NATIONAL BUREAU OF STANDARDS REPORT

6105

PROPOSED CRITERION FOR DEFINING

LOAD FAILURE OF BEAMS, FLOORS, AND ROOF

CONSTRUCTIONS DURING FIRE TEST

by

A. F. Robertson

J. V. Ryan

<0>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT

1002-12-1029 August 1, 1958

NBS REPORT

6105

PROPOSED CRITERION FOR DEFINING

LOAD FAILURE OF BEAMS, FLOORS, AND ROOF

CONSTRUCTIONS DURING FIRE TEST

by

A. F. Robertson

J. V. Ryan

NATIONAL BUREAU OF

Intended for use within tl to additional evaluation ar listing of this Report) eith the Office of the Director)

however, by the Governmt

to reproduce additional cc

IMPORTANT NOTICE

Approved for public release by the

Director of the National Institute of

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on October 9, 2015.

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C. Such permission Is not needed,

illy prepared If that agency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
t t • ' i |
Proposed Criterion for Defining Load Failure of Beams,

Floors, and Roof Constructions During Fire Test

A. F. Robertson and J. V. Ryan

ABSTRACT

A brief account is presented of procedures used

in development of criteria for defining the

point at which fire test specimens fail to sustain load during test. It is proposed that both

a deflection of X /800d and an hourly rate of

deflection of 5-1/3 times this value be required

as an indication of load failure. In these

formulae, 1 is the span between supports in the

direction of principle reinforcement and d is

the distance between upper and lower extreme

fibers of the structural component or assembly.

INTRODUCTION

Recently an investigation was performed on the effect

of variations in ceiling f abrication_on_the fire endurances

of a number of floor constructions /_ 1_/*. During initial

tests in this study it became apparent that in many cases

failure might be expected to result from structural effects

rather than heat transfer_to_the unexposed surface. Since,

the test procedure used /_ 2_/ is not specific in defining

methods for determining the point at which the specimen

fails to "sustain the applied load", it appeared desirable

to adopt laboratory procedures which would provide an

objective method of determination of this end point. The

first attempt at selection of a criterion_of load failure,

that of fixed deflection of 3 inches /_ 1_/, was selected

for the particular type of floor construction used. It

was chosen because it seemed to represent a significant

indication of deflection and in addition the data then

available, Figure 1, example III, showed that the corresponding rate of deflection was so great that collapse of

the construction might be expected to occur rather promptly.

*Numbersin brackets refer to references at the end of the

text.
-

.
2

It is evident that a fixed deflection method of

specifying the time at which failure to carry the load

occurs is not likely to be generally applicable to a wide

variety of construction types. This brief paper outlines

some considerations made in developing more general

criteria of load failure for beam, floor, and roof

constructions during fire endurance tests.

LOAD FAILURE CRITERIA

1. Fixed Deflection

The selection of a critical deflection for defining

point of load failure, while possibly useful in specific

cases, is not applicable to the general case because of

differences in specimen construction, span, and materials

of construction used. It would be preferable to specify

a deflection in terms of the construction design. This

was considered but found deficient in properly allowing

for variations in longitudinal restraint at the ends of

the load carrying members of the construction.

2. Increase in Rate of Deflection

Tests performed in which heavy steel beams were

incorporated as load carrying members showed the shortcomings of simple deflection as a criterion of load

failure. In tests such as this it was not uncommon to

find very large deflections develop without any indication

of rapidly increasing rate of deflection with resultant

impending collapse. Some analysis of fire endurance

data was accordingly made to determine the feasibility of

using an increase in rate of deflection as an indication

of load failure. Figure 1, example IV, illustrates the

method used. The initial nearly constant rate of deflection Ri was determined and then the time of load failure

was assumed to occur at a time when this initial rate had

been exceeded by a fixed percentage. In the case

illustrated R2 = 1.5Ri> The difficulty with this

procedure was largely that of determining the point on the

curve at which Rp was to be measured. Therefore it

seemed desirable that the limiting rate of deflection be

defined on some other basis, preferably dependent only on

the structural features of the design. Also, it seemed

apparent that rate of deflection alone was not an adequate

criterion.

