Showing papers in "Aci Structural Journal in 1991"
TL;DR: In this paper, a series of 16 under-reinforced beams were tested to study the effectiveness of external strengthening using fiber reinforced plastic (FRP) plates, and an interative analytical method was developed to predict the stiffness and maximum strength in bending of the plated beam.
Abstract: A series of 16 under-reinforced beams was tested to study the effectiveness of external strengthening using fiber reinforced plastic (FRP) plates. Plates of glass, carbon and aramid fibers were bonded to the tension side of the beams using a 2-part epoxy. An interative analytical method was developed to predict the stiffness and maximum strength in bending of the plated beam. Increases in stiffness (over the working load range) from 17 to 99 percent and increases in strength (ultimate) from 40 to 97% were achieved for the beams with FRP plates. Predicted and acutal load-deflection curves showed fairly good agreement, although generally the theoretical curves were stiffer. Experimental failure did not occur in the maximum moment region on many of the beams, despite attempts at end anchorages to postpone local shear failure. The ulltimate loads of the beams that did fail in the maximum region were within about 5% of predicted values.
365Â citations
TL;DR: In this article, the authors present the results of recent tests on diagonal shear failure of reinforced concrete beams without stirrups, which indicate a significant size effect and show a good agreement with Bazant's law for size effect.
Abstract: The paper presents the results of recent tests on diagonal shear failure of reinforced concrete beams without stirrups. The results indicate a significant size effect and show a good agreement with Bazant's law for size effect. Scatter of the test results is much lower than that previously found by studying extensive test data from the literature, which have not been obtained on geometrically similar beams. The tests also show that preventing bond slip of the longituudinal bars causes an increase of the brittleness number of the beam. It is concluded that the current design approach, which is intended to provide safety against the diagonal crack initiation load, should be replaced or supplemented by a design approach based on the ultimate load, in which a size effect of the fracture mechanics type, due to release of stored energy must be taken into account.
253Â citations
TL;DR: In this paper, a research program investigating the retrofit of circular columns by encasing plastic hinge regions with a bonded steel jacket was presented, and results from 6 large-scale column tests showed that steel jacketing results in column ductilities as high as those available from confined columns designed to current codes and inhibits bond failures in lap splices of longitudinal reinforcement in the hinge regions.
Abstract: This paper reports results of a research program investigating the retrofit of circular columns by encasing plastic hinge regions with a bonded steel jacket. Results from 6 large-scale column tests showed that steel jacketing results in column ductilities as high as those available from confined columns designed to current codes and inhibits bond failures in lap splices of longitudinal reinforcement in plastic hinge regions. A secondary result from the tests was the identification of joint shear failure at the footing/column junction as a potential failure mechanism for old and current footing designs.
249Â citations
TL;DR: In this paper, a model for the structural behavior from cracking until failure of reinforced concrete-prestressed concrete slabs is presented, in which a simple distribution for the friction stresses along the crack is assumed and the resulting state of stress in the web of the member is described.
Abstract: For slender members without transverse reinforcement, like reinforced concrete-prestressed concrete slabs, a mechanical model is presented that explains the structural behavior from cracking until failure. The shear force is mainly transferred in the web. A simple distribution for the friction stresses along the crack is assumed and the resulting state of stress in the web of the member is clearly described. From a condition for the crack width in the web, an explicit equation for the ulltimate shear force is derived, which consistently describes the influence of axial forces and prestress. This model gives a clear explanation for the influence of the depth of the member on the ultimate shear force as well as that of the reinforcement ratio. Finaly, dimensioning diagrams are presented for the common case of reinforced concrete members like slabs.
149Â citations
TL;DR: In this paper, the authors reviewed existing prediction equations for the stress f sub pm in unbonded tendons at nominal bending resistance, compared prediction equations with experimental results from 143 beam tests reported in the technical literature, and pointed out some of the drawbacks in existing prediction models for f sub ps.
