The temperatures attained by unprotected structural steelwork in experimental natural fires

Fire Safety Journal, 12 (1987) 139 - 152

139

The Temperatures Attained by Unprotected Structural Steelwork in Experimental Natural Fires D. J. LATHAM*, B. R. KIRBY and G. THOMSON British Steel Corporation, Swinden Laboratories, Moorgate, Rotherham $60 3AR (U.K.) (Received January 9, 1987; in revised form April 29, 1987)

SUMMARY

Current research is shedding new light on the behavoiur o f steel structures in fire. The salient features o f a collaborative test programme, funded jointly by the British Steel Corporation and the Department o f the Environment, are described in which the heating rates o f a wide range o f structural sections in fires o f different but k n o w n severities were measured. For the purposes o f this work a large fire c o m p a r t m e n t was built, in which w o o d and wood/plastic fuels were burnt under different ventilation conditions and thermal properties o f the enclosing surfaces. The experimental observations are compared with various relationships proposed for the determination o f the equivalent time o f fire exposure. Design tables highlight those situations where unprotected steel beams and columns have sufficient inherent fire resistance to be used in both single and multistorey buildings.

1. INTRODUCTION In recent years renewed interest has been shown in the use of structural steel frameworks within the construction industry, due largely to a significant reduction in the costs of production and fabrication. As a result, steel has greagly enlarged its market share in the face of intensive competition. This trend is particularly noticeable in the multi-storey building sector. If the cost-effectiveness of steel-framed buildings is to be improved still further, one of the greatest opportunities lies *Author to whom correspondence should be addressed. 0379-7112/87/$3.50

in new approaches to structural design to resist collapse in fire. The heart of the problem stems from the reduction in strength experienced by all building materials at high temperature. Engineers have traditionally designed steel frameworks using the ambient temperature properties of the material. Insulation is then applied to the structure to ensure that, in a fire environment, the load-bearing capacity is not impaired as a result of the steel temperature exceeding a critical value. However, experimental evidence suggests that it is often more economic to design in steel using more comprehensive high temperature properties, obtained for example from anisothermal creep tests, and so minimize the need for protective insulation. Such concepts form the basis of a fire engineering design approach, which in essence involves the determination of the temperature changes in any particular member or sub-assembly and comparison with a limiting temperature based on the required load-bearing capacity. The simplest and traditional m e t h o d of assessing structural stability in fire is based on the standard fire exposure of structural elements in the BS476:Part 8 test. The design criterion is that the fire resistance is equal to or exceeds the time of fire duration required by the Building Regulations. The m e t h o d is essentially a classification system but has produced valuable information on the temperatures t h a t structural members can withstand in fires of different severities. This work has culminated in the concept of limiting temperature which is to be introduced in the forthcoming structural steelwork code BS5950: Part 8. The limiting temperature at which failure occurs varies and is dependent upon the loading which the member is carrying, its support conditions, the change in its proper© Elsevier Sequoia/Printed in The Netherlands

140 ties as the temp er at ur e rises, and the temperature gradients through the cross-section. This c o n c e p t makes the greater use of u n p r o t e c t e d steelwork possible. The standard fire resistance test, as adopted in Europe, can be criticized on the basis t hat the procedures are insufficiently specified to prevent significant variations in the measurem e n t o f fire resistance from one laboratory to another. Also, the standard heating curve does n o t represent the heating regime in the real fire due to marked differences in the time -- temp er atur e cycle of the combustion gas atmosphere. There is t her e f or e a need to gather additional information on the behaviour of u n p r o t e c t e d steel members in welld o c u m e n t e d natural fires. The data can then be used in an improved assessment of structural stability in fire by means of the equivalent time of fire exposure, which relates the m a x imu m temperature achieved by a steel member in a real fire to the time taken to reach the same temperature in the BS476: Part 8 fire resistance test. The heating rates achieved by steel members in natural fires can be predicted from a knowledge o f the size and shape of the section and the rate of heat transfer to the steelwork using theoretically derived equations [1, 2]. Where possible it is preferable to use data from full-scale fire tests. A series of fire tests was carried out in a specially constructed c o m p a r t m e n t by BISF and the Fire Research Station (FRS) in 1964 [3] and a separate programme was performed by CTICM in 1973 using a much smaller c o m p a r t m e n t [4]. In both studies the only u n p r o t e c t e d steelwork examined comprised columns of similar size. Therefore, in 1983, a collaborative test programme was initiated jointly by Swinden Laboratories of the British Steel Corporation (BSC) and the Fire Research Station of the Building Research Establishment, DoE, to measure the heating rates of a wide range of unprotected but unloaded structural steel sections in fires o f different but known severities. For this work, a c o m p a r t m e n t measuring 8.6 X 5.9 X 3.9 m 3 high was built inside the FRS hanger at RAF Cardington so t h a t tests were independent o f weather conditions. Low fire loads and comparatively large ventilation conditions were selected as being typical of buildings with specified occupancies such as schools and multi-storey office blocks.

This paper examines a number of features from the work to highlight situations where u n p r o t e c t e d steelwork has adequate inherent fire resistance to enable the fire-resistant design approach to be extended to a variety of fire exposure conditions.

