Redundancy of Structural Systems in the Context of Structural Safety
Redundancy of Structural Systems in the Context of Structural Safety
(OP)
In light of various discussions on this forum regarding recent failures of decks, balconies, etc., I would like to open a discussion on the topic of redundancy in structural systems in the context of structural safety. My impression is that this design consideration is not well defined and is too often overlooked in critical situations.
A paper has been included in The Proceedings of the Twelfth East Asia-Pacific Conference on Structural Engineering and Construction — EASEC12, entitled "Redundancy of Structural Systems in the Context of Structural Safety". Please review the attachment. I would like to hear your comments on this paper in particular and on the subject in general. Thank you.
Link
A paper has been included in The Proceedings of the Twelfth East Asia-Pacific Conference on Structural Engineering and Construction — EASEC12, entitled "Redundancy of Structural Systems in the Context of Structural Safety". Please review the attachment. I would like to hear your comments on this paper in particular and on the subject in general. Thank you.
Link
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.






RE: Redundancy of Structural Systems in the Context of Structural Safety
another day in paradise, or is paradise one day closer ?
RE: Redundancy of Structural Systems in the Context of Structural Safety
This paper, and many of the responses to the recent threads on collapses in buildings, reinforce in my mind some changes that I think need to be made in the way we approach structural design:
- Structural design codes state that prevention of collapse is a key objective of design, but almost all code provisions relate to the strength of individual sections or members in as-constructed condition, assuming good construction practice, with a nominal capacity reduction factor.
- There are limited provisions for consideration of load redistribution and collapse mechanisms, but these are either stated in very general terms, or are limited to certain specific extreme load conditions.
I believe that the Limit State design approach needs to be made more rational and consistent, and those codes that do not specifically adopt the Limit State approach should do so:Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Redundancy of Structural Systems in the Context of Structural Safety
This is not true at all in the context of any seismic design. Or, for that matter, in most high rises; the World Trade Center buildings were designed for catastrophic failures, up to and including airplane collisions. And even though the buildings were only designed with the existing aircraft of the day in mind, they survived the initial impacts with large planes long enough to keep fatalities to less than 4000. What the designers did not anticipate was a deliberate attack with planes with full fuel loads.
I won't, however, claim that a balcony is a "structural" system, so to that degree, balconies are not necessarily designed to survive something like an earthquake. I would, however, expect that the apartment building itself, is designed to handle something like a magnitude 7 or 8 earthquake, and should survive sufficiently that the people inside could effect a safe evacuation after a large earthquake.
That said, it should be pointed out that building design has a continual process improvement in the sense that previous systems deemed adequate may be revealed as inadequate, as demonstrated during the Loma Prieta quake, where it was revealed that a new, heretofore, unexpected shaking mode made certain design standards pathetically inadequate and fatal. Those structures have been torn down and replaced with better ones.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
[IMG http://tinyurl.com/7ofakss]
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
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: http://www.engineering.com/AskForum/aff/32.aspx
RE: Redundancy of Structural Systems in the Context of Structural Safety
I agree that procedures for prevention of collapse due to earthquake loads are well defined, at least in high seismic zones (much less so in areas where large earthquakes are very infrequent, but do happen).
My point is that the same approach should be applied to all the other events that can cause collapse.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Redundancy of Structural Systems in the Context of Structural Safety
So, can you define ALL the types of events which could possibly cause collapse - that is, beyond the types of loads, and load cases already required? This is beginning to sound a lot like making all structures "foolproof". I don't think such an approach is feasible, for as we used to say in the mechanical design of operable equipment: "You can't make a thing foolproof, because fools are so damn ingenious."
Thaidavid
RE: Redundancy of Structural Systems in the Context of Structural Safety
1. Subsidence at any time due to poor soils.
2. Category 5 hurricane winds (157 MPH +)
3. Magnitude 7.0 earthquake
How to blend technical compromises and economic considerations for optimum results?
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Redundancy of Structural Systems in the Context of Structural Safety
No, that is not what I am suggesting (in fact it is pretty close to the opposite).
I am saying that when the unexpected happens, structures should be designed to limit the consequences.
The extent to which you do that depends on the extent of the consequences.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Redundancy of Structural Systems in the Context of Structural Safety
That's our job, isn't it?
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Redundancy of Structural Systems in the Context of Structural Safety
Sometimes, until what should be done (for redundancy) exceeds the budget.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Redundancy of Structural Systems in the Context of Structural Safety
Florida had proposed that Architects inspect their own jobs. Now this would pose a problem in the real world, but would make a big difference in safety.
Government inspectors have no vested interest in babysitting incompetent contractors and being attacked for doing their jobs.
Contractors can not possibly know how most things must go together, only how they can.There will be errors on the order of 15-80%.
