Post-tensioned Flat Plate Research Articles

Experimental and numerical investigation of punching and post-punching shear capacities of post-tensioned flat plate slab-column joints influenced by prestressing levels

Post-tensioned slab systems serve as an effective method to enhance the progressive collapse resistance of concrete flat plate structures by significantly improving the punching shear and post-punching capacities of their slab-column joints. While the existing research mainly focused on the punching shear behaviour of post-tensioned slab-column joints in small deformation stages, the post-punching behaviour in large deformations has been overlooked. This study aims to fill this research gap thorough experimental and numerical studies. Experimentally, four slab-column joints, including three post-tensioned specimens (with varied prestressing levels) and one specimen without tendons, were designed and tested. Detailed failure process and load-resisting mechanisms of the specimens at different deformation stages were discussed. The test results confirmed the efficiency of post-tensioning in improving the punching shear performance. Varying prestressing levels have influenced the development of punching shear capacity but did not affect the growth rate of load resistance in the initial post-punching stage. Numerically, validated finite element models were used to predict the post-punching capacities of the specimens. The numerical results indicated that post-tensioning helped increase post-punching capacities, but such an enhancement was independent of the prestressing levels. Additionally, the influence of the rupture strains of both tendons and reinforcement on the post-punching capacity was investigated through a parametric study.

Assessment of the embodied carbon performance of post-tensioned voided concrete plates as a sustainable floor solution in multi-story buildings

With growing concerns for the building industry’s contribution to climate change, designers are considering the global warming potential and corresponding embodied carbon (EC) emissions of building structures. However, conventional reinforced concrete (RC) floors are one of the highest contributors to the EC in a multi-story building. A solution to reduce EC that combines proven construction technologies is post-tensioned (PT) voided plates, where void formers are placed in areas where less material is required, therefore increasing the effectiveness of the pre-stressing, and reducing substantial structural material. This paper assesses the EC of PT voided plates for three different tendon layouts and compares them to RC flat plates, RC voided plates, and PT flat plates. 3280 different design conditions are assessed to replicate the range of common applications of each concrete floor system. The minimum required material quantities were determined for each design condition, which were used to calculate and compare the corresponding EC. The EC results for the floor systems were also compared to other sustainable floor designs. This paper found that PT voided plates have an EC savings up to 51 % compared to conventional floors and are a sustainable floor system that can be built worldwide using current construction practices.

Experimental study of punching shear in post-tensioned slabs with unbonded tendons

Post-tensioning in flat plate slabs provides the advantage of deflection and crack control under service loads, and can reduce slab thickness. However, due to the reduced slab thickness, post-tensioned flat plate slabs may be susceptible to punching shear failure under ultimate load. This study performed experimental research of the punching shear resistance of posttensioned flat plate slabs with unbonded tendons. A total of four 3,100 mm square specimens of 150 mm thick slab were fabricated with the variables of average compressive stress in concrete, tendon layout, and aspect ratio of the column stub. Based on the test results, the load-deflection relationship, patterns of cracks, strains of flexural reinforcement, and force increments in tendons were compared. In addition, the equations for punching shear strength of unbonded post-tensioned flat plate slabs specified in the design codes (ACI 318, Eurocode 2, Model Code) were evaluated in detail.

Unified equivalent frame method for post-tensioned flat plate slab structures

The post-tensioned (PT) flat plate slab system is commonly used in practice, and this simple and fast construction method is also considered to be a very efficient method because it can provide excellent deflection and crack control performance under a service load condition and consequently can be advantageous when applying to long-span structures. However, a detailed design guideline for evaluating the lateral behavior of the PT flat plate slab system is not available in current design codes. Thus, typical design methods used for conventional reinforced concrete (RC) flat plate slab structures have inevitably been adopted in practice for the lateral load design of PT flat plate structures. In the authors\' previous studies, the unified equivalent frame method (UEFM) was proposed, which considers the combined effect of gravity and lateral loads for the lateral behavior analysis of RC flat plate slab structures. The aim of this study is to extend the concept of the UEFM to the lateral analysis of PT flat plate slab structures. In addition, the stiffness reduction factors of torsional members on interior and exterior equivalent frames were newly introduced considering the effect of post-tensioning. Test results of various PT flat plate slab-column connection specimens were collected from literature, and compared to the analysis results estimated by the extended UEFM.

