برآورد ضريب مقاومت افزون سازه‌هاي مسلح به ديوارهاي برشي فولادي

Estimation of Overstrength Factor for Steel Plate

ديوارهاي برشي فولادي از جمله سيستم‌هاي باربر جانبي با دو خصوصيت دلخواه سختي جانبي همراه شكل‌پذيري بسيار بالا است. با وجود داده‌هاي تحقيقاتي با ارزش درباره ديوارهاي برشي فولادي، اطلاعات مربوط به طراحي لرزه‌اي اين سيستم‌ها در بيشتر آيين‌نامه‌هاي لرزه‌اي (همچون استاندارد 2800 ايران) نيست و يا بسيار محدود است. از جمله پارامترهاي عملكرد لرزه‌اي اين سازه‌ها ضريب مقاومت افزون است. بيشتر مطالعات قبلي روي اين ضريب، آزمايشگاهي بوده و محدود به قاب‌هاي كوتاه‌مرتبه و يا با مقياس‌هاي كوچك آزمايشگاهي است. هدف اين مقاله، ارايه فرمولي بسته براي تعيين ضريب مقاومت افزون ديوار‌هاي برشي فولادي براي قاب‌ها با ابعاد و ضخامت‌هاي ورق مختلف و با فرض اندركنش قاب و ورق است. نتايج آزمايشگاهي ديگر محققين و همچنين تحليل‌هاي اجزا محدود در اين پژوهش، مويد درستي نتايج استفاده از فرمول بسته پيشنهادي است.

Steel plate shear walls are very effective lateral load resisting systems with high lateral stiffness and ductility capacity. Although there are valuable experimental data available for such systems, most of the current seismic codes (including Iran’s Std. 2800) provide none or limited design provisions for such structural systems. One of the important seismic performance parameters of the structures is “over-strength factor” which is implicitly or explicitly part of seismic design base shear formulation. Most of the available data on this factor are obtained from experimental research and therefore results are limited to low-rise structures and/or with reduced scaled structures. The main objective in this research is to assess the over-strength factor for the steel shear walls. A closed-form-solution is proposed for obtaining this factor based on plate-frame interaction. It was found that the over-strength factors obtained by this method are in line with available experimental data and analytical results obtained in this study. In derivation of this closed form solution the following assumptions are made: 1) beam-to-column connection is rigid, 2) SPSW behaviour is affected by the interaction of the steel plate and its surrounding frame members, 3) the first storey, usually the soft storey, controls ductility and strength of the structure in general, 4) the beam remains rigid and the plastic hinges only form in the columns, 5) stresses due to the bending action in the plate do not interfere with the stresses due to buckling in the plate, 6) stress-strain relationship of the steel plate and the steel frame members are elastic-perfectly plastic. The experiments on SPSWs used in this study were conducted in four major universities three of which in Canada and USA. The analytical model used in this study for finite element verification of the results is a three storey building that is adopted from a previous study on SPSW structures which was based on SAC buildings. It was found that the overstrength factor of a multistorey SPSW structure can be estimated by that of only the first storey of that. Also the results for the overstrength factor are not that sensitive to the angle of tension field in the plate. Moreover, it was found that this factor decreases with a decrease of plate thickness and the yield stress of the plate.