Bridge Components Loading Codal Provisions Suhasini Madhekar College of Engineering Pune Faculty Development Program on Fundamentals of Structural Dynamics and Application to Earthquake Engineering 12 th December 2015 Sanjay Ghodawat Group of Institutions Atigre, Kolhapur 1
Bridge Components Bridge Bearings: Supported on a bridge pier, which carry the weight of the bridge and control the movements at the bridge supports, including the temperature changes. Types : Metal rockers, rollers or slides or merely rubber or laminated rubber, POT - PTFE Bridge Dampers & Isolators: To absorb energy generated by earthquake waves and lateral load Bridge Pier: A wide column or short wall of masonry or plain or RCC for carrying loads as a support for a bridge, founded 2 on firm ground
Bridge Cap: The highest part of a bridge pier on which the bridge bearings or rollers are seated. Bridge Deck: The load bearing floor of a bridge which carries and spreads the loads to the main beams. (RCC / PSC / Steel plate girder / Composite) Bridge Components Abutment: A support of bridge which may carry a horizontal force as well as weight. Expansion Joints : These are provided to accommodate the translations due to possible shrinkage and expansions due to 3 temperature changes.
4 Bridge - Components
Bridge Components Superstructure Bearings (Connections) Soil Stratum Pier Cap Well Cap Foundation Substructure 5 The FOUR Components:: Foundation :: Well and Well Cap; Pile and Pile Cap Substructure :: Pier(s) and Pier Cap; Wall; Frame Connections :: Fixed, Free and Guided Bearings Superstructure :: Slab; Girder-Slab; Box; Truss; Frame
6 Bridge Cap and Damper
7 Loading on Bridges
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9 Cars on a suspension bridge over a river : Colorado
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Loading on Bridges Permanent Loads: remain on the bridge for an extended period of time (self weight of the bridge) Transient Loads: loads which are not permanent - gravity loads due to vehicular, railway and pedestrian traffic - lateral loads due to water and wind, ice, ship collision, earthquake, etc. 11
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20 Mass of deck = 3,278,404 kg ( DL = 32784 kn) LL = 3850 kn D = 65658 kn, F= 324 kn
Behaviour: Longitudinal shaking Bridge Vibration Units: Single-span Multi-span Simply-supported Continuous Overall Structural Behaviour 21
Behaviour: Transverse shaking Connections Superstructure Substructure Foundation Vertical cantilever action Mass lumped at the top Foundation flexibility 22
Capacity Design of Bridge Components Ductile Link Plastic Moment Hinges Brittle Link 23 Damage only in piers: mandatory ductile detailing Elastic design of other components
Bridge Performance in past Indian Earthquakes Gawana Bridge (1991 Uttarkashi Earthquake) - Shearing off of anchor bolts of roller cum rocker bearings 24
Past EQs... Gawana Bridge - Unseating of superstructure from abutments 25
Past EQs Gawana Bridge 26
Past EQs Old Surajbadi Bridge (2001 Bhuj Earthquake) - Bearing damage due to jumping of superstructure 27
Past EQs New Surajbadi Bridge (2001 Bhuj Earthquake) - Jumping of Girders Damage to girders 28
Past EQs Toe Crushing of Stone Wall Masonry Piers - Old Highway bridge (2001 Bhuj earthquake) 29
Past EQs Vertical Splitting of Stone Wall Masonry Piers - Old Highway bridge (2001 Bhuj earthquake) 30
Past EQs Collapse of Superstructure - Aman Setu (2005 Kashmir earthquake) 31
32 Analysis of Bridges : Issues in Modeling Superstructure No ductility demand Usually, stiff in vertical direction Connections Simple Bearings :: Rocker, Roller Model as rigid, with usual freedom Flexible Bearings :: Neoprene/Rubber/Lead Rubber Model as Flexible Substructure Only structural component with ductility Detailed idealisation required Effect of shear deformations to be included Foundation Main concern is modeling soil
Levels of earthquake shaking LOW :: Functional Evaluation Earthquake Un-cracked Section (EI gross ) HIGH :: Safety Evaluation Earthquake Cracked Section (EI eff ) Properties for Modeling M EI gross Spectral Acceleration Sa/g 2.0 1.5 Safety 1.0 0.5 Functional 0.0 0 1 2 3 4 Natural Period T (sec) M u 0.6M u EI eff ϕ 33
Properties for modeling Modulus of Subgrade Reaction k Layered Soil N Value Rigid Foundation 34 Distributed Springs Lumped Springs
Modeling: Summary Overall model for Longitudinal Shaking 35 Cantilever model for Transverse Shaking
Analysis Methods of Dynamic Analysis Seismic Coefficient method Response Spectrum analysis for other bridges Time History analysis for special bridges Push over analysis Geometric and material nonlinearities 36
IRC Codes: Flexure and Shear Design Design lateral force calculation (Interim IRC: 6-2014) - Structural flexibility - Response Reduction Factor (R) for nonlinear response Working Stress Design for bridge substructures (IRC:21-2000) - Not applicable for explaining seismic behaviour - Contradiction with the lateral force calculation method 37
IRC Codes: Flexure and Shear Design No provision on explicit design against lateral shear force (IRC:21-2000) - Shear design prescribed only for beams and slabs - Horizontal steel provided as per the prescribed minimum amount - No provision on confinement of concrete Capacity design not prescribed for any bridge component (IRC:21-2000, IRC:78-2000) - No plastic hinge formation in case of extreme seismic event Limit State Design for bridge (IRC:112-2011) 38
