The Third International Bridge Seismic Workshop (III IBSW)
Gian Michele Calvi - Professor at the IUSS Pavia, Italy, and Adjunct Professor at the North Carolina State University
Re-Visiting Earthquake Resistant Design of Bridges
This presentation will discuss the issues raised in seismic design, considering a number of practical examples and trying to raise some parallelism with the case of buildings, in which the perception of different performance levels’ pertinence on total losses following an earthquake has become increasingly relevant. Numerous developments on the convolution of hazard-vulnerability-exposure-risk have contributed to produce frameworks to quantify and manage seismic risk.
This lecture will thus aim to re-discuss the challenges induced by seismic demand to bridges, focusing on the following main topics:
a) A rational definition of earthquake action demand, considering the digital records today available and the specific features of bridge response.
b) A re-visitation of the connections between bridge response parameters and resulting global losses by considering damage to both structural and non-structural elements, effects of down time, traffic limitation, regional loss of competitiveness and societal impact.
c) A consequent revision of seismic design philosophy and methods. In the past, these moved from a strength–acceleration comparison, followed by a ductility demand–capacity check, to displacement–based approaches. The possibility of designing new bridges and strengthening intervention based on a minimization of the expected average annual loss (EAL) will be discussed, to identify feasible structural solutions that align with the conceptual goals of performance-based design.
Lee Marsh - Deputy Director – America’s Technical Excellence Center, WSP
Performance-Based Seismic Design of Bridges - What Is It and How Will It Change Design Practice?
Performance-based seismic design has emerged as a major advancement for helping owners provide both seismic safety and performance of bridges exposed to earthquake hazard. Overall, the approach seeks to provide communities with economic stability and resilience in the face of seismic hazard. While great strides are being made in developing the approach, many challenges remain for the engineering community in order to deliver on the concept. This presentation provides an overview, current status assessment, and broad-based perspective on this important aspect of bridge engineering.
Jianzhong Li - Deputy Dean of the college of civil engineering of Tongji University and the Director of Tongji’s Multi-Functional Shake Table at Jiading Campus, Shanghai, China
Seismic Damage Mechanism and Control of Long-Span Bridges
An experimental full bridge model for shake-table testing is designed and constructed based on a prototype of a thousand-meter cable-stayed bridge. Multi-support, non-uniform ground motion excitations are employed in the shake-table experiment. The effect of viscous dampers on longitudinal and lateral seismic responses of this thousand-meter cable-stayed bridge is studied and verified. Additionally, together with the pseudo-static experiment of cable-stayed bridge tower, the damage progress and damage mode of the thousand-meter cable-stayed bridge are studied as well. Seismic damage and damage characteristics of this thousand-meter cable-stayed bridge are illustrated. Finally, according to the overall structural responses and local curvature responses, multi-level performance design theory and method are developed.
Bijan Khaleghi - State Bridge Design Engineer, Washington State Department of Transportation
Seismic Design Requirements and Construction Challenges of Lifeline Essential and Critical Bridges
This article presents the major design issues addressed in the design of lifeline bridges for serviceability and functionality after design seismic events. To address these issues the Washington State Department of Transportation (WSDOT) has established deign policy for lifeline bridges classified as essential and critical bridges. The result is a design criteria that attempts to ensure the structure will remain serviceable following the maximum credible earthquake.
Major seismic events during the past few decades have continued to demonstrate the destructive power of earthquakes, with damages to structures such as bridges, as well as giving rise to great economic losses. Economic losses for bridges very often surpass the cost of damage and should therefore be taken into account in selecting seismic design performance objectives. The structural engineering community in its transition to more refined seismic design codes has proposed methodologies for seismic design of lifeline bridges.
This paper also discusses challenges encountered for bridge modification or widening where substructure bents are modified and new columns or piers are added, or an increase of bridge deck width or widenings to the sidewalk or barrier rails of an existing bridge resulting in significant mass increase or structural changes.
Kuo-Chun Chang - A Distinguished Professor of the Department of Civil Engineering of National Taiwan University (NTU), Taipei, Taiwan
Capacity-based Inelastic Displacement Spectra for Reinforced Concrete Bridge Columns Subjected to Far-Field and Near-Fault Ground Motions
Capacity-based inelastic displacement spectra that comprise an inelastic displacement ratio (CR) spectrum and the corresponding damage index (DI) spectrum are proposed in this study to aid seismic design and evaluation of reinforced concrete (RC) bridges. Nonlinear time history analyses of SDOF systems are conducted using a versatile smooth hysteretic model when subjected to far-field and near-fault ground motions. It is demonstrated that the Park and Ang’s damage index can be a good indicator for assessing the actual visible damage condition of column regardless of its loading history, providing a better insight into the seismic performance of bridges. The computed spectra for near-fault ground motions show that as the magnitude of pulse period ranges increases from NF1 (0.5 - 2.5 s) to NF2 (2.5 - 5.5 s), the spectral ordinates of the CR and DI spectra increase moderately. In contrast, the computed spectra do not show much difference between NF2 and NF3 (5.5 - 10.5 s) when the period of vibration Tn ≤ 1.5 s, after which the spectral ordinates of NF3 tend to increase obviously whereas those of NF2 decrease with increasing Tn. Moreover, when relative strength ratio R = 5.0, nearly all of the practical design scenarios could not survive NF3. Based on the computed spectra, CR and DI formulae are presented as a function of Tn, R, and various design parameters for far-field and near-fault ground motions. Finally, application of the proposed spectra to the performance-based seismic design of RC bridges is presented using DI as the performance objective.
