Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design (2020) / Chapter Skim
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From page 37... ...
37 Goals of PBSD The motivation behind PBSD is to provide bridge owners and designers with a better way to influence the performance of a bridge during an earthquake. The current provisions of the AASHTO guide specifications provide for only one level of performance, that of life safety at a single-hazard level.
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38 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design design and evaluation is the most complete methodology, the most widely used, and the type closest to full probabilistic design, as seen by the work Caltrans is now advancing in their latest seismic design criteria. However, a full probabilistic methodology is not universally ready for deployment in a nationwide fashion.
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Development of the AASHTO Guidelines for Performance-Based Seismic Design 39 Element No. Description Table No.
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40 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Figure 20. Flowchart of basic steps in framework for PBSD of bridges (GM = ground motion)
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Development of the AASHTO Guidelines for Performance-Based Seismic Design 41 Life Safety, PL2 or Operational, and PL3 or Fully Operational. Additionally, as seen in Table 14, the three operational categories have increasing performance for each motion level and each operational category.
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From page 42... ...
42 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design The research team adjusted the performance level for the lower level event, in order to better align with past practices and limit the burden on designers and owners as much as possible. The simplest approach was to set the performance level as PL3 Fully Operational for all bridges at the lower level event.
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Development of the AASHTO Guidelines for Performance-Based Seismic Design 43 Strain Limits The primary EDPs used to influence the performance of a bridge under seismic loadings are strain limits. These were chosen over other possible parameters, such as ductility ratios or drift ratios, because they appear to be more directly tied to key performance limits.
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44 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Table 3.1-4 (Table 16) of the proposed AASHTO guidelines details the strain limits for each performance level.
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Development of the AASHTO Guidelines for Performance-Based Seismic Design 45 • 1 is the first yield of the transverse steel. The location on the pushover curve of this limit state depends on transverse steel content, axial load, and curvature demand.
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46 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design longitudinal steel. This damage state is not entirely definable by strain limits alone, and each column may need a case-by-case assessment of damage.
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Development of the AASHTO Guidelines for Performance-Based Seismic Design 47 Translating Limit State Strains to Member Deformations The limiting strains defined for the EDPs will need to be interpreted in terms of member deformations. This assessment will depend on whether reinforced concrete (RC)
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From page 48... ...
48 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Lpc = kLc + Lsp ≥ 2Lsp Rectangular Compressive Hinge Length Lpt = Lpc + γD Rectangular Tension Hinge Length γ = 0.4 for bidirectional loading and γ = 0.33 for unidirectional loading Note that for simplicity, the 0.4 value in the tension hinge length for bidirectional loading is recommended, since it is rare to have unidirectional ductility demands. As the preceding equations indicate, the tension hinge length is simply the compressive hinge length plus 0.4x the diameter for circular sections.
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From page 49... ...
Development of the AASHTO Guidelines for Performance-Based Seismic Design 49 0.2 1 0.08k f f u y = − ≤ Factor for Plastic Hinge Length LSP = 0.15fye • dbl Strain Penetration Length Approach Fill Rotation and Displacement Limits An EDP also needs to be developed to represent relative movement between the approach fill and the bridge abutment. Rather than defining limiting strains, this EDP is defined in terms of rotations and permanent displacements that occur between the abutment and the fill.
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50 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design For a pile-supported abutment, the distortion results from the settlement of the fill and the underlying soil during or following seismic shaking. Normally, the pile-supported abutment would be assumed to undergo negligible settlement during the seismic event.
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From page 51... ...
Development of the AASHTO Guidelines for Performance-Based Seismic Design 51 If the interior bents for the bridge are also supported on spread footings, the allowable movement in terms of angular rotation of the footing relative to adjacent bents can be estimated using the methodology in NCHRP Web-Only Document 245. Lateral Displacement Limits Lateral displacement of the approach fill can occur with or without liquefaction.
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52 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design settlement. If ground improvement is used to limit the development of liquefaction, then the potential for lateral movement can be evaluated using either simplified pseudo-static stability analysis method with Newmark-type displacement estimates or by numerical modeling.
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From page 53... ...
Development of the AASHTO Guidelines for Performance-Based Seismic Design 53 of the structural system, structure type, amount of nonlinear behavior expected in the soils, and magnitude of the constructed value. These divisions are a step beyond what is currently addressed in the Seismic Guide Specifications, but the research team thought this was needed to ensure the appropriate level of analysis and assessment was applied for structures whose behavior may be complex.
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54 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Seismic Design Category The Seismic Design Category (SDC) is determined according to Table 19 based on (1)
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Development of the AASHTO Guidelines for Performance-Based Seismic Design 55 Table 20. Basic requirements for each seismic design category (Guidelines Table 3.1-8)
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56 Proposed AASHTO Guidelines for Performance-Based Seismic Bridge Design Bridge Attributesa Performance Level Hazard Level SDC Demand Analysis b Capacity Assessmentc Basic • Regular geometry • Conventional type • Linear soils • Low-to-moderate value of construction Life Safety, PL1 I A1 None Connection Strength Check II B1 Equivalent Static Analysis Implicit Method B III C1 Elastic Dynamic Analysis Implicit Method C IV D1 Nonlinear Static Procedure Operational, PL2 I B2 Equivalent Static Analysis Demand Capacity Ratio Method II C2 Elastic Dynamic Analysis Nonlinear Static Procedure III-IV D2 Fully Operational, PL3 I C3 Elastic Dynamic Analysis Demand Capacity Ratio Method II-IV D3 Intermediate • Irregular geometry • Conventional type • Nonlinear soils • Liquefaction hazard • Moderate-to-high value of construction Life Safety, PL1 I A1 None Connection Strength Check II B1 Elastic Dynamic Analysis Implicit Method B III C1 Elastic Dynamic Analysis Implicit Method C IV D1 Nonlinear Static Procedure Operational, PL2 I B2 Equivalent Static Analysis Demand Capacity Ratio Method II C2 Elastic Dynamic Analysis Nonlinear Static Procedure III-IV D2 Fully Operational, PL3 I C3 Elastic Dynamic Analysis Demand Capacity Ratio Method II-IV D3 Complex • Irregular geometry • Nonconventional type • Nonlinear soils • Liquefaction hazard • High value of construction Life Safety, PL1 I C3 Elastic Dynamic Analysis Demand Capacity Ratio Method II D2 Nonlinear Static Procedure III-IV d Nonlinear Response History Nonlinear Response History Operational, PL2 I C3 Elastic Dynamic Analysis Demand Capacity Ratio Method II D2 Nonlinear Static Procedure III-IV d Nonlinear Response History Nonlinear Response History Fully Operational, PL3 I-IV d Linear Elastic Response History Demand Capacity Ratio Method a, b, c See Steps 9 and 10 of the AASHTO guidelines for definitions of terms and explanation of methods. d Design requirements selected by owner on a case-by-case basis.
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From page 57... ...
Development of the AASHTO Guidelines for Performance-Based Seismic Design 57 Capacity Design There is an underlying assumption in the displacement-based seismic design methodology utilized in the Seismic Guide Specifications that seismic loadings will control the design of the main elements of the ERS and that inelastic behavior up to and beyond plastic hinge formation will occur. This forms the basis of the capacity design philosophy, wherein the remaining elements of the structure are proportioned to resist the maximum loads that can be developed by the ERS.
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