CONCEPTUAL DESIGN AND CONTROL OF BRIDGE STRUCTURES IN SEISMIC AREAS

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CONCEPTUAL DESIGN AND CONTROL OF BRIDGE
STRUCTURES IN SEISMIC AREAS

• Dr Radomir FOLIC, Professor
• Institute for Civil Engineering
• Faculty of Technical Sciences University of Novi
  Sad
• E-mail folic_at_uns.ns.ac.yu

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INTRODUCTION

• The extensive damage of the recent earthquake
  have led to a significant damage of B Ss.
• The cause is often the error of conceptual
  design, i. e. the choice of the structural and
  foundation system, spacing of piers and
  connections between them, deck and abutments, the
  spacing of joints, etc.
• This presentation reviews philosophies of seismic
  design and protection which can be used in the
  conceptual phase of bridge design (Eurocode
  8-part 2 provisions and recommendations used in
  U.S.A. and Japan).

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Traditional design procedureEarthquakeStruc
tureResponse

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Buckling long. bars caused by bed confinement
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INTRODUCTION

• Beam system is used for small and medium spans,
  arch and suspension system for large spans.
• Importance of structure, site conditions and
  regularity of structure influence on methods of
  analysis. Based on regularity in plane and
  elevation structures are classified as regular or
  non-regular.
• Based of the need for the B, to maintain
  emergency communications after the design seismic
  event, classified greater than average (?I1.3)
  average (?I1.0) less than average (?I0.7)- (EC
  8-2).

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INTRODUCTION

• In the most current seismic code aim is to
  prevent collapse of the structure under the
  design earthquake. The importance of conceptual
  analysis in B designing problems cannot be
  stressed enough.
• Choice of appropriate earthquake resisting
  structural system (ERS) must provide in early
  phase of design.

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DESIGN

• Three steps in design of bridge structures (BS)
  are
• Conceptual design,
• Analysis,
• Detailing.
• Three approaches in design of BS are
• Force - Based Seismic Design FB SD,
• Displacement - Based Seismic Design DBD (N.
  Priestley), and
• Performance - Based Seismic Design PB SD.

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DESIGN

• Performance requirements depend on the importance
  and configuration-regularity of bridges (B's). We
  can divided (B's) on
• normal (B's) special bridges arch bridges,
  cable-stayed B's, B's with extreme geometry, and
  B's with distinctly different yielding strengths
  of piers.
• Special B's designed to behave elastically under
  the design earthquake or use seismic isolation to
  achieved elastic response.

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Elastic and inelastic response (Rq)
Design Force-reduce
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BEHAVIOUR OF Bs IN EARTHQUAKE and BASIC DEISGN
PHILOSOPHIES (BDPh)

• The BDPh is to prevent B from collapse during
  severe earthquake with small probability of
  occurring during service life of the B.
• The ductility behaviour? using elastic calcul.
  with reduced seismic forces (with behaviour
  factor qR) lead to economic solutions.
• The alternative is use of elastic systems on the
  isolated base or? used devices for dissipation of
  input seismic energy.
• In concrete B?inelastic damage located in the
  pier and abutments, and plastic hinges develop
  simultaneously in as many piers as possible
  ?greater energy is dissipated.

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Demand for seismic performance of
infrastructures-Japan
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BEHAVIOUR OF BRIDGES IN EARTHQUAKE AND BASIC
DEISGN PHILOSOPHIES

• According EC 8
• in regions of low and moderate seismicity
  frequently chosen limited ductile behaviour It is
  needed access for inspection and repair of the
  pot. plastic hinges and the bearings.
• In regions of moderate and high seismicity the
  ductile behaviour is required.

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BEHAVIOUR OF BRIDGES IN EARTHQUAKE AND BASIC
DEISGN PHILOSOPHIES

• The performance-based crit. to provide ductile
  failure ? usually require two level design
• to ensure service performance of B for
  earthquake with small magnitude that can occur
  several times during service life
• is to prevent collapse under severe earthquake
  with small probability of occ. during service
  life of bridge.

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Development of performance-based criteria is
obtained through following steps

• Establish post-earthquake performance
  requirements.
• Determine B specific loads and various
  combinations.
• Determine materials and their properties.
• Determine analysis method for evaluation of
  demands.
• Determine detailed procedures for evaluation of
  capacity.
• Establish detailed performance acceptance
  criteria.

