Mat Foundations

[wirelesstom]

I have a 10'x18.5' floating slab foundation w/minimal loading (12,600 lbs of equipment in various cabinets). The slab is 6" thick w/#4 placed both ways at 12" O.C.. The slab is placed on compacted crushed stone.

The loading equates to 185-lbs/SF and the soil pressure is approx 1000lbs/SF. Should I worry about placing footings too or is the floating slab sufficeint?

The grade is level and not sloped. I am in the Northeast (NJ) however....

 

[Ron]

You don't need a footing for load purposes, but you might consider a thickened edge slab to prevent scour under the slab by drainage cross-flow. Without a thickened edge, the slab is more susceptible to undermining and tilting with the types of loads you will have.

 

[steve1]

wirelesstom

Not sure how you arrived at your soil bearing pressure. If the load is distributed uniformly then weight of equipment plus weight of slab produces a uniform pressure of 143 psf.

How is the load from the equipment delivered to the slab? Is it on skid or flush mounted or what?

If this is an outdoor installation have you considered wind loading? How about seismic? How about equipment operating loading? Rotating equipment induces an overturning moment, piping loads may be significant. How about live loads on the slab?

Once you have established the bearing pressure then you need to design the reinforcement based on an analyss of the moments induced into the foundation from the upward pressure (bearing stress) and the reaction points (location of applied load). In ts simplist form you have a cantilever from the edge of the slab to the edge of the equipment. Depending on how the equipment is supported you may have reverse bending under the equipment (between skid beams for instance). Ultimate strength methods are used for the concrete and reinforcing design

 

[MotorCity]

wirelesstom,

As ron said, for strength purposes you probably don't need footings but it would help with scour issues. If you do add footings, it is no longer a floating slab.

Also, I'm guessing that the weather in NJ is similar to that here in Detroit. We usually add a layer of compressible foam below the slab to help minimize the effects of frost heave.

steve1 raises a valid point about wind loading but if your equipment is fairly small you will probably find that the effects of wind load are neglible with respect to the foundation design.

Good luck.

 

[dicksewerrat]

Is this bldg. going to be heated? You may need frost footing for the bldg. inspector. What kind of bldg going on top?

 

[wirelesstom]

I have three outdoor cabinets. All are 77" tall. The first is 54" wide and the other two are 30" wide. The rest of the pad is left open for future growth of similiar cabinets. The first cabinet is 4500lbs and the other two are 1200lbs each. Any future cabinets would be assumed to be approx 1500lbs. the cabinets aresimply bolted directly to the slab w/four anchor bolts.

The cabinets are self heated during running conditions and have their on HVAC systems for cooling purposes.