Lessons from small failures – Part II: Designing for durability

If you get to design and build generation after generation of essentially the same structure and you get to study the old problems, then you end up with a mature well-designed product. Over the decades automobiles have clearly become both more reliable and longer-lasting. Not necessarily so with structures, especially when they are not built repetitively, as might be a standardized overpass/bridge over an interstate; but put it in an urban area, jazz it up with architectural features and you may be adding problems. DOT’s have test tracks to learn more about particular pavements. No such luxury applies to anything with even a little bit of novelty. Lest you think the immense challenges of designing for durability apply only to the minor stuff Cary Concrete Products is normally involved with, here are snippets from a recent story.

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After Only Seven Years, New I-35W Bridge in Minneapolis Showing Signs of Age

07/27/2014

Star Tribune (Minneapolis, MN)

July 27 –Cracks emerged in concrete girders. A drainage hole on the bridge deck plugged up. Rust showed above piers. Seven years after the collapse of its predecessor, the new Interstate 35W bridge has been showing its age…Repairs to the system were eventually made under warranty, but not before years of finger-pointing between the state and the lead contractor. Taxpayers also covered some costs.

“Contractors are always very reluctant to give anything,” said Tom Styrbicki, bridge construction and maintenance engineer for the Minnesota Department of Transportation (MnDOT). “They always try to drag MnDOT into it: ‘We did it per your specifications. If it leaks, it leaks.’ ”

[My comments: 1) of course there is finger-pointing. Often problems arise from issues in both design and execution; 2) most contractors eventually fatigue from the battles over specifications that are poorly written (my own experience and informal polls peg error-free specs at 1%) and occasionally enforced to our surprise and detriment. The result of that fatigue is unfortunate but understandable to any subcontractor: ‘We did it per your specifications. If it leaks, it leaks.”]

Full story is at http://www.startribune.com/politics/statelocal/268746561.html or http://enr.construction.com/yb/enr/article.aspx?story_id=id:ZDAtwZvbDEgPAf81w4aA1jHIfPjJuLGNDcMwvg2Pfds8xYrzQnvP5LykSce0mHPo

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Our microcosmic story concerns a minor failure and then thinking in detail what it takes to avoid creating designs with inherent problems. The previous post in this series went into detail about the specific problem we recently analyzed and on how surprised we were by the cause of the failure: https://planetcommercialconstruction.wordpress.com/2014/07/21/lessons-from-small-failures-part-i-searching-for-truth/

To summarize: enough moisture entered an unusually large mortar-filled void. When  frozen it was able to generate enough force to crack concrete.  We say that the piece “failed” but in fact the piece performed exactly as it was designed to perform.  The curbs at the posts in this railing system were not durable because the design did not anticipate what occurred. With hindsight we can easily see what happened.

  • The rail assembly sits on the sidewalk (and the sidewalk on the deck)
  • Water on the back of the curb flows to a small overhang that is part of the sidewalk
  • The sidewalk, including the overhang, slopes to the roadway for drainage.
  • Water flows into the mortar that is the bed joint of the curb pieces.
  • Moisture in the bed joint moves into the mortar-filled cavity (blockout)
  • The mortar gets saturated
  • The mortar  gets frozen and expands
  • Frozen mortar in the bed joint is not a problem, too small to generate large forces
  • Frozen mortar in the block out starts to crack the precast curb
  • The crack takes in water, ice forms and enlarges the crack
  • Visible large cracks form and the curb piece falls apart.
  • In 6 years, about 15% of the pieces at this condition have already  failed.
  • All other curb pieces, piers, etc., are in excellent condition.

The foresight to anticipate this is highly improbably. For 25+ years I have been regularly involved with mortar-filled openings, and never before saw a failure of this type. Couldyou reasonably expect someone to anticipate this? Failure required both a large volume of mortar and a large volume of water (and a freezing climate). Rules simply about the size of mortar-filled blockouts would not help. (I am sure such rules exist in <0.05% of architectural firms and <0.5% of engineering firms.)  Avoiding this failure would require you to have imagined this scenario. It only is an issue at the pieces at the posts. If you were using this exact railing on many bridges you would be in a different business and in 5 years or less you would have learned that this particular detail does not work. And you would never use it again.

This can also be a reminder that a BIM with intelligent objects needs both software that can store and apply the knowledge and  an organization that can capture and organize little tidbits like what we learned from this minor failure.  We are not close to doing that in non-repetitive structures.

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About Leo Schlosberg

Graybeard with experience in commercial construction and IT, and an interest in information flow and process. Aware and respectful of the enormous complexity, technical, legal, and other, embedded in every structure that is part of the built environment.
This entry was posted in durability, knowledge and tagged , , , , . Bookmark the permalink.

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