RC beams under blast load: Reliability and sensitivity analysis

The effects of blast loading on structures can be very dangerous: damages and failures are expected with serious threats to structural safety and human life. Materials stresses and strains are often pushed to the limit and the modelling of these phenomena can be very complex. In order to design blast-resistant structures it is very important to determine what are the key parameters of this problem.This paper presents a reliability and parametric analysis of the response of reinforced concrete (RC) beams under blast loads. The main aim is to highlight the key parameters of the problem in order to produce information useful for the design of reliable blast-resistant RC structures.The beam has been idealised as an equivalent SDOF system, in which strain-rate effects are accounted for. This approach is convenient from a computational point of view and it has been validated by a direct comparison with a more sophisticated finite element model and with experimental results found in literature. Then a sensitivity analysis of the parameters involved in beam response under blast load has been developed. Slenderness (which has a direct effect on stiffness) and peak load prove to be the most important parameters, but span length (which has an important influence on the mass) is also a key parameter. Other variables such as concrete strength and reinforcement ratio do not seem to have a strong correlation with the beam response.     

Flexural behavior of reinforced concrete (RC) beams retrofitted with hybrid fiber reinforced polymers (FRPs) under sustaining loads

This paper reports experimental studies of reinforced concrete (RC) beams retrofitted with new hybrid fiber reinforced polymer (FRP) system consisting carbon FRP (CFRP) and glass FRP (GFRP). The objective of this study is to examine effect of hybrid FRPs on structural behavior of retrofitted RC beams and to investigate if different sequences of CFRP and GFRP sheets of the hybrid FRPs have influences on improvement of strengthening RC beams. Toward that goal, 14 RC beams are fabricated and retrofitted with hybrid FRPs having different combinations of CFRP and GFRP sheets. The beams are loaded with different magnitudes prior to retrofitting in order to investigate the effect of initial loading on the flexural behavior of the retrofitted beam. The main test variables are sequences of attaching hybrid FRP layers and magnitudes of preloads. Under loaded condition, beams are retrofitted with two or three layers of hybrid FRPs, then the load increases until the beams reach failure. Test results conclude that strengthening effects of hybrid FRPs on ductility and stiffness of RC beams depend on orders of FRP layers.     

FRP-strengthened RC beams under sustained loads and weathering

Glass FRP-strengthened RC beams were subjected to sustained loads and placed for different periods outdoors, indoors, and in chambers that accelerate the effects of outdoor tropical weathering by a factor of six. Beams subjected to outdoor weathering had up to 18% larger crack widths and 16% larger deflections compared to those kept indoors at the end of 1 and 2¾ years, respectively. The increase in deflections and crack widths was lesser for beams with a higher FRP reinforcement ratio. The residual flexural strength and ductility of the beams decreased with longer weathering periods. Also, the failure mode of the beams changed from concrete crushing to FRP rupture, indicating a deterioration in the mechanical properties of the FRP laminates. Analytical methods which account for material degradation in concrete and FRP laminates are presented and found to predict the long-term flexural characteristics of the beams well.     

Numerical simulation of rebar/concrete interface debonding of FRP strengthened RC beams under fatigue load

The aim of this paper is to simulate the rebar/concrete interface debonding of FRP strengthened RC beams under fatigue load and also, to ascertain the influence of design parameters such as the elastic modulus, thickness and length of the FRP plate on the debonding performance. In order to simplify the simulation, some basic equilibrium equations are formulated and then the stresses of the rebar and FRP plate are numerically solved, and stress intensity factor is avoided in the simulation by fundamentals of fracture mechanics because of its complexity around the crack tip of bi-material interface. With the combination of finite element method and difference approximation, authors program the degradation model of coefficient of friction, debond criterion, propagation law and loop of load process into a commercial finite element code to investigate the fatigue debonding. The relationships between the debond length as well as other fatigue parameters and number of cyclic load are obtained and discussed.     

