Optimum design of concrete cable-stayed bridges with prestressed decks

This article presents an optimization-based numerical method for the design of concrete cable-stayed bridges with prestressed decks. This method includes a structural analysis module and a sensitivity analysis and optimization module. The structural analysis considers concrete time-dependent effects, construction stages and geometrical nonlinearities. The discrete direct method is used for sensitivity analysis and an entropy-based approach is used for structural optimization. The design is formulated as a multi-objective optimization problem with objectives of minimum cost, minimum deflections and stresses. Numerical examples concerning the optimization of a real sized cable-stayed bridge are presented to illustrate the applicability of the proposed method.     

Seismic performance behavior of beam-column connections in prestressed concrete bridges

Test results for four prestressed concrete beam-column connections subjected to reversed cyclic loading are reported. The specimens were part of an experimental program designed to investigate the effect of design provisions/specifications and concrete strength on seismic performance behavior of such connections. The major design parameters varied in the specimens were the design code (AASHTO versus New Zealand codes) and the concrete strength (high versus normal strength concrete). From the test results, it was concluded that both the AASHTO and New Zealand codes provide sufficient capacities for earthquake-resistant design, and before spalling of the concrete cover, the stiffness degradation of the specimen with high strength concrete was less serious than that with normal strength concrete. To account accurately for the hysteretic behavior, a new set of rules for such connections to predict the hysteretic loops is proposed.

---

Résumé On présente les résultants des essais menés sur quatre connections poutres-colonnes en béton précontraint soumises à des sollicitations cycliques inversées. Ces essais faisaient partie d’un programme expérimental ayant pour but d’étudier l’effet des spécifications pour la conception et de la résistance du béton sur la performance sismique de telles connections. Les principaux paramètres de conception que l’on a fait varier furent les codes (codes AASHTO et codes de la Nouvelle Zélande) et la résistance du béton (béton de haute résistance versus béton ordinaire). Sur la base des résultats, il a été conclu que les deux codes (AASHTO et de la Nouvelle Zélande) fournissent des capacités suffisantes pour une conception résistante aux séismes, et que, avant l’éclatement de la couverture du béton, la dégradation de la rigidité de l’éprouvette en béton de haute résistance était inférieure à celle du béton de résistance normale. Afin de prendre en compte de manière précise le comportement d’hystérésis, de nouvelles règles pour de telles connections sont proposées afin de prédire les boucles d’hystérésis.

     

Different CFRP strengthening techniques for prestressed hollow core concrete slabs: Experimental study and analytical investigation

This study presents an experimental–analytical investigation on the structural behavior of precast prestressed hollow core RC slabs strengthened in flexure by CFRP laminates. Externally bonded and near surface mounted (NSM) laminates were used. The CFRP area and using transverse anchorage were also investigated. Results demonstrated that NSM technique resulted in optimum strengthening efficiency. The increased bond strength also resulted in full activation of the NSM laminates at failure. However, the NSM flexural strengthening level should be carefully designed to avoid unfavorable shear-tension failure mode. Moderate efficiency was associated with the externally bonded technique due to the premature de-bonding. However, this efficiency was optimized by using transverse CFRP laminates as anchorage, which re-directed de-bonding further away from the laminates’ ends and delayed failure, but at much lower deformations than those of the control slab. A rational analytical study was conducted. Comparisons between the experimental and analytical results demonstrated satisfactory agreements.     

预应力混凝土箱梁桥竖向预应力孔道对腹板截面

以两片安装竖向筋试验简支梁为基础,进行了预应力混凝土箱梁桥竖向预应力孔道对腹板截面削弱的研究,试验表明在不灌浆情况下梁开裂荷载提前,箍筋应力、混凝土表面主拉应增加明显,同时对试验梁进行了有限元分析,试验结果与有限元分析一致。研究表明加强竖向预应力灌浆质量的检测与管理对防止预应力混凝土箱梁桥的开裂有重要意义,同时为分析这类桥的开裂原因提供了理论与试验依据。     

Experimental and analytical study of corroded shear-critical reinforced concrete beams

