Podium Building - Expansion Joint 1

[RFreund]

I am looking at a podium building - approx. 400' x 150'. 4 levels of parking below 8 stories of residential. The parking levels will be concrete - either cast in place or post tensioned. The upper floors are cold formed steel.

We will likely add an expansion joint in the building however I wanted to know if you were consider not having an expansion joint. I have seen some other building almost this size/length without expansion joints. I assume without an expansion joint you would need to consider some additional moment in the exterior columns due to either thermal changes and/or shrinkage. These would exist with or without the expansion joint but obviously larger without.

Any thoughts on this?

EIT

http://www.HowToEngineer.com

 

[manstrom]

400' is pushing it. Over 300', I usually look to add an expansion joint.

If the parking levels are mostly temperature controlled (not open on all sides, temperate climate), then thermal movement won't be a huge concern and maybe you can get away with just a pour strip to accommodate concrete shrinkage during curing.

Even if an enclosed garage is unheated, it won't swing temps dramatically.



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[XR250]

8 Stories using light gage? I hope you have a very experienced engineer and contractor on that one.

 

[KootK]

Any thoughts on this?

Yeah, it sounds like a cool project!

How many of the parking levels are below grade?

Is it just one tower for the super structure?

Which of these conditions are you considering an expansion joint for?

1) Below grade concrete?

2) Above grade concrete?

3) Above grade light gauge?


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[RFreund]

@Manstrom:
That's pretty much are thoughts as well.

@XR250:
Fingers crossed on the contractor part. We do quite a bit of LG though. However, I'm a lot less familiar with Post Tensioned structures.

@Kootk:
All parking is above grade.
Yes just one tower.
The expansion joint would extend through the entire structure. Somewhere in the middle third.

Some quick calcs on displacement due to temperature change indicates almost 4" total. So 2" at each end. Which would be some pretty large moments induced in the columns.

EIT

http://www.HowToEngineer.com

 

[hokie66]

"Either cast in place or post tensioned" makes no sense, as by definition, post tensioned structures are cast in place.

Agree with manstrom that a movement joint should divide the 400' length.

 

[theonlynamenottaken]

At 400' I'd relish an expansion joint not just for lateral temperature strains, but to help deal with differential vertical foundation settlements too. Maybe that's not a concern where the OP is, but differential foundation movement wreaks havoc on a highly redundant structure like a CIP concrete frame.

 

[dik]

Watch for post-tensioning restraints by the foundation walls, and location of stress ends. 400' is a no brainer with poly sheathed unbonded tendons... might look into use of these for a parking garage... use good concrete and lots of cover, and maybe a membrane.
Dik

 

[RFreund]

"Either cast in place or post tensioned" makes no sense, as by definition, post tensioned structures are cast in place.

Touché - I really meant a mildly reinforced system vs PT.

So it seems that most are in agreement with the expansion joint. They are such a pain in the rear though. Well sorta, usually, mostly because of the architectural constraints.

Just for fun - Anyway you would forge ahead without one?


EIT

http://www.HowToEngineer.com

 

[calvinandhobbes10]

Great question here. Developers often cater to the engineers that "forge ahead" so it's a tough one to tackle.

Were I to forge ahead, I would mostly look at facade impact. Brick/panel/eifs/precast/glass, whatever it is, it needs to be able to accept the movement of the structure (in-plane displacement and racking). This probably requires having input from these subs early on, and good documentation on your drawings. The caulk seals will be the first thing to fail.

My gut for 400' is do an expansion joint. 325' to 350' is about as far as I'd go. The cost of the joint will not be substantial compared to the construction cost. Focus effort elsewhere to optimize structural cost.

A few other comments:
1) I recc mild reinf for your transfer podium, PT for the parking decks below. (I am guessing you have a series of ~60' residence wings, as opposed to a rectangular diaphragm, for the upper levels. Likely with a pool and paver-patio on the transfer level. The slab steps this level often defeat the economy of PT, plus you have CF wall anchors that need to avoid tendons.)
2) You said you do a lot of cold-formed, would be curious your experience---I've found CIP stair/elevator towers, if you can get them in the layout, are better than CF strapped panels. This is assuming you have a concrete diaphragm to span (ecospan or epic deck, etc). CF panels are on the critical construction path, while the CIP towers are not. Savings in time erecting and finishing the CF panels outweighs tower concrete cost. Plus the CF straps buckle or stretch from taut when they bring the panels to the field, or you make them go out and weld the straps.
3) Exp joint in the middle might dictate another central shearwall opposite the elevator core, as well as the double-columns.
4) I don't think I would worry too much about the "extra moment" from shrinkage into the columns, although a check is certainly OK. In theory all your floors shrink or stretch, and the foundation is all that sees this P-delta. PT will shrink the floors on the same magnitude during construction. I'd just go bigger on the columns (helps punching shear too) and focus optimization on the parking floors.

Good stuff!