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Review of design parameters for FRP-RC members detailed according to ACI 440.1R-06
Abstract
This paper investigates the parameters that control the design of Fiber Reinforced Polymer (FRP) reinforced concrete flexural members proportioned following the ACI 440.1R-06. It investigates the critical parameters that control the flexural design, such as the deflection limits, crack limits, flexural capacity, concrete compressive strength, beam span and cross section, and bar diameter, at various Mean-Ambient Temperatures (MAT). The results of this research suggest that the deflection and cracking requirements are the two most controlling limits for FRP reinforced concrete flexural members.
... Shear failure mode is as much as flexural failure importance because of its rapid progression (El-Chabib et al. 2005). Although shear resistance mechanisms of FRP-RC beams with and without stirrups are mainly similar to the steel reinforcement case, the material differences between FRP and steel result in different shear resistance performances (Jnaid and Aboutaha 2013). Available design codes are essentially developed by modifications on the shear design equations of steel reinforced beams with the aim of minimizing these differences. ...
In the present study, group method of data handling networks (GMDH) are adopted and evaluated for shear strength prediction of both FRP-reinforced concrete members with and without stirrups. Input parameters considered for the GMDH are altogether 12 influential geometrical and mechanical parameters. Two available and very recently collected comprehensive datasets containing 112 and 175 data samples are used to develop new models for two cases with and without shear reinforcement, respectively. The proposed GMDH models are compared with several codes of practice. An artificial neural network (ANN) model and an ANFIS based model are also developed using the same databases to further assessment of GMDH. The accuracy of the developed models is evaluated by statistical error parameters. The results show that the GMDH outperforms other models and successfully can be used as a practical and effective tool for shear strength prediction of members without stirrups (R²=0.94) and with stirrups (R²=0.95). Furthermore, the relative importance and influence of input parameters in the prediction of shear capacity of reinforced concrete members are evaluated through parametric and sensitivity analyses.
... Therefore, deflections and crack width should be controlled for the serviceability limit state specified in ACI 440. 1R-06 (2006) (Jnaid and Aboutaha 2013). Deflection should be estimated for both short term and long term loads that might sustained under service loads condition and it must be less than the allowable deflections permitted by the code. ...
This paper presents a generalized formulation for optimizing the design of concrete beam reinforced with glass fiber reinforced polymer bar. The optimization method is formulated to find the design variables leading to the minimum weight of concrete beam with constraints imposed based on ACI code provisions. A simple genetic algorithm is utilized to solve the optimization task. The weights of concrete and glass fiber reinforced polymer bar are included in the formulation of the objective function. The ultimate limit states and the serviceability limit states are included in formulation of constraints. The results of illustrated example demonstrate the efficiency of the proposed method to reduce the weight of beam as well as to satisfy the above requirement. The application of the optimization based on the most economical design concept have led to significant savings in the amount of the component materials to be used in comparison to classical design solutions.
Strut-and-tie model (STM) has been recommended by many codes and standards as a rational model for discontinuity regions in structural members. STM has been adopted in ACI building code for analysis of reinforced concrete (RC) deep beams since 2002. However, STM recommended by ACI 318-11 is only applicable for analysis of ordinary RC deep beams. This paper aims to develop the STM for CFRP strengthened RC deep beams through the strut effectiveness factor recommended by ACI 318-11. Two sets of RC deep beams were cast and tested in this research. Each set consisted of six simply-supported specimens loaded in four-point bending. The first set had no CFRP strengthening while the second was strengthened by means of CFRP sheets using two-side wet lay-up system. Each set consisted of six RC deep beams with shear span to effective depth ratio of 0.75, 1.00, 1.25, 1.50, 1.75, and 2.00.The value of strut effectiveness factor recommended by ACI 318-11 is modified using a proposed empirical relationship in this research. The empirical relationship is established based on shear span to effective depth ratio.
Synopsis: Compared to ordinary steel reinforcement, Fiber-Reinforced Polymer (FRP) reinforcing bars have a lower stiffness, display a brittle-elastic response, and possess particular bond characteristics. The dependence on these distinctive features makes deflection control in FRP-reinforced concrete beams and one-way slabs a more elaborate process compared to the traditional serviceability design of steel-reinforced members. This paper reports the rationale and fundamental concepts backing the indirect deflection control procedure for concrete beams and one-way slabs reinforced with FRP bars adopted by ACI 440.1R-06. The fundamental procedure can be applied regardless of the type of reinforcement; it is independent of the member's stiffness through the cracked stage; and it is expressed as a function of the deflection-span ratio, which allows designers to fully control deflections depending on applicable serviceability limits. The paper also explains the simplifications made to the fundamental procedure that led to the development of the indirect deflection control procedure in tabular form found in ACI 440.1R-06, including the method by which tension stiffening effects are accounted for.
