بررسي مشكلات تحليلي دال‏هاي پس‏كشيده‌ي بتني به كمك تحليل اجزاي محدود

INVESTIGATION OF ANALYTICAL PROBLEMS OF POST-TENSIONED CONCRETE SLABS USING FINITE ELEMENT METHOD

طراحي ايمن و اقتصادي همواره مورد توجه محققان قرار گرفته است و به‌تبع آن آيين‏نامه‏هاي مختلف در زمينه‏هاي گوناگون ايجاد شده‌اند. در اين ميان، آيين‏نامه‏‌ي 2005-318ACI تاكيد بر قرارگيري دال‏هاي بتني پس‏كشيده در ناحيه‌ي ترك‌نخورده دارد (بند 18.3.3)، به‌گونه‌يي كه مقطع كاملاً در حالت انتقال و بهره‏برداري بدون ترك باشد. همچنين دال پس‏كشيده‌ي دوطرفه بايد در هر دو راستا، براي 100% بار پس‏‌كشيده شود و اين به‌معناي يك طرفه درنظرگرفتن رفتار اين‌گونه دال‏هاست. در اين نوشتار، براي بررسي موارد مذكور، دال‏هاي پس‏كشيده با دهانه‏هاي مختلف تحت شدت بارهاي گوناگون قرار گرفته و طراحي شده‏اند. سپس دال طراحي‌شده با استفاده از مدل آسيب خميري همسان‌گرد لي و فنوينيز براي بتن و روش اجزاي محدود شبيه‏سازي عددي مي‏شود. نتايج به‌دست‌آمده از شبيه‏سازي عددي حاكي از عدم برآورده‌شدن بندهايي از آيين‏نامه‌ي 2005-318 ACI از جمله ترك‌خوردن دال در حالت انتقال و بهره‏برداري است. همچنين پس‏تنيده‌كردن زياد سبب ايجاد خيز منفي مي‌شود كه باعث غيراقتصادي‌شدن طرح مي‌شود.

Researchers have always paid great attention to safe and economical design and, therefore, different codes in different areas have been formed. The ACI 318-2005 code insists that two-way post-tensioned slabs shall be classified as class U (without cracking), which means the section must be uncracked in both service and transient conditions (code reference 18.3.3). In addition, two-way post-tensioned slabs must be post-tensioned in both directions for 100% of the load, and this means that we assumed one-way behavior for this type of slab. In this study, two-way post-tensioned slabs with various spans under different loadings have been designed, based on the ACI code. Then, using the Lee-Fenves isotropic damage plasticity model for concrete, the numerical simulation of post-tensioned slabs was simulated through the finite element method. The isotropic damage plasticity used a yield criterion that matches the experimental data for both elastic and plastic parts of the stress-strain curves of concrete. The onset and propagation of cracking can be evaluated by simple postprocessing of the finite element plasticity solution. The accuracy of the model was checked by previous researchers and the simulation was precise. The linear elastic-perfectly plastic stress strain relation was used for steel using the Von-mises yield criterion, while the tendons were simulated by the bilinear stress strain curve and the Von-mises yield criterion. By using nonlinear finite element software (Abaqus) and the mentioned yield criterion, several slabs are simulated. Numerical simulation shows that some laws of the ACI code are not satisfied in some cases, including cracked slabs in both service and transient. In addition, over post-tensioning of slabs results in negative deflection and non-economical design. Finally, some suggestions for better designs are expressed.