مقايسه‌ي آيين‌نامه‌هاي مختلف براي تعيين ظرفيت باربري محوري شمع ها

Comparison of Different Codes for Determination of the Axial Bearing Capacity of Piles

علي رغم پيشرفت هاي اخير در ارايه‌ي روش هاي علمي در طراحي شمع ها، تعيين ظرفيت باربري شمع ها به دليل لزوم لحاظ‌كردن اندركنش خاك و سازه در روش هاي علمي، همچنان به روابط تجربي وابسته است. آيين‌نامه هاي مختلف، روابط متفاوتي را براي محاسبه‌ي ظرفيت باربري محوري شمع ها ارايه كرده‌اند. ولي هر يك از روش هاي مذكور با فرضياتي تنظيم شده اند كه ممكن است تمامي آن ها در شرايط كلي برقرار نباشد. در اين نوشتار، فلسفه‌ي طراحي بر مبناي روش-هاي قطعي و احتمالاتي و نيز مفهوم ضرايب اطمينان در آيين‌نامه هاي طراحي: ساختمان، راه، سازه‌ي دريايي، و پل مورد بحث قرار گرفته است. همچنين اصول طراحي شمع تحت بار محوري بر مبناي آيين‌نامه هاي API (انجمن نفت آمريكا)، 7Eurocode (استاندارد اروپا) و آيين‌نامه‌ي كانادا بيان و در قالب يك مثال عددي، رويكردهاي مختلف طراحي براساس آيين‌نامه هاي API و 7Eurocode مقايسه شده‌اند.

Despite recent developments in proposing a scientific method in pile design, determination of the capacity of piles is still complicated due to the importance of soil-structure interaction in piles. Different codes have proposed various methods for the axial capacity of piles; however, these methods are based on assumptions that are not applicable in all situations. Traditionally, engineers use safety factors to compensate for the uncertainties and balance between reliability and economy. This traditional method is called deterministic. However, deterministic methods are very subjective and are generally not based on a systematic assessment of reliability. Therefore, these methods may produce structures with some overdesigned components. For elimination of these deficiencies, reliability-based methods have been developed. The load resistance factor design (LRFD) is one of the most applicable reliability-based methods. Its advantage is consideration of the uncertainties in strengths and loads separately. In this article, design philosophies and safety factors in different codes are first explained. By comparison of load factors of the LRFD in 8 different codes, it can be concluded that offshore and bridge codes have wider ranges for load factors than building codes. Then, the axial design of piles, in accordance with the API code, Eurocode 7, and Canadian codes, is discussed. API proposes both LRFD and working stress design (WSD) methods for the design of offshore structures. A numerical example for the design of axially loaded pile following API is conducted to compare LRFD (reliability-based) and WSD (deterministic) methods of API. Results show that the required penetration depth computed in the LRFD method is a little higher, however; this conclusion is not generalized to other situations. The Eurocode suggests three approaches for the design of piles. The difference between these approaches is related to the way they use load and resistance factors. A numerical example for design of axially loaded pile, according to three approaches of the Eurocode, indicates that these three approaches give approximately similar answers; however, the third approach is rather more conservative.