Assessment of Progressive Collapse Behaviour of Moment Frames Strengthened with Knee Elements

Knee element can be used in junction of beam and column elements for strengthening steel moment frames against progressive collapse. Two different types of knee elements, namely strong and weak knee elements have been used to strengthen the frames. Strong knee element has been designed in a way that it prevents the occurrence of plastic hinge on knee element before beam element. But in weak knee elements, formation of plastic hinge on knee element will be prior to formation of the plastic hinge on the beam element. The nonlinear static alternate path analysis in accordance with the UFC guideline has been used to determine the resistance of structures against progressive collapse. The results indicate the priority of strong knee elements compared to weak knee elements in strengthening the structure against progressive collapse. Installing strong knee element reduces the effective length of beams and columns and changes the direction of flow of forces from the beam–column connection to out of the knees area and changes the position for the formation of plastic hinges. Eventually, the installation of strong knee elements will increase stiffness and structural strength and result in better behaviour against progressive collapse.