This paper investigates the design of fiber reinforced polymer (FRP) reinforced concrete based on ACI 440.1R serviceability requirements related to deflection control of one-way slabs and rectangular beams, and uses this information as the basis for evaluating the minimum member thickness requirements needed to satisfy ACI 318 deflection limits. Serviceability is shown to govern design in most cases, as flexural members designed for deflection control are usually stronger than required. Slabs satisfying deflection requirements have a service load that ranges from 20 to 30% of the nominal member capacity, while service loads for beams range from 35 to 45% of the member capacity. Recommended minimum member thickness values for slabs are too conservative and require revision, while those for beams appear reasonable. A practical approach for design of FRP reinforced concrete members is proposed based on selection of member thicknesses needed to satisfy deflection and strength criteria.
The structural reliability of concrete flexural members reinforced with fiber reinforced polymer (FRP) reinforcement is investigated. Reliability indices based on the equations for flexure in ACI 440.1R-03, which uses the load factors from ACI 318-99 are presented. Choice of a resistance factor for flexure for ACI 440.1R-06, which uses the load factors from ACI 318-02 is also presented. Flexural designs using either ACI 440.1R-03 or ACI 440.1R-06 provide sufficient reliability, with reliability indices between 3.5 and 4.8, with the older versions of ACI 440.1R yielding higher reliability. An analysis of curvature of the beams at failure showed that flexural members that fail by FRP reinforcement rupture have ductilities similar to those that fail by concrete crushing, indicating that FRP reinforcement fracture is not necessarily a more brittle failure mode than concrete crushing.
Serviceability related to deflections and cracking often controls design of fiber reinforced polymer (FRP) reinforced concrete. The existing approach prescribed in ACI 318 for computing deflection of steel reinforced concrete overestimates member stiffness when FRP is used as the reinforcement. Deflection is then underestimated. Numerous proposals have consequently been made for computing deflection of FRP reinforced concrete, and have mostly involved modifications to Branson's original ACI expression for the effective moment of inertia Ie. This paper reviews the different procedures used in the past by ACI Committee 440 to compute deflection of FRP reinforced concrete members, and discusses deficiencies of past and present ACI 440.1R deflection calculation guidelines. A case is made for the need to adopt a more rational approach to compute deflection and the basis for proposed changes are reviewed and explained in detail. A statistical comparison of past, present, and proposed approaches for computing deflection are compared with an experimental database that justifies the need for a more rational approach to computing deflection. The paper ends with a clear set of deflection procedures for carrying out serviceability design of FRP reinforced concrete.
The newly expanded and revised edition of Fiber-Reinforced Composites: Materials, Manufacturing, and Design presents the most up-to-date resource available on state-of-the-art composite materials. This book is unique in that it not only offers a current analysis of mechanics and properties, but also examines the latest advances in test methods, applications, manufacturing processes, and design aspects involving composites. This third edition presents thorough coverage of newly developed materials including nanocomposites. It also adds more emphasis on underlying theories, practical methods, and problem-solving skills employed in real-world applications of composite materials. Each chapter contains new examples drawn from diverse applications and additional problems to reinforce the practical relevance of key concepts. New in The Third Edition: • Contains new sections on material substitution, cost analysis, nano-and natural fibers, fiber architecture, and carbon-carbon composites • Provides a new chapter on polymer-based nanocomposites • Adds new sections on test methods such as fiber bundle tests and interlaminar fracture measurements • Expands sections on manufacturing fundamentals, thermoplastics matrix composites, and resin transfer molding Maintaining the trademark quality of its well-respected and authoritative predecessors, Fiber-Reinforced Composites: Materials, Manufacturing, and Design, Third Edition continues to provide a unique interdisciplinary perspective and a logical approach to understanding the latest developments in the field.