A contractor should have some knowledge of engineering and architecture and codes, but from my experience 15% do.
So structural redundancy might be an extra 100% (actually 300-400%)factor of safety, but may not matter when water attacks the members.
If the joist in Berkeley were doubled how much time would that have bought? Water it seems equalizes factors of safety.
So weep holes and wicks that can be seen would be a better option. Hot tar between membranes and up the wall would work. A balcony sees maximum loads 99% more that a standard roof or floor assembly.
So all aspects of the construction should be redundant.
Inspector Jeff
RE: Redundancy of Structural Systems in the Context of Structural Safety
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
RE: Redundancy of Structural Systems in the Context of Structural Safety
It is clear that from review of the recent deck and balcony failures that potential failure modes for both structures could have been very easily identified and the failure probability for both structures could have easily been reduced by the designer. In either case, the cost to the project would have been negligible.
RE: Redundancy of Structural Systems in the Context of Structural Safety
Agreed.
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
RE: Redundancy of Structural Systems in the Context of Structural Safety
This is not quite so cut and dry in my opinion. I believe that the answer to the cantilevered wood balcony problem is that there simply should not be cantilevered wood balconies. Building envelope concerns and structural concerns for cantilevered balconies are always at odds and the results have been catastrophic. Of course, the "cost" of not having cantilevered wood balconies is a major sacrifice in aesthetic.
It is important to recognize an important truth about our profession: it is, and always has been, reactive. We don't check and rule out every conceivable mode of failure. We're not even smart enough to know what all the possible failure modes are. Rather, we check the failure modes that have given us grief in the past and for which we have devised methods of assessment.
To fly headlong into trying to preclude all modes of failure, including those that haven't yet given us grief, is to incur unnecessary costs and fall prey to hubris. I truly believe that there will come a point where the code bodies decide to preclude the use of cantilevered wood balconies. On the flip side, I would like it very much if the same code bodies would stop requiring me to calc out diaphragm capacities for wood structures as nothing ever goes wrong in that realm. I feel a bit ridiculous knocking my self out trying to determine diaphragm chord stresses while people seem to be plumetting to their death from cantilevered wood balconies every other week.
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: Redundancy of Structural Systems in the Context of Structural Safety
RE: Redundancy of Structural Systems in the Context of Structural Safety
So what if nobody said this? I was expressing my own opinion, not railing against anyone else's.
That's the trouble with hubris, it rarely makes anybody any safer. I'll take humility and faith in the collective wisdom of those that came before me.
I also believe in not attempting to fix things that aren't broken. 99% of the building columns out there are utterly non-redundant. Yet column failures are extremely rare. Nothing there to fix in my opinion.
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: Redundancy of Structural Systems in the Context of Structural Safety
I think this overstates the case. Literally any structure has a non-zero probability of failure resulting in loss of life under some circumstances. There are three key points that I think tend to get overlooked in these discussions:
- All structures should be designed to eliminate or reduce risk to life "So Far As Is Reasonably Practicable". Indeed, this is now a legislated requirement in Australia.
- Following existing codes of practice to the letter does not come close to satisfying this requirement.
- Determination of what is "reasonably practicable" is the hard part, but that does not mean we can ignore the requirement. In my opinion codes of practice need a radical change of approach so that engineers who have followed the requirements of the code ("So Far As Is Reasonably Practicable") will not be found to be negligent, even if the structure does fail with a resulting loss of life.
Earthquake design provisions (in high seismic regions) show that codes can be written to satisfy this requirement. There is no reason why codes should not adopt a similar approach to other unpredictable risks.Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Redundancy of Structural Systems in the Context of Structural Safety
RE: Redundancy of Structural Systems in the Context of Structural Safety
RE: Redundancy of Structural Systems in the Context of Structural Safety
By the sound of it, we have a structure designed to code, that would have easily taken the required load in its as designed and as built condition. It appears to have failed due to poor workmanship. Someone has claimed that pressure treated wood has a lifespan of twenty years if it is not maintained.
Marginal structures are used extensively in aviation, motor racing, and bicycle racing. The low safety factors in the structural design are accommodated by high quality workmanship, by aggressive inspection, and by the fact that the structures have known, finite lifespans. The structures are retired and either scrapped or placed in a museum, depending on how successful they were.
Apparently, construction people and contractors are not all that skilled. I know a number of people who rent out apartments, and they all do their own maintenance. I don't know how qualified they are. Then you get home handymen.
Do building codes account for the quality of workmanship and inspection that the building is going to get?
