Update on Seismic Behavior and Design of

Transcription

1 Update on Seismic Behavior and Design of Steel Plate Girder Bid Bridges Ahmad M. Itani, Ph.D., P.E., S.E. Professor Department of Civil and Environmental Engineering University of Nevada, Reno

2 Background Current taashto Specs and Si Seismic i Guide Specs have limited information on the Seismic Design of Steel Plate Girder Superstructures. t The AISI Report presents the state of the art information on seismic design of steel plate girder bridges, proposed seismic provisions, and seismic design examples.

3 AISI Seismic Report Table of Contents Chapters 1. Introduction and Background 2. Proposed Seismic Provisions and Commentary 3. Seismic Design Examples Appendix 3A, 3B, 3C

4 Lateral Load Path in Steel Superstructures Prototype Single Span 150 ft Model 60 ft (2/5 scale) 2 Plate Girders R/C Deck Actuators Steel Girder E X F I X F

5 Seismic Response of Steel Plate Girder Superstructure Shake Table Experiments

6 Seismic Lateral Load Path

7 Behavior and Design of End Cross Frames and Diaphragms Elastic Response Ductile Response

8 Lateral Load Path at End cross Frames P P

9 Cyclic Response of Single Angles Variables End Conditions Welded Bolted A n /A g b/t ratio KL/r

10 Cyclic Behavior of End Plate Girder Region

11 Shear Connectors on Plate Girders

12 12 Shear Connectors on Top Chord

13 13 R/C Deck Damage at 4% Drift

14 14 R/C Deck Damage at 7% Drift

15 R/C Deck Damage at 7.5% Drift Connectors on Plate Girders 15

16 R/C Deck Damage at 7.0% Drift Connectors on Top Chord 16

17 Why Integral Abutment Bridge (IAB)? Jointless bridges where the deck is continuous and connected monolithically to the abutment with a moment resisting connection. The rationale for integral abutments is clear as presented by Wasserman, Burke, etc. : Efficient design, reduced substructure costs No bearings and djoints (The e best joint is no joint) Simplified construction Minimized deterioration Added redundancy and capacity 17

18 Scope and Motivation FHWA through a contract (DTFH61 07 D Task Order 003) with the HDR Engineering, Inc (Pilot Study) Task 1: Literature Review Task 2: Mathematical Models for Steel Bridges with Integral Abutments: Local and Global Task 3: Seismic Load Path in Straight and Skew Bridges Task 4: Seismic Response of Steel Bridges with Integral Abutments Task 5: Seismic Design Guidelines Lack of information/studies on the seismic system response of steel bridges with integral abutments Steel bridges with integral abutments with well detailed piles for large cyclic deformation can act as fuses during seismic events 18

19 Seismic Loads in Integral Abutments Elastic Behavior Steel Plate Girder to Abutment Connection Steel Piles to Abutment Connections Controlled Inelastic Behavior Steel Piles

20 Girder to Abutment Connection Common assumption in analysis and design is that the superstructure is rigidly connected to the abutment. Embedment length is usually based on rule of thumb procedures and standard details. No specific guidelines for the proper design of the connection and under what condition this connection can be assumed as rigid.

21 Girder to Abutment Connection Effect on Bridge Dynamic Characteristics Detailed finite element of a typical connection was developed on ADINA to assess the connection flexibility and obtain equivalent springs that can be implemented to a global bid bridge model dl 2.5 ft 27 ft ft Girder top flange thickness 1inch Girder top flange width 18 inch Girder bottom flange thickness 1.5 inch Girder bottom flange width 22 inch Girder web thickness 0.5 inch Girder web width 78 inch thickness of web stiffener 05inch 0.5 location of web stiffener (X-coordinate) 20 inch girder embedded length 24 inch

22 Girder to Abutment Connection: Embedment Length Sufficient i embedment bd tlength this required so that t the rigid idconnection assumption is valid. SimplifiedMechanism (Shama, et al., 2002) lemb F P ' Cm 0. 5 fc b M C jd c m assuming jd = 05l 0.5l emb M f c f l emb ' cbf lemb M l emb M 2 f ' c b f Cm for max. concrete strain cu = 0.003, and are both equal to 0.85, and with = 0.9, jd Cm f'c fc l emb 2.5 f M ' c b f

