When do you use isolation joints for steel columns surrounded by concrete slabs? 1

[Revv]

Hey Guys, Just noticed sometimes there are isolation joints detailed on steel columns and I'm wondering what thought goes into deciding if you need one. Is it always, or are there conditions you consider? Thank you in advance!

 

[phamENG]

Pretty much always, though there are some things to consider.

They are typically blocked out for a couple of reasons. One is to keep that section of the floor level. The footing is poured and the steel is erected. If you pour the slab right up to the column, a portion of it is supported on the footing and the rest on the soil around it. Well as the building keeps going up, short term settlement can cause the footing to move. By waiting until all or a majority of the dead load is in place before you pour around the column you minimize the localized differential movement between the slab as a whole and the block over the footing.

Then there's the practicality of getting a crack control joint right up to the column. If you have a round column and the plan is to saw cut the joists, how are you going to get the saw blade up to the column? You're not. So it sets it off a bit and makes it easier for the contractor to line everything up.

If you have wide flange columns and no isolation, where do you put the joint to? There's lots of spots that a crack would love to propagate from. By having a small block of concrete, you minimize the strains from shrinkage and minimize the potential for cracking in the immediate vicinity of the column.

Why wouldn't you use them? probably a floor finish issue where the finish can't accommodate the joint pattern in the slab. If your column bears on the slab with an integral footing there's not much point. If the slab is structural and supported on the same pile system as the column. That's all I can think of off the top of my head.