Ten rules of fire endurance rating

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Building Research Note (National Research Council of Canada. Division of Building Research); no. BRN-46, 1964-07-01

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC :

https://nrc-publications.canada.ca/eng/view/object/?id=50780cbf-1b03-425a-ba52-d7de5735e4a3 https://publications-cnrc.canada.ca/fra/voir/objet/?id=50780cbf-1b03-425a-ba52-d7de5735e4a3

NRC Publications Archive

Archives des publications du CNRC

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/40000662

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Ten rules of fire endurance rating

C A N A D A Ser TH1 B92 no.

46

c * 2 BrnG

ZCN-AIYf

ED'

TEN

RULES

OF

FIRE ENDURANCE RATING

HATIONAL RESEARCH COUPlClL

( I D I V I S I O N O F B U f L D I N Q R E S E A R C H * N A T I O N A L R E S E A R C H C O U N C I L

-

O T T A W A

-

C A N A D A

T E N RULES

OF

FIRE ENDURANCE RATING

It is becoming increasingly evident that all the information necessary f o r

classifying building elements from the point of view of their performance in fire

camnot be derived from standard f i r e endurance t e s t s alone. It i s hardly conceiv- able that a11 various constructions can ever be subjected t o Eire t e s t s , Even in

those cases where test results are available for m o r e or l e s s similar constructions,

the classification may not be i m m e d i i t e l y apparent. It must be clearly under stood

that c e r t a i n variations in the dimensions, loading conditions, materials, o r w o r k m a n -

ship may markedly affect the performance of the individual constructions, and

the extent of such a possible effect cannot be evaluated f r o m the fire t e s t report. The value of t h e p i e c e s of information obtained f r o m standard fire t e s t s

i s very limited without a theory that is capable of cementing the p i e c e s into a

consistent unit. Fortunately, the theory of f i r e endurance rating has already

advanced far enough not only to offer some guidance in _estimating

_the

_{effect of}

certain variables on fire endurance but, very often, even to provide r i g o r o u s

methods of designing building elements for s o m e prescribed performance.

Although the design f o r f i r e endurance, whenever possible, is a fairly

complex proceduxe, involving extensive laboratory investigations and s o m e t i m e s

very laborious heat flow and stress -deflection analyses, it will always prove of

extreme: value when developing new building products for maximum economy.

h

the case of products already on the market, the problem of e c o n o m y no longer

remains, and the question of whether the product will yield a f i r e endurance

higher than some given value may not d e s e r v e an extensive theoretical study.

In this r e p o r t a number of rules will be discussed which may prove

useful in the quick appraisal of th e f i r e endurance of building elements. Some

of the rules are concerned with the geometry of the construction, some with the

materials. All of them are based on experimentally and theoretically w e l l -

f omded facts. It is hoped that these r u l e s will provide some guidance to those charged with administering building bylaws, and to those who are engaged in

either the design o r production of building components.

In the second half of the r e p o r t , several examples are given to illustrate

the wide applicability of these rules.

Rule 1 : The "thermal" f i r e endurance of a construction consisting of a n u m b e r

characteristic of the individual l a y e r s when exposed separately to fire.

This

rule is probably the most important

aid in

the assessment of the performance of building elements in fire, and suggests that the result of fire

tests conducted on individual components of building elements (e.g. on gypsum

b o a r d , plywood, brick veneer, plaster on expanded metal lath, etc. ) with the

purpose of determining their "thermal" performance, may a l s o be of considerable

value.

It

is very convenient to use small-scale specimens f o r these %on- standard" informative t e s t s .

Because of the difficultie s involved in the analytical treatment of the

problem of heat flow through composite slabs, at present it is not possible t o

provide a rigorous proof of this rule. Nevertheless, its validity has been

c o n f i r m e d by the result of s e v e r a l numerical analyses and small-scale fire

t e s t s conducted on both combustible and non-combustible constructions, There is a special case ia which the validity of the above rule is

immediately obvious: the case of a "quasi -composite" construction which consists

of layers made from the s a m e material. It is well-known (and can be proved by

the method described in Ref. ( I ) t h a t by doubling the thickness of a slab, the

f i r e endurance becomes m o r e than twice the original value. Similarly, for a

slab of nA thickness (in other words, consisting of n identical layers of

thickness 1 ),

w h e r e t is the time of f i r e endurance and the subscripts denote the thickness of the slabs. This inequality is obviously an expression of Rule 1 f o r such a "quasi-

compo sitett construction,

It may be noted that there are a f e w cases jn. which the above rule may

not be applicable. If, f o r example, the unexposed surface is covered with a thin,

shiny, metal sheet, the heat transfer t o the surroundings may b e c o m e very Low.