Analysis and Comparative Study of Unbonded Post-tensioned Cast-In-Place Parking Floor on the Effects of Tendon Layout using Safe

In recent years post-tensioned floors are commonly used for parking purposes in residential and commercial high rise buildings. This paper includes result on a study of un-bonded post tensioned cast-in-place parking floor subjected to various arrangements of tendon layout based on FEM analysis. Modelling and analysis of post-tensioned flat plate is done by using SAFE software. Equivalent loads based on cable profiles are applied to the flat plate according to the tendon layout. Design moments, service moments, hyper-static moments, short term deflection, long term deflection, and punching shear are compared for the various tendon layouts at service and ultimate limit state.

Flexural Behavior of Posttensioned Flat Plates Depending on Tendon Layout

This paper discusses the experimental results on the flexural behavior and deflections of posttensioned concrete flat plates depending on tendon layout. One reinforced concrete flat plate and two posttensioned concrete flat plates were manufactured and tested. One-way posttensioning layout and two-way posttensioning layout were considered in this paper. The load-deflection behavior and modes of crack are presented from the test results. Posttension systems effectively controlled crack and deflection. One-way and two-way posttensioning layouts both showed similar maximum load. However, serviceability improved with two-way posttensioning layout compared to one-way posttensioning layout. Also, the yield-line theory was applied to predict the ultimate load for the posttensioned flat plates. The comparison between the test results and estimation by yield-line analysis generally showed good agreement.

Effect of slab bottom reinforcement on seismic performance of post-tensioned flat plate frames

The purpose of this study is to evaluate the seismic performance of gravity-designed post-tensioned flat plate frames with and without slab bottom reinforcement passing through the column. In low and moderate seismic regions, the seismic demands may not control the design, and buildings are often designed considering only gravity loads. This study focuses on the seismic performance of gravity load designed post-tensioned flat plate frames. For this purpose, three-, six- and nine-storey post-tensioned flat plate frames are designed considering only gravity loads. For reinforced concrete flat plate frames, continuous slab bottom reinforcement (integrity reinforcement) passing through the column should be placed to prevent progressive collapse; however, for the post-tensioned flat plate frames, the slab bottom reinforcement is often omitted since the requirement for the slab bottom reinforcement for post-tensioned flat plates is not clearly specified in ACI 318-05. This study evaluates the seismic performance of the model frames by conducting non-linear static pushover analyses and non-linear response history analyses. For conducting non-linear response history analyses, six sets of ground motions are used as input ground motions, which represent two different hazard levels (return periods of 475 and 2475 years) and three different locations (Boston, Seattle and Los Angeles). An analytical model is developed for emulating the non-linear hysteretic behaviour and the failure mechanism of post-tensioned slab–column connections. This study shows that gravity-designed post-tensioned flat plate frames have some seismic resistance. In addition, the seismic performance of post-tensioned flat plate frames is significantly improved by placing slab bottom reinforcement to pass through the column.

Stiffness reduction factor for post-tensioned flat plate slabs

The effective beam width model (EBWM) has been used for estimating force and drift demands on flat plate frames under lateral loads. The accuracy of this model depends strongly on how to estimate the effective stiffness of flat plate slabs. With an increase in slab moments due to lateral loads, slab cracks occur and propagate on the slab, which leads to a reduction in slab flexural stiffness. For post-tensioned (PT) flat plate slabs, the initiation and propagation of slab cracks can be delayed due to slab in-plane compressive stress induced by PT tendon forces. This study investigates the contribution of slab in-plane compressive stress to the slab stiffness reduction factor, which is defined as the ratio of slab stiffness obtained from a cracked slab section to that from the corresponding uncracked slab section. For this purpose, this study collected the test results of PT flat plate slab–column connection specimens and estimated the slab stiffness reduction factors (β) for those specimens with respect to a level of applied slab moment. Non-linear regression analysis is used to propose an empirical equation computing β for PT flat plate slabs using β estimated from the test results. The accuracy of the empirical equation for β is verified by comparing the lateral stiffness of a two-storey PT flat plate frame, obtained using the EBWM with the empirical equations for β, with that estimated from shaking table test results on the frame.