Abstract: The first part of this study reviewed existing prediction equations for the stress f sub pm in unbonded tendons at nominal bending resistance, compared prediction equations with experimental results from 143 beam tests reported in the technical literature, and pointed out some of the drawbacks in existing prediction models for f sub ps. In the second part, the background for a new rational methodology for the analysis of beams prestressed with unbonded tendons is covered, and a new prediction equation for f sub ps at ultimate is developed. The equation is shown to account for most of the variables found important in the analysis, and to predict experimental results with much better accuracy than any of the prediction equations reviewed in the first part. The developed equation is proposed for adoption in the ACI Building Code.
140Â citations
108Â citations
TL;DR: In this article, a limited truss model design approach with variable angle of inclination diagonals and a concrete contribution for beams with web reinforcement is proposed as a viable and economic design tool.
Abstract: A limited truss model design approach with variable angle of inclination diagonals and a concrete contribution for beams with web reinforcement is proposed as a viable and economic design tool. Computed values were compared with a wide range of test results of reinforced and prestressed concrete beams failing in shear. The modified truss model approach was shown to be conservative and in good agreement with test results. A diminishing concrete contribution is proposed to supplement the strength of the variable angle truss model for reinforced concrete beams.
84Â citations
TL;DR: In this paper, three reinforced concrete panels were subjected to cyclic pure shear and the results indicated that if reinforced concrete is subjected to repeated cycles of shear stress at any level above that which causes yield in the weaker reinforcement, it will eventually fail by concrete crushing.
Abstract: Three reinforced concrete panels were subjected to reversed cyclic pure shear. As the stress state over the panels was approximately uniform, an average stress-versus-average strain response could be measured and from this detailed data on the stresses in the concrete and steel components deduced. The results indicate that if reinforced concrete is subjected to repeated cycles of shear stress at any level above that which causes yield in the weaker reinforcement, it will eventually fail by concrete crushing. This has direct implications for the seismic-resistant design of regions such as beam-column joints. A constitutive model for reinforced concrete subejcted to general membrane loading is proposed.
82Â citations
TL;DR: In this paper, a review of existing experimental and analytical investigations dealing with the stress at ultimate in unbonded tendons is presented, with a summary of prediction equations for fpm suggested by different investigators in various North American and European codes.
Abstract: This study deals with the stress at ultimate fpm in unbonded tendons of prestresed and partially prestressed beams for the purpose of computing their nominal bending moment resistances. The first part comprises mainly: (1) some background information followed by a comprehensive review of existing experimental and analytical investigations dealing with the stress at ultimate in unbonded tendons, (2) a summary of prediction equations for fpm suggested by different investigators in various North American and European codes, and (3) an evaluation of typical prediction equations achieved by comparing predicted with experimentally observed results. The experimental results were obtained from 9 different investigations totaling 143 beam tests described in the technical literature and carried out since 1960 in various parts of the world. In the second part of this study, the background for a new rational methodology for analysis of beams prestressed with unbonded tendons is covered and a new prediction equation for fpm at ultimate is developed.
75Â citations
TL;DR: In this article, the authors provided data on the behavior of noncontact lap splices subjected to repeated inelastic tensile loading and proposed guidelines for the design of spaced splices.
Abstract: Noncontact lap splices are common in reinforced concrete construction and, although the ACI Building Code allows use of the spaced splices, design guidelines are limited. Previous experimental research on noncontact lap splices indicates a behavior similar to that of contact lap splices, but these earlier tests had small spacings of the spliced bars and were loaded under monotonic loading. No experimental tests or guidelines existed for seismic loading. The study reported in this paper provides data on the behavior of noncontact lap splices subjected to repeated inelastic tensile loading. The results show that spaced splices can sustain somewhat more inelastic load cycles than contact lap splices and that a truss model is an accurate description of behavior. Guidelines are proposed for the design of spaced splices.
