独立基础设计及相关问题的理解与建议

论述了钢筋混凝土柱下独立基础以及独立基础加防水板的设计过程及方法,对设计中需注意的相关问题提出了作者的见解和建议。可供类似工程参考。     

多失效模式下基于区间非概率的岩质隧道稳定可靠度分析

对于岩质隧道工程的稳定可靠度分析,一方面由于岩体参数的统计数据获取困难使得概率可靠度方法难以适用,另一方面其工程稳定与多种失效模式密切相关,须考虑隧道结构体系的可靠度问题。首先,基于区间理论,通过区间变量形式表征不确定性参数;然后,针对岩质隧道工程中多种失效模式并存的情况,引入结构体系可靠度理念,建立基于区间非概率的岩质隧道结构体系可靠度指标计算及其稳定性评价方法;在此基础上,通过工程实例验证了该方法的合理性;最后,定义不确定性参数的波动幅度,进一步分析各失效模式中不同参数对相应可靠度指标以及结构体系可靠度指标的影响。分析结果表明,各失效模式相应的非概率可靠度指标均随区间变量范围的增大而降低,且同一参数在不同失效模式中表现出不同的影响;另外,不确定性参数的变化还将导致影响岩质隧道结构体系稳定的主要失效模式发生相应的变化。     

桩撑与基础托换联合支护的设计与施工

概述了湖北洪山宾馆基坑支护工程采用桩撑与基础托换联合支护技术的设计及施工。实践证明,该基坑支护设计合理,施工技术可靠,成功利用支护桩对临近建筑物的天然基础进行了基础托换,确保了周围建筑物的安全,可为类似工程的设计与施工借鉴。     

大底盘高层建筑基础的分析设计

阐述大底盘高层建筑基础设计中流行的几项关键技术,包括基础沉降控制方式;后浇带施工、基础沉降缝、基础整体设计;变刚度调平技术;考虑上部结构刚度设计等,解答了基础电算设计中应注意的问题。     

柔性基础下水泥搅拌桩复合地基分析与设计

结合工程试验及模型试验结果,对水泥搅拌桩复合地基在柔性基础及刚性基础下的工作性状进行了分析,并对柔性基础下水泥搅拌桩复合地基设计方法进行了探讨.     

破坏包络面理论在多基础系统中的应用研究

由多个分离基础组成的多基础系统是常用的海洋结构基础型式。基于破坏包络面理论,分析了砂土地基多基础系统的失效模式,建立了相应的承载力计算方法,并验证了计算方法的可行性。对比分析了单一基础和多基础系统不同荷载路径下的荷载安全系数,探讨了破坏包络面理论与分项系数法相结合的基础承载力计算方法。失效模式的分析表明,由于水平荷载的增大,四腿平台结构迎浪侧基础首先到达破坏包络线,其失效模式属于滑动失稳,但由于基础间的运动约束,其并不会出现真正的滑移破坏。随着水平荷载进一步地增大,迎浪侧基础承担的水平和竖向荷载不断减小,导致背浪侧基础受到不断增大的荷载。最终,背浪侧基础也到达破坏包络线,多基础系统失效。分析表明,荷载路径对基础的荷载安全系数有决定性的影响,计算基础的荷载安全系数需指明相应的荷载路径。鉴于破坏包络面的大小和形状取决于众多因素,基础设计时需采用特定工况下的破坏包络面进行承载力计算。     

辽阳市某化工公司工程地质勘察与基础设计探讨

本文以某化工公司工程地质勘察与基础设计为例,阐述了该工程的岩土工程地质勘察、测试工作及数据处理,并对地基承载力进行计算,最终对地基基础做出科学设计,为工程建设的合理规划设计、施工提供可靠的地质科学依据,为相关工程设计积累经验。     

局部桩筏基础的设计与施工

以加拿大多伦多市某工程为例,介绍了在复杂地质条件下采用局部桩筏基础（PPRF）的设计及施工问题;探讨了设计PPRF的决定性因素;在保持PPRF设计的完整性前提下,提出了单位沉降量的准则,并用于筏板和桩的设计;计算了PPRF的滑移及转动;最后,采用有效方法对该工程采用的局部桩筏基础进行了计算分析。结果表明,PPRF的设计主要取决于侧向土压力、分布不均的建筑荷载以及地基土的非均匀承载力,工程桩应主要布置在沉降较大的区域,即位于筏板基础承受高压力而土体承载力较低的西北部。探讨局部桩筏基础的设计与施工为该类型工程的基础设计提供了一个新的解决途径。     

