Column end Conditions

[sybie99]

I have a edge column with following end conditions, what you use in design:

Bottom: 550mm square columns with starter bars that were cast into a 1000mm diameter pile. The pile designer says that the pile would be able to carry 50kNm in bending.

Top: 550mm square column tied into a 550mm deep slab.

The column is tied into a hoist and stair shaft, but this shaft comprises concrete columns and ring beams, no concrete shear walls, with infill brick work that is tied to the columns with hoop irons and a 20mm soft joint between column and wall.

I am not sure if this would provide sufficient restraint, as in for the column to be designed as braced.

What effective length would you use for a column in this case as the end conditions and braced or unbraced condition are a bit of an uncertainty. The column is 8.25m tall.

 

[Bagman2524]

I would use k=1.0

 

[JoshPlumSE]

My tendency is to look at this as a pinned-fixed condition which is subject to side sway. K = 2.0.

But, it depends on the degree of restraint provided by the pile. It is providing rigid lateral restraint against translation? Is it providing rigid restraint against rotation?

 

[aaronPTeng]

My instinct is 2. Unless, you have relied on the concrete stairs to connect to the columns/slab around the shaft, which can preformed restraint for the structure (and checked), and then you can make it 1.

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[sybie99]

The pile top is tied to surrounding piles with ground beams so cannot move laterally. If designed as partly fixed bottom and fixed top ( as slab is as thick as column) and designed as unbraced k is 1.3 according to bs8110. Why would you use 2? Can you not assume the end conditions I suggest above? The core is 20m from the column and will have concrete stairs that tie into the slab. It is only a two storey building.

 

[sybie99]

See attached a layout, the columns in question are the top 3 columns on gridline j. See the stairs core below, the landing of which will tie into the slab. Its a 3 storey building, ground to 1st is 8m and 1st to 2nd is 6m.

 

[sybie99]

sorry, now attached in engineering.com

 

[JoshPlumSE]

In the original post, it wasn't clear that the top of the piles were connected to grade beams that provide rotational restraint.

I would just use the nomograph tables (Ga, Gb vs K) that are given in the AISC manuals. I think some copies of ACI have these nomographs as well.

If the column stiffness exceed 4 times the rotational stiffness of the grade beam connection, then I would tend to use a K value of between 1.5 and 2.0.

If, however, the column is weak compared to the grade beams moment resistance then I would use a K of between 1.0 and 1.5.

 

[sybie99]

Josh, thanks, there are no ground beams at present, but to ensure the pile head does not move laterally I will add ground beams. These beams will be deeper than the column, say 600mm deep and 300mm wide and will be of similar length to column. There will also be 200mm ground slab that will be cast up against the column. I am hoping this will provide sufficient restraint at the bottom to consider this connection as partly fixed. If I can use a k of 1.5 the column will work

 

[aaronPTeng]

Sybie,

If you design the pile caps to prevent rotation then you can use k = 1.0. However I find this an expensive solution. For a little bit of concrete on the stair shaft you
can remove the requirement for these ground beams,


Regards,

"Structural Engineering is the Art of moulding materials we do not wholly understand into shapes we cannot precisely analyse, so as to withstand forces we cannot really assess, in such a way that the community at large has no reason to suspect the extent of our ignorance." Dr. Dykes, 1976

 

[sybie99]

Thanks Aaron, so you would think for a 2 storey building the stair shaft, if I provide RC walls, will be sufficient to design the columns as braced?

 

[sybie99]

Thanks Aaron, so you would think for a 2 storey building the stair shaft, if I provide RC walls, will be sufficient to design the columns as braced?