Diaphragm/Chord Reinforcement Design in Precast Concrete Structures

[JohnRwals]

Hi, everyone!

I would like to ask if exterior PRECAST concrete walls (and connections) can be utilized
as chord reinforcement.
Sometimes precast concrete diaphragm is too thin or hard to add continuous chord reinforcement inside.
If wall to wall connections like PLAN 2 near diaphragm levels can function as chord reinforcement,
precast concrete will be more popular.

Is there any experiment or research related with this idea?

-JRW

 

[Once20036]

Hoping that KootK shows up for this one.
He and I discussed this concept years ago. I think his contention was that even the chord reinforcing in the walls wasn't required.
I`m still not sure that I agree with him, but still think about that conversation.

While I won't be as cavalier as he, I think the the wall chords you present would work, as long as there's sufficient shear connection between the slab and the wall

 

[DaveAtkins]

The reason why chords are not technically required in a case like this is you can divide the building in half, and treat each half as a three sided diaphragm. What would have been chord forces are resisted by shear walls.

DaveAtkins

 

[Greenalleycat]

Technically I don't think you require chord reinforcement in either the diaphragm or the walls, in this scenario
This assumes that you have adequate connection to the perpendicular walls, and that those walls can transfer the loads to ground

Practically, I have a couple of concerns
1) If this is seismic, the system critically relies on the wall-floor connections, which are often quite brittle and have low redundancy
2) Due to the aspect ratio (of this building at least) you'll get large forces in the perpendicular walls at the edges - can these walls actually transfer the loads to ground?

 

[KootK]

Hoping that KootK shows up for this one.

Thanks for the summons. I believe that this is the thread that Once20036 alluded too and, in particular, the sketch shown below whipped up by yours truly. It's pretty bitchin' if I do say so myself.

 

[KootK]

I would like to ask if exterior PRECAST concrete walls (and connections) can be utilized as chord reinforcement.

They can, and frequently are. Typical morphologies of that:

1) Run a continuous angle along the wall to deck joint to function as the chord or;

2) Let reinforcing in the tops of the walls be the chords. Somehow or other, the "chords" in each wall panel need to be spliced for this to be legit.

While these things work on paper, they tend to cause some problems in the real world. The chords interfere with natural panel shrinkage, or vice versa, depending on your perspective. There have been quite a few threads here about this topic.

Per my sketch above, the wall panels -- or any VLFRS really -- can be used to effectively make the foundation or the level below the "chord". We like to conceptualize diaphragms as horizontal beams with flanges that become chords. There's value in that model but we sometimes do ourselves a disservice in forgetting that, often times, there will be VLFRS located along the chords that restrain their movement / strain longitudinally. Taking advantage of this can be useful.

When you understand load path, a whole world opens up to you.

That's from the referenced thread in my previous post. It's a tad pretentious sounding but wholly accurate none the less. I was thinking it well before JLNJ went and said it.

One snag to watch out for is that some codes and standards stipulate that chords shall not be vertically offset too much from diaphragms. I consider that direction to be violable when the design is in the hands of an engineer capable of properly handling the implications.