846

THERMAL BOWING OF A MODEL BRICKWORK PANEL

T J ShieIds, D J O'Connor, G W H Silcock, H A Donegan University of UIster, Jordanstown, Co Antrim BT37 OQB

ABSTRACT

The results of a fire test carried out on a model brickwork panel are presented and these highlight some of the different modes of thermal bowing possible in such a structural system. An explanation of the recorded behaviour is postulated reIating to exising theory. This problem is recognised to be complex and requires further investigation.

INTRODUCTION

The investigation of the thermo-physical behaviour of brickwork panels in

fire situations is not entirely new. In a fire, brick partition walls

behave as barriers retarding the spread of fire beyond the room of origino

As a resuIt they are subjected to high thermal gradients, retaining

temperatures that can be in excess of lOOO°C. Walls can be elements of

structure, either supported by a variety of mechanisms, or be designed as

free standing. AIternatively, load bearing brickwork exists where the

brick panel while being restrained by the structure also forms a part of

the structure, and as such can carry a considerable load.

The combination of brick and mortar forms a non homogeneous compos­

ite, which as a result introduces additional complications into the

analy sis of the thermo-structural behaviour.

There are obvious difficulties of size and cost in repeated testing

of large, full scale walls. A possible answer may lie in the scaling down

of the test environment. This is the viewpoint the authors have taken in

preparing this paper, which describes the initial findings of such a test,

and presents the results of a preIiminary investigation into the fire

testing of a model brick wall.

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BACKGROUND

There is a lack of data regarding the properties and behav iour of brick­

work at elevated temperatures. Current codes (1) concentrate on the

strength and dimensional instability of units due to moisture movement and

shrinkage . This is similar for alI types of brickwork including the

calcium silicate type as used in the model brick panel discussed herein.

However, the masonry design code BS 5628 (2) comments on both thermal

movement and shrinkage and quotes typical coefficients of linear

expansion.

Current findings regarding research into the performance of building

components under thermal stresses due to fire are detailed by Cooke(4,5),

who investigated the thermal bowing of structural steel and also several

generic fire protecting materiaIs. Deflection data were presented in

relation to temperature and fire duration, also allowing some comparison

of response to be made. Additionally, two full scale fire tests were

described relating to brick wall strips, in the first instance, and pre­

stressed concrete floor units in the other. This documented response data

provides suitable comparison for the results presented in this paper.

THEORETICAL BEHAVIOUR

Thermal Movement

Specific responses relating to thermal bowing are illustrated in Fig.1 .

Free bowing occurs when a structural member is subjected to a temperature

gradient 6T = (T 2 - TI) across its thickness d, a linear distribution pro­

moting constan t curvature. The maximum deflection for a particular

situation depends on boundary conditions. The midspan deflection of a

member with pinned ends is:

aL6T 3d (.1)

Whilst a cantilever member with fixed feet will incur a maximum

deflection at its end, given by:

6 c

aL6T 2d

(2)

The above equations are also relatively accurate for non linear

temperature distributions if the average temperature gradient is used in

place of the expression 6T/d .

[~ -]

L L ~p -

o(i:T 1

848

~( = o( L ~T

2d

(i) Pinned (ii) Contllever

(o) Free bowing

I ~,=

-frLfa(T

(b) Fixed bowing

Figure 1. Thermal bowing displacements for different end conditions.

Restrained bowing occurs when a structural member is prevented from

axial extension and the component buckles under the thermal end restraint

forces. The enforced central bow relates to the thermal extension of the

neutral axis and is dependent on the average cross-sectional temperature

T = (T z + T 1)/2. The midspan displacement may be evaluated from the

change in geometry to give the relationship:

L'lF ~ L raT 11

This equation assumes a sinousoidal deflected shape and can be

compared with the equivalent equation due to Cooke (4):

L'lF L 10.375aT

(3)

(4)

Both free and restrained bowing may occur along the same length of

elemento In such cases, superposition does not take place and repsonses

are mutually independent. Whichever deflection is greatest (eqns.l or 3)

is indicative of the dominant mode, that completely governs response. It

is more likely that the restrained bow is greater than the free bow and as

this behaviour is associated with buckling it is also probable that

stresses in excess of yield will be generated, causing permanent defor­

mation. As the free thermal response is related to the thermo-elastic

behaviour this type of response is reversible.