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BEHAVIOUR OF BRIDGES IN EARTHQUAKE AND BASIC
DEISGN PHILOSOPHIES

• EC 8 ?seismic resistance (SR) requir. That
  emergency communications shall be maintained,
  after the design seismic event (SDE).
• Non-collapse req. (ultimate limit state) after
  SDE the bridge shall retain its structural
  integrity, at some parts considerable damage may
  occur.
• Deck shall be protected from plastic hinges and
  unseating under extreme displacements? only minor
  damage without reduction of the traffic or the
  need of immediate repair. Capacity design shall
  be used to provide the hierarchy configuration of
  plastic hinges in piers.

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CONCEPTUAL DESIGN

• Majority of Codes relates to modeling and
  analysis elements and structures (E/S). Only
  rarely they deal with conceptual design (Russian
  and Swiss).
• Russian Code ?beam system are recommended. The
  arch bridges can be applied only in rock
  terrains. In the IXth zone MCS scale? precast
  concrete, composite-monolithic and concrete
  structure bearings is recom.
• Swiss Code local damage - destruction of bearings
  or expansion joints tolerated provided that the
  superstructure is prevented from falling

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CONCEPTUAL DESIGN

• Bridges should be as straight as possible. Skew
  angle should be as small as possible. Curved
  bridges complicate seismic responses.
• Vibrations along the axis of a skew bridge cause
  torsional response - large rotation demands on
  piers heads. In single pier bridges, an
  eccentricity between the deck and pier axis would
  also lead to torsional response.
• Behaviour of continuous Bs is better than other
  types. Necessary restrainers and sufficient seat
  width should be provided between adjacent bents
  at all expansion joints.

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Balance mass and stiffness distribution FRAME
STIFFNES
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CONCEPTUAL DESIGN

• Bs are long period structures - effected by
  higher modes.
• Adjacent bents or piers should be design to
  minimize the differences in fundamental periods,
  and to avoid drastic changes in stiffness and
  strength in both longitudinal and transverse
  directions.
• Stiffer frame receives greater part of load.
• The pier causing the most irregular effect due to
  its stiffness and damaged first (unequal pier
  heights) in special situation of full isolation
  applied.

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CONCEPTUAL DESIGN

• It is recommended that
• Effective stiffness between any two columns
  within a bent, does not vary by a factor of more
  than 2.
• Ratio of the shorter fundamental period to the
  longer ones for adjacent frames in the
  longitudinal and transverse directions should be
  larger than 0.7.
• Balanced mass and stiffness distribution in a
  frame results in a good response. Irregularities
  in geometry increase complex nonlinear response.

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Unfavorable distribution of transverse seismic
action
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Permissible Earthquake Resistance systems -ATC
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Permissible Earthquake -resisting elements- ATC

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require owner's approval - ATC
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require owners approval - ATC
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Earthquake-resisting elements that are not
recommended for new bridges- ATC
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Methods of minimizing damage to abutment
foundation ATC
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Location of primary plastic hinge, a)
conventional design, b) menshin-seismic isolation
design, c) bridge on a wall type pier (Japan Code
1996)
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MODELING AND ANALISYS
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MODELING AND ANALISYS-without base isolation
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MODELING AND ANALISYS-WITH BASE ISOLATION
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PROTECTION OF BRIDGE STRUCTURES

• Concrete B design to direct inelastic damage into
  columns, pier walls, and abutments.
• The superstructure should sufficient
  over-strength to remain essentially elastic if
  piers reach plastic M capacity
• Seismic protection devices-energy dissipation and
  isolation at approp. location provide good
  behaviour.

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PROTECTION-CONROL OF BRIDGE STRUCTURES
Spri-ng
Spring
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Bridge control system devices, advantages and
disadvantages
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BASE ISOLATION
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FRICTION DAMPER
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Deformation response spectra/with variation
damping ratio ? for SDOF system
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Pseudo-acceleration spectrapeak value of A(t)
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CONTROL OF STRUCTURES
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Three-span C. Frame B. S. of MDOF ex. b) Long.
Degree of freedom, c) Tran. DOF,d) rotational
DOF, e) mode shape I, f) mode shape 2, g) mode
shape 3. WITHOUT PROTECTION
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Three span bridge with active control system (a)
b) B model for analysis c) SDOF
system controlled by actuator
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Controllable sliding bearing
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Base isolation Active control
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Simple-span bridge with hybrid control system
b) lumped mass system model c) four-degree-
of-freedom system
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Multi column structures offer the option of
fixed or pinned base solutions. Displacements at
the deck level are reduced, especially in the
transverse direction.
Options for lateral force resisting systems
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• Monolithic connections between deck and abutment
  are more
• commonly used for small bridges, solution b) is
  more reliable
• Than of a). Bearing supports have many
  configurations c) and d).
• For both configurations the bearings may be
  substituted by isolators.