智能CFRP加固RC梁荷载效应的实时模拟与测评

制备了4 根炭纤维复合材料(CFRP) 加固钢筋混凝土(RC) 实验梁, 并在梁内钢筋、混凝土及加固CFRP中预置了布拉格光栅光纤传感器(FBG) 和电阻应变片两种传感器。根据钢筋混凝土理论和ANSYS 有限元软件编制了实验梁受弯荷载效应模拟计算程序。实验表明, 实验梁在受弯承载过程中, 布拉格光栅光纤传感器与传统应变片有完全一致的线性关系; 模拟计算出的实验梁受拉钢筋、压区混凝土应变值及挠度与荷载的关系与CFRP 中FBG的实测值吻合较好。由于对既成RC 结构不能在内部装置传感器(会破坏结构降低抗力) , 采用智能CFRP 加固RC 结构可实现加固和实时健康测评双重功能。      

Serviceability characteristics of flowing repairs to propped and unpropped bridge structures

The paper presents the results of a field investigation of repairs to Sutherland Street Bridge on B6080 in Sheffield, England. Flowing repair materials were used to apply large patch repairs to propped and unpropped columns of the bridge. Watertight shuttering was securely attached to the structure and flowing repair materials were either pumped or poured into the shuttering. The elastic modulus of the repair materials (E_rm) used was greater than that of the substrate concrete (E_sub). The results show that free shrinkage of the repair patch is effectively transferred to the substrate when E_rm>E_sub and the risk of restrained shrinkage cracking is reduced. In a propped repair, long-term strain distributions in the repair patch are erratic and are predominantly governed by the depropping process; properties of the repair materials only have a secondary influence. The repair patches applied with flowing materials did not, in the 60 week monitoring period, redistribute any significant external loading from the substrate member.     

Structural analysis of RC circular grouped silos under patch actions

The actions and assumptions for structural analysis of reinforced concrete grouped silos (called also silo batteries) are not precisely specified in technical references and relevant standards of design. The internal forces appearing in the cross-sections of cylindrical silo battery walls due to silo pressure may be evaluated by superposition of symmetrical actions and free patch load representing accidental asymmetries of pressure. The paper deals with the numerical analysis of sectional forces and moments in R C cylindrical silo battery subjected to silo pressure and patch load, taking into account a silo bin model that includes the interaction between the wall structure and stored particulate solid.This paper has been prepared at Bialystok Technical University within the framework of research projects sponsored by the Polish Committee for Scientific Research (KBN). The authors acknowledge the financial support for this paper.     

Interfacial shear stress in FRP-plated RC beams under symmetric loads

A recently popular method for retrofitting reinforced concrete (RC) beams is to bond fibre-reinforced polymer (FRP) plates to their soffits. An important failure mode of such plated beams is debonding of the FRP plates from the concrete due to high level interfacial stresses near the plate ends. A closed-form rigorous solution for the interfacial stresses in simply supported beams bonded with thin plates and subjected to arbitrary loads has been found, in which a non-uniform stress distribution in the adhesive layer was taken into account. This paper uses the rigorous solution to investigate the impact of symmetric loading configurations on the interfacial shear stress distributions, and concludes that the bending moments on the cross sections at the plate ends play a significant role in terms of stress concentration, while the shear forces on the same cross-section contribute little to the concentration. On the basis of this observation, this paper proposes a simplified approximate solution to the shear stress along the interface between concrete and adhesive layer. Compared with the rigorous and other approximate solutions, the simplified solution exhibits sufficient accuracy in terms of stress distribution and stress concentration localized near the plate ends. Due to its compact feature, the simplified solution is more suitable for engineering applications using a portable calculator and to be adopted in the codes of practices.     

Behavior of RC T-section beams strengthened with CFRP strips,subjected to cyclic load

This paper presents results of an experimental investigation on T-section reinforced concrete (RC) beams strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) strips. Specimens, one of which was the control specimen and the remaining six were the shear deficient test specimens, were tested under cyclic load to investigate the effect of CFRP strips on behavior and strength. Five shear deficient specimens were strengthened with side bonded and U-jacketed CFRP strips, and remaining one tested with its virgin condition without strengthening. The type and arrangement of CFRP strips and the anchorage used to fasten the strips to the concrete are the variables of this experimental work. The main objective was to analyze the behavior and failure modes of T-section RC beams strengthened in shear with externally bonded CFRP strips. According to test results premature debonding was the dominant failure mode of externally strengthened RC beams so the effect of anchorage usage on behavior and strength was also investigated. To verify the reliability of shear design equations and guidelines, experimental results were compared with all common guidelines and published design equations. This comparison and validation of guidelines is one of the main objectives of this work. The test results confirmed that all CFRP arrangements differ from CFRP strip width and arrangement, improved the strength and behavior of the specimens in different level significantly.     

