Presentation on theme: "I. Truss Bridge Gussets Background"— Presentation transcript:
1 I. Truss Bridge Gussets Background Truss Gusset Plates and Connections of Truss Members to the Gusset Plates are Usually Stronger than the Truss Members to which they are Connected.Load Ratings of Trusses Have not Usually Included a Check of the Gusset Plate CapacityAfter I-35W Bridge over the Mississippi River in Minneapolis, Minnesota Collapsed in 2007, NTSB Decided on Under-designed Gusset Plate
6 2008 FHWA TECHNICAL ADVISORY (T 5140.29) RECOMMENDATIONS Load ratings required according to the AASHTO Manual for Condition Evaluation of Bridges. The following supplemental actions are recommended:New or replaced truss bridges – check gusset plate capacity as part of the initial load rating.Existing truss bridges – check gusset plate capacity when load rating after condition or load changes of the structure.Review previous load rating calculations for truss bridges that have undergone significant changes is stress levels previously in their service life.
7 Gusset Plate Rating The best available knowledge… FHWA compiles “Load Rating Guidance and Examples For Bolted and Riveted Gusset Plates in Truss Bridges”
8 CAUSES of DEAD LOAD INCREASE Increased Deck ThicknessDeck Overlay IncreaseBridge Deck WideningWidening Roadway Width (Curb to Curb, Rail to Rail)Addition of Sidewalk OverhangAddition of Major UtilitiesAddition of Concrete
9 CAUSES of LIVE LOAD INCREASE Current Live Loading > Design Live LoadingIncrease in Number of Lanes Since Original ConstructionIncrease in Deck Roadway Width (Increased LL Distribution to Trusses)Known to be in an Area of Increased Overweight Loads
10 THE RESISTANCE OF FASTENERS Fasteners Shear FailureBolt: AASHTO LRFD ArticlesRivet: φR=φF x m x ArPlate Bearing Failure(AASHTO LRFD Articles)Bolt: AASHTO LRFD Articles and shall apply for determining the resistance of bolts to prevent bolt shear and plate bearing failures.Rivet: φR=φF x m x ArφFConstructed prior to 1936 or of unknown origin 18Constructed after 1936 but of unknown origin 21ASTM A 502 Grade IASTM A 502 Grade II 32
11 THE RESISTANCE OF GUSSET PLATES The resistance of a gusset plate shall be determined as its least resistance inTension including block shearShearCombined flexural and axial loadsCompression.
12 1. GUSSET PLATES UNDER AXIAL TENSION Gusset plates subjected to axial tension shall be investigated for three conditions (Whitmore section is adopted):Allowable Tensile Force on Gross Section (Yielding Resistance)Allowable Tensile Force on Net Section (Fracture Resistance)Block shear rupture
15 2. GUSSET PLATES IN SHEAR Gross Shear Yield Sections • Ω = 1.00 when the gusset plates are of ample stiffness to prevent buckling and develop the plastic shear force of the plates, or• Ω = 0.74 in the absence of a more rigorous analysis or criterion to assure and quantify the stiffness requirements to develop the plastic shear force of the plates.
16 GUSSET PLATES IN SHEARNet Shear Fracture Sections
17 3. GUSSET PLATES IN COMPRESSION Effective Length Factor, K, varied from 0.65 to In most cases, K=1.0 (Pinned-Pinned)
18 4. GUSSET PLATES UNDER COMBINED FLEXURAL AND AXIAL LOADS FHWA Guide: The Maximum Elastic Stress may be Taken as φf FyGusset plates behave as deep members. Therefore, the application of flexural theory to the analysis of gusset plates is questionable and not required in this Guidance.