Guidelines for the Load and Resistance Factor Design and Rating of Riveted and Bolted Gusset-Plate Connections for Steel Bridges (2013) / Chapter Skim
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5 CHAPTER 1. BACKGROUND The collapse of the I-35W Bridge has focused significant attention on the reliability and safety of truss bridges in the U.S.
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6 when analyzed with respect to the FHWA Guide. This is not surprising considering that the original bridge designers had considerable discretion and probably did not follow or know of the exact procedures outlined in the FHWA Guide.
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7 investigation will be discussed as they pertain to the limit-states examined. The discussion will also reflect results from a literature review of gusset plate research.
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8 section from one member to overlap another, raising the question of how to handle interaction effects between members. There is no evidence to suggest a problem exists due to the Whitmore method providing unconservative results, but additional verification of results from a wide range of geometries would be valuable.
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9 Figure 1. U10 gusset plate from the I-35W Bridge showing the plane for plotting stress distribution (A-A)
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10 Gusset Plates in Shear The fundamental shear resistance of gusset plates is (BDS Section 6.13.5.3)
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11 peak shear stress exceeds that predicted by the uniform shear distribution and is slightly less than that predicted by the parabolic shear model. It may be fortuitous, but the AASHTO flexural shear provisions (Ω = 0.74)
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12 = 1.0, but will be less than 1.0 when = 0.74. Therefore, the method used to calculate shear resistance can have a substantial impact on evaluating bridge safety and determining the need for strengthening retrofits.
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13 Clearly an uncomplicated and consistent method of checking stability, even if the results are conservative, is a needed addition to the specifications. Initially, to address stability, the model that best predicts buckling in gusset plates needs to be determined.
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14 behavior observed by Brown.(13) The FHWA model also shows that adding initial geometric imperfections along the gusset plate edge produces a significant reduction in the critical buckling load.
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15 deterioration mechanisms are corrosion and fatigue. Of the two, corrosion is clearly more important from a strength perspective.
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16 to the gusset. While effective, this is typically a very costly and time consuming operation and is best avoided unless absolutely necessary.
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17 a) Original I-35W Bridge over Mississippi River (MN)
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18 c) HW-23 Bridge over the Mississippi River (MN)
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19 in Figure 8) in the I-80 Bridge over the Clarion River in Pennsylvania.
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21 In general, the load versus displacement curve from the analysis has a loading and a postpeak unloading path. The peak ALF value from this plot is another important limit for the joint response.
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22 Figure 13 shows the shear and normal stresses occurring along the B-B plane. The B-B plane stresses are harder to interpret because of the out-of plane bending effects around the compression diagonal.
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23 Figure 10. Von Mises stress contour normalized to the yield stress at the peak load for I-35W U10 connection.
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24 Distance From Left Edge of Gusset (inch) 0 10 20 30 40 50 60 70 80 90 100 110 N or m al iz ed S tre ss -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Shear (fv/vy0.58Fy)
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25 Figure 13. Section B-B mid-thickness shear and normal stresses at failure.
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26 Figure 14. Plot of out-of-plane displacement motion of Point 6 versus the ALF.
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27 Figure 16. Equivalent plastic strain response contours at the limit load (ALF=9.72)
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28 Figure 17. Plot of out-of-plane displacement motion of Point 8 versus the ALF.
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29 Figure 19. Equivalent plastic strain response contours at the limit load (ALF=2.70)
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30 Figure 20. Plot of out-of-plane displacement motion of Point 7 versus the ALF.
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31 I-80 Bridge (Joint L3) Important load levels on the ALF versus out-of-plane displacement plot are shown in Figure 23.
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32 Figure 24. Response contours at Strength-1 level (ALF=1.03)
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33 Table 1 shows the resulting LRFR rating factors for each of the five connections. The I-94 connections used high-strength A325 bolts and therefore the slip capacity of the fasteners was checked, but not on the other joints which used rivets.
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34 there may be the possibility that the two checks are redundant. The sensitivity of the Whitmore section crossing over multiple members should also be evaluated.
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35 Table 1 Rating Factors for Five Representative Joints Type of Check I-80 I-40 I-94 HW-23 I-35W TENSION CHORD Whitmore Section 2.60 3.71 9.75 N/A 5.62 Block Shear 2.30 2.96 9.92 N/A 5.63 Fastener Shear Capacity 2.75 4.73 9.67 N/A 3.38 Fastener Slip Capacity N/A N/A 4.28 N/A N/A COMPRESSION CHORD Whitmore Buckling N/A N/A N/A 3.27 9.43 Fastener Shear Capacity N/A N/A N/A 4.38 7.58 TENSION DIAGONAL Whitmore Section 2.86 6.93 14.87 4.41 2.25 Block Shear 2.66 3.82 6.17 3.58 1.38 Fastener Shear Capacity 2.31 2.32 9.42 2.66 2.54 Fastener Slip Capacity N/A N/A 4.30 N/A N/A COMPRESSION DIAGONAL Whitmore Buckling 3.88 5.83 8.84 2.98 0.29 Edge Slenderness(a)
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36 Table 2 Professional Factors for Five Connections, (RFEA/Rnominal) Type of Check I-80 I-40 I-94 HW-23 I-35W TENSION CHORD Whitmore Section 0.91 0.95 0.91 N/A 0.26 Block Shear 0.82 1.01 0.88 N/A 0.26 Fastener Shear Capacity 0.77 0.91 1.34 N/A 0.46 Fastener Slip Capacity N/A N/A 3.10 N/A N/A COMPRESSION CHORD Whitmore Buckling N/A N/A N/A 0.37 0.06 Fastener Shear Capacity N/A N/A N/A 0.63 0.09 TENSION DIAGONAL Whitmore Section 0.94 0.69 2.09 0.73 0.66 Block Shear 0.83 0.90 3.42 0.74 0.66 Fastener Shear Capacity 1.14 1.51 1.10 1.21 0.67 Fastener Slip Capacity N/A N/A 4.11 N/A N/A COMPRESSION DIAGONAL Whitmore Buckling 0.63 0.85 3.63 1.29 1.00 Edge Slenderness Pass Pass Pass Fail Fail Fastener Shear Capacity 0.74 1.18(a)
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