This paper presents the experimental results of two highly corroded shear-critical deep beams subjected to a chloride environment to assess the shear behavior of long-term-corrosion damaged beams. A 26-year-old reinforced concrete beam corroded by exposure to a chloride environment was cut into two small short-shear-span beams, or deep beams, which were tested under three point bending until failure, along with a control beam of the same age, same length and same cross-section (115 × 28 × 15 cm). Cracking and corrosion maps were drawn for the corroded beams in order to assess the corrosion. Force displacement curves were drawn for the corroded and control beams. After testing the beams until failure, the main steel bars and the stirrups were extracted from the beams and loss of mass was measured and plotted for both longitudinal and transverse reinforcement. It was noted that the loss of mass of transverse and longitudinal reinforcements had not had an important influence on the failure mode and it had not affected the load-bearing capacity of the beam. It was found that the stirrups came into action after the failure of the compression strut and thus decreased the ductility of the corroded beams. Analytical investigations were made to compare the experimental shear capacity with the theoretical value. Strut and tie model predictions were more accurate than the conventional sectional methods provided for in ACI 318-08 and Eurocode 2.     

加强型预应力混凝土管桩抗弯剪性能试验研究

采用ABAQUS软件,选取适合模型的本构关系,在验证模型正确性的基础上,建立21个预应力高强混凝土管桩有限元模型,考察剪跨比、轴压比、混凝土强度、增配普通钢筋以及预应力筋配筋率等参数对管桩极限抗剪承载力和P−Δ关系曲线的影响。分析结果表明：剪跨比、轴压比、预应力筋配筋率、增配普通钢筋对管桩的抗剪性能有影响,随着构件剪跨比的减小和预应力筋配筋率的增加,管桩的极限抗剪承载力有较大提高;增配普通钢筋后的管桩,与仅配预应力筋的管桩相比,其极限承载力更高,不同剪跨比下的曲线变化规律也与后者基本一致;增加混凝土强度可以提高管桩的抗剪承载力,但P−Δ关系曲线的弹性阶段几乎没有变化;管桩的极限抗剪承载力随着轴压比的提高而增大。     

预应力T梁束孔管道压浆质量的无损检测试验研究

采用探地雷达技术、超声波透射法等多项技术方法对T梁的压浆质量进行试验性检测研究.首先采用超声波透射法在未压浆的钢束管道位置和正常混凝土位置（即避开波纹管位置）进行测试,然后将这两种类型的测试数据结果作为学习样本提供给神经网络进行学习;再将已压浆预应力T梁上的实测结果作为网络测试数据进行计算,根据神经网络的计算结果可预测钢束管道内的压浆质量,判断管道中是否有空隙存在.根据检测结果和开窗验证的情况来看,超声波透射法检测和神经网络的预测结果是准确可靠的,探地雷达技术不适合于T梁束孔管道的压浆质量检测.     

Experimental and analytical studies in crack initiation under fatigue: Part I — Experimental studies

Mechanistic investigations of damage evolution before crack initiation in an amorphous polymer show that damage consists of a core of highly dense crazing and a peripheral less dense zone of crazing. Damage characterization is carried out at consecutive configurations of the damage zone. Analysis of the kinematics of damage at different times involves comparisons of the inertia moments of damage distributions. The results indicate that damage evolution between consecutive configurations can be approximated by a linear transformation of the space variables. Thus, the process of damage growth can be described by translation and deformation of the damage zone. The growth rates of the damage zone movements decrease until crack initiation. In all cases, the average densities exhibit a damping type behavior with the number of cycles. The crack initiates within a core zone immediately ahead of the stress concentrator. The experimental results suggest that damage density within the core zone is independent of the loading conditions considered herein. This value is approximately equal to the damage density around the crack tip during slow crack propagation. The crack length at initiation is found to increase exponentially with the stress level. A simple decaying exponential relationship relates the crack initiation times and the applied stress level. This result is consistent with the fracture models based on absolute reaction theories.     

GPR inspection of concrete bridges

Ground-penetrating-radar (GPR) has become an important method for the non-destructive testing of concrete bridges. Although there are standards and guidelines available today, the quality of results does not only depend on the object inspected but also on the qualification and experience of the team carrying out the radar survey. This situation is unsatisfactory for engineers that have to decide whether a radar survey is suitable to solve their problem.EMPA has been active in the field of non-destructive testing of concrete structures for more than ten years. During this time, procedures for efficient data acquisition, processing interpretation and reporting have been developed. In addition, a large number of concrete structures has been inspected for research and services.In a research project completed in 2005, radar inspections using the EMPA approach were carried out on bridges designated for demolition. Results were laid open before the bridges were taken down. After the demolition radar results were verified with the help of the bridge parts. Thus, the accuracy and reliability of radar surveys was quantified under realistic circumstances.     

