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Numerical Investigation of Base Metal Fracture in Square HSS Brace Members

Morshedi, Hossein | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58562 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Maleki, Shervin
  7. Abstract:
  8. Hollow Structural Sections (HSS) are widely used as bracing members in braced frames, trusses, and bridges because of structural advantages such as low weight, large radius of gyration, high compressive strength, and high torsional stiffness, as well as architectural and aesthetic benefits. The typical connection of HSS braces to a gusset plate is made by cutting a longitudinal slot in the brace, inserting the gusset plate into the slot, and welding four fillet welds along the length of the slot. this detail commonly referred to as a “knife” connection. This study numerically investigates the base-metal failure limit state of square HSS brace members made from S235 steel with welded knife connections to gusset plates, and proposes an appropriate design criterion for the brace base metal while addressing current ambiguities in practice and codes. Eleven seismically compact HSS sections of varying widths and thicknesses were selected to represent the practical range of brace sections used in Special Concentrically Braced Frames (SCBFs). Brace-to-gusset and gusset plate were designed according to AISC 341 seismic provisions for special concentrically braced frames (SCBFs) for the expected tensile yield strength of the brace section (Py). Section J4 of AISC 360 introduces the shear yield (SY) and shear rupture (SR) limit states to determine the base metal design strength of the brace member. While these limit states provide two different strength levels in welded connections. AISC SDM design examples, contrary to the recommendation of AISC 360, calculate brace base-metal strength based on SR only and ignoring the SY criterion. AISC 360 applies a resistance factor (φ) of 0.75 to SR limit state, whereas part 10 of the Iranian National Building Code, contrary to AISC, permits increasing φ from 0.75 to 0.90 for non-ductile limit states such as SR under a capacity-design philosophy. This discrepancy has led to uncertainty among designers and among codes regarding appropriate criterion for the design of the brace base metal and appropriate φ for SR limit state. In this study, for each HSS section three optimized designs were produced—based on SY, SR with φ = 0.75, and SR with φ = 0.90—resulting in 33 specimens. Nonlinear 3D finite-element models incorporating ductile damage model for metals were developed in Abaqus, and model predictions were validated against several existing experimental studies. Specimen performance under monotonic and cyclic loading was evaluated: force–displacement responses, fracture sequence and failure mode of the brace base metal, and stress and plastic strain distributions were examined. Reliability analysis using the First-Order Second-Moment (FOSM) method was also performed. Results indicate that specimens designed based on SR limit state with φ = 0.75 achieve the target reliability index and design of the base metal based on the SY criterion, that provides less resistance to shear rupture, is conservative and leading to increased connection length, larger gusset plates, and uneconomical design. The results also showed that using φ=0.9 instead of φ=0.75 for SR limit state, which is allowed in part 10 of the Iranian National Building Code, contrary to AISC, It is relatively acceptable but requires further investigation and studies. Finally, it is recommended that the base metal design strength of the brace members determined based on the SR limit state with φ=0.75 and ignoring the shear yield criterion
  9. Keywords:
  10. Shear Rupture ; Shear Yielding ; Finite Element Analysis ; Base Metal Strength ; Hollow Stractural Sections (HSS) ; Special Concentrically Braced Fram System (SCBFS)

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