Thermal restraint and structural steel assemblies in fire

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Thermal restraint and structural steel assemblies in fire

I r ?mnI

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no.

191

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LUILDING

RESEARCH

NOTE

THERMAL _{RESTRAINT AWD} STRUCTLTRAL STEEL ASSEMBLIES IN FIRE

K. Bardefl

TEIERMAL _{RESTRAINT AND STRUCTURAL STEEL ASSEMBLIES IN FIRE}

INTRODUCTION

The behaviour of a steel assembly Ln f i r e can be signi.ficantly a£ fected by the details of i t s structural support, in p a r t i c u l a r by

restraint. This fact w a s recognized in the 1950's, when the concept of a

difference in fire resistance ratings for restrained and unrestrained

floor

of

assembly, the desfgner can decide whether he should a i m at acquiring a

ftre resistance rating for the _{assembly with or without restraint.}

This Mote describes the mechanism of thermal restraint and

explains the d i f f errence between f i r e resistance ratf ngs for restrained

and unrestrained assemblies.

Definition of Thermal Restraint

A thermally restrained condition, in the context of f i r e

resistance, may be defined as one in which expansion or rotation at the supports of a load-carryiw element or assembly, resulting from t h e

effects of f i r e e.xposure, is resisted by forces or moments external t o

the element or assembly.

When structural steel assemblies are exposed to Eire, the steel members undergo axf a1 expansion. (The cocf f i c i e n t of linear expansion of

steel is about I x 1 6 cm/cm/"G+)

The effect that restraint of t h i s expansion has on the behaviour

of the assembly in f i r e depends on I t s configuration.

Beams

Restrained expansion of a simple beam w i l l create mantents at the

supports which

will

(pinned or f i x e d ends)

,

will lead to P-& effects; in other words they w i l l generate additional

mamnts in the ember. Factors that reduce lateral load deflections, euch ae structural continuity over supports, will reduce these F A

effects.

Isolated beam evenly heated under r i g i d restraint (as is common

t h e restraiuing frame has e l a s t i c i t y , as i s often the case in a real

structure, axial forces will be reduced a d di~tributed t o the rest of the structural frame.

Floor Assemblies

If a steel beam is part o f a concrete s l a b steel deck floor

assembly, heating of the beam and f ts expansion will n ~ t be uniform

across the cross-section (Figure 2). In this case, the sense of moments

generated by t h e m 1 restraint is opposite t o those generated by lateral loads; i.e., restraint, as in the case of a simple beam, is beneficial.

If the floor assembly is composite, or i f s o w composite action

occurs because of friction between the beam and deck, restraint-induced moments w i l l be distributed to the floor s l a b , The reductkon in lateral load beam d e f l e c t i o n s d u e to c o q o s i t e action also results in reduction of P-A mmnts. However, the beneficial d f e c t s of composite action i n fire continue only until beam deflectisas b e e m large enough t o sever the shear connection between the beam and slab.

Columns

As indicated in F i g u r e 3, axial forces due to thermal restraint increase the loads already a p p l i e d to columns and are not beneficial. In

certain situations however, thermal expansion may creaFe end fixity,

which leads to overall improvemnt in the r i g i d i t y of the steel fie.

In addition, if thermal exposure is uneven, moments may result which

counteract those generated by eccentricity in axial load,

In an analysfs of

be considered.

restrained beam fire t e s t where a perfectly r i g i d restraining frame

permits no expansion. Real structural behadour is between these t w o and

can be influenced by the location of the fire in relation to the assembly or hm mch of it is directly exposed t o the fire. The overall stiffness

o f the steel frame, t h e location of the assembly in the frame, the t y p e

of end and slab cannectf ons and t h e behaviour of other parts of the fire-

exposed structure, such as shrinkage and cracking in the concrete s l a b s , also have a bearing on the behaviour of the structure.

Computerfzed analytical techniques have been developed t o predict the degree of restratnt provided by a structure add the response of an

assembly i n a real f ire1. Hawever, until these methods gafn general

acceptance, designers mu st rely oa engineertag judgment and guidance

offered in fire t e s t standards (far example ULC 510l "Fire Endurance

a restrained rating is applicable. Assemblies must then be f i r e tested

according t o that decision.

