Suspended Slab Restraint - Movement Joints

[Drapes]

I started a thread a couple of years ago seeking some advice on the relationship b/w the overall length of a suspended slab and the need to provide movement joints (link provided herewith for those interested).

https://www.eng-tips.com/viewthread.cfm?qid=446169

Following on from this, I was wondering what length of slab is actually considered in determining whether a movement joint will be required. I have provided a crude sketch below giving 5x scenarios to help facilitate discussion. As a default, most engineers would simply and conservatively take the max overall length of the slab without considering the points of high restraint offered by say the core walls.

For case no 1, I understand you would take the full length of the slab as part of this assessment (that's a given), however for case no 2, I would argue you could take the effective length b/w the core walls as the extent/degree of expansion/contraction in the slab would be dictated b/w points of high restraint. The length of the outstanding portions beyond the core would then be treated separately.

Similarly, for case no 5 you would consider the triangulated lengths b/w the core walls, and the outstanding portions beyond the core to the corners of the slab would be treated separately.

For case no 3, we have an L-shaped slab with the core walls located at the far ends which will almost always warrant a movement joint (or at the very least a delayed pour strip depending on the length of each wing) given the fact we have a re-entrant slab arrangement, however for case no 4 the stresses imposed at the re-entrant point would be significantly reduced as the core walls have been brought in which would intercept or negate any high re-entrant stresses, so each wing could arguably be treated separately and a movement joint may not be required.

Look forward to hearing other peoples thoughts on this.

 

[KootK]

That's an excellent summary and I agree with you on all counts. The only corollary that I'd add is that, in some cases, it may be prudent to consider the restraing effect of your columns as well as your shafts even if your columns are ostensibly non-lateral.

 

[Drapes]

Thanks KootK, always nice to hear your insights.

I was wondering what your thoughts would be on case no 5 below, a situation with a long enough slab that would usually warrant a movement joint (not dissimilar to a below grade basement). Would it be acceptable to not take the full length of the slab in this situation in determining if movement joints can be avoided, as the the in-plane stresses would strut directly into the walls and not accumulate over the full length of the slab?

 

[Tomfh]

It all comes down to restraint and the amount the slab wants to move at that location.

It’s complex stuff.

We have seen one like 3 which we reviewed during design phase. PT guys had no joint or pour strip. We said to put one in. They said no. They were right and it worked.

We have also seen one like case 1 with columns only along “free” ends. The designers didn’t really consider the column stiffness and the column restraint accumulated and cracked the slab very badly.

 

[rapt]

Drapes,

is your new 5 showing continuous walls along the length? If yes, the slab is fully restrained. Unless you have very very close movement joints (obviously unacceptable) you need to reinforce the slab for the tension caused by the restraint to try to control the crack widths.

 

[KootK]

I would actually seek to avoid post-tensioning in the long direction for scenario #5. Unless you can create good slab/wall temporary slip joints, which is difficult, you'll wind up with most of your pre-compression bleeding out into the wall systems and, therefore, not actually pre-comopressing the slab as one would normally hope. You would still have the balancing load effect at play.

 

[Drapes]

Thanks to everyone for their thoughts.

rapt, correct this new case is showing continuous walls along the full length. Understand a high-level of crack control reinforcement will definitely be required, however I was more interested in understanding if it would make a difference to the level of reinforcement if you had a long compared to a short fully restrained condition (refer below), as my argument would be the length would be irrelevant in this instance as the in-plane stresses would strut directly into the walls and not accumulate over the full length of the slab.

In my view and in general, the length only becomes critical in determining if you need movement joints and the level of reinforcement, depending on if you give the slab the opportunity to shrink and build-up the stresses over the length of slab.

 

[Tomfh]

I would put more reo in the long restrained slab.

 

[Drapes]

Thanks Tomfh, I think I would intuitively put more reo in to the longer restrained slab as well. But in principle is it really required, based on the reasons I mentioned in my earlier post?

KootK, rapt - always keen to hear your thoughts as well.

 

[KootK]

KootK, rapt - always keen to hear your thoughts as well.

Alright, I see it as shown below. In this instance, it seems to me that:

1) The presence of the longitudinal PT will not exacerbate restraint shrinkage but.

2) The presence of the longitudinal PT also will not be effective in helping to control restraint shrinkage.

As I mentioned previously, I'd likely not post-tension a slab of this configuration as a result of the diminished effectiveness and potential complexities.