I think you are right, BS2, but think that is irrelevant. But then I also think the design loading is irrelevant in this case, but some others disagree.
ChiefInspectorJeff showed us a framing plan purporting to be of the building in question. It appears that the balcony joists are cantilevered with a fairly small back-span but I tend to agree with hokie66 that it isn't relevant.
You may be correct, hokie66, in stating that the design loading is not relevant in this case. However, as was the intent of the article, "What can we learn from this?" I saw an opportunity here to describe two basic methods of framing balconies that are in use today. My claim that this is an "outrigger" method of framing was based primarily upon one photo that showed the cut-away view of the balcony directly below the one that failed. I have searched in vain to recover that one photo, but basically, we can note from the available photos that the perimeter framing is somewhat larger than the joists themselves, and this suggests the outrigger method of framing was employed. The other hint at this was the way in which the balcony failed: it was a "catastrophic" failure - meaning that the entire balcony gave way rather than merely a portion of it. The other common method of framing - where all the joists are cantilevered - would have been a more redundant system which may have allowed a more localized type of failure (one or two joists failing, for example) rather than a total loss of the balcony. In terms of life safety, the difference between these two types of balcony framing should be addressed.
However, I have no intent upon interfering with the investigation. As stated in the article, I give all due respect to the ongoing forensic investigation and I certainly do not intend to interfere with that effort. It is not my intention here to suggest that the failure had anything to do with the chosen method of framing. I think we are all clear on that point and we are looking at cause related to the flashing resulting in deterioration of the wood framing.
The article was presented as an opportunity to learn something from this experience - focusing upon a certain characteristic that will likely be "brushed aside" as not relevant to the investigation.
I also wish to express gratitude to all who have contributed to this thread.
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
How could it fail in any other way? Look at the simplified example of two joists with a rigid plate on top. Assume the joists are of equal strength and there is some asymmetric weight that overloads one of them. Either the rigid plate tilts dropping all the people on it or the weight formerly shared is now more concentrated to the remaining one, causing it to fail and dropping all the people on it. There is no condition in which the rigid plate experiences a small, telling motion that gives people on the balcony a warning in time to vacate.
For the redundant cantilevers, the end result might have been the same. While there is redundancy, that would only be for as long as the remaining member stay sound. If they were all rotted, then you would get a cascade failure that might happen so quickly that it would be indistinguishable from a sudden catastrophic fail. Moreover, it's likely that the failure was exacerbated by the 13 people on it at the same time. Had there only been, say, 4 people, the balcony might have started to give way slowly, allowing the people to recognize the danger and leave before it collapsed altogether. Certainly, prior to the actual failure, the balcony gave the appearance of being able to support its own weight, which is not inconsiderable, even though the members were already rotted.
While there was some speculation that this balcony shielded the one below, which exhibited, as near as I can tell from photos, almost no rotting, rain water would still have gotten onto the lower balconies. This possibly suggests that the actual construction on that one particular balcony was deficient, which aggravated the less than ideal design. I had that sort of thing happen in my new house; we got some water intrusion, and it turned out that the tar paper had gotten damaged and torn in ONE spot, and water managed to get down to the hole and entered into the wall.
TTFN
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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:
My point in asking about the type of construction is that the posted article seems to indicate that a more redundant system (where all of the joists are cantilevered) would be safer. However, from what I can see, all of the joists do appear to be cantilevered. In my opinion, this type of sloppy writing misleads its readers and is not useful in our pursuit of learning from past mistakes.
BS2: "All the joists appear to be cantilevered." I would like to know what leads you to that conclusion.
Found the pic I was referring to. This pic is what led me to think that this is an outrigger system. If anyone has more information on the type of framing system employed (a pic of the structural drawings showing the framing system would be ideal), please feel free to post it on this thread.
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
I agree with BS2 that the joists appear to be cantilevered. What leads me to that conclusion is the photo of the failed joists. They all failed at the exact same location along the span as the two edge cantilevered members that are larger in cross section (i.e. the two you refer to as outriggers). In the system you refer to as an outrigger system, I assume that you are implying that only the two edge members are cantilevered, and that the infill joists are simply supported spans between a ledger at the exterior face of the building and an edge beam spanning between and perpendicular to the two cantilevered outriggers at their free ends. If what I have described is an accurate description of what you refer to as an outrigger system, then I don't think that the infill joists would fail in the manner that we see in the photo. Don't you think they would fail at the simple support at the ledger at the face of the building?, especially if it was a toe nailed connection, which it would have had to be since we don't see any joist hangers at the face of the building. I don't think this is the system that we see in the photo. They clearly look like cantilevered joists to me.
