Seismic Vulnerability Study of the Alaskan Way Viaduct: Typical Three-Span Units Marc Eberhard (J. De la Colina, S. Ryter, P. Knaebel) Lacey, Washington

Outline Scope of Study Description of Typical WSDOT-Designed Unit Analyses of WSDOT Unit –3D Response Spectrum –2D Nonlinear Static Vulnerability Assessments of WSDOT Unit –Flexure –Shear –Anchorage –Splices –Joints –Footings Comparison of SED and WSDOT Typical Units

Scope of Structural Evaluation WSDOT and SED Typical Three-Span Sections Ground Motions –ATC-6 spectrum, Type III soil with Ag = 0.25g –Site-specific spectra for 20-ft fill/20-ft tidal deposit Analyses –3D Response Spectrum Analysis –Nonlinear 2D Static Analysis ATC-6-2 and Priestley Procedures for: –Flexure –Shear –Anchorage –Splices –Joints –Pile-Supported Footings As-Designed Unit (no deterioration or construction errors) Retrofit Priorities

Outside Scope Failure Modes –Torsion –Effect of anchorage deficiencies on beam/slab shear capacity –Effect of splice failure on shear resistance –Piles –Fracture at welded lap splices –Behavior of square bars Procedures Developed Since 1994 Specific Retrofit Recommendations Atypical Sections –North-end and South-end single-deck structures (Bents 1-53) –Double-deck to single- deck transition structures (e.g., outrigger bents) –Ramps –Curve Interactions among three-span frames Variations in Properties –Material properties –Reinforcement

WSDOT Unit: Longitudinal Elevation Interior Frame Exterior Frame

WSDOT Interior Bent: Transverse Elevation Short Splices No Top Reinforcement in Footings Short Beam Bar Anchorage Little Joint Confinement Little Column Transverse Reinforcement No. 12 in.

WSDOT Interior Bent: Column Cross- Section

Analyses Assumed Material Properties Description of Typical WSDOT-Designed Unit Analyses of WSDOT Unit –3D Response Spectrum –2D Nonlinear Static Vulnerability Assessments of WSDOT Unit: –Flexure –Shear –Anchorage –Splices –Joints –Footings Comparison of SED and WSDOT Typical Units

Model for Dynamic Analysis WSDOT UNIT Exterior Girders Transverse Beams Interior Stringers Slab (Shell Elements) Columns (Frame Elements)

20' Fill, 20' Tidal 30' Fill, 30' Tidal 40' Fill, 40' Tidal 38' Fill, 26' Tidal ATC-6 Modal Periods Fixed Base T L Pinned Base T L WSDOT-UNIT

Summary of Minimum Flexural C/D Ratios Response Spectra DirectionRec, First Story Rec, Second Story Rec, Beams, Girders ATC-6Transverse Longitudinal Site-SpecificTransverse Longitudinal Depending on site and direction, expect first-story column displacement ductility demands in the range of 2-4. Location of max. demands depends on column base-fixity. Low ductility demands in 2 nd story, with exception of M+ in some beams. WSDOT UNIT

WSDOT-INT FRAME Two-Dimensional, Nonlinear Analysis

Total One Exterior Frame One Interior Frame WSDOT-INT FRAME W unit = 4800 kips V trans /W unit = 0.25 Force-Deflection Relationship

WSDOT-INT FRAME Column Moments

Beam Bending Moment (first level) WSDOT-INT FRAME

Curvature Ductility Demands at Vu ( /  y, Fixed Base ) Interior Frame WSDOT Unit Exterior Frame Unclear if first-story or beam mechanism controls. First- story controls for pinned base and for longitudinal frames.

Response Spectra DirectionC/D (Fixed) C/D (Pinned) ATC-6Transverse Longitudinal Site-SpecificTransverse Longitudinal  cu =0.005 corresponds to low level of damage. Depending on site and base fixity, expect low to moderate level of flexural damage. Unlikely to lead to catastrophic collapse of unit. WSDOT UNIT Flexural Failure C/D Ratios

Shear Failure = 2 = 4

Shear Failure (Min. C/D Ratios) StoryDirectionC/D (  =2) C/D (  =4) FirstTransverse Longitudinal SecondTransverse Longitudinal First-story columns probably OK, but margin of safety is small at large ductility demands. Second story columns OK as long first story is not strengthened. Could check with updated shear- strength procedures WSDOT UNIT

Anchorage Assume hooked bars are OK Follow Priestley (1992) recommendations (No method available to account for square bars in SED Unit). Bar development OK if splitting failure suppressed If not …… lsls lsls lsls lsls

Ts/Tu lsls lsls lsls lsls Bottom reinforcing bars of first-level beams have the most critical anchorage conditions. Consequences on vulnerability (e.g., shear resistance) unclear. Also need to look at development of top-story column bars through joint.

Lap Splices ATC-6-2 (r cs ) Priestley (Mb/My) Priestley (Mb/Mu) First Story Second Story Short (20-22 Db) Poorly confined Located at base of first- and second-story columns Bottom splice critical, because even if Priestley procedure is correct, deformation demands will be large. Top splice probably OK. (Interaction with joint?)

Joints Priestley (1992) suggests limit on maximum tensile stress of 3.5(f’c)^.5 to prevent joint cracking. Others limit shear stress. Did not consider results of recent tests by Stanton and Lehman (2000) Calculate: with and without slab reinforcement at ultimate conditions and using forces from nonlinear analysis Neglect interaction with splice Assume only part of joint is effective

Joint Effective Areas Girder Beam Column Effective Joint Area Interior Exterior Beam Girder Column Exterior Effective Joint Area Depth Girder Beam Column Effective Joint Area Interior Exterior Beam Girder Column Interior Exterior Effective Joint Area Transverse Longitudinal

Normalized Joint Stresses (Transverse) Similar for interior and exterior WSDOT frames Normalized v ult

Normalized Joint Stresses (Longitudinal) Normalized v ult Normalized v nonl WSDOT Unit, including slab reinforcement Joint shear cracking is expected in transverse direction (1 st and 2 nd level) and in longitudinal direction (1 st level only). Cracking does not necessarily lead to collapse.

Footings F1F1 F5F5 F4F4 F3F3 F2F2 P Mu Flexure Shear (ACI-ASCE 426) Anchorage Joints ( 3.5(f’c) ^.5 )

Footings C/D Ratios C/D min Transverse C/D min Longitudinal Flexure 0.9 on My 1.1 on Mu 1.3 on My 1.6 on Mu Shear Anchorage of Starter Bars Joints Using conservative criteria, it appears that the WSDOT footings are vulnerable to shear failure. Detailed analyses and testing might change that diagnosis. Few footings have failed during earthquakes.

Priorities for Retrofitting LocationFailure Mode LikelihoodConsequenceCost 1 st -Story Splices Flexure Shear 1?1? JointsDiag. Tens.213 ColumnsFlexure Shear FootingsShear2*23 2 nd -Story Splices Flexure422 * Modified