Kang-Su Kim

Evaluation of Horizontal Shear Strength of Prestressed Hollow-Core Slabs with Cast-in-Place Topping Concrete

Prestressed hollow-core (PHC) slabs are structurally-optimized lightweight precast floor members for long-span concrete structures, which are widely used in construction markets. In Korea, the PHC slabs have been often used with cast-in-place (CIP) topping concrete as a composite slab system. However, the PHC slab members produced by extrusion method use concrete having very low slump, and it is very difficult to make sufficient roughness on the surface as well as to provide shear connectors. In this study, a large number of push-off tests was conducted to evaluate interfacial shear strengths between PHC slabs and CIP topping concrete with the key variable of surface roughness. In addition, the horizontal shear strengths specified in the various design codes were evaluated by comparing to the test results that were collected from literature.

Applicability of Partial Post-Tension Method for Deflection Control of Reinforced Concrete Slabs

Recently, it is getting into a good situation for the flat-plate slab system to be applied. The flat-plate slab without beam, however, is often too weak to control deflection properly compared to other typical slab-beam structures, for which the post- tension method is generally regarded as one of best solutions. The post-tension (PT) method can effectively control deflection with- out increase of slab thickness. Despite this good advantage, however, the application of PT method has been very limited due to cost increase, technical problems, and lack of experiences. Therefore, in order to reduce difficulties on applying full PT method under the current domestic circumstances and to enhance constructability of PT system, this research proposed the partial PT method with top jacking anchorage applied in a part of span as need. For the top jacking anchorage system, the efficiency of deflec- tion control shall be considered in detail because it can vary widely depending on the location of anchorage that can be placed any- where as need, and tensile stresses induced at back of the anchorage zone also shall be examined. Therefore, in this study, analysis were performed on the efficiency of deflection control depending on the location of anchorage and on tensile stresses or forces using finite element method and strut and tie model in the proposed top jacking anchorage system. The proposed jacking system were also applied to the floor slabs at a construction site to investigate its applicability and the analysis results of slab behavior were compared to the measured values obtained from the PT slab constructed by the partial PT method. The result of this study indicates that the partial PT method can be very efficiently applied with little cost increase to control deflection and tensile stresses in the region as a need basis where problem exists.

Minimum Torsional Reinforcement Ratio of Reinforced Concrete Members for Safe Design

Dept. of Architectural Engineering, Sungkyunkwan University, Suwon 440-746, KoreaABSTRACT Current design codes regulate the minimum torsional reinforcement requirement for reinforced concrete members to prevent their brittle failure. The minimum torsional reinforcement ratio specified in the current national code and ACI318-11, however, have problems in the minimum longitudinal reinforcement ratio for torsion, the equilibrium condition in space truss mo del, and a marginal strength, etc. Thus, in order to overcome such shortcomings, this study presents a rational equation for minimum torsional reinforcement ratio that can provide a sufficient margin of safety in design. The minimum torsional reinforcement ratio proposed in this study was compared to the test results available in literature, and it was confirmed that it gave a proper margin of safety for all specimens studied in this paper.Keywords : minimum torsional reinforcement, reserved strength, reinforced concrete, torsional cracking strength

Allowable Compressive Stress of Pre-Tensioned Members with Tee or Inverted Tee Sections at Transfer

In a previous research performed by the authors, the allowable compressive stress coefficient (K) in pretensioned members with rectangular section at transfer was proposed based on strength design theory. In this study, a subsequent research of an enormous analysis was performed to determine the K factor for Tee and inverted Tee section members, considering the effect of section height (h), section type, amount of tendons (), and eccentricity ratio (e/h). Based on the analysis results, the allowable compressive stress coefficients (K) for Tee and inverted Tee section members at transfer were derived, which limit the maximum allowable stresses as 80% and 70% of the compressive strengths at the time of release for Tee section and inverted Tee section, respectively. And these were larger than the allowable stresses specified in domestic and other international codes. In order to verify the proposed equations, they were compared to the test results available in literature and other codes, which showed that the allowable stresses in domestic and international codes are unconservative for the cases with low eccentricity ratios while conservative for those with high eccentricity ratios. The proposed equations, however, estimate the allowable stresses of the Tee and inverted Tee section members reasonably close to test results.

