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Several resources give guidance on the temperature profile through composite slabs; BS 5950-8,
EN 1994-1-2 and NCCI PN005C-GB. Ricardo Pimentel of the SCI discusses the impact of these
alternative profiles on the design of composite beams at elevated temperature.
Composite beams are one of the most common structural
elements in the UK construction market. Steel and concrete are
connected by mechanical devices (shear connection – usually
studs), allowing the two materials to work together. Composite
beams are usually simply supported elements, allowing the steel to
be mainly in tension and the concrete in compression.
The re design of composite beams is often required, which
demands an assessment of resistance of the concrete, steel and
studs at elevated temperature. The main topic of this article is
to evaluate the impact of alternative temperature distributions
in the slab to obtain the critical temperature or the allowable
re exposure period of composite beams. For a composite
beam design at elevated temperature, there are three possible
ways to model the temperature distribution in the slab in
the UK: (i) EN 1994-1-2 Annex D Table D.5; (ii) BS 5950-8 Table
12; (iii) NCCI PN005C-GB. However, note that the UK National
Annex to EN 1994-1-2 states that Annex D should not be used,
recommending the use of non-contradictory complementary
information (NCCI).
The eect of dierent temperature proles will be assessed
based on two worked examples, comprising 6 m and 12 m span
beams, both optimized for an adequate performance under
Serviceability Limit States, Ultimate Limit States and Fire Design.
The geometry and design conditions for the two worked examples
are summarized in the data presented in Figure 1 and Table 1.
According to EN 1994-1-2, to take into account the ribs of
a trapezoidal deck, an eective slab depth can be calculated
(he - Figure 1), allowing a more realistic uniform temperature
distribution in the concrete ange. According to equations D.15a
and D.15b of EN 1994-1-2, an eective depth of 100 mm can be
obtained for the slab shown in Figure 1 (he = 100 mm). Basically,
this eective depth means that the temperature of the top
concrete bre is obtained assuming a depth of 100 mm in table D.5
of EN 1994-1-2.
There are no recommendations in the NCCI or BS 5950-
8 for assessing an eective slab depth for composite oors.
When estimating the resistance of the concrete ange at
elevated temperature using NCCI, a weighted average between
temperatures above ribs and between ribs can be considered (using
l2 and l3 to calculate the weighted average). If BS 5950-8 is used,
the approach of equations D.15a and D.15b of EN 1994-1-2 can be
assumed to be valid. An alternative (and conservative) measure can
be to disregard the ribs, i.e., assuming that he = h1 = 70 mm.
The temperature on the unexposed (top) side of the slab
is required to be no more than approximately 140°C to full
insulation requirements[6]. A minimum slab thickness is imposed
to full this requirement. For the beam analysis, according to
EN 1994-1-2, 4.3.4.2.2 (16), it may be assumed that for concrete
temperatures below 250°C, no strength reduction is necessary.
For these reasons, according to some references[7], assuming
room temperature for assessing the sagging bending resistance
of composite slabs and beams is suggested, as, in general, only
a modest depth at the top of the slab will be necessary to obtain
section equilibrium at elevated temperature. Thus, an example
assuming room temperature in the slab will also be considered
(note that if oor screed is considered for the minimum insulation
thickness, the temperature in the top concrete bre can be slightly
higher).
For 90 minutes of re exposure, the minimum insulation
thickness according to EN 1994-1-2 Annex D would be he ≥
100 mm (note that the prole falls outside the scope of Annex D
of EN 1994-1-2, which limits l3 to 115 mm, compared to the actual
value of 125 mm). According to the NCCI, a minimum thickness of
h1 ≥ 70 mm is imposed, while BS 5950-8 suggests h1 ≥ 70 mm for
Composite beam design at elevated
temperature: comparisons between
different temperature distributions in
the concrete flange
h1 [mm] 70
h2 [mm] 60
l1 [mm] 175
l2 [mm] 125
l3 [mm] 125
Figure 1 – Composite slab geometry.
Characteristic Description/value
Steel section for the 6 m beam: UB 203 x 133 x 25
Steel section for the 12 m beam: UB 406 x 178 x 67
Eective slab breath to 12 m span: 3000 mm
Eective slab breath to 6 m span: 1500 mm
Floor usage: Oce
Beam spacing [m] 3.50
Slab weight [kN/m2] 2.65
Additional permanent loads [kN/m2] 2.00
Imposed Load [kN/m2] 2.70
Steel: S355 JR
Concrete: C30/37
Slab mesh: A142
Ribs direction: Perpendicular to the steel beam.