--
JHG
RE: Redundancy of Structural Systems in the Context of Structural Safety
Redundancy is not "fail-safe", yet it could be argued that in some cases it would be negligent to ignore. I concede that in fields such as aerospace and other large-scale industries, redundancy can well lead to unnecessary cost overruns, etc. However, in the construction industry - particularly in specific situations in residential construction - redundancy can be cost viable as well as life-saving. The recent deck failure in North Carolina I think provides a fine example of a lack of redundancy that is far too common in residential construction. There are often a number of reasons for such failures to occur: lack of quality control, negligence, zero maintenance, owner's ignorance, etc. In such an environment where quality control and maintenance is largely ignored, it is all the more important to explore opportunities to design with redundancy where the very real possibility of negligence could result in disastrous consequences.
With regards to wood construction, the building code has built into itself a tabulated list of requirements commonly known as conventional construction practice. Included in this list is a variety of redundant requirements. I have cited one such requirement on the thread regarding the recent deck collapse in Florida: namely, that of floor joists to be lapped-spliced over supporting beams/purlins. It could well be argued that if that deck was constructed in this manner then the likelihood of this disaster occurring could have been minimized (note: not "eliminated" but "minimized"). What I wish to stress here is that although the building code cites such redundant construction methods, it does not press this requirement upon the structural engineer. It is implied, therefore, that the structural engineer would have the wherewithal to employ redundancy where deemed necessary while being mindful of the expected serviceable life of every specified part of the structure.
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
RE: Redundancy of Structural Systems in the Context of Structural Safety
I think you will agree with many of the recommendations. Home builders and the public needs to be educated on the reasons for "good practice" and that they are worth paying for. Time has shown that just making the information available, without explanation, goes nowhere.
A complicating factor is that there appear to be ways to get reliability by using shortcuts. This may be true, but there are pitfalls for the unwary. For example, in the thread about the North Carolina (not Florida) deck (porch floor) collapse you commented that a Simpson Strong-Tie representative advised you that hot dip galvanized connectors / fasteners (which includes Simpson's ZMAX) were satisfactory for service in typical ocean front service. I saw that you and Hokie66 correctly concluded that this recommendation was wrong. In fact, the Simpson rep missed an important note in his own published literature. Download the Simpson Strong-Tie Deck Connection & Fastening Guide
See Note 6, page 18, an excerpt of this page is shown below:
My point is that taking the Simpson representative's recommendation, without independently checking its accuracy would lead to a design that, for ocean salt-air exposure, was worse than the 1986 concept used in North Carolina.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Redundancy of Structural Systems in the Context of Structural Safety
RE: Redundancy of Structural Systems in the Context of Structural Safety
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.
RE: Redundancy of Structural Systems in the Context of Structural Safety
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: Redundancy of Structural Systems in the Context of Structural Safety
With the minor semantic edit to reflect probabilistic realities, I am in complete agreement with this. The tricky bit, of course, is what constitutes proper mitigation.
I agree that the dominant issue seems to be durability. But then, is durability one of the parameters that goes into reliability, which is a structural issue? Or does reliability, in the structural sense, only include parameters affecting the initially built state?
I see your point. I see the failure mechanism generally being the environmental corruption of an already weak moment connection between the balcony and the main structure. A simply supported balcony eliminates the moment connections and replaces them with simple shear connections which I would expect to be more reliable although, by no means invincible. Surely balconies continuous over the exterior support would be an improvement over the more typical scenario as the moment connection would be less weak. But I agree, without protection from the elements, there would still be an "expire by" date on the system.
I believe that they account for tolerances as defined in the applicable standards. I don't think that they account for truly shoddy workmanship which would fall outside of those tolerance standards. Off of the top of my head, the only instance that I can think of where the level of inspection is built into the design capacities is the construction of masonry walls in the US. If block walls are to receive "special" inspection, higher capacities can be used. I would be very interested to hear of other examples.
...and...
...and...
...and...
I believe that structural engineering is a perpetual race to the bottom, at least in the private sector. Any shortcut and/or questionable interpretation will be exploited by my competitors and, therefore, must be exploited by me for me to survive. I don't believe that vague philosophical code clauses are of much use. They need precise definition or they will have little impact. I also don't believe that engineers can be, or should be, expected to "do the right thing" when doing the right thing involves going significantly beyond the code requirements.
One of my favorite things about the high seismic requirements in north america is that many of the requirements are prescriptive in nature. Some folks resent that but I think that it's glorious. It means that I can do smart things like insist that column ties be spaced at 4" o/c and there's not a damn thing that my competition can do to undermine me. It's liberating. If we're to have redundancy requirements in the code, I'd very much like to seem them be prescriptive in nature.
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: Redundancy of Structural Systems in the Context of Structural Safety
This is disconcerting. Do smaller components not require proportionally less galvanizing?
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: Redundancy of Structural Systems in the Context of Structural Safety
RE: Redundancy of Structural Systems in the Context of Structural Safety
As both a structural engineer and safety engineer, I think the balance is struck when a base code-level design is modified for extenuating circumstances. That is the key to engineering judgement. The structural design should address known issues not addressed by code. This might be something like design to prevent catastrophic collapse under tornadic winds while allowing significant damage. In the case of cantilever, wood balconies, going forward after these recent events have demonstrated what can happen, we should be planning for water intrusion.
