Physical modeling of behaviors of cast-in-place concrete piled raft compared to free-standing pile group in sand

Similar to free-standing pile groups, piled raft foundations are conventionally designed in which the piles carry the total load of structure and the raft bearing capacity is not taken into account. Numerous studies indicated that this method is too conservative. Only when the pile cap is elevated from the ground level,the raft bearing contribution can be neglected. In a piled raft foundation, pileesoileraft interaction is complicated. Although several numerical studies have been carried out to analyze the behaviors of piled raft foundations, very few experimental studies are reported in the literature. The available laboratory studies mainly focused on steel piles. The present study aims to compare the behaviors of piled raft foundations with free-standing pile groups in sand, using laboratory physical models. Cast-in-place concrete piles and concrete raft are used for the tests. The tests are conducted on single pile, single pile in pile group, unpiled raft, free-standing pile group and piled raft foundation. We examine the effects of the number of piles, the pile installation method and the interaction between different components of foundation. The results indicate that the ultimate bearing capacity of the piled raft foundation is considerably higher than that of the free-standing pile group with the same number of piles. With installation of the single pile in the group, the pile bearing capacity and stiffness increase. Installation of the piles beneath the raft decreases the bearing capacity of the raft. When the raft bearing capacity is not included in the design process, the allowable bearing capacity of the piled raft is underestimated by more than 200%. This deviation intensifies with increasing spacing of the piles.

Field study of plastic tube cast-in-place concrete pile

The compositions, technical principles and construction equipments of a new piling method used for ground improvement plastic tube cast-in-place concrete pile were introduced. The results from static load tests on single piles with different forms of pile shoes and on their composite foundations were analyzed. The distribution patterns of axial force, shaft friction and toe resistance were studied based on the measurements taken from buried strain gauges. From the point of engineering application, the pile has merits in convenient quality control, high bearing capacity and reliable quality, showing higher reasonability, advancement and suitability than other ground improvement methods. The pile can be adopted properly to take place of ordinary ground improvement method, achieving greater economical and social benefits.

Field observation of the thermal disturbance and freezeback processes of cast-in-place pile foundations in warm permafrost regions

The bearing capacity of pile foundations is affected by the temperature of the frozen soil around pile foundations.The construction process and the hydration heat of cast-in-place(CIP)pile foundations affect the thermal stability of permafrost.In this paper,temperature data from inside multiple CIP piles,borehole observations of ground thermal status adjacent to the foundations and local weather stations were monitored in warm permafrost regions to study the thermal influence process of CIP pile foundations.The following conclusions are drawn from the field observation data.(1)The early temperature change process of different CIP piles is different,and the differences gradually diminish over time.(2)The initial concrete temperature is linearly related with the air temperature,net radiation and wind speed within 1 h before the completion of concrete pouring;the contributions of the air temperature,net radiation,and wind speed to the initial concrete temperature are 51.9%,20.3%and 27.9%,respectively.(3)The outer boundary of the thermal disturbance annulus is approximately 2 m away from the pile center.It took more than 224 days for the soil around the CIP piles to return to the natural permafrost temperature at the study site.

Influence of the penetration of adjacent X-section cast-in-place concrete(XCC)pile on the existing XCC pile in sand

A series of small-scale 1g X-section cast-in-place concrete(XCC)pile-penetration model tests were conducted to study the effects of soil density and pile geometry on the lateral responses of an existing pile and the variations in surrounding soil stress.The results showed that the bending patterns of existing XCC piles varied with penetration depth.The lateral response of the existing pile was sensitive to the change in relative density and pile geometry.For example,the bending moment of the existing pile increased along with these parameters.The development of the radial stressσ′r/σ′v0 of the soil around an existing pile showed different trends at various depths during the penetration of the adjacent pile.Moreover,the change in radial stress during the penetration of the XCC pile did not exhibit the“h/R effect”that was observed in the free-field soil,due to the shielding effect of the existing piles.The peak value of radial stressσ′r_max/σ′v0 decreased exponentially as the radial distance r/R increased.The attenuation ofσ′r_max/σ′v0 with r/R in the loose sand was faster than in the medium-dense or dense sands.Theσ′r_max/σ′v0 at the same soil location increased with the cross-section geometry parameter.