23 Behavior of Steel Piles under Large Burdette et al. (2003) Df Deformations Tests conducted on HP10x42 piles driven into undisturbed clay show that large displacements can be imposed without breaking the piles, but the maximum sustainable displacement is limited by the pile to abutment connection. Ingram et al. (2003) AASHTO and AISC equations, which considers length effects, had limited applicability and in most cases are inappropriate for steel H piles embedded in soil. Pile resistance can be evaluated with more accuracy by the plastic limit of the section. Pilescan sustainlargedisplacements providedthere issufficientembedmentsufficient embedment length coupled with the confinement provided by the soil. Pile resistance is evaluated at plastic limit.

24 University of Tennessee Experiment University of Tennessee Prof. Burdette et al

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26 Piles: Seismic Design Recommendations Pile Rotation Capacity: = 10% rotation (0.10 radian) anywhere along the pile length. Note: maximum moment in the pile occurs not only at the top (i.e. below the cap) but also at about 1/2 the distance between the top and location of zero deflection and zero moment. Embedment Length: l emb 2.5 f M ' c b f where, M = 1.2 M p M p = plastic moment 1.2= overstrength factor

27 IBA and Damping Ratios Large damping occurs in a bridge with integral abutments during a seismic event. Factors contributing to the increase in damping include yielding of the piles, columns, abutment soil interaction, soil pile interaction, as well as radiation damping occurring when the abutment soils are engaged. Because of the its relatively large stiffness, most of the forces are attracted to the integral abutment. Field analysis on damping ratios of IBA, on a skew two span bridge, during moderate earthquakes indicates that the damping ratio in the transverse is around 15% (Chopra and Goel, 1997)

28 Damping Evaluation The total damping in bridge with integral abutments can be expressed as: inh sys where, inh is the inherent structure damping usually taken equal to 5%, sys is the system damping associated with the yielding of the piles, columns, and abutment soil Equivalent hysteretic damping can be expressed as (Mander and Cheng, 1997): sys 1 where, is the energy absorption efficiency factor, is the post yield stiffness ratio, and is the ductility demand ratio max / yield. The factor can be assumed to be equal to 0.3 in the absence of complete information but can be refined by a further study. Note: sys is evaluated in the system level instead of in the component level

29 Proposed Seismic Analysis Procedure for seismic analysis and evaluating sys : 1. Determine modal properties (periods and mass participation ratios) 2. Identify the main vibration modes corresponding to the directions where the earthquake is applied. 3. Perform modal pushover analysis in these modes. 4. Establish the system yield displacement and post yield stiffness. 5. Run seismic analysis (response spectrum analysis) 6. Determine the bridge displacements. 7. Calculate the total damping and update the effective stiffnesses of the yielding components. 8. Rerun the response spectrum analysis. Apply the damping in (7) to the corresponding modes. 9. Determine bridge displacements, calculate damping, update stiffnesses. 10. Repeat above procedure until the displacements, damping and stiffnesses are within +5% of previous.

30 30 Benchmark IAB (Wasserman and Walker, 1996)

31 3D Finite Element Model deck = shell elements Columns, bent cap, girders, cross-frames, pile, abutment = beam elements Fiber Hinge at top & bottom of column Gap + soil springs (long. dir. only) Nonlinear p-y springs (long. & trans. dir.) at 12in spacing from top of pile Fiber Hinge at: top, depth = 84 in, and depth = 108 in. (Pile Length = 216 ) Pin support at bottom of pile. 216 from top of pile to location of zero moment & zero displ.

32 Results of Response Spectrum Analysis Design spectrum was applied in the longitudinal and transverse directions. Orthogonal combination of seismic demands is based on the maximum of: 100X + 30Y 30X + 100Y Iterative procedure described previously was used. Calculated total damping 6% applied to Mode 5 13% applied to Mode 1 Pile displacement demands 2.9 in longitudinal direction 7.5 in transverse direction

33 Current Status Seismic analysis, design guidelines and design example are finalized Final report is under review.