104. Wind Effects on Long Span Cable Stayed Bridges: Assessment and Validation
The well known collapse of Tacoma Narrows Bridge in 1940 clearly identified the importance of aeroelastic effects on long span bridge performance. Extensive research has been carried out since then to better understand the effects of wind on long span bridges, producing various analytical response prediction techniques. An example of the application of such techniques is presented. However, due to challenges related with full scale measurements, these prediction techniques have commonly been validated using only wind tunnel experiments. Recent research has revolved around the conduct of long term full scale measurements on a cable stayed bridge to compare actual bridge performance with those of analytical predictions. In order to ensure the reliability of predicted response, the input parameters, such as wind conditions at the site and modal properties of the bridge are also calibrated using corresponding measured quantities. This paper summarizes some of the preliminary results and outlines their implications.
103. Validation of the 2000 NEHRP Provisions’ Equivalent Lateral Force and Modal Analysis Procedures for Buildings with Damping Systems
Equivalent lateral force and modal analysis procedures for yielding buildings with damping systems were developed, validated, and incorporated in the 2000 NEHRP Provisions. Key to the implementation of the procedures was the validation process that demonstrated the accuracy of the proposed procedures. The procedures for implementing yielding, viscoelastic, linear viscous, and nonlinear viscous dampers were tested using the results of nonlinear response history analysis on sample three- and six story frames and were found to be robust.
102. TORSIONAL CONTROL OF TWO ADJACENT OFFICE BUILDINGS USING VISCOUS DAMPERS
Two adjacent wings of a three story office building in Southern California were found by analysis to be excessively responsive in torsion under an earthquake on the near-by Newport-Inglewood fault, some five miles from the site. The generous 4.5″ seismic separation between the two office building segments was found to be inadequate to prevent heavy pounding even in a moderate event, having a high probability of occurrence at this location. A variety of structural retrofit schemes were evaluated to mitigate the excessive torsional responses of the two building segments. These included converting the perimeter gravity frames to moment resisting frames, adding diagonal bracing to the perimeter frames, tying the two structures together at each floor level, and using viscous dampers as attachments between the buildings. The best solution from a cost, schedule, construction disruption, and earthquake performance standpoint, turned out to be joining the two building segments with horizontally oriented viscous dampers at a single floor level. This paper describes the analysis and retrofit solution that was used, and discusses the advantages and disadvantages of the retrofit options studied.
101. SHOCK CONTROL OF BRIDGES IN CHINA USING TAYLOR DEVICES’ FLUID VISCOUS DEVICES
Adding damping with various energy dissipating devices has become an accepted method to reduce wind induced vibrations in tall buildings. An example of a 39-story office tower is presented where large projected accelerations generated by the vortex shedding of an adjacent existing 52-story building are reduced by a passive system composed of viscous dampers and a motion amplification system. A description of the damping system and its analytical complexities are discussed. Non-linear analysis of the tower, using time history forcing functions derived from the wind tunnel is presented. Cost data for the damper system is also presented.
100. SHOCK CONTROL OF BRIDGES IN CHINA USING TAYLOR DEVICES’ FLUID VISCOUS DEVICES
Fluid Viscous Devices have been found to be a highly effective protection system for bridges. Introduced to China in 1999, the Taylor Devices damper systems have been successfully installed or will be installed in both large and super large bridges in China for protection from earthquake, wind, vehicle and other vibration. Seventeen different bridge projects include the Sutong Yangtze River Bridge, the longest cable stayed bridge in the world, the Nanjing 3rd Yangtze River Bridge, the fifth longest suspension bridge in the world, and the Xihoumen across Sea Bridge, the second longest suspension bridge in the world. The performance of the bridges and dampers have been reported as “very good” during the May 12, 2008 Wenchuan earthquake. All of the dampers produced have been subjected to rigorous static and dynamic testing, which show the dampers will perform well for the next 50 years and possibly much longer.
