IRJET-Analysis of PT Flat Slab with Drop- Considering Seismic Effect

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 06 | June-2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 119
Analysis of PT flat slab with Drop- Considering Seismic Effect
Anusha. I. Koti1, Dr. S. B. Vanakudre2
1PG Student, Department of Civil Engineering, SDMCET, Dharwad, Karnataka, India
2Principal, Professor, SDMCET, Dharwad, Karnataka, India
---------------------------------------------------------------------***---------------------------------------------------------------------
Abstract - As we know that in the present scenario
buildings with post-tensioned flat slab are gaining more
popularity thanconventional RCCbuildingsforflooringsystem
due to its increased number of floors and even due to its
capability to the resistance during earthquake. In this papera
plan of residential building with post-tensioned flat slab with
drop considering seismic effect is considered for the analysis
using CSI SAFE 2016 software. Afloorsystemofpost-tensioned
flat slab with drop model is considered for the analysis. The
modeling and analysis is done using CSI SAFE 2016 software.
An attempt is made to study the parameters such as moments,
stresses, time period, frequency and behaviorofflatslabunder
seismic effect for Equivalent Frame method and strip base
method.
Key Words: PT flat slab with drop, Equivalent Frame
method, Strip base method, Stresses, Moments, Shear
forces, Frequency, Time period.
1. INTRODUCTION
Post tensioning system is probably the latest discovery in
man's continuing search for new construction materialsand
methods. The concept of pre-stressing is employed to
express the mechanism of introducing internal
stresses(forces) within a concrete or masonry element
during construction procedure in order to counteract the
external loads applied when the structure is allowed to use
(known as service loads). The internal stresses are
practically activated by tensioning high-strength steel, and
are done before or after concrete placed. When the steel is
tensioned prior to the concrete placement,themechanism is
called pre-tensioning. The time when the steel is tensioned
posterior to the concrete placement,themechanismiscalled
post-tensioning. For the reason that pre-tensioning requires
personally planned casting beds, it is second hand by and
large in the precast manufacturing administer to
manufacture clear-cut shapes that be able to be transported
to a work site. Post-tensioning is finished onsite by
introducing post-tensioning tendonsinsidethedefiniteform
work in a conduct comparable to introducing rebar.
The floor plan of residential building has been chosen to
analyze using SAFE 2016 software for thesis work of post
tensioned flat slab with drop considering seismic effect.
1.1 Objectives of the Present Work
 Analysis of the PT flat slab with drop in CSI SAFE
2016 software.
 To determine stresses, moments,forcesoftheptflat
slab.
 To evaluate the frequency and time period of the
structure.
 Equivalent frame analysis of PT slab with drop.
1.2 Material Properties and Loads
This work has been analysed using CSI SAFE 2016
software. For the analysis the parameters considered are
given below:
Grade of concrete: M30
Grade of steel: Fe400
Modulus of elasticity of steel, Es: 2x105N/mm2
Ultimate stress of tendons, fu: 1860 N/mm2
Live load: 2kN/mm2
Super imposed dead load: 1kN/mm2
Slab thickness: 150mm
Drop thickness: 250mm
Storey height: 3m
Drop panel size at corners: 1200mm x 1200mm
Drop panel size in remaining parts: 1050mm x 1050mm
Size of columns at corners: 600mm x 600mm
Remaining size of columns: 450mm x 450mm
1.3 Model Description
Fig- 1: Typical Floor plan of residential building