Thermal and Physical Properties

It has been shown experimentally elsewhere (6) that the thermal properties

of masonry type materials vary with temperature in a nonlinear manner. In

this investigation of the thermal bowing of a brick panel quantities such

as the thermal diffusivity and linear expansion coefficient are important.

Typical varia tions in these quantities are shown in Fig.2.

--

-

;:: 1'0 '> Vi :;) u. u. Ci

;i 0'5 ::[ a: w I ....

100

849

200 300

TEMPERATURE O(

400

i:: '" Z W ...J

O'4~ z

º a: O u. O

0 '2 ;-' z O Vi z

~ w

500

Figure 2. Variation of diffusivity and expansion of masonry type materiaIs with temperature.

Thermal diffusivity(which is defined as Ó = À/pC where À = the p

thermal conductivity and C p

the specific heat capacity, are func tions

of temperature) is important in the determination of the transient

behaviour of the thermal gradien ts and average temperatures whichin turn

control the expansion and bowing of the panel.

Dimensional Analysis

The flexural behaviour of a panel under the influence of a thermal stress

may be stated in a generalised manner:

(5)

in t erms of the following quantities:

6n

bowing deflection or extension

a coefficient of linear expansion

L characteristic leng th or height

d thickness of panel or strip

68 temperature difference

Using dimensional analysis it can be shown that:

(6)

where 1T) (aL68/d) are the relevant dimensionless

groups . lf the necessary condition (1T2) = (1T2) , where m and p refer m p

to the model and prototype respectively, is satisfied then:

(7)

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For this necessary condition, the following equalities must be simul ­

taneously satisfied :

a = a m p "'0 m "'0 p

S S m p

where S d/L is the slenderness ratio of the panel. In this case:

d /L d /L 1/20 m m p p

and L /L p m

d /d 2 p m

When alI these condi tions are sa t isf i ed the deflection of a 2m wide

prototype panel under the same thermal regime as the 1m wide model can be

predicted using the equation:

/:,n p (L /L ) "'n p m m

LABORATORY MODEL

(9)

The te st was set up as shown schematically in Figs.3 a,b . Themodel wall

was built t o half scale (10Smm x SOmm x 30mm calcium silicate bricks),

with appropriate cement mortar. A panel 1m square, one brick thick was

chosen to maintain the same slenderness ratio as an equivalent 2m high

prototype panel .

J~o~mtl (c)

Position of Thermocouples

9BO A

850! B

650 C ~ retOll (C)

.§\ 'f FURNACE '

... A} I Thermo ... B Olai I I -couples@l') Gouges

I -+ C I I I I I I I I

I o

LJt:f ~ , -I Extent of furnoce behlnd

(o) Elevot 'l on (b) Sect ion

Figure 3. Schema tic layout of testo

The model wall wasbuilt on a firm steel base and encased wi thin a

light steel frame, composed of SOmm x 6mm steel pIate at the sides and

top, the frame being bolted to the base by means of angle cleats. There

was a large gap of approximately 40mm at the top but only a slight gap of

I 1

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less than 2mm at the sides. Initially, the frame could be displaced in

the direction normal to the wall, indicating no initial lateral restraint

from the side ver ticals.

The wall was then placed as close as possible to a one cubic metre

lightweight furnace . All gaps between the wall and furnace were filled

with a ceramic insulation material. The furnace wasenergised using a

propane gas flame, the temperature of which was controlled in order t o

follow the BS 476 Part 8 temperatu r e time curve. The furnace fire pro­

file can be compared with th e BS 476 test in Fig.4.