Ultimate Stress of Unbonded Tendons in Post-Tensioned Flexural Members

It is quite difficult to predict the flexural strength of post-tensioned members with unbonded tendons (unbonded posttensioned members, UPT members) because of debonding behavior between concrete and prestressing tendons, which is different from that with bonded tendons. Despite many previous researches, our understanding on the flexural strength of UPT members is still insufficient, and thus, national codes use different methods to calculate the strength, which quite often give very different results. Therefore, this paper reviews various existing methods, and aims at proposing an improved rational strength model for UPT flexural members having better accuracy. Additionally, a database containing a large number of test data on UPT flexural members has been established and used for verification of the proposed flexural strength model. The analysis results show that the proposed method provides much better accuracy than many existing methods including the rigid-body model that utilizes the assumption of concentrated deformation and plastic hinge length, and that it also gives proper consideration on the effects of primary parameters such as reinforcement ratio, loading pattern, concrete strength, etc. Especially, the proposed method also well predicts the ultimate stress of unbonded tendons of over-reinforced members, which are often possible in construction fields, and high strength concrete members.

Analysis on the Influence of Moment Distribution Shape on the Effective Moment of Inertia of Simply Supported Reinforced Concrete Beams

Dept.of Architectural Engineering, University of Seoul, Seoul 130-743, KoreaABSTRACT The concept of the effective moment of inertia has been generally used for the deflection estimation of reinforcedconcrete flexural members. The KCI design code adopted Bransons equation for simple calculation of deflection, in which a rep-resentative value of the effective moment of inertia is used for the whole length of a member. However, the code equation for theeffective moment of inertia was formulated based on the results of beam tests subjected to uniformly distributed loads, which maynot effectively account for those of members under different loading conditions. Therefore, this study aimed to verify the influencesof moment shapes resulting from different loading patterns by experiments. Six beams were fabricated and tested in this study,where primary variables were concrete compressive strengths and loading distances from supports, and test results were comparedto the code equation and other existing approaches. A method utilizing variational analysis for the deflection estimation has beenalso proposed, which accounts for the influences of moment shapes to the effective moment of inertia. The test results indicatedthat the effective moment of inertia was somewhat influenced by the moment shape, and that this influence of moment shape tothe effective moment of inertia was not captured by the code equation. Compared to the code equation, the proposed method hadsmaller variation in the ratios of the test results to the estimated values of beam deflections. Therefore, the proposed method is con-sidered to be a good approach to take into account the influence of moment shape for the estimation of beam deflection, however,the differences between test results and estimated deflections show that more researches are still required to improve its accuracyby modifying the shape function of deflection. Keywords : effective moment of inertia, deflection, moment of inertia of the cracked transformed section, flexural rigidity

Theoretical Evaluation of the Post Tensioning Effect in Continuous Slabs

Dept. of Architectural Engineering, University of Seoul, Seoul 130-743, KoreaABSTRACT Reinforced concrete (RC) structures have been most widely used because of their good economic efficiency. However,it is very weak in tensile stresses and difficult to control deflection due to the heavy self-weight of concrete. On the other hand, it isgenerally known that prestressed concrete structures can be the most effective to overcome the demerits of RC structures by using var-ious tendon lay-out and its amount. In the prestressed concrete members, the inflection points of tendons should be placed effectivelyfor the deflection control and the moment reduction. Therefore, in this study, the equations of tendon profiles are derived in terms ofpolynomials that satisfy essential conditions of tendon geometries such as inflection points and natural curved shapes of tendons placedin continuous members, from which vertical components of prestressing forces can be also calculated. The derived high order poly-nomial expression for the distributed shape of the upward and downward forces was transformed to an simplified equivalent uniformvertical force in order to improve the applicability in the calculation of member deflection. The influences of vertical forces by tendonsto deflection and moment in a continuous slab were also considered depending on the distance from column face to the location oftendons. The applicability of the proposed method was examined by an example of deflection calculation for the cases of slabs withand without tendons, and the efficiency of deflection control by tendons was also quantitatively estimated.Keywords : post tension, continuous slab, tendon shape, inflection point

Prediction Model of Unbonded Tendon Stresses in Post-Tensioned Members

As the demand on long span structures increases more in recent years, the excessive deflection, in addition to the ultimate strength, in horizontal members becomes a very important issue. For this reason, as an alternative method to effectively solve the deflection problems, the application of post-tensioned structural system with unbonded tendon increases gradually. However, most of the existing researches on post-tensioned members with unbonded tendons (UPT) focused on the ultimate flexural strength, which would be impossible or improper to check serviceability such as deflections. Therefore, this study aims at proposing a stress prediction model for unbonded tendons that is applicable to the behavior of UPT members from the very initial loading stages, post-cracking states, and service to ultimate conditions. The applicability and accuracy of the proposed model were also evaluated comparing to the existing test results from literature. Based on such comparison results, it was verified that the proposed model provided very good predictions on tendon stresses of UPT members at various loading stages regardless their different characteristics; wide range of reinforcement index, different loading patterns, and etc. The proposed model especially well considered the effect of various loading types on stress increases of unbonded tendons, and it was also very suitable to apply on the over-reinforced members that easily happened during strengthening/repairing work.