RIESTRAIRT IN FIRE TESTS

In a Eire t e s t , restraint is provided to an assembly by a rigid restraining f ram.

Isolated beams can be fire t e s t e d with load a d restraint. To o b t a i n an unrestrained-assembly rating from t h i s test, a steel

temperature criterion is applied, the so-called "critical temperature" of

538°C. Research has shawn that the strength of loaded s t e e l members is

reduced to design strength at approximately t h i s temperature. To obt-ain a restrained assembly rating, structural perf o m n c e c r i t e r i a are

applied. (Collapse or rapid rate of deflection, and large values of deflection are the criteria.) However, because test restraint i s not

necessarily duplicated in the f i e l d , as an added degree of s a f e t y a m t n h m fire protection thidcness is required on restrained beams,

irrespective of their structural behaviour. This requirement is enforced by a less s t r i n g e n t temperature criterion for t h e s t e e l members.

Beams can also be tested without load to o b t a i n an unrestrained r a t i n g where c r i t i c a l temperature is the s o l e criterion.

Floor systems consisting of steel beams and floor deck can be t e s t e d as a unit. For f l o o r a y e t m , unlike for beams, there is another performance criterion: in a fire test they must a c t as a barrier to

prevent the spread of f i r e * Restraint does n o t d i r e c t l y affect

performance according t o t h i s criterion. Both restrained and

unrestrained asaembly ratings can be dertved from a t e s t on a restrained assembly. The restrained assembly rating is based on structural

performance wf th m i n i r m m cover for the s t e e l beams s p e c i f i e d , 'L'he

unrestrained assembly rating is determined by the attainment of c r i t i c a l steel temperatures. The steel b e a m in the assembly can receive an

independent unrestrained beam rating based 04 temperatures from the

assembly t e s t . Floors can be tested unxestrained, in which case collapse or c r i t i c a l temperature are the criteria.

Colums, like beam, are ftre tested t o ensure only that they

maintain their structural function (i.e,, not as f i r e barriers). Steel

columns are generally t e s t e d without load and critical temperature is the performance criterion. They may be t e s t e d under load, in which case the

structural performance criteria are applied.

References

1, Wise, Janney, Elstaer and Assoctates, Effect of F i r e Exposure on

S t e e l Frame BuildXtqp

,

2. Standard Hethods of Fire Endurance Tests of Bufldirrg C~nstmction and

Ma

.

BIBLIOGRAPHY ON RESTRAINT

1. Ashton,

L.A.,

Bauverlag GmbH, Wiesbaden, 1966, pp. 20-25.

2. Bletzacker, R.W., "Effect of Structural Restraint on the F i r e

Resistance of Protected Steel B e a m and Floor and Roof Assemblies",

Ohio State UQiversity, F i n d Report, EES 246/2666 of Building

Research Laboratory, 2966.

3. Bletzacker, Raw., "Fire Resistance af Prot~cted Steel Beam Floor and

Roof Assemblies as Aff eeted by Structural Restratnt"

,

Society for Testibg and Haterials Spactai Technscal Publication 422,

1967.

4, Ratmathy, T. 2. arid T.T. Lie:, "Fire Test Standard in the Light of Fire

Researchw, American Society for Testing and Materials S p e c i a l

Technical Publication 4 6 4 , 1970.

5. Lie, T.T., "Fire a d Buildings", Applied Science Publishers, England, 1972, p, _47.

6 . Pearce, N.S. and W.W. Stauzak, "Load and Fire Test Data on S t e e l Supported Floor Assemblies"

,

Materials S p e c i a l Technical Publication 422, 1967.

7. Selvaggho, S.L. and C.C. Caslson, "Effect of Restraint on F i r e

Resistance

of

344,

8, Selvaggio, S.L. and

C.Cc

SIMPLE BEAM

MOMENT Dl AG RAM IN NORMAL SERVICE

MOMENT DIAGRAM

P COLUMN FIGURE 3 MOMENT DIAGRAMS

lN

EM