68Â citations
TL;DR: In this paper, the authors conducted flexural tests on 6 singly reinforced concrete beams and concluded that to achieve a ductility index of 3, rho/rho sub b should not exceed 0.40 for beams with concrete strengths of 8000 psi or 0.20 for beams having concrete strength of 11,000 psi.
Abstract: Flexural tests were conducted on 6 singly reinforced beams. The variables were strength of concrete and the ratio of tensile steel content r as a ratio of the balanced steel content. No compression or lateral reinforcement was used in this study. Test results are presented in terms of load-deformation behavior, ductility indexes, and cracking patterns. It is concluded that to achieve a ductility index of 3, rho/rho sub b should not exceed 0.40 for beams with concrete strengths of 8000 psi or 0.20 for beams with concrete strength of 11,000 psi. The flexural design provisions of the ACI Building Code are found to be adequate to predict the strength of reinforced high-strength concrete beams.
TL;DR: In this paper, an approximate analytical approach based on the observation that flexural-shear cracking results from a severe localization of internal stress after the onset of flexural cracking was proposed.
Abstract: An hypothesis is proposed for the shear cracking mechanism in point-loaded reinforced concrete beams with no web reinforcement. The hypothesis results from an approximate analytical approach based on the observation that flexural-shear cracking results from a severe localization of internal stress after the onset of flexural cracking. This local stress concentration is associated with the nature of bond between concrete and flexural reinforcement, and the development of arch action in the end region of the beam. Analytical expressions are developed for predicting the shear cracking load and the corresponding location of the critical shear crack. The adequacy of these expressions is then established by comparison with published test data.
TL;DR: The experimental results indicated that increasing the span-to-depth ratio from 8 to 20 resulted in a drop in the measured stress results by about 35%. On the other hand, the effect of load application was found insignificant as mentioned in this paper.
Abstract: The test results of 26 concrete beam specimens prestressed with unbonded tendons and reinforced with and without ordinary reinforcing steel are described. Several parameters and their effect on the magnitude of stress in the prestressing steel f sub ps at nominal flexural strength of the members were examined. The experimental results indicated that increasing the span-to-depth ratio from 8 to 20 resulted in a drop in the measured stress results by about 35%. On the other hand, the effect of load application was found insignificant. The experimental results were combined with extensive experimental data reported in the technical literature and used to develop a general design equation for f sub pm.
Journal Article•
TL;DR: In this article, a structural model that explains the structural behavior from cracking until failure of reinforced concrete-prestressed concrete slabs is presented, where the shear force is mainly transferred in the web.
Abstract: For slender members without transverse reinforcement, like reinforced concrete-prestressed concrete slabs, a mechanical model is presented that explains the structural behavior from cracking until failure The shear force is mainly transferred in the web A simple distribution for the friction stresses along the crack is assumed and the resulting state of stress in the web of the member is clearly described From a condition for the crack width in the web, an explicit equation for the ulltimate shear force is derived, which consistently describes the influence of axial forces and prestress This model gives a clear explanation for the influence of the depth of the member on the ultimate shear force as well as that of the reinforcement ratio Finaly, dimensioning diagrams are presented for the common case of reinforced concrete members like slabs
TL;DR: In this article, a column failure is defined for design purposes as the peak of the diagram of axial load versus mid-length bending moment at constant load eccentricity, and the tangent modulus load is found to be apporximately equal to the peak load of column with load eccentricities 0.01 of the cross-sectional thickness.
Abstract: The paper presents a simple new method to calculate column-interaction diagrams, which takes into account slenderness effects. The method consists of a simple incremental loading algorithm that traces the load-deflection curve at constant eccentricity of the axial load. The column failure is defined for design purposes as the peak of the diagram of axial load versus midlength bending moment at constant load eccentricity. The tangent modulus load is found to be apporximately equal to the peak load of column with load eccentricity 0.01 of the cross-sectional thickness and represents a lower bound for the maximum loads at still smaller eccentricities. Strain irreversibility at unloading can be taken into account but its effect is very small. The method is compared with the ACI moment magnification method and with the CEB Model column method based on moment-curvature relations. The agreement with the CEB method is very close, but with respect to the ACI method there are large discrepancies.
