An evaluation of force-based design vs.direct displacement-based design of jointed precast post-tensioned wall systems

The unique features of jointed post-tensioned wall systems, which include minimum structural damage and re-centering capability when subjected to earthquake lateral loads, are the result of using unbonded post-tensioning to attach the walls to the foundation, along with employing energy dissipating shear connectors between the walls. Using acceptance criteria defined in terms of inter-story drift, residual drift, and floor acceleration, this study presents a multiplelevel performance-based seismic evaluation of two five-story unbonded post-tensioned jointed precast wall systems. The design and analysis of these two wall systems, established as the direct displacement-based and force-based solutions for a prototype building used in the PREcast Seismic Structural Systems （PRESSS） program, were performed at 60% scale so that the analysis model could be validated using the PRESSS test data. Both buildings satisfied the performance criteria at four levels of earthquake motions although the design base shear of the direct displacement-based jointed wall system was 50% of that demanded by the force-based design method. The study also investigated the feasibility of controlling the maximum transient inter-story drift in a jointed wall system by increasing the number of energy dissipating shear connectors between the walls but without significantly affecting its re-centering capability.

Rock engineering design of post-tensioned anchors for dams - A review

High-capacity, post-tensioned anchors have found wide-spread use, originally in initial dam design and construction, and more recently in the strengthening and rehabilitation of concrete dams to meet modern design and safety standards. Despite the advances that have been made in rock mechanics and rock engineering during the last 80 years in which post-tensioned anchors have been used in dam en- gineering, some aspects of the rock engineering design of high-capacity rock anchors for dams have changed relatively little over the last 30 or 40 years. This applies, in particular, to the calculations usually carried out to establish the grouted embedment lengths required for deep, post-tensioned anchors. These calculations usually make simplified assumptions about the distribution and values of rock-grout interface shear strengths, the shape of the volume of rock likely to be involved in uplift failure under the influence of a system of post-tensioned anchors, and the mechanism of that failure. The resulting designs are generally conservative. It is concluded that these aspects of the rock engineering design of large, post- tensioned rock anchors for dams can be significantly improved by making greater use of modern, comprehensive, numerical analyses in conjunction with three-dimensional （3D） models of the rock mass structure, realistic rock and rock mass properties, and the results of prototype anchor tests in the rock mass concerned.

Numerical study of a multiple post-tensioned rocking wall-frame system for seismic resilient precast concrete buildings

A multiple rocking wall-frame(MRWF)system,in which the wall panels are directly connected to the adjacent beams and foundation is presented herein.In the MRWF system,the unbonded post-tensioned(PT)tendons are used to promote the self-centering ability,and O-shaped steel dampers are applied to enhance the energy dissipation capacity and reparability of the structure.First,analytical equations are proposed to determine the behavior of the O-shaped dampers.Then,the MRWF system is numerically evaluated for five different models consisting of rocking walls with varying numbers and arrangements while keeping the total effective width of wall panels constant.The numerical results show that with an increase in the number of wall panels and a decrease in the wall width,the hysteretic behavior of the MRWF system tends to the ideal flag-shaped pattern,resulting in little damage to the beams,insignificant strain in the wall toe,negligible residual drifts and damage index of less than 0.2 under severe earthquakes.In contrast,the conventional model demonstrates extensive damage to the structural elements due to undesirable wall-to-frame interaction,which leads to a damage index of 0.78 and residual drifts of 0.42%under seismic loads.

Stresses induced by post-tensioned anchor in jointed rock mass

A new analytical study on stresses around a post-tensioned anchor in rocks with two perpendicular joint sets is presented. The assumptions of orthotropic elastic rock with plane strain conditions are made in derivation of the formulations. A tri-linear bond-slip constitutive law is used for modeling the tendon-grout interface behavior and debonding of this interface. The bearing plate width is also considered in the analysis. The obtained solutions are in the integral forms and numerical techniques that have been used for evaluation. In the illustrative example given, the major principal stress is compressive in the anchor free zone and compressive stress concentrations of 815 k Pa and 727 k Pa(for the anchor load of 300 k N) are observed under the bearing plate and the bond length proximal end, respectively. However, large values of tensile stresses with the maximum of-434 k Pa are formed at the bond length distal end. The results obtained using the proposed solution are compared very those of numerical method(FEM).

Approach to Determine Corrosion Propensity in Post-Tensioned Tendons with Deficient Grout

After corrosion failure of post-tensioned tendons was identified in a Florida bridge in early 2011, laboratory tests were conducted in this study on extracted sections from the failed tendons to identify the grout properties and makeup leading to the failure and also to elucidate the mechanism of corrosion. The initial steps in identification of PT tendons with a high propensity for corrosion initiation or damaged included a detailed visual inspection and identification of voids in the grout. Voids in tendon can be a result of bleed water formation or construction problems. General characteristics of the deficient grout and corrosion behavior of steel in the affected bridge gave a first approach to assessing grout deficiency and corrosion susceptibility. However, refinements in the understanding of the mechanisms causing grout segregation and the elucidation of the role of sulfates, oxygen content, and pore water pH in corrosion development are required.

