January 2011 Design Approach For Multiple Hazard Load Condition Hossein Ghara, P.E., M.B.A.

Transcription

1 January 2011 Design Approach For Multiple Hazard Load Condition Hossein Ghara, P.E., M.B.A.

2 The Common Phenomenon Earthquake & Wind Scour Vessel Collision Storm Surge & Wave Action Debris Flow Fire Tsunami Blast Load Overload Construction & Erection Issues Transportation or Shipment Issues

3 Special Workshop initiated by FHWA Project 012 On Regional Limit state Equations & Extreme Load Combinations for the Development of Multiple Hazard LRFD Participated by: WV, LA, NC, MD Arora, M&M, UB, FHWA, UCI Analyze the survey data on extreme Load combinations for Establishing a regional-based MH-LRFD Shape up Limit state equations for the different regions in U.S. based on Survey Results

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18 Not all extreme event Limit states are necessary for every bridge. Factors include: Regional variation Bridge function/importance Tolerable risk may vary in different regions and different bridges. The survey reflected such variation. Critical parameters in producing adequate reliability assessment include: Recurring Frequency & intensity Duration Design considering cascading effects is complex, however potentially more important than concurrent events.

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20 These measures indicate little overlap challenges in analysis & developing design strategies to effectively address multiple hazards.

21 Earthquake vs. Blast vs. Scour Structural mass creates greater internal seismic forces while it reduces internal forces from a blast. The case of an abutment bent.

22 For Bridges in the U.S.A.

23 Incorporated a factor of safety between stresses induced by the applied Loads vs. the member s stress carrying capacity Φ/Y Rn Q i

24 Applied different factors to different loads such as Live & Dead loads, computed various stresses and made sure these stresses did not exceed the member s stress capacity. Φ (Rn) Y[Β O + Β L (L+I)

25 A reliability-based design approach Not just differentiate between the various loads but also differentiate between the various materials by developing reliability indices. LRFD also considers the variability in the behavior of structural elements. Adjustments to design factors to reflect the variable predictability of loads and materials is included in the specifications. Not present in LFD & WS. LRFD also provides a move accurate distribution factors for Live load than prior specifications.

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27 Where: ηi Υi Qi Φ Rn Qi = Load of Force effects Rn = Nominal resistance Load Modifier: ηi = A factor relating to ductility, redundancy and operational importance. Load Factor: Υi = Load factor, a statistically based multiplier. Φ = Resistance factor, a statistically based multiplier. Load & resistance factors developed for this equation have been calibrated by trail designs to provide a high and uniform level of safety in new bridges.

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29 The reliability index of β=3.5 provides a 1 in 10,000 notional failure probability. Compare the older AASHTO standard specs. Did not provide a consistent and uniform safety level by providing a reliability index as low as 1.5 or as high as 4.5. A β of 2.0 is a high rate representing 4 out of 100 design elements & components that would overload and experience a problem during the design life. A β of 4.5 result in a design that would be very conservative & costly.

30 LRFD adopts a limit state philosophy or a state beyond which a component cease to satisfy the provisions for which it was designed. The limit states provide a systematic approach to structural design to ensure satisfactory short and long term performance of bridges. LRFD defines Four main limits states to be satisfied by design: 1. Strength Limit state: Assures adequate ultimate load capacity. 2. Extreme Event Limit state: Earthquake, scour or other hydraulic events, Ice loads, or ship collisions. 3. Service Limit State: Imposes restrictions on stress, deformation, and crack width under service conditions. Assure elastic behavior to minimize need for maintenance during service life. 4. Fatigue and Fracture Limit State: imposes restrictions on stress range due to a design truck occurring at a number of expected stress cycles.

31 Only Strength Limit state has been calibrated using the reliability method Reason? Scope & Cost of Calibration

32 AASHTO Load Cases Provide Comfort Zone but not current state of the Art beyond Strength Limit State.

33 AASHTO Load Cases assume simultaneous application of loads to be resisted. Reality: Structures realize multiple & cascading events leading to inelastic responses to the bridge damages. Aviation and Nuclear Engineering have addressed these types of events.

34 Wave Action, Fires, and Blast Post Katrina Guide specs for Hurricane Surge & Wave action pooled Fund Study. Fire, NCHRP Blast, NCHRP 12-72(01) Blast resistant Highway Bridges Design & Detailing Guidelines Outcome: A new guide specification on Bridge security. Louisiana s participation?

35 Sponsorship of several projects to improve structural performance and examining Structural Redundancy Seismic Analysis of sup & sub Scour & Extreme events in general Coastal scour Wave action & Hurricane Surge & Tsunami Ship & Barge Collision Fire Blast Load & vehicle collision Tunnel safety Overload Construction Loads/Erection Issues

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