Dawn E. Lehman - Department of Civil Engineering, University of Washington, Seattle, WA, USA
Concrete Filled Steel Tubes for Accelerated Bridge Construction and Enhanced Structural Performance
Concrete filled steel tubes (CFST) offer many opportunities to accelerate construction while improving structural performance and reducing cost. CFST may be used for bridge piers and deep pile or drilled shaft foundations. The steel tube provides composite resistance without requiring internal reinforcing cages and expensive formwork or shoring. The concrete fill delays buckling of the steel, and the steel tube is at the optimal location for developing the maximum stiffness and resistance with the minimum weight and material requirements. This results in superior system performance with the inelastic deformation capacity and resistance necessary to resist seismic and other extreme loadings. The use of CFST has historically been limited in bridge construction, because of misunderstanding of the performance of CFST, and limited connections available to join the CFST elements to other steel, reinforced and precast concrete construction. Recent research has addressed many of these issues, and this paper will summarize the results of this recent work. The work will demonstrate the resistance and inelastic performance of CFST members under flexure, shear and axial loading. Recent research on connections between reinforced concrete piers and columns and CFST columns and reinforced or precast concrete superstructures will be noted. It will be shown that these connections are strong with good inelastic performance, and facilitate rapid field construction to permit accelerated bridge construction. Design guidelines have been developed for these CFST members and connections, and will be briefly described.
Kenji Kosa - Professor Emeritus, Kyushu Institute of Technology, Kitakyushu, Japan & Technical Advisor, Hanshin Expressway Technology Center, Osaka, Japan
Failure Mechanism of the Furyo Daiichi Bridge in the 2016 Kumamoto Earthquake
In the 2016 Kumamoto Earthquake that occurred in western Japan, the Furyo Daiichi Bridge, a 60 m-long overbridge, collapsed onto the Kyushu Expressway. The bridge, having a skew angle at one end, was supported by two abutments and two rocking piers. The rocking piers had pivot bearings at the upper and lower ends of the columns to provide a rotating function and a vertical load support function. Displacement confining devices were also installed on both sides of the main girder. When Mw 7.0 shaking struck, however, the superstructure fell to the ground due to a large displacement after causing a punching shear failure at the confining device-bridge seat connection area.
We inferred the failure mechanism of the bridge from the observed damage and then conducted dynamic analysis to identify the behavior of the bridge under seismic loading. Analysis results show that immediately after the superstructure was deformed by crashing against a displacement confining device, the pivot bearings lost a vertical load support function by exceeding their rotation limit, causing the fall-off of the superstructure. It was also found that the damage of the displacement confining device was caused not only by the rotation of the superstructure due to a skew angle but also by the movement of the superstructure to the perpendicular direction due to the seismic force.
Bruce Johnson - Former State Bridge Engineer, Bridge Engineering Section, Oregon Department of Transportation
State DOT Seismic Resiliency Assessment Process and Mitigation Program
Scientists have documented a long history of earthquakes and tsunamis on the CSZ. Since damage is expected to be so widespread throughout western Oregon and the Cascade Mountain range, in 2004 ODOT developed and adopted a “Performance Level” criteria for highway structures. ODOT’s seismic design criterion requires bridges in western Oregon to remain serviceable after a CSZ earthquake and sustain only minor damage. Although the construction cost for bridges meeting our seismic criteria are a bit higher (compared to the typical bridge construction cost,) the long term benefits are considered to be justified. This investment will facilitate rescue and economic recovery after a major event.
Once ODOT had defined the highway segments that would be priority candidates for seismic investments, a more detailed vulnerability assessment of bridges and unstable slopes was performed and improved mitigation costs were obtained. The evaluation criteria for bridges developed for this study considered other existing structural deficiencies and the age of structures. The cost for replacing nearly 140 bridges, retrofitting 580 others, and mitigating nearly 1,200 unstable slopes along our lifeline corridors was estimated to be $5 billion.
Once it became clear that the State could not support a level of investment to achieve the needed level of resilience, ODOT began a series of new studies using a “triage” approach that uses regional and local agency routes to achieve minimal levels of mobility for rescue and economic recovery.
The extended abstract and presentation will provide details about the Oregon vulnerability studies and resilience planning.