TL;DR: In this article, the effect of superimposed slab loading on the behavior of exterior slab-column connections was studied by testing 2-bay slab-Column subassemblies, where each subassembly consisted of an interior and two exterior connections.
Abstract: The effect of superimposed slab loading on the behavior of exterior slab-column connections was studied by testing 2-bay slab-column subassemblies. Each subassembly consisted of an interior and two exterior connections. Three identical slab-column subassemblies were subjected to the same cyclic lateral-displacement history while each supported a different superimposed slab load. The tests show that increased slab load substantially reduces the lateral drift capacity of the connections. A limiting shear stress is suggested to insure adequate drift capacity for exterior slab-connectiions. A rational method to calculate the moment capacity of exterior connections is developed. The test results are compared with present ACI 318-83 Building Code provisions and ACI Committee 352's recommendations for design of slab-column connections.
TL;DR: In this article, moment-curvature relationships are obtained for reinforced concrete beams using the developed algorithm, which is based on stress-strain curves for materials (steel and concrete) and demonstrated on a typical concrete bridge T-beam.
Abstract: Reinforcing steel area may be reduced due to corrosion or mechnical damage. Moment-curvature relationships are obtained for reinforced concrete beams using the developed algorithm. The analysis is based on stress-strain curves for materials (steel and concrete). The developed iterative numerical procedure is demonstrated on a typical concrete bridge T-beam. Moment-curvature relationships are obtained for various degrees of reinforcing steel area loss.
TL;DR: The complete load-deformation response of reinforced fiber concrete deep beams was investigated both analytically and experimentally in this article, where the softened truss model theory for nonfibrous concrete was modified using new stress-strain relationships for fiber concrete, and a series of tests was carried out to generate relevant information.
Abstract: The complete load-deformation response of reinforced fiber concrete deep beams in investigated both analytically and experimentally. The softened truss model theory for nonfibrous concrete has been modified using new stress-strain relationships for fiber concrete, and a series fo tests was carried out to generate relevant information. Test results indicate that including discrete fibers in concrete significantly improves the strength and deformation characteristis of deep beams. A comparison of test results with theoretical predictions of the load-deformation response shows very good agreement for the entire range of loading.
TL;DR: The ACI-ASCE Committee 352 for the design of ductile moment-resistant beam-column connections limit the joint-shear stress to a gamma factor relationship, where the gamma factor is a function of the type of joint and loading condition as discussed by the authors.
Abstract: The present recommendations of ACI-ASCE Committee 352 for the design of ductile moment-resistant beam-column connections limit the joint-shear stress to a gamma factor relationship, where the gamma factor is a function of the type of joint and loading condition. Test results of corner connections subjected to inelastic cyclic loading show that when a frame is constructed with high-strength concrete with compressive strength between 8000 and 14,000 psi, the equation becomes unconservative. In addition, the current recommendations for joint confinement, which were devloped for normal strength concrete, cannot be satisfied for high-strength concrete frames. Based on the test results, new requirements for allowable joint-shear stress as well as joint confinement are presented to insure ductile behavior of frames.
TL;DR: In this paper, an efficicent algorithm is developed to solve the 13 equations derived in the softened truss model theory for the nonlinear analysis of membrane elements, which not only simplifies the analysis of concrete elements reinforced with mild steel but is also very suitable for prestressed concrete.
Abstract: An efficicent algorithm is developed to solve the 13 equations derived in the softened truss model theory for the nonlinear analysis of membrane elements This method not only simplifies the analysis of concrete elements reinforced with mild steel, but is also very suitable for prestressed concrete
TL;DR: In this paper, an experimental investigation was carried out on a total of 28 simply supported ferrocement I-beams loaded in flexure under two symmetrical point loads.