2. DESIGN OF THE COMPARTMENT AND EXPERIMENTAL PROGRAMME The BSC/FRS fire c o m p a r t m e n t was considerably larger than the buildings used in earlier tests of this nature. The floor area o f 50 m 2 and ceiling height of 3.9 m were typical of an office in a multi-storey building or a six-bed hospital ward. The front elevation is shown in Fig. 1.

Fig. 1. Fire test compartment.

Ventilation was provided by means of shutters placed within the long walls. The precise dimensions were chosen to be directly comparable with those used in theoretical studies. For the majority of the tests the yen1 to ¼ and ~ of the tilation was changed from ~total area of one wall as in the B I S F / F R S approach. On the basis of the theoretical opening factor (Avv/h)/At, where Av = area of ventilation, h = height of opening and A t = total bounding surface area: these alterations corresponded to opening factors of 0.12, 0.06 and 0.03 m 1'2. A num ber of steel section sizes typical of present day construction were incorporated in the form of columns and beams, as shown in Fig. 2. Where appropriate, a specific section size, or form of construction, that had been evaluated previously in a BS476:Part 8 fire

141 Shelf Angle F l o o r Beam

203 • 133 x 25 k g / m

533 • 2 1 0 , - - , • 122 k g / m

254 •

mm

B2

B1

B~

1~6 • ~3 k g / m

mm

II

i Window

Window

Not

scale

to

8660

U---

j~

Back r---

Window

Window

FSC3

H FSC2 Z

4O6 .~ •356634x kg/m

FSCI

Z

r'OO

Water Filled

Column (8 m) Window

Window F--

7-

Front

Not

to

scale

Fig. 2. F r o n t e l e v a t i o n a n d plan o f fire-test c o m p a r t m e n t s h o w i n g l o c a t i o n o f s t e e l w o r k a n d t h e sizes o f t h e structural s e c t i o n s discussed.

test was selected. T he steelwork was evenly distributed t h r o u g h o u t the c o m p a r t m e n t to minimize its effect as a localized heat sink. Th e constructional materials were selected on the basis that t h e y would withstand repeated exposure to high t e m p e r a t u r e w i t h o u t collapsing or changing their thermal characteristics. In particular, the inner walls comprised spall-resistant MPK insulating refract o r y bricks and the precast concrete r o o f slabs were lined with ceramic fibre tiles. In later experiments the thermal characteristics o f the c o m p a r t m e n t walls were altered before each fire, by lining them with two layers of 12.7 mm G y p r o c Fireline plasterboard m o u n t ed o n 50 mm X 25 mm w o o d e n battens with the fibre tiles removed f r o m the ceiling. T h e thermal properties are given in Table 1.

Wood was used as the fuel for several tests in densities of 10, 15 and 20 kg/m 2 of floor area. Twelve equally spaced cribs comprised kiln-dried 50 m m X 50 mm X 1 m Western Hemlock sticks spaced 1:1 with alternate rows at right angles to each other. In view of the widepsread use o f plastic and laminated products in m o d e r n building materials and contents, mixed fuels were also examined by adding 25 mm X 25 m m X 1 m polypropylene sticks to the soft w ood cribs. Safety considerations and limitations on smoke emission restricted most o f these cribs to t he equivalent of 75% w ood and 25% plastic, taking their respective calorific values into account. T he usual procedure was to ignite the cribs simultaneously but growing fires were also developed in a few cases.

142 TABLE 1 Thermal properties of lining materials inside Cardington fire compartment Position

Material

Density (kg]m 3)

Thermal conductivity at 20 - 200 ~'C (W/m K)

Mean specific heat (J/kg K)

Ceiling

'Unifelt' ceramic fibre tiles (38 ram) Concrete slabs (200 mm) MPK 125 insulating refractory bricks 'Fireline' plasterboard (12.7 mm thick) Ceramic fibre blanket 'Fireline' plasterboard (12.7 mm thick) Insulating concrete (Monocast 115 - 149)

50 - 250 2400 650 825 - 945

0.06 1.3 0.19 0.24

1070 1200 1046

96 825 - 945

0.06 0.24

1070

1570

0.44

1045

Walls

Shutters

Floor

A t o t a l o f 21 fire tests w e r e carried o u t [5]. In each t e s t t h e t e m p e r a t u r e d i s t r i b u t i o n o f t h e c o m b u s t i o n gas t h r o u g h o u t t h e c o m p a r t m e n t and t e m p e r a t u r e gradients across t h e respective s t r u c t u r a l steel sections w e r e m o n i t o r e d e v e r y 30 s using 3 - m m - d i a m e t e r c h r o m e l / alumel m i n e r a l - i n s u l a t e d t h e r m o c o u p l e s placed 0.5 m a n d 2 m b e l o w t h e ceiling. This present a t i o n is c o n c e r n e d with t h e b e h a v i o u r o f specific u n p r o t e c t e d s t r u c t u r a l sections within t h e c o m p a r t m e n t , as i t e m i z e d in Fig. 2. When t h e e x p e r i m e n t a l n a t u r a l fire prod u c e d a c o m b u s t i o n gas t e m p e r a t u r e rise similar to t h a t o f t h e s t a n d a r d f u r n a c e curve, t h e partially p r o t e c t e d steel m e m b e r s also r e c o r d ed similar e x p o s e d - f l a n g e t e m p e r a t u r e s . H o w ever, t h e r e p e a t e d cycling t o elevated t e m p e r a t u r e a n d t h e resulting lateral d i s t o r t i o n e n a b l e d h o t gases t o p e n e t r a t e t o the p r o t e c t ed areas. T h e e x p e r i e n c e w i t h t h e water-filled c o l u m n e m p h a s i z e d t h e c o m p l e x n a t u r e of the design o f such a cooling s y s t e m and is considered separately.

