Column shortening in tall buildings

[Lion06]

Does anyone know of a good reference that talks about column shortening in tall buildings generally and differential shortening specifically? I'm getting total axial shortening at the upper floorof 2.5" for gravity columns, but only 3/8" for lateral columns. I'm concerned about what this is going to do to the levelness of the floors and how much extra concrete the contractor will pour.

 

[hokie66]

Agree with Ron that the actual load path differing from the assumed can make a big difference. It's the old axiom "the structure is smarter than the designers". I don't think E changes enough to make a difference. Big "if" in concrete buildings, along with creep and shrinkage, but the properties of steel by and large take these issues out of the equation.

Casting slabs to a uniform thickness rather than level may be finicky, but it is doable. This is one tool which has been used to address this issue.

 

[bookowski]

Are you sure this is even a problem? The non-structural components are attached after the majority of this movement has occurred, i.e. partition walls, curtain wall, mep equip etc. The little movement that will occur after should easily be accommodated by the connections of these elements.

Beyond that you need to look at how much that differential movement really amounts to. If you are talking about say a 30' span between the two columns in question and 1" differential movement that's the equivalent of (2x30'/1") = L/720 which is not much.

It depends on your layout. In general differential shortening is not such a big problem in steel building and 30 stories isn't that tall where I would expect an issue unless you have some strange condition of very heavily loaded columns a short distance from lightly loaded ones. I work mostly on high rise and we wouldn't normally even check this for a 30 story steel building.

 

[csd72]

As I expect that most of the shortening will be in the lower floors that you may consider increasing the size of the lower 5 or so floors of columns to help reduce this.

Also, check if you need the full depth of concrete for strength in the composite floor, if it is purely for fire reasons then you may consider pouring a thinner structural slab and making up the rest with self levelling screed. Not easy but it could work if done right.