Abstract: An experimental investigation was carried out on a total of 28 simply supported ferrocement I-beams loaded in flexure under two symmetrical point loads. The major parameters considered in the study were the shear span-to-depth ratio a/h, compressive strength of the mortar, and amount of longitudinal and transverse reinforcement. The results of these tests are presented and discussed. Test results indicate that the beams fail in shear only when the a/h is less than or equal to 1.5. Beyond this value, failure occurs in flexure. Analyses are presented to predict the cracking and ultimate strength of such beams in shear as well as flexure. A comaprison of theoretical predictions with experimental results shows good agreement.
TL;DR: In this paper, the failure modes associated with soft impulsive loads for reinforced concrete slabs are analytically predicted using a nonlinear dynamic layered finite element method and the failure mechanism at the ultimate states is also predicted and found to be in good agreement with the actual phenomenon.
Abstract: The faillure modes that can be associated with soft impulsive loads for reinforced concrete slabs are analytically predicted using a nonlinear dynamic layered finite element method. The failure mechanism at the ultimate states is also predicted and found to be in good agreement with the actual phenomenon. The effects of loading rates during impulsive loadings are also considered, and it is found that the failure modes are affected by the loading rates and also impulse from an impulsive load function. The failure modes can be predicted using the proposed analytical method. The failure modes can be classified into three different regions based on the loading rates. It can be concluded that this method may be used in future as a tool for a dynamic design method for reinforced concrete structure under soft impulsive loads.
TL;DR: In this article, a triaxial failure criterion is applied together with the theory of plasticity for modeling concrete in reinforced concrete (RC) slabs subjected to impulsive loads, and the transverse shear stresses are extrapolated and then included into the dynamic layered finite element procedure, as it is assumed to affect not only the ultimate behaviors but also the failure modes.
Abstract: A triaxial failure criterion is applied together with the theory of plasticity for modeling concrete in reinforced concrete (RC) slabs subjected to impulsive loads. The transverse shear stresses are extrapolated and then included into the dynamic layered finite element procedure, as it is assumed to affect not only the ultimate behaviors but also the failure modes. A provision for material nonlinearity, cracking in concrete elements, and the loading and unloading phenomena are adopted in this study. Verification of the analytical procedure is carried out by means of comparisons with test rsults on full-scale RC slabs. It is found that ultimate behaviors, the post-failure behaviors as well as the failure modes, can be predicted accurately using the proposed procedure. The procedure could serve as a tool for a dynamic design method for concrete slab structures subjected to impulsive loads.
TL;DR: In this paper, the deflection of cracked reinforced concrete beams under various types of loading is investigated and the effect of the type of loading on their effective moment of inertia is discussed.
Abstract: The deflection of cracked reinforced concrete beams tested under various types of loading is investigated and the effect of the type of loading on their effective moment of inertia is discussed. Four type of loading were considered in this study, which include: midspan and third-point concentrated loads, uniformly distributed load, and a combination of a midspan concentrated and uniformly distributed load. The value of the effective moment of inertia was found to be significantly affected by the type of loading used. Therefore, to account for such an effect, a newly developed model for estimating the effective moment of inertia of cracked reinforced concrete beams under any type of symmetrical loading is proposed.
Journal Article•
TL;DR: In this article, the effect of steel fibers on the behavior and strength of reinforced concrete beams under shear was studied, and the first-crack strength ratio of fibrous and plain concrete beams was a function of fiber spacing.
Abstract: The effect of steel fibers on the behavior and strength of reinforced concrete beams under shear is studied. Nine fiber reinforced beams and one plain reinforced concrete beam were tested by 2-point loading. The fiber content ranged from 0.4 to 1 %, and the aspect ratio of the fiber was between 60 and 120. Improvement of first crack load and ultimate load over that of plain concrete was noted. The first-crack strength ratio of fibrous and plain beams was a function of fiber spacing. The ultimate strength of fibrous beams under shear was much higher than that of the plain concrete beam, depending on the energy absorbed in debonding and stretching the fibers, which is directly proportional to the fiber volume content and aspect ratio.