3. COMPARTMENT TEMPERATURES AND FIRE SEVERITY T h e t e m p e r a t u r e s a t t a i n e d in t h e c o m p a r t ment depend upon the balance between the rates at w h i c h h e a t is p r o d u c e d in a fire and lost to its surroundings. As also f o u n d in earlier studies [ 3 ] , t h e p o t e n t i a l fire severity increased w i t h the fire load a n d w i t h a decrease in t h e e x t e n t o f v e n t i l a t i o n . A change in t h e t y p e o f fuel h a d a m a r k e d e f f e c t o n the

c o m b u s t i o n gas t e m p e r a t u r e as did a change in t h e c o m p a r t m e n t lining.

3.1. Fuel type Much o f t h e e x p e r i m e n t a l w o r k was based on t h e s i m u l t a n e o u s ignition o f w o o d e n cribs. An increase in t h e fire load d e n s i t y raised t h e m a x i m u m average c o m b u s t i o n gas t e m p e r a ture. This e f f e c t is d e m o n s t r a t e d in Fig. 3 where, f o r t h e i v e n t i l a t i o n o f o n e wall, the m a x i m u m t e m p e r a t u r e rose f r o m 691 °C {at 10 k g / m 2) to 966 °C (at 20 k g / m 2 ) . Also f o r each value o f fire load d e n s i t y , t y p i c a l l y the 15 k g / m 2 for Fig. 3, a smaller w i n d o w o p e n ing p r o d u c e d a fire t h a t was h o t t e r a n d o f longer d u r a t i o n . T h e B S 4 7 6 : P a r t 8 s t a n d a r d t i m e - t e m p e r a t u r e curve is s u p e r i m p o s e d o n the results for c o m p a r i s o n . T h e e f f e c t on fire d e v e l o p m e n t b y dividing a given v e n t i l a t i o n b e t w e e n the t w o o p p o s i t e walls o f the c o m p a r t m e n t instead o f o n e wall was c o m p a r a tively small. Work by F R S has s h o w n t h a t t w o t y p e s o f burning rate exist inside c o m p a r t m e n t s w h i c h d e p e n d u p o n processes b y w h i c h air is d r a w n into a fire [6]. W h e n t h e w i n d o w area is large, t h e burning rate d e p e n d s o n t h e characteristics o f the fuel itself {stacked in t h e f o r m o f cribs}, b u t for small o p e n i n g s t h e b u r n i n g rate relies o n t h e air s u p p l y t h r o u g h t h e w i n d o w and the e x p u l s i o n o f h o t gases. T h e t r a n s i t i o n f r o m o n e regime t o t h e o t h e r o c c u r s w h e n the ratio o f fuel load (L) t o w i n d o w area (Av) exceeds a p p r o x i m a t e l y 150 k g / m 2. O n this basis the c o m b u s t i o n of a fire load d e n s i t y o f

143 Temperature,

°C

1000 P

BS476: Pt 8 Standard heating

curve

800 i

600

I

40O

15(~)

%

20(~) 200

"x

I0(~)

"15(½) 15 (¼) of

0

0

floor

Insulated l 10

denotes

area

a fire load density o f 15 k g o f wood/m 2 with ventilation provi_ded by ~ area of one

Compartment l 20

! 30 Time,

l 40

| 50

I 60

waiI°

| 70

rain

Fig. 3. Average combustion gas temperatures inside compartment for wooden fire load densities and different ventilation conditions. 15 kg/m 2 at-~ ventilation, shown in Fig. 3, was ventilation-controlled. By mixing polypropylene with w o o d in ratios typical of their utilization in many modern designs of furnishing, very high combustion gas temperatures were recorded in a comparatively short time. A comparison between the two fuel types, based on a nominal density of 15 kg/m 2, is shown in Fig. 4. All the mixed-fuel fires were characterized by maximum average temperatures of 1000 1100 °C reached after 5.5 min compared to temperatures of 800 - 850 °C attained in w o o d fires after 13 min. The burning characteristics of polypropylene had been found by FRS to produce 40 times the volume of smoke in

comparison with soft w o o d [7]. A similar effect was observed in the fire tests, Fig. 5. Once the polypropylene had been burned, the combustion gases fell to temperatures that were typical of the cellulosic fire. The conversion of mixed fuels to an equivalent quantity of w o o d depends on an accurate knowledge of their respective calorific values. On this basis alone, the high temperatures recorded for the mixed-fuel tests would not be expected to arise and it is the burning characteristics of the fuel itself which predominates. The superposition of the standard heating curve onto Fig. 4 shows the inability of the standard test to typify such extreme conditions of combustion.

