Performance of flat slab structures subjected to fire

8/11/2019 Performance of flat slab structures subjected to fire

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Presented by

K. Saiguru Raghavendra Guptha

Enrl no. 12523008

Structural engineering

Under the guidance of

Dr. Pradeep Bhargava


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Sequence of Presentation:

1. Introductionon the RC Flat slab framed structure.

2. Objective and Scopeof Flat slab structure performance when it

subjected to fire.

3. Modellingof prototype structure in ABAQUS.

4. Theory of Heat transfer and Thermal analysis on ABAQUS model.

5. Thermal stress analysis on the Model based on the temperature

distribution

that is carried out in the thermal analysis.

6. Future work that has to be carried out in Research.


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INTRODUCTION:

In the context of Structural engineering understanding the

performance of response of structures in fire has become very

essential after the collapse of World Trade Centre in 2011.

RC Flat slab system is a beamless slab directly support on a

columns

Flat slab is a typical structural system generally used for office

buildings and Residential buildings.

The slabs of flat-plates generally have very thin concrete cover

leaving steel reinforcement more sensitive to thermal loads.

Less information available in the engineering community about

the structural performance of flat Slab structures subjected to fire.


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Aims and Objectives:

To understand the various aspects of modelling techniques and

features which are defined in ABAQUS.

To understand the variation of temperature across the structural

members in Flat slab structure by carrying out heat transfer analysis.

Performing the Non linear finite element thermal stress analysis on

the protoype model .

To investigate response ( deflections, membrane forces and punching

shear) of Flat slab structure at elevated temperatures.

Analyse the difference in behaviour of Flat slab frame and regular

RC structure of same configuration at elevated temperatures.


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Scope of work:

1.This Research is on the Flat slab structures where slabs

design has been primarily governed by gravity load.

2. Shear reinforcement has not been provided in slab -column

connection .

3. The Flat slab is analyzed for standard ISO 834 time-temperature curve.

This is analyzed for only single storey single bay RC flat

slab frame


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METHODOLOGY

Modeling in ABAQUS :

ABAQUS software is capable of simulating the real behavior of

Engineering materials such as rubber, polymer, reinforced concrete and

geotechnical materials such as rocks and soil etc. The modeling of

prototype structure has been doing in this software because of tworeasons.

1. when slabs are modelled using solid or shell elements reinforcement

can be modelled conventionally.

2. Reliability of Non-linear analysis solvers has been acknowledged.

.


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Element types used

ABAQUS has extensive element library. The behaviorof elements is characterized by five aspects namelyfamily, degrees of freedom,Number of nodes, Formulation and Integration

Element library in ABAQUS


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Different analysis may require different types of

elements based on the required degree of freedom at

nodal level. This can be explained

below example.

C3D8 is used for continuum elements for

stress/displacement analysis.

DC3D8 used for a heat transfer analysis as their d.o.f

are different.

The order of the element is also highlighted in the element

name by indicating the number of nodes.


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Modeling of prototype structure:

The concrete part of slab, columns, plinth beams aremodelled by 3D solid continuum elements andReinforcement is modelled by wire shape element.

The concrete frame model and reinforcement cage models

are created as different parts and they are merged in theinstances which is available in ABAQUS software.

The various elements that are used in this study are C3D8,T3D2, B31, S4R, DC3D8 and DC1D2. The usage of these

elements are mentioned in at the appropriate analysis.


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DIMENSIONS OF FRAME MODEL:

Slab - 1.96 X 1.96 X 0.12 m

Columns0.3 X 0.3 m

Plinth beams-0.23 X 0.23mIn slab Reinforcement grid is at 0.3 and 0.8 m from

bottom face.

12mm diameter bars are used

In Columns 16mm diameter bars are used.

In Plinth beams 12mm diameter bars are used.

M35 grade concrete and Fe415 grade steel is used for

whole structure.


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Concrete Frame modelled by

Solid elements

Rebar cage modeled by

truss elements


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Concrete flat slab frame after merging with reinforcement


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THERMAL PROPERTIES OF MATERIALS:

Temperature dependent thermal properties of concrete and reinforcing steel

areimportant for understanding the fire response of RC structures.

These properties include

1.Thermal conductivity.

2.Specific heat.

These properties determine the extent of heat transfer inside the material.

Thermal conductivity of concrete is given as input parameters with respect

to temperature

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 500 1000 1500

conductiv

ity

Temperature

concrete conductivity

Thermalconductivity

0

10

20

30

40

50

60

70

0 500 1000 1500

conductivity

Temperature

Steel conductivity

Steelconductivity


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THERMAL ANALYSISThermal stress analysis is carried out in two phases.

1.Conducting a time dependent heat transfer analysis inwhich temperature distributions at various timestages are computed.

2. By using the Temperature distributions that areobtained in the first stage Thermal stress analsyishas been carried out.

In ABAQUS heat transfer can be done in two ways. oneis by giving surface film condition and another by

giving boundary conditions to the exposed andunexposed surfaces. But researches have shown that 1stmethod represents better realistic condition .