Effect of concrete creep on pre-camber of continuous rigid-frame bridge

The effect of concrete creep on the pre-camber of a long-span pre-stressed concrete continuous rigid-frame bridge constructed by cantilever casting method was investigated.The difference of creep coefficients calculated with two Chinese codes was discussed.Based on the calculations,the pre-camber of a pre-stressed concrete continuous rigid-frame box bridge was computed for construction control purpose.The results show that the short-term creep coefficient and long-term creep coefficient calculated with the CC-1985 are larger than those calculated with the CC-2004,while the medium-term creep coefficient calculated with the CC-1985 is smaller than that calculated with the CC-2004.The difference of creep deformation calculated with these two codes is small,and the influences of concrete creep on the pre-camber for most of the segments are negligible.The deflections and stresses of the box girder measured during the construction stages agree very well with the predictions.

Influence of concrete differential aging time on the construction phases analysis of the Fenghua River Bridge

The Fenghua River Bridge is a major structure on the highway between Hengzhang and Guojiachi, which is to be built with a four-span prestress concrete (PC) box girder and symmetrical cantilever castings. In this paper, a finite element method (FEM) model is set up to study the effects of concrete differential aging time on the construction phases of the Fenghua River Bridge by calculating the vertical displacement of the folding segment of the middle span and the longitudinal bending moment of Pier 12#. In the model, the girders are classified into 150 changing sections based on the desgn scheme, and their construction is to be carried in 16 phases respectively to build 12 blocks connected by a side folding segment and a middle folding segment, covered with a second dead load and in completion for 20 years. It is found that the internal forces and deformations of the concrete structures at the aging time of 60 d are quite different from those of 0 d aging time while the behaviors of the structures of 120 d aging time is nearly the same as those of 60 d aging time― the differences are so small that can be neglected, suggesting that the creep develops obviously about one month after the cement is hardened and the development fades later on.

长阳天池口清江特大桥主拱圈悬臂浇筑斜拉扣挂系统设计

长阳天池口清江特大桥主桥为净跨径242 m的非对称上承式钢筋混凝土拱桥,主拱圈采用单箱单室截面,宽9.5 m、高4.0 m。资丘岸和五峰岸半跨主拱圈各分为24个和21个节段,第1个节段采用支架现浇,其余节段采用斜拉扣挂悬臂浇筑法施工。扣索采用星式和扇形组合方式布置,位于墩顶和扣塔的扣索为星式布置,位于交界墩墩身的扣索为扇形布置,并将墩身上的扣锚索设计成连续索以避免在墩身上搭设张拉平台;扣塔由8根∅800 mm×16 mm钢管组成,两岸扣塔分别高52.68 m和60.12 m;根据锚索最大索力,设计了能承受800 kN张拉力的岩锚索。建立主拱圈悬臂浇筑施工阶段MIDAS Civil有限元模型,对斜拉扣挂系统扣锚索索力进行计算,通过拆除部分扣索和调整扣锚索索力,施工期间主拱圈截面最大拉应力、扣塔最大偏位、主拱圈成拱后线形均满足要求。

预制模壳填芯混凝土悬臂盖梁承载力分析

盖梁的轻量化发展是实现全预制桥梁的必然要求。基于已有的预制模壳填芯混凝土悬臂盖梁静力试验,采用有限元软件ABAQUS对试件PRC1进行数值分析,有限元计算结果和试验结果吻合较好,验证了有限元模型的可靠性。通过修改已验证的有限元模型参数,进一步分析外壳混凝土强度、纵筋配筋率和交界面接触设置对预制模壳填芯混凝土悬臂盖梁力学性能的影响。基于钢筋混凝土一般构件的承载力计算方法建立试件的理论计算公式,并对提出的计算公式进行验证。研究结果表明:当外壳材料为普通混凝土时,混凝土强度等级从C35增加到C60,峰值承载力仅提高1%,而当外壳材料为超高性能混凝土(UHPC)时,峰值承载力相较C35提高了22%。纵筋配筋率与峰值承载力呈线性增长趋势,当顶部第1排纵筋直径从14 mm增加到22 mm时,峰值承载力提高了34%。交界面接触设置为绑定约束比面面接触的峰值承载力更接近试验值,且后者是相对保守的。基于钢筋混凝土一般构件建立的承载力计算方法所计算结果与试验结果和有限元分析结果吻合较好。

