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Experimental and Numerical Investigations of Base Metal Strength in Welded Bracing Connections

Ghaderi Garekani, Majid | 2023

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 56378 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Maleki, Shervin
  7. Abstract:
  8. The strength of welded joints is determined by the lower value of the weld metal and base metal strength. In welded bracing connections, the latter includes the connecting element (i.e., the gusset plate) and the bracing member. According to the American steel design standard AISC 360, the base metal strength is determined based on the limit states of tensile rupture, shear yielding, shear rupture, and block shear. While extensive research has been conducted on the limit state of tensile rupture, limited attention has been given to the limit states of shear yielding, shear rupture, and block shear in welded bracing connections. Furthermore, the design strength equations adopted by AISC 360 for these limit states were initially developed based on research results on bolted joints and later extended to include welded connections. This dissertation investigates the limit states of shear yielding, shear rupture, and block shear in welded bracing connections through experimental and numerical studies. Three independent research programs were conducted, and relevant conclusions were drawn. The first program examines the shear yielding and shear rupture failures in welded channel and angle brace members. To achieve this, a nonlinear finite element model with ductile damage simulation is developed and validated against available test results. An extensive parametric study involving 432 models is then conducted to determine the yield and ultimate failure loads of the members. The results reveal that the code-specified approach to predict the shear strength of members is overly conservative and fails to consider the existing tensile strength contribution of the members. Additionally, the commonly assumed failure patterns are inaccurate. The finite element findings indicate that the shear yielding limit state always occurs prior to shear rupture, with approximately 50% less strength. Moreover, it is observed that the yielding mechanism in the shear yielding limit state is limited to the connection region. The shear failure behavior under compression loading and the effects of connection geometry are also examined. Based on extensive finite element analyses, it is recommended to disregard the shear yielding limit check when designing brace members. Furthermore, a modification is proposed to the conventional shear rupture formula, using a block shear framework that includes tensile areas to provide a more accurate prediction of the member's strength. Applying these recommendations in the seismic design of brace-to-gusset connections leads to more economical connections. The second program is dedicated to assessing the block shear failure in welded gusset plates under combined loading. A review of the relevant literature reveals a lack of attention given to the block shear failure of welded connections, particularly the failure mechanism and strength calculation of this limit state under combined loading. Combined loading refers to simultaneous loading parallel and perpendicular to the weld lines (or an inclined loading) in the plane of the connecting plate. A nonlinear finite element model with ductile damage prediction capability is employed to study the effect of connection geometry and weld group configuration on the block shear strength of welded connections under combined loading. Current design standards assume that the block shear failure planes consist of shear and tension planes, parallel and perpendicular to the applied load, respectively. However, the results indicate that inclined failure planes are possible for combined loading, and the stress state in such planes is a combination of tensile and shear stresses, the extent of which is proportional to the angle of loading. The results are utilized to describe the behavior of block shear rupture planes in such cases. Finally, to provide a better estimation of block shear strength in welded connections subjected to combined loading, a new equation is proposed that considers the actual shape of the displaced block. In the third program, experimental and numerical investigations of block shear failure in gusset plates welded to double angle members are conducted. Double angle braces are extensively used in structures to resist lateral loads such as wind, earthquake, and blast loads. A potential failure mode in the gusset plate, when an all-around weld is used for the angles, is the block shear failure. To date, only limited test data on welded gusset plates failing in this mode have been reported in the literature, and all of them considered only concentrically loaded gusset plates. The test results, including failure loads, fracture sequences, and load-displacement responses, are reported in detail. The results indicate that tensile fracture initiates adjacent to the angle heel weld and then propagates along the tension plane. Due to the existing in-plane load eccentricity, the shear planes do not rupture simultaneously, and the shear plane adjacent to the heel weld fails earlier. Following the testing, a numerical investigation is performed on the tested specimens. Subsequently, a parametric study is conducted to generate additional numerical data across a wide range of gusset and angle dimensions, welding configurations, and connection lengths. The existing design equations for block shear strength specified by the AISC specification are evaluated. It is demonstrated that the AISC specifications provide excessively conservative and scattered failure loads due to the neglect of stress triaxiality and the use of shear yield instead of shear rupture in their strength equations. Additionally, equations specified for welded gusset plates in previous studies are deemed unsafe for general use. To address these shortcomings, an improved block shear strength equation is proposed, which accurately predicts the gusset strength in block shear
  9. Keywords:
  10. Laboratary Study ; Finite Element Method ; Welded Joint ; Base Metal Yielding ; Weld Metal ; Limit State

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