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Uncoupled Heat Transfer Analysis :

The ABAQUS Standard capability for uncoupled heat transfer analysis

is intended to model solid body heat conduction with general,temperature dependent conductivity, internal energy (including latentheat effects) and quite general convection and radiation boundaryconditions. This section describes the basic energy balance,constitutive models, boundary conditions, finite elementdiscretization, and time integration procedures used.

ENERGY BALANCE:

Where V= Volume of solid material with surface area

S= Density of the materialq= Heat flux per unit area of the bodyr = Heat supplied externally into the body per unit volume

VSV

rdVqdSdVU


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Boundary conditions:-1) Prescribed temperature = (x, t)

2) Prescribed surface heat flux, q = q(x, t)per area

3) Prescribed volumetric heat flux, q = r(x, t)per volume;

4) Surface convection q = h ( 0), where h = h(x, t)is the filmcoefficient And 0 = 0(x, t)is the sink temperature: and

5) Radiation q = A (( Z)4(0Z)4), where A is the radiation

constant (emissivity times the Stefan-Boltzmann constant) and Zis

the absolute zero on the temperature scale used.The boundary

conditions 4 and 5 are used in this analysis.

Time integration:

Abaqus uses the backward difference algorithm

)/1)(( tUUU ttttt

Introducing the operator into the energy balance equation gives

Sq

N

V

N

V V

N

ttt

NqdSNrdVNdV

xk

x

NdVUUN

t0..)(

1


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This Nonlinear system is solved by a modified Newton method. The

method is modified Newton because the tangent matrix (the Jacobian

matrix) i.e.the rate of change of the left-hand side of equation withrespect to is not formed exactly.

The first-order heat transfer elements use a numerical integration rule

with the integration stations located at the corners of the element for

the heat capacitance terms. This approach is especially effective when

strong latent heat effects are present.

The second-order elements use conventional Gaussian integration.

Thus second-order elements are to be preferred for problems when the

solution will be smooth (without latent heat effects), whereas the

first-order elements should be used in non smooth cases (with latent

heat).

N

tt


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Analysis Procedure:

The assumption made with the boundary conditions used in this analysis is

that the temperatures achieved in the surrounding gases and that of the surface

are equal.

Heat transfer from the gas phase to the structural elements (beam/column) was

modelled by applying appropriate convection and radiation boundary

conditions.

A convection coefficient of 25 W/m20C for exposed surface and 9 W/m20C for

other ambient exposed surfaces and emissivity of 0.7 are given as input

parameters as shown in below.

Exposed surface


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UNEXPOSED SURFACE:

Input parameters for unexposed surface

In Heat transfer analysis concrete is modeled using continuum elementswith thermal degree of freedomIn general a tie constraint can be used in between concrete and rebars inorder to transmit temperatures in to the bars from surrounding concrete.But it must be done by picking the nodes of concrete and rebarsimultaneously and then the tie constraint must be defined. In case of a frameit is not practical to define the tie constraint.


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In Heat transfer analysis special properties called stringers is used. All the

rebars in the frame are replaced with stringers with the same geometric and

material properties.

Usage of any other constraints between concrete and rebars is unnecessary ifStringers are implemented.

Mesh details:

Solid concrete has been discretized using DC3D8 element which is having

Nodal temperatures are only active degrees of freedom. Reinforcement has been

discretized using DC1D2 element having Node temperature as the only activedegree of freedom.

The approximate mesh size used for concrete and rebar elements is 90mm

Total no. of elements = 66868

ELEMENT TYPE NUMBER OF ELEMENTS2 Node heat transfer link(DC1D2)

10645

8 Node Heat transfer brick(DC3D8)

28816


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For Heat transfer analysis temperature is applied on the model according to

the

standard ISO-834 time-temperature curve as shown below.

ISO 834 time-temperature Equation:

ISO -834 time-temperature curveTemperature is applied on the exposed surface of the model upto 60 min

according toThe curve shown above.


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Before Heat transfer entiremodel is given initial

temperature as 20C.

Initial state of Heattransfer


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Final state of Heat transfer


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Slab sectional temperature distribution withrespect to time:

1.Initial temperature(t=0)

2.At time t=455sec


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3. At time t=1062sec

4. At time t=2209sec

5.At time t=3600sec

Distribution of temperature across slab


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For validation of Modeling techniques and Thermal loads the

results of heat transfer are compared with the eurocode.

0

100

200

300400

500

600

700

800

900

1000

0 50 100 150

TemperatureinC

Distance from exposed surface across slab in 'm'

Temperatures profiles forslabs-EURO code

temperature profile forslabs-analytical+She+Sheet1

As shown in above curve the results are in good agreement with the

Euro code values.


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THERMAL STRESS ANALYSISTheory and Analysis procedure:The temperature gradients induced in the structural elements giverise to thermal loads because of different thermal distributions and

coefficient of expansion of materials.