Experimental behaviour of existing precast prestressed reinforced concrete elements strengthened with cementitious composites

Strengthening of reinforced concrete (RC) members by means of fibre reinforced polymers (FRP) has gained increasing importance in the last few decades. On the other hand the necessity of skilled labour, high costs and particularly the weak response under high temperature conditions represent critical issues for the effective application of this technique. The use of fibre reinforced cementitious matrix (FRCM) composites applied to RC members seems to be a promising technique since it combines cost economy and high performance. Despite the fact that a number of experimental investigations on strengthening of RC elements by means of fibre reinforced polymers (FRP) composites are available in the literature, very little information is available about fibre reinforced cementitious matrix composite (FRCM). Hence, the use of cementitious composites in strengthening of RC structures is strongly limited by the lack of design models, guidelines, and recommendations and by the few available experimental investigations.This work aims to better understand the behaviour of FRCM strengthened RC full-scale elements through experimental tests on precast prestressed double-T beams. In addition to investigating the experimental behaviour of an innovative and promising strengthening system, a further element of novelty of the work is that the tested beams belong to an actual existing industrial building, since the few experimental tests available in the literature are mostly related to small-scale and cast-in-place RC elements.     

Experimental and analytical study on grouted duct connections in precast concrete construction

Owing to its forgiving tolerances and eliminating the need for welding, grouted dowel in-conduit connections are widely used for connecting various precast concrete elements, for instance in precast wall construction and bridge bent cap systems. Current design recommendations for such a connection treat it similar to a conventional reinforcing bar-in-concrete and do not account for the restraining effect of the duct. In the present study, a series of experimental and analytical approaches have been adopted to explore the disparity between grouted dowel connections and bar-in-concrete. The experimental program consisted of testing twenty-four pull-out specimens under monotonic loads. The main parameters investigated included the embedment length, concrete compressive strength and corrugated duct. Results from the experimental and analytical procedures showed that grouted dowel in-conduit connections behave markedly different from bars in concrete. Different failure mechanisms occurred in the grouted connections due to the confinement effect of the duct. Moreover, an increase in load carrying capacity and ductility of the connections was observed at all embedment lengths, regardless of the concrete compressive strength. Based on the experimental findings, an analytical model for predicting the embedment length of the connection was derived, calibrated and proven to be more accurate than state-of-the-art design procedures.     

Experimental and analytical assessment of the behavior of stainless steel reinforced concrete beams

This paper reports the results of a comprehensive experimental test program on high performance and standard AISI 316L stainless, steel reinforced concrete beams. Experimental results were compared with theoretical analysis data using current reinforcement mechanical feature standards and experimental testing-based ones, to identify any resistance, ductility and membrane collapse mode difference. It is also proved the importance of establishing a specific standard on the use of stainless steel reinforcement, in order to enable its more widespread, structural use, and not only for its corrosion resistance.

---

Résumé Dans ce travail sont présentés les résultats d'un vaste programme d'essais expérimentaux réalisés, sur des poutres en béton armé normal et à haute résistance; toutes les poutres sont renforcées de barres d'acier inoxydable. Les résultats obtenus à partir de l'analyse expérimentale ont étés comparés avec ceux obtenus par une analyse théorique dans laquelle on a utilisé les valeurs des caractéristiques mécaniques de l'acier, soit standardisées, soit obtenues à partir de l'expérimentation, afin de mettre en évidence pour les poutres, les différences en termes de résistance, de ductilité et de mode de rupture.

     

Experimental and analytical studies on the prediction of forming limit diagrams

Metal forming processes are widely used in industrial productions, automobile bodies, food industries, oil refineries, and liquid and gas transmission systems. Analyzing these processes is very important to reduce wastes and optimize the processes. Study of some main factors such as physical and mechanical properties of material and its formability, die geometry, die material, lubrication and pressing speed has been the topic of many research projects. In this paper, forming limit diagrams (FLDs) for LC and ULC steels and the effect of different parameters like the work-hardening exponent, n, and the plastic strain ratio, r, on these diagrams have been evaluated and simulated using ABAQUS/Standard. In this case, Hill’s quadratic anisotropy function is assumed to be the yield function and the Atkins criterion is used as the failure criterion.     