144 Temperature,

i" /

jl

1000

(:

Polypropyler~e/Wood Fuel

I

t: 800

° c o

i I

•

600

.

•

.

.

/

400 IIII

//`

",\~

\ Wood, Growing Fire

oofl/ ~ L O0

-.

I n s u l a t e d Compartment I 10

I 20

I 40

I 30 ']~ime ~

1 50

| 60

min

Fig. 4. Average combustion gas temperature inside compartment for a fire load density of 15 kg/m 2 and-~1 ventilation for different fuels and types of ignition.

As the cribs in each fire test were usually ignited simultaneously the development of the fire showed no 'growth' period. However, as also indicated in Fig. 4, when only four cribs were lit and fire allowed to spread were the temperatures attained in the compartment reduced due to the greater time for heat loss to occur.

3.2. Compartment lining The inner walls of the compartment comprised refractory bricks and the precast con-

crete roof slabs were lined with ceramic fibre tiles. It was recognised that the highly insulating properties of these materials would increase the maximum combustion gas temperature to an extent not normally found in modern buildings. The subsequent addition of the Gyproc 'Fireline' plasterboard to the walls and the removal of the ceramic fibre roof tiles had a significant effect on the m a x i m u m average temperature attained in fire. The effect is shown in Fig. 6 for a wooden fire load density of 15 kg/m 2 and the¼ ventilation of one wall. The introduction of the plasterboard reduced

145

(a)

(b)

Fig. 5. (a) Fire t e s t in progress w i t h 100% w o o d fire load ( v e n t i l a t i o n = 1 f r o n t wall -: 0.12 m2). ( b ) Fire test in progress w i t h 75% w o o d / 2 5 % plastic fire load ( v e n t i l a t i o n = ~8 f r o n t wall ~ 0 . 0 3 m2).

Temperature, 900 r

°C

800

700 g ../ r~

l I l l i

600

l i

500

l t

400

\

,

\ N

300

200

Tnsulating 100

°o

.....

Gyproc

~o

firebrick

'Fireline'

2o Time,

30

walls plasterboard

4o~o

min

Fig. 6. Average c o m b u s t i o n gas t e m p e r a t u r e s as influe n c e d b y c o m p a r t m e n t lining for a w o o d e n fire load d e n s i t y o f 15 kg]m 2 a n d t h e ~1 v e n t i l a t i o n o f o n e

wall.

the maximum average c o m p a r t m e n t temperature in a cellulosic fire from 851 °C to 732 °C. This effect was confirmed in other tests in the series. These observations had similarities with the records obtained in the early B I S F / F R S tests where the compartment had been lined with a gypsum/vermiculite plaster on the walls [ 3 ]. Insulating fire bricks are comparatively stable and exhibit specific heat and thermal conductivity values that increase slowly with temperature. Gypsum-containing plasters provide a degree of fire protection due to the release of free water and water of crystallization, as shown in Fig. 6. These processes result in an irregular change in specific heat and thermal conductivity with temperature. In view of these observations it was considered that the temperatures recorded in the insulated compartment represented the most severe conditions likely to be experienced b y unprotected steelwork in a fire. Extensive theoretical studies have been carried o u t in Sweden to relate fire load density and the opening factor with the combustion gas heating curves under ventilationcontrolled conditions. The effect of various types of construction material are taken into account. On this basis, the theoretical combustion gas heating curves for the t w o fires close to the transition point between fuelload-controlled and ventilation-controlled burning gave good agreement with experimental measurements, as shown in Fig. 7.

146 F~mporatur~,

~("

tempera ture,

'°Ft

.....

oC

',C

:::%

> , ...... ,

1~,(:) )

'1~ll.,st,,r't,,,a~t

I t:litlt;

3} ,)

()

T*

:im*',

mlr~

:'7

)

V,

Fig. 7. Measured and predicted c o m b u s t i o n gas temperatures. )

kl:

i o:1

4. T E M P E R A T U R E S OF U N P R O T E C T E D

10

STEELWORK

As the increase in fire load density and a reduction in the degree of ventilation produced fires of increasing severity it was to be expected, from the examples given in Fig. 8, that the behaviour of the unprotected steelwork would respond in a similar manner. This was also shown for the range of beams and free-standing columns in Fig. 9, in which the maximum average steel temperature measured inside the insulated compartment is plotted against the section factor. The Hp/A value, which is the ratio of exposed perimeter: cross-sectional area, is a concept used to estimate the heating rate of a steel section. The rate of heat transfer is governed predominantly by radiation which depends on the interrelationship between the emissivities of the bare steelwork and the flames. The temperature rise of the steel member depends on its position in relation to the flames. For this reason a degree of scatter was to be expected in the results of Fig. 9, becoming less marked at the higher fire loads. The change from fuel load-controlled to ventilation-controlled fires in the test series resulted in only a small increase in steel temperature. Despite the fact that high combustion gas temperatures were observed in the early stages

i~

PC}

•

*JC) imr•,

rnlr 1

Fig. 8. The effect of fire load density and ventilation on the temperature of an unprotected b e a m w i t h an Hp/A = 169 m - I .

I ' X) O

&

a

,,,o! •

•"

d

..

"

--

.,.,.,m,

0 -

i., .., ~.