S ~ h c e the insulating v a l u e of the sheet i s negligible, the fire endurance of the construction might be lower than it would be without the presence of the sheet,

It

is also conceivable that the inclusion of a layer which in itself

exhibits n o appreciable f i r e endurance, but is liable to undergo some chemical reaction accompanied by large exothermic effects at elevated temperatures,

will upset the validity of the rule. The s c are, however, examples not likely t o

be met in practice.

Rule 2: The f i r e endurance of a colnstruction d o e s not d e c r e a s e with the addition

of further layers.

validity also follows from the fact that, by the addition of further layers, both

the

resistance t o heat flow and the heat capacity of the construction increase, which,

i n t u r n , reduce the rate of temperature r i s e at the unexposed surface.

Although this rule s e e m s quite obvious, a rigorous examination will

reveal that it may be subject to certain limitations. Thus, in the l i g h t of what

has already been noted it is clear that the addition of a thin metallic layer to the

unexposed surface does reduce the f i r e endurance of the existing construction.

A l s o , since in this rule "fire endurancer' is considered in its most general

meaning, certain restrictions must be imposed on some properties of the materials to be added, and on the load conditions.

Am apparent r e s t r i c t i o n i s that the new layer, if applied to the exposed

surface, must not produce additional thermal s t r e s s e s in the construction, i, e.

its thermal expansion characteristics must be similar to those of the adjacent

layer.

It i s also obvious that the new layer must be capable of contributing to

the load-bearing capacity of the construction at least as much as is required to

carry the increased dead load.

If

this requirement i s not fulfilled, the allowable

live load must be reduced by an amount equal to the weight of the new layer.

Because of these limitations, this rule should n o t be applied without due

criticism*

Rule 3 ;

The

f i r e endurance of constructions containing continuous air gaps or

cavities is greater than the f i r e endurance of similar constructions of the same

weight, but containing n o air gaps or cavities

.

The validity of this rule rests on the fact; that by the insertion of voids,

additional resistances are produced in the path of heat flow, Numerical heat

flow analyses indicated that a 10 t o 15 per cent i n c r e a s e inf i r e endurance can

be achieved by creating an air gap at t h e midplane of a brick wall (2).

Since the g r o s s volume of constructions is also increased by the presence

of voids, the air gaps and cavities have a beneficial e f f e c t on the stability as

well.

Constructions containing combustible materials along an air gap may

be regarded as exceptions to this rule, because of the possible development

of burning in the gap.

Rule 4 : The farther am air gap o r cavity is located from the exposed surface,

the more beneficial is its effect on the fire endurance.

In the

heat

transfer through a n air gap or cavity, radiation is the

with the average level of temperature in the void, an a i r gap

or

cavity is

a

very

poor insulator i£ it is located in a region which attains'high temperatures during

f i r e exposure.

Rule 5 : The f i r e endurance of a construction cannot be increased by increasing

the thickmess of a completely enclosed air layer.

T h e r e is evidence ( 2 ) that

if

the thickness of the air layer is larger

than about 1/2 in., the heat transfer through the air layer depends o n l y on the

temperature of the bounding surfaces, but is practically independent of the

distance between them.

Rule 6 : L a y e r s of materials of l o w thermal conductivity a r e better utilized on

that side of the construction on which fire i s m o r e likely to happen.

The validity of this rule has been demonstrated (2). The rule may not

be applicable to materials undergoing physico-chemical changes accompanied

by significant heat absorption o r heat evolution.

R u l e 7 : The fire endurance of asymmetrical constructions depends on the

direction of heat flow.

This rule is a consequence of R u l e s 4 and 6, which point out the

importance of the location of air gaps o r cavities and of the sequence of

different layers of solids.

Rule 8 : T h e presence of moisture, if it does not result in explosive spalling, i n c r e a s e s the f i r e endu~ance.

The

flow of heat t h ~ o u g h the construction is greatly hindered by the

absorption of the heat associated with moisture desorption. It has been

shown (2,3) that the gain in fire endurance may b e as high as 8 per cent per

e v e r y per cent (by volume) of moisture

.

As the latent heat required f o r the desorption of moisture is roughly

p r o p o r t i o n a l to the amount of moisture present in a unit volume of the material,

there is no direct relationship between the relative humidity (in the p a e s ) of

the s o l i d and the increase in f i r e endurance.

