THE BEHAVIOUR OF FIRE-EXPOSED COMPOSITE STEEL/CONCRETE SLABS

Citation for published version (APA): Hamerlinck, A. F. (1991). The behaviour of fire-exposed composite steel/concrete slabs. [Phd Thesis 1 (Research TU/e / Graduation TU/e), Built Environment]. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR348360
DOI: 10.6100/IR348360
Document status and date: Published: 01/01/1991
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COMPOSITE STEEL/CONCRETE SLABS
Hamerlinck, Alphonse Florentinus
slabs / Alphonse Florentinus Hamerlinck. - (Eindhoven :
Technische Universiteit Eindhoven]
in het Engels.
COMPOSITE STEEL/CONCRETE SLABS
aan de Technische Universiteit Eindhoven,
op gezag van de Rector Magnificus, prof. dr. J.H. van Lint,
voor een commissie aangewezen door het College van Dekanen
in het openbaar te verdedigen .
op vrijdag 22 maart 1991 om 16.00 uur
door
prof.ir. J.W.B. Stark
Voor Sandra,
Sabine en
Ir ene
The author wishes to record bis sineere gratitude to all who have contributed to the research
reported in this thesis.
Prof.ir. J.W.B. Stark, ir. L. Twilt and ir. F. van Pelt are cordially thanked fortheir support
and fruitful discussions with the author. The dedicated work of ing. P.W. van de Haar, ing. J.W.P.M. Brekelmans and others in
TNO Building and Construction Research, where the experimental part of the research was
carried out, is very much appreciated.
Financlal support from the European Coal and Steel Community under grant 721Q-SA/509
and the Dutch organisations CS and CUR is gratefully acknowledged.
CONTENTS
2.1 Thermal behaviour
2.2 Meehamcal behaviour
2.2.1 Material properties
2.2.2 Cross-flectional behaviour
2.2.3 Structural behaviour
2.2.3.2 Continuous slabs
3.1 Fire-resistance criteria
3.2 Experimental research
3.3 Numerical research
3.4 European recommendations
4.2.1 Heat transfer at the exposed side of the slab
4-2.1.1 Temperature development in the fire campartment
4.2.1.2
4.2.1.3
4.2.1.4
4.2.2
4.2.2.1
4.2.2.2
4.2.2.3
4.2.2.4
4.2.2.5
Heat transfer in the slab
Ge ometry of the cross-s eetion
Thermal properties
without moisture
into account
V
1
1
2
2
4
4
7
7
7
8
9
10
13
13
16
20
22
24
24
27
27
27
28
28
34
35
35
36
39
40
40
4.2.3 Heat transfer at the unexposed side of the slab 40
4.2.3.1 Convection-heat transfer 40
{2.3.2 Radiation-heat transfer 41
slab bonndarles 41
Chapter 5 Mechanical model 47
5.1 Introduetion 47
5.2.1.1 Parameters 49
5.2.1.2 Behaviour of a cross-section in positive bending 50
5.2.1.9 Behaviour of a cross-section in negative bending 51
5.2.1.4 M -x diagram for both positive and negative bending 54
5.2.2 Cross-sectional behaviour during fire exposure 55
5.2.2.1 Thermal expansion 55
5.2.2.9 Behaviour of a cross-section (positive and negative bending) 58
5.3 Structural behaviour 61
behaviour 61
5.3.4 Cantilever slabs 63
5.3.5 Continuons slabs 66
5.4.1 Assumptions 70
5.5 Mechanical model for a structural analysis 75
5.5.1 Description of the mathematica! model 75
5.5.2 Description of the numerical model 77
5.5.3 Iteration process for continuous slabs 79
vi
6.1 Description of tests
6. 3. 2 Heat transfer to the slab ( convection
and radiation)
6.4.1 Heat transfer intheslab ( conduction)
6.4.2 Heat transfer to the slab ( conveelion
and radiation)
7.1 Test program
7.3 Tests on simply supported slabs
7.9.1 Description of tests
1.9.4 Additional calculation results
1.4.1 Description of tests
1.4.4 Additional calculation results
1.5.1 Description of test
7.5.4 Additional calculation results
8.1 Mechanica! calculations basedon temperatures
calculated with the thermal model
8.2 Influence of mechanica! properties of the reinforcement
at elevated temperatures
vii
81
81
86
92
92
94
96
96
96
96
100
100
105
108
108
110
112
115
120
120
123
125
132
135
135
135
139
143
146
146
150
153
References
Samenvatting
B Review of thermal tests and calculations
C Meehamcal properties of concrete and steel
at elevated temperatures
viii
157
164
172
179
1.1 General
Composite steel/concrete slabs are slabs normally spanning in one direction, in which profiled
steel sheeting acts as a permanent formwork a.nd as reinforcement to the concrete placed on
top, cf. Fig. 1.1. The use of these composite slabs in buildings has been common in North
America for many years, but less so in Europe. Since 1980, however, a rapid increase in their
use has occurred in the UK. There the overall depth is usually 100 to 150 mm, with spans
ranging from 2.5 to 3.6 m. when the slab is not propped during the construction stage [Cooke
et al., 1988]. The height of the steel sheet commonly used varies between 45 and 80 mm, sheet
thickness between 0.7 and 1.5 mm. The sheet is hot-dip galvanized for durabîlity. A common
feature of the applied steel sheet is the use of indentations or embossments to strengthen the
mechanica! bond between concrete and steel. Speed of construction and structural efficiency
are the main advantages of composite slabs.