Unbonded Post Tensioned Concrete Slabs in Fire - Part II - Modelling Tendon Response and the Consequences of Localized Heating

This is Part II of a two part paper dealing with the current state of knowledge of the fire-safe structural design and construction of unbonded post-tensioned (UPT) flat plate concrete structures. Part I provided detailed results of nineteen transient high temperature stress relaxation tests on restrained UPT tendons of realistic length and parabolic longitudinal profiles. Experimentation identified several credible concerns for UPT concrete structures in fire, most notably the potential for premature tendon rupture due to localized heating, which may result from a number of possible causes in a real structure. The real world response of continuous UPT tendons both during and after heating is largely unknown, and is dependent on factors which are not currently accounted for either in standard fire tests or by available prescriptive design guidance. This second part of the paper presents and applies a numerical model to predict the time-temperaturestress-strength interdependencies of stressed UPT tendons under localized transient heating, as may be experienced by tendons in a real concrete building in a real fire. The model is used, along with previously developed and validated computational models for heat transfer and prestress relaxation in UPT tendons, to assess existing prescriptive concrete cover requirements for UPT slabs. It is shown that localized heating of UPT tendons is likely to induce premature tendon rupture during fire, and that current prescriptive code procedures based on concrete cover alone are, in general, insufficient to prevent this. Based on the data presented it appears that minimum code prescribed concrete covers for UPT structures require revision if premature tendon rupture during fire is to be avoided.

Unbonded Post Tensioned Concrete Slabs in Fire - Part I - Experimental Response of Unbonded Tendons under Transient Localized Heating

The fire-safe structural design and construction of unbonded post-tensioned (UPT) flat plate concrete structures has recently come under debate in the UK, and questions are being raised regarding the response to fire of post-tensioned concrete slabs. Related to these concerns is the real world response of continuous UPT tendons inside such structures both during and after a fire, which is largely unknown and depends on many potentially important factors which are not currently accounted for in standard fire tests. Several credible concerns exist for UPT concrete structures in fire, most notably the potential for premature tendon rupture due to localized heating which may result from a number of possible causes (discussed herein). The research presented in this paper deals specifically with the time-temperature-stress-strength interdependencies of stressed UPT tendons under localized transient heating, as may be experienced by tendons in a real UPT building in a real fire. Nineteen high temperature stress relaxation tests on UPT tendons of realistic length and parabolic longitudinal profile are reported. It is shown that localized heating of UPT tendons is likely to induce premature tendon rupture during fire, even in structures which meet the prescriptive concrete cover requirements imposed by available design codes.

중력하중으로 설계된 포스트텐션 플랫플레이트 골조의 내진성능

The purpose of this study is to evaluate the seismic performance of gravity-designed post tensioned (PT) flat plate frames with and without slab bottom reinforcement passing through the column. In low and moderate seismic regions, buildings are often designed considering only gravity loads. This study focuses on the seismic performance of gravity load designed PT flat plate frames. For this purpose, 3-, 6- and 9-story PT flat plate frames are designed considering only gravity loads. For reinforced concrete flat plate frames, continuous slab bottom reinforcement (integrity reinforcement) passing through the column should be placed to prevent progressive collapse; however, for the PT flat plate frames, the slab bottom reinforcement is often omitted since the requirement for the slab bottom reinforcement for PT flat plates is not clearly specified in ACI 318-08. This study evaluates the seismic performance of the model frames, which was evaluated by conducting nonlinear time history analyses. For conducting nonlinear time history analyses, six sets of ground motions are used as input ground motions, which represent two different hazard levels (return periods of 475 and 2475 years) and three different locations (Boston, Seattle, and L.A.). This study shows that gravity designed PT flat plate frames have some seismic resistance. In addition, the seismic performance of PT flat plate frames is significantly improved by the placement of slab bottom reinforcement passing through the column.