Fig- 2: Sectional elevation
2. ANALYSIS AND DESIGN METHODOLOGY
The design of post-tensioned slab is done by two methods,
 Load balancing method
 Equivalent frame method
2.1 Load balancing method
The concept of load balancing is introduced for pre stressed
concrete structures, as per T.Y Lin a third approach after the
elastic stress and the ultimatestrength methodof design and
analysis. It is first applied to simple beams and cantilevers
and then to continuous beams and rigid frames. This load-
balancing method represents the simplest approach to pre
stressed design and analysis, its advantage over the elastic
stress and ultimate strength methods is not significant for
statically determinate structures. When dealing with
statically indeterminate systems including flat slabs and
certain thin shells,load-balancingmethodofferstremendous
advantage both in calculating and visualizing. According to
load-balancing method, pre stressing balances a certain
portion of the gravity loads so thatflexural members,suchas
slabs, beams, and girders, will not be subjected to bending
stresses under a given load condition. Thus a structure
carrying transverse loads is subjected only to axial stresses.
2.2 Equivalent Frame method
The equivalent frame method of analysis is known as the beam
method. This method of analysis utilizes the conventional elastic
analysis assumption and models the slab or slab and columns, as
a beam or as a frame, respectively. This is the most widely used
and applied method of analysis for the post-tensioned flat plates.
According to Y. H. Luo, A. Durrani et the effect of
vertical of lateral services and design loading on post-
tensioned flat plates, bonded or unbonded, may be analyzed
as for rigid frames in accordance with the provisions of the
code ( IS, ACI etc.). When the columns are relatively slender
or not rigidly connected to the slab, their stiffness may be
neglected and continuous beam analysis applied. As per A.C.
Scordelis, Lin, T.Y, and R Itaya et al the moment induced by
pre stressing may also be determined bya similaranalysisof
a rigid frame or continuous beam, using equivalent load or
load balancing concept. However it should be kept in mind
that the distribution of moments due to loads may differ
considerably from the distribution of moments due to pre
stressing. Service loads produce very pronounced moments
peaks at columns, whereas the moment curve produced by
post-tensioning hasa moregentleundulatingvariationofthe
same form as the tendon profile.
According to A .Pan, and J. P. Moehle the effects of
reversed tendon curvature at supports are generally
neglected in applying the load balancing methodtodesignof
flat plates since the reverse curvature has only a minor
influence on the elastic moments (in the order of 5 to 10
percent), and does not affect the ultimate moment capacity.
It is necessary to consider reverse tendon curvature tort
adequately evaluate the shear carried by the tendons inside
the critical section.
2.3 Modeling of structural floor system
In SAFE, tendon essentials are utilized to endow with the PT
technique. Diagram represents a plan of the essentials
convoluted in counting post-tensioning, from definition of
material up- to comprehensive results.
Post-Tensioning - A course ofactionwhenthesteel tendons
are stressed after the concrete casting.
Tendon - It consists of a quantity of high-strength steel
wires or strands covered by a duct, sited wherever
applicable in the slab or beam. Tendons are a kind of rebar
which are inserted in concrete to stand for the end product
of post-tensioning. Every tendon is designated as a category
of line object between two joints, i and j. The two ends of the
tendon are denoted end I and end J, respectively..
Post-Tension Load-Thestresseswhichthetendonexecutes
on the structure. This includes incorporation with the
vertical loads because of profile of tendon and end forces
because of anchorages.
Dead load - The structure weight due to gravity, SAFE
calculates automatically from sectional parameters and
according to material density.
2.4 Analysis and Design Procedure
The SAFE model has been accomplished by all of the
properties of material and sectional definitions, prototype
(including tendon layouts, profiles, and jacking force
assignments), member assignments, and loading criteria
have been specified, an analysis is ready to be performed.
While the examination phase, SAFE calculates reactions,
displacements, forces, stresses for all particular load
patterns and its combinations. Then SAFE the executes a

output according to the required code of design and
calculates the essential parameters.
Fig- 3: Tendon layout of residential building for post
tensioned flat slab with drop
3. POST TENSIONING TECHNIQUES AND SYSTEMS
3.1 Post-tensioning systems
Post tensioning systems are bifurcated as bonded and
unbonded type for fabrication purposes. There are various
types of post tensioning systems which are listed below
 Unbonded system
 Bonded system
3.1 Unbonded systems
This setup is immediate for fixing; tendons are deflected
easily and to make comfortable with slab shapes by
neglecting opening. The system minimizes abrasion and
amplified irregularity. Grouting is not required.
It consists of 7 cable strands of13mmor15 mmdia.
which are glazed with corrosion preventing grease and
encased in high – density polyethylene (HDPE) sheathing.
Tendons are positioned in the slab in order to suit
the profiles prior to pouring of concrete. The lubricant
minimizes abrasion and the cover permits for free motion of
the steel wire around concrete while stressing operation.
Fig- 4: Unbonded monostrand system
3.1 Bonded systems
The setup incorporate groups of 2, 3,4,5 or 6 set of wires
enclosed in a tendon in a flat conduit fixed at both end by
anchorages.
The conduits are made full by a cement grout
mortar to bond the tendons around the concreteinsideup to
the tendon length. This initiates frictionless medium
surrounding to the steel for everlasting decay preservation.
The cables are closed at one end, and can be left
exposed at the other end and implanted in the concrete
through adequate distance to make sure their bonding. This
system needs a lesser quantity of ordinary reinforcing such
that bonding permits the strands to make contact with
maximum stress at ultimate state.
Fig- 5: Bonded mulitstrand system
4. RESULTS
4.1 Structural results of pt slab with drop
Seismic analysis–Modal analysis showing natural periods
and frequencies of pt slab.
TABLE: Modal Periods And Frequencies
OutputCase ModeNum Period Frequency
CircFre
q
Eigen
value
Text Unitless Sec Cyc/sec rad/sec rad2/sec2
MODAL 1 0.066277 15.088 94.802 8987.4
MODAL 2 0.059353 16.848 105.86 11207
MODAL 3 0.057232 17.473 109.78 12052
MODAL 4 0.053144 18.817 118.23 13978
MODAL 5 0.051645 19.363 121.66 14802
MODAL 6 0.050164 19.935 125.25 15688
MODAL 7 0.049516 20.196 126.89 16102
MODAL 8 0.048265 20.719 130.18 16947
MODAL 9 0.046973 21.289 133.76 17892
MODAL 10 0.04609 21.697 136.32 18584
MODAL 11 0.043014 23.248 146.07 21338
MODAL 12 0.041319 24.202 152.06 23123
Table- 1: Frequencies and natural periods