As

pointed out by Shorter and Harmathy (41, and Harrnathy (31, materials

of low permeability (dense concretes) are liable to undergo explosive spalling

if the moisture content, i s higher than a critical value. The permeability of

mature portland cement pastes i s several orders of magnitude lower than that

Rule 9 : Load-supporting elements, such as beams, g i r d e r s , and joists, ~ i e l d

higher f i r e endurances when subjected to f i r e endurance t e s t s as parts o f floor,

roof or ceiling assemblies, than they would when t e s t e d separately.

Before being able t o prove the validity of this rule it is necessary to

define the meaning of a few terms. Figure l(a) is a diagrammatic picture of

a

common floor construction, From the point of view of their role in transferring the load to some vertical bearing construction (wall or c o l u m ) , the constituents

of this f l o o r can be divided into three groups. The upper layer m a y be t e r m e d

as the "load-receiving" element. This l a y e r is supported at short spans, t h e r e -

f o r e it is generally a light consfruction. The components of the second layer

will be called "load-transmitting" elements in this report. A s they are bridging somewhat l a r g e r spans, they usually contain some steel reinforcement. The

components that span t h e whole distance between two walls or columns will be

t e r m e d "load-supporting

''

elements, T h e s e are either made of steel ( t r u s s e s ,

girders, beams, joists), or a r e heavily reinforced with steel. Finally, the

walls or c o l u m n s to which all the load imposed on the floor is last transferred, will b e t e r m e d "load-bearinglt elements. It m a y be noted that very often a single layer plays the part of both the " l o a d - r e c e i v i n g ' k d "load-transmitting1' elements.

Figure Z(a) (which m a y b e called "load flow diagram1') shows how,

under the conditions contemplated by the design, the various elements contribute

t o transferring the load t o the ground.

During the f i r e t e s t the supporting elements are, as a rule, under the

most adverse temperature conditions, and since they are spanning the largest

distances, their deflection b e c o m e s significant at a s t a g e when the strength of

the transmitting a d receiving elements is hardly affected b y t h e heat. T h e s e

e l e m e ~ t s will, of course, follow the deflection af the supporting elements, but

as a result of theix e l a s t i c bending, an increasingly l a r g e r portion of the load

will be switched d i r e c t l y to the bearing elements (walls, colnmns). Thus the

load carried by the suppartkg elements will gradually decrease.

The variation of the load imposed on the supporting elements during

the f i r e t e s t is further discussed in Appendix A.

Figure 2(b) shows how the "load flow diagram" may appear towards the

end of the f i r e t e s t ,

When load-supporting elements are tested separately, the imposed load

is constant and equal to the design load throughout the t e s t , U n d e r such c i r c u m -

stances they cannot yield higher Eire endurance than they d o when t e s t e d as

parts of a floor, roof, or ceiling assembly.

Rule 10 : The load-supporting elements (beams, girders, joists, etc. ) of a

f l o o r , roof or ceiling assembly can be replaced by such other load-supporting

elements which, when t e s t e d separately, yielded fire endurances not l e s s than

The validity of this rule rests on Rule 9 . A beam or g i r d e r , i f capable of yielding a certain performance when t e s t e d separately, will obviously yield an

equally good or better performance when it forms a part of: a f l o o r , roof, or

ceiling assembly.

It must be emphasized that the supporting element of one assembly must

not be replaced by the supporting element of another assmbly if the performance

of this latter element is not known f r o m a separate (beam) t e s t . Because of the

load-reducing effect of the receiving and transmitting elements, from the result

of a t e s t performed cm

an

assembly, the performance af the supporting element

(beam, e t c . ) alone cannot be evaluated by simple arithmetic.

This

rule clearly indicates the advantage of performing f i r e t e s t s on

supporting elements separately and proves the validity of the concept that the

results are more widely applicable

if

smaller units rather than complex

assemblies are subjected to f i r e t e s t s .

F i g u r e 3 i s a diagrammatic illustration of t h e rules discussed in this

paper. Althoughthese r d e a do n o t eliminate the need for accurate and elaborate

d e s i g n methods, they m a y give some assistance in solving many problems

encountered in everyday practice. The following part of this report contains

a

number of examples that show how the rules can be applied t o practical

cases.

Example 1: A contractor would like to u s e a partition consisting of a 3 314-in-thick

layer of r e d clay brick, a 1 1/4-in. -thick layer of plywood and a 3 / 8 - i n . gypsum

wallboard, at a location where 2-hr f i r e endurance is required,

Question: Is t h i s assembly capable of providing 2-hr

protection?

Answer: Y e a . The f i r e endurance of a 3 314-in.-thickbrick

wall i s approximately 80 min,

h

small-scale experiments

the f i r e endurance of a 1 114-in. -thick plywood was found

t o be 30 min, and that of a 3/8-in. gypsum wallboard 1 3 rnin.