Fire protection: Yes
Temperature gradient: Uniform temperature in the steel prole.
Fire rating: 90 minutes
Steel Critical temperature – 6 m span: 620°C
Steel Critical temperature – 12 m span: 621°C
Miscellaneous: Cambered beam; restrained by steel sheet in
construction stage.
Table 1 – Design conditions
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Technical
lightweight concrete and h1 ≥ 80 mm for normal weight concrete.
In Figure 2, for 90 minutes of re exposure, the dierent
temperature distributions in the concrete ange according to
the three dierent UK resources can be found for normal weight
concrete. Slab depth is measured from the face exposed to re.
For 90 minutes of re exposure, the temperatures in the top
(Table 2) and the bottom (Table 3) bres of the concrete ange
above the steel sheet can be obtained (i.e. X = 130 mm, and X = 60
mm, respectively, according to Figure 1). Eight possible approaches
are presented. Once the temperatures have been obtained, the
respective concrete resistance reduction factors (Kc) according
to Table 3.3 of EN 1994-1-2 can be obtained. In the top concrete
bres, according to EN 1994-1-2 and BS 5950-8 approaches, the
top temperature is in fact close to 140°C (Cases 2 and 3). Even with
conservative approaches (Cases 5, 6 and 7), the temperature in
the top concrete bre is generally below 250°C, so no concrete
strength reduction would be needed for the top concrete bres. On
the other hand, for lower concrete bres, the strength reduction
can be up to 29 % for Cases 2 and 3 and 83 % for Case 6. Thus,
depending of the depth of the concrete ange required for section
equilibrium, the concrete resistance may have some signicant
reductions.
To evaluate the impact of dierent temperature distributions
in the slab, the critical steel temperatures shown in Table 1 were
assumed as xed. The plastic bending resistance under re, for each
slab proles temperatures (Cases 1 to 8) were then evaluated, and
are presented in Table 4 and Table 5 for the two worked examples.
The degree of shear connection (η) can vary between 0 and 1 in a
composite beam. Results for dierent degrees of shear connection
are presented in steps of 0.25 between those two extreme cases,
obtained through a stress block analysis. Partial interaction curves
are presented for both worked examples in Figure 3, for 6 m and
12 m worked examples.
Conclusions
1. The UK NCCI gives temperature proles at/above ribs and
between ribs for composite slabs; in the paper, a weighted
average temperature is suggested to assess the sagging
bending resistance of the composite beams design under re.
2. The temperature distribution prole in the composite slab
has generally minimal impact in the composite beam sagging
plastic bending resistance because: (i) only the top concrete
strips are usually needed to obtain section equilibrium, which
are not signicantly aected by the slab temperature; (ii)
dierences in the position of the plastic neutral axis are usually
small between the approaches; (iii) as the concrete ange tends
to be more resistant at elevated temperature than the steel,
even if the slab temperature is actually higher than considered,
26
RAINHAM
STEEL
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Phone: 01708 522311 Fax: 01708 559024
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GRADES S355JR/J0/J2
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email: sales@rainhamsteel.co.uk www.rainhamsteel.co.uk
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Channel • Angle
Flats • Uni Flats
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Hot & Cold Structural
Hollow Sections
Full range of advanced steel sections available ex-stock
Figure 2 - Temperature distribution according to dierent UK resources.
Case Methodology (90 minutes of re exposure) θc,top Kc
1Room Temperature 20 1.00
2 EN 1994-1-2 Annex D (he = 100 mm) 160 0.97
3 BS 5950-8 with EN 1994-1-2 Annex D (he ≥ 100 mm) 160 0.97
4 Medium value according to NCCI (weighted average) 224 0.93
5 Ignoring Ribs According to EC (he = 70 mm) 246 0.90
6 Ignoring Ribs According to BS 5950-8 (he = 70 mm) 260 0.89
7 Ignoring Ribs According to NCCI (he = 70 mm) 244 0.91
8Assuming 40% of steel top ange temperature (θtop ange =
620°C) EN 1994-1-2, 4.3.4.2.5 (2) – for shear studs resistance. 248 0.90
Table 2 – Top concrete bre temperature according to dierent approaches (X = 130 mm).
