A procedure for creep and shrinkage analysis of frames with low beam stiffness

Adjacent vertical members in tall buildings undergo differential time‐dependent deformations due to creep and shrinkage. These deformations result in differential deflections at floor levels and redistribution of forces. An available practical procedure in the literature to estimate these deformations, taking into account the sequential nature of the loading, utilizes the rate of creep method (RCM) to evaluate the stress transfer between concrete and steel. This procedure is designated as Procedure(RCM) and is prone to errors since the creep recovery of concrete is neglected in RCM. A more accurate procedure designated as Procedure(AEMM) using the age‐adjusted effective modulus method (AEMM) as a stress transfer method, and taking into account the sequential nature of the loading, is proposed. The proposed procedure is more accurate than Procedure(RCM) since creep recovery of concrete is considered in AEMM. Numerical studies are carried out in this paper to compare the results from these two procedures. Copyright © 2007 John Wiley & Sons, Ltd.     

Evaluation of creep and shrinkage effects in composite tall buildings

In a composite tall building, adjacent vertical members undergo differential time‐dependent deformations due to creep and shrinkage. Highly elaborate, complex and requiring large computational effort procedure, utilizing stress transfer method, age‐adjusted effective modulus method (AEMM) is available to evaluate creep and shrinkage effects in a composite building. Recently, a simple procedure and requiring lesser computational effort, utilizing AEMM has been presented for concrete and composite buildings. Studies comparing deflections and axial forces in reinforced concrete frames with those obtained from the procedure available in literature (using stress transfer method, RCM) have been reported. In this paper, suitability of the above procedures for evaluation of creep and shrinkage effects in composite buildings has been demonstrated. Studies are reported for typical creep shrinkage parameters. It is shown that the error to determine axial forces from AP(AEMM) is small for composite frame–shear wall systems with very low beam stiffness (I_b = I_c/100). Use of CP(AEMM) is warranted for the frame–shear wall systems even with low beam stiffness (I_b = I_c/20). Copyright © 2010 John Wiley & Sons, Ltd.     

Nichtlineare Analyse der zeitlichen Antwort von Stahlbeton‐, Spannbeton‐ und Verbundkonstruktionen auf die mechanische Einwirkung und Temperatureinwirkung

Nonlinear Analysis of the Time‐Dependent Response of Reinforced and Prestressed Concrete Structures to Mechanical and Temperature Loads The developed computational procedure for the nonlinear analysis of the time‐dependent response of structures to mechanical loads and to elevated temperature in fire up to failure is limited to plane frame structures. They can be carried out as reinforced or prestressed concrete structures or as steel‐concrete composite structures. The analysis takes into account all types of structural nonlinearities, resulting from the geometry of the structure and from the material properties, including rheology. The computational procedure for the analysis is based on the finite element method. A highly effective finite element P4 is used. Its curvature along the element axis is interpolated using fourth order polynomial. The stress‐strain state of the structure is analysed gradually and iteratively according to the time intervals. The nonlinear equation systems of the structure are solved numerically using the Newton‐Raphson procedure.     

Stochastic Tension‐Stiffening Approach for the Solution of Serviceability Problems in Reinforced Concrete: Constitutive Modeling

A number of studies have indicated that the tension‐stiffening law is an important input parameter in a numerical analysis of serviceability (deformations and cracking) problems of reinforced concrete (RC) structures. The stochastic nature of concrete cracking, which results in a large scatter of experimental results, renders the constitutive modeling a very difficult task. Even data obtained from short‐term tests are to some degree uncertain due to time‐dependent processes occurring in concrete, such as shrinkage and creep relaxation. This article provides statistical analysis tools that can be readily applied to engineering practice. Stochastic principles are applied to modeling of tension‐stiffening for the purpose of predicting not only the average deformation response of RC elements, but also bounds of these predictions that are of vital importance for practical problems. Unlike common practice, shrinkage of concrete is taken into account in the short‐term numerical simulation.     

Time‐dependent analysis of steel‐reinforced concrete structures

The time‐dependent behavior is a major consideration in the design and construction of tall buildings, especially in concrete and composite structural systems. To make an analysis of long‐term effect of steel‐reinforced concrete structures, the method of using master–slave constraint to deduce substructure element model of composite members was introduced, and the problem of co‐work between steel and concrete was solved. The creep calculation method of combined Age‐adjusted Effective Modulus Method (AEMM) and finite element method was adopted. Steel Reinforced Concrete Construction Modeling (SRCCM), a calculation program based on Visual C++ and ObjectARX, was developed for simulating the construction process of high‐rise composite structures. The use of the method is illustrated through one computation example of Shanghai Center Tower, which is a super high‐rise steel‐reinforced concrete structures. The method provides valuable information about time effects that may be used in designing new structures or in diagnosis existing structures. The results also indicate that the vertical shortening of Shanghai Center Tower between column and core‐tube is significant. Such differential length changes should be compensated during the construction process of high‐rise composite structures. Copyright © 2013 John Wiley & Sons, Ltd.     