基于零位移与无应力状态控制的斜拉扣挂施工扣索力确定方法

大跨度拱桥常采用斜拉扣挂法开展拱肋施工,但在节段拼装过程中扣索索力和拱肋空间位形之间互相影响,索力优化计算以及扣索状态控制问题复杂。提出改进零位移法结合无应力状态控制法的复合扣索力计算方法:基于改进型零位移法快速得到满足成拱线形要求的扣索力大小,基于成拱索力采用无应力状态法反推各施工阶段扣索索力。该复合方法可避免传统零位移法扣索力不准和反复迭代计算的缺陷,施工中扣索可实现一次张拉,后续节段拼装中无需对扣索进行索力调整。以某跨径430 m大跨度钢管混凝土拱桥缆扣合一施工为工程应用背景,采用的零位移法结合无应力状态法对大跨度非对称式钢结构拱肋进行斜拉扣挂拼装的扣索力求解和施工模拟。分析拼装过程中扣索力变化规律,并对比验证实际线形与目标线形偏差,以及拱肋应力与设计应力之间偏差程度。结果表明,所提出的复合算法理论简单且索力求解计算便捷。所求的扣索力可有效保证拱肋拼装完成及松索后拱肋线形和受力均满足要求。

大跨度预应力混凝土连续梁悬臂浇筑施工技术

本文探究了悬臂浇筑施工技术在某大跨度预应力混凝土连续梁中的应用。采用悬臂平衡浇筑法进行施工,在悬臂端设计了三角形挂篮,利用挂篮做吊架进行悬臂浇筑、完成合拢段施工,降低了施工难度,提高了施工质量。在本工程中,贯彻全过程、精细化管理理念,分别从支架搭设与预压、挂篮制作与安装、钢筋绑扎、混凝土浇筑预养护、孔道压浆与封锚等方面采取了控制措施,确保了预应力混凝土连续梁悬臂施工的顺利完成。

大跨度预应力桥梁工程中挂篮悬臂浇筑施工技术

为解决大跨度预应力桥梁工程中存在悬臂预应力损失过多的问题,对大跨度预应力桥梁工程中挂篮悬臂浇筑施工技术进行研究。根据关门江水道大桥项目施工情况,分析大跨度桥梁工程中挂篮悬臂浇筑施工技术的具体应用,包括挂篮制作与安装、模板拼接、混凝土浇筑、悬臂桩预应力张拉等方面。对实际工程的支架位移进行检测,结果显示:测点处模板最大位移值为4.2 mm,混凝土浇筑完成二次收面处理,悬臂桩单桩预应力为5 000 kN。由此表明,模板连接紧密、混凝土浇筑均匀密实、预应力结果承提供受荷载效果良好,可有效解决悬臂预应力损失。

济南华润万象城抗震性能分析与设计

济南华润万象城为一栋结构高度412m的超限高层商业建筑,采用框架-剪力墙结构。结构中同时存在扭转不规则、平面开洞、大跨度结构、大悬挑结构、斜柱、桁架悬挑等多项平面及竖向不规则情况。采用多种计算程序进行了抗震性能分析,其结果均满足既定的抗震性能目标。对复杂商业中的各关键部位设计进行了多方案比选,确定了合理的结构布置和适当的构造措施,取得了良好的效果。

