Technical Digest 2018
the slab of the general form illustrated in Figure 2. Different coefficients are
presented depending on whether the deck is re-entrant or trapezoidal.
The slab can be divided into a number of thin horizontal strips whose
temperatures can then be determined using the equations discussed
previously. The temperature of any mesh reinforcement can be determined
from the temperature of the concrete strip at the level of the mesh.
Separate equations are given for determining the temperature of bar
In addition, by applying these equations to a number of locations along
the top surface of the slab, the acceptability of the slab with regard to
the insulation criteria (also specified in PN005c) can be determined by
evaluating the maximum and average temperature rises along this surface.
A typical surface temperature profile is shown in Figure 3.
The scope of PN005c is limited to a specified range of deck geometries,
however, this range covers the majority of deck profiles available within the
UK. For trapezoidal decks, the profile height is required to be in the range
60-80mm whilst the bottom flange width should be between 100-130mm.
For re-entrant decks, the profile height should be in the range 50-60mm
and the bottom flange width in the range 120-150mm.
Figures 4 and 5 show a comparison of the temperature profiles given in BS
5950-8, BS EN 1994-1-2 and PN005c for fire resistance periods of 90 minutes
and 120 minutes respectively. The profiles are plotted for a typical 60mm
trapezoidal deck with a bottom flange width of 125mm and a total slab
depth of 140mm. The profiles are also plotted against physical test data
obtained from tests on a slab of the same geometry and the results of an FE
analysis. All data is for normal weight concrete.
It can be seen that BS 5950-8 and BS EN 1994-1-2 provide very similar
profiles. However, at some depths into the slab, both profiles are unconservative
when compared to the physical test data. This is even more
noticeable for the 120 minute fire resistance period where temperature
differences can be as high as 100°C.
It would appear on first inspection that PN005c produces overly
conservative results. However, it should be noted that these equations
are required to take account of a range of deck and slab geometries. The
PN005c curves provide an envelope on the actual temperature profile
through the composite slab in all of these cases.
The FE results tend to provide relatively good agreement with the test
data, being slightly conservative in most cases. Where the FE results are
un-conservative, they indicate only very slightly lower temperatures than
the test data. In cases where a deck or slab geometry falls outside of the
scope of the codes or PN005c, FE analysis is an accurate alternative for
determining an appropriate temperature distribution through a composite
slab and is especially useful for profiles of unusual geometry.
1. BS 5950-8 and BS EN 1994-1-2 provide tabulated data for temperature
profiles which in some cases can be un-conservative. The two codes
produce very similar temperature profiles but the UK National Annex
does not allow the use of the profiles provided in BS EN 1994-1-2.
2. PN005c provides a set of calibrated quadratic equations to describe the
temperature profile in different locations within a composite slab. It
takes account of a range of deck geometries to provide safe temperature
3. FE analysis is a suitable alternative to the curves presented here and is
particularly suited to unusual deck profiles which may fall outside the
scope of the codes and NCCI document.
The follwing article considers how the temperature profile through the
slab can be used in the design of composite beams in fire.
1 BS 5950-8:2003
Structural use of steelwork in building - Part 8: Code of practice for fire
2 BS EN 1994-1-2:2005
Eurocode 4 - Design of composite steel and concrete structures - Part 1-2:
General rules - Structural fire design
NCCI: Fire resistance design of composite slabs
The Steel Construction Institute
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 fire design
Figure 2: Illustration of a typical temperature distribution through a composite slab.
Figure 3: Illustration of a typical temperature distribution along the surface of a
Figure 4: Comparison of temperature profiles for a 90 minute fire
(using normal weight concrete)
Figure 5: Comparison of temperature profiles for a 120 minute fire
(using normal weight concrete)