Nonlinear time history analysis of a super‐tall building with setbacks in elevation

Standing 260 m above the ground, the super‐tall building employs steel reinforced concrete frame and reinforced concrete core wall system strengthened by a belt truss story to resist lateral and vertical loads. It has two setbacks in elevation. One is structurally designed by direct termination of vertical members, and the other is realized by inclining columns. Because of these characteristics, the building is classified as an irregular and complex structure. To investigate the seismic behavior of the structure under rare earthquake action, a refined finite element model was developed by using ABAQUS (Dassault Systèmes Simulia Corp., Providence, RI, USA). Nonlinear time history analyses were conducted using explicit integration method. The results show that the structural system has sufficient seismic capacity and ductility to resist rare earthquake. The plastic deformation capacity of this building can meet the requirement of Chinese code, and seismic performance objective of no collapse under rare earthquake can be reached. However, deformations were found concentrated in members within and adjacent to setback stories, at the bottom strengthening portion of core walls and its upper story where lateral stiffness suddenly changed. It was suggested that transfer stories should be placed above or below these stories to improve the concentration of strain and deformation. Copyright © 2011 John Wiley & Sons, Ltd.     

Outrigger system analysis and design under time‐dependent actions for super‐tall steel buildings

The outrigger system has been widely adopted as an efficient structural lateral‐load resisting system for super‐tall buildings in recent years. Although the outrigger system has many structural advantages, it has a significant defect due to differential shortening, which cannot be neglected. Due to the shrinkage and creep of concrete, as well as the differential settlement of foundation, the shortening of the structural member is an important time‐dependent issue, which leads to additional forces in the outriggers after the lock‐in of the outriggers. As a result, it will increase the size of the structural member cross section in the design. In a real project, engineers can delay the lock‐in time of the outrigger system to release the additional forces caused by the differential shortening during the construction phase. The time‐dependent actions, such as the column shortening and the differential settlement of the foundation, were estimated. A mega frame steel structure was employed to illustrate the analysis and design of the outrigger under the time‐dependent actions. Furthermore, a simple optimal method, considering the structural stability and overall stiffness, was proposed to optimize the construction sequence of the outrigger system.     

A temporal hybrid dynamic integration algorithm strategy for inelastic time history analysis of high‐rise reinforced concrete structures under strong earthquakes

A complete earthquake time history analysis (THA) requires a stable, accurate, and efficient dynamic integration algorithm. It is not rare to encounter numerical divergence when some implicit algorithms are used to deal with severe materially or geometrically nonlinearities. For explicit algorithms, computational efficiency is always a major concern. A temporal hybrid dynamic algorithm (THDA) strategy, which is specialized in the inelastic THAs of high‐rise reinforced concrete (RC) structures experiencing severe plasticity development, is developed herein. A preliminary evaluation is carried out on three low‐rise structural models, that is, two frame structures and one wall‐frame structure, for each group of collected implicit algorithms and explicit algorithms. From the evaluation, four alternatives are generated for the subsequent detailed assessment. A general framework for the THDA is proposed and implemented on a finite element analytical platform. The four alternatives are assessed based on their performance on a high‐rise frame core‐tube RC structure. The assessment indicates that the proposed THDA strategy can give rise to a more compatible dynamic integration algorithm for the complete THAs of high‐rise building structures when they are experiencing severe damage. The concerns about the computational stability, accuracy, and efficiency of the dynamic algorithms can be well balanced by the THDA.     

