Technical Notes 16 - Fire Resistance
April 2002

Key Words:  brick, fire ratings, fire resistance, fire test procedure.

INTRODUCTION

Provisions relating to fire safety represent the most important regulations of every building code, so far as the safety of the public is concerned. In contrast with the few structural failures that occur, fire collects a heavy toll of lives and a high loss of dollars every year.

The performance of walls, columns, floors and other building members under fire conditions is of major importance to fire safety. However, these single elements are only part of the code requirement. It is also important for the code to provide for a balance between fire ratings of members and other provisions of equal importance, such as exit facilities, floor area limitations, separation minimums and height limitations.

This Technical Notes discusses the fire test procedure used for brick masonry walls and lists the fire ratings of brick masonry walls and fire protection ratings for beams and columns.

FIRE RESISTANCE RATINGS

It has become standard practice to express the degree of fire resistance required for any member in terms of its ability to withstand exposure to fire as prescribed by the American Society for Testing and Materials' Standard Methods of Fire Tests of Building Construction and Materials, E 119.

Specifications in ASTM E 119 provide that the temperature on the exposed (fire) side of the test panel shall be controlled by the standard time-temperature curve, as shown in Fig. 1. The points on this curve that determine its character are:

Wall Specimens. The area exposed to fire shall be at least 100 sq ft (9 m²) with no dimension less than 9 ft (2.7 m). Non-bearing walls and partitions are restrained at all four sides, but bearing walls and partitions are not restrained at the vertical edges. (See Fig. 2.)

Columns. Two methods of test are permitted. If the fireproofing is structural, the column specimen must be at least 9 ft (2.7 m) long and acceptance is based on ability to carry an axial load. If the fireproofing is not structural, the minimum column length is 8 ft (2.4 m) and acceptance is based on temperature rise in the structural column.

Hose Stream Test. For most ratings, (Strictly speaking, the ASTM procedure determines ultimate fire resistance periods, not fire ratings) ASTM E 119 requires that walls undergo a fire and hose stream test as well as the fire endurance test. (Two alternates are available. The hose stream test may be performed on a duplicate wall sample which has been subjected to a fire exposure test for one half of the determined fire resistance period (but not less than 1 hr); or the hose stream test may be performed on the original wall specimen. The latter is the usual case for brick walls.) The test subjects a specimen to impact, erosion and thermal shock over the entire area of the surface which has been exposed to the fire. The specifications carefully denote nozzle size, distance, duration of application and water pressure at the base of the nozzle. (Some of these requirements vary with the fire resistance period). (See Fig. 3.)

Loading. Throughout the fire endurance and hose stream tests, a constant superimposed load is applied to bearing walls to simulate a maximum load condition. The applied load shall be as nearly as practicable to the maximum load permitted by design in reference to nationally recognized structural design criteria.

Columns are loaded to develop the design stresses and then subjected to the standard fire on all sides. Where the fire protection is not designed to carry loads, an alternate method of test may be used in which the column is not loaded. Temperature rise, the sole criterion here, is measured by at least three thermocouples located at each of four levels.

CONDITIONS OF ACCEPTANCE (FIRE TESTS)

Non-Bearing Walls and Partitions. (No combustible members framed in). The test is, successful and a fire resistance period may be assigned to the construction, if:

1. The wall or partition withstands the fire endurance test without passage of flame or gases hot enough to ignite cotton waste for a period equal to that for which classification is desired.

2. And, the wall or partition withstands the fire and hose stream test without passage of flame, of gases hot enough to ignite cotton waste, or of the hose stream. (Although ASTM E 119 does not specifically state what is meant by "passage of the hose stream", this is generally accepted to mean that no collapse of the wall occurs. For example, water passing through masonry joints is not a cause for rejecting the specimen).

3. And, the average temperature of nine thermocouples on the unexposed surface has not increased more than 250 ^oF (121 ^oC) above its initial temperature.

Bearing Walls. The conditions of acceptance for bearing walls are essentially the same as for non-bearing walls and partitions (above), with the following addition:

1. The specimen must also sustain the applied load during the fire endurance test.

Columns. Columns with integral structural fireproofing may be assigned a fire resistance period if they successfully sustain the superimposed load during the fire endurance test.

For fireproofing of columns not designed to carry loads, a fire resistance period may be assigned if the average temperature rise does not exceed 1000 ^oF (538 ^oC) and the maximum temperature rise does not exceed 1200 ^oF (649 ^oC) at any one point.

TERMINATION POINTS OF TEST METHOD

When an assembly under test reaches any one of the acceptance criteria; (1) ignition of cotton waste, (2) average temperature rise on unexposed side above 250 ^oF (121 ^oC), or (3) fails to carry the design load if it is a load-bearing test; the test is terminated. The first two criteria relate to the function of providing a barrier against the spread of fire by penetration of the assembly, the third relating to structural integrity.

