عوامل موثر در لنگر خمشي سينماتيك در گروه شمع‌ها

Factors effecting the kinematic bending moment in the piles group

اين نوشتار نتايج مطالعه‌ي پارامتري رفتار لرزه‌يي گروه شمع مدفون در توده‌ي خاك لايه‌يي را ارايه مي‌دهد. هدف از اين پژوهش، بررسي اندركنش سينماتيك خاك– شمع و چگونگي تاثير عوامل مختلف در توسعه‌ي لنگرهاي خمشي سينماتيك در هنگام زلزله در گروه شمع‌ها بوده است. براي ساخت مدل‌‌ها از نرم‌افزار تفاضل محدود سه بُعدي FLAC3D استفاده شده است. صحت استاتيكي با مقايسه‌ي مدل با پژوهش‌هاي قبلي و صحت لرزه‌يي با استفاده از نرم‌افزار 2000 SHAKE سنجيده شده است. تجزيه و تحليل لرزه‌يي پارامتري با تغيير عوامل اصلي و تاثيرگذار در پاسخ گروه شمع مانند: مشخصات خاك، قطر شمع‌ها، فاصله‌ي شمع‌ها، و عمق لايه‌ي خاك با استفاده از شتاب‌نگاشت زلزله‌‌ي ال‌سنترو و در حوزه‌ي زماني انجام شده است. نتايج تجزيه و تحليل‌ها نشان داده است كه عوامل ذكرشده تاثير زيادي در مقدار لنگر خمشي سينماتيك دارند. همچنين در شمع‌‌هاي مياني، لنگر خمشي سينماتيك بيشتري نسبت به شمع‌‌هاي كناري توسعه مي‌يابد.

Kinematic soil – pile interaction during earthquake motions has received large attention in recent years. In fact, post-earthquake underground field measurement by many researchers proof that bending moments due to kinematic interaction an addition to superstructure inertial effects damaged the pile foundation in recent earthquake. Kinematic forces arise from the passage of seismic waves through the surrounding soil and become significant in the presence of strong discontinuities in stiffness of the soil profile. In this case, the large curvatures imposed to the piles by the vibrating soil in turn generate bending moments; these moments will develop even in the absence of a substructure and are referred to as “kinematic” moments, to be distinguished from moments generated by structural loading at the pile head (“inertial” moments). Kinematic soil – pile interaction is a complex problem involving a number of factors, such as soil profile, soil properties, nonlinear soil behavior, induced pore – pressure and pile properties. Evaluation of the above factors and impact of each on the final response of piles can help engineers to better design. In this paper the results of parametric study of seismic behavior of pile groups embedded in layered soil is evaluated by focusing the attention on the kinematic bending moments induced by the transient motion using the three dimensional finite difference program FLAC3D. Elcentro earthquake excitation that has a low predominant frequency was applied as an acceleration–time history at the base bedrock of the finite difference mesh. Lateral side grids were horizontally fixed for static analysis. In dynamic analyses, lateral boundaries were considered as "Quite Boundaries", a built-in boundary condition in FLAC for dynamic analysis. Analysis is carried out by varying the main parameters governing the dynamic response of piles like the soil properties, the diameter, the spacing between pile and the depth of soft soil layer.The results showed that 1) Kinematic bending moment is increased by increasing the diameter of the pile and increasing the depth of soft soil layer or increase the space between the piles enhance the effect of pile diameter on the bending moment; 2) Increase the space between the piles reduce kinematic bending moments; and 3) Increasing the depth of soft soil layer is increase the kinematic bending moment and by increasing the space between the piles effects of soft soil layer depth on bending moment increases slightly.