Deflection of reinforced concrete is typically computed with an effective moment of inertia I(e) that accounts for nonlinear behavior after the concrete cracks. Existing expressions for I(e) tend to overpredict the member stiffness of concrete reinforced with fiber-reinforced polymer (FRP) bars, and an alternative expression is used as the basis for developing a practical design approach to compute deflection. The proposed expression has a rational basis that incorporates basic concepts of tension stiffening to provide a reasonable estimate of deflection for both steel and FRP-reinforced concrete without the need for empirically derived correction factors. Calculation of deflection with the proposed expression for I(e) is recommended using the code value for the elastic modulus E(c) of concrete because computed values of deflection are relatively insensitive to variations in E(c), and shrinkage restraint is taken into account by using a reduced cracking moment less than the code-based value of the cracking moment M(cr). I(e) is conservatively based on the moment at the critical section (where the member stiffness is lowest), unless more accuracy is required with an integration-based expression that gives an equivalent moment of inertia I(e)(') to account for the variation in stiffness along the member length. Recommendations are validated by comparison with a database of deflection test results for FRP-reinforced concrete. DOI: 10.1061/(ASCE)CC.1943-5614.0000195. (C) 2011 American Society of Civil Engineers.
This paper reports on the key features of the "Guide for the Design and Construction of Concrete Reinforced with FRP Bars" issued by the American Concrete Institute (ACI). For new construction, FRP bars have been used as the internal reinforcement in concrete members to replace conventional steel rebars for a host of reasons. The principles for design and construction were first established and proposed to industry in 2001. The third revision of the guide is now under preparation.
The corrosion of reinforcement in concrete bridge deck has been the cause of major deterioration and of high costs in repair and maintenance. Glass Fibre reinforced polymer (GFRP) reinforcement is a more durable alternative to steel reinforcement and has higher strength to weight ratio. Due to the low value of elasticity and brittle behaviour of GFRP, the service behaviour of GFRP reinforced concrete structure is critical. However, laterally restrained slabs, such as those in bridge deck slabs, exhibit arching action or compressive membrane action (CMA) which has a beneficial influence on the service behaviour such as the deflection. This paper presents the results of experimental tests and numerical analysis of laterally restrained GFRP reinforced concrete slabs with varying some structural variables. The analysis results are discussed and conclusions on the compressive membrane action in GFRP reinforced concrete slabs are presented.
Fundamental concepts of tension stiffening are used to explain why Branson's equation for the effective moment of inertia I, does not predict deflection well for fiber reinforced polymer (FRP) reinforced concrete beams. The tension stiffening component in Branson's equation is shown to depend on the ratio of gross-to-cracked moment of inertia (I-g/I-cr), and gives too much tension stiffening for beams with an I-g/I-cr, ratio greater than 3. FRP beams typically have an I-g/I-cr ratio greater than 5, leading to a much stiffer response and underprediction of computed deflections as observed by others in the past. One common approach to computing deflection of FRP reinforced concrete beams has been to use a modified form of the Branson equation. This paper presents a rational development of appropriate modification factors-needed to reduce the tension stiffening component in Branson's original expression to realistic levels. Computed deflections using this approach give reasonable results with the right modification factor, and compare well with a more general unified approach that incorporates a realistic tension stiffening model. Comparison is made with the existing and past correction factors recommended by ACI 440 for predicting deflection of FRP beams. The method presently used by ACI 440 gives reasonable estimates of deflection for glass and carbon FRP reinforced beams. However, this method underestimates deflection of aramid FRP reinforced beams and is restricted to rectangular sections. A proposal is made for adoption of a simple modification factor that works well for all types of FRP bar and beam cross-sectional shape.
Corrosion of the steel reinforcement in a cold and saline environment leads to the overall deterioration of reinforced concrete structures. To avoid such deterioration, fiber-reinforced polymers (FRP) rebars are used in place of steel reinforcement. In the present paper, test results of 12 concrete beams reinforced with FRP rods are reported. The main parameters investigated in the study include the reinforcement ratio and the concrete strength. Theoretical models are proposed for the prediction of crack width, crack spacing, load-deflection response, ultimate capacity, and modes of failure. The concept of deformability is also discussed.