The termination point for fire tests of brick masonry walls is almost invariably due to temperature rise (heat transmission) of the unexposed surface. Brick masonry walls successfully withstand the load during test and the hose stream test conducted immediately after the wall has been subjected to the fire exposure. This structural integrity of brick masonry walls is attested to in many fires where the masonry walls have remained standing when all other parts of the building have been destroyed or consumed during the fire.

COMBUSTIBLE FRAMING OR FACING

Conditions of Acceptance. The test procedure for walls supporting combustible framing or facing is essentially the same as for other walls. However, no hose stream test is required. The test is successful and a fire resistance period may be assigned if:

1. The protection withstands the test and no combustible members ignite.

2. And, the contact-surface temperature of combustible members does not increase more than 250 ^oF (121 ^oC). For members closely embedded on three sides in masonry, concrete, etc., the permissible temperature rise is 325 ^oF (163 ^oC).

AMOUNT OF COMBUSTIBLES RELATED TO SEVERITY OF FIRE

Burn-out tests conducted at the National Bureau of Standards, which were performed in fireproof structures with various concentrations of combustibles having calorific values in the range of wood and paper (7000 to 8000 Btu per lb) and assembled to represent building occupancies, indicate that the relation between the amount of combustibles and the fire severity is approximately as given in Table 1 which is reproduced from National Bureau of Standards Report, BMS92, Fire-Resistance Classifications of Building Constructions, October 1942.

 

COMBUSTIBLE CONTENTS IN BUILDINGS

In 1947, the Office of Technical Services of the Department of Commerce sponsored an investigation of the weights of combustible contents in various occupancies. This survey was made by the Public Buildings Administration under the supervision of the National Bureau of Standards and the results are reported in National Bureau of Standards Report, BMS149, Combustible Contents in Buildings, published July 1957. Regarding the weights of combustibles reported, the report states:

Table 2 gives average combustible contents of various occupancies as included in the authors' summary of BMS149.

In order to determine the total fuel content or total conflagration hazard represented by a building and its contents, the fire hazard represented by the combustible materials incorporated in the structure itself must be added to the combustibles incident to the occupancy of the building.

FIRE RESISTANCE

The fire resistance ratings of walls and partitions are usually less than ultimate fire resistance periods as determined by test, since most building code requirements are in multiples of 1 hr with a 4-hr maximum. Table 3 gives ultimate fire resistance periods for solid brick loadbearing walls. These walls were tested under working loads of 160 psi of gross area, except the 4-in. walls which were loaded to 80 psi. Ratings for 8-in. or thicker solid brick walls apply when units are laid in any of the mortars included in ASTM Specification C 270. Ratings for 4-in. solid walls apply when the units are laid with type M, S or N mortar.

^aAdapted from BMS92, Reference 7.

^bTo achieve these ratings, each plastered wall face must have at least 1/2-in., 1:3 gypsum-sand plaster.

^cBased on load failure.

^dBased on temperature rise (for non-loadbearing walls).

Estimating Effects of Combustible Members. Where no fire tests are available for unplastered walls with combustible members framed in, the rating may be approximated from tests on similar walls without combustible framing by using the factors in Table 4.

^aAdapted from BMS92, Reference 7.

^bFor walls plastered with 1/2-in., 1:3 gypsum-sand plaster on the side opposite combustible framing, add 1 hr to computed ratings over 3 hr and 1/2 hr to lesser ratings. No increase is permitted for plaster on the same side as combustible framing nor where combustible members enter from both sides.

^cMultiply ratings of unplastered walls with no combustible framing by the appropriate factor to compute ratings for similar unplastered walls which support combustible framing.

^dWalls of units which are not cored more than 25 percent.

Fire Resistance Ratings. Figures 4 through 7 list fire resistance ratings for various brick walls. Ratings listed are for loadbearing walls tested with working loads of 160 psi of gross area, except the 4-in. wall in Fig. 5 was loaded to 92 psi and the 4-in. wall in Fig. 4 was loaded to 80 psi.

Table 5 lists the fire resistance ratings for steel columns covered with brick.

^aColumn fire resistance varies with the cross-sectional area of solid material; the larger the area, the greater the fire resistance for a given thickness of protection around the structural steel. Column dimensions are outside dimensions. Smaller columns may require more cover to achieve equal ratings. For columns that are not square, protection should equal that of a square column having equal or lesser cross-sectional area.
^bThicknesses do not include plaster.

SUMMARY

This Technical Notes provides the fire resistance ratings of several brick masonry assemblages based on ASTM E 119.  The conditions of this standard test are described.  Calculated methods for determining fire resistance of brick masonry are found in other Technical Notes in this series.

The information and suggestions contained in this Technical Notes are based on the available data and the experience of the engineering staff of the Brick Industry Association.  The information contained herein must be used in conjunction with good technical judgment and a basic understanding of the properties of brick masonry.  Final decisions on the use of the information contained in this Technical Notes are not within the purview of the Brick Industry Association and must rest with the project architect, engineer and owner.