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Tilt-up Wall Diaphragm Chord Connector

Tilt-up Wall Diaphragm Chord Connector

Tilt-up Wall Diaphragm Chord Connector

(OP)
I have heard, but still cannot fathom, that the roof or floor diaphragm chord for a tilt-up building does not have to be developed for the building system to work.

There has always been the dichotomy between the lateral movement of the side connectors and the rigidity of the chord connector that has bugged me, and I believe an explanation of the above could solve the dichotomy. It gets into each panel acting separately rather than one unit.

Anyone run across this? Any explanations?

Mike McCann, PE, SE (WA)


RE: Tilt-up Wall Diaphragm Chord Connector

I'm not sure that I know the answer but I'm interested in the discussion. What exactly do you mean by a chord being "developed"? Can you describe a typical situation?

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.

RE: Tilt-up Wall Diaphragm Chord Connector

(OP)
Well, the problem has always been the rigidity of the chord connector as opposed to the flexibility of the side connectors, both to the lateral expansion/contraction issues due to temperature changes.

The dichotomy comes in the that I have always learned that the top connector (Chord) must be rigid, otherwise the chord force cannot be generated to keep the diaphragm intact. This is a different issue than shear collection to a drag strut.

It is hard to explain what you do not understand, but I have heard, through my son and others, that there is another way to look at individual panels to solve the issue. That thinking is what I am trying to discover.

Mike McCann, PE, SE (WA)


RE: Tilt-up Wall Diaphragm Chord Connector

We have developed the chord force within the PC panel, but we have not deleted the chord element per-say. Given the lateral loads we are forced to deal with today it is rare that we do not have panel to panel connections to utilize the wall as a full length shear wall. Without such connections the uplift forces are always a problem for simple footing designs that most EOR's employ.

RE: Tilt-up Wall Diaphragm Chord Connector

If you split a diaphragm into two, separate three sided diaphragms, then you can perform the lateral analysis without counting on continuous chords.

DaveAtkins

RE: Tilt-up Wall Diaphragm Chord Connector

(OP)
OK.

I can envision a detail that uses a continuous angle at the diaphragm edge with 2 to 4 fixed bolts at or near the center of the panel length, and bolts with horizontally slotted holes from there to the edges of the panels. This would allow for panel thermal movement, but keep the diaphragm chord force intact through the continuity of the angle. Say the internal five feet of a 25 foot panel would have fixed holes, and the ten feet to either side would have horizontally slotted holes.

Is that a detail anyone has ever used?

If that is the case then, the chord force is never seen by the panel, only the transfer of horizontal shear to the panel.

Mike McCann, PE, SE (WA)


RE: Tilt-up Wall Diaphragm Chord Connector

I feel that a "chord-less" system could generally be workable:

1) With an untopped steel deck, I would think that the diaphragm could tolerate the small amounts of differential lateral movements between panels. You get something similar in both conventional steel and wood diaphragms where you have to accommodate localized nail/bolt slip. Obviously, one would have to place reasonable limits on the amount of tolerable differential deflection but, for common tilt up structures, I wouldn't expect this to be a problem.

2) At a floor with a composite deck, I'd have to think that your chord would be integral with the floor and you'd be forced to use a detail such as Msquared has suggested to accommodate panel thermal movement.

With the truly "chord-less" diaphragm, I essentially see the tilt up panels as an extension of the diaphragm with the the real chord being some element of the foundation.

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.

RE: Tilt-up Wall Diaphragm Chord Connector

(OP)
Ok... but then the same logic could apply to any diaphragm. I find that hard to fathom.

Mike McCann, PE, SE (WA)


RE: Tilt-up Wall Diaphragm Chord Connector

Can you elaborate on the fathoming difficulties? The systems that we've been discussing certainly satisfy equilibrium. The only question, in my mind, is strain compatibility between diaphragm chord and diaphragm web. And really, that's a question that I wonder about all the time, even for normal diaphragms. How stiff must a chord be in order to shield the adjacent web from tearing apart? We don't seem to give that explicit consideration. I've always kind of hoped that frugal span to depth ration limits took care of that to some extent.

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.

RE: Tilt-up Wall Diaphragm Chord Connector

When I was designing a lot of small, wood framed buildings, which many times don't have a well defined diaphragm chord (think of exterior walls which jog in and out a lot), I realized the diaphragm could not literally tear itself apart without overturning exterior shear walls.

DaveAtkins

RE: Tilt-up Wall Diaphragm Chord Connector

The most common is weld plates and welds to the deck angles. Bolts and precast (or tilt) are difficult due to tolerances. We use a 1/2" design gap and some use 3/4" to account for field variations or casting irregularities. The Aussies have a great connection that allows for 20mm of misalignment in bolted connections, but that connector has not made it here.

In the connection we have used our panel thickness reduces from the deck bearing elevation to the top of the panel to form a ledge. They cast continuous plates into the panels that the decking is attached to. That plate along with panel to panel connections are deemed to be the chord. We build insulated panels and there are other reasons for the ledge. This did not turn out that great because of the detailing around joist pockets and the responsibility shift for the chord. It also does not save much because one just basically shifts whom is paying for the chord material and installation.

RE: Tilt-up Wall Diaphragm Chord Connector

@Brad: can you comment on the nature of the panel to panel chord connection? I don't have a good feel for what form that takes. If I understand Mike's issue correctly, there is concern that connection might be flexible with regard to panel thermal movement and therefore not rigid with regard to chord action.

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.

RE: Tilt-up Wall Diaphragm Chord Connector

The panel to panel connection we have used is rigid and does not allow for thermal movement. When we are the seismic force resisting system the forces are generally too large to use a cantilevered panel design. None of the EOR dwgs we ever see have any ability to resist uplift forces at the footing elevation. In essence we mimic a CIP wall. Expansion joints are typically shown at spacing similar to what one might do for CIP.

To achieve what Mike has heard seems difficult, but then again, I have reviewed buildings from the 60's that have no perimeter chord member and they have performed ok.

RE: Tilt-up Wall Diaphragm Chord Connector

Thanks Brad.

Quote (Brad805)

but then again, I have reviewed buildings from the 60's that have no perimeter chord member and they have performed ok.

Yeah, I think that we can all agree that, in many instances, a designated perimeter chord probably isn't really necessary. In one direction, I'd argue that the deck flutes could probably do the job. In the other, interior framing a bay in from the edge could surely get the job done.

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.

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