Structures used in buildings havetomeet fire--;safety requirements. Normally these structures
are judged on the basis of standard fire tests [ISO, 1975]. However, such tests are very
expensive and time-consurning, for which reason methods have been developed to determine
the fire resistance of steel and concrete structures by calculation [CEB, 1987; ECCS, 1983a].
In Europe, recommendations for composite steel/concrete slabs, were introduced by the
European Convention for Constructional Steelwork (ECCS, 1983b]. These recommendations
enable the fire resistance of composite slabs to be quickly and simply calculated. With
properly designed slabs, no additional measures need be taken and no calculation is required
to achleve a fire resistance of 30 minutes [ECCS, 1983b]. For fire resistance of 60 minutes or
Jonger, additional measures usually have to be taken, such as additional reinforcernent,
insulating coatings or suspended ceilings, as shown in Fig. 1.2.
acdit:onal reinforcement
1
The calculation method in the ECCS recommendations provides a satisfactory predietien of
the fire resistance, notwithstanding the great number of assumptions and simplifications. The
calculation method applies to a specific field of application and is ba.sed on the available
knowledge. As this knowledge was incomplete, conserva.tive a.ssumptions have been generally
adopted. Under certain circumstances, this could lead to uneconornic solutions. In other cases
the recommendations could be unconservative. Hence, a mathematica} model which can
predict the behaviour of fire-exposed composite slabs on a more fundamental basis, is felt to
be preferable. An additional advantage of such a model is that it can be used for developing
new types of slabs.
1.2 Aim
The aim of the present research is to develop a mathematica! model to analyse the thermal
and mechanica! behaviour of fire-exposed composite steel/concrete slabs and verify the model
experimentally.
1.3 Scope and contents
This thesis deals with a model descrihing the behaviour of fire-exposed composite
steel/concrete slabs and its experimental verification.
The model camprises a thermal submodel, enabling temperature profiles to be calculated as a
function of fire-exposure time (Chapter 4), and a mechanica! submodel for calculating the
mechanica! response of the slab structure (Chapter 5).
The mechanical behaviour is influenced by the thermal behaviour, but it is assumed that the
thermal behaviour is not influenced by the mechanical behaviour. Although thermal
behaviour could be influenced by concrete craclcing or loosening of the steel sheet, the effects
are considered to have only minor, local implications. Usually general thermal behaviour is
not significantly affected. Therefore thermal and mechanica! behaviour can be analysed in
separate submodels, as illustrated in Fig. 1.3.
Fig. 1.3
model into two submodels
Chapter 1 Introduetion
The roodels are verified on the basis of fire tests performed in the scope of an international
research program. This program, ccr-;;ponsored by the European Coal and Steel Community
(ECSC), bas been carried out at the Centre for Fire Research TNO Building and
Construction Research [Hamerlinck and Twilt, 1989).
The test program camprises detail tests for studying the thermal behaviour and system tests
for studying the mechanica! behaviour of loaded systems during fire. The detail tests serve to
verify the thermal submodel (Chapter 6), the system tests to verify the mechanica! submodel
(Chapter 7).
The primary object of the tests is verification of the numerical model rather than coverage of
the full area of application of composite slabs. The most important parameters are considered
in the test program. After verification of the model, the effect of parameters that are not
varled in the test program will he numerically investigated in fut ure. Hence it is expected that
simple calculation rules can be developed with a limited number of fire tests, see Fig. 1.4.
The final aim of this ECSC-llponsored research is to establish practical design rul es for simple
and rapid determination of the fire resistance of composite steel/concrete slabs. These rules
will be proposed as revisions of the present European recommendations.
In Chapter 2 the behaviour of fir~xposed composite steel/concrete slabs is described
qualitatively. A review of liter at ure is given in Chapter 3.