多塔楼下整体大面积筏板基础设计及反力测试分析

多个塔楼下整体大面积筏板基础反力分布特征仅局限于室内模型试验资料。通过北京三里屯SOHO工程地基反力原位测试并与已有模型试验数据对比分析,结果表明,当地基土比较均匀、上部结构刚度较好,高层外框柱荷载及柱间距的变化不超过20%,核心筒荷载与高层外框柱总荷载相差不超过10%,且筏板厚度超过1/6柱跨时,计算高层下筏形基础的内力时,基底反力可按直线分布;裙房基础可有限扩散高层建筑荷载,实际工程的原位测试结果表明,高层荷载的扩散范围约为2跨裙房的距离,3跨裙房之外没有影响;对于多个高层建筑下整体大面积筏板基础的地基反力可应用叠加原理计算     

“硬-软-硬”三层黏土上圆形基础的承载力分析

海洋浅基础设计所遇地层条件通常由于基础大尺寸的特点而涉及多土层剖面,其中“硬-软-硬”三层黏土是最常见的地层条件之一。现行海洋浅基础设计常用的API和ISO规范尚无针对“硬-软-硬”黏土竖向承载力的计算方法。本文基于自升式平台基础设计规范推荐的“bottom-up”预测模型,假定“硬-软-硬”黏土上基础的承载力为“硬-软”黏土冲剪破坏产生的侧向摩阻力与“软-硬”黏土挤压破坏提供的承载力之和。通过有限元模拟分析,验证了“bottom-up”方法所假定破坏模型的合理性,但由于规范推荐的相关公式忽略了上覆土层对挤压破坏模式的影响,使其严重低估了“软-硬”黏土挤压破坏产生的承载力。基于变参数有限元分析结果,本文对挤压破坏承载力公式进行了修正,提出了“硬-软-硬”三层黏土上圆形基础竖向承载力的预测方法。     

Reliability-based robust geotechnical design of spread foundations using multi-objective genetic algorithm

This paper presents a new geotechnical design concept, called robust geotechnical design (RGD). The new design methodology seeks to achieve a certain level of design robustness, in addition to meeting safety and cost requirements. Here, a design is considered robust if the variation in the system response is insensitive to the variation of noise factors such as uncertain soil parameters and construction quality. When multiple objectives are considered, a single best design may not exist, and a trade-off may be necessary. In such a case, a genetic algorithm is adopted for multi-objective optimization and a Pareto Front, which describes a trade-off relationship between cost and robustness at a given safety level, is established. The new design methodology is illustrated with an example of spread foundation design. The significance of the RGD methodology is demonstrated.     

Simplified procedure for reliability-based robust geotechnical design of drilled shafts in clay using spreadsheet

This paper provides a simplified procedure for reliability-based robust geotechnical design (RGD) using spreadsheet. In the RGD methodology, design robustness is achieved by adjusting “design parameters” without reducing the uncertainties in noise factors. This design approach generally involves a multi-objective optimisation, which is computationally challenging. To improve the efficiency of the RGD methodology, the design robustness is evaluated in terms of sensitivity index and the safety requirement is evaluated using mean value first order second moment (MFOSM). To ease the concern that the reliability index obtained with MFOSM may not be sufficiently accurate, a mapping function that relates MFOSM to a more accurate method such as first order reliability method is introduced. To further improve the efficiency of the proposed simplified RGD method, a new simplified procedure along with a more accurate robustness measure is developed that eliminates the need for multi-objective optimisation. With these modifications, the proposed simplified RGD method can efficiently be implemented in a single Excel spreadsheet. The proposed simplified method, which goes beyond any existing reliability-based RGD methods in terms of ease of use and computational efficiency, is illustrated in this paper with an example of robust design of drilled shaft in clay.     

Development of partial factor design method on bearing capacity of pile foundations for Japanese Specifications for Highway Bridges

ABSTRACT

This paper shows the development of a partial factor design method on the bearing capacity of pile foundations for Japanese Specifications for Highway Bridges. Firstly, estimation design equations on the bearing capacities of pile foundations are improved by analysis of pile load test results and uncertainties in the bearing capacities are evaluated. Secondly, the reliabilities of pile foundations designed by the former specifications are evaluated based on reliability analysis considering the uncertainties in the bearing capacities and coefficients of subgrade reaction. Finally, a partial factor design method is developed based on the target reliability index obtained based on the conventional pile installation method by the pile installation methods. The factors are different for each pile installation method.     