99. APPLICATION OF ENERGY DISSIPATION TECHNOLOGY FOR RETROFITTING STEEL STRUCTURES WITH VULNERABLE PRE-NORTHRIDGE CONNECTIONS
Prior to the 1994 Northridge earthquake, modern welded moment resisting steel frame structures were regarded as highly resistant to earthquake induced damage and few engineers regarded earthquake induced collapse of such structures as credible. This paradigm changed following the 1994 Northridge, California and 1995 Kobe, Japan earthquakes, creating a new class of potentially hazardous structures. In response to this new information, the Federal Emergency Management Agency retained a consortium of the Structural Engineers Association of California, the Applied Technology Council and the California Universities for Research in Earthquake Engineering, known as the SAC Joint Venture, to research the cause of the unexpected poor performance of these buildings and develop recommended design criteria. The resulting FEAM-351 publication provides performance based design criteria for the evaluation and upgrade of these structures. This paper presents the application of the FEMA-351 criteria to the design of structural upgrades employing energy dissipation technology to an existing 10-story steel structure. The East Bay Municipal Utility District (EBMUD) administration building is nine stories tall, with three below grade basement levels and a rooftop, mechanical penthouse. It was completed in 1991, employing the standard vulnerable moment connections. Alternative upgrade strategies were investigated, including modification of individual connections and application of energy dissipation criteria. The latter approach, using viscous dampers, was selected.
98. SEISMIC PROTECTION SYSTEM AND ITS ECONOMIC ANALYSIS ON THE BEIJING HIGH-RISE BUILDING PANGU PLAZA
Pangu Plaza, located at Beijing close to 2008 Olympic main stadium, is a 191 meter, 39-story steel high rise building. It was analyzed under earthquake and wind loads with both Fluid Viscous Dampers (FVD) and Buckling Restrained Braces (BRB or UBB) as the seismic protection system. The complete seismic response on the horizontal and vertical directions showed that the Fluid Viscous Dampers are highly effective to reduce the structural response, as well as the secondary system response. A comparative analysis of structural seismic performance and economic effect was considered, by the traditional method of increasing steel columns and beams size; by using BRB’s and by using FVD’s to absorb the seismic energy. Structural response analysis showed that using FVD’s to absorb the seismic energy made the structure satisfy the Chinese seismic design code for the “rare” earthquake and also greatly improved the seismic performance. Economic analysis showed that FVD’s were the most economic approach for both one-time direct investment and long term maintenance.
97. SCISSOR-JACK-DAMPER ENERGY DISSIPATION SYSTEM
Installation of damping devices has been limited to diagonal or chevron brace configurations until the recent development of the toggle brace. This configuration magnifies the effect of damping devices, thus facilitating their use in stiff framing systems. This paper introduces the scissor jack damper system that was developed as a variant of the toggle-brace damper system, with the added advantage of compactness. The effectiveness of the scissor jack configuration is demonstrated through testing of a large scale steel framed model structure on an earthquake simulator. Experiments showed that despite the small size of the damping device considered, the scissor jack system provided a significant amount of damping and substantially reduced the seismic response of the tested structure. Response history and simplified analyses produce results that are consistent with the experimental results.
96. ROOF ISOLATION SYSTEM TO REDUCE SEISMIC RESPONSE OF BUILDINGS
A roof isolation system is proposed as a means to reduce the detrimental effect of earthquakes in buildings. This roof isolation system entails the insertion of flexible laminated rubber bearings between a building’s roof and the columns that support it and the addition of viscous dampers connected between the roof and the rest of the building. The properties and dimensions of the rubber bearings and viscous dampers are selected in a way that makes the roof, bearings, and dampers form a highly damped vibration absorber. Presented also is a comparative study with a simple five story steel building under a strong earthquake ground motion that is carried out to assess the effectiveness of the proposed system. In this comparative study, it is found that the roof isolation scheme reduces the floor displacements and interstory drifts of the analyzed building by as much as 83 percent. On the basis of these results and in view of its simplicity, it is concluded that the proposed roof isolation system has the potential to become a practical and effective way to reduce earthquake damage in buildings.
95. OPTIMIZED DAMPING DEVICE CONFIGURATION DESIGN OF A STEEL FRAME STRUCTURE BASED ON BUILDING PERFORMANCE INDICES
Energy dissipation devices (EDDs) have been accepted as one of the viable strategies for enhancing the seismic performance of building structures. However, current design provisions do not provide guidelines for optimizing the EDD configurations. For many building structures an efficient configuration of EDDs may provide considerable performance improvement. Similarly, an optimized configuration may reduce the number of EDDs required to achieve a target performance objective. In this paper an existing building with added linear viscous dampers is redesigned based on different performance index optimization. The results indicate that the optimal device configurations are highly related to the dynamic properties of the structure and its required performance index. In one instance, where the cost is the major concern and a performance requirement is placed on story drift limitation, the total device damping coefficient can be reduced by 26%.