950

900

850

800

750

700

650

600

550

~ 500 ·C

450

400

350

300

/í /

/ I /

G) I I @

I I

250 I 200 I 150 I 100

26m ~ ffi n ~ ~ ~ m ~ " ~

t/min-

Figure 4. Temperature-time variations of furnace and interior of panel.

Temperatures at various locations throu gh the thickness of the wall

and displacements at various locations were monitored throughout the

duration of the testo The location of instrumentation is as shown in

Fig.3.

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Three displacement transduce r s were fixed at centre span a t varying

heights within the top half of the panel. Gauges A and C were of the

normal dial t ype, read and recorded manually, the central gauge was a

linear vol t age displacement transducer (LDVT), connected to an X-Y

plotter.

Type K thermocouples were utilised t o record the t emperature through­

out the wall thickness, within the same central panel region. Two were

positioned on the wall surfaces, TS to record the temperature inside the

furnace while T1 displayed the t emperature on the externai face. The

other thermcouples, T2, T3 and T4, were embedded within the wall at

quarter thickness points (Fig.3c).

Thermocouple T4 was connected direc tly t o the X-Y plotter and its

temperature recorded against the displacement of the LVDT gauge B,

enabling a con t inuous monitoring throughout the testo The other displace­

ments (gauges A and C) were recorded manually against the temperature of

thermocouple T4. The externai thermocouple T1 was used to monitor the

externai unexposed surface tempera ture . Thermocouples T2, T3 and TS were

connected to a time base chart recorder and provided a comparative assess­

ment of the generation of thermal gradients through the wall thickness

with time.

DISCUSSION OF RESULTS

Thermal Bowing

The response of displacement at the three gauge positions in relation to

the assigned control temperature T4 is presented in Fig.S. Ali curves

exhibit two distinct zones. The initial response a-b is somewhat mixed,

ranging from the top displacement gauge A recording movement away from

the fire source to the bottom gaug e C identifying displacement towards

the source in linear fasion. The second part of the curve b-d shows a

linear response of displacement towards the source, with ali gauges

presenting qui te similar gradients. On removal of the fire source, d-e

the continuous graphical output from gauge B shows some reversal of move­

ment from a maximum displacement of approximately 38mm with reducing

temperature, seemingly cur tailing to a permanent se t, of the order of 27mm.

'"

---

853

8oo r-__ ,-__________________________________ -,

600 U o

~ :J "ê [,00

'" Q.

E

'" f- 200

c

b

~ d

c,cV

c c

,c (00"

e

20 L---~~----~,0~------~:~O--------f.30~------~4·0

Displocement (mm)

Figure 5. Thermal bowing in relation to wall temperature T4.

A further appreciation of behaviour may be ob tained b y relating the

displacement t o tempera ture gradient 6T . To thi s end, Fig.6 shows a

graph of temperature gradient - average values assessed fcom the temper­

ature profiles of Fig.? - plotted against the displacement of gauge B.

Zones of response arise which can be usefully compared with the previous

graph . After the initial transit ion zone a-b, there is a distinct

portion of graph which indicates displacement at nearly constant temper­

ature gradient b-c, proceeded b y a further portion c-d of almost linear

response .

600 r-------------------------------~~, G 0~d ~500 0 ~~-o c 0Y - . S--n~.a.Y . ~l.OO c/

~ / 6300

1:' I .2200

~ $ ~'00 I ~ i f- o

°0~------~,O--------~20~------~3~O------~40

Displocement (mm)

Figure 6. The rmal bowing in relation to tempera ture gradient.

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A possible explanation for this behaviour is outlined as follows. It

is thought that the primary mode of behaviour is the restrained bow case,

identified by the almost linear displacement response against a single

temperature source (Fig.S), and reinforced by the presence of a period of

test where considerable deflection was still occurring at constant temper­

ature gradient (Fig.6). Displacement responses of the gauges at differing

heights aIs o indicate similar rates Df change with temperature although

some difference might be expected due to free bowing within the height of

the panel. It is assumed that the panel was never completely unrestrained

in width and the initial portions of response curves a -b are indicative of

the panel expanding horizontally and quickly maintaining compatibility

with a fixed end condition. Notably, the initial reverse displacement of

the top gauge A cannot be attributed to free cantilever bowing as the

magnitudes of displacement are not large enough. Such initial reverse

effects were recordedbyCooke (4) for other materiaIs and there is a

slight possibility that these may be rela ted to initial drying shrinkage.