- 3

-

3. Deflection and Rate of Deflection Method

Previous experience had shown that to be useful a

criterion of load failure must be applicable to a variety

of construction variables including various types of

end restraint, loading, and construction dimensions and

materials. The large number and complexity of these

variables, not to mention the effects of thermal strains,

seem to require a special analysis of each structure.

This seemed impractical for the purposes intended and as

a result a compromise method was developed. This

involves the requirement that both a deflection and rate

of deflection be exceeded as an indication of load

failure. The requirement of both criteria is believed to

provide a practical substitute for detailed analysis of

each structure tested.

It can be shown that the maximum deflection of

beams involving various types of end support and load

application methods is given by the formula:

y max = K ( § )

Ji d

Here

:

K = a numerical constant

S = working stress

E = modulus of elasticity

t = length of beam between supports

d = depth of beam

This suggests that since permissible values of S,

E, and K will be used for design of a structure to be

tested, the maximum deflection and rate of deflection

which might be considered permissible should be a function of J?/yd. To explore the feasibility of such

criteria a number of tests were studied in which a decision had been reached that load failure could be

assumed to have occurred. As a result it was decided

that the following relationships might well be used for

defining time of load failure:

a deflection of D = i

2/800d and an

hourly deflection rate equal to 5-1/3

times this value or:

R = l

2/150d

- 4 -

To investigate the effect of applying such criteria

for identification of time of load failure of structures

during fire test a group of 50 experimental data were

analyzed. The results are presented in Figure 2. Two

columns of data are presented under the main caption

"Time to Failure". The first of these entitled "Reported"

lists the reported failure time for the construction. In

some cases this was limited by load failure, in others by

temperature rise, ignition of waste, etc. The second

subcaption entitled "New Criteria" indicates the time at

which load failure would be indicated by application of

the criteria proposed. In cases where heat transfer etc.

limits performance the endurance would be shorter than

the values shown in this column. The column entitled

"Net Change of Fire Endurance" indicates the change which

would occur on application of the proposed criteria. The

entries here recognize the fact that in some cases tempera

ture rise, etc. , may limit endurance. In other respects

the table is believed to be self explanatory.

Study of the table indicates that application of the

criteria to the specimens listed would have the effects

of increasing the endurance in 13 instances, reducing it

in 13, and in 24 instances there would either be no effect

or it would be uncertain. It thus becomes evident that

use of the criteria is quite successful in selection of

load failure times which are reasonably consistent with

behavior as analyzed by the operator in charge of the test

The requirement that both a given deflection and rate of

deflection be achieved is believed to present a useful

method of defining the point of load failure of beams,

floors, and roof constructions tested on end supports

but regardless of the type of restraint applied at these

ends

.

CONCLUSIONS

The investigation performed seems to justify use of

the following criteria for defining the time at which

a specimen should be considered as having failed to

sustain the load during a fire test.

A beam, floor, or roof construction mounted on end

supports and subjected to a fire endurance test will be

considered to fail to sustain the applied load when both
-
- 5 -

the net deflection resulting from fire exposure has

exceeded £v800d, and the hourly rate of deflection has

exceeded 5-1/3 times this deflection.

In these formulae:

JL is the span of the structural component

between supports and in the direction of

the principle reinforcement or structural

element.

d is the distance between the upper and lower

extreme fibers of the structural component

or assembly.

REFERENCES

/_ 1_7 "Fire Endurance of Open-Web Steel-Joist Floors

With Concrete Slabs and Gypsum Ceilings" by

J. V. Ryan and E. W. Bender, Building Materials

and Structures Report 1*+1, 195^+? U. S. Government

Printing Office.

L 2_/ "Standard Methods of Fire Tests of Building

Construction and Materials", ASTM Designation

E-119, ASTM Standards.
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U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its headquarters in Washington,

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