Development of LRFD Specifications for Horizontally Curved Steel Girder Bridges (2006) / Chapter Skim
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From page 6... ...
2.3 Calibration 2.3.1 Scope The objective of this section is to document the calibration of the design code for steel curved girder bridges, consistent with the AASHTO LRFD specifications. It is assumed that load factors for curved girders remain the same as for straight girders.
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From page 7... ...
The bridge resistance model depends on the statistical parameters of materials and geometry. Therefore, the latest available material test data are reviewed and used in derivation of the bias factors and coefficients of variation for moment and shear capacity of curved girders.
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7. Calculation of resistance factors: It is assumed that load factors remain the same as in the AASHTO LRFD specifications.
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The live load model was originally determined during the development of the AASHTO LRFD specifications. For curved girder bridges, the spans are mostly 60 to 150 feet (18 to 45 meters)
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For the maximum 75-year values,the corresponding dynamic load factor (DLF) does not exceed 0.15 of live load for a single truck and 0.10 of live load for two trucks side by side.
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, the 2003 Guide Specifications (9) , and the 2006 Interim AASHTO LRFD specifications that resulted from NCHRP Project 12-52.
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One of the other NCHRP 12-50 "use cases" that was documented in the 12-50 report was the use of similar techniques for the purpose of comparing the results obtained from different design specifications (or proposed changes to an existing set of design specifications) .This particular application of the NCHRP 12-50 methodology fits in well with the curved girder bridge design comparisons outlined herein.
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• Among the existing bridges, the longest span of the bridges in the sample was 190 feet, and the minimum radius was 239 feet. 13 Bridge A Bridge B Bridge C Bridge A Bridge B Bridge C 0.80 5.52 5.31 5.40 5.67 6.04 5.59 0.85 5.10 4.89 4.98 5.26 5.64 5.17 0.90 4.70 4.48 4.58 4.85 5.25 4.77 0.95 4.31 4.08 4.19 4.46 4.87 4.38 1.00 3.93 3.70 3.82 4.08 4.51 4.00 1.05 3.56 3.34 3.46 3.71 4.15 3.64 1.10 3.21 2.98 3.11 3.35 3.80 3.28 1.15 2.87 2.65 2.78 3.00 3.47 2.94 1.20 2.54 2.32 2.46 2.67 3.15 2.61 1.25 2.22 2.01 2.15 2.35 2.83 2.30 1.30 1.92 1.71 1.86 2.03 2.53 1.99 1.35 1.63 1.42 1.58 1.73 2.24 1.70 1.40 1.35 1.14 1.31 1.44 1.96 1.42 1.45 1.08 0.88 1.05 1.17 1.69 1.15 1.50 0.82 0.63 0.80 0.90 1.43 0.89 Reliability index, β Construction Operation Resistance factor, φ Table 3.
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Three-Span Continuous Composite Plate Girder 6 17.0 ft40 ft 90 ft Wyoming DOT (Hard Copy) 5 Bridge Over Tongue River Sheridan County (WY)
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Fore River Bridges, Portland, ME Main Span Unit Between Piers 95 and 135 - Simulated Bridge 60 in Two-Span Continuous Composite Plate Girders 3 4 7 5 . 2 1 f t 63 in 15 TNDOT (Electronic)
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Minnesota Bridge No. 62705 - Simulated Bridge One-Span Composite Plate Girder 4 12.0 ft 9 7 .
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The main differences among the three specifications are as follows: • The maximum transverse stiffener spacing has been progressively increased from D, the depth of the web, in the 1993 Guide Specifications to 3D in the 12-52 recommended specifications. • The 12-52 recommended specifications allow for the consideration of the additional post-buckling strength from tension-field action in the shear critical stress calculations.
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For these bridges, the 12-52 recommended specifications allow the additional post-buckling strength from tension-field action to be considered. This post-buckling strength is also recognized in the 1993 Guide Specifications andAASHTO LRFD specifications straight girder provisions.
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• Hybrid sections are not allowed by the 2003 Guide Specifications, but are allowed by the 1993 Guide Specifications and the 12-52 recommended specifications, which have similar provisions for the consideration of stresses in hybrid sections. The provisions for hybrid sections in the 1993 Guide Specifications consider only yielding of the tension flange in positive bending regions or only yielding of the compression flange in negative bending regions, while the 12-52 19 Specifications Compact Flange Noncompact Flange Slender Flange 1993 Guide Specifications f f t b ≤ 14.31 14.31 < f f t b ≤ 19.68 f f t b > 19.68 2003 Guide Specifications f f t b ≤ 18 18 < f f t b ≤ 23 f f t b > 23 12-52 Recommended Specifications f f t b ≤ 18.3 18.3 < f f t b ≤ 24 f f t b > 24 Table 5.
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From page 20... ...
This post-buckling strength is also recognized in the 1993 Guide Specifications and AASHTO LRFD specifications straight girder provisions and results in a higher critical stress for the majority of the stiffened bridges in the sample. Strength I load case, flexural design: • Flexural analysis, according to all of the specifications, is divided between composite and noncomposite sections, positive and negative flexure, and compression and tension flanges.
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• The 1993 Guide Specifications and the 2003 Guide Specifications consider the lateral bending stress to reduce the critical stress, while the 12-52 recommended specifications consider the lateral bending stress as a load. The final effect of including the lateral bending stress in all of the specifications is to reduce the useable stress limit for gravity loads.
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22 Figure 9. Box-girder bridge example, cross-section.
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