1000

900

BOO

700

600

G 500

400

)00

:00

100

°5~1----7---~~--~--~ ---------SOmm----__ __

Figure 7. Variation Df thermal gradients with time.

Using the typically quoted va lue for the coefficient of linear expan­

sion of calcium silicate bricks as 11 x lO-6/ oC,the calculated restrained

bow of this panel horizontally, using the maximum recorded average temper­

ature of S7SoC is SO.6mm. The projection oJ the reIevant graph of gauge B

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(Fig.S) on to the X axis, gives a projected displacement of S2mm. Similar­

ly, the calcula t ed free bow for t he same section in relation to th e maxi­

mum recorded tempe rature differential of S60°C over th e SOmm t hickness

equa tes to lS .4mm. The r ecorded reverse movement of gauge B in Fig.S was

11.2mm.

Thermal Gradient s

As shown in Fig.? the transient thermal response corresponds quite well t o

that expec t ed fo r conduction within bulk ma ter ia l of a similar t ype t o

tha t of the panel . Some ir re gu l ari t ies exist between the theoretical

estima tions and ac tual measured value s . This is due t o the difficulty of

the inclusion in the thermal ana l ys is o f ac tual boundary conditions and

possibly also t o varying thermal proper tie s . It is suggested t ha t if the

a ctual furnace temperature had been maintained a t lOOO °C for a longer

period a steady sta t e condi t ion would have occu rred wi thin the thickness

of the panel.

CONCLUSIONS

1 . The observa tions of the beh aviour of the mo del panel during this pre­

liminary investigation seems to ident ify two fu ndame ntal components

of bowing, namely restrained a nd unrestrained.

2. These hav e been shown both by analysis and by experimen t a t ion t o act

independently of each other.

3 . Res trained bowing promotes permanent d eforma t ion, whils t unrestrained

bowing seems t o be r eversib l e.

4 . It is difficult to forsee with any certainty, a t this point in time,

wha t type of bow i ng is likely t o oc cur in a practical si tua tion as a

consequ e nc e of a severe enclosur e f ire .

S . Th e utilisation of model ana l ys i s indicates tha t it may be possible

to predict the flexural behaviour o f f ull scale walls on the basis

of model te sts .

RE COMMENDATIONS

1 . Further investigations should be initiated t o progress these prelim­

inary findings.

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2. The behaviour of mode1 pane1s with differing edge constraints shou1d

be further investigated .

3. The performance of wa11s having different sca1ing factors shou1d be

assessed.

4. The detrimenta1 effects of therma1 bowing regarding 10ss of inte­

grity, particu1ar1y at boundaries of different structura1 e1ements

shou1d be studied.

5. The inc1usion of the effects of therma1 bowing in structura1 codes

shou1d be considered.

REFERENCES

British Standards Institution, Ca1cium si1icate (sand1ime, f1int1ime) bricks. British Standards Institution, BS 187, 1978.

2. British Standards Institution, Use of masonry. British Standards Institution, BS 5628, Part 3, 1985.

3 . British Standards Institution, Fire tests on bui1ding materia1s and structures : tes t methods and criteria for the fire resistance of e1ements of bui1ding construction. British Standards Institution, BS 476: Part 8, 1972.

4. Cooke, G.M.E., Fire engineering of ta11 fire separating wa11s-part 1. Fire Surveyor, June 1987, pp.13-28.

5. Cooke, G.M.E., Fire engineering of ta11 fire separating wa11s-part 2. Fire Surveyor, Aug. 1987, pp.19-29.

6. Shie1ds, T.J. and Si1cock, G.W . H., Bui1dings and Fire, Longmans, London, 1987.