat...
RE: Generator Pad Design Questions
Check this video as what I envision.
www.youtube.com/watch?v=-EEbnMrSwAc
Numerous options.
RE: Generator Pad Design Questions
RE: Generator Pad Design Questions
Another option is grouted auger holes, done by injecting grout in hollow stem augers as they are withdrawn. But that takes some modification of standard test boring equipment. I saw that done once by a test boring contractor. A regular grout piling job takes a crane and the associated drilling gear and grout pumps, all pretty expensive for this small job.
I'd bet there are underpinning contractors in the area that can do these micro piles or similar at much less fuss and cost. Ask around.
RE: Generator Pad Design Questions
Oldestguy is right on the construction issues. No one ever wants to pay the cost of geotech investigation so they assume things will work just fine and then run into collapsing holes, buried debris, groundwater, etc during foundation installation. Also, the weight of equipment necessary to install such pier/pile systems may cause you more of a problem than your generator.
RE: Generator Pad Design Questions
RE: Generator Pad Design Questions
RE: Generator Pad Design Questions
As for finding re-bars in concrete, I've tried several methods and have not found any that are good. A surveyor's dip needle may do OK for those near the surface, as would a compass.
RE: Generator Pad Design Questions
I would favor some type of piers penetrating well into the native soil below the backfill. That sidesteps the question of lateral pressures against the wall as well as the backfill question. Chance anchors may be a good choice. They can be installed with portable equipment if access is restricted. But without either extensive experience in the vicinity plus knowledge of the geology, I could not defend a design nor the assumption that the conditions will allow it to be installed. Exploration is needed.
RE: Generator Pad Design Questions
RE: Generator Pad Design Questions
Kieran
RE: Generator Pad Design Questions
RE: Generator Pad Design Questions