ارزيابي اثرات وابسته به زمان در پل‌هاي يكپارچه با عرشه‌ي مركب بتن فولادي

Time-Dependent Effects Evaluation of Composite Integral Abutment Bridges

در اين نوشتار، براي محاسبه‌ي اثرات وابسته به زمان در پل هاي يكپارچه ، از ضرايب 2 آيين نامه‌ي AASHTO و ACI استفاده شد. نتايج روسازه‌ي پل موردي نشان مي دهد كه گذشت زمان باعث افزايش لنگر خمشي منفي در محل تكيه‌گاه‌ها و نيز كاهش لنگر خمشي مثبت در محل وسط دهانه شده است. نسبت كاهش لنگر وسط دهانه و نيز افزايش آن در محل اتصال عرشه- كوله در پل با تكيه‌گاه گيرداري انتهايي برابر 28/1 و مفصلي برابر31/1 در آيين‌نامه‌ي AASHTO نسبت به آيين‌نامه‌ي ACI است. مي توان گفت كه در هنگام محاسبه‌ي مقادير لنگرهاي خمشي وابسته به زمان عرشه، استفاده از ضرايب آيين‌نامه‌ي AASHTO نسبت به آيين‌نامه‌ي ACI باعث تغييرات 30 درصدي در نتايج خواهد شد. مقايسه‌ي نتايج مربوط به خيز وسط دهانه براي پل با شرايط تكيه‌گاهي انتهايي متفاوت (گيرداري و مفصلي) نشان مي دهد كه با گذشت زمان مقادير خيز وسط دهانه ها 2 برابر شده‌اند.

Construction of integral bridges solves difficulties due to the maintenance of expansion joints and bearings during serviceability, hence, causing the integral bridge to become more economical compared with conventional bridges. However, there is some ambiguity in the design of integral bridges, which has led to some limitations in construction design. Some ambiguities are time-dependent effects, such as creep, shrinkage, superstructure thermal change, backfill pressure and soil-bridge interaction. In this paper, to investigate the above effects, two different models were made for sub and superstructures. Time-dependent internal forces obtained from the superstructure analysis, as well as deck positive thermal change effects, were applied to the substructure. Integral abutment bridges should be designed under vertical and lateral loads. Also, during the design procedure, gradually, a stage construction mode should be considered. For composite integral bridges, in which the deck is made of concrete and steel, three stages of construction should be considered: First, when the concrete has not yet been hardened and there is no composite function. Second, when the composite function exists, but by considering time-dependent effects, the concrete modulus of elasticity is about 0.33 its ultimate value, and, finally, when the concrete has completely hardened and the composite function has been achieved. The maximum length of integral bridges is limited by temperature changes. Seasonal temperature change causes expansion and contraction on a superstructure. This causes a pressure subjected to abutments from backfill. When the temperature increases, a passive pressure is created behind each abutment, whereas, when the temperature decreases, an active pressure is created behind each abutment. For the second case, a gap is created between the backfill and abutment. In the construction of skewed integral bridges, non-uniform force distribution will exist and lead to additional internal forces. Therefore, for skewed integral bridges with a skew angle greater than 25 degrees, the temperature and time-dependent effects should be considered. Superstructure analysis results, for a case study, showed that the passing of time caused a negative bending moment increase in supports and decreasing positive bending moment in the middle of the spans. ACI and AASHTO coefficients were used to calculate time-dependent effects and considerable differences were observed in the results. Using AASHTO coefficients gave bigger changes in moment amount. It can be said that for calculating the amount of time-dependent deck bending moment using AASHTO coefficients, about 30% change will be obtained in results compared with ACI code.