According to Rule I the f i r e endurance of the assembly must

be l a r g e r than 8 0

+

30

+

1 3 = 123 min.

Example 2: A snanufacturer of roof slabs would like to obtain experimental evidence

that his products - are capable of yielding - 2-hr f i r e endurance. According to a

rigorous interpretation of ASTM E 119, however, only roof assemblies, including

the roof slabs as well as the cover and the supporting elements, can be subjected

to f i r e t e s t s , and therefore a fire endurance classification cannot be issued f o r

the slabs separately. The manufacturer thinks, however, that his slabs will

yield 2-hr fire endurance even without the cover, and any beam of at least Z -hr

prescribing a particular cover and supporting system to be used with the slabs,

the sale of his product will suffer.

Que stian: Is it possible f o r the manufacturer to obtain

classification for the slabs separately?

Answer: Yes. According ta Rule 10 it is not c o n t r a r y to

c o m m o n sense ta t e s t and classify roofs and supporting

elements separately. Furthermare, according to Rule 2 ,

if the roof slabs actually yield a 2-hr f i r e endurance, the

endurance of an assembly including the. slaba cannot be

less than 2 hr either. The recommended procedure is to

assemble the roof slabs on any convenient supporting

system (which, however, shall not be r e g a r d e d as part of the specimen) and to subject them to a load which,

besides the usually required superimposed h a d ,

includes some allowance for the weight of the cover.

Example 3: The f i r e endurance of a floor construlction containing simple steel floor units was found to be 2 hr 40 m k .

Question: Is there any hope that by replacing the simple f l o o r

units by cellular units the fire endurance can be

increased to 3 hr?

Answer: According to Rule 3 this replacement willfavourably

affect the perforrnamce of the constructi'on, but there is n o simple way of estimating the actual gain in f i r e

endurance.

A

steel-joisted floor and ceiling assembly is known to have yielded

a f i r e endurance of

I

hr 35 min. At a certain location 2-hr endurmce is required.

Question: What is the most economical way of increasing its

fire endurance by at least 25 min?

Answer: A thorough examination of the drawings would be

necessary. Slightly increasing the thiclmess of the

ceiling plaster is always very effective. In this way

there will be a twofold gain

in

fire endurance:

(i) a gain due to the greater thiclaess of plaster, and

(ii) a gain due to shifting the air gap farther from the

The fire endurance of a particular brick cavity wall consisting of

-thick

layers separated by a 2-in. air gap, is 4 hr 40 rnin.

Question: C a n the f i r e endurance of the

wall

be extended to 5 hr by increasing the thickness of the air gap to 4 in?

Answer: No, by virtue of Rule 5 .

Example 6:

In

o r d e r to increase the insulating value of its precast roof slabs a

company - has decided to make the slabs f r o m two layers of different c o n c r e t e s .

T h e lower half of the slabs where the strength of the concrete is immaterial

(since all the tensile load is c a r r i e d by the steel reinforcement), i s n o w made

from

a

concrete

of

low strength but good insulating value. F o r the upper layer,

where the concrete is supposed to carry the compressive load, the original high strength, high thermal conductivity concrete has been retained.

Question: H o w will the fire endurance of the slabs be

affected by the change?

Answer: The effect on the t h e r m a l fire endurance is

beneficial f o r two reasons:

(i) the total resistance to heat flow of the new slabs

has

been increased due to the replacement

of

a

layer of high thermal conductivity by one of

low conductivity, and

(ii) the layer of low conductivity is on the s i d e to be exposed to f i r e , w h e r e it is m o r e effectively

utilized according t o Rule

6 .

The change is a l s o

beneficial f o r the resistance of the slabs to

collapse. The layer of 40w thermal conductivity

provides a better protection f o r the steel

~ e i n f o r c e m e n t , therefore the time of attaining the temperature at which the creep of steel b e c o m e s significant will be extended.

-7: The f i r e endurance of

an

exterior wall consisting of a 3 3/4-in.

l a y e r of brick and a 3 3/4-in. l a y e r of sandstone is known. The wall was tested

with the brick layer exposed to the fire.

Question: Can the r e s u l t be applied to the c a s e when tbe sandstone i s to be exposed to f i r e ?

A n s w e r ; No, by virtue of Rule 7. From Rule

6

it is

evident that the construction will yield a lower

fire endurance when t e s t e d from the direction of sandstone. (The thermal conductivity of

sandstone is higher than that of the brick. )

Example 8 : A floor construction with concrete on s t e e l f l o o r units w a s t e s t e d in

1956 and was found to give 3 h r 18 min f i r e endurance, The t e s t report reveals

t h a t the age of the specimen w a s 35 days on the d a y of the t e s t . T h e r e is no

information available concerning the moisture content of the concrete slab.

