Deck Collapse in North Carolina 6

[Ron]

Another wood structure collapse. This one has clean drop at joist buckets. Hmmmm.....fastening issue perhaps?

News story notes it as a "balcony collapse"; however, it is an elevated deck.

Deck Collapse

A photo is attached as well.

 

[bridgebuster]

SRE had it: Deteriorated nails according to this story

http://www.foxnews.com/us/2015/07/0...rth-carolina-deck-collapse/?intcmp=latestnews

 

[msquared48]

So the better detail would have been to bear the joists on the beams, not set them in hangers or nailers.

Mike McCann, PE, SE (WA)

 

[rlflower]

Agreed, msquared48. I ask myself: why didn't they simply drop the beam below the joists and pass the joists over the beam in a conventional manner with lap-splices?

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[JAE]

Here's a close up of a joist end - taken from this video: Video Link - go to the 30 second mark

With nails sticking out up above - it seems like the bond between the nail and joist was such that many simply pulled out of the end grain.
This may not be the initial cause of the collapse - perhaps something gave way in shear and that started a domino effect as the deck joists pulled away from the supporting beam.

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[rlflower]

Interesting comments made on this thread regarding the use of galvanized nails in this situation. The attachment indicates a recent (2004) change in how pressure treated lumber is manufactured and how this change affects the proper fasteners to be used.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[Archie264]

>>>It seems that we read about at least one of these a year.<<<

Years ago I made that very comment to a representative of Simpson Strong-tie. He, being more directly involved in that specific corner of our industry, replied, "More like every week." And I didn't get any sense that he was joking or using hyperbole.

 

[hokie66]

The ledgers seem to have stayed attached. Maybe they were fixed with different fasteners.

The edge beam rolled when loaded, and there were no nails left to resist.

 

[SlideRuleEra]

Hokie - I believe the left ledger is gone. Take a look at the enlargement of IRstuff's photo below.

I propose all the corroded nail on the left side sheared (the nails into the ends of the joist and the nails holding the ledger in place).
As the left side of the floor descended the joists rotated downward... pulling away from nails remaining in place on the right.
This explains why no full length nails are visible on the left side, yet the right ledger and full length nails are visible on the right.
The photo JAE furnished, showing nail holes, must be of the right end of the joists.

Here is the enlargement of the left side:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Brad805]

It almost looks like there was a bottom bearing at the last beam that is now missing.

How many times have we all wanted to say to a client, please put down the hammer and tool belt?

 

[rlflower]

Continuing with the adventure regarding proper usage of fasteners and connectors engaged with pressure treated wood: I just spoke with a technical rep for Simpson Strong-tie. The general synopsis is that except for more extreme marine environments, they would be satisfied with the specification of "Z-Max or hot dipped galvanized equivalent" for all their connectors and fasteners that are engaged with PT wood and are exposed to such an environment.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[IRstuff]

Seems to be another case where < $100 worth of joist hangers would have kept the thing from coming apart that way.

TTFN
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[hokie66]

SRE,
Yes, I agree. That stands to reason, as the exterior one is the most susceptible to the weather.

IRstuff,
No, joist hangers are nailed, so the same corrosion problems exist.

rlflower,
The outer islands of North Carolina, as well as all barrier islands, are an extreme marine environment. Simpson's satisfaction would not satisy me.

 

[rlflower]

I will concede to your assessment with regards to the location, hokie66.

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[69Firebird]

There is another consideration to look at that can contribute to the fastener corrosion. After manufacturers switched to ACQ instead of CCA for lumber treatment most lumber was treated with ACQD as it was probably cheaper. What most people including reputable contractors weren't made aware of for several years after the switch, was how corrosive ACQD is to fasteners and connectors. While this house is older it is harder to tell the age of the deck, but the lumber doesn't appear to be weathered that bad. Also the story makes mention of 24 family members injured. Overloading may be a contributing factor, with the joists being toe nailed instead of hangered or supported on beams its hard to say if the deck would have held without the corrosion.

 

[msquared48]

The connections at the end of the joist shows as two side toenails and one top toenail. Proof not to rely on toenails.

Mike McCann, PE, SE (WA)

 

[Ron]

Much better photos!
Looks like the left ledger dropped and allowed rotation that pulled the right side of the joists out from the beam.

The ledgers were installed on the bottom of a double beam, with all the fasteners in tension (however few of them there were). Ledger fasteners should be in shear, not tension. Even if the fasteners did not corrode, it would be easier to overload the pullout in this orientation.

 

[hokie66]

I didn't get that, Ron. Looks like to me the ledgers were about 2"x2", nailed to the side of the beams. The one on the right side which was still in place seemed to have nails horizontal.

 

[SlideRuleEra]

Ron - From everything that I have seen, read, and know (first hand: 50 years+) about SC, NC beach house construction, here's my take on it:

Double 2 x 10 beams, approximately 10' long.
2 x 8 floor joists @ 16" O.C, each approximately 12' long.
2 x 2 ledger with 3 nails under each joist (probably 16d or 20c common). Nails loaded in shear.
Remains of toenails from joists into beams (probably 12d or 16d common).
5/4 x 6 deck boards.

All lumber color and condition (29 years old) consistent with CCA-C treatment.
All nails hot dip galvanized.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Ron]

SRE...that's what I would have expected as well except that the right side looks like a solid piece of lumber under the double 2x10's. I see the 3 nails under each joist, as you would expect, but it looks strange. Maybe I'm just not seeing it correctly from the angle or resolution.

I think you'd have to be nuts to construct it as I described above, but considering the amount of bad construction I've investigated over the last 35 years, not much surprises me anymore.

Looking at the band below the sliding glass doors, the left side looks like maybe a 2x12 and the right side narrower and is pieced with a 2x2. Why? That member picks up essentially no load.

 

[3DDave]

Ron, I think it is a visual illusion - the 2x2 shadow is farther down the wall, indicating it is protruding farther from the wall than the 2x12. There seems like some anti-pest netting underneath and maybe the little strip was part of that.