动剪应力折减系数影响因素及计算公式

动剪应力折减系数r_d的计算是国际上最为通用的液化判别方法中的核心技术,研究发展理论上更合理同时具有可操作性的r_d计算方法,不仅对液化判别技术的发展具有重要意义,同时也是土层地震响应认识水平提升的重要标志。基于Ki K-net的32个实际场地模型508条实际地震动输入的场地反应分析结果,通过方差分析,研究r_d各影响因素的显著性,揭示r_d变化形态以及形态参数变化规律;遴选出必要因素并提出三因素和单因素r_d计算公式,并与已有方法进行对比。结果表明:深度对r_d值影响最为显著,震级和土层剪切波速次之但其作用不可忽视,地表峰值加速度对r_d值影响不显著;r_d随深度变化服从Logistic模式,由渐近线、参考影响深度和形状变量3个参数控制,3个参数随土层剪切波速和震级变化呈现良好的规律性。以此提出的r_d三因素新计算公式由深度、土层剪切波速和震级确定,单因素简化计算公式由深度确定,均适用于0~50 m深度范围,且具有连续曲线形式,克服了以往公式沿深度拼接的弊端。较Seed曲线和NCEER曲线,单因素新公式准确性有所提高;与目前最复杂的Cetin公式相比,提出的三因素新公式浅层准确性相同,在深层有所提高,且新公式表达简单,便于工程应用。

Influence factors and formula for dynamic stress reduction coefficient

The reduction coefficient of dynamic shear stress is the key in the methods of liquefaction evaluation. The reliable and easy method of calculating is very important for liquefaction evaluation and seismic response analysis. The influencing factors of were therefore studied based on the results of site response analysis to 508 records of seismic motion at 32 actual sites. The variation pattern of and the trends of shape parameter are uncovered. The necessary factors are selected and thus a new 3-parameter formula and a single-parameter formula of are proposed. The proposed formulas are compared with the existing formula. The results show that the depth is the most significant factor affecting ，the seismic magnitude and the shear wave velocity are the second significant factors which cannot be neglected，and PGA is the least significant factor. The curve follows the Logistic model，and is controlled by three shape parameters：asymptotic line，reference depth of influence and shape index. The proposed 3-parameter formula depends on d， and ，and the single-parameter formula depends on d. Both formula are applicable in the depth range of 0–50 m，and change continuously with depth，which overcome the disadvantages of existing formula in terms of splicing lines. The single-parameter formula is more accurate in comparing with the curve recommended by Seed and by NCEER. Comparing with the formula proposed by Cetin，the 3-parameter formula has the similar accuracy in the range of shallow depths，and has the higher accuracy in the range of deep depths. The new 3-parameter formula has a simpler form than Cetin curve and is more applicable for engineering applications.