Seismic response of semi‐active friction‐damped reinforced concrete structures with self‐variable stiffness

The influence of structural self‐variable stiffness and semi‐active friction dampers on the behavior of reinforced concrete (RC) buildings during strong earthquakes is discussed. A fully braced six‐story beamless RC frame is analyzed. The effect of concrete braces (with only constructive reinforcement) as a self‐variable mechanism is studied. It is shown that up to a certain limit the frame itself controls its behavior by adapting its dynamic characteristics in the real time of the earthquake. This self‐adaptation is achieved by autonomous disengagement of the braces under tension and their further nonlinear action under compression. The system has several levels of seismic adaptation, and it selects one of them for enhanced response to the given earthquake. However, when the limit is reached, further self‐adaptation of the frame becomes impossible. The occurrence of an earthquake of higher magnitude can then lead to disengagement of the concrete braces under compression, intensifying structural damage and even causing collapse. The use of semi‐active controlled friction dampers is proposed as a means of preventing the collapse of braces under compression, thereby enabling structures to withstand earthquakes. The forces in the friction dampers are regulated according to an optimal control algorithm. Modulation of the friction level in real time during the earthquake yields additional improvement of structural seismic behavior and obviates the need for retrofitting. Copyright © 2007 John Wiley & Sons, Ltd.     

Seismic performance of steel braced frames with self‐centering buckling‐restrained brace utilizing superelastic shape memory alloys

Buckling‐restrained braced frame (BRB) is one of the newest seismic force‐resisting systems used in buildings. However, one of the requirements for designing a structure is to provide a ductility behavior of structures to dissipate earthquake energy and to avoid residual drifts. These days, self‐centering seismic lateral force‐resisting systems have drawn attention due to their potentials to solve the above mentioned issues. On the other hand, shape memory alloys (SMAs) are characterized by unique superelastic behavior, which enables the material to recover its original shape after experiencing large deformations. The goal of this study is to assess BRBs whose ductility are improved by utilizing SMA. Nonlinear time history and incremental dynamic analysis techniques are applied to investigate the behavior of the two frames with different stories (four and eight stories) under different ground motion records. The results showed that utilizing BRB made of hybrid steel and SMA resulted in increasing ductility of the structure and decreasing residual displacements of the structures.     

Echtzeit‐Röntgenuntersuchungen an duktilen stabförmigen Holzverbindungen bei dynamischer Beanspruchung

Real time radioscopy investigations on ductile dowel‐type fastener under dynamic load. This article discusses an application of real‐time radioscopy in the investigation of dynamically loaded connections. Real‐time radioscopy is especially suitable for connections of wood structures due to differences in density that are required for a radiographic image. With a real‐time system, one can observe deformations of fasteners inside the investigated specimen. Theoretically, deformations can be quantified using image‐processing techniques. The evaluation of deformations is, however, relatively complex due to the dimensionality of the problem where a 3‐dimenional object is collapsed into its 2‐D projection. A radiographic system capable of 30Hz frame acquisition frequency was used to study monotonically and cyclically loaded spliced and beam‐to‐column connections and an image‐processing technique was used to estimate strains and study failure modes of the fasteners.     

Leistungsfähigkeit von Betonen mit Flugasche

Performance Capacity of Concrete containing Fly Ash The conception to take into account the addition of fly ash to structural concrete presented in the German Standard DIN 1045‐2 should guarantee a high resistance of concrete structures against corrosive attack. In this context, criteria such as compressive and flexural strength of concrete, modulus of elasticity, bond strength, shrinkage and creep as well as the pore structure are focused in this paper. The obtained results on concretes containing a maximum of fly ash according to DIN 1045‐2 compared with concretes produced only with ordinary Portland cement show similar strength values at the age of 28 days while the strength development is somewhat lower for the fly ash concretes. However, at the age of one year the strength of fly ash concretes in particular also the flexural strength is considerably higher. Shrinkage and creep of the fly ash concrete is lower resulting in an overall reduced risk of shrinkage and thermal cracking.     

Beton‐Beton Verbindungssystem in einer Festen Hochgeschwindigkeits‐Fahrbahn

Concrete Connection System for a High Speed Rail Track The concrete‐concrete connections for HSRT's have to perform not only the requirements in terms of the “fatigue load capacity” but also of the maximum deformation. In normal case the proof of deformation is controlling the design. Because of the complex task a structural analysis cannot be performed at the moment. Therefore a design based on experimental methods is necessary. During the development of a practical application the deformations increased super proportional with an increasing load range and is connected to a multiplicity of parameters. The load history is a parameter with a special importance. This report describes how the task can be solved constructive and how to proceed to get reliable results.     