Robust geotechnical design of shield-driven tunnels

This paper presents a fuzzy set-based robust geotechnical design (RGD) methodology for the design of shield-driven tunnels. Here, uncertain geotechnical parameters required for analysis of tunnel performance (referred to herein as the structure safety and serviceability performance of tunnel cross section) are represented as fuzzy sets. Given fuzzy input parameters, the performance of a shield-driven tunnel will be uncertain, which is expressed in this study as a fuzzy factor of safety, according to the analysis of vertex method. Then, the fuzzy factor of safety for a given design is used to evaluate the failure probability and design robustness, which are, in turn, employed in the proposed RGD framework. Note that a design is considered robust if the performance of the shield-driven tunnel is insensitive to the variation of its uncertain geotechnical parameters. Within the RGD framework, each candidate design in the design space is analyzed for its safety state (in terms of failure probability), design robustness, and cost. The goal of the RGD of a shield-driven tunnel is to bring the safety state to an acceptable level, while maximizing the robustness and cost efficiency simultaneously. To this end, a multi-objective optimization is performed and a Pareto front is obtained, which provides a trade-off that may be used to select the most preferred design. Through an illustrative case, the effectiveness and significance of this new robust design methodology is demonstrated.     

南京地区复合桩基设计思考

根据笔者参与设计的南京地区复合桩基工程实例,对复合桩基的设计方法和适用情况等作了进一步的探讨,重点分析了复合桩基单桩承载力的确定、桩的布置及施工、桩基优化设计、沉降预测和安全系数等方面的内容,对推广复合桩基更为广泛的工程应用,具有积极的实际意义.     

CFG桩复合地基承载力可靠度分析

利用可靠度理论对从国内收集到的25组CFG桩复合地基承载力试验数据进行了概率统计处理。借助无量纲计算模式,计算不同载荷组合下CFG桩复合地基承载力的可靠度指标,并分析了各随机变量对可靠度指标的影响程度。为评价常 用经验公式 的可靠性提供一定的参考。     

扩展基础冲剪破坏特征试验研究

通过10个基础模型试验,研究了钢筋混凝土扩展基础的冲剪破坏特征和破坏规律。试验结果表明,当基础宽度大于柱宽加两倍基础有效高度时,将发生冲切破坏;当基础宽度小于或等于柱宽加两倍基础有效高度时,若基础底板只配置抗弯钢筋时,基础将发生剪切破坏;当基础板按梁式配筋时,基础将发生弯曲破坏,在基础尺寸、底面纵筋相同时,柱下条形基础按基础梁配筋与按板式配筋相比,基础的延性性能和承载力都将有较大提高。建议在柱下条形基础设计时按基础梁的形式进行配筋,以避免基础发生脆性的剪切破坏。     

Reliability of shallow foundations designed to Eurocode 7

Eurocode 7, the new European standard for geotechnical design, is based on the limit state design method, with partial factors and characteristic parameter values. An objective of the Eurocodes is that the chosen partial factors should achieve reliability levels for a structure close to a prescribed target value. The target ultimate limit state reliability index for a medium risk structure for 50 years is 3.8. This paper examines the reliability of an inclined-eccentrically and vertically loaded square foundation designed using all three Design Approaches in Eurocode 7 with the recommended partial factor values given in EN1997-1. The reliabilities obtained using the three Design Approaches are compared with the target reliability value and the reliabilities of traditional designs using overall factors of safety. The paper provides evidence that Eurocode 7 gives more consistent reliabilities for a greater range of parameter values than the traditional allowable stress method and demonstrates the importance of selecting appropriate characteristic values.     

Calibration of resistance factor for design of pile foundations considering feasibility robustness

The resistance factor for pile foundations in load and resistance factor design (LRFD) is traditionally calibrated considering target reliability index (β_T) and statistics of load and resistance bias factors. However, the resistance bias factor is hard to quantify statistically. Consequently, the design obtained using the calibrated resistance factor can still miss β_T if the variation in resistance bias factor has been underestimated. In this paper, we propose a new resistance factor calibration approach to address this dilemma by considering “feasibility robustness” of design in the calibration process. Herein, the feasibility robustness is defined as a probability that the β_T requirement can still be satisfied even in the presence of uncertainty or variation in the computed bearing capacity. For illustration, LRFD approach for pile foundations commonly used in Shanghai, China is examined. Emphasis is placed on re-calibration of resistance factors at various feasibility robustness levels, with due consideration of the variation in the resistance bias factor. A case study is presented to illustrate the use of the re-calibrated resistance factors. The results show that the feasibility robustness is gained at the expense of cost efficiency; in other words, the two objectives are conflicting. To aid in the design decision-making, an optimal feasibility robustness level and corresponding resistance factors are suggested in the absence of a designer’s preference.     

浅基础竖向极限承载力的可靠度分析

结合各种地基土的原始载荷板试验资料,用太沙基理论公式、汉森理论公式和魏西克理论公式进行了浅基础极限承载力的计算,对计算模式的不确定性、土性固有的变异性及由其引起的极限承载力的不确定性等进行了研究;根据以上计算结果,计算了各类地基土承载力的可行度指标;结合上部结构的荷载标准及分项系数,计算了各类地基土承载力的分项系数。