4.2 Diagrams showing stresses for applied loads &
its combinations:
Fig- 4.2 a) Stresses of Dead Load for stresses at top face
with maximum of(2.42 N/mm2) at (5.86m, 5.68m) &
minimum of (-0.692N/mm2) at (15.105m, 4.7875m)
Fig- 4.2 b) Stresses of Live Load for stresses at top face
with maximum of(1.063 N/mm2) at (5.86m, 5.68m) &
Fig- 4.2 c) Stresses of Load combination
{1.5DL+1.5LL+1PT-FIN-HP-1E} for stresses at top face
with maximum of (12.9 N/mm2) at (16.91m , 6.11m) &
minimum of (-8.44N/mm2) at (1.789m , 12.245m)
Fig- 4.2 d) Stresses of Load combination {1.5DL+1PT-FIN-
HP-1.5E} for stresses at top face with maximum of (11.473
N/mm2) at (16.91m , 6.11m) & minimum of (-9.28N/mm2)
at (1.789m , 12.245m)
Fig- 4.2 e) Stresses of Dead Load for stresses at bottom
face with maximum of (1.468 N/mm2) at (14.015m,
6.41m) & minimum of (-1.397N/mm2) at (16.495m,
6.41m)
Fig- 4.2 f) Stresses of Live Load for stresses at bottom face
with maximum of (0.647N/mm2) at (14.015m, 6.41m) &

Fig 4.2 g) Stresses of Load combination {1.5DL+1PT-FIN-
HP-1.5E} for stresses at bottom face with maximum of
(15.216 N/mm2) at (20.978m , 12.1m) & minimum of (-
3.51N/mm2) at (16.495m , 6.11m)
Fig- 4.2 h) Stresses of Load combination
{1.5DL+1.5LL+1PT-FIN-HP-1E} for stresses at bottom face
with maximum of (14.5 N/mm2) at (20.978m, 12.1m) &
4.3) Diagrams showing strip moment for applied
loads & its combinations
Fig- 4.3 a) Strip moment of Dead Load with maximum of
(5.72kN-m/m) at (14.015m, 6.635m) & minimum of (-
22.71kN-m/m) at (11.535m, 6.635m)
Fig- 4.3 b) Strip moment of Live Load with maximum of
(2.522kN-m/m) at (14.015m, 6.635m) & minimum of (-
9.97kN-m/m) at (11.535m, 6.635m)
Fig- 4.3 c)Strip Moment of Load combination {1.5DL+1PT-
FIN-HP-1.5E} with maximum of (36.72kN-m/m) at
(1.72m , 10.045m) & minimum of (-50.26kN-m/m) at
(16.945m , 3.05m)
Fig- 4.3 d) Strip Moment of Load combination
{1.5DL+1.5LL+1PT-FIN-HP-1E} with maximum of
(31.88kN-m/m) at (1.72m , 10.045m) & minimum of (-
62.32N/mm2) at (16.945m , 3.045m)

4.4) Diagrams showing strip shear force for
applied loads & its combinations
Fig- 4.3 a) Strip Shear force of Dead Load with maximum
of (31.94kN/m) at (16.2m, 6.635m) & minimum of (-
30.9kN-m/m) at (6.628m, 6.635m)
Fig- 4.3 b) Strip Shear force of Live Load with maximum of
(13.94kN/m) at (16.2m, 6.635m) & minimum of (-
13.95kN-m/m) at (6.275m, 6.635m)
Fig- 4.3 c)Strip Shear force of Load combination
{1.5DL+1PT-FIN-HP-1.5E} with maximum of
(65.42kN/m) at (16.19m , 6.635m) & minimum of (-
61.02kN/m) at (6.28m , 6.635m)
Fig- 4.3 d)Strip Shear force of Load combination
{1.5DL+1.5LL+1PT-FIN-HP-1E} with maximum of
(86.33kN/m) at (16.19m , 6.635m) & minimum of (-
81.25kN/m) at (6.28m , 6.635m)
4.4) Equivalent frame analysis results
4.4.a) Graph showing variation of Stress along Distance for
Dead load
4.4. b) Graph showing variation of Stress along Distance
for Live load

