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Practical Application of limiting temperatures for the
specification of intumescent fire protection.
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PRACTICAL APPLICATION OF LIMITING
TEMPERATURES FOR THE SPECIFICATION OF
INTUMESCENT FIRE PROTECTION.
By
Mr. Peter Scott MEng CEng MIStructE MIFireE
(Fire Protection Structural Engineering Manager)
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Prescriptive Design Approach
Our profession encourages designers to specify a limiting
steel temperature together with the fire resistance period as
part of the overall steelwork specification.
Passive fire protection is generally specified post design-
stage at a contractor or applicator level and sometimes very
little is known about the limiting steel temperature (Critical
Core Temperature)
Passive fire protection is a critical safety element of any
structure. If specified incorrectly it could have serious
consequences in the event of a fire.
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Overview
Due to the lack of information for the majority of jobs that
Intumescent manufacturers receive the fire protection
industry uses prescribed limiting temperatures (550˚C &
620˚C) based on assumed load ratios, which may or may not
be conservative.
Although this is not the most accurate way of specifying, it is
in the majority of cases the only way to quantify fire
protection on jobs with no structural fire engineering.
The presentation will demonstrate the prescribed
temperatures calculated to Eurocodes allowing comparison
against current prescribed industry values, thus
demonstrating quantified fire protection savings.
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Background
Current UK guidelines produced by the ASFP in the ‘Yellow
Book” outline an industry practice of prescribed limiting steel
temperatures of 550C for columns and 620C beams based
on assumed load ratios and designed to British Standard
5950:8. For instance the load ratio for the column is 0.6.
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Prescribed BS Load ratio
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Eurocode Method
The introduction of the Eurocodes in the UK has brought
about greater awareness and flexibility to enable calculation
of actual limiting steel temperatures based on structural
capacity during a fire event with a view of providing structural
integrity in the event of a fire and optimisation of fire
protection materials
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Eurocode Method
Despite the tools at hand, limiting temperatures are rarely
considered at the ULS design stage. They are generally an
afterthought taken up by a structural fire engineer after the
building has been designed.
Fire protection is usually specified by the architect and no
fire engineering done whatsoever.
Also Intumescent manufacturers generally don’t publish their
full multi temperature assessment information.
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Prescribing with Eurocodes
As stated the current industry practice prescribed limiting
steel temperatures are based on British Standard BS5950:8
With the introduction of the Eurocodes in the UK there are no
current prescribed limiting temperatures with which to
provide estimation of fire protection.
In some countries, such as Belgium and France the National
Annexes limit the use or calculated temperatures:-
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Prescribing with Eurocodes
Belgian NBN-EN NA & French NF-EN NA method
For members of class 1 to 3:
- 540 °C for Isostatic beams or elements in tension
- 570 °C for Hyperstatic beams
- 500 °C for Elements in compression or in compression
and bending
In case the member has class 4 the default EN method is
applied. (350 °C )
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Example
Typically a fire engineer may try and reduce the scenario in
an attempt to reduce the fire protection material.
In this example we see a comparison of prescribed values.
203x203x60UC Column R120
500C Product thickness = 3.402mm
203x203x60UC Column R90
500C Product thickness = 1.861 mm
Saving on Fire protection 45.3%
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Example
A structural fire engineer can look at the utilisation and make
similar or even greater savings without the need to reduce
the scenario:-
203x203x60UC Column R120
500C Product thickness = 3.402mm
203x203x60UC Column R120
80% utilisation 653C Product thickness = 1.783mm
Saving on Fire protection 47.58%
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Example
A structural fire engineer can look at the utilisation and make
similar or even greater savings without the need to reduce
the scenario:-
203x203x60UC Column R120
500C Product thickness = 3.402mm
203x203x60UC Column R90
80% utilisation 653C Product thickness = 1.156mm
Saving on Fire protection 66%
Plus reduced to single coat system
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Column Table - Office Loading
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Design Approach
on Application saving
Member Analysis
Section
Factor,
Am/V #
Steel
Temperature,
Dry Film
Thickness †
No. of
Coats ‡
Fire protection
material saving ¥
(1) 150 x 150 x 5 RHS
UK industry standard
temperature 205 m-1 550oC 8.279 mm 8 0 %
(2) 150 x 150 x 5 RHS
Calculated limiting temperature
for a given applied loading 205 m-1 700oC 4.125 mm 4 50 %
(3) 150 x 150 x 8 RHS
Calculated limiting temperature
as in (2) but with wall thickness
increase 129 m-1 700oC 2.643mm 2 70 %
(4) 150 x 150 x 10 RHS
Calculated limiting temperature
but further wall increase 105 m-1 700oC 2.149 mm 2 74 %
Application could be counted in days per coat of
application (depending on drying time)
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Conclusion
When a structural Engineer designs the structure, the design
is normally only considered at ambient temperature and
reliance is placed on the architect to specify the fire
protection.
No regard is made for the structural ability of the building at
Fire Limit State.
Savings can be made if fire protection is designed into
schemes (Should be considered more)
However predetermined levels of utilisation at ULS can also
provide savings over current industry standard practice
(fast and very easy for industry to adopt)
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Benefits
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Benefits
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