Dock High Foundations

[humanengr]

I'm designing a dock high (truck) foundation for the first time.
Since the top of floor is elevated 4ft. above grade, I'm planning to provide a
structural grid (conc., CMU, or steel) framing beneath the floor. The floor will
be designed as struct. slab elevated on and supported on the framing.

Question: Is there a more economical solution? For example, the area / volume
beneath the floor could be filled with compacted fill and the floor slab would
be similar to slab on grade. However, that is probably not a good option. Constructability, soil consolidation and degradation of slab
support could be a problem. Any comments / alternative approaches are
appreciated.

 

[Eldorado]

I design dock high slab on grade structures all the time. Your geotech report should include site improvement recommendations that you include in your contract documents. Normally, the projects with docks I work with have docks one side only and have near grade slabs at the remaining (3) sides. In general, the dock ramp areas are excavated below the original grade, so you specify retaining walls at the end of the ramps.

With the above construction scenario, your building pad doesn't have any more fill than a building without a dock. You just form the dock side as needed. I usually like to keep a minimum grade beam depth below grade of 18" even if the geotech calls for a 12" min., which is common in my area of the country.

Review existing soils and specify soils with at least some cohesiveness. Non-plastic sands will sluff off into the relatively deep beam excavations.

 

[msquared48]

I would still throw some extra steel in the slab as this area will get a lot more use than the rest of the slab, and slightly more prone to settlement, very salightly.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JAE]

I think most docks are built with retaining wall type foundation walls with slab-on-grade above and adjacent to it. Most docks have dock leveler pits but alternative levelers are now being sold that stand upright on the dock and flop down on the trailer floor once the doors are opened.

There are two options on the slab-on-grade method - 1) cantilever retaining wall with separated (expn. joint) slab-on-grade above and 2) traditional foundation wall on smaller spread footings where the wall depends on the slab-on-grade to tie the top of it back.

Using the retaining wall option results in larger footings but avoids the issue of having to brace the walls prior to placing the backfill soils that support the upper slab-on-grade.

If you use a suspended structural floor above the dock walls, be very careful as these kinds of floors are subjected to significant forklift traffic with heavy, hard wheel loads. You have to control your beam/slab deflections very tightly as the continuous loading/unloading can degrade a slab very quickly and you'll end up with a mess.