L shape column 2

[ENGUCR]

Hi guy,
Simple question......One of my project, My building has an L shaped column. A single pile caps are proposed for the said column. I would like to know if I place the pile centre at the geometric centre of the L shape column(Which is at the re-entrant corner of L shape), Will it there any eccentric moments to pile apart from any accidental eccentricity. Assume that no ground beams are present. I seem it little bit odd to place the pile centre away from the column footprint.

 

[r13]

I think the shear center of the column shall be aligned with the pile center.

 

[phamENG]

1) I disagree with the shear center comment. Geometric centroid would be correct for maintaining axial loads free of intentional eccentricity.

2) I disagree with assuming there are no grade beams (or ground beams as you refer to them). A single driven pile and no grade beams is inherently unstable. Bridge construction will use single drilled shafts or augercast pile with special detailing, but building construction usually relies on grade beams to stabilize the pile cap to column connection when there are less than three piles.

 

[Agent666]

Yeah 100% agree on geometric centroid, shear center isn't right at all. Under uniform axial load this is the line of action of the axial force.

 

[BAretired]

Maybe it should depend on the location of the c.g. of applied load. If a very short L_shaped column is loaded on one leg only, it would seem reasonable to treat it as a rectangular column and place the c.g. of the rectangle over the pile center.

Most columns have moment applied at the top, so eccentricity at the base can't easily be avoided.

If load is applied equally to both legs, the c.g. of the 'L' should be over the pile center but it is likely that moments will still be present at the base. The top of pile should be braced in two orthogonal directions.

BA

 

[ENGUCR]

Tx all
@phemENG, to make the question simple, I put the no grade beam assumption.
@BAretired
What will be the best generalisable point to place the pile center to minimize eccentric moments, if the column is a corner column with equal span beams either side and it has some top moment due to gravity and lateral loads. Will it resonable to place it at a average point considering both no eccentricity and with eccentricity condition since moments due to lateral loads are not always present?

 

[r13]

It real depends on the problem you would like to deal with - bending due to eccentrically loaded, or torsion due to lateral loads. For odd shapes, I think the latter will give me more headache. I could be wrong though.

 

[BAretired]

If the goal is to minimize eccentricity at the column/pile junction, the best location for the pile is directly under the resultant load. If Mx and My are the applied moments at the top of column and P is the axial force, then e[sub]x[/sub] = Mx/P and e[sub]y[/sub] = My/P. The resultant load, and the ideal location for the pile, is at e[sub]x[/sub] and e[sub]y[/sub].

The eccentricity for dead load, live load and combined dead and live load will all be different, so it is not possible to satisfy all conditions simultaneously.

You could consider the column hinged at the base and provide minimal reinforcement in the pile.

Alternatively, you could design the grade beams to carry all or part of the calculated moment.


BA

 

[r13]

Agree with BA. Dominate force and physical line of action should control.

 

[ENGUCR]

Got it. Tx all

 

[KootK]

What are the dimensions of your L and pile?

 

[KootK]

Also, how many stories are supported and how many of them have the L shaped corner columns?

 

[ENGUCR]

Dear kootK
It's a 400x400x200(leg thickness) L Column with 300D pile
5 storey building with each storey has 4 corner L columns. I wonder how the no of storey matters

 

[KootK]

It's a 400x400x200(leg thickness) L Column with 300D pile

At such a small scale, it probably doesn't much matter what you do. It's always nice to get some overlap between your column and your wall but, with these proportions, it's a safe bet that you'll have some even with the centroid positioning.

I wonder how the no of storey matters

For me, this might affect how I handled things. For a light axial load, I'd be inclined to have the column overlap the pile to preclude any punching shear mechanism in the pile cap. Then I'd deal with any implied lateral kick at the connection in the pile design or via restraint provided by grade beams. Alternately, if this were a fifty story building, then I'd put the pile on the centroid and just make darned sure that punching shear worked comfortably by the numbers. Of course, with a fifty story building, we'd likely be talking about more than one pile.