Structural assessment of freight bridges in Queensland Ross Pritchard, Rob Heywood and Peter Shaw Austroads Bridge Conference 2014 | 22 – 24 October, 2014 1| 2| Outline of presentation 1. Introduction 2. Inventory and access 3. Design code versus assessment code 4. Assessment methodology 5. Tier 2 6. Conclusions. 3| Transport and Main Roads (TMR) bridge inventory 4| Period Design class % of stock 2004 – Present SM1600 (160t) 5% 1976 – 2004 T44 (44t semi) 44% 1954 – 1976 H20-S16 (76% T44) 33% 1922 – 1954 A-Class (62% T44) 14% 1922 – 1954 B-Class (33% T44) 3% Various Other 1% As-of-right multi combination vehicle access Maximum permitted mass on axle group (t) Bridge Formula 140 120 100 GML • 6t, 16.5t, 20t 80 HML 60 • 6t, 17t, 22.5t 40 20 0 0 10 30 40 50 Distance between extreme axles of axle groups under consideration (m) Roadtrain 5| 20 B-Double General Access B-Double General Access 60 Access to bridge network For multi-combination vehicles (MCVs) on road train routes: M ≤ 3L + 12.5 ≤ 132t For MCVs on B-Doubles routes: M ≤ 3L + 12.5 ≤ 46.5t M ≤ 1.5L + 29.5 for 46.5t < M ≤ 62.5t For MCVs on general access routes: M ≤ 3L + 12.5 ≤ 42.5t M ≤ L + 32.5 for 42.5t < M ≤ 50t 6| Austroads (1994) • 1994 Austroads only considered mid-span moment of superstructure • Critical items not considered: 7| Shear in girder Shear in substructure headstock Total reaction in substructure Moment in substructure Shear formula error in 1960s and 1970s bridge code. Evolution of Australian Design Codes • • • • • 8| Design loads Dynamic load allowance Working stress design versus limit state design Methods of calculating structural capacity Errors in codes. BD 21/01 “The Assessment of Highway Bridges and Structures” AS 13822 “Assessment of Existing Structures” BD 79/06 “Management of Sub-Standard Highway Structures” AS 5100.7 “Rating of Existing Structures” 9| AASHTO 2011 Bridge Assessment Manual NEN 8700 “Assessment of Existing Structures…” TMR bridge assessment SIA 269 “Existing Structures – Basis of examination and intervention” Assessment versus design • Design codes cater for next 100 years, while assessment is current loads • Bridge design: code simplifications may be structurally conservative – results in minor increase in initial construction cost. • Bridge assessment: more detailed and theoretically correct assessment of existing structure. 10 | ISO/AS 13822:2001 ‘Assessment of existing structures’ • “The establishment of principles for the assessment of existing structures is needed because it is based on an approach that is substantially different from the design of new structures” • “… requires knowledge beyond the scope of design codes…” • “… current codes are normally design codes and therefore cannot be used directly for assessment.” 11 | Tiers for assessment Tier Description Tier 0 Comparison of design load and reference vehicle based on line model Bridge condition not considered Tier 1 Grillage model for load distribution Condition assessed Structural capacity assessed Tier 2 Higher Tier code for example, NEN 8700, SIA 269 Higher Tier assessment for example, fib Model Code, CSA S6-06 Numerical modelling Testing 12 | Load assessment – existing bridges Freight Route Bridge Tier 0 Assessment Tier 1 Assessment Tier 2 Assessment Structure Management Plan Restrict access 13 | Strengthen / replace Structurally adequate Tier 0 Assessment – Bridge design and road train network Excludes timber and continuous bridges 14 | Tier 1 Assessment Tier 1 Bridge Heavy Load Assessment Criteria based on AS 5100.7 and AS 5100.1 to AS 5100.6. Tier 2 Assessment Higher Tier Codes for example, NEN 8700 and SIA 269 Higher Tier Assessment for example, fib Model Code, CSA-S6-06 MCFT Shear, Numerical Models Testing 15 | Outcome 1. Restrict access 2. Replace 3. Strengthen (based on Draft AS 5100.8) Tier 2 shear strength concrete members Tier 2 methodologies for assessing shear in concrete structures 16 | Level 1 Modified compression field theory shear assessment in accordance with Canadian Highway Bridge Design Code (CSA, 2006) Level 2 Modified compression field theory shear assessment in accordance with fib Model Code Level III shear assessment (ceb-fip, 2012) Level 3 Non-linear finite element assessment in accordance with fib Model Code Level IV shear assessment Strut and tie assessment of concrete members Tier 2 methodologies for strut and tie assessment of concrete structures. 17 | Level 1 Strut and tie assessment in accordance with AS 5100.5 (2004) Level 2 Strut and tie assessment in accordance with Canadian Highway Bridge Design Code (CSA, 2006) Tier 2 reliability factors Case New bridge After repair * Existing bridge * * Based on NEN 8700 β Reliability Index 18 | β 4.3 3.6 – 3.8 3.3 Condition Desired normal operating Special approval Short term with special monitoring Tier 2 ultimate load factors Case Dead load Steel Concrete Superimposed DL New bridge 1.1 1.2 2.0 2.0 After repair 1.1 1.1 2.0 1.72 Existing bridge 1.1 1.1 2.0 1.66 * Multi combination vehicle . B-double 19 | Live Load Truck * Risk and reliability index • Short-term reduced reliability • Assumed relationship between Australia and Dutch codes • Dutch code based on higher level enforcement • Limited duration NEN 8700 is supported by legal framework. 20 | Improving reliability • Weigh-in Motion (WIM) • Structural Health Monitoring (SHM) • Comparing the axle spacing of the vehicle fleet. 21 | Actions • • • • • • 22 | Review access by existing vehicles Ban any increases in loading Increased enforcement for compliance Level 3 inspection based on NEN 8700 and SIA 269 Increased Level 2 inspection frequency Test loading / Structural Health Monitoring. Conclusion • Bridge assessment different to bridge design • Bridge design is conservative • Higher Tier Assessment required for some bridge (for example, numerical analysis, assessment code, structural capacity) • Bridge assessment methodology is required in Australia. 23 | Thank you 24 |
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