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Seismic Design of Concrete Bridges
Kyle Turner, PE, SPRAT I, IRATA I
In September 2020, we hosted a webinar, “How I Design Bridges : Seismic Design of Concrete Bridges" by Kyle Turner, P.E. from Michael Baker International.
He presented the lesson about Seismic Design of Concrete Bridges. There are differences between building a new bridge and widening or replacing an existing bridge.
When you build a new bridge, there are more alternative options and fewest constraints. When you widen the bridge, the widen structure needs to meet current design standards and match the existing bridge's behavior. You also need to be aware of the existing structure analysis and the potential retrofit necessity.
When you replace a bridge, you need to pay attention to construction staging, existing structure stability, potential overbuild, and temporary existing utility realignments.
Kyle Turner will explain the challenges of bridge projects which engineers may face based on different conditions. He will present the I-10 Design/Build Corridor Express Lanes Project, which involves a precast and prestressed bridge with a unique integral seismic bent connection.
The project will lead to the objectives of seismic analyses, response spectrum analysis for determining longitudinal & transverse displacement demand, and pushover analysis for determining longitudinal & transverse displacement & ductility capacity.
1. Seismic Design Overview
1.1 Bridge Category
- Capacity Protected Elements
- Earthquake Resisting Elements
- Sacrificial Elements
2. Project Example: Vineyard Ave OC (Replace) over I-10
2.2 Bent Connection
2.3 Midas Seismic Models
1.1 Seismic Design Overview – Bridge Category
• Safety Evaluation Earthquake
5% Probability of Exceedance in 50 years (975-year return period)
i.e. 7% in 75 years (correlates to 75-year bridge lifespan)
• Functional Evaluation Earthquake
20% Probability of Exceedance in 50 years (225-year return period)
• Major Damage
Bridge Replacement Likely
• Moderate Damage
Bridge Repair Likely – Replacement Unlikely
• Minimal Damage
1.2 Seismic Design Overview – Philosophy
Capacity Protected Elements
• Resists Overstrength Moment (Essentially Elastic)
- Foundations (typically)
• Designed to be Replaced
- Shear Keys
- Abutment Backwall
Earthquake Resisting Elements
• Ductility (Deformation > Yield)
- Local Ductility (Minimum Limit)
- Global Ductility (Maximum Limit)
• Expected Material Properties
- Greater than Specified Properties
• Effective Section Properties
- Iterative Calculation Procedure
- RC Columns ~0.35 Ig, Bent Cap ~0.70 Ig
• Foundation Flexibility
- Contributes to Global Drift
- Remove from Local Rotation Calculations
• Plastic hinges are highly confined finite regions within the ERE that provide its ductile response
• Plastic rotation occurs within a defined length or lengths of the ERE and, combined with elastic displacements, produces total displacement at the superstructure level
1.3 Seismic Design Overview – Analyses
Displacement Demand Analyses
• Equivalent Static Analysis (ESA)
- Lumped-Mass Single-Degree-of-Freedom Oscillator
• Elastic Dynamic Analysis (EDA)
- Response Spectrum Analysis (90% Mass Participation)
- Linear Elastic & Multi-Modal
- Applied in orthogonal directions, combined with CQC3
• Nonlinear Time-History Analysis (NTHA)
- Useful for Complex Bridge Types
- Should be checked against simpler method
- Models soil-structure interaction, joints, multiple support excitations, bearings, and nonlinear soil, material, and hysteretic behavior
Displacement Capacity Analyses
• Inelastic Static Analysis (ISA)
- Incremental Quasi-Static Pushover to Collapse
- Considers Defined Analytical Plastic Hinges
- Considers Soil-Structure Interaction
• Local Displacement Capacity
- A simplified ISA for use where ESA is used for Demand
- Column Elastic Flexibility and Plastic Hinge Deformation Capacity
• Moment-Curvature Analysis (M-Φ)
- Ductile Element Plastic Moment Capacity
- Considers effects of axial load
- Considers effective stiffness properties
Global Ductility = Bridge Drift / Column Yield Displacement
- Limits Maximum Drift
Local Ductility = Column Displacement / Yield Displacement
- ERE’s Required to Achieve Minimum Ductility of 3 or meet minimum transverse reinforcement requirements.
The content discusses seismic design of concrete bridges, including the differences in building new bridges versus widening or replacing existing ones. The article also covers seismic design principles, appropriate design devices, and parameter values, as well as the challenges faced by engineers in bridge projects. A specific project, the I-10 Design/Build Corridor Express Lanes Project, is used as an example to highlight the objectives of seismic analyses, including response spectrum analysis and pushover analysis. The article concludes with an overview of seismic design factors, capacity protected elements, sacrificial elements, earthquake resisting elements, and displacement and ductility analyses.
When designing buildings and structures in seismically active regions, it is important to consider a variety of factors to ensure the safety and integrity of the structures. Seismic design devices, such as dampers and base isolators, can be incorporated into the design to mitigate the effects of earthquakes. These devices work by absorbing or redirecting seismic energy, reducing the overall forces and stresses on the structure.
Seismic design principles should also be taken into account when designing buildings for earthquake resistance. One important principle is ductility, which refers to a structure's ability to deform and absorb energy without failing. Ductility is a key factor in ensuring that a structure can withstand strong earthquake shaking.
Another critical aspect of seismic design is selecting appropriate parameter values. These values are used to determine the expected ground motions and other seismic hazards at a site. Parameter values must be carefully chosen based on site-specific conditions, such as soil type, geological features, and topography.
To ensure the best seismic design outcome, it is essential to work with competent professionals who have expertise in seismic design factors and buildings seismic terminology. These professionals can help ensure that the proper seismic design factors are being considered, and that the design is optimized to meet local building codes and standards.
Overall, designing for seismic resistance involves considering a range of seismic design parameters and principles, such as appropriate seismic design devices, ductility, and parameter values. Working with qualified professionals and understanding buildings seismic terminology can help ensure that the resulting structure is safe and reliable in the event of an earthquake.