Case Methodology (90 minutes of re exposure) θc,top Kc
1Room Temperature 20 1.00
2 EN 1994-1-2 Annex D (he = 100 mm) 428 0.71
3 BS 5950-8 with EN 1994-1-2 Annex D (he ≥ 100 mm) 430 0.71
4 Medium value according to NCCI (weighted average) 559 0.51
5 Ignoring Ribs According to EC (he = 70 mm) 738 0.24
6 Ignoring Ribs According to BS 5950-8 (he = 70 mm) 790 0.17
7 Ignoring Ribs According to NCCI (he = 70 mm) 747 0.23
8Assuming 40% of steel top ange temperature (θtop ange =
620°C) EN 1994-1-2, 4.3.4.2.5 (2) – for shear studs resistance. 248 0.90
Table 3 – Bottom concrete bre temperature according to dierent approaches (X = 60 mm).
29
NSC
Nov/Dec 18
Technical
RAINHAM
STEEL
Nationwide delivery of all Structural Steel Sections
Phone: 01708 522311 Fax: 01708 559024
MULTI PRODUCTS ARRIVE ON ONE VEHICLE
GRADES S355JR/J0/J2
Head Office: 01708 522311 F ax: 01708 559024 Bury Office: 01617 9 62889 F ax: 01617 962 921
email: sales@rainhamsteel.co.uk www.rainhamsteel.co.uk
Beams • Columns
Channel • Angle
Flats • Uni Flats
Saw Cutting
Shot Blasting
Painting • Drilling
Hot & Cold Structural
Hollow Sections
Full range of advanced steel sections available ex-stock
only small changes in the neutral axis are expected, as a small increase in
the assumed slab depth increases considerably the slab resistance.
3. For assessing the resistance of the slab, generally no reduction in strength
is needed (ambient temperature may be assumed). An alternative often
used, which is to assume the slab temperature is equal to 40% of the steel
top ange temperature (a rule used to assess studs resistance under re),
can be seen as a conservative solution.
References
[1] BS 5950-8:2003
Structural use of steelwork in building - Part 8: Code of practice for re
resistant design
BSI, 2003
[2] BS EN 1994-1-1:2004
Eurocode 4 - Design of composite steel and concrete structures - Part 1-1:
General rules and rules for buildings
BSI, 2005
[3] BS EN 1994-1-2:2005+A1:2014
Eurocode 4 - Design of composite steel and concrete structures - Part 1-2:
General rules - Structural re design
BSI, 2005
[4] NA to BS EN 1994-1-2:2005
UK National Annex to Eurocode 4: Design of composite steel and concrete
structures - Part 1-2: General rules - Structural re design
BSI, 2008;
[5] PN005c-GB
NCCI: Fire resistance design of composite slabs
The Steel Construction Institute
[6] Steel and composite structures: behaviour and design for re safety
Y. C. Wang, Spoon Press, 2005
[7] AS/NZ 2327.1
Composite Structures – Composite steel-concrete construction in buildings
Standards Australia/Standards New Zealand, 2017
Mpl,rd,re
[kNm] Slab temperature prole case
η1 2 3 4 5 6 7 8
0.00 37.71 37.71 37.71 37.71 37.71 37.71 37.71 37.71
0.25 60.22 60.22 60.22 60.22 60.22 60.22 60.22 60.22
0.50 76.24 76.24 76.24 76.24 76.24 76.24 76.24 76.24
0.75 91.41 91.41 91.41 91.41 91.35 91.30 91.36 91.34
1.00 105.37 105.25 105.25 105.08 105.00 104.95 105.01 104.99
Mpl,rd,re
[kNm] Slab temperature prole case
η1 2 3 4 5 6 7 8
0.00 199.13 199.13 199.13 199.13 199.13 199.13 199.13 199.13
0.25 284.60 284.60 284.60 284.60 284.60 284.60 284.60 284.60
0.50 335.08 335.08 335.08 335.08 335.08 335.08 335.08 335.08
0.75 375.67 375.44 375.44 375.10 374.93 374.82 374.95 374.92
1.00 413.06 412.83 412.83 412.49 412.33 412.22 412.34 412.31
Table 4 – Results for 6 m span beam: UB 203 x 133 x 25; Steel critical temperature: 621°C. Table 5 – Results for 12 m span beam: UB 406 x 178 x 67; Steel critical temperature: 620°C
Figure 3 – Partial shear connection curves for the 6 m (left) and 12 m (right) worked examples.