Seismic tests of post-tensioned self-centering building frames with column and slab restraints

Post-tensioned(PT)self-centering moment frames have been developed as an alternative to typical moment-resisting frames(MRFs)for earthquake resistance.When a PT frame deforms laterally,gaps between the beams and columns open.However,the gaps are constrained by the columns and the slab in a real PT selfcentering building frame.This paper presents a methodology for evaluating the column restraint and beam compression force based on the column deformation and gap openings at all stories.The method is verified by cyclic tests of a full-scale,two-bay by one-story PT frame.Moreover,a sliding slab is proposed to minimize restraints on the expansion of the PT frame.Shaking table tests were conducted on a reduced-scale,two-by-two bay one-story specimen,which comprises one PT frame and two gravitational frames.The PT frame and gravitational frames are self-centering throughout the tests,responding in phase with only minor differences in peak drifts caused by expansion of the PT frame.When the specimen is excited by a simulation of the 1999 Chi-Chi earthquake with a peak ground acceleration of 1.87 g,the maximum interstory drift and the residual drift are 7.2%and 0.01%,respectively.

Rock engineering design of post-tensioned anchors for dams-A review

High-capacity, post-tensioned anchors have found wide-spread use, originally in initial dam design and construction, and more recently in the strengthening and rehabilitation of concrete dams to meet modern design and safety standards. Despite the advances that have been made in rock mechanics and rock engineering during the last 80 years in which post-tensioned anchors have been used in dam engineering, some aspects of the rock engineering design of high-capacity rock anchors for dams have changed relatively little over the last 30 or 40 years. This applies, in particular, to the calculations usually carried out to establish the grouted embedment lengths required for deep, post-tensioned anchors.These calculations usually make simpli fi ed assumptions about the distribution and values of rock e grout interface shear strengths, the shape of the volume of rock likely to be involved in uplift failure under the in fl uence of a system of post-tensioned anchors, and the mechanism of that failure. The resulting designs are generally conservative. It is concluded that these aspects of the rock engineering design of large, posttensioned rock anchors for dams can be significantly improved by making greater use of modern,comprehensive, numerical analyses in conjunction with three-dimensional(3D) models of the rock mass structure, realistic rock and rock mass properties, and the results of prototype anchor tests in the rock mass concerned.

Parametric study of hexagonal castellated beams in post-tensioned self-centering steel connections

The effects o f important parameters (beam reinforcing plates, initial post-tensioning, and material properties o f steel angles) on the behavior o f hexagonal castellated beams in post-tensioned self-centering (PTSC) connections undergone cyclic loading up to 4% lateral drift have been investigated by finite element (FE) analysis using ABAQUS. The PTSC connection is comprised o f bolted top and bottom angles as energy dissipaters and steel strands to provide self-centering capacity. The FE analysis has also been validated against the experimental test. The new formulations derived from analytical method has been proposed to predict bending moment o f PTSC connections. The web-post buckling in hexagonal castellated beams has been identified as the dominant failure mode when excessive initial post-tensioning force is applied to reach greater bending moment resistance, so it is required to limit the highest initial post-tensioning force to prevent this failure. Furthermore, properties o f steel material has been simulated using bilinear elastoplastic modeling with 1.5% strain-hardening which has perfectly matched with the real material of steel angles. It is recommended to avoid using steel angles with high yielding strength since they lead to the yielding o f bolt shank. The necessity o f reinforcing plates to prevent beam flange from local buckling has been reaffirmed.

Estimation of Load-Induced Damage and Repair Cost in Post-Tensioned Concrete Rocking Walls

Post-tensioned concrete rocking walls might be used to avoid severe seismic damage at the base of structural walls, decrease residual drift, and lessen post-earthquake repair costs. The prediction of load-induced damage to the rocking wall resulting from seismic loading can provide an extremely valuable tool to evaluate the status and safety of structural concrete walls following earthquakes. In this study, the behavior and the damage state of monolithic, self-centering, rocking walls, as a new type of concrete rocking wall, are investigated. The nonlinear mechanical behavior of the wall is first modeled numerically, and subsequently the mechanical parameters from the numerical simulation are used to generate the local damage index. The results from the damage index model are compared with the full-scale test results, confirming the viability of the numerically based damage index method for estimating the seismically induced damage in concrete walls. Moreover, the estimated damage can be utilized as a qualitative and quantitative scale to assess the status of the wall following seismic loading events.Finally, an equation is proposed to estimate the repair cost based on the predicted damage state for the studied structural system.

Secondary transfer length and residual prestress of fractured strand in post-tensioned concrete beams

An experimental study is performed on five post-tensioned concrete beams to explore the effects of different fracture positions on secondary transfer length and residual prestress of fractured strand.A numerical model is developed and used to predict the secondary transfer length and residual prestress of fractured strand in post-tensioned concrete beams.The model change interaction,which can deactivate and reactivate the elements for simulating the removal and reproduction of parts of the model,is used to reproduce the secondary anchorage of fractured strand.The numerical model is verified by experimental results.Results shows that the fractured strand can be reanchored in concrete through the secondary anchorage,and the secondary transfer length of fractured strand with the diameter of 15.2 mm is 1133 mm.The residual prestress of fractured strand increases gradually in the secondary transfer length,and tends to be a constant beyond it.When the fractured strand is fully anchored in concrete,a minor prestress loss will appear,and the average prestress loss is 2.28%in the present study.