1/4 mile from the beach will still be a moist, salty exposure. Corrosion of steel will accelerate once chloride reaches the surface in the presence of moisture. You don't need a lot of salt, only a molecule-thick layer for the reaction to progress.
RE: Redundancy of Structural Systems in the Context of Structural Safety
I would think for the most part, the thinner the element in the same environment, the more corrosion protection is required. You can't realistically control how much zinc is applied in a bath, unlike applying coatings by other means.
RE: Redundancy of Structural Systems in the Context of Structural Safety
This is exactly why we need a new approach to how the limit state format is presented in codes (even those codes where the words "limit state" are for some reason avoided). There should be specific guidance on how to consider the behaviour of the whole structure in the event of the failure of any element, how to minimize the consequences of such an event, and what is considered to be "practicable" for different classes of structure.
This is already done for earthquake loads (which at any given location have a low probability), but it is largely or totally ignored (in most cases) for comparatively high probability events such as loss of strength due to corrosion, poor construction practice, or vehicle impact.
I don't agree with that at all. I think that engineers should be expected to do the right thing. We should however consider the consequences when they don't.
Finally, in reply to the many comments along the lines of: "we can't be expected to design for every eventuality" this is exactly why structures should be designed and detailed to minimize the adverse effects of unexpected events. To quote from the book "The Black Swan":
“the idea is not to correct mistakes and eliminate randomness … The idea is simply to let human mistakes and miscalculations remain confined, and to prevent their spreading through the system, …”
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Redundancy of Structural Systems in the Context of Structural Safety
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: Redundancy of Structural Systems in the Context of Structural Safety
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: Redundancy of Structural Systems in the Context of Structural Safety
Can't argue with that, as a statement of how it is (and how it will always be, I guess).
I do wonder if we engineers make full use of the potential competitive advantage to be gained from working along-side insurance people.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Redundancy of Structural Systems in the Context of Structural Safety
RE: Redundancy of Structural Systems in the Context of Structural Safety
This may sound peculiar but I believe that the paucity of structural failures actually works against us when it comes to improving our designs and processes. With failures being as rare as they are, there's just no feedback loop in place to reward good design and punish the bad. There are no tangible consequences.
Firms that ride the line regarding code minimums, and take a few short cuts to boot, are rewarded with prosperity and happy clients. Clients simply can't tell good design from bad because very little ever goes wrong. Worse, they often equate cheap construction and a "never say no" attitude with good design. That's just basic client service, right? Of course it is.
Imagine a world world where every community had a CAT5 tornado or a 9.0 magnitude earthquake every five years. Poor designs would wreak havoc on society and people would be "willing to pay" for good design. And corner cutting firms would be driven out of the market places. This would be a structural engineering utopia! You know, other than the appalling loss of life and property.
I find it quite frustrating that our safety margins, which I wholly agree with, prevent the people who pay for my services from truly being able to "see" my work. A buddy of mine works as an electrical engineer for a major cell phone manufacturer. When something goes wrong and prompts a safety recall, there are consequences. Heads roll. The faulty people who made the faulty products disappear. It's great.
With the one semantic edit to reflect probabilistic realities, this is essentially a perfect logical summary of our duty regarding life safety concerns. It should be that simple. Unfortunately, the nature of free market economies and the near absence of consequences for bad design muddy the waters pretty badly.
Funny story regarding hubris and structural redundancy.
Back in the 90's, I was inspecting a metal plate connected wood truss as it was leaving the jig. The design called for 8" x 10" plates at heel joints where the demand gets crazy. The joints were plated with 6"x8" plates instead. I asked the foreman why and he replied "We've run out of 8x10's but it's okay, I made up for it by doubling the size of the plate at the peak joint!". AOK.
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: Redundancy of Structural Systems in the Context of Structural Safety
This is as it should be. Consider all the planes that have had accidents in the past because of design, maintenance, or materials failures; they all resulted in corrections in design and processes, which means that fewer planes have failed in the last few years because of the plane itself. I think that housing is in that same category, which is what we all want. Engineers hate the notion of "good enough," but that's the basis of most of what we do. NASCAR, because of its desire to not really have people dying in the middle of a race, has taken "good enough" to a level where last weekend's crash allowed the driver to WALK AWAY with only bruises. Had that been a passenger car, they would have been mopping the remains of the occupants off the track. We tolerate that difference because of the valuations applied to the potential loss, and that's what we as a society accept. To do thing beyond that is simply going to price you out of any meaningful business.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
[IMG http://tinyurl.com/7ofakss]
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
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: http://www.engineering.com/AskForum/aff/32.aspx