Question: Would the f i r e endurance of this construction be

higher o r lower if the t e s t w e r e repeated with the

observance of the 1958 revision of ASTM E119?

Answer: Probably it would be Lower. According to ASTM

E

1

19-

58, the construction shall not be t e s t e d until

the dampest section of the assembly attains a

7 0 per cent relative humidity.

It

is the experience

af this laboratory that this -humidity level cannot

be attained in 35 days.

It

i s also known ( 5 ) that the sorption curve of concretes is very steep in

the 80 to 100 per cent relative humidity range; in

other words, above 80 per cent a small change in

relative humidity m a y mean a significant difference

in the amount of adsorbed moisture, and thus

,

in

accordance with Rule 8, a significant d i f f e r e n c e

in the fire endurance.

Example 9 :

In

a t e s t of a floor assembly the deck failed thermally at 2

hr

21 min.

The submitter asked the testing authorities to continue the t e s t to see whether the

beam would be capable of sustaining the applied load f o r a 3

-Br

period. The construction collapsed at 3 hr 1 5 min.

Question: Can the beam be qualified as one of 3-hr f i r e endurance?

Answer:

No.

According t o Rule 9 the beam was not

t e s t e d under the most adverse conditions.

h

is possible that in combination with another deck,

which exhibits less rigidity

and

contributes in a

l e s s e r d e g r e e t o supporting the load, the same

beam will yield a considerably poorer f i r e endurance. It is obvious from Rule 10 that if a beam i s intended to be used in conjunction

with a variety of floor or roof constructions,

References

I , Harmathy,

T.

2 . Temperature distrib~tion in homogeneous slabs during

f i r e t e s t . Transactions,

Eng.

& s t . of Canada, Vol.

6 ,

No.

B-6,

O c t . 1963. (Paper No. EIC-63-Mech

6).

2. Harmathy,

T.

2. A Treatise on theoretical fire endurance rating. ASTM

Special Technical Publication

No.

301, 1961, p. 10.

3, Harmathy,

T.

2, Effect of moisture on the fire endurance of building

elements. Paper presented at the

67th

Annual Meeting of ASTM,

21 -26 June, 1964, Chicago,

4, Shorter, G . W.

,

and T. Z. Harmathy. Discussion of The f i r e resistance

of prestressed concrete beams by L . A . Ashton and S .

C.

C.

Bate,

Proc., Inst. Civil Engrs., Vol. 20, No. 313, 1941.

5. Menzel, C . A . Fallacies in the current per cent of t o t a l absorption

method for determining and limiting the moisture content of

concrete block. Bulletin 84, R e s e a r c h Department, Portland -

Cement Association, Chicago, 1 9 5 7 .

6

Parcel,

J.I.,

and R.B.B. Moorman, Analysis of statica1ly:indeter~inate

w

Ground

v

Ground

FIGURE

2

" L O A D

FLOW

"

D I A G R A M S

a )

BEFORE FIRE TEST

b3

NEAR THE EN0 OF FIRE TEST R E = RECEIVING ELEMENT, T E = TRANSMITT IN6 ELEMENT,

SE

= SUPPORTlNG ELEMENT, B E =

APPENDIX A

DECREASE

OF

LOAD ON SUPPORTING ELEMENT

DUE T O

DEFLECTION

Figure 1(b) shows a section of a common f l o o r construction under normal

service conditions.

I£

the dimension of the floor perpendicular to the section is

considerably larger than

24

the reactions A and l3 (in lb/in., at same distance

from the ends in the direction perpendicular to the section) are h o w to be

where p = total load, lb/sq in.

,

= half of the distance between the side walls,

in in.

These equations indicate that 62. 5 p e r cent of the total load ( 2 p a ) is

c a r r i e d by the beam and 1 8 - 7 5 per cent by each of the side walls.

Figure 1(c) shows a section of the floor at some not t o o advanced stage

of the fire test. At this s t a g e the deflection of

the

beam, y, m a y be partly

e l a s t i c , partly plastic but, f o r the sake of simplicity, the deflection of the deck

is assumed to b e purely elastic. B y means of the t h e o r e m of three moments (see, e. g . , (Ref. 6 ) ) it can be shown that with a deflection y the reactions are as follows:

4

w h e r e

I

= moment

of

inertia of the deck, in. /in., and E = modulus of elasticity

f o r the material of the deck, l b / a q in.

It is seen that part of the load formerly supported by the beam is now

t r a n s f e r r e d directly to the s i d e walls, and the transfer of load is proportional