مقاوم‌سازي قاب‌هاي خمشي فولادي با استفاده از مهاربندي كابلي

RETROFITTING OF FLEXURAL STEEL FRAMES USING CABLE BRACING

در اين نوشتار، رفتار قاب‌هاي خمشي فولادي مقاوم‌سازي‌شده با مهاربند بررسي شده است. مهاربندهاي ضربدري با نيمرخ نبشي، مهاربند ضربدري با كابل و مهاربند با دو كابل عبورداده‌شده از يك غلاف فولادي استوانه‌يي در محل برخورد كابل‌ها، كه سه نوع مهاربندي موردبررسي براي مقاوم‌سازي قاب‌هاي خمشي فولادي را تشكيل مي‌دهند. مدل‌هاي المان محدود قاب خمشي تنها و سه قاب خمشي مقاوم‌شده ايجاد شده است. تحليل غيرخطي قاب‌ها تحت اثر بارگذاري چرخه‌يي با دامنه‌هاي افزايش‌يابنده انجام شده است. مقايسه‌ي نتايج تحليل المان محدود با داده‌هاي آزمايشگاهي، دقت مناسب مدل‌هاي المان محدود را نشان مي‌دهد. با تعيين برش پايه، نيروهاي محوري ايجادشده در ستون‌ها، رفتار چرخه‌‌يي نيرو‌ي جابجايي و مكانيزم خرابي قاب‌ها، مزايا و معايب هر كدام از شيوه‌هاي مقاوم‌سازي بررسي شده است. نتايج نشان داده است كه استفاده از نيمرخ نبشي و كابل براي مقاوم‌سازي قاب، سختي اوليه و بار نهايي قاب را درمقايسه‌ با قاب خمشي به صورت قابل‌توجهي افزايش و شكل‌پذيري آن را كاهش مي‌دهد. در اين روش به‌دليل افزايش قابل‌توجه نيروي محوري ستون‌ها، نياز به تقويت ستون‌ها و پي بيشتر است. در قاب با مهاربند كابلي غلافدار، سختي اوليه با قاب خمشي يكسان بوده و قاب رفتار شكل‌پذير از خود نشان داده است. در اين حالت افزايش نيروي محوري ستون درمقايسه با ساير روش‌هاي بادبندي كمتر بوده و تقويت لازم ستون‌ها و پي كمتر از ساير روش‌هاي مقاوم‌سازي است.

In this paper, the behavior of flexural steel frames using cable bracing is investigated. X bracing with angle profiles, X bracing with a cable, and bracing with two cables passing cylindrical steel sheaths at the intersection of the cables, are the three types of retrofitting of flexural steel frames studied in this paper. The finite element model of a steel frame, with and without the three types of retrofitting has been made. Non-linear analysis of frames under cyclic loading with increasing oscillations has been undertaken. Comparison of the results from the finite element model with experimental data shows that the finite element model has acceptable accuracy. By determining shear base, the axial force of columns, and the cyclic behavior of force-displacement, the failure mechanism of frames and the advantages and disadvantages of each type of retrofitting have been investigated. According to results, using the Cornerstone profile for retrofitting steel flexural frames increases remarkably the initial stiffness and ultimate load of the frame compared to flexural frame and reduces ultimate displacement. Reduction in plasticity and frame energy absorption capacity shows the unacceptability of this type of retrofitting, from the perspective of plasticity and energy depreciation capacity. In the frames retrofitted by a cornerstone, because of the bracing function and a significant increase in the axial force of columns, the need to reinforce the columns and foundation is greater. Due to buckling of the compressive member in this type of retrofit, permanent deformation occurs in the frame. In retrofitted frame using cross cables, cables from the early stages of loading begin to pull, which increases the frame’s initial stiffness compared to the stiffness of flexural frames. In this case, ultimate load increases and failure displacement reduces. In this frame, the cable brace converts frame behavior from ductile to brittle. Being narrow and unstable in hysteresis cycles shows the meager capacity of this frame to resist lateral forces. In this method of retrofitting, the required reinforcement of columns and foundation is considerable. The initial stiffness of the frame with a cable brace with sheath and the flexural frame is the same. Up to the fixed amount of displacement and the brace being straight, the cable is ineffective in flexural frame behavior, and, after that, if lateral forces invade lateral forces, the brace contributes with a delay in frame behavior. By adding the brace to these frames while maintaining the plasticity of the frame, its resistance will increase. In this case, the increase in the axial force of the column is less than other methods of bracing, and the existing possible overload in columns and foundation can be responsive to the increase in axial force in columns, or, if retrofitting is needed, the amount of reinforcement is slight.