NUMERICAL liDDEL covered by
this thesis EXPERIIENTAL VERIFICATION
3
2.1 Thermal behaviour
In practice, the behaviour of fire-exposed slabs is generally determined with respect to fire
exposure from below. Exposure at the upper si de of the slab is less critica.L
During fire exposure, heat is transferred from the fire to the bottom surface of the slab.
lf the slab is not thermally insulated, the steel sheet is directly exposed to the fire. The heat
transfer totheslab causes a quick temperature rise of the steel sheet.
If the slab is thermally insulated with a coating material or a suspended ceiling, as shown in
Fig. 1.2, the heat tran5fer to the slab is considera.bly less, so that the tempera.ture rise of the
steelosheet is slower.
In the following description a direcily exposed (noninsulated) composite slab is considered.
Ideally, heat. Ja.nsfer to all pa.:ts of a.n infinitely large, flat slab is uniform a.nd ca.n be
ana.lysed with a one-dimensional model. Due to the profiled shape of the sheeting, this is not
the case for composite slabs. The heat transfertoparts of the sheet (upper fla.nge and web) is
more or less obstructed, depending on the shape of the steel sheet. Temperatures of these
pa.rts are therefore lower than in the lower fla.nge of the steel sheet, a.s illustra.ted in Fig. 2.1 (1
a.nd 2) [Bah and Fulop, 1975].
Fig. 2.1 Measured teinperatures in a fire-exposed composite steel/concrete slab
[Bah and Fulop, 1975]
Chapter 2 Qualitative description of behaviour
A significant part of the heat transfer to the sheet goes into heating up the concrete. Hence,
less heat is ava.ilable to increase the temperature of the sheet. Steel-sheet temperatures are
therefore significantly lower than those in the :lire compartment, cf. Fig. 2.1.
Owing to the profiled shape of the cross-section, the heat transfer in the slab is essentially
two-dimensional. This is illustrated in Fig. 2.1 by point 3, showing higher temperatures (than
point 4) as aresult of heat transfer from both the lower flange and the webs. Another effect of
the profiled shape is a variation of thickness in the cross-section, causing temperature
differences, as can be seen by camparing temperature development in the points on lines
above lower and upper flanges (points 5 vs. 4 and 7 vs. 6 in Fig. 2.1).
In many cases the heat transfer in the third dirneusion ( along the ax:is of the slab) can be
neglected. Hence temperature distributions are appro:ximately the same for each
cross-section. This is not the case near supports. The cross-sections above the support are
thermally shlelded by the supporting structure, thus yielding lower temperatures in these
sections.
The temperature of the directly exposed steel sheet (1 and 2) rapidly increases, the
temperature of the additional reinforcement (for instanee located in point 3), important with
regard to mechanica! behaviour, increases less rapidly. The temperature increase of the mesh
reinforcement (for instanee located in points 4 and 5) and the unexposed side (6 and 7) is
relatively slow.
The geometry of the steel sheet has an important influence on the therm al behaviour.
Furthermore, the thermal properties of concrete are important and what is more, they change
at elevated temperatures. The thermal conductivity decreases as a function of temperature,
thus giving better insulation. The specific heat increases as a function of temperature, which
means that more heat is required to heat up the concrete.
The thermal properties also depend on the type of concrete used. Therefore slabs of
lightweight concrete have higher temperatures at the exposed si de (steel sheet) and lower
temperatures at the unexposed si de (top of slab and negative reinforcement) than slabs of
normal-weight concrete, see Fig. 2.2 (Bah and Fulop, 1975].
For a more deta.iled description of the therrnal properties, see 4.2.2.2.
By increasing the concrete depth, the total thermal resistance and heat capacity of the
concrete increase, resulting in lower temperatures at the unexposed side.
5
T [0 ( I ~~-r--~~---r--r-~~~
=d:::o<!~~""""':o:__l-----l -- LWC ----NWC
~--:!::---:-':--!:::---::eo'::--::100!-:--:1~20:--:140-:':--~t [minl
Fig. 2.2 Influence of type of concrete (NWC: norma.l-weight concrete; L WC:
lightweight concrete) on the iempera.ture development in a. Hibond-55
sla.b (Ba.h a.nd Fulop, 1975]
6
2.2 Mechanica] behaviour
During fire exposure, flexural faîlure is the faîlure mode occurring most commonly in
composite slabs. Mechanica! behaviour is therefore described on the basis of pure bending.
Due to the profiled shape of the slab, this bending behaviour is uniaxial.
With regard to mechanica! behaviour, a distinction is made between the behaviour of the
cross-section and that of a structural system, the second being dependent on the first. Before
descrihing cross-sectional behaviour, material properties are first discussed.
~. ~.1 Material properties
The properties of the steel and concrete components of a composite slab change at elevated
temperatures.
First, temperature increase causes thermal expansion. In contrast to room-temperature
conditions, the thermal expansion of concrete and steel at elevated temperatures is not the
same. Thermal expansion is discussed in 5.2.2.1.
