Technical
Composite beam design at elevated
temperature: comparisons between
different temperature distributions in
the concrete flange
26 NSC
Nov/Dec 18
28
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 fire 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
fire 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 effect of different temperature profiles 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 effective slab depth can be calculated
(heff - Figure 1), allowing a more realistic uniform temperature
distribution in the concrete flange. According to equations D.15a
and D.15b of EN 1994-1-2, an effective depth of 100 mm can be
obtained for the slab shown in Figure 1 (heff = 100 mm). Basically,
this effective depth means that the temperature of the top
concrete fibre 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 effective slab depth for composite floors.
When estimating the resistance of the concrete flange 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 heff = h1 = 70 mm.
The temperature on the unexposed (top) side of the slab
is required to be no more than approximately 140°C to fulfil
insulation requirements6. A minimum slab thickness is imposed
to fulfil 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 references7, 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 floor screed is considered for the minimum insulation
thickness, the temperature in the top concrete fibre can be slightly
higher).
For 90 minutes of fire exposure, the minimum insulation
thickness according to EN 1994-1-2 Annex D would be heff ≥
100 mm (note that the profile 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
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
Effective slab breath to 12 m span: 3000 mm
Effective slab breath to 6 m span: 1500 mm
Floor usage: Office
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 profile.
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
/Composite_construction#Types_of_composite_beam
/Composite_construction#How_and_why_composite_construction_works
/Design_using_structural_fire_standards
/Design_codes_and_standards#National_Annexes
/Design_codes_and_standards#National_Annexes
/Design_codes_and_standards#NCCI
/Floor_systems#Composite_slabs