Post Tensioned Concrete Slabs Pdf Download : Exemples de réalisations de dalles post-tensionnées en

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Photo 2 shows part of a building of post-tensioned concrete construction. Note that the beams and columns are widely separated, that the floor-ceiling slab is relatively thin, and that it is flat on both top and bottom.

Post Tensioned Concrete Slabs Pdf Download

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Builders like using post-tensioned concrete because it allows for longer spans, fewer columns, and larger open spaces in the building; faster construction than some other methods; and uses one-fourth to one-third less concrete than waffle slabs or prestressed double-tees. It is used most frequently in high-rise buildings, parking structures, and bridges.

The section contains Prestressed Concrete Structures multiple choice questions and answers on beams self weight estimation, partially prestressed members design, pretensioned and post tensioned beams design.

Post-tensioned (PT) slabs are typically flat slabs, band beam and slabs or ribbed slabs. PT slabs offer the thinnest slab type, as concrete is worked to its strengths, mostly being kept in compression. Longer spans can be achieved due to prestress, which can also be used to counteract deflections.

Post-tensioned slabs use high-strength tensioned steel strands to compress the slabs, keeping the majority of the concrete in compression. This gives a very efficient structure which minimises material usages and decreases the economic span range when compared to reinforced concrete.

In this experimental study, six post-tensioned light weight concrete (LWC) continuous one-way slabs were tested in the following manner: the flexural behaviors of the members were compared with the calculations from the existing standards. The test also examined the effect of prestressing in tendons and proper prestress conditions to reduce the deflection and crack width, and to enhance the flexural capacity and ductility of LWC members. Flexural capacity and stress increments in unbonded tendons of the specimens were compared with those of the simply supported normal and the lightweight concrete members. The suggested safety limit from the American Concrete Institute (ACI) regulation on the maximum capacity and the stress incremental in unbonded tendons were also compared with the test results under simple and continuous supporting conditions.

Most of the research using prestressed concrete flexural members has been conducted with normal weight concrete (NWC) (Warwaruk et al. 1962; Harajli and Kanj 1990; Campbell and Chouinard, 1991; Chakrabrti 1995; Mainsekar and Senthil 2006; Lou and Xiang 2007; Ellobody and Bailey 2008; Cai et al. 2009). Yang and Mun (2013) initiated experimental studies on the flexural capacity of post-tensioned LWC beams. They found the stress increase in the unbonded strands (Δf ps ) at the ultimate strength of the post-tensioned LWC beam was higher than that of the post-tensioned NWC beams under same reinforcing index values. This indicates that the ACI 318-11 provisions for Δf ps would be unconservative for post-tensioned LWC members.

Currently, most codes and recommendations, including American Concrete Institute standards (ACI Committee 318 2011), evaluate the flexural behaviors of the post-tensioned concrete members based on studies with NWC; therefore, there could be a question about how to evaluate the flexural behavior of post-tensioned LWC members. As it is well known, LWC has lower stiffness than NWC. LWC also shows many different material properties from NWC including creep and drying shrinkage deformation. The flexural behaviors of post-tensioned LWC members are distinguished from those of NWC in bending strength, ductility and stress in unbonded strands.

One of the most important factors in the design of post-tensioned flexural members is to evaluate the stress incremental in unbonded tendons to which the sectional strain compatibility condition is not applicable. The stress increase in unbonded tendons is suggested by the member compatibility condition based upon the test results using simply supported NWC beams (ACI Committee 318 2011; Yang et al. 2013). However, when it comes to the LWC, insufficient data have been provided to assure the safety of the existing models and current design standards. For safe design of post-tensioned LWC beams, the effects of variables on the stress incremental in unbonded tendons must be studied under both simple and continuous support conditions. Therefore, the bond reduction coefficients of plastic hinge length proposed to evaluate stress increase in unbonded tendon need to be adjusted in LWC members because the proposed equations are empirically derived using limited test data from the NWC members. However, available test results and information are very rare for LWC flexural members.

