Embedment / fixity

[IsaacStructural]

I'm reviewing another engineers calculations,

They are Analyzing 11foot tall concrete columns/piers that are attached to 1 foot thick shallow foundations 3 feet below the surface of 110pcf soil (detailed soil properties are not provided), so the column projects 8 above the ground surface. The column has a significant later load at the top, and the engineer is take the column as fully fixed right at the surface of the soil, so they are analyzing as an 8 foot cantilever.

To me this seems a little generous, I'd rather see the bending moment taken at the connection to the footing, for the full 11 foot length , am I being too conservative?

Any thoughts would appreciated.






M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[ztengguy]

I have seen it done with walls, but not a column. Maybe its similar to a light post foundation with passive pressure, etc.

 

[IsaacStructural]

thanks Ztengguy

If it were a continuous wall, would it be common to take the fixity at the ground surface for out of plane bending?

I've not encountered this construction before.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[JAE]

I wouldn't count on the soil in the top 2 feet or so.
When it freezes it will expand - then shrink away from the column over time in cycles. The compaction and ability to develop passive pressure is questionable.

 

[IsaacStructural]

that was my thinking JAE, I'm inclined to tell them to just analyze from the column base. I appreciate the input, glad to see I'm not alone in questioning this.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[briancpotter]

It's an 11 ft cantilever. Soil pressure is simply no where near the conditions required for fixity. Quite a bit of deflection is required to engage passive pressure, at which point your concrete column has already failed.

If it was necessary to reduce the load (for whatever reason), it could probably be modeled with soil springs over the bottom 3 ft. But even this is less than conservative, for the reasons JAE mentions. I'd avoid it if at all possible.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[msquared48]

If there was at least a 6" slab present with concrete grade beams between the columns, OK. Otherwise no.

Mike McCann
MMC Engineering

 

[IsaacStructural]

Thanks everyone, I appreciate the sanity check. I hate reviewing another engineers calculations and having to bleed all over them over something like this, so I want to make sure my comment is legitimate.

M.S. Structural Engineering
Licensed Structural Engineer and Licensed Professional Engineer (Illinois)

 

[StructuralEd]

Full height here too if there are no other foundation elements creating lateral restraint above the footing.

 

[hokie66]

If the footings are just pads 12" thick, the columns should be modelled as pinned at the base. Maybe you can get the fixity required at the top.

 

[Splitrings]

A typical rule of thumb is the embeddment depth divided by 3 below the surface is the point of fixity. Check out ASABE EP 486.