跨座式单轨独柱大悬挑钢-混组合框架高架车站抗震设计探讨

针对跨座式单轨独柱大悬挑钢-混组合框架高架车站结构抗震冗余度低,在高烈度地区应用较少,为更好地分析高架车站抗震受力全过程,以某跨座式单轨独柱大悬挑钢-混组合框架高架车站工程实例为背景,根据建筑规范和城市轨道交通规范要求设定合理的抗震性能目标,并研究构件抗震受力历时过程中的受力情况。采用大型有限元Midas作为主要计算软件,选取合理的地震波,进行多遇地震和设防地震有限元计算时采用弹性时程分析方法,且对振型分解反应谱法地震作用进行相应的放大,结果分析构件满足抗震性能目标要求。进行罕遇地震计算时,为更好地模拟工程实际情况,计算模型考虑桩土共同作用,采用结构-承台-桩基整体计算建模方法,进行弹塑性时程分析,关键构件采用截面纤维模型进行计算,能够反映关键构件在大震作用下进入破损阶段之后的行为,可以查看关键构件截面混凝土的开裂和压碎历史,钢材或钢筋的屈服和硬化过程。研究结果表明结构构件均满足抗震性能目标:(1)关键构件的钢材纤维受拉受压均未超过屈服强度,保持弹性工作状态,混凝土纤维受拉开裂,受压未达到屈服强度,同时延性系数D/D1小于1,均说明关键构件满足抗震性能为压弯、抗剪不屈服的目标;(2)桩基的钢筋纤维受拉、受压均未超过屈服强度,混凝土纤维受拉开裂,受压未达到屈服强度,满足抗震性能为压弯、抗剪不屈服的目标;(3)地震作用下,带有桩模型与无桩模型的基底剪力有一定差距,表现为带有桩基模型的地震剪力较大,设计时应考虑桩土共同作用。

考虑车辆荷载对预应力大悬臂盖梁开裂的影响

随着路面车辆的不断增加以及混凝土盖梁使用年限的增加,部分桥梁的盖梁出现裂缝。以石家庄某桥梁为例,通过研究在运营阶段可能对桥梁开裂产生的影响,研究大悬臂预应力混凝土盖梁在车辆荷载作用下,车辆荷载位置、车辆荷载大小以及上部结构形式对盖梁产生裂缝的影响与桥梁在无车辆荷载时进行对比,确定车辆荷载对盖梁开裂的影响程度。结果表明:车辆荷载位置越靠近悬臂端,盖梁拉应力越大,结构的下挠越严重,右车道发生严重超载时与左、右车道同时超载相比,拉应力变化较小,但盖梁挠度较大,约为左、右车道超载时两倍。

大悬臂预应力混凝土箱梁桥拼宽方案受力分析

以沪武高速公路太仓至常州段的望虞河大桥改造工程为背景,研究大悬臂预应力混凝土箱梁的拼宽方案,利用Midas软件建立全桥的有限元模型,分析桥梁在活载、基础沉降、收缩徐变作用下的结构受力特点,总结各拼宽方案的关键问题,并提出施工建议。

预应力混凝土梁桥关键施工技术分析

为保证预应力混凝土梁桥的施工质量,文章总结了连续梁桥的形式、受力特点等,分析了悬臂浇筑法的施工流程和特点。同时,以某公路连续箱梁桥为研究对象,从0#块支架计算和墩梁临时固结、挂篮预压、合龙等方面分析了施工要点,最后从线形控制和应力控制两方面阐述了连续梁桥的施工控制方法,研究成果可供类似桥梁结构施工借鉴。

预应力混凝土梁桥关键施工技术探讨

预应力混凝土梁桥中波纹管、混凝土浇筑及钢绞线等的施工质量直接影响项目整体的施工品质、周期及后期的运营维护环节。应用预应力混凝土梁桥施工技术,可提高桥梁的刚度及抗拉强度等,但应用过程中也出现了部分问题。文章总结了预应力混凝土梁桥常见的施工工法,并针对相关问题提出了改进措施。分析表明,采用合理的防治措施可有效减少出现波纹管堵塞、混凝土裂缝及滑丝或断丝问题的风险;挂篮悬臂法能在避免使用重型设备的前提下保障施工安全。

基于屈服线理论的混凝土护栏和桥面板悬臂设计研究

为提高桥梁混凝土护栏和桥面板悬臂抗撞性能,文章论述了混凝土护栏的车辆横向撞击力的三要素,建议了其最小作用高度;依据屈服线理论和虚功原理推导了混凝土护栏对横向荷载的抗力标准值和屈服线发生的临界长度。结果表明:通过实例验证了混凝土护栏和《公路桥涵通用图》桥面板悬臂的抗撞性能,混凝土护栏的极限承载力满足车辆横向碰撞的要求,而空心板桥面板悬臂的抗撞性能不足,并提出了设计建议,以为桥梁设计提供借鉴和参考。