횡하중하의 포스트 텐션 플랫 플레이트 해석을 위한 강성감소계수

횡하중을 받는 포스트 텐션(PT) 플랫 플레이트 슬래브 골조의 해석은 일반적으로 유효보폭모델을 많이 사용한다. 횡 변위비와 불균형 모멘트을 예측하기 위한 유효보폭모델의 정확성은 PT 플랫 플레이트 슬래브의 유효강성을 어떻게 평가하느냐에 달려 있다. 슬래브 강성은 횡하중에 의한 작용모멘트의 증가와 함께 감소되기 때문에, 슬래브의 강성 감소현상은 플랫 플레이트 골조의 해석에 반영되어야 하며 균열의 영향 또한 고려되어야 한다. 횡하중을 받는 PT 플랫 플레이트 슬래브 구조의 정확한 해석을 위해 슬래브 강성감소는 유효보폭모델이 정확하게 되어야 한다. 이 목적을 위해 이 연구는 기존 연구자들에 의해 실시된 PT 플랫 플레이트 내부 및 외부 접합부의 실험 결과를 수집하였다. 그리고 이 연구는 시행착오를 통해 각 실험체의 횡강성에 유효보폭모델의 강성이 수렴하도록 슬래브의 폭을 감소시켰다. 슬래브의 모멘트 크기에 따라 비선형 회귀 분석을 수행함으로서 슬래브에 대한 강성감소계수 계산식을 제안하였다. 이 연구에서는 제안된 식의 정확성을 검증하기 위해서 PT 플랫 플레이트 골조의 실험 결과와 비교하였다. 제안된 식을 적용한 유효보폭모델은 작용하중의 크기에 따라 변화하여 PT 실험체의 실제 강성을 잘 예측하는 것으로 나타났다.

한계상태모델을 이용한 철근콘크리트와 포스트텐션 무량판의 통합해석

2층 규모의 철근콘트리트조 및 포스트텐션조 무량판구조를 1/3 스케일로 축소하여 제작한 실험체의 진동대 실험 결과를 바탕으로 무량판구조의 모델링 기법을 향상하고자 하는 연구를 수행하였다. 이 연구에서 적용한 모델링 방법은 슬래브의 휨모멘트에 의한 휨파괴, 불균형모멘트의 전달에 의한 휨파괴 및 펀칭전단파괴에 의한 슬래브-기둥 접합부의 모멘트 전달능력 상실등의 영향을 반영하는 매우 포괄적인 구조해석 방식이다. 펀칭전단파괴에 대해서는 중력비와 층간변위각에 기초한 한계상태 모델이 적용되었다. 이 논문에서 제안된 비선형 모델은 무량판구조의 진동대 실험 결과와 잘 부합하는 것으로 나타났다.

Effects of Bottom Reinforcement on Hysteretic Behavior of Posttensioned Flat Plate Connections

Flat plate systems are commonly designed as a gravity-force-resisting system (GFRS) along with stiffer lateral-force-resisting systems (LFRS) such as shear walls, and moment resisting frames; however, deformation compatibility between the GFRS and LFRS results in positive moments (producing tension at the slab bottom) at the slab-column connections in flat plate systems. Continuous slab-bottom reinforcement (integrity reinforcement) passing through the column is required to prevent progressive collapse; however, for posttensioned flat plate (PT-flat plate) systems, there have been arguments among engineers on applying continuous slab-bottom reinforcement since ACI 318 and 352 provisions do not clearly specify the requirement on the slab-bottom reinforcement for PT flat plates. The purpose of this study is to investigate the effects of continuous slab-bottom reinforcement passing through the column core on the hysteretic behavior of the PT slab-column connections. Test results for six, 3/5 scaled specimens for interior PT flat plate connections subjected to constant gravity loads and quasistatic reversed cyclic lateral loads are presented and indicate that the slab-bottom reinforcement conforming to ACI-ASCE 352R.1 is not required to prevent collapse, but does significantly improves the hysteretic energy absorption capacity.

Transient high-temperature stress relaxation of prestressing tendons in unbonded construction

Unbonded post-tensioned (UPT) flat plate concrete slabs are popular for modern continuous multiple bay floor assemblies due to economic and sustainability benefits (reductions in slab thickness and building self-weight) and structural advantages (decreased deflections over larger spans). Only limited research has been conducted on the performance of UPT flat plate slabs under fire conditions, yet the inherent fire endurance of these systems is sometimes quoted as a benefit of this type of construction. One concern for these structures in fire is that high-temperature stress relaxation of the unbonded prestressed reinforcement may cause considerable and irrecoverable prestress loss, with subsequent structural consequences. This paper uses a computational model which has been developed to predict the transient high-temperature stress relaxation (i.e., prestress loss) for typical UPT multiple span flat plate slabs in fire, to study the potential prestress relaxation behaviour under various plausible temperature conditions as might occur during exposure to a standard fire. The model is validated using experimental data from relaxation tests performed on locally heated unbonded seven-wire prestressing stand. The initial prestress level, concrete cover to the prestressed reinforcement, and ratio of heated length to overall tendon length are varied to investigate the potential implications for prestress loss, and subsequently for flexural and punching shear capacity. The results highlight the need for particular care in the construction of UPT slabs to ensure adequate concrete cover for structural fire safety.

