مدلسازي ماكرو رفتار پس پانچ دال هاي تخت در خرابي پيش رونده

در اين مقاله، روش هاي تحليلي مدلسازي ماكرو اتصالات دال- ستون براي شبيه سازي رفتار پس پانچ آنها ارايه شده است. اين روش هاي مدلسازي مي تواند براي ارزيابي خرابي پيش رونده ساختمان هاي دال تخت به كار رود. در سيستم هاي دال تخت، عمده آسيب هاي سازه اي در اتصالات دال- ستون متمركز است. در مدل پيشنهادي از المان هاي پوسته براي شبيه سازي دال و المان هاي تير اتصال دهنده براي شبيه سازي شكست پانچ استفاده شده است. براي مدلسازي انتقال نيروهاي داخلي بين دال و ستون و همچنين بررسي رفتار اتصالات، شش منحني نيرو- تغييرشكل با رفتارهاي خطي و غيرخطي براي شبيه سازي خمش، برش، پيچش و نيروي محوري تيرهاي اتصال دهنده معرفي شده است. دو روش مدلسازي ماكرو نيز براي شبيه سازي رفتار پس پانچ ارائه شده است كه شامل مدلسازي با ميلگرد و مدلسازي با المان اتصال مي باشد. براي شبيه سازي رفتار پس پانچ، يك منحني دوخطي نيرو- تغييرشكل به المان اتصال اختصاص يافته است. ميلگرد و يا المان اتصال در مجاورت المان تير اتصال دهنده قرار مي گيرد و پس از شكست پانچ فعال مي شود. برخي پارامترهاي به كار رفته در معرفي اين منحني ها از طريق كاليبره كردن با نتايج آزمايش به دست آمده است. روش هاي پيشنهادي توسط نمونه هاي آزمايشگاهي اعتبارسنجي شده است. نتايج نشان مي دهد كه هر دو روش مدلسازي رفتار پس پانچ، دقت قابل قبولي را در پيش بيني مقاومت پس پانچ ، سختي پس پانچ و ظرفيت تغييرشكل دارند.

Reinforced concrete flat slabs are simply a plate of uniform thickness placed on columns without the help of beams or capitals or drop panels. Due to the direct transfer of slab loads to the supporting column, the column tends to punch through the slab. Flat slabs without shear reinforcements often have a shear failure with very little ductility and no sign of warning. Most studies of flat plate performance were attended to punching shear failure, and very little research was conducted on the flat plate behavior after punching failure and its subsequent progressive failure. Consequently, the literature on the behavioral characteristics of flat slabs following punching failure is very restricted. Over the past years, researchers have proposed different models of grid model and shell element model for 3D modeling of flat slabs. In the grid model, the slab is simulated by a grid of beam elements, Because the load-bearing process in the slabs is somewhat similar to the load-bearing process in the beams. This method can be used to analyze the progressive collapse but requires much effort in modeling the slabs. The use of multilayer shell element for modeling slabs can be used with less effort and higher accuracy. In the present study, two improved methods of macro modeling were proposed to predict the post-punching behavior of the slab-column connections. These modeling techniques can be used to analyze the progressive collapse of reinforced concrete flat slab buildings. Liu et al. (2015) proposed a macro model to analyze the progressive collapse of flat plate buildings. In this macro model, the slab-column joint region is simulated by the inflexible shell element. The critical section of the punching shear around the joint region is considered at distance half slab effective depth from the edge of the column. To simulate the slab away from the punch environment, a multilayered shell element consisting of concrete and rebar with nonlinear material properties is used. The junction area between the critical punch section and the edge of the column is modeled with two beam elements for each column face. Then flexural, shear, torsional, and axial behaviors are defined with six degrees of freedom for the connector beam elements. This model can be used to evaluate the potential for progressive collapse of flat slab buildings, but this model ignores the post-punching resistance of flat slabs. The post-punching resistance of flat slabs without transverse reinforcement, without taking into account the interaction of aggregates, the sum of the shear transfers through tensile reinforcements and integrity reinforcements. In the present study, the model presented by Liu et al. (2015) was modified to assess the post-punching response of the slab-column connections. In the proposed model, constant residual shear strength is assumed after the punching shear failure for the connector beam element to consider to the post-punching shear transfer through the flexural reinforcements. The remaining shear strength (𝑉􀯋,􀯣􀯣,􀬴) improves by increasing the diameter of the integrity reinforcement. The remaining shear strength for integrity bars with diameters of 8 to 14 mm is recommended about 30% to 40% of the punching strength, respectively.