NOVEMBER 2010 MEETING
What Caused it to Fail?
Dr. Fowler has earned B.S. and M.S. degrees in Architectural Engineering at The University of Texas at Austin, and a Ph.D. in Civil Engineering at the University of Colorado at Boulder. Since 1964 he has been a professor at University of Texas at Austin in the Department of Civil, Architectural and Environmental Engineering. He teaches courses in construction materials, forensic engineering, repair of concrete, and concrete structures. His research has been in polymer concrete and other special concretes for repair, repair of cracks, use of aggregates in concrete, and repair requirements for concrete bridge decks and pavements.
Dr. Fowler was accompanied by Ryan Chancey, Ph.D., Senior Associate with Nelson Architectural Engineers, Inc., Dallas, TX. Tel. 877-850-8765
To an audience of about 65 at the HESS Club, Dr. Fowler gave a slide presentation titled, "What Caused it to Fail?" Dr. Fowler started by saying there are five main causes of failure for a structure that occur as follows:
He briefly described the role of the forensic engineer and what we can learn from failures. He then presented two famous failure cases. Afterwards, one of his former students at UT presented a third case study. The case studies presented were:
Tacoma Narrows Bridge
The replacement bridge had deeper truss girders that allow wind to pass through thereby reducing the airfoil effect. This time, wind tunnel testing was performed to verify the design. Dr. Fowler presented an excellent video of the failure, which may be viewed on YouTube by clicking here.
Big Dig Ceiling Panel Failure
Dr. Fowler describes the failure of epoxy embedded anchor bolts that held the ceiling panels in place and the comedy or errors that ultimately resulted in a panel falling and killing a passenger in a car being driven through the tunnel while the driver escaped with minor injuries. This failure is shown in the photo, with the crushed care barely visible below the fallen panels.
The result of the legal case was that the family was awarded $28 million from the contractor and the Mass Turnpike Authority among others. The contributing parties agreed to pay a $450 million settlement to avoid criminal charges. The fastener company and contractor faced criminal charges.
The contractor ignored other warning signs as long as five years before when creep was identified as an issue. However construction continued for three months while a decision was being made and most of the epoxy anchors were installed by this time. Due to cost only in the HOV lane was tested and 10% of the bolts failed in this area and were replaced. None were checked in other areas.
Later over 200 bolts were found to be loose in the tunnel but not in the HOV lane where they were tested. The testing showed that 81 of 471 bolts were loose in the eastbound lane (including 20 that failed when the ceiling collapsed) and 63 of 109 were loose in the west bound lane and none of 243 in the HOV lane.
Other issues discussed were the set time for the epoxy, standard set versus fast set as a function of the creep characteristics of the epoxy, fast set has higher creep but was still used. Some bolts were found to have very little covering of the epoxy. Also the holes had dust which reduced bond strength.
The main points for failure were:
NTSB determined that probable cause of failure was the use of an epoxy anchor with poor creep resistance and was not capable of sustaining long-term loads.
Foundation Failure in Texas
Dr. Chancey described this case as the combination of a inadequate geotechnical engineering, bad structural engineering, and poor quality construction combined with poor decisions during construction by the structural engineer. The results was a $5 million cost to either demo the foundation and leave the frame standing and rebuild the remaining structure or completely rebuild the foundation and structure from scratch. This was design build project where there apparently was no independent checks and it was driven by the motive to reduce engineering and construction costs to a minimum so as to maximize the profit. The building was then sold to the current owner.
Reported distresses were:
Floors were found to be 4.25" out of level using both a water level and (independently) a laser level. There were many fractures in the floor and cracks in interior gypsum walls and separations in the exterior stone masonry.
Some discussion on construction:
Dr. Chancey said causes of failure were: