Movement Joints - Suspended Slabs

[Drapes]

I have a couple of questions re: movement joints:

1) When deciding whether or not a movement joint is required in a suspended slab, the overall length/size of the slab is the first thing that is often considered. Is this simply due to the fact that, for a given restrained strain (resulting from temp fluctuations and shrinkage etc), the in-plane movement of the slab (and resulting movement of walls/columns/cladding) will be higher if the slab is longer?

2) Can movement joints be avoided altogether in a suspended slab, if the slab is designed with a high degree of reinforcement to control cracking due to temp and shrinkage restraint, and assuming the slab is a regular shape? I understand the resulting movement and possible distress this may exert on walls/columns/cladding etc, which would obviously need to be accounted for as well.

 

[oldestguy]

You might look at highway continuously reinforced concrete slabs. There are no joints for miles. However, where a bridge is present they can't continue that continuous slab over the bridge. So a hundred feet or so back of the bridge,they stop the reinforcing. However the end of that long slab is tied in to a series of cross trenches, filled with reinforced concrete that are anchorages of sorts. Even so, that final joint does have room to move some. So if you can effectively resist that temperature caused movement, maybe so. I'd first want to see where some other job has accomplished this however.

 

[hokie66]

Drapes,
1) Yes, but other factors like shape in plan and reentrant corners must be considered.

2) If it is not restrained, concrete will shrink without cracking. A high degree of reinforcement controls the width of individual cracks, so the number of cracks will be greater, just not as wide.