하부 철근 유무에 따른 포스트 텐션 플랫 플레이트 골조의 내진성능 평가

This study evaluates the seismic performance of post-tensioned flat plate structures with or without slab bottom reinforcement. For this purpose, 3 and 9 story frames were designed only considering gravity loads. This study conducts a nonlinear static pushover analysis. This study was an analytical model that is able to represent punching shear failure and fracture mechanism. The analytical results showed that the seismic performance of a post-tension flat plate is strongly influenced by the existence of slab bottom reinforcement through column. By placing slab bottom reinforcement in a PT flat plate frame, lateral strength and max drift capacity are significantly increased.

Fracture Model for PT Flat Plate Connections under Seismic Loading

Post-tensioned (PT) flat plate frames are commonly used to support gravity load in connections in high seismic regions. But test results of many studies indicated that PT flat plate connections were possessed of later-resisting capacity for lateral load though this capacity is not enough to be used by itself in high seismic regions. So this study evaluated the cyclic behavior of PT flat plate connections and developed an analytical model for predicting nonlinear behavior of those to estimate the seismic performance of PT flat plate frames accurately. For this purpose, the test results of static pushover test and shaking table test of 2 story PT flat plate frame were used to evaluate the characters of failure mode for PT flat plate connections and to compare with analysis results. The test results indicated two failure modes which are classified the shear failure mode or the moment failure mode as for shear strength of PT connections to resist lateral loads acted on the specimens. So the analytical model was designed to consider these failure modes of PT connections and the results of comparing with test results predict the nonlinear behavior of PT connections.

Comparison of Seismic Behaviors of Interior Joints in PT and RC Flat Plate Systems

The purpose of this study is to compare the seismic behaviors of interior post-tensioned (PT) and reinforced concrete (RC) flat plate slab-column connections designed to resist only gravity loads. In general, flat plate systems have been used to support gravity loads, which should be constructed with lateral system such as shear walls or moment frames. Flat plate systems should retain the ability to undergo the lateral drift associated with the lateral system without loss of gravity load carrying capacity. Although these systems are common, relatively little experimental study has been conducted to assess actual behavior under lateral loads. Therefore, this study was undertaken to assess the seismic performance of two PT and one RC slab-column interior connection under high gravity loads. This study observed that PT connections had a better seismic performance than corresponding RC connections in terms strength, deformability, energy absorption.

Cyclic behaviour of interior post-tensioned flat plate connections

In high seismic regions, post-tensioned (PT) slab–column frames are commonly used to support gravity loads in conjunction with a lateral-force resisting system (LFRS) such as a core wall. The LFRS is designed to resist 100% of the design lateral forces as well as to limit lateral displacements to an acceptable level, whereas the slab–column frame must sustain the gravity loads under the expected (design) displacements. Given the relatively sparse data on the seismic performance of PT flat plate slab–column frames, cyclic tests of four interior PT slab–column connections were conducted. Primary test variables were the level of gravity shear at the slab–column connection and the slab tendon arrangement. Test results indicate that both the test variables strongly influence the cyclic behaviour of the PT connections, and that the use of slab bottom reinforcement at the slab–column connection was effective in resisting positive moment developed under lateral loading as well as improving the hysteretic energy dissipation capacity.

Hysteretic behavior of exterior post-tensioned flat plate connections

The purpose of this study is to evaluate hysteretic behavior of exterior post-tensioned flat plate slab–column connections (PT connections) designed to resist only gravity loads. For this purpose, experimental studies were conducted using three approximately two-thirds-scale test specimens; two exterior PT connection specimens having different tendon layouts and one exterior reinforced concrete flat plate slab–column connection (RC connection) specimen. Quasi-static cyclic loading was applied to the specimen with a constant gravity load. All specimens had bottom bonded reinforcement around the column as required by ACI 352.1R-89. This study collects previous test results to draw a general conclusion for the hysteretic behavior of the PT exterior connections. This study observed that the tendon layout influenced the hysteretic behavior of PT exterior connections, which means that lateral drift capacity, dissipated energy, failure mechanism, and ductility vary with respect to tendon layouts. Moreover, this study shows that the amount of bottom reinforcement specified by the ACI 352.1R-89 is sufficient for resisting positive moments resulting from moment reversal under the cyclic loading. Shear strength of the test specimens is more accurately predicted by the shear strength equation considering average compressive strength ( f p c ) due to post-tensioning tendons than that without considering f p c .