,

~.~L:I,,,I : , t t . h , a }

k,:, m" kl:, ='

{,.-sl,~d

Pit,.

,,+al,l.lf:.," 1

%;I

(

,,]um.s

-

,,(t.ck~

;..,d ,,[

1

',,r,.,tv

o.

. . . .

11 I

14p'afm-1~

Fig. 9. Steel temperatures measured in w o o d fires in an insulated c o m p a r t m e n t .

of the mixed-fuel fires the influence on the maximum average temperature reached by the steelwork was small, as shown in Fig. 10, for 1 a fire load density of 15 kg/m 2 and the-~ ventilation of one wall. The simultaneous ignition of the 12 cribs gave the highest steel tempera-

147 Lower

Flange

oC

T~mperature,

25%

~CK

PoIypropyleno/75%

-

Wood -- -

-

Fue~ - -

Lower 1000

x _ _

Flange

T~porature,

800 f,O{

o

~ / "

x

°C

,•

-

_ _÷-

-

- - . . . . . .

_ ~-

- . . . . . . . .

,-

~-

-

~

~'o--'o~dodFuel o00

~

~

Uoo ! 2OO

Insulated

C~par

S

S lmu~ ¢aneou8

G

Growln~.

tmo,zt 1 ~litlon

Fifo

-----

Insulating

- -

Plaiterboara

200

A +

510

l IO0

I

I

0

,

l 2OO

,

~,

J 250 °0

Fig. 10. Steel b e a m t e m p e r a t u r e s in d i f f e r e n t fires based o n a n o m i n a l fire load d e n s i t y of 15 k g / m 2 a n d _1 v e n t i l a t i o n . 4

tures, but in the more typical situation of a growing fire, referred to earlier, these temperattires were reduced. The significance of the thermal properties of the walls of the compartment on the maxim u m steel temperatures recorded in the fire tests was a most important observation. This was particularly the case as there was a serious lack of experimental data on the effects of different b o u n d a r y material.~ in fire compartments of large volume. For a wood fire load density of 15 kg/m 2 and 1 ventilation, the m a x i m u m average temperature of a 203 mm × 133 mm × 25 kg/m universal beam fell by 173 °C on changing the wall linings from refractory bricks to 'Fireline' plasterboard. This behaviour together with the drop in temperature experienced by other beams in different fires is shown in Fig. 11. BS475:Part 8 fire tests on fully loaded unprotected beams (BS449:Part 2:1969) exposed on three sides have shown that the lower flange temperature reaches about 630 °C at a limit of deflection of (span/30) [8]. If similar members are subjected to a natural fire, the change in the b o u n d a r y surfaces of the c o m p a r t m e n t in the way described enables lighter sections to be exposed w i t h o u t exceeding the limiting temperature. For instance, for a wood fire load density of 15 kg/m 2 and 1 ventilation, the limiting section factor changes from approximately 90 m -~ to 210 m -1 by lining the walls with plasterboard, i.e., a serial size of 305 mm × 165 m m × 40 kg/m may therefore be left unprotected.

I

~

50

1~

I 5 kg/m 2 k~jm:2

[~r~.cks

Flre

Lond

DensLty

20 ~,

~ vo,ltilalL,,~ l 150

ar,!a

I 2o0

~r

o,i-

~all

I 250

HpI*(.- * )

Fig. 11. Steel b e a m t e m p e r a t u r e s m e a s u r e d in w o o d fires a n d d i f f e r e n t c o m p a r t m e n t linings.

5. T I M E - E Q U I V A L E N T A P P R O A C H

One concept employed by fire engineers is to relate the behaviour of a natural fire to an equivalent heating time in the BS476:Part 8 fire resistance test. In this way the ability of a steel member to withstand a real fire under load can be assessed. A comparison between the m a x i m u m average temperatures recorded on the different steel sections in the Cardingt o n experiments with their respective heating curves in the standard fire test provided a series of experimentally derived time-equivalent values. Examination o f the data enabled an average value of Te to be determined for each fire test, as given in Table 2. These figures showed that, in the insulated compartment, wood fire load densities of 15 k g / m : and 20 k g / m : at ¼ ventilation were equivalent to 30 min and 1 h standard fires, respectively. Also, replacing the wall lining by a more conventional building material reduced the severities of b o t h fires to the respective timeequivalent values of approximately 20 min and 30 min. A number of relationships have been proposed for calculating the equivalent fire severity, Te. An early CIB programme proposed that: T~ = K - -

L

x/AvAt

(min)

(1)

where A t is the total bounding surface, Av is the ventilation area, L is the fuel load and K is a constant usually taken as unity [9]. A sec-

148 TABLE 2 Time-equivalent values for steelwork in Cardington fire tests using simultaneous ignition of cribs. Ventilation of one wall occurred with two types of compartment lining, refractory brick and plasterboard Experimental and theoretical time t~quivalent

Refractory brick

10 k g / m :

. . . .. . . . . . . .

.

I)l~L~t~.rboard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . W . o. o d. .+ .p(,lypropyi~'n~' . . . . . . . . . . . V,ood . . . fuel . [h.nsity

15 k g / m -~

Ventilation of

E x p e r i m e n t a l value Equation (1) Equation(2) Equation(3)

.

