USING ULTRA-HIGH PERFORMANCE CONCRETE FOR ELIMINATING SOFT AND WEAK STORY IRREGULARITIES IN SPECIAL REINFORCED CONCRETE MOMENT RESISTING FRAMED STRUCTURES
Mohammad Sharawneh, Ali
An Najah National University
Multistory buildings are common in Palestine and sometimes are built with a ground floor that has an open space or is higher than the rest floors for commercial purposes. Therefore, the soft and weak story irregularities may occur on the ground floor. This research aims to eliminate the soft or weak story irregularities in the design stage without affecting the architectural requirements by changing the columns material from normal strength concrete (NSC) to ultra-high performance concrete (UHPC) in the soft or weak story. To quantify the effect of the material switch from NSC to UHPC on the column strength and stiffness, a parametric study using sectional stress analysis is performed. Overall, 216 NSC and UHPC columns cross-sections are analyzed under the following parameters: axial load levels, longitudinal reinforcement ratio, cross-section width, and cross-section depth to width ratio. The effectiveness of using UHPC on the column stiffness is studied where the change in the flexural rigidity is represented using the ratio of the effective flexural rigidity (EIe) of UHPC columns to NSC columns. Also, the validity of cracking analysis modifiers of NSC columns is established for UHPC columns. The effectiveness of using UHPC on the column strength is investigated using the ratio of the moment capacity of the UHPC columns to NSC columns. After that, the adequacy of the column shear capacity is checked, and found that the lateral strength of the UHPC column is still controlled by the moment strength. Moreover, regression analysis is performed for the parametric study results to create equations that predict the increase in the sectional stiffness and strength of the columns. Finally, a 3D sway special moment-resisting framed building is used as a case study to confirm the sectional analysis results and to investigate the frame's overall behavior before and after using UHPC in the soft/weak story columns. The frame is designed according to ASCE7-16 and ACI318-19 and has both an extremely soft story and weak story irregularities on the ground floor. Nonlinear static analysis (pushover) is performed for the frame using SAP2000. The frame analysis results agree well with the parametric study results. In addition, the overall behavior of the frame is enhanced when the UHPC is used since the displacement and the plastic hinges do not concentrate on the soft/weak story. In summary, switching the columns material in the soft/weak story from NSC to UHPC can be safely used to eliminate the soft/weak stories irregularities at the design stage without changing the architectural or functional restriction.
vertical irregularity, stiffness, strength, soft story , weak story, flexural moment rigidity, XTRACT, cracking analysis modifiers , ultra-high performance concrete (UHPC), plastic hinge, nonlinear static (pushover).