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A Study on the Behavior of Post Tensioned Flat Slabs with Various Tendon Distribution Systems

post tensioned concrete floor
M. Kamalakannan,
Managing Director, Utracon Structural System Pvt Ltd (India)
General Manager, Utracon Structural System Pvt Ltd (India)

Abstract: The use of post tensioned concrete floor in buildings has been growing consistently in recent year over conventional slab systems due to their advantages. This paper will presents comparative study on the behaviour of flat slab with various tendons distribution systems analysed by Finite element analysis. The slab was model and designed by ADAPT BUILDER software package for the typical office building. The flat slab was modelled for various distribution systems such a uniformly distributed, banded in column line & distributed other direction and banded in column line in both direction & mixed system. The analysis was done for the gravity load case only. The   upward moments due to tendons, service moments, tensile stresses and deflections are compared.

I. Introduction

The post tensioned slabs were used in building sectors due to its advantages the conventional R.C.C slabs. some of the advantages are  increased spans , thinner sections, reduced storey height, lighter  structures, better control on deflections and cracking, rapid construction and large reduction in conventional rebar. Due to the material savings, headroom reductions and rapid construction these are more economical than the conventional flat slabs.  By partial prestressing   part of the service load or major part of self weight was balanced by tendons and rest by the non prestressed steel.  The various tendon distribution systems can be used for load balancing technique. Tendons can arranged in four   kinds of distribution systems to load balance such as closely placing near to column in both directions, one direction closely banding & other direction uniformly distributing, both directions uniformly distributing and mixed distributions in  both direction.

A. Scope and Objectives

The scope of this paper consists of studying the behavior of flat slabs for uplift moment due to tendons, stresses at bottom fiber and deflections for service load combination and ultimate flexures for various tendon distribution systems. The objective of this works is to study the results and compare for various parameter and to opt for the feasible options to suit for the Design as well as construction friendly to the Building sector.


A. Geometric Properties of Flat slabs

To study the behavior of post tensioned flat slabs typical office floor plate of 9.0mx9.0m was considered. The thickness of slab and drop was 200/400mm considered based on the standard span / depth ratio of post tensioned flat slabs. Size of drops is 3.0m x3.0 x0.4m. Loading considered as per the standard office floor loading of live load of 4 KN /m2 & SDL = 3.0 KN/m2.  . Rest of properties like Grade of concrete, covers to rebar and tendons considered as per the codal requirements.

B. Tendons Distribution Systems used for the Study

Four type of tendon distribution systems were considered in the study.  Those are

  1. Tendons uniformly distributed in both directions
  2. Tendons Distributed 50 % banded in column and 50% uniformly distributed in both directions.
  3. Tendons 100% banded in column in one direction and uniformly distributed in other direction.
  4. Tendons were fully banded in columns in both directions.

The four tendon layouts arrangement are summarized in Table 1 and illustrated in Figure 2.

The amount average pre-stressing force used in the typical floor plate was 1.2MPa which was greater than minimum force recommending by TR-43 which is a hand book of post tensioned designs.

C. Analysis and Design

For analysis and design the structural analysis soft ware ADAPT Builder was used for finite analysis of flat slab. Which is integrated structural analysis and design for concrete Buildings and used for both RCC and post tensioned slabs. The modeling was done for the four different tendon distribution systems with the same amount of pre stressing force.  The flat slab system was analyzed and designed for the four tendon distribution systems. For studying the behavior for various tendon distribution systems

Four typical panel was considered as shown in Fig.1, those  are  A-corner panel, B- External panel, C first internal panel and D- Interior panel.

III. Results

The result for the various tendon distribution systems was obtained from the analysis and design.

Bending moment distribution due to various tendon systems due to pre-stress alone was tabulated in Table-2. The deflection due to service load conditions was tabulated in table-3. The stresses at span bottom and ultimate bending moments were tabulated in Table4 & 5 respectively.

IV. Comparison of Study

From the above results it shows that the average moment distribution due to 100C – 100C tendon distribution shows more positive bending moments at supports than other distribution systems of tendons. It is very closer to the 100C-Uniform distribution system in direction of banding tendons. The negative moments at spans more or less same but the uniform distribution shows higher negative moments than other systems. The same pattern following in other direction also.

The deflection value for the spans showing clearly that the uniform distribution of tendons having better control than other systems. As the tendons moving toward the column line the deflection values are increasing. The deflection increment was there approximately 10% for the external bay and 15% for the interior panel.

The ultimate bending moments was not shown much difference for the various tendon distribution systems.

The stresses at bottom fiber of spans was increasing as the tendons moving towards the column line . but the variation are within the range of around 5% only.

V. Conclusions

This analytical study on the behaviour of flat slab with various distribution systems of tendons acknowledged the following conclusions.

  • All tendon distribution systems are fulfilling all structural requirements as per codal provisions. Such as ultimate limit state and serviceability criteria’s.
  • Tendon distribution systems of 50 % C & 50% uniformly distributed in both directions are similar to conventional flat slab distribution system of 65% capacity in column strip and 35% in middle strip.
  • Tendons with uniformly distributed are showing better sensitivity to deflections than other distribution systems.
  • The tensile stresses are more or less same for all the tendon distribution systems but uniformly distribution system having sensible than remain systems and increasing as tendons moving toward the column line.
  • The ultimate BMDs are insensitive to the tendon distribution systems.
  • The uplift BMD’s due to load balancing from tendons was sensitive to 100 C-100C at supports than other systems and uniform distribution system was sensible at spans than other systems.
  • In erection point of view tendon Distribution system with 100 C – Uniform in other direction will avoid the tendon hitting problems at span locations as well as it reduce the  laying time of tendons.
  • Uniformly Distributed system can be used at exterior spans to control the deflection and interior spans than other systems.
  • 100C – Uniform distribution in other direction resemble the beam slab option.
  • 100C -100C Distribution system can be used in slabs where the requirement future openings in flat slabs.

Further experimental studies required on the 100C-100C to understand the load transferring systems to the column tendons as all tendons was banded in column line.


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