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Thin-walled steel sheets with indentations in composite
steel-concrete
structure under different types of loading
JOSEF HOLOMEK, RADEK KARÁSEK, MIROSLAV BAJER, JAN BARNAT
Department of Metal and Timber Structures
Brno University of Technology Veveří 95, Brno 602 00
THE CZECH REPUBLIC [email protected],
[email protected], [email protected],
[email protected]
http://www.kdk.fce.vutbr.cz Abstract: This article is focused on
a special type of thin-walled steel sheets with shear connection.
These are especially suitable for roof and floor-bearing
structures, allowing the building process to be easy and fast. The
shear connection between the sheet and the concrete, which is
realised by moulded dimples, ensures the composite action of the
steel and concrete material without any need of additional
reinforcement. In this article you can find a short review of
thin-walled sheets, which are manufactured in Western Europe, and
results from our experimental testing of specimens. These specimens
were tested by vacuum loading and by cyclic loading in four-point
bending tests.
Key-Words: thin-walled, steel, connection members, vacuum,
loading, shear connection
1 Introduction Steel structures have a greater load capacity
than those made of concrete, but have lower spatial stiffness.
By contrast, concrete structures are sufficiently rigid, but with
very low tension bearing capacity. Combining these two materials
can maximize their advantages and suppress their disadvantages. In
a combined cross section the concrete takes the pressure and the
steel takes the tensile stress. For these structures, a shear
connection between steel and concrete is a prerequisite [1].
2 Type of shear connection The profiled steel sheet must be
capable of
transmitting horizontal shear at the interface between the sheet
and the concrete; pure bond between the steel sheeting and the
concrete is not considered effective for composite action.
Composite behavior between profiled sheeting and concrete should be
ensured by one or more of the following means; see Fig. 1: a)
mechanical connection using hitch elements pre-pressed into dimples
in the sheets; b) frictional connection via profiled self-locking
sheeting; c) connection by terminal headed studs in combination
with (a) or (b);
d) end anchorage by deformation of the ribs at the end of the
sheeting; only in combination with (b).
Fig. 1: Typical forms of interlock in composite
slabs.
The text below describes the thin-walled corrugated sheets which
are used in shear connections where possible to realize composite
behaviour via mechanical connections using hitch elements
pre-pressed (moulded) into dimples in the sheets; see Fig 1a.
This type of combined structure stands out above all for its
simplicity and the speed at which it can be constructed. The
coupling between the sheeting and the concrete provides sufficient
load capacity without further reinforcement of the concrete. The
system also adds just structural reinforcement. The bearing
capacity of this type of connection is determined by the stress
concentration near the dimples, which can be assessed using
experimental research [2].
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3 Characteristics of selected thin
sheets 3.1 Materials 3.1.1 Characteristics of sheeting
It is a soft steel sheet, cold rolled, galvanized continuously
with a surface of Z275 according to P 34.310, or galvanized
pre-painted B 15 / E prepared according to Standard P 34-401.
Another production process is identical to that used in the
production of classic trapezoidal sheets with a smooth surface. A
track rolling mill is used to roll the sheet into the desired final
shape. Sheets are manufactured as trapezoidal sheets or wavy
sheets. The most commonly-used sheets of this type are for example
COFRASTA, COFRAPLUS or HAIRCOL sheets manufactured in several
modified versions. COFRASTA and COFRAPLUS sheets are produced in
the Czech Republic, while HAIRCOL sheets are imported from France
[3]. 3.1.2 Other steel parts
The concrete slabs of every specimen were reinforced by
Ø8/100/100 mm reinforcing mesh, which should prevent concrete
shrinkage cracks in the upper surface. This reinforcement is not
considered as a load bearing part of the structure. 3.1.3
Concrete
We used C20/25 concrete. 3.2 Geometry of thin-walled sheets
We used COFRAPLUS 60 steel sheets with a plate thickness of 1.0
mm (Fig. 2).
Fig. 2: Geometry of COFRAPLUS 60
Fig. 3: Geometry of a COFRAPLUS 60 steel sheet 3.3. Description
of assembly
- the individual components are placed and fixed on the beams
using appropriate pistols and fillings, or self-tapping screws, or
nuts and bolts; - sheets are supported only at the ends; no middle
abutments; - over the whole area of the sheet there must be
reinforcing mesh, which should prevent concrete shrinkage cracks in
the upper surface; - the concrete is poured using traditional
methods; - it is necessary to vibrate the concrete moderately due
to the fact that the metal transfers vibrations better than
traditional formwork.