4.4. c) Graph showing variation of Stress along Distance
for load combination {1.5DL + 1PT-FIN-HP -1.5E}
4.4. d) Graph showing variation of Stress along Distance
for load combination {1.5DL+1.5LL+1PT-FIN-HP-1E}
5. CONCLUSIONS
This chapter presents the conclusions that are concluded
depending upon the study executed. Further the scopes for
future work have also been discussed.
 CSI SAFE software has got a new approach for
EQUIVALENT FRAME ANALYSIS for PT slab than
any other civil software.
 EQUIVALENT FRAMEMODEL (EFM)analysistool in
SAFE software analysis the PT slabasperthedesign
strips provided while modeling.
 Each individual strip is analyzed separately in EFM
analysis for more accurate and better results than
compared to overall basic analysis.
 PT slab with drop helps to reduce the stress
concentration in the slab and column junctions.
 High strength tendons provided in the slab resists
the stresses induced due to due to self weight and
varying dynamic loads (live load).
 Due to parabolic profile of tendons provided as
main reinforcement and reverseparabolicprofileof
tendons provided in distribution direction of slab
nullifies the secondary moments arisen in the slab.
 Because of tensioning of flat plate slab there is no
effect at great extent on axial force but shear and
moment on column increases.
 The curvature at middle of flat plate slab is handled
high efficientlybyparabolic andTrapezoidal tendon
than triangular tendon.
 EFM method is also applicable when the columns
are quite slender, not rigidly linked or stiffness of
the column is neglected.
 The moment calculated for Post-tensionedflatplate
slab is less when compare to moment calculated for
RCC flat plate slab by equivalent frame procedure
because as depth of Post tensioned flat plateslab30
to 35% less than RCC plate slab, due to which self
weight of slab get reduced.
 The stresses calculated by strip method are greater
than stresses calculated by equivalent frame
method. Hence EFM gives mosteconomical sections
for the designers.
5.1) Scope for future work:
The study presented here can be improved further by
considering various other factors of analysis also some
of which are listed below:
 Further analysis can be carried out by providing
different profile for tendons such as partial
parabola-left, partial-right, trapezoidal, etc.
 Analysis of PT flat slab with drop is also done by
time history analysis, equivalent staticanalysis,and
response spectrum analysis other than push over
analysis.
 Even analysis can also be done using finite element
method.
 It can be analyzed using different software such as
STAAD-pro, ETABS, ANSYS, and ADAPT.etc.
REFERENCES
[1] Y. H. Luo and A. Durrani, “Equivalent Beam Model for
Flat-Slab Buildings: Interior Connections,” ACI Structural
Journal, vol. 92, no.1, pp. 115-124, 1995.
[2] Y. H. Luo and A. Durrani, “Equivalent Beam Model for
Flat-Slab Buildings: Exterior Connections,” ACI Structural
Journal, vol. 92, no. 2, pp. 250-257, 1995.

[3] A. J. Duran, S. T. Mau and A. A. Abouhashish, “Earthquake
response of flat-slab buildings.” Journal of Structural
Engineering, vol. 120, no.3, March, 1994.
[4] Y. H. Luo, A. Durrani, and J. Conte, “Seismic Reliability
Assessment of Existing R/C Flat slab Buildings,” Journal of
Structural Engineering, ASCE, vol.121,no.10,pp.1522-1530,
1995.
[5] Code of Practice for Pre-stressed Concrete Is: 1343 –
1980, BIS, Indian Standard Institution, and New Delhi.
[6] Plain and Reinforced Concrete Code of Practice Is: 456 -
2000, BIS, Indian Standard Institution, and New Delhi.

IRJET-Analysis of PT Flat Slab with Drop- Considering Seismic Effect

More Related Content

What's hot (19)

Similar to IRJET-Analysis of PT Flat Slab with Drop- Considering Seismic Effect (20)

More from IRJET Journal (20)

Recently uploaded (20)

IRJET-Analysis of PT Flat Slab with Drop- Considering Seismic Effect