Experimental study on scaling of RC beams under close-in blast loading

Damage effects analysis and assessment of buildings under blast loading is an important problem concerned by the area of explosion accident analysis, blast-resistant design, anti-terrorist and military weapon design.The damage character of RC beam under close-in blast loading is investigated through experiments. The damage modes and damage levels of RC beams are studied under different blast loads. The results show that the spallation area increases with the decrease of the scaled distance. The concrete beams are prone to be damaged in flexure mode with concrete crushed on the front face, concrete spallation on the back surface and concrete flake off on the side surface. The scaling of the dynamic response of reinforced concrete beams subjected to close-in blast loadings is also studied. The test results showed a similar macrostructure damage and fracture in all experiment conditions. But the local damage degree of RC beams with smaller size has been reduced a little as compared with that of beams with larger size. Based on the results, empirical equations of the center deflection to height ratio are proposed to correct scaling model considering size effects.     

冲击作用下钢筋混凝土深梁动力性能试验研究

为了探讨在冲击荷载作用下钢筋混凝土深梁的动力性能,利用大型落锤试验机对两组具有不同静载破坏特性的简支钢筋混凝土深梁进行了不同冲击速度下的动力性能试验研究,并考虑了二次冲击的影响。通过对高速摄像机所记录的各试件在冲击过程中裂缝的发生、发展直至试件破坏的全过程进行分析和不同荷载下裂缝形态差异的对比分析,表明不同的冲击速度下试件裂缝的发生、发展过程及裂缝形态表现出明显的差异,二次冲击下的主要裂缝基本遵循一次冲击产生的裂缝路径发展。详细分析了冲击力和跨中位移时程曲线以及冲击力-跨中位移曲线的特征,发现冲击力峰值与冲击速度、最大跨中位移和跨中残余位移与冲击速度在不发生严重剪切破坏时均满足近似线性关系。分析还表明,具有较好延性的深梁具有更好的抗冲击性能。最后,通过对比分析冲击力、支座反力和惯性力时程曲线,得出采用冲击力峰值和支座反力峰值作为深梁的抗冲击承载力均不准确的结论。     

Behaviour of RC beams strengthened by partial jacketing under cyclic loading

This work presents a study on the behaviour under cyclic loading of reinforced concrete beams strengthened in bending by the addition of concrete and steel on their tension side using expansion bolts as shear connectors, denominated here as partial jacketing. The experimental program included tests on six full scale reinforced concrete beams, simply supported, with rectangular cross section 150 mm wide and 400 mm high, span of 4,000 mm and total length of 4,500 mm. All the beams, after receiving two cycles of static loading in order to create a pre-cracking condition, were strengthened in bending by partial jacketing and then subjected to cyclic loading until the completion of 2 × 10⁶ cycles or the occurrence of fatigue failure. Following the cyclic loading, the beams that did not fail by fatigue were subjected to a static load up to failure. The main variables were the flexural reinforcement ratio in the beam and in the jacket, the beam–jacket interface condition (smooth or rough) and cyclic load amplitude. On the basis of the obtained test results and the results of previous study of similar beams tested only under static loading, the behaviour of the strengthened beams is discussed and a proposal for their design is given.     

Numerical study on damage mechanism of RC beams under close-in blast loading

RC structures have been widely used in civil engineering, and are vulnerable under blast loading. Improved understanding of damage mechanism of RC components helps advances the damage evaluation of RC structures under blast loading. In this paper, damage mechanisms of RC beam under close-in blast loading are investigated numerically. The FE model is validated through the experimental data reported by other researchers. A comprehensive investigation is carried out to investigate the damage mechanisms of RC beam, including the propagation of the main cracks, spallation at the bottom and the exfoliation of the side-cover concrete.     

Modelling of creep in continuous RC beams under high levels of sustained loading

A nonlinear theoretical model is developed in this paper for the long-term analysis of continuous reinforced concrete beams. The model accounts for creep, cracking, nonlinear behaviour in compression, shrinkage, aging, yielding of the reinforcement. The constitutive relations follow the modified principle of superposition, which are presented in the form of nonlinear rheological generalized Maxwell models with strain and time dependent springs and dashpots that account for material nonlinearity and aging of the concrete. The governing equations are presented in an incremental form, and are solved through a step-by-step algorithm in time along with the numerical shooting method for the solution along the beam. An iterative procedure is implemented at each time step for the determination of the rigidities and the creep strains. The capabilities of the model are demonstrated through numerical examples. The results show that creep and shrinkage have various influences on the structural response, and they may decrease the load carrying capacity and the factor of safety of continuous reinforced concrete beams with time.     