4 Current design methods 4.1 Longitudinal shear for slabs
without end
anchorage [4] The design resistance against longitudinal
shear
should be determined by the m-k method, or by the partial
connection method. The partial connection method should only by
used for composite slabs with ductile longitudinal shear
behavior.
The longitudinal shear behavior may be considered as ductile if
the failure load exceeds the load, causing a recorded end slip of
0,1 mm by more than 10%. If the maximum load is reached at a
midspan deflection exceeding L/50, the failure load should be taken
as the load at the midspan deflection of L/50.
If the m-k method is used it should be shown that the maximum
design vertical shear VEd for a slab width b does not exceed the
design shear resistance V1,Rd determined from the following
expression:
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+
⋅
⋅⋅= k
Lb
AmdbV
s
p
Vs
p
Rd γ,1 (1)
where: b, dp are in mm; Ap is the nominal cross-section of the
sheeting
in mm2; m, k are design values for the empirical factors
in N/mm2 obtained from slab tests meeting the basic requirements
of the m-k method;
Ls is the shear span in mm; γVs is the partial safety factor for
the ultimate
limit state.
If the behaviour is ductile, the representative experimental
shear force Vt should be taken as 0.5 times the value of the
failure load Wt. If the behaviour is brittle this value shall be
reduced, using a factor of 0.8.
From all the test values of Vt the characteristic shear strength
should be calculated as the 5% fractile by using an appropriate
statistical model and drawn as a characteristic linear regression
line, as shown in Fig. 4.
Fig. 4: Evaluation of test results
4.2 Manufacturers’ documentation
In the documentation for these types of structures, their
manufacturers and suppliers limit the burden on ceilings or floors
for the static scheme and the range, thickness and number of
assembly supports. Reported values (for COFRASTA and COFRAPLUS
elements) are based on scientific and technical reports prepared by
the "Center Scientifique et Technique du Batiment" located in
France. This collection of technical expertise defines relations
for the determination of marginal deflection and load torque. The
effectiveness of the shear connection between steel and concrete in
the technical report (CSTB 3/93-390, 2004) is defined as
follows:
RS ττ ≤ (2) where τS is acting shear stress and τR is shear
resistance.
( )els
szb
V
⋅=τ (3)
( )klhm
R +⋅⋅
=ρ
τ (4)
where: Vs is the shear force acting in the connection
after the concrete hardens, l is the span of the structure, b is
the width of the element, zel is the arm of the internal forces,
(dp-x/3) , dp is the average thickness of the floor, x is the
thickness of the compressed part of
the RC slab, m, k are coefficients determined from
experiments (CSTB 3/93-390, 2004), h is the total thickness of
the floor, ρ is the ratio between metal and concrete in
the cross section (b.dp).
Static and dynamic load
Static load Dynamic load
m m1=3228 m2=1775 m3=1420 k k1=0.1286 k2=0.5302 k3=0.4242
Table 1: Experimental coefficients “m” and “k”
from [1]
Fig. 5: Diagram of the problem
The studies were prepared according to French
standards and regulations. There is no widespread methodology
for calculating these types of structures in Central and Eastern
Europe, nor is there such a methodology in EC4.
5 Experiments verifying deflection
and shear connection The first series of experiments verifying
the
actual behaviour of thin-walled sheets combined
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with a concrete slab were performed in March 2010. The second
series were performed in December 2010. These experiments were
focused mainly on the deflection of the samples tested and on
verification of the effectiveness of the shear connection between
the steel sheet and the concrete slab [5].
The test specimens were prepared with the following dimensions:
width 1.08 m and span 2.0 m. The total thickness "h" of all
specimens was 110 mm (50 mm of concrete above the top edge of the
steel sheet). The slabs were made without any support in the middle
of the span during the concreting. The concrete slabs of each
specimen were reinforced by Ø8/100/100 mm reinforcing mesh, which
should prevent concrete shrinkage cracks in the upper surface. This
reinforcement is not considered as a load bearing part of the
structure. We used C20/25 concrete and COFRAPLUS 60 steel sheets
with a thickness of 1.0 mm (Fig. 3). After the concreting of the
slabs, the test samples were moved to the laboratory and fitted
with measuring devices (Fig. 8, Fig 10).