This paper provides a critical evaluation of equations commonly used to compute short-term deflection for steel and fiber reinforced polymer (FRP) reinforced concrete beams. Numerous proposals have been made for FRP in particular, and the different approaches are linked together by comparing the tension-stiffening component of each method. Tension stiffening reflects the participation of concrete between cracks in stiffening the member response. The Branson equation used in North America and other parts of the world is based on an empirically derived effective moment of inertia to calculate deflection. The tension-stiffening component with this method is highly dependent on the applied level of loading relative to the cracking load as well as the ratio of uncracked-to-cracked transformed moment of inertia (I-g/I-cr) for the beam section. Tension stiffening is overestimated for the high I-g/I-cr ratios typical with FRP concrete, leading to a much stiffer response and underprediction of member deflection. Deflection of steel reinforced concrete with reinforcing ratios less than 1% is also likely to be underestimated because of higher I-g/I-cr ratios at these lower reinforcement levels, but not to the same extent. In both cases, service loads are less than twice the cracking load where tension stiffening is most significant. Modifications to Branson's equation for deflection prediction of FRP concrete soften the member response by reducing the tension-stiffening component, mostly by introducing empirical factors that effectively decrease the I-g/I-cr ratio. An alternative expression for calculating beam deflection is developed with a rational approach that incorporates a tension-stiffening model adopted in Europe. The proposed equation gives an effective moment of inertia that is independent of I-g/I-cr and works equally well for either steel or FRP reinforced concrete.
This paper evaluates the benefits of computing deflection with an equivalent moment of inertia based on integration of curvature to account for changes in member stiffness along the span. Results are evaluated for steel and fiber-reinforced polymer reinforced (FRP-reinforced) concrete flexural members with different loading arrangements and support conditions. Closed-form solutions of integrated expressions for deflection are expressed in terms of an equivalent moment of inertia I-e' and compared to deflection computed with an effective moment of inertia I-e based on the stiffness at the critical section. Results from this comparison are validated with measured deflections from an experimental database for FRP-reinforced concrete. Current code-related approaches are also compared to the experimental database. It is shown herein that the use of an integration-based expression for the moment of inertia can lead to improved prediction of deflection, though the use of an effective moment of inertia based on member stiffness at the critical section gives a reasonably conservative estimate of deflection in many cases. The benefits of taking account of changes in stiffness along the member span are more evident when low reinforcing ratios are used in combination with FRP reinforcement, and use of the integration-based expression I-e' may be warranted when deflection control is critical in such cases. DOI: 10.1061/(ASCE)CC.1943-5614.0000164. (C) 2011 American Society of Civil Engineers.
Deflection control is an important performance criterion that needs to be satisfied to ensure serviceability of the structure for its intended use. The extent of cracking and amount of reinforcement affects the flexural rigidity, E1, of a reinforced concrete member and both the Canadian concrete design standard (CSA A23.3-04) and ACI Building Code (ACI 318-05) use an effective moment of inertia, I, that was originally proposed by Branson to compute beam deflection. This is an empirically derived equation that works well within a narrow range of limits corresponding to steel-reinforced concrete beams with a reinforcing ratio between 1% and 2%. However, the equation underestimates deflection for steel-reinforced concrete beams and slabs with a reinforcing ratio less than 1% and for most beams reinforced with low-modulus, fibre-reinforced-polymer (FRP) bars. Deflection of slender tilt-up wall panels can also be underestimated with Branson's equation. This paper provides an explanation of why the Branson equation does not always work well in predicting deflection, and presents a rational approach to develop an alternative expression for the effective moment of inertia that works equally well for both steel- and FRP-reinforced concrete at all reinforcing ratios. A rational expression is also introduced for continuous beams that uses an averaged moment of inertia, to calculate beam deflection. Changes are included in a proposed revision to deflection prediction requirements specified in clause 9.8 of CSA A23.3-04.
The characterization of fiber reinforced polymer (FRP) bars for concrete reinforcement is necessary for design purposes as required by structural engineers, and for quality control/optimization purposes as required by bar manufacturers. This paper reports on a test protocol and the results obtained from a replicated experiment intended to yield a statistically valid estimate of the distribution of tensile strength in FRP bars. Four selected types of glass FRP (GFRP) bars with the same diameter were tested. In total, 32 bars from the same manufacturer were investigated. Instead of a polymeric resin-based anchor, a steel pipe filled with expansive cementitious grout was used as the end restrainer. An experiment based on a randomized complete block design was carried out to obtain data for statistical analysis. The analysis was carried out using a commercially available data analysis software program. This research project indicates that the suggested test procedure provides reliable data for tensile characterization and confirms that a Gaussian distribution can represent the tensile strength of the GFRP bars as tested.
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