Application of Strut-and-Tie Model for Design of Interior Anchorage Zone in Post-tensioned Concrete Structure

This paper presents an application of strut-and-tie model(STM) to design the interior anchorage zone(IAZ) in the post-tensioned concrete structure.The STM theory and range of IAZ are introduced.Then,based on the finite element analysis,a series of simplified equations to calculate internal forces in IAZ are presented.Finally,the STM model for IAZ is given.In the proposed STM model,internal forces in ties vary with the dimension ratio and the eccentricity of load.The U-turn of internal forces is suggested to allocate rebar to resist bearing flexural tensile force.Compared with the FIP(International Federation for Prestressing) model,the proposed STM model is more reasonable and applicable.

Application of Strut-and-Tie Model Zone in Post-tensioned for Design of Interior Anchorage Concrete Structure

This paper presents an application of strut-and-tie model(STM) to design the interior anchorage zone(IAZ) in the post-tensioned concrete structure.The STM theory and range of IAZ are introduced.Then,based on the finite element analysis,a series of simplified equations to calculate internal forces in IAZ are presented.Finally,the STM model for IAZ is given.In the proposed STM model,internal forces in ties vary with the dimension ratio and the eccentricity of load.The U-turn of internal forces is suggested to allocate rebar to resist bearing flexural tensile force.Compared with the FIP(International Federation for Prestressing) model,the proposed STM model is more reasonable and applicable.

不同气候条件下预应力木梁柱节点长期转角计算方法

梁柱节点长期转角是评估预应力木框架结构长期适用性能的重要指标,过量的梁柱节点长期转角会引起木梁竖向变形的增大,降低结构适用性;因此准确预测预应力木梁柱节点转角的时变规律是保证预应力木结构长期安全服役的重要前提。该文提出了不同气候条件下预应力木梁柱节点长期转角的数值计算方法,该方法包括木材水分传输、时变预应力计算、弹性转角计算、蠕变和环境变形计算、节点长期转角更新五个模块。随后利用长期试验数据验证了计算方法的准确性。最后,基于验证的数值计算方法分析了不同气候条件、木材初始含水率和构件尺寸对预应力木梁柱节点长期转角的影响规律,依据50年服役期内的最大转角蠕变系数评估了节点长期变形的性能等级。结果表明:该文所提出的数值计算方法与试验结果吻合良好;气候年平均相对湿度越大,节点发生中等或严重变形的比例越高;木材初始含水率越高,节点发生中等或严重变形的比例越低;构件理论尺寸越大,节点长期变形越小。

基于20辊轧机冷轧带材的后张力机制研究

重点讨论了张力机制在20辊轧机生产工艺中的作用,适宜的张力可以有效控制产品尺寸和形状,影响轧制力的大小,提升产品的表面质量和机械性能;稳定的张力调节可以减少表面缺陷和不均匀性,并影响轧制速度和冷轧温度的控制。在实验中通过改变后张力预设值,发现后张力的调节可以对工艺参数产生显著影响,进一步实现产品质量和生产效率优化。使用三位形貌轮廓仪对成品带材进行了表面粗糙度检测,发现调整后张力预设值对带材表面粗糙度有较大的影响。

自复位混凝土剪力墙研究进展

自复位剪力墙在遭受地震时能够实现大位移而不会有明显的残余变形或大量损伤,适用于地震后的快速恢复结构。为研究自复位混凝土剪力墙的发展趋势,文中介绍了国内外自复位混凝土剪力墙结构的相关研究进展,对相关研究进行展望和总结,表明在自复位混凝土剪力墙中加入耗能部件或选择合适的配筋方式,可以改善墙体的力学性能并控制损伤。

高速公路桥梁工程后张法预应力砼箱梁施工技术研究

文中将后张法预应力砼箱梁施工技术应用于高速公路桥梁建设项目中,设置预应力孔道及锚垫板,以便于进一步的箱梁模板安装施工;在箱梁的混凝土强度达到设计强度的90%后,进行箱梁砼钢绞线穿束;确定施工流程,根据工程概况,对箱梁进行静荷载测试。结果显示,测试点在静荷载下达到的累计沉降量均在10 mm以内,说明箱梁抗压能力较强,施工质量较好,满足设计要求。

后张法预应力混凝土桥梁施工技术应用研究

为了提升桥梁工程施工技术水平,总结了预应力混凝土的基本理论,从检查工作、张拉方式、张拉顺序等方面阐述了后张法张拉预应力混凝土构件的准备工作。同时,以市政某空心板桥梁为研究对象,分析了后张法张拉预应力混凝土的具体施工工艺,并对张拉效果进行评价。结果表明,板梁的伸长率均小于6%,满足规范和设计文件要求。