Axially loaded steel column directly in footing (no pedestal, just bending rebar)

[octagie]

Is there any particular consideration on having an axially loaded steel column (compression and tension) placed directly over a deep footing (1.6 m deep)? without either a pedestal or pedestal rebar inside the footing (embedded pedestal)? Appart from punching shear, is there any other consideration to consider that are not on codes? Anchor bolts will end deep with an end plate (they are 2 inches diameter), so i don't need any rebar parallel to the anchor bolts. Also, I don't need to confine the concrete, since the base plate is generous, so no stirrups forming an embedded pedestal. So the footing will have just bending rebar (bottom and top). All my calculations are OK (pull out, punching shear, endplate thickness, etc.), it is just I haven't seen similar things with big axial loads. Any comments?

 

[phamENG]

I'm with BA on the rotation. I don't like the columns in line with the corners of your square donut. Feels like a crack waiting to happen. Also, why not center up the columns?

Other than the potential crack at the corner and what could end up being mass concrete depending on how you pour it and the actual thicknesses of the various parts, I wouldn't be too worried about it.

 

[r13]

Put this way, though I don't know the exact edge distance, from the 3D base layout, it looks like the 4 columns fall on x and y axes do not have adequate edge distance, at outboard edges, to provide pullout resistance. So, your punching shear/anchor pullout capacity should be relied on 3 sides only. But even that (3 side) scenario works, the outboard side concrete would feel tension/compression, therefore, reinforcement may be required.

 

[BAretired]

The proposed column locations are indicated by white circles centred on the upturn.

@retired13,

I'm not convinced that columns being off centre is a big deal, but you may be right. This would eliminate that concern. It would also be easier for the contractor to measure and place the anchor bolts accurately.

BA

 

[r13]

BA,

My concern is only on edge distance of that 4 columns, no mater the orientation. But, the rotation suggested by you could result in more concrete by the columns, thus alleviate my concern.

 

[r13]

The concern is on the right side edge.

 

[KootK]

Yes, there are things to check. That said, it sounds as though you've done the appropriate checking and it's working out. As such, I see no need for the local pier cages at the column locations. I assume that your detailing is as I've shown it below and, therefore, you have a robust load path for transmitting uplift load into the base foundation element.

...78 tonf uplift taken by 6 1+1/2 inch A36 bolts, with an endplate deep)

- If "deep" means all the way down as I've shown below, you should be fine.

- If "deep" means part of the way down, some extra checks may be required if pier cages are to be avoided.

- If "deep" means means only down into the upper plinth, you probably do need pier cages.

- Taking the anchor bolts all of the way down can, itself, be a constructability issue that might be best resolved by adding pier cages and only taking the anchor bolts down to the bottom of the upper plinth.

 

[BAretired]

retired13,
I agree, pullout could be a concern but we don't know the dimensions of the concrete or the axial tension in the column and the OP claims to have checked pullout; placing the columns on the centre of the upturn is certainly preferable.

BA

 

[r13]

BA,

Correct, we don't know the dimension, so I can only say it is a concern to me. As the OP said "

So the footing will have just bending rebar (bottom and top).

", when I don't think he needs cage around the anchors, I somehow feel there is need of vertical reinforcement around perimeter to keep the two lifts together (Big mat below and small footing above), and protect the edge concrete from premature crack/spalling. Just a detailing concern.


[Correction] I saw the rebar now, the blue line and dots. My mistake, forget the noise I made.

 

[BAretired]

Okay retired 13; all is well.

BA