This research provides meaningful test results on the behaviors of the post-tensioned continuous lightweight slabs. Here, the applicability of the stress incremental equations for unbonded tendons suggested by design standards was evaluated. The stress increments in unbonded tendons were also reviewed under simple and continuous support conditions. Authors believe the results can provide basic data for development of a future design manual of post-tensioned continuous lightweight concrete slabs.

Six LWC one-way continuous slab specimens post-tensioned with unbonded tendons were prepared for flexural tests, as shown in Table 1. The nomenclature for each specimen is shown in Fig. 1. Each specimen had three supports with two spans. Two critical parameters affecting the stress increase in unbonded tendons were considered in experiments: span-to-effective tendon depth ratio (L/d p ) in Group 1; and steel ratio in member section (ρ s ) in Group 2.

As known well from the study by Mojtahedi and Gamble (1978), L/d p of the concrete member can affect the stress in tendons. ACI 318 equation (18-4) overestimates Δf ps in NWC members when L/d p is greater than 35. In this study, L/d p varies from 25, 35, and 45 to evaluate the stress changes in tendons. It was Warwaruk et al. (1962) who conducted an experiment about the loading pattern for the first time. Harajli and Kanj (1990) found the length of the plastic hinges of beams, as well as the stress in tendons, can be changed according to the loading patterns. When it comes to continuous slabs, a uniform loading method has been suggested for ideal member behaviors (Burns et al. 1978), so the use of uniformly distributed loading was applied in this test.

The mean and standard deviation of the ratios of the measured and predicted moment capacities of the post-tensioned NWC beams are 1.05 and 0.22, respectively (Yang et al. 2013). Hence, the ACI 318-11 procedure for predicting the M n value of post-tensioned flexural members can be considered conservative for LWC. In other words, the ACI 318 design equations generally underestimated Δf ps . The Δf ps predictions from ACI 318 are as conservative as in NWC. In addition the predictions obtained from the AASHTO 1998 were in better agreement with the test results than ACI 318-11, showing that the mean, standard deviation, and coefficient of variation of the ratios between measured and predicted stress increases (Δf ps ) are 1.70, 0.53, and 0.31, respectively. The predictions obtained from CAN 3 were the least conservative among three standards showing that the mean, standard deviation, and coefficient of variation of the ratios between measured and predicted stress increases (Δf ps ) are 1.10, 0.23, and 0.21, respectively. The ratios of measured to predicted incremental stresses are plotted against ω accompanying with the NWC test results by Lim et al. (1999) in Fig. 9.

To evaluate the flexural capacity and the ductility of post-tensioned LWC continuous slabs, six specimens were tested and compared with the design standards. Through the test the followings have been found: (1) The initial crack happened at the center of negative moment and then developed at the positive moment areas on both sides simultaneously; (2) As the steel ratio in member section (ρ s ) increased, the width of the crack decreased while the number of micro cracks increased. Also the maximum capacity increased while the ductility decreased; (3) As the span-to-effective tendon depth ratio (L/d p ) increased, the maximum strength (P u ) decreased. The initial crack load (P cr ) and the yielding capacity (P y ) also decreased while the deflection increased with L/d p growth; (4) As the quantity of minimum required rebars increased, the initial crack load, yielding capacity, and maximum strength increased; (5) Even after reaching the ultimate stress of flexural members, the stress in the tendons continuously increased sharing the tensile stresses. The stress increases stopped when the flexural member finally ruptured; (6) As the L/d p increased, the stresses in tendons decreased. That means L/d p is a critical factor affecting the tendon stresses; (7) In all specimens, before the initial crack, no stress change was observed in the tendons. After initial crack happened, the tendon stresses increased gradually as the applied load was increased; (8) With fewer rebars, the stress-increasing rate in the tendons was relatively high. The ACI standard evaluated the stress increment conservatively.

The expected stress increase in unbonded tendon especially by ACI and AASHTO were less than the test results in all specimens. In other words, the normalized flexural capacity of the post-tensioned LWC slabs was higher than the predictions based on the ACI code provision. The differences were not negligible quantity so it may cause overdesign. 2ff7e9595c