Second, strength and stiffness of steel and concrete decrease. This change in properties is
illustrated by means of stress-straîn dîagrams as a function of temperature ( a-f(T)) in
5.2.2.2.
~.~.~ Cros-sectional beha'11iour
The cross-sectionat behaviour of a composite slab changes at temperatures above room
temperature. In each fibre of the cross-section, temperature increase causes a thermal
expansion and a deercase in strength and stiffness. Consequently, the moment capacity and
flexural stiffness of the cross-section decrease.
Thermal straîns, stresses and thermal curvature develop out of nonuniform temperature
distribution. This is illustrated in Fig. 2.3, in which a cross-section with reinforcement is
considered. Along the lines A-A and B-B, nonuniform temperature distributions are given
and straîn dîstribution conesponding to a nonloaded cross-section (bending moment M=O).
The strain distribution is linear, indicated by the thermal curvature K0 . The difference
between this linear strain distribution and the strain distribution that would occur in free
thermal expansion causes thermal stresses. Meeting the equilibrium conditions (moment and
sum of horizontal farces equal 0), compressive thermal stresses occur in the upper and lower
parts of the cross-section and tensile stresses take place in the rniddle part.
At the beginning of a fire, temperature differences in the cross-section increase, causing a
rapid increase of thermal curvature K0 • After some time of fire exposure, temperature
7
Chapter 2 Qualitative description of behaviour
differences increase less rapidly, causing a more moderate increase of fi.o. Alter long
fire--a:posure times, ~o may stabilize or even decrease.
Curvatures higher or lower than K.o correspond to positive, respectively negative bending
momentst. Strain and stress distributions change accordingly.
Fig. 2.3
a) cross-flection with reinforcement
b) nonuniform temperature distribution along lineA-A
c) strain distribution along A-A and thermal curvature in a nonloaded
cross-flection (M=O} d) nonuniform tempera.ture distribution along line B-B
e) strain distribution along B-B and thermal curvature in a nonloaded
CIOSS-flection (M=O)
3. !. 3 Strru:tval beho.viour Normally, it is assumed that the load remains the same during fire exposure. However,
deformations increase, on the one hand, due to the decrea.se of flexural stiffness and on the
other, due to the thermal curvature.
At the beginning of a fire the thermal curvature increases rapidly. Deflections increase
accordingly (stage I in Fig. 2.4). After a certain period of time the deflection rate drops, as
thermal curvature increases less rapidly (stage II}. Near failure, the deflection rate increa.ses
again due to plastic material behaviour (stage III in Fig. 2.4). These three stages are
characteristic of structural behaviour during fire.
Due to the lossof strengthof the materials, the load-bearing capacity of a sirneture decreases
lJn this thesis the sagging and hogging (bending) moments are indicated as positive or
negative, respectively.
Chapter 2 Qualitative description of behaviour
as a function of fire-exposure time. Failure occurs if this capacity equals the load level (at the
end of stage III in Fig. 2.4).
Simply supported and continuons systems are the two static systems considered. These
systems along with cross-sections of the slabs at mid span and at a centre support are shown
in Fig. 2.5.
Fig. 2.4 Schematic presentation of increasing defl.ections during fire exposure
a) l A or ~A
b) A
Fig. 2.5 Static systems and cross-sections of composite steel/concrete slabs:
a) simply supported
2. 2. 3.1 Simply supported slabs
In simply supported slabs the load-bearing capacity is determined by the positive moment
capacity. Owing to the high temperatures of the steel sheet the positive moment capacity
decreases rapidly, as shown in Fig. 2.6. Therefore normally additional positive reinforcement
is applied in composite steel/concrete slabs if fire resistance of 60 minutes or longer is
required. Tensile farces in the sheet are transferred to the reinforcement. The load-bearing
capacity is mainly ensured by the concrete and the reinforcement.
9
Chapter 2 Qua.litative description of behaviour
Jtl I I I I I I I I I I I I I !~q
Fig. 2.6
t
Decrease in positive plastic moments MP during fire. Increase in fire resistance l\tF of a simply supported slab due to the applica.tion of additional reinforcement
Although the steel sheet may, in general, scarcely contribute to the load-bearing capacity for fire resistances of sa.y more than 30 minutes, its presence is important. Apart from the positive influence on the thermal behaviour, as will be discussed in 4.1, it prevents concrete spalling and acts as a diaphragm preventing the passage of flames and hot gases.
However, where there are great differences between the temperatures of upper a.nd lower flanges of steel sheet, e.g. re-entrant profiled sheets, the upper flange (and web) of the sheet may contribute significa.ntly to the load-bearing capacity.
Applying positive…