شبيه سازي عددي عملكرد زنجيري اعضاي بتن‌آرمه با ميلگردهاي فولادي و هيبريدي (تركيب فولادي و FRP) تحت اثر خرابي پيشرونده

در اين مقاله با توجه به نقش عملكرد زنجيري اعضاي سازه­اي در انتقال بارهاي اضافي ناشي از حذف ستون باربر مياني در تغييرشكل­‌هاي بزرگ براي جلوگيري از وقوع خرابي پيشرونده، روش عددي المان محدود (FEM) سه بعدي (3D) براي شبيه‌سازي اين رفتار معرفي مي­‌گردد. اين روش در عين سادگي، براي پيش‌­بيني پاسخ عناصر بتن‌آرمه­ تا مرحله خرابي نهايي بسيار قابل اعتماد مي­‌باشد. براي صحت­‌سنجي مدل عددي از دو سري نمونه كه توسط محقيقين قبلي به روش تجربي آزمايش شده­‌اند، استفاده گرديده است. سري اول، زيرمجموعه‌­هاي تير-ستون با سناريو حذف ستون مياني، براي مقايسه و نمايش قابليت مدل عددي در پيش­‌بيني سه سطح از عملكرد شامل مرحله خمشي، قوس فشاري و زنجيري و سري دوم شامل تيرهاي بتني مسلح­‌شده با تركيب آرماتورهاي فولادي و ميلگرد­هاي از جنس الياف پليمري تقويت شده (FRP) مي­‌باشد. مقايسه نمودار­هاي بار-تغييرمكان حاصل از داده­‌هاي تجربي و نتايج عددي براي هر دو سري نمونه، نشان از دقت بالاي روش شبيه‌­سازي پيشنهادي دارد. در ادامه تحقيق، بر اساس مدل­‌هاي عددي صحت­‌سنجي شده، به مطالعه تأثير ميلگردهاي FRP در تركيب با آرماتورهاي فولادي بر عملكرد عناصر بتني تحت خرابي پيش­رونده پرداخته و تأثير خصوصيات مكانيكي و آرايش مختلف آنها بر مقاومت و شكل­پذيري و رفتار زنجيري بررسي مي شود.

Progressive collapse is defined as the spread of an initial local failure from element to element, resulting eventually in the collapse of an entire structure or a disproportionately large part of it. When a column of structure is suddenly removed by an explosion or other effects, axial compressive forces in columns above the removed column will be redistributed to adjacent frames quickly. This transfer is generally possible through the system of floor and beam connected at the top of the removed column. The beam above a removed column undergoes three structural mechanisms: flexural action (FA), compressive arch action (CAA) and catenary action. Initially, all beams mobilize flexural action. when a column is removed, the span of the beam increases and in most cases leads to large deflection occurring in the beam. Compressive arch action, which enhances the flexural strength at critical sections, can be mobilized in the presence of axial compression provided by stiff lateral restraints. At large deflections, catenary action can be mobilized. In this stage, both tensile and compressive rebars are subjected to tension and the beams start to act like suspended cables. In this paper, a simple and reliable three-dimensional (3D) Finite Element (FE) model was defined. This proposed numerical model can be used with high accuracy to predict the response of reinforced concrete elements to the final failure stage and catenary action. It can also be used to predict hybrid reinforced concrete elements with both steel and Fiber Reinforced Polymer (FRP) reinforcements. Concrete damaged plasticity model (CDP) available in ABAQUS Finite Element Code was used for defining concrete behavior in plastic range. This model is based on the Lubliner studies and modifications made by Lee and Fenves. Tensile cracking and compressive crushing are two main mechanisms of the concrete failure in CDP model. Two series of experimental specimens tested by other researchers are used for verification of the numerical model. The first series consists of four RC beam-column subassemblages with the middle column removal scenario for comparing and displaying numerical modeling capabilities in the prediction of three-phase behavior mentioned above, and the second series includes concrete beams with hybrid FRP-steel reinforcements. The comparisons between the load-displacement diagrams obtained from the experimental responses and the numerical results of both series as well as the failure mechanism of the samples indicates the high accuracy of the proposed simulation method. In the following of research, based on validated numerical models, the effect of Glass Fiber Reinforced Polymer (GFRP) and Carbon Fiber Reinforced Polymer (CFRP) bars in combination with steel reinforcement on the performance of beam-column subassemblages under progressive collapse and the effect of mechanical properties of FRP bars and their arrangement in concrete beams section on the strength and ductility and catenary action are investigated. It can be seen that the use of hybrid reinforcement in the different elevation of the beam section greatly increases the catenary resistance of the specimens.