Movement Joints - Tower & Podium Interface 2

[Drapes]

Say we have a 40-storey tower with a 3-storey above grade podium structure and double volume basement. Assuming the overall length of the podium slab does not warrant a movement joint to accommodate thermal and shrinkage effects:

1. Will a movement joint still be required at the interface b/w tower and podium structure to accommodate differential settlement of the foundation?

2. Will differential axial shortening b/w the tower and podium columns also play a role in warranting a movement joint? Does this add to the effects seen from differential settlement?

3. If yes to either 1 or 2 (or both), does the movement joint need to extend through both the podium levels (above grade) and the basement levels (below grade)? I understand that it will be required to the podium levels, but have heard mixed opinions on whether it will need to extend into the basement box - any guidance on this?

For example one publication quotes the following: "It is usually unnecessary, and also not desirable, to isolate the tower from the basement unless the basement is shallow. In such cases, consideration may need to be given to isolating the tower structures through the basement/podium levels".

4. In what situations would a movement joint need to extend through the foundation to form an isolation joint? Obviously this would be undesirable in most instances due to hydrostatic uplift, high water table levels etc, but wondering what would warrant full isolation.

5. Does seismic pounding typically dictate the width of the movement joint b/w the two buildings?

6. Is it possible to avoid movement joints altogether in this scenario if the resulting curvatures and stresses in the slabs are within acceptable limits? After all, the effect on the slabs at the interface b/w the tower columns and the podium/basement columns is not dissimilar to the differential axial shortening effects seen in a typical slab b/w the columns and core in the main tower which are obviously detailed and resolved without any permanent joints.

Sorry for all the questions, but this topic has particularly interested me for quite some time now and am yet to give myself adequate closure - I look forward to hearing feedback from those who have had experience in this area.

 

[slickdeals]

I would provide a long-delay pour strip between the low rise and the tower and allow the dead load settlement to occur before closing pour strip. It would be advisable not to have a low rise column very close to the high rise to allow the slab to "flex" between columns. A short span could lead to some slab cracking.

 

[Retrograde]

I would not provide a movement joint. Also, I would not bother with a pour strip. You need to design your building such that the differential axial shortening is within an acceptable limit. Thus I agree with your point 6.

If you do not limit the differential and provide a movement joint (or pour strip) you may end up with a permanent step in the slab at the joint location.

 

[Drapes]

Thanks for your feedback slickdeals and retrograde.

Makes a lot of sense, and seems like the practical way to go - I try to avoid resorting to movement joints if possible. My only reservation here is that there are numerous sources that recommend adopting a movement joint in this situation (at least to accommodate point 1 above on differential settlement).

So would you only typically provide movement joints in long and/or irregular shaped slabs to accommodate thermal and shrinkage effects? And in a situation like this, would the movement joint need to extend through both the podium levels (above grade) and the basement levels (below grade)? Or can it just be applied to the podium levels (above grade)?

Under what situation (if any) would the movement joint be required to extend through to the foundation to form a full isolation joint? Have you ever had to deal with this?

 

[KootK]

I'm glad that you asked this question as it's been a point of frustration for me for years. I'll leave the question of what should be done to others but, instead, speak to what I see getting done. And that would be nothing. I've seen dozens of 30+ story podium buildings go out with no special detailing at all -- no problems that I know of. Ditto for long buildings for which the literature would seem to suggest that movement joints would be required. Ditto for two to four towers on one combined podium. I'll sometimes see and use delay strips but even those never carry through the basement walls. So there always seems to be this discrepancy between:

1) What the literature and reason suggests and;

2) What the competition is doing and, effectively, forcing you to do.

If joints are simply never required, I'm cool with that. I just don't like the mixed messages.