The EERI student chapter of the University at Buffalo (UB-EERI), the MCEER Student Leadership Council, the Networking and Education Programs of MCEER, and the University at Buffalo’s Department of Civil, Structural and Environmental Engineering jointly sponsor a series of seminars on a variety of topics related to earthquake hazard mitigation. The purpose of the seminar series is to widen accessibility to timely, technical presentations by students, researchers, visitors and affiliates of MCEER. Most seminars have been broadcast over the Internet in real-time, and can be viewed at http://mceer.buffalo.edu/SLC.
On February 28, 2003, Dr. Michael Bartlett, an Associate Professor in the Department of Civil & Environmental Engineering at the University of Western Ontario, gave a lecture titled “Testing Full-Scale Houses Subjected to Simulated Extreme Wind Loads.”
Dr. Bartlett began his presentation with some examples of residential wood-framed structures destroyed by extreme wind events. A phenomenon known as “down burst” generates high velocity winds to which residential wood-framed construction is vulnerable. Dr. Bartlett discussed the complex response of wood-frame roof, wall, and floor systems and the effect of connections on the interaction of these systems. Full-scale testing provided an effective means to investigate how load paths develop and are maintained up to failure. A full-scale test of a corrugated fiberboard shelter approximately 16 by 20 ft in plan subjected to simulated hurricane-force wind loads was conducted. The fiberboard shelter performed well but failed prematurely where the tension was applied through the thickness of the corrugated fiberboard and where the quality of construction was imperfect. Dr. Bartlett demonstrated that finite-element analysis conducted in conjunction with the full-scale experimental test identified regions of high stress concentration. This type of analysis could be effectively used to identify possible failure points of the structural system but could not predict failure due to construction quality. Dr. Bartlett discussed the design and effectiveness of the testing apparatus and the instrumentation used in the full-scale test and briefly described some preliminary plans for a more effective test facility. The seminar concluded with a question and answer session, which segued into an informal discussion regarding the complexities of full-scale testing and alternative testing configurations.
--Submitted by Gordon Warn, UB-EERI secretary
Diego Lopez Garcia, Ph.D. Candidate in the Department of Civil, Structural and Environmental Engineering at the University at Buffalo, presented a seminar entitled “Tri-Center Field Mission 2002: Taiwan” on March 7, 2003, which was webcast.
Diego began by stating that the field trip to Taiwan was one of many enriching experiences at UB. The field mission was supported by MCEER, MAE and PEER and coordinated by Taiwan’s NCREE at the local level. In this mission, students studied some civil engineering structures that were damaged or destroyed by the 1999 Chi-Chi earthquake and have been replaced or rebuilt, as well as structures not affected by the earthquake but retrofitted to mitigate damage during future seismic events.
Diego presented several examples of damaged bridges. Most bridges in service at the time of the earthquake had simply supported spans. The deck was made of reinforced concrete members. Almost all had a system of two parallel bridges. In many cases, the simply supported spans became unstable and the bridge collapsed completely. These were demolished and the replacement bridges that were built had decks made of concrete slabs and steel girders. The deck is continuous over the pier and has elastomeric bearings connecting it to the substructure. Adding steel jackets to columns and repairing them wherever necessary retrofitted some bridges.
Diego then discussed typical residential buildings. Important characteristics of these buildings are that they had an open front and the first floor was weaker and softer in comparison to the higher floors. Usually only one wall was located at the back of the building. In addition, there were stiffness and strength eccentricities. As a result, most of the buildings suffered soft story failure. Many owners performed non-engineered retrofit measures and repairs. Columns were retrofitted using steel jackets, typical thickness being 25 mm. In many of the beam column connections, steel plates were added.
Diego concluded by saying that using modern techniques and procedures has significantly reduced the seismic vulnerability of major bridge structures and school buildings. However, seismic vulnerability of typical residential buildings remains the same as they were repaired using non-engineered techniques.
--Swapna Phadnis, UB Graduate Student
Douglas P. Taylor, President, Taylor Devices, Inc. presented a seminar entitled “Damper Retrofit of the London Millennium Footbridge: A Case Study of the Biodynamic Design Issues” on April 4, 2003 and was webcast. He described the retrofit of the London Millennium footbridge with fluid viscous dampers.
Mr. Taylor began by explaining a field of engineering called Biodynamics, which studies the interaction of humans and structures in the presence of dynamic motions. The London Millennium footbridge is a classic example of biodynamics as people in motion caused a problem. The Thames River, on which it was built, is a navigable waterway and has high velocity current flow. The Millennium Bridge, which he describes as ‘absolutely stunning,’ is 330m in length, with a 4 m wide deck. It has two concrete piers in the water and a lateral support cable.
As a major tourist attraction, nearly 100,000 people used the bridge during its first day of operation. Yet, on June 12, 2000, it was ordered closed – due to large vibrations of the bridge deck under pedestrian loadings. As people began to randomly walk across the bridge it would begin to vibrate. This vibration would cause the people to begin walking in a more synchronized fashion, which would cause the amplitude of the vibrations to increase, etc. Up to five distinct structural vibration modes were excited. Peak lateral deck accelerations were in the range of 0.25 g, at frequencies of 0.5 to 1.1 Hz. At these levels, people on the bridge found walking to be extremely difficult, if not impossible.
Taylor described the urgent retrofit operation that was initiated and the challenges that operation faced. A retrofit method was conceived and implemented in 2001, using an array of 37 fluid viscous dampers providing 20% critical damping to control gross lateral motions of the bridge. Viscous fluid dampers were chosen because they reduce both the stress and deflection in a structure subjected to dynamic loadings. They are predictable, small in size, relatively easy to install, and have a long life.
The dampers suggested were unique to structural applications and used flexible metal willows to seal the fluid chambers by flexural motion rather than sliding seals. Several different damper locations were chosen to control the various vibration modes that were being excited and causing problems. Dampers were used in a chevron configuration under the deck to control the lateral modes, vertical dampers between the bridge deck and the ground were used to control both lateral and vertical modes, and dampers at the piers between the bridge support towers and the bridge deck were used to suppress the lateral and torsional modes. Finally, three tests were done by loading the bridge with people, ranging from 700 to 2,000. They were asked to march, dance and jump up and down on the bridge and the results were observed. The behavior of the bridge was reported as “Rock Solid.” No resonance or amplified responses were noted.
Taylor concluded by saying that using passive energy dissipation, the deck acceleration of the Millennium Bridge was reduced to 0.006g from the original 0.25g, the damping was increased from 0.5% to 21%, and dynamic displacement response was decreased from 40 to 1.
-- Swapna Phadnis, UB Graduate Student
Editor’s Note: The Seminar by Thomas D. O’Rourke, Cornell University, “Lessons Learned from the World Trade Center Disaster for Critical Infrastructure” was not reviewed.