Practical Application of limiting temperatures for the...

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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.



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.



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.



Prescribed BS Load ratio



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



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.



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:-



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 )



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


500C Product thickness = 1.861 mm

Saving on Fire protection 45.3%



Example

A structural fire engineer can look at the utilisation and make

similar or even greater savings without the need to reduce

the scenario:-




80% utilisation 653C Product thickness = 1.783mm

Saving on Fire protection 47.58%



Example

A structural fire engineer can look at the utilisation and make

similar or even greater savings without the need to reduce

the scenario:-




80% utilisation 653C Product thickness = 1.156mm

Saving on Fire protection 66%

Plus reduced to single coat system



Column Table - Office Loading



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


as in (2) but with wall thickness

increase 129 m-1 700oC 2.643mm 2 70 %

(4) 150 x 150 x 10 RHS


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)



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)



Benefits

• Fetter Lane London 30% savings



Benefits

• 2012 Olympics Broadcast Centre, Stratford

30% savings



Benefits

• Wigmore Street London

35% savings



Benefits

• Rotterdam

25% savings



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