TL;DR: In this article, the web crushing strength of 16 reinforced and prestressed concrete I-beams is reported and compared with the upper limit on shear strength given by ACI Building Code, the Australian and Canadian standards, and the expression proposed by Nielson et al.
Abstract: The web crushing strengths of 16 reinforced and prestressed concrete I-beams are reported The principal variables of the test specimens are the ratio of yield force in the prestressed steel to yield force in the nonprestressed steel and the area of vertical stirrups The test strengths are compared with the upper limit on shear strength given by ACI Building Code, the Australian and Canadian standards, and the expression proposed by Nielson et al The code proposals are found to be always conservative
TL;DR: In this article, the authors investigate the stress in the prestresing tendons at ultimate in unbonded partially prestressed concrete beams with particular emphasis on the effects of nonprestressed bonded reinforcement.
Abstract: An ivestigation of the stress in the prestresing tendons at ultimate in unbonded partially prestressed concrete beams is described with particular emphasis on the effects of nonprestressed bonded reinforcement. Load tests on 6 rectangular section unbonded beams are reported. It is shown that the presence of bonded nonprestressed reinforcement has a marked influence on the flexural behavior of unbonded beams and the related increase of stress in the prestressed steel at ultimate. Test results are compared with predictions from 2 relevant design codes, and some limitations in the application of these codes to unbonded partially prestressed concrete construction are identified.
TL;DR: In this paper, an analytical model is proposed for predicting the relative flexural capacity, i.e., the ratio of the moment capacity with interaction to moment capacity in pure flexure, for longitudinally reinforced concrete beams or one-way slabs without shear reinforcement failing under the combined influence of fleuxure and shear.
Abstract: An analytical model is proposed for predicting the relative flexural capacity, i.e. the ratio of the moment capacity with interaction to moment capacity in pure flexure. The study concerns longitudinally reinforced concrete beams or one-way slabs without shear reinforcement failing under the combined influence of fleuxure and shear. For these structural elements, it is possible to obtain the 'valley of diagonal failure.' Moreover, an expression is determined giving the a/d ratio at which the minimum value of the flexural capacity under shear and moment interaction is attained. At the minimum flexural capacity, the moment contribution due to beam action is analytically found to be 60% of the total capacity. It is further shown that the expression for the a/d ratio corresponding to minimum flexural capacity also gives the critical a/d ratio distinguishing failure at diagonal cracking from failure by crushing or splitting of the concrete.
TL;DR: In this paper, a modified empirical approach is developed to predict the flexural strength of fiber concrete composites, where the direct tensile strength of the composite is used as the basic parameter in this approach.
Abstract: Several methods are available for predicting flexural strength of steel fiber concrete composites. In these methods, direct tensile strength, split cylinder strength, and cube strength are the basic engineering parameters that must be determined to predict the flexural strength of such composites. Various simplified forms of stress distribution are used in each method to formulate the prediction equations for flexural strength. In this paper, existing methods are reviewed and compared, and a modified empirical approach is developed to predict the flexural strength of fiber concrete composites. The direct tensile strength of the composite is used as the basic parameter in this approach. Stress distribution is established from the findings of flexural tests conducted as part of this investigation on fiber concrete prisms. A comparative study of the test values of an earlier investigation on fiber concrete slabs and the computed values from existing methods, including the one proposed, is presented.
TL;DR: In this paper, the authors developed limited experimental data on the flexural behavior of doubly reinforced high strength lightweight concrete beams with web reinforcement, and the test results were presented in terms of load-deformation behavior, cracking, and ductility indexes.
Abstract: This paper develops limited experimental data on the flexural behavior of doubly reinforced high strength lightweight concrete beams with web reinforcement. Flexural tests were coducted on 6 doubly reinforced beams. The test results are presented in terms of load-deformation behavior, cracking, and ductility indexes. The experimentally obtained load-deflection curves are compared to those predicted analytically, and good comparisons are obtained. Conclusions drawn from the study results are discussed.