W o o d fuel d e n s i t y

one

l0

k~{.,'m2

l:) k~,,m:

15 k g ; m :

20

kg.lnl -"

wall

1,'2

1/-t

1/8

1/2

1/.t

1/8

1!2

Ii-1

I;.l

1/~

1 2

I.'.t

i. 1

I.'~

l'2

1.'1

9.5 ~.7 13..1 l l 2

17.1 12.3 19.1 16.2

23.7 17.3 27.3 22.~

17.2 13.2 20.0 16.7

29.7 1~.5 28.6 24.2

333 25.9 43.9 34.2

18.6 17 5 26.7 22.1

61.0 24.6 38.4 32..t

1~ I 12 3 19.1 16.2

2~.(; 17 :l 27.3 22.v.

20.t 13.2 200 167

;15 4 1~..-~ 286 2.1.2

I.~ 0 1~.1 1~7 lS~

22.! 25.4 27 3 265

1-,9 1",'.:~ 15.2 17 I

27 ; ?-1 l 21, ~ "[; 1

ond expression which takes account of the geometry of the opening, based on Swedish work is: Te = 0.067q[---~---1__

(min)

(2)

where A t is the total bounding surface including openings and q is the fire load density in MJ/m 2 of bounding surface [2]. Both eqns. (1) and (2) predict similar time equivalents for the different Cardington fire tests based on a 'standard' compartment. However, eqn. (2) can be modified to take account of the thermal properties of the enclosing surfaces by using theoretically derived conversion factors. For the compartment with a plasterboard lining on the walls, the conversion factor is only about 10% greater than the 'standard' enclosure and therefore the calculations using either equation are similar: eqn. (2) gives the better agreement with experimentally derived values of Te in the compartment having a refractory lining. A recent approach prepared by CIB W14 [10] based on DIN Standard 18230:1982

[111 is: Te = q , c w

20 kg/m:

(min)

(3)

where w is a dimensionless ventilation factor which allows for the profile of the opening, and c is a conversion factor related to the thermal properties of the boundary materials. However, in contrast to eqn. (2), this conversion factor takes one of three specific values depending upon the thermal inertia of the construction. The values of T~ derived from the three formulae are compared with the experimental

data in Table 2. Good agreement exists between theoretical and experimental fires inside the compartment lined with plasterboard. For example, the required value of c in the CIB W14 relationship, 0.07 min/(MJ/m:), compared favourably with 0.065 min/(MJ/m 2) as estimated from the tests. This information, together with an analysis of the BISF/FRS data, suggests that the behaviour of more severe fires in a compartment of similar thermal characteristics could be assessed by calculation. However, the behaviour inside the insulated compartment is not predicted with the same accuracy. Of the two relationships that take the properties of the enclosing surfaces into consideration, eqn. (2) is perhaps more accurate for severe fires but neither equation predicts the behaviour observed for a fire load density of 20 kg/m 2 and 1 ventilation. The required value of c becomes 0.09 min/ (MJ/m 2) whereas the average figure derived from experiments is 0.105 min/(MJ/m 2) with a maximum of 0.17 min/(MJ/m2). From a regulatory standpoint the timeequivalent expression in the CIB W14 document provides guidance on the fire-rating category of different types of compartment, i.e., '/~ h, 1 h, 2 h, etc. However, the use of only three gradation values for c is not sufficiently sensitive to enable fire engineering calculations to be performed in all likely situations; therefore, the approach given by eqn. (2) is preferred at the present time.

6. LIMITING SECTION FACTORS From the available experimental evidence, sufficient confidence can be placed on the

149 estimation of the time equivalent to enable the influence of other fires on steel temperature to be determined. A series of limiting Hv/A values for unprotected beams and columns has been derived for different fire severities in a plasterboard-lined c o m p a r t m e n t and are included in Tables 3(a) and 3(b). The data are based on experimental measurements and extrapolated to a low fuel-load density of 7.5 kg/m 2 and up to 60 kg/m 2 for woodburning and mixed-fuel fires, respectively. The limiting temperatures relate to simply supported universal beams, acting non-compositely and supporting reinforced concrete or composite steel deck floors and also members in compression uniformly heated on all faces with a slenderness ratio < 1 0 0 . Such a Table can be used in the following manner. Consider the proposed design of a twostorey junior school in which 203 mm × 203 mm × 46 kg/m universal columns 3 m in length support 406 mm × 178 mm × 54 kg/m universal beams over a span of 5 m. Each classroom is rectangular in plan with a concrete floor and ceiling, with the walls rendered in plaster and the potential total ventilation equivalent to half of the area of the longer wall. A compendium of European data suggests that a typical ' w o o d ' fire load density for a junior classroom is about 20 kg/m 2 (at 80% fractile [12]): when beams exposed to a three-sided fire attack are tested to BS476: Part 8 at m a x i m u m design load (BS449:1969) the temperature of the lower flange at failure (span/30) is approximately 630 °C. From Table 3(a) the limiting Hp/A value corresponding to this temperature is about 255 m- ~. The universal beam in the design has an Hp/A value of 191 m -~ and can therefore be left unprotected w i t h o u t risk of collapse in a fire. The corresponding limiting temperature of fully loaded free-standing columns in the BS476:Part 8 test is approximately 580 °C, which from Table 3(a) is equivalent to an Hp/A value of 185 m -~. Therefore, the 203 mm × 203 m m × 46 kg/m columns with an Hp/A value of 200 m ~ would require protection; this would not be necessary if the section weight were to be raised to 52 kg/m. The limiting temperature of any structural element is influenced by the applied stress; if the 46 kg/m column had been stressed to only 75% of the m a x i m u m allowed in (BS449: 1969) then it could be left untouched.