Options for abutment- deck connection
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Mechanisms of resisting forces at the abutment
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For piers the circular section is desirable (L
T demands are similar)? provides uniform
confinement and restrains the L bars from
buckling. In the rectangular sec. the protection
of long. bars against buckling must be provided
with add. S tie.
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DETAILING-CONNECTIONS
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Comparative provisions for aseismic design
Provisions Caltrans (USA) Eurocode 8 Japan
1. Performance Criteria Structural integrity to be maintained and collapse during strong shaking to be prevent. No collapse under safety-level event (ULS). No damage under frequent earthquakes (SLS). To be maintained in small and moderate earthquakes (EQ). Collapse to be avoided for large EQ.
2. Design philosophy Adequate duct. capacity to be provided and failure of non-duct. el. and inaccessible to be prevented. Sufficient strength of elastic str. In order to avoid damage. Brittle types of fail. To be avoided in all structures. Component to perform elastically under functional earthq. Detailing specific components to avoid damage
3. Design approach Single-level design. Desired perf. at lower earthquake load is implied. Single-level design. Desired performance at lower earthq. load is implied. Utility level earthq. and working stress. Detailing to avoid collapse of girders shacked. All review.
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CONCLUSIONS

• The basic philosophy for seismic design of
  ordinary bridges is that for small to moderate
  earthquakes the bridges should resist within the
  elastic range without significant damage, while
  for large earthquake must prevent collapse.
• In current design practice the changes are
  necessary to incorporate improved design
  procedure, especially Perf. B S D.
• It is very important to analyse plane layout and
  layout in elevation of BS in preliminary phase to
  respect presented recommendations.

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References

• ATC, Improved Seismic Design Criteria for
  California bridges Provisional Recommendations,
  ATC - 32, Applied Tech. Council, Redwood City,
  CA, USA, 1996
• AASHTO (American Association of State Highway and
  Transportation Officials) Bridge Design
  Specifications, 1998.
• Bridge Engineering-Seismic Design (BESD) Ed. W.
  F. Chen and L.Duan, B. R. 2003.
• CALTRANS (California Department of
  Transportation) SEISMIC DESIGN CRITERIA, VERSION
  1.2, (p.121), December, 2001
• Duan, L., Wai-Fah, C. Bridges, in Earthquake
  Engineering Handbook, Ed. W.F. Chen and C.
  Scawthorn, CRC Press, Boca Raton, 2003. pp.
  18.1-18.56.
• Duan, L., Li, F., Seismic Design Philosophies and
  Performance-Based Design Criteria, (p. 5.1-5.35)
  in BESD, Ed. W. F. Chen and L. Duan, CRC, B.
  Raton, 2003
• Elnashai, A., Seismic Response and Design of
  Bridges, in Manual of Br.Eng., 2002.
• EC8/2 Eurocode 8 Design of Structures for
  Earthquake Resistance Part 2 Bridges, prENV
  1998-2, May 1994, CEN, Brussels.
• EC8/2 Eurocode 8 Design of Structures for
  Earthquake Resistance Part 2 Bridges, prEN
  1998-2200X/ Draft 5 (pr Stage 51) June 2004,
  CEN, Brussels.
• Folic R., Ladinovic Ð. Some current methods and
  tendency in seismic design of concrete bridges.
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  Bridges Across the Danube, Novi Sad, Serbia
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• Pristleey, J.M.N., Seible,F. and Calvi,G.M.
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  Interscience, New York, 1995.
• Regulations for Seismic Design a World List-1996,
  Supplement IAEE, 20002004.
• Troitsky, M.S. Conceptual Bridge Design, in
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  Duan CRC Press, Boca Raton, Florida, 1999. Chap.
  1. pp 1.1-1.19
• UNJOH, S., Seismic Design Practice in Japan, (p.
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  CRC, Boca Raton, 2003.