مهندسی عمران شریف (Feb 2019)
DETERMINATION OF TOLERABLE DIFFERENTIAL SETTLEMENTS IN MOMENT-RESISTING STEEL FRAMES BASED ON PERFORMANCE LEVEL OF STRUCTURE
Abstract
Knowledge of allowable differential settlement is necessary for foundation design. Differential settlements may cause additional stresses and strains in structural members. The stresses induced by differential settlements are not conventionally considered in designing superstructure. The differential settlements are generally determined from separate analysis of foundation slab overlaid on soil by applying the structural loads onto it. Then, the calculated settlements are limited to the allowable values. The allowable differential settlements in construction codes are not strongly hooked on the type, design method, and application of structures. The current applied criteria are mainly based on the observations on actual problematic buildings. It is necessary to numerically determine the limiting values of differential settlements which endanger the performance of structures. In this research the allowable settlements of steel moment resisting frames are determined based on seismic performance level. Four moment-resisting steel structures were analyzed and designed according to conventional methods without applying any differential settlements. Then, different patterns of differential settlements (point, axial, and block patterns) were applied to the structures and the structures were analyzed by nonlinear finite-element method. Based on the analysis results, the limiting values of differential settlements at life safety performance level were calculated according to FEMA-356 code. The results of this research revealed that the height of structure considerably affects the allowable differential settlements such that increasing structure height can reduce the allowable differential settlements up to 50\%. The results of this research also revealed that the geometrical shape and properties of building plan affect the amount of allowable differential settlements, such that moment-resisting structures with rectangular plan tolerate higher differential settlement values compared with the structures with square plan shape. According to the results of the present study the allowable angular distortion at life safety performance level of moment-resisting steel frame structures is suggested to be limited to 1/94 in the buildings with maximum five stories, and 1/200 in five to ten-story buildings. The results indicate that the height of structure should be taken into consideration in the allowable differential settlement criteria.
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