冲击作用下CFRP修复损伤RC叠合梁动力性能试验研究

采用落锤冲击试验机,进行了现浇梁、叠合梁以及碳纤维增强复合材料(carbon fiber reinforced polymer,CFRP)修复的损伤梁的动力性能对比试验。冲击作用下试件破坏形态以及冲击力与变形时程曲线表明,叠合面在一定程度上抑制了跨中裂缝向上开展的趋势,损伤修复后的叠合梁在冲击荷载下的破坏程度明显减轻,初始损伤减弱了梁跨中位移的滞后现象;相比完好梁,现浇与叠合修复梁抗冲击承载力有着同等程度的提高。基于有效应变计算的修复梁承载能力提升与试验结果的对比显示,CFRP能有效改善并增强构件的整体受力性能,在冲击作用下对构件的实际抗力贡献大于原有构件承载力与FRP受拉能力简单叠加时的理论抗力贡献。     

Web-bonded FRPs for relocation of plastic hinges away from the column face in exterior RC joints

In an RC building subjected to lateral loads, the beam–column joints constitute one of the critical regions. In existing frames, which were not adequately designed, a practical way of controlling plastic hinging and implement the strong-column weak-beam concept is through the use of a web-bonded FRP retrofitting system. This paper presents the results of an experimental and numerical study carried out in order to evaluate the ability of CFRP sheets in preventing the plastic hinge formation at the face of the column in exterior RC joints. Seven scaled-down RC exterior joints of a typical Ordinary Moment Resisting Frame are tested under moderately monotonic/cyclic loads. Two specimens are used as control while the other five are CFRP-strengthened/repaired of different lengths and thicknesses. The results show that carbon fibre sheets can effectively relocate the plastic hinge away from the face of the column. Non-linear numerical results using ANSYS are also presented and discussed.     

Degradation of bond between FRP and RC beams

Beams and slabs externally reinforced with FRP are often in contact with moisture and temperature cycles that reduce the expected durability of the system. Bond degradation is a frequent cause of premature failure of structural elements and environmental conditions are known to relate to such failures. The study shows the effects of cycles of salt fog, temperature and moisture as well as immersion in salt water on the bending response of beams externally reinforced with GFRP or CFRP, especially on bond between FRP reinforcement and concrete. Temperature cycles (−10 °C; 10 °C) and moisture cycles were associated with failure in the concrete substrate, while salt fog cycles originated failure at the interface concrete–adhesive. Immersion in salt water and salt fog caused considerable degradation of bond between the GFRP strips and concrete. However, immersion did not lower the load carrying capacity of beams, unlike temperature cycles (−10 °C; 10 °C) that caused considerable loss. No significant differences were detected on the behavior of the systems strengthened with GFRP and CFRP, perhaps because the design of the tests impeded failure of the fibres.     

Design and validation of composite patch repairs to cracked metallic structures

This paper describes an approach to a patch design for a crack repair, based largely on the simple closed-form analyses. For simplicity, the design approach does not include the effect of the out-of-plane bending nor polygon-shaped patch. However, once the basic design was brought out by that design approach, it can be re-analysed by using other advanced analytical models to account for those latter complexities. These advanced analytical methods are therefore also reviewed briefly in the paper. In addition, test data to validate the mentioned design and analysis methods are also presented and discussed.     

Failure diagrams of FRP strengthened RC beams

Amongst various methods developed for strengthening and rehabilitation of reinforced concrete (RC) beams, external bonding of fibre reinforced plastic (FRP) strips to the beam has been widely accepted as an effective and convenient method. The experimental research on FRP strengthened RC beams has shown five most common modes, including (i) rupture of FRP strips; (ii) compression failure after yielding of steel; (iii) compression failure before yielding of steel; (iv) delamination of FRP strips due to crack; and (v) concrete cover separation. In this paper, a failure diagram is established to show the relationship and the transfer tendency among different failure modes for RC beams strengthened with FRP strips, and how failure modes change with FRP thickness and the distance from the end of FRP strips to the support. The idea behind the failure diagram is that the failure mode associated with the lowest strain in FRP or concrete by comparison is mostly likely to occur. The predictions based on the present failure diagram are compared to 33 experimental data from the literature and good agreement on failure mode and ultimate load has been obtained. Some discussion and recommendation for practical design are given.