Column shortening in tall buildings
Column shortening in tall buildings
(OP)
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.






RE: Column shortening in tall buildings
RE: Column shortening in tall buildings
RE: Column shortening in tall buildings
RE: Column shortening in tall buildings
Shouldn't there be people in your office who have dealt with this before? For a job of that magnitude, I would sure hope so.
RE: Column shortening in tall buildings
RE: Column shortening in tall buildings
If you look at articles about burj dubai you'll see that they sized all of the elements to have a similar stress ratio even though this was overkill to deal with this problem. In your case this obviously doesn't make sense.
Most of this can be built out. The steel fabricator and erector can take a portion of this differential into the columns so that under your dead load (or a portion of it) you level out, this is common. You can work with them to provide the info that they need. Also, your slabs will be poured level so the only differential affecting floor level will be from superimposed loads which should be quite a bit less.
RE: Column shortening in tall buildings
If you pour say the 15th floor level, then it shortens another 1" compared to the adjacent lateral column (due to the additional 20 stories on top of it) won't that result in a floor that is substantially out of level?
RE: Column shortening in tall buildings
If this differential from the subsequent loading is too much the fabricator can build this out by adjusting the location of the connections, i.e. move each connection up 1/8" every 3 floors or whatever you require and adjust the slab as well - similar to cambering. You should first see if it's even a problem though - if you look in the code of standard practice they talk about column shortening and tolerances (do a search in the pdf if you have it), you may not even have an issue.
There's a document titled "Column Shortening in Tall Structures" by Mark Fintel, S.K. Gosh and Hal Lyengar. It's mostly geared towards concrete since this is a bigger problem with concrete construction but it still has some useful info.
RE: Column shortening in tall buildings
SEIT: You can also try to shift more vertical load to the lateral columns to balance the system.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Column shortening in tall buildings
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Column shortening in tall buildings
When there is a differential strain profile across the building there is also problems associated with stiff outrigger systems transferring load from the highly strain elements to the lightly strained elements.
RE: Column shortening in tall buildings
SEIT.....I think you might be overthinking this one. Yes it's a consideration. No, it is not a big deal on any individual floor....just the total.
RE: Column shortening in tall buildings
It is a big deal on the upper floors if the external columns have shortened 2.5" and the core hasn't. Or the other way around. Gets tricky, with lots of ways to compensate. Have to also consider affects on facade.
RE: Column shortening in tall buildings
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Column shortening in tall buildings
RE: Column shortening in tall buildings
Looking at say the 20th floor of a 35 story building. Let's say there is 1.5" of differential shortening between adjacent columns at the 16th floor due to total DL only. Let's also say that this 1.5" is accounted for with elongated columns and moving the connection up at the problem column incrementally such that at the 16th floor the top of steel is higher at the problem column (say 1" higher since some of the shortening has already happened) . This means that the contractor needs to provide a sloped slab of constant 6" thickness so that when the final 1" of shortening takes place the slab will be level. That seems very finicky. I also imagine this would be extremely problematic for F(F) and F(L) numbers.
Hokie, this makes me definitely see your point about the problem being worse at half-height than at either top or bottom. The top has little shortening left to go by the time the slab is cast and the bottom never sees much accumulated shortening to begin with. The real problem (in terms of compensating for it) is where there is a fair amount of shortening that happens AFTER the slab at a given floor takes place.
RE: Column shortening in tall buildings
RE: Column shortening in tall buildings
That's very interesting. I wonder if E comes in higher than the 29000 ksi used in design. I know we often see Fy come in higher.
RE: Column shortening in tall buildings
You might consider some in-place strain monitoring during the construction of the building. For a few thousand bucks you can get a lot of data that will allow you to modify some things as necessary as the building goes up. Cheap compared to some of the problems you can have. Hokie66 noted a particular concern of facades. Strain compatibility is sometimes difficult to control, thus more "slop" is put in the design/connection of facades, thus the more difficult the waterproofing....a "vicious cycle" to say the least.
RE: Column shortening in tall buildings
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.
RE: Column shortening in tall buildings
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.
RE: Column shortening in tall buildings
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.