I agree with gte, they are all more likely than not cantilevered. I often put in a bigger edge beam because it makes other things easier, like connecting guardrails and such.
nackra, precisely right. Several commenters have already made the same point. Accessible, visual, hands-on observation is really the only practicable inspection method that can be routinely carried out expeditiously and cost effectively for the tens of thousands (at least) or hundreds of thousands of these types of balconies that exist across the country. Some of the other suggestions of load testing and measuring in-situ natural frequencies and the like would never work in the real world, in my opinion.
I fail to see how everyone on this thread concludes that these joists are cantilevered. These photos can only give us certainty that these joist bear upon the wall; there has been no means presented on this thread that can suggest that these joists extend beyond the wall - a backspan - to another line of support. Indeed, we are only looking at photos taken from the outside of the wall.
Yes, the stubs remaining of these joists have not significantly rotated as a result of the failure, and that does suggests some measure of restraint to keep them somewhat horizontal; however, there was not much strength left in these members to cause such a rotation at the moment of failure.
We have a lack of information, gentlemen, and we are jumping to conclusions. Again, all I have to go on - as shown on the pic I have posted above - is that the outlying "joists" are thicker than the other joists. This leads me to the likelihood (not certainty of course) that these are outriggers. Why else would these members be thicker than the others?
So, yes, of course I may be incorrect in stating that this is an outrigger system. But the point of my article was NOT to insist on this, rather, to suggest an opportunity to explore two types of balcony construction.
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
With respect, I think you are the one who has jumped to conclusions, and placed your conclusions in an article for public consumption. Did you see the framing plan? It seems to show cantilevered joists, but that is just my conclusion. A cantilevered balcony is a cantilevered balcony, no matter how many members cantilever.
Thank you hokie66. I see that it was posted three days before the article was posted; however, I wrote that article on the 23rd. Had I realized that the floor framing plan was posted, I would have reviewed it before writing the article.
So, yes, that is a true cantilever system rather than an outrigger system. I was fooled by the larger members in the pictures. The joists were "ripped" to allow the shallower depth required for the concrete deck on the balcony.
Anyways, although I have stated in the article that this is an outrigger system, the intent of the article does not hinge upon what this particular system actually is.
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
There are 2 balcony framing types. One is joist supported on each end by hangers.
The 2nd is the type that collapsed which consist of cantilevered from an inset inside the floor edge lam beams and between them non p.t. joist.The blocking at the interface of the building makes it hard to see the inset joist.
My theory is that the stucco leaked into the wall from an area about 16' high and with no deck above to block the rain dripped into the wall behind the flashing and floor membrane.Also flashing nails are highly suspect.
I see no weep screed at the base of floor 5 to drain water.
Also the membrane looked short and not going up behind the metal flashing on the detail at the door.It also looks like the decay occurred at the membrane lap.
I'll have info from the man at the scene.
I've been following this from the start and is the below a reasonable summary of what the forum has found / posted / noted?
1) The balcony was designed and built from non pressure treated wood as a cantilever design with no significant level of redundancy
2) It was highly unlikely that it was designed to hold 13 people in a 6 x 4 space even at new condition, but no notices existed to that extent
3) The failure of ALL the beams at seeming the same location close to the building indicates a failure of the water proofing, the details of which are seen in posts above
4) Confining the wooden supports within a structure which did not allow either inspection or drainage of any water entering the envelope is poor design
5) Placing a 2 1/2" concrete screed above the beams is not a good idea
6) Similar structures are recommended to be either removed or inspected to show adequate strength of the beams on a regular (1/2 year) basis, especially for those locations where there is no rain cover above them
7) Load limits should be noted / posted on similar balconies
8) More info to come from the scene in due course
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
9) Any structure where collapse could lead to human deaths must be designed with sufficient redundancy so that it does not collapse when any element fails.