If this Table is to be used as a general guide it should be recognized t h a t the fire tests were based on simultaneous ignition which resulted in the most severe heating rate and that steel frameworks can exhibit a greater fire resistance than their individual steel elements. A similar table of information could be derived from the test data for highly insulating surfaces.

7. CONCLUSIONS The low fire loads and comparatively large ventilation conditions used in this investigation are typical of m a n y modern steel-framed buildings with compartments similar in size to the Cardington rig. Valuable and hitherto unavailable experimental data have been obtained on the behaviour of a variety of steel building elements in experimental natural fires of different severities as compared to idealized heating conditions. The factors that determine the rise in combustion gas temperature, such as the fire load density and reduction in the extent of ventilation, influenced the rise in temperature of the steelwork depending upon the Hp/A value of the section and its location in the compartment. Changes in the burning characteristics of wood fuel by the addition of plastic had only a small effect on the m a x i m u m average steelwork temperatures; the change from simultaneous ignition to a growing fire situation was beneficial. A change in the properties of the c o m p a r t m e n t lining had a significant influence on heating rate. If the behaviour of a real fire is to be related to an equivalent heating time in the BS476:Part 8 fire test, an allowance must be made for the thermal capacity of the compartment and the height of the ventilation openings. The experimental observations suggest that the CIB W14 relationship goes part way in satisfying this requirement but the fixed gradation values given to the conversion factor c are restrictive in certain fire situations. Apart from this reservation, the time-equivalent approach can now be used with greater accuracy to estimate the behaviour in more severe fires. The provision of design tables related to the temperature sensitivity of structural steel sections in a range of natural fire environments

9.0

Time equivalent (min)

(2) Four-sided attack

(1) Three-sided attack

500 550 600 650 700 750

500 550 600 650 700 750

(~C)

0.06

Opening factor

Limiting t e m p e r a t u r e

1/4

Ventilation o f one wall

11.0

0.03

1/8

7.5

9.5

0.12

1/2

10.0

> 300 > 300 > 300 > 300 >300 :'300

> 285 > 285 >285 >285 >285 >285 220 275 > 300 > 300 ~300 ::,300

278 > 285 >285 >285 >285 >285 290 :> 300 7 300 :. 300 >300 >300

> 285 > 285 >285 >285 >285 >285

Limiting Hp/A value

7.5

Fire load density ( k g / m 2)

168 195 225 265 >300 >300

196 241 >285 >285 .-~285 >285

13.4

0.06

1/4

10.0

Limiting section factor values (a) For wood fires burning in plasterboard-lined compartment

TABLE 3

96 118 144 182 220 -300

116 138 166 206 293 :>285

18.9

0.03

1/8

10.0

250 300 > 300 > 300 >300 >300

> 285 > 285 >285 >285 >285 >285

10.5

0.12

1/2

15.0

108 133 158 192 240 '300

126 150 181 232 >285 >285

18.0

0.06

1/4

] 5.0

80 96 113 140 80 245

91 109 130 158 215 >285

22.0

0.03

1/8

15.0

140 175 192 220 ::-300 >300

149 180 222 288 >285 >285

16.0

0.12

1/2

20.0

60 70 83 97 118 170

69 84 J00 121 154 275

27.0

0.06

1/4

20.0

53 60 71 83 102 138

60 63 78 94 118 180

30.0

0.03

1/8

20.0

84 101 120 150 190 265

98 115 138 169 232 >285

21.0

0.12

1/2

30.0

40 47 54 62 77 96

< 60 ~ 60 -~60 68 86 121

36.0

0.06

1/4

30.0

< 30 34 40 47 55 72

• 60 (-: 60 (~60 ~60 <60 80

44.0

0.03

1/8

30.0

30 36 43 48 60 77

< 60 < 60 460 ~60 64 88

42.0

0.12

1/2

60.0

---< 30 32 42

< 60 .~ 60 <60 ~60 <60 ,~60

72.0

0.06

1/4

60.0

9.9

Time equivalent (rain)

500 550 600 650 700 750

500 550 600 650 700 750

(1) Three-sided attack

(2) Four-sided attack

(°C)

0.06

Opening factor

Limiting t e m p e r a t u r e

1/4

V e n t i l a t i o n o f one wall

12.1

0.03

1/8

7.5

10.5

0.12

1/2

10.0

> > > > >

270 300 300 300 300 300

>244 >285 >285 >285 >285 >285 205 240 275 > 300 > 300 > 300

>285 >285 >285 >285 >285 >285

> > > >

250 300 300 300 300 300

>285 >285 >285 >285 >285 >285

Limiting Hp]A value

7.5

Fire load d e n s i t y ( k g / m 2)