Supports and rigid connections don't allow the structure to undergo deformation so there need to be study of thermal stress analysis.

The temperature distributions across entire structure can beimported from heat transfer odb file as shown below.

THERMAL STRESS ANALYSIS


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Mechanical properties of Materials :

The concrete damaged plasticity model in ABAQUS:

Provides a general capability for modeling concrete and other quasi-brittlematerials

in all types of structures (beams, trusses, shells, and solids);

Uses concepts of isotropic damaged elasticity in combination with

isotropic tensile

and compressive plasticity to represent the inelastic behavior of concrete;

Can be used for plain concrete, even though it is intended primarily for the

analysis

of reinforced concrete structures;

Designed for applications in which concrete is subjected to monotonic,

cyclic,

and/or dynamic loading under low confining pressures;

Allows user control of stiffness recovery effects during cyclic load

reversals;

Can be defined to be sensitive to the rate of straining;

Requires that the elastic behavior of the material be isotropic and linear


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The model is a continuum, plasticity-based, damage model for concrete. It

assumes

that the main two failure mechanisms are tensile cracking and compressive

crushing

of the concrete material. The evolution of the yield (or failure) surface is

controlled bytwo hardening variables, and , linked to failure mechanisms under tension and

compression loading, respectively. and represent tensile and compressive

equivalent

plastic strains, respectively

(a) Uniaxial tensile behavior o f concrete (b) Uniaxial compressive behavior

of concrete


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The Model used in this study is the modified version of the model

proposed by Scott et.al

The tensile behaviour of concrete is assumed to be bi-linear in this study

i.e. the behavior assumed is to be linear up to peak stress, and the

descending branch is also taken as linear.

Abaqus assumes the value of E to be same in both compression and tension

For reinforcing bars the constitutive model used is according to the Euro code


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Meshing of structure for structural analysis is done by taking stress

elements as shown in below table.

The approximate mesh size used for concrete elements and re-bar elementsis 90mm

Total no. of elements =39461

Element Type

Number

Truss element (T3D2) 10645

Continuum element (C3D8) 28816

Reinforcement is assigned as T3D2and Concrete element is

assigned with C3D8


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Results of Structural analysis:

The Flat slab is designed for self weight, Super imposed dead load 1.44

KN/m^2

and live load of 2.39 KN/m^2.

It is assumed that in the event of a fire, the prototype structure is subject

to a uniformly distributed gravity load of 1.0D + 0.5L..

The deflected shape is as shown in below figure.


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RESULTS OF THERMAL STRESS ANALYSIS

Presentation of total analysis results will be difficultfor entire structure. So for convenience results arepresented at specified points as shown in belowfigure. point 'C' is located at 150 mm from columnface. Point 'B' is the mid-point of column centre

line. Pont 'A' is the centre of slab panel.

Reference diagram of Prototype structure plan

Sl b i l d fl i


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Slab vertical deflection :

Initially gravity load causes a deflection of 1.3 mm, 0.6 mm at the points

A and B.becauseof thermal stress anslysis after the 1 hr deflections at

A and B is to 62mm and 25mm.

The largest deflection always occur at the centre of the heated slab panel.

As shown in below increase in the deflection rate for the midpoint of slab

panel 'A' is much higher than the midpoint of centre line 'B' and column

face point 'C'

At this loading stagethere is nosign of generating a

collapse mechanismassociated withflexural yieldingbecause the slab hasnot experienced arapid increase indeflection at either

location.

0.00E+00

1.00E-02

2.00E-02

3.00E-02

4.00E-02

5.00E-02

6.00E-02

0 1000 2000 3000 4000

Deflectioninm

Time (sec)

Deflection at point 'B'Deflection at point 'B'


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In plane slab Expansion

The figure shows the In plane displacement in X-direction.


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The Thermal expansion of heated panel leads to the steadily increased

outward Displacements at all three locations i.e. A, B and C.

At location A the displacement is 10mm and at B is 16mm. A large

deformation enough to cause column cracking.

0

2

4

6

8

10

12

14

16

18

0 20 40 60 80

displacementsmm

Time (min)

In plane slabdisplacement at 'A'

Inplane slabdisplacement at 'B'


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The main steps of this research Heat transfer and thermal stressanalysis is carried out.

FUTURE WORK :

1.In depth scrutiny of thermals stress analysis has to be done.

2.The thermal stress analysis has to be carried out under the load of

gravity load and live load and lateral loads.

3.Response and Risk of punching shear failure of flat slab at elevated

temperature has to be studied based on the results of analysis andvalidity of equations on punching failure has to be studied.


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REFERENCES:

1.ABAQUS analysis users Manual,version-6.10

2. Eurocode 2, "Design of concretestructures.Part1.2.

Generalrules-Structural Fire design", Commission of

European communities, Brussels,2004

3. Journal structural fire engineering, Multi-sciencepublishing ISSAN 2040-2317.,vol 4., November 4.,2013


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Performance of flat slab structures subjected to fire

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