Fig. 6: Test specimen set-up in the mould for the pouring of
concrete
Fig. 7: Test specimen after pouring of concrete
5.1 Ideal uniformly distributed load In the experiment the
specimens were loaded by
an ideal uniformly distributed load. This was provided by the
use of a unique vacuum loading system, Fig. 9, Fig. 10 [6].
Fig. 8: Configuration of the measuring device for monitoring
sliding on the contact between the
concrete and the steel sheet
Fig. 9: Arrangement of formwork around the support structure for
the vacuum loading
experiment
Fig. 10: Vacuum loading. Configuration of the measuring devices
for monitoring slab deflection
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Fig. 11 and Fig. 12 show the specimen after the
failure. The failure occurred because the ultimate strength of
the connection between the steel sheet and the concrete was
reached. The slip of the concrete slab and the steel sheet is shown
in Fig. 12.
Fig. 11: The specimen after the test. Total loss of load
carrying capacity
Fig. 12: Mutual sliding between the concrete and the steel sheet
after the failure
5.2 Four-point bending tests
Tests should be carried out on simply supported slabs. The test
set-up should be shown in Fig. 13 or equivalent. Two equal
concentrated line loads, placed symmetrically at L/4 and 3L/4 on
the span, should be applied to the specimen.
Fig. 13: Geometry of test specimens and loading by four-point
bending
The distance between the centre line of the
supports and the end of the slab should not exceed 100 mm. The
width of the bearing plates and the line loads should not exceed
100 mm.
When the tests are used to determine m and k factors, for each
variable to be investigated two groups of three tests (indicated in
Fig. 4 by regions A and B) or three groups of two tests should be
performed. For the specimens in region A, the shear span should be
as long as possible while still providing failure in longitudinal
shear and for the specimens in region B, the shear span should be
as short as possible while still providing failure in longitudinal
shear, but not less than 3ht in length. The following figure, Fig.
14, shows the four-point bending test setup, including the location
of sensors recording the vertical deformation of the test specimen.
Loading was carried out in repeated press-and-release cycles. 5000
load cycles were performed. After these cycles the specimen was
subjected to increasing static loading until it collapsed.
Fig. 14: Four-point bending test setup, with tested
specimen.
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6 Conclusions
Fig. 15: Slip between steel sheet and concrete desk at the edge
of the specimen in 4-point bending
(black dashed lines – static test, red lines – cycling, blue
lines – static test after cycling)
Fig. 16: Comparison of edge slip in 4-point bending (values on
left) and vacuum testing (values
on right)
Longitudinally profiled thin-walled sheeting with a concrete
slab (without any load-carrying reinforcement) is a modern ceiling
structure system. The main advantage of this type of structure is
in particular its very simple and fast construction process.
However, its use is not widespread in Central and Eastern Europe.
One of the reasons why this type of structure is not so often used
is that there is no generally known and widespread methodology for
its design. Test results from uniformly distributed loading are on
average 35% higher in comparison to static 4-point bending. Using
cyclic loading significantly
influences the profile of the load deflection curve in
comparison to static loading. The deflection grows more uniformly
from low loading values without any clear level of major
deformations. Acknowledgement These results were achieved with the
financial assistance of Ministry of Education, Youth and Sports
project No. 1M0579, within the activities of the CIDEAS research
centre and projects GAČR 103/09/1258 and GAČR 103/09/H085
References:
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Plates with Moulded Connection Members in Composite Steel-Concrete
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and Techniques, Vilnius Gediminas Technical University, 19-21.
May 2010, Lithuania, pp. 880-885, ISBN 978-9955-28-594-6
[2] Ducháček J., Bajer M., Barnat J., 2009; Tenkostěnné
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[3] CSTB Technical report Cofraplus 60, 3/03-390, PAB, ARCELOR
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[4] EN 1994-1-1; 2004, Eurocode 4 – Design of composite steel
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[5] Z. Kala, J. Kala, Variance-Based Sensitivity Analysis of
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[6] MELCHER, J. Full-Scale Testing of Steel and Timber
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