Redundancy of Structural Systems in the Context of Structural Safety 6

[rlflower]

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

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

 

[drawoh]

I am a mechanical designer, not civil or structural so this is outside my area of expertise. Please be kind.

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

 

[rlflower]

In answer to your concerns, AminusVox, with regards to any damage assessment, we should be concerned with more than merely the root cause. There may be only one root cause, but there may be subsequent damage as a result.

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.

 

[SlideRuleEra]

RLF - You are bringing up some really important issues. For residential applications, many of the technical aspects of reliable design/ construction have been available for years, but not taken seriously. Take a look at the Forest Products Lab's Construction Guides for Exposed Wood Decks, published 44 years ago (1971).

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.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[hokie66]

A fundamental problem with using galvanized components like nails, joist hangers, etc. is that they don't have much zinc because the components are small. Regardless of how it is specified, the amount of zinc deposited in the hot dipping process increases with base material thickness.

 

[rlflower]

Thank you for your post, slideruleera. For clarification, the Simpson strong-tie technical rep did not recommend "Z-Max or hot-dipped galvanized equivalent" for marine environments. My call to him was concerning a project that is in an environment that is not that extreme - in Monterey near the 17-mile drive, but about a 1/4 mile from the beach, up in a hilly forested area. I expect this area to get heavy fog on occasion, but not the highly corrosive salt spray one would expect for a structure located right on the beach. In fact, he agreed that stainless steel would be appropriate for marine environments.

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

 

[SlideRuleEra]

RLF - Thanks for the clarification. A 1/4 mile inland makes a huge difference on the southeast coast of the USA, too.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

Regardless, for any type of project, if there is a failure mode that can be anticipated, it is up to the designer to evaluate the risk and if that failure mode is potentially life threatening, it should be eliminated mitigated (regardless of budget)

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.

All this said, I believe that these recent problems lie outside of the structural domain.

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 still don't follow how a simply supported deck would be much better, as if one support fails the whole thing comes down anyway. Or should wood balconies be continuous over the exterior support? I still see the same type of issue occurring if the building envelope doesn't properly protect the balcony.

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.

Do building codes account for the quality of workmanship and inspection that the building is going to get?

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.

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.

...and...

Home builders and the public needs to be educated on the reasons for "good practice" and that they are worth paying for.

...and...

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.

...and...

Earthquake design provisions (in high seismic regions) show that codes can be written to satisfy this requirement.

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.

 

[KootK]

A fundamental problem with using galvanized components like nails, joist hangers, etc. is that they don't have much zinc because the components are small. Regardless of how it is specified, the amount of zinc deposited in the hot dipping process increases with base material thickness

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.

 

[Tomfh]

I agree that notions of redundancy and ductility are often dealt addressed with wooly motherhood statements which we all sagely nod in agreement to, before we immediately proceed to document our next concrete panel/plank building held together with steel clips.

 

[TXStructural]

The building code is a minimum. Cost and time are a barrier to entry for owners and renters, so the code minimums stay intentionally low. Designers are responsible for determining what in excess of code is needed to assure a reasonably safe building. We do not want a code that mandates every possible contingency - not least of all because those provisions would be developed by committees loaded with special interests. The problem with any new code provision would be covering all exposures and situations without being excessive. Even sophisticated owners usually buy code-minimum buildings, which is a shame.

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.

 

[hokie66]

KootK,
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.

 

[IDS]

I don't believe that vague philosophical code clauses are of much use. They need precise definition or they will have little impact.

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 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.

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/

 

[KootK]

@IDS: how about this...

I also don't believe that engineers can be, or should be, expected realistically be expected to "do the right thing" when doing the right thing involves going go significantly beyond code requirements when doing so would put them at a competitive disadvantage within their marketplace.

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.

 

[KootK]

@Hokie: the Canadian steel code has a provision that, regardless of galvanizing, steel used outdoors should be at least 3mm thick. This would seem to be consistent with your view.

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.

 

[IDS]

I also don't believe that engineers can be, or should be, expected realistically be expected to "do the right thing" when doing the right thing involves going go significantly beyond code requirements when doing so would put them at a competitive disadvantage within their marketplace.

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/

 

[Leftwow]

It is a good thing to have in mind. Clients want to hand the cheapest firm the job. So perhaps we could sneak in some redundancy when front-end is finished. In my line of work I don't understand why corners are cut when it comes to entire communities being at risk but they are still cut.

 

[KootK]

Despite the forensic cases that we discuss here, and our collective fear regarding lawsuits, there are very few structural failures to speak of. And for every failure that does occur, there are probably a thousand design errors that never manifest themselves as failures. And these are design errors in simply meeting code minimums, never mind designing beyond them. When I look critically at our profession, it isn't the lack of redundancy that scares me, it's the lack of quality mentorship.

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.

Regardless, for any type of project, if there is a failure mode that can be anticipated, it is up to the designer to evaluate the risk and if that failure mode is potentially life threatening, it should be eliminated mitigated (regardless of budget).

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.

 

[IRstuff]

"Clients simply can't tell good design from bad because very little ever goes wrong. "

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
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