IRC Codes: Flexure and Shear Design Wall piers and column piers (IRC:78-2000) - No difference in design methodologies Pier Cap Pier Cap Pile Cap :: Shear deformations :: No plastic hinge :: Flexural deformations :: Plastic Hinge Region Pile Cap Wall Pier Column Pier 39
IRC Codes: Flexure and Shear Design Well Foundations (IRC:78-2000) - Three dimensional finite element analysis of the foundation - Tensile and compressive stresses checked at the critical sections - No formal flexure and shear design methodology prescribed - Nominal vertical and horizontal steel prescribed - Proportioning of foundation prescribed on an empirical basis - Seismic design procedure not available 40
Earthquake Force Generated where the mass is (at deck level) Needs to be transferred safely to ground 41
Ground vibrations Vertical vibrations Vertical inertia force Adds and subtracts to the gravity force Generally not a problem due to FS in gravity design 42 Gravity Loads Vertical EQ-Induced Inertia Force
Ground vibrations Horizontal vibrations Horizontal inertia force Need load transfer path Need adequate strength Deck Slab Inertia Forces Piers Foundations Soil 43 Earthquake Shaking Flow of EQ inertia forces through all components
The Bridge Example Capacity Design Concept Superstructure EQ Design Good Ductility Adequate Strength Connections Substructure Foundation 44
The Example The Bridge Example (F EQ ) max P (F EQ ) max M 45
The Example The Bridge Example Shear Design (F EQ ) max M ( F ) EQ = max H 0 H 0 P ( ) EQ max If F > V design additional steel for the balance shear u (F EQ ) max M 46
The Example Ductile Link Plastic Moment Hinges Brittle Link 47
Reinforced concrete bridge : Slab bridge: span < 12 m Carriageway Cross section of solid slab bridge deck Slab 48 48
Reinforced concrete bridge : T-Beam bridge : span 12 to 24 m Carriageway Footpath D=1200-1800 mm T-beam Cross beam 49 Cross section of T-beam bridge deck 49
Reinforced concrete bridge : Slab on girder bridge : Carriageway Footpath D=1200-3000 mm I-beam Cross beam (Diaphragm) 50 Cross section of I-beam bridge deck 50
Reinforced concrete bridge : Box girder bridge : span: 20 to 50 m Carriageway Footpath D= 1000-3000 mm 51 Cross section of box girder bridge deck 51
Steel bridge : Steel I-beam bridge : Span: upto 20 m Carriageway Footpath 52 Cross section of steel I-beam bridge deck 52
Common types of failure observed under seismic excitation: Seismic displacement failure Abutment slumping failure Column failure Joint failure 53 53
Displacement failure : Unseating 54 Unseating failure of main approach of Nishinomiyako bridge in Kobe earthquake (Japan) 54
Displacement failure: Pounding 55 The longitudinal movement of the new Surajbadi bridge superstructures led to pounding at the deck slab level in Bhuj Earthquake, 2001 India. 55
Abutment Slumping failure Deck Pile foundation 56 56
Column failure due to improper detailing of plastic hinge region 57 Crushed column of Santa Monica Freeway Northridge earthquake 1994 (USA) 57
Column failure due to improper detailing of plastic hinge region Column failure in Mission-Gothic under crossing at Simi Valley 58 San Fernando Freeway in Northridge earthquake 1994, USA 58
Column shear failure. Failure of column of Hanshin Expressway, Japan in 59 Kobe Earthquake, 1995 Japan. 59
Joint failure due to poor detailing Cypress viaduct joint failure in 60 Northridge earthquake in 1994 USA.
Conceptual seismic design: The bridge should be straight as curve bridge complicates the seismic response. Deck should be continuous with few movement joints. Simply supported spans are prone to unseating. Foundation material should be of rock or firm alluvial. Soft soil amplifies seismic response. Pier height should be constant along the bridge. Non-uniform height results in stiffness variation and attraction of more forces to stiffer pier. 61 Pier stiffness should be uniform in all direction. 61
Conceptual seismic design: Span length should be kept short. Long span results in high axial forces on the column with potential for reduced ductility. Plastic hinges should be developed in the column rather than in the cap beam or in superstructure. The abutment and the pier should be oriented perpendicular to the bridge axis. Skew supports tend to cause rotational response with increased displacement. 62
Connection of pier and superstructure : Bearing (a) Moment resisting conection (b) Bearing supported connection Support alternative for pier and superstructure 63
Beneficial effect of consideration of soil flexibility Consideration of soil flexibility effect on foundation gives lesser forces due to shift of period of vibration of structure because of added flexibility by soil from higher acceleration zone to lower acceleration zone of design spectrum. 64 64
Outcome: The substructure of bridge are more vulnerable under seismic excitation. Non consideration of inelastic action of structure led to the failures in plastic hinge region of column. Seismic deflection of bridge calculated using elastic theory of design will lead to underestimation of actual deflection and will result into unseating or pounding of girders during seismic excitation. 65 65
Outcome (contd..) Comparative study of possible alternative models of same type of bridge are required Comparative results of fixed base and detailed model for bridge with well foundation considering SSI Difference in seismic response of bridge model with actual and simplified location of bearing Effect of scour of river bed on seismic response Effect of hydrodynamic pressure on seismic response using global model. 66 66
Thank you.. 67