Missing Coil Straps!!

[Martin_O]

We have a light gauge bearing wall project in New Jersey (seismic Design Category B) with floor joists at 24" oc and 9/16" x 24 gauge decking with 1.5" light weight concrete topping.

We are the EOR, but the contractor hired a separate engineer to design the bearing walls for panelized shop construction that were shipped to the site. Everyone has agreed that the lateral system remains per our drawings. We assumed that the floor diaphragm carries lateral loads horizontally to the shearwall lines and various elements along those lines (wall top tracks, steel beams, ledgers etc) "collect" and deliver the loads to the individual shearwalls. In addition, we detailed continuous coil straps to ensure continuity at various transitions. Unfortunately the contractor (and special inspectors) missed the straps when they should have been installed!!

The panel engineer's position is that the floor deck likely has the strength and stiffness to act as both a diaphragm and a collector and, given the concrete topping, the most natural load path is for the diaphragm to deliver the lateral loads directly to the shearwalls without relying on the straps. As a result, they believe the focus should be on verifying the deck capacity followed by checking the local connections at the floor to shearwall transfer and not necessarily forcing the contractor to install the coil straps.

The (untopped)deck diaphragm shear capacity is 180 plf (ASD) which is higher than any of the shear wall loads from an individual level. So it appears at first glance that this approach may have merit?

 

[r13]

A plan and cross section, with locations of the straps in the design, may better describe your question, and generate responses.

 

[Martin_O]

We can get into the specifics if people are willing to join a deep dive. For now, more interested in the philosophy of using the diaphragm to satisfy the code requirement to provide or assign elements as collectors.

 

[r13]

I think you shall first check if the roof deck can span between the end supports without anything in the middle. If it can, obviously you have over designed. But then you don't want to have anything in the middle rigidly connected to the roof, that would block the shear flow, which might tear the deck apart. Just speak loud.

 

[Aesur]

If there is chord reinforcing in the concrete then it's possible, but it's tough to fit much reinforcing in 1.5" concrete on form deck. I personally wouldn't use the deck itself as the deck is non-composite, however I would be willing to entertain adding reinforcing in the slab for the tension loading and the concrete for the compression. How did you connect the top track to the deck/concrete?

 

[Martin_O]

The panel supplier changed the detail to top track of lower wall = bottom track of upper wall = bottom of metal deck.
The deck screws down to a continuous ledger track/clip system that supports the floor joists and the concrete topping runs over everything (including track)

A quick sketch should be linked below.

Your file's link is:

https://files.engineering.com/getfi...-15d496a143e4&file=Typ_Floor_to_Wall_Conn.pdf

 

[Aesur]

@Martin_O is the ledger continuous for the full length of the diaphragm at the walls? if so, the ledger could act as the chord. A floor plan would help a lot. Are the wall studs in-line framing and do the upper wall studs align to the bottom wall studs to avoid placing bending on a track? How does the shear from the wall above get to the wall below with framing like that - typically the bottom track would act as the cont. chord and drag into the diaphragm/wall below - it appears that with the concrete over the track the sheathing would not extend to the chord and you could need blocking and straps to create a new cont. bottom chord that would be attached to the concrete.

 

[JLNJ]

I’ll give it a shot.

If the shear walls are of the same stiffness and the diaphragm can deliver the forces to each wall, then maybe there is nothing to “collect”. It seems to me that your collectors function to deliver/distribute the load between shear walls. This makes the collectors more or less redundant.

If the collectors are essential for force distribution I would be more hesitant to rely solely on the metal deck. Especially in the cross-rib direction.

For seismic, there also might be some overstrength combinations to consider. If they control, it’s easier to beef up the collector than the diaphragm itself.

 

[RFreund]

So I'm sure there are buildings that are constructed the way the specialty engineer is suggesting (i.e. without the straps). There is a load path there, just depends on how far you want to get into it. Things that make is easier - are the walls spread out over the plan area? If yes, then you feel a little better about it. If there are areas where your diaphragm has to transverse a change in deck direction before it can be "picked" up by a shear wall, then things get a little worse. If you have LW concrete (as opposed to gypsum) I suppose that is even a little better.
Having said that I'm a fan of coil straps as collectors and would use them, but in low-seismic areas, I have seen projects without them.

EIT

http://www.HowToEngineer.com

 

[XR250]

If you are hurting for diaphragm chords, see if you can justify a 3-sided building design. It will load up your shearwalls and tie-downs more, but has the benefit of not needing chords.
Also, might want to check that the deck was fastened to the shearwalls adequately for the load transfer as the demand on the fasteners would increase w/o the benefit of the collector.

 

[phamENG]

In general, sure. If your deck has sufficient diaphragm capacity and the connection between the diaphragm and the shear wall is sufficient, it should be okay. But there are geometric issues to consider. Do you have openings? Where are they in relation to the walls. Do the walls stop and start, or is the diaphragm continuously supported? You could end up with some interesting stress concentrations that may cause you some problems.

Without specifics, that's all I've got.

 

[Martin_O]

Thanks everyone for the thoughts.
We're a west coast design firm, so seismic load path is of critical importance to our way of thinking. Having said that, we realize that there are a lot of unaccounted for strengths and stiffnesses that can be significant when compared to the demand in low seismic areas.