Uplift on Bases: Is there a construction technique to anchor them?

[HermanJordaan]

Hi,

My design requires 2.75x2.75x0.75m bases. Fractured rock starts at 1.3m progressing to more solid rock so I don't really want to go past 1.5m to keep costs down. The client has a very tight budget.

The 2.75x2.75x0.75m size is only due to uplift. So what I'm asking: Is there a construction technique to anchor bases (therefore allowing smaller bases) thus reducing costs?

 

[JedClampett]

You've given us very little to go on. Is this a concrete foundation? Is this for a piece of equipment? What's causing the uplift? Is it seismic? Operating forces? Something else?
And one last thing. If this is a slab on grade, the concrete costs are very cheap. The formwork is simple, the reinforcing costs are small and there's only one side to finish. It's costing your client more to refine the design or anchor it than to just pour it.

 

[msquared48]

Using Rock Bolts, but that would probably be more expensive to install rather than the mass concrete you are most likely describing.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[HermanJordaan]

JedClampett: Yes it's concrete foundations. The Uplift is due to design wind forces. No Seismic forces. No operating forces (It's a smallish warehouse).

 

[HermanJordaan]

For me it's about saving money. If it would be cheaper to somehow put in anchors instead of bring in the big stuff to break up the rock? I would try that.

 

[humanengr]

Do u have horiz loads adding to the problem? I so try taking the horiz forces
out by anchorage into slab. you may also be able to use passive soil pressure against the grade beams. If you can take horiz loads off your footing, it will be easier to deal with uplift.

 

[ukengineer58]

i wouldnt rely on pasive presure on the sides of beams. in order to mobilise the resistance too much lateral movement is normally required.

 

[dik]

What is the magnitude of your uplift force?

Dik

 

[racookpe1978]

Your bases are so small that it makes little extra expense to try to keep them arbitraily "shallow" by spending some exotic (and very expensive!) design time and methods.

Instead of 3/4 meter deep, dig all the way down to the solid rock. (1.3 meter apparently, but this will vary at each baseplate. Drill into that solid rock with 10 inch to 16 inch (250 mm to 400 mm) deep anchors. Insert anchor bolts in those holes and grout/epoxy them in, then cover the hole to your nominal surface with concrete. The anchor bolts then go through the concrete into the rock, but are available to bolt into on top of the baseplate. If you don't "trust" your layout and drilling enough to "risk" extending the anchored bolts all the way through your baseplate holes, then anchor rebar into the rock holes, and anchor the baseplate concrete to the rebar, then anchor the baseplate bolts tot he concrete.