Column Fixed support

[WiQi]

Hi,

how to make a column have fixed support, is longitudinal bars bending pattern at foundation level effect the fixity condition, mean if bars are bent inside and if bars are bent outside or bars are not bent at all they are erected without bending what will be the difference in behavior.

 

[BAretired]

If longitudinal bars are fully developed, with or without bends, the column will have fixity if the foundation itself is fixed.

BA

 

[JAE]

...if the foundation is itself fixed...which is a hard thing to do unless you are on rock.
So typically footings on earth (dirt) have some level of rotational flexibility. Getting a number to represent that flexibility is almost impossible to get correct.
So you have to do a parametric set of analyses (assume soft, then hard soils) and design based on the worst cases of those assumed ranges.


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[KootK]

Although I doubt that it affects the behaviour very much, I believe that a fixed base concrete column should rightly have it's hooks turned inward. This, based on the conventional wisdom that hook bends should generally contain concrete struts.

You may want to consult the PCI Design Hanbook. They have a good section on calculating base fixity considering footing rotation.

Another thing that you may want to consider is the effect of permanent dead load on the column. This will create an effective prestress at the joint between the column and the footing. That prestress will need to be overcome before the rebar detailing comes into play.

If I really want to call a column fixed, I'll consider connecting it to a neighbouring column with a stiff grade beam. It's expensive but sometimes warranted if you're trying to create a dependable plastic hinge at a column base.

@JAE: I agree with the approach that you've described. Is it what you actually do in practice? From what I've seen, most designers ignore column fixity at foundation connections unless the columns are part of the lateral system. Technically, one ought to follow through and design the footing for moment transfered via punching shear and varying soil bearing stress.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[JAE]

KootK - it depends on the situation of course (as to whether to do a floor-ceiling parametric approach on anything). It is simply a way to be conservative in a rational manner.

For a cast-in-place concrete frame building I would probably start with a pinned base for the frame design. Then for the footings look at a few cases with fixity perhaps.
A lot of times the shears in the footings (one way and punching) are pretty conservative to start with.

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[delagina]

i've always been a little confused which is the better assumption here.. i know there is no true pinned or true fixed..

if you have cantilever column with 4 bolts outside the column, you have to assume it as fixed, otherwise it will be unstable.

if you use the same base plate detail of that cantilever column on a framed column, why is it assumed as pinned..

the assumption that base plate will bend, foundation will rotate, how come you are not doing that assumption on a cantilever column with just 4 bolts outside the column.

 

[BAretired]

A cantilever column should not be considered fixed but its rotation does not affect any other members, only its own performance. The base anchorage needs sufficient strength to resist the cantilever moment and if there is axial load, the P-delta effect should be considered.

The same detail, used in a frame is not pinned but a small rotation will result in moments being transferred to the opposite end of the column. The assumption of a pinned base is a conservative assumption made by some engineers, but is not precise and is not mandatory. To rely on a moment at the base of a framed column using a rotational spring constant to represent the stiffness of the foundation is acceptable engineering practice.

BA

 

[KootK]

I think that you've sort of answered your own question here Delagina. A cantilever has a fixed base precisely because equilibrium requires/ensures it. Once you introduce a top side support, the system is indeterminate and relative stiffness determines your degree of fixity.

I share your frustration with indeterminate situations however. It's always seemed to me that column bases, as typically detailed, are pretty fixed until some manner of concrete breakout failure makes them pinned. And, if you move the anchor bolts closer together to encourage pin behaviour, you really just reduce the lever arm on the bolts and promote concrete breakout earlier in the load history.



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[jike]

For steel or precast columns, 3 items will affect fixity......
1) anchor bolt elongation
2) base plate rotation
3) footing rotation

It is fairly easy to set up equations to determine the percent of fixity. I believe the PCI Precast Design Handbook shows how to develop these equations.

I prefer to use a rectangular footing, wall or grade beam to minimize footing rotation. Use stiffen base plates to minimize base plate rotation. Use larger size or quantity anchor bolts or larger moment arm to minimize AB elongation.