Time-dependent analysis of RC frame structures considering construction sequences

This paper deals with a time-dependent analysis of reinforced concrete (RC) frame structures considering the construction sequences. Because of the non-mechanical deformations induced by the time-dependent deformations of concrete, concrete structures usually present different behaviors when the construction sequences are changed, despite having the same structural configurations. Therefore, the time-dependent effects of concrete such as creep and shrinkage must be taken into consideration to simulate the actual behavior of RC frame structures. The material nonlinearity including the cracking of concrete is taken into consideration, and geometric nonlinearity due to the P-Δ$\mathrm{\Delta }$ effect is also taken into account by using the initial stress matrix. In addition, the creep deformation of concrete is described in accordance with a first-order algorithm based on the expansion of a degenerated kernel of the compliance function. Finally, correlation studies with previous numerical results and experimental data are conducted to verify the validity of the proposed model. An analysis of a 10-story RC frame structure is carried out to assess the differences in structural responses according to the construction sequences.     

Simplified multi‐degree‐of‐freedom model for estimation of seismic response of regular wall‐frame structures

A simplified multi‐degree‐of‐freedom (MDOF) model is developed for estimation of seismic response of tall wall‐frame structures. By using the continuum technique for the structure and adopting the bilinear hysteretic model for material properties, procedure for the development of the simplified MDOF model is derived. The numerical study for a 20‐storey reinforced concrete (RC) wall‐frame structure is conducted to investigate the accuracy of seismic response predicted by the proposed model. Results from the nonlinear response history analyses based on the proposed MDOF model and the detailed structural model with member‐by‐member representation are compared and show very good agreement. The proposed simplified MDOF model is shown to provide a simple, efficient and accurate method for estimation of seismic performance of tall wall‐frame structures. Copyright © 2009 John Wiley & Sons, Ltd.     

Erratum

Erratum: Creep and shrinkage analysis of reinforced concrete frames by history‐adjusted and shrinkage‐adjusted elasticity moduli The above mentioned article was published online on the 2 August 2007 (DOI: 10.1002/tal.391). An error was subsequently identified on page 5 in equation 11 . Please find the corrected version below: (11)     

Spannungen durch Verformungsbehinderung in gebundenen Aufbetonen

Bonded concrete overlays‐stresses resulting from restrained deformations The bonded concrete overlay technique is one of the most widely used methods for repair, retrofitting and rehabilitation of concrete structures. Differential shrinkage is commonly the most crucial factor influencing the performance of the composite system and may result in extensive overlay cracking. Based on strain measurements, fundamental characteristics of overlay strain and stress development under restrained conditions were identified. The restraint of overlay deformations was found to depend less on the relative cross‐sectional dimensions of substrate and overlay than is commonly assumed. A new analytical approach for the design of bonded overlays subjected to differential shrinkage is introduced, based on localised strain conditions at the interface between substrate and overlay. A parameter study of the most important influences on overlay stress development is performed.     

Analytical model and evaluation of maximum crack width for unbonded PSRC frame beam under short‐term service load

Prestressed steel reinforced concrete (PSRC) beam members have the advantages of both ordinary prestressed concrete and SRC members and are usually applied to the structures with large span or heavy load. They are often designed to crack under service load. In this paper, the service‐load behavior is studied based on the experimental results of four unbonded PSRC frame beam specimens. The cracking behavior, deflection, and strains in tensile reinforcement during service‐load stage are described in detail. A computer program for a simple macroelement analysis approach, based on conventional matrix‐displacement method, is written to predict the response of unbonded PSRC frame beam members under service load. The calculation results by this method agree well with the observed experimental results. Moreover, an approach based on two enacted Chinese codes, one for ordinary concrete members (GB50010‐2010) and another for steel‐concrete composite members (JGJ138‐2001), is provided to calculate the short‐term maximum crack width of PSRC beam members. By comparing with the test results, it implies that this approach can be applied to the evaluation of short‐term maximum crack width.     

混凝土早龄期性能与裂缝控制

从混凝土微观结构出发,研究普通混凝土、高性能混凝土成型过程的时变温度场;对裂缝持续增长的早龄期混凝土物理、力学特性进行深入、系统的研究;综合考虑混凝土材料、结构特征,研究混凝土结构早期裂缝的成因机理、分析方法与控制措施;提出裂缝开展的预测和控制方法,建立裂缝扩展过程的损伤模型;在理论分析与试验研究的基础上,总结了混凝土早龄期的水化、温度、收缩、徐变、力学性能和断裂性能随时间的发展规律;综合分析各种因素对混凝土早期开裂的影响,推导了混凝土结构内部应力的增量计算方法。对室内试验与实际工程的研究表明：对混凝土早期开裂的分析应该是对混凝土温度、收缩、徐变、力学性能、结构特征等因素的综合动态分析,提出的混凝土早龄期开裂分析模型与试验结果吻合良好,采用理论模型结合数值模拟的方法可以有效提高分析过程的效率与准确性。图18参22