Analysis of prestressed concrete slab-and-beam structures

 In this paper a solution to the problem of prestressed concrete slab-and-beam structures including creep and shrinkage effect is presented. The adopted model takes into account the resulting inplane forces and deformations of the plate as well as the axial forces and deformations of the beam, due to combined response of the system. The analysis consists in isolating the beams from the plate by sections parallel to the lower outer surface of the plate. The forces at the interface, which produce lateral deflection and inplane deformation to the plate and lateral deflection and axial deformation to the beam, are established using continuity conditions at the interface. The influence of creep and shrinkage effect relative with the time of the casting and the time of the loading of the plate and the beams is taken into account. The estimation of the prestressing axial force of the beams is accomplished iteratively. Both instant (e.g. friction, slip of anchorage) and time dependent losses are encountered. The solution of the arising plate and beam problems, which are nonlinearly coupled, is achieved using the analog equation method (AEM). The adopted model, compared with those ignoring the inplane forces and deformations, describes better the actual response of the plate–beams system and permits the evaluation of the shear forces at the interfaces, the knowledge of which is very important in the design of prefabricated ribbed plates. Received 1 June 2000     

Mechanical properties of prestressed self-consolidating concrete

Since the mix design of self-consolidating concrete (SCC) differs from that of conventional concrete, mechanical properties of SCC may differ from those of vibrated concrete. An experimental program was performed to evaluate mechanical properties of SCC used for precast, prestressed applications. Sixteen SCC mixtures with a fixed slump flow of 680 ± 20 mm were prepared with different mixture parameters, including binder content and binder type, w/cm, dosage of viscosity-modifying admixture, and sand-to-total aggregate volume ratio. Two high-performance concrete mixtures that represent typically concrete used for precast, prestressed applications were investigated for the control mixtures. They were proportioned with 0.34 and 0.38 w/cm and had slump values of 150 mm. Mechanical properties of SCC were compared to code provisions to estimate compressive strength, elastic modulus, and flexural strength. The modified ACI 209-90 and CEB-FIP MC90 codes are found to provide good estimate for compressive strength prediction. The AASHTO 2007 model can provide good prediction of the elastic modulus and flexural strength of SCC.     

Optimum design of concrete cable-stayed bridges

The design of cable-stayed bridges involves a significant number of design variables and design objectives. The concrete cable-stayed bridge optimization is formulated here as a multi-objective optimization problem with objectives of minimum cost, minimum deflections and minimum stresses. A numerical method is developed to obtain the optimum design of such structures. This numerical method includes: structural analysis, sensitivity analysis and optimization. The structural analysis accounts for all the relevant effects (concrete time-dependent effects, construction stages and geometrical nonlinear effects). The structural response to changes in the design variables is achieved by a discrete direct sensitivity analysis procedure, and an entropy-based approach was used for structural optimization. The features and applicability of the proposed method are demonstrated by numerical examples concerning the optimization of a real-sized concrete cable-stayed bridge.     

Classification of damage in concrete bridges

Resume Le groupe de travail de la Commission Technique 104-DCC, qui traite des ponts, a présenté quatre rapports lors de la dernière réunion de la Commission qui s’est tenue à Munich en juin 1990. On commence par une classification générale des dommages des ponts en béton selon la nature de la détérioration et la partie du pont où elle est susceptible d’intervenir. On donne ensuite une classification des influences déterminant les dommages, de ceux causés par les tremblements de terre, et de ceux qui interviennent dans les tabliers en béton. On donne enfin une classification des dommages en Chine et en Finlande.

---

---

Dr Jokela died recently.     

Compressive-force path and behaviour of prestressed concrete beams

A study of the performance of prestressed concrete beams designed either by conventional design methods or to a physical model proposed in compliance with the ‘compressive-force path’ concept is presented. Results obtained from testing heavily prestressed beams are reported, which show that members designed to the latter concept may be safer than their Code counterparts. The behaviour of these beams has been compared with similar prestressed concrete beams (also designed either to current code provisions or to the proposed method)— but subjected to a smaller amount of prestressing—in an effort to monitor the effect of the level of prestressing on the behaviour of members designed by different methods. In contrast to the current way of thinking, it emerges that an additional amount of prestressing, instead of increasing the shear contribution of concrete, may, in fact, limit the load-bearing capacity of the member itself.