Experimental and Analytical Study of Connection Strength Effects in X-Type Braced Frames

SCBF (Special Concentrically Braced Frame) is a common structural system for steel construction in Iran. This system could be used where increasing of strength and stiffness is the main concern for a structural system over the ductility. In SCBF, it is assumed that most of stiffness and strength are provided by the brace element. The contribution of gusset plates in total stiffness and strength values is even overlooked in the structural analysis and design. Ignoring these effects is conservative for controlling of seismic responses and is on the safe side. But, in practice, this hypothesis is not valid since this extra portion of stiffness and strength values is not included in the design of gusset plates and the connections. The importance of this phenomenon will be more noticeable when the brace element material behavior significantly enters the nonlinear phase. Based on previous descriptions, experimental and analytical study of this kind of structural system under seismic loading seems necessary in order to evaluate the contribution of stiffness and strength of each mentioned element i.e. gusset plates and double- seat angle connections. Three test specimens were built in half scale and designed regarding the AISC seismic provisions (2005); the first test specimen consisted of one-by-one story frame with simple connection, i.e. double seat angle connections, known as simple frames. The second specimen was simple frame with gusset plate connections without brace elements. The third specimen was braced frame; the specimens were tested cyclically and companion Finite Element (FE) analysis was then conducted. Verification of FE models in ANSYS finite element program using “Solid 45” element with proper mesh dimensions, obtained by sensitivity analysis, showed good agreement between the analytical and experimental responses. The experimental results showed that the stiffness value of simple frame and simple frame with gusset plates reached 60% to 65% and 86% to 98% of stiffness values of the braced frame, respectively. Also energy absorption values of simple frame and simple frame with gusset plate reached 4 % and 10% of the energy absorption value of braced frame, respectively. Moreover, ultimate strength value for simple frame and simple frame with gusset plate reached 11% and 25% of the ultimate strength values of braced frame. As a result, it could be mentioned that gusset plates and double seat angle connections have significant contribution in the total stiffness values of a braced frame whereas their contributions in strength value of a braced frame are insignificant. The results of FE analysis also confirmed the above-mentioned results with a definite low tolerance.