146 180 204 249 > 300 > 300

175 212 268 >285 >285 >285

14.7

0.06

1/4

10.0

86 104 128 158 193 270

100 119 142 175 248 >285

20.8

0.03

1/8

10.0

19.8

0.06

1/4

15.0

> > > >

220 257 300 300 300 300

94 114 140 178 208 300

,?,44 109 >285 129 >285 154 >285 192 >285 274 >285 >285

11.5

0.12

1/2

15.0

(b) For wood + polypropylene (75:25) fires burning in plasterboard-lined compartment

71 83 94 118 144 210

79 93 111 134 175 >285

24.2

0.03

1/8

15.0

108 132 156 198 245 > 300

130 155 190 248 >285 >285

17.6

0.12

1/2

20.0

54 60 72 84 106 144

<60 66 80 96 121 181

29.5

0.06

1/4

20.0

46 52 60 72 88 114

<60 <60 66 81 102 148

33.0

0.03

118

20.0

76 90 107 132 170 228

85 101 120 146 195 >285

23.0

0.12

112

30.0

34 41 47 55 66 85

<60 460 <60 <60 72 101

39.6

0.06

114

30.0

< 30 < 30 35 40 49 60

460 460 <60 <60 <60 <66

48.4

0.03

118

30.0

< 30 31 38 43 52 67

460 <60 ~60 <60 <60 75

46.2

0.12

112

60.0

----> 30 > 30

<-60 460 460 <60 <60 <60

79.2

0.06

1/4

60.0

152

highlights those situations where unprotected beams and columns can be installed and is essential if the new concepts of fire-resistant design are to be developed. The current investigation is therefore an important contribution towards more effective analysis. An extension of the use of bare steelwork in buildings, with the added protection afforded by adjacent elements of structure, will have a significant impact in the reduction in overall costs of the multi-storey structure. ACKNOWLEDGEMENTS

The authors wish to thank Dr. R. Baker, Director of Research and Development, British Steel Corporation, for permission to publish this paper. Thanks are also expressed to Mr. G. M. E. Cooke of the Fire Research Station (BRE) for his cooperation and for helpful discussions during the course of this work and also to Mr. D. E. Wainman of Swinden Laboratories who assisted in the data analysis. The provision of funding from the British Steel Corporation and also from the Department of the Environment, Building Research Establishment, is gratefully acknowledged. LIST OF SYMBOLS

A At Av c

Hv

h k L q

area of cross-section of a steel member (m 2) area of bounding surfaces (m 2) area of vertical openings (m 2) conversion factor which accounts for the thermal properties of the fire enclosure (min/(MJ/m 2 )) heated perimeter of a steel member exposed to fire (m) height of vertical openings (m) constant, often taken as unity (kg/ sm 5,2 ) fire load in wood (kg) fire load density related to the total bounding surface (MJ/m 2)

qf

T~ w

fire load density related to the floor area (MJ/m 2 ) equivalent fire severity (min) dimensionless ventilation factor.

REFERENCES 1 European Recommendation for the Fire Safety o f Steel Structures -- European Convention for Constructural Steelwork, Committee T3 -- Fire Safety of Steel Structures, May 1981. 2 0 . Pettersson, S. E. Magnusson and J. Thor, Fire Engineering Design o f Steel Structures, Swedish Institute of Steel Construction, Stockholm, Sweden, 1976. 3 E. G. Butcher, T. B. Chitty and L. A. Ashton, The Temperature Attained by Steel in Building Fires, Fire Research Technical Paper No. 15, HMSO, 1966. 4 P. Arnault, H. Ehm and J. Kruppa, Rapport Resumd sur les Essais avec des Feux Naturels Ex& cut,s dans la Petite Installation de Maizi~res-lesMetz, Doc. CECM 3 - 73/18F, CTICM, November. 1973. 5 D. J. Latham, Practical fire engineering -- The use of unprotected steelwork in buildings, Proc. National Structural Steel Conference on New Developments in Steel Construction, London, December, 1984, The British Constructional Steelwork Assoc. Ltd., Part 1, P5/C/1 to 5/C/11. 6 P. H. Thomas, A. J. M. Heselden and M. Law, Fully Developed Compartment Fires .... Two Kinds o f Behaviour, Fire Research Technical Paper No. 18, HMSO, 1967. 7 W. D. Woolley, M. M. Raftery, S. A. Ames and A. I. Pitt, The Behaviour o f Stacking Chairs in Fire Tests, Paper CP10/79, BRE, October, 1979. 8 J. T. Robinson and D. J. Latham, Fire-resistant steel design -- The future challenge, Proc. International Conference Design o f Structures Against Fire, Aston University, Birmingham, April, 1986, Elsevier Applied Science Publishers, Barking, U.K., pp. 225 - 236. 9 M. Law, A Relationship Between Fire Grading and Building Design and Contents, Note No. 877. FRS, 1971. 10 Design Guide Structural Fire Safety Workshop, CIB W14, Fire Safety J., 10 (2) (1986). 11 DIN 18230:Part I, Structural Fire Protection in Industrial Buildings Analytically Required Fire Resistance Time, November, 1982. 12 S. Bryl, Brandbelastung in ltochbau, Schweizerische Bauzeitung, April 24, 1975, Reprint from 93, Jahrgang, Heft 17.