تحليل استاتيكي پل‌هاي خاكي فولادي تحت اثر بارهاي اجرايي

STATIC ANALYSIS OF SOIL-STEEL BRIDGES UNDER CONSTRUCTION LOADS

پل‌هاي خاكي- فولادي با تركيب صفحات فولادي موج‌دار و خاك، سيستم جديدي را براي پل‌سازي ارايه كرده است. در تحليل اين نوع پل‌ها عموماً تنش‌ها و تغييرشكل‌هاي ايجادشده در لايه‌هاي خاك و ديواره‌ي سازه تحت اثر بارهاي اجرايي و بهره‌برداري مورد توجه قرار مي‌گيرد. در اين نوشتار، پس از طراحي يك نمونه پل خاكي فولادي مطابق آيين‌نامه‌ي كانادا با استفاده از نرم افزار اجزا محدود PLAXIS، بارهاي اجرايي بر پل موردنظر اعمال شده و نتايج باربري با پروفيل پيشنهادي آيين‌نامه‌يي مقايسه شده است. اين مقايسه نشان مي دهد كه كنترل تنش و تغييرشكل مذكور با پروفيل پيشنهادي آيين‌‌‌نامه ‌يي جوابگوي تمامي ضوابط طراحي است، وليكن دقت در بارگذاري و تحليل گام به گام، استفاده از مدل‌هاي رفتاري غيرخطي، و درنظرگرفتن لغزش بين پوشش خاكي و جداره مي تواند منجر به انتخاب پروفيل اقتصادي-تري در مقايسه با آيين‌نامه هاي طراحي مبتني بر روش‌هاي كشسان شود.

Soil-steel structures are composite structures made by steel rings and a soil envelope, which are constructed in spans of, maximum, thirty meters. Soil-steel interaction causes flexible steel plates to interact with the surrounding backfill for suitable distribution of external loads in a radial direction. Internal forces and deflections are of the most concern in analytical study of soil-steel structures. But, only precise modeling makes results reliable. The current methods of analyzing these types of structures are principally based on the elasticity theory, in which nonlinear interaction of soil-structure and stage construction of local effects are not taken into account. These are the main concerns of the present research. In this paper, introducing soil-steel structures for bridge construction, the behavior of such structures is investigated under stage construction loads. As a case study, a single span soil-steel bridge has been analyzed and designed, based on the Canadian highway bridge design code (CHBDC) and results are compared with the PLAXIS finite element code. Stage construction consists of 1) the initial phase where a natural trench has been idealized; 2) filling and compaction of soil under foundations; 3) construction of foundation and installation of plates; 4) backfilling and compaction of both sides of the steel structure up to crown level; 5) backfilling the soil cover up to a minimum height of cover for construction loads; 6) backfilling and compaction of soil cover up to the final level for performance loads. In addition to investigate the deformations, axial and shear forces, bending moments and stresses in each of the above construction stages, the stability of the natural trench before construction, the interface between corrugated steel plates and the soil, the assumption of the elastic behavior of backfilling, the effect of unbalanced backfilling, and the effect of the asymmetry of construction loads, have also been studied. The comparison indicates that horizontal and vertical displacements and the compressive stresses obtained from FE stage construction are 64, 43 and 56 percent, respectively, greater under standard limitations. Therefore, a more optimum and economic design, in comparison to elasticity based analysis methods, can be achieved by accounting for stage construction and the nonlinear behavior of soil-steel bridges.