Residential Deck Collapses in NC - 21 Injured 1

[SlideRuleEra]

This article is about a deck that collapsed, injuring 21 people. From a strictly engineering viewpoint, looks like evidence is growing that the copper based treated lumber (ACQ), one of several treatments that replaced arsenic based compounds (CCA) about 10 years ago, may be expediting fastener corrosion. This has been known all along but, unfortunately, may take incidents like this to "prove" it. Time will tell.

http://www.charlotteobserver.com/2013/07/10/4156649/21-hurt-in-ocean-isle-beach-deck.html

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[msquared48]

OK, and how do you figure this?

"Peter Combs, an Atlanta-based architect who has testified in more than 90 court cases involving structural integrity, said Wednesday that in most deck collapses, the problem actually is with the connection between the deck and the house."

Does anyone see anything wrong with this picture? This guy is dangerous...



Mike McCann
MMC Engineering

 

[SlideRuleEra]

Ok, here is the way I figure this... be skeptical of a newspaper article and do some independent reading from authoritative sources (before I posted this thread). Here is one from the American Wood Council:

http://awc.org/helpoutreach/faq/faqFiles/Corrosion.html

another from the North American Deck & Railing Association:

http://www.nadra.org/acqarticle.html

... in this case, looks like the newspaper article MAY be correct. As I said in the original post, "Time will tell."

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[Ron]

Interesting...I'm going to a site tomorrow to start investigation of similar. In this case, 8 people were hurt on July 4.

There is usually not a single cause, but multiple errors...most commonly, poor construction...but could be poor design or lack of design altogether. As for fasteners, corrosion will occur in exposed decks without regard to the treatment. Any copper-based treatment will exacerbate corrosion of steel nails/screws...at the ionic level, they are dissimilar metals. The rate of corrosion has numerous variables that are often site specific. Increased moisture, increased chlorides...all lead to corrosion.

 

[msquared48]

And another thing that my have contributed is music, causing the patrons to move in rhythm, causing a natural frequency of the deck to occur, accentuating the forces to any bad connections. I would be very interested in seeing how that deck was braced laterally, if at all.

Mike McCann
MMC Engineering

 

[hokie66]

Whatever the actual failure mechanism, these external decks have a habit of falling and hurting people. This applies in many parts of the world. I think the theme of the report is that the connections fail, not the wood. That agrees with my experience. But then that applies to any type of wood structure, not just decks. The connections are the key. When decks are stacked post, bearer, joist, they don't fail. When the joists are supported on shear connectors, they fail, sometimes.

 

[JAE]

This is why I added a new beam line about 2 feet from my house wall to negate the need for the deck-house connection except for some lateral stability.

 

[Ron]

As usual, either the report got Combs' qualifications wrong or Combs misstated his qualifications....according to his website he has only testified 24 or 25 times in deposition and 8 times at trial....not 90 as stated. He shows conflicting statements on his website in the numbers.

I just thought....90 times testified??? How old is this guy. I've been doing this a long time and it would be difficult to fit in 90 testifying events since the pace of construction litigation is so slow.

He's an architect! What are his qualifications to opine on structural integrity? Yes, I know architects are allowed to "design" structures up to a certain limit, but that usually does not involve anything rigorous.

We do have to consider; however, that he's a Gator! (Go Gators).....he can therefore leap tall buildings in a single bound.

 

[JAE]

Ron - architects wear black. Black only. They don't wear blue and orange.

 

[SlideRuleEra]

I'm concerned about the many specifications for treated lumber construction that I have come across that only require "galvanized" fasteners. In the past, just about any fasteners that were galvanized would be more or less suitable for use with CCA. However, now that ACQ and similar treatments are in common use and the chemicals used damage fasteners (no help needed from Mother Nature) it makes a BIG difference exactly what type galvanized fasteners are used. Hot-dipped galvanizing is acceptable, zinc plated and mechanical galvanizing are not.

Many Engineers, Architects, and Contractors are going to need "re-education" into the details and importance of correct fastener usage.

In coastal areas hot-dip galvanized fasteners are common because of seawater and "salt-air" corrosion. Is hot-dip common on more inland areas?

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[hokie66]

Even with hot dipped galvanized fasteners, I wouldn't be confident long term in external decks. The lighter the steel section, the less zinc is deposited in the galvanizing process. In design of timber decks, it is preferable if all the important members are supported in bearing rather than in shear. JAE has the right idea...add another beam line rather than supporting off the house. Stainless steel fasteners are readily available now, so they are another tool to consider.

 

[Triangled]

From the OP article...
"Marzano said 25 people were on the deck when it gave way.
“I do not think the N.C. building code anticipates 20 to 30 people in a small deck at one time,” she said. “It happens often for photo ops with large families at the beach.”"

25x150lbs average / 60 psf = 62.5 sf >> 6x10 deck or so. Article says "small deck". Anyone know deck size?

But what has always concerned me is that same 25 people crowded together on the edge of the deck jumping up and down excitedly, same as msquareds rhythms above, occupying maybe 1.0 to 1.5 sf per person, generating 100 to 150 or more psf live load locally.

 

[Ron]

JAE...orange and blue is the new black.

Do architects wear black to match their minds or their hearts?

 

[Ron]

JAE...orange and blue is the new black.

Do architects wear black to match their minds or their hearts?

 

[msquared48]

Repetition for emphasis?

Mike McCann
MMC Engineering

 

[JAE]

I'm hard of hearing.

 

[Ron]

Thumbs on tablet....no finesse in these hands

 

[dik]

What kind of meds? oh, wrong tablet!

I did a report on a deck collapse about 15 years ago. The backyard sloped away from the house and there was a ravine below which increased the height of fall... several people were injured and a few hospitalised.

The deck was horizontal and the 3'6 guard were all around, except for the area where the stair and an enclosed area existed. The deck was elevated about 15' and braced back to the building at the base; the sloped brace was on an angle about 6' hor and 15' vertical. The deck had never been loaded as heavily; there was a fairly large international crowd. The deck had been used with light loads for over 20 years...

The large number of people put a horizontal outward load on the fasteners to the wall. These failed in 'pullout' and as initiating slip occurred, the horizontal force increased, precipitating failure. There was a minor trace of dry rot (brown rot). As the sloped brace rotated, the horizontal force increased, and precipitated the total failure. It was a total collapse, the deck load was pretty much distributed over the area.

If the outer supports for the deck had been vertical, there would not likely have been a failure.

Dik

 

[hokie66]

And if the tiebacks had been adequate (both at the time of construction and 20 years later) to resist the horizontal force, it also likely would not have failed.

 

[dik]

Agreed... but they weren't and failure was catestrophic... The force on the fasteners increased as the sloped brace rotated about the base.

Dik