The Problem of "Noncombustible"

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Technical Note (National Research Council of Canada. Division of Building Research), 1964-12-01

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The Problem of "Noncombustible"

Ferguson, R. S.; Shorter, G. W.

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DIVISION OF BUILDING RESEARCH

No.

NATIONAL RESEARCH COUNCIL OF CANADA

428

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PREPARED BY

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APPROVED By NBH

DATE December 1964

PREPARED FOR Comment

SUBJECT The Problem of "Noncombustible"

Over the past centuries noncombustibility has been almost the only means of fire protection. Cities have turned from wood to brick, stone and steel as a consequence of the tragic experience of fire.

In recent times research and testing have shown that the problem has many sides. Stability and prevention of collapse can be acquired by fire resistance. Fire spread can be controlled by compartmenting.

Hazards to life safety can be controlled by limiting flame spread and pro-viding adequate exits. Fire resistance, compartmenting, flame spread, and exits are safety measures that are applied to both combustible and noncombustible buildings. Everything else being equal a completely non-combustible building is safer than a completely non-combustible one because its heat generating capacity is"less, but the difference is sometimes marginal; in a modern building the hazard that the contents contribute

may exceed the possible hazard of the construction by many times. Because of this it is true to say that to a large extent the four methods described have supplemented noncornbustibility as a safety measure.

This change in emphasis has been underlined by an interesting change in the concept of noncombustibility. There was a time when every-one knew what was noncombustible and what wasn't. It was the difference between stone and wood. Manufactured products, however, have changed that. Gypsum wallboard is noncombustible but has a paper skin, cinder block incorporates up to 30 per cent combustible material, and sandwich panels of noncombustible materials are held together with combustible adhesives. Even steel is painted. What are considered to be entirely noncombustible materials have some combustibles. A completely non-combustible building is as rare as a completely non-combustible one.

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-To accommodate the new situation building code authorities have defined noncombustibility. Developed by the National Board of Fire Under-writer s, this definition appeared in the National Building Code of Canada

1960 (see Appendix A). In essence, it describes certain materials with limited combustibility as noncombustible. As the demarkation line was always arbitrary - everything burns under some circumstances - there was no reason why it shouldn It be moved a little bit in the direction of combustibility.

This move had tremendous repercussions. Forms of treated wood and plywood that met the definition were developed and building codes could be said to have fathered noncombustible wood. This was not the intention, and it is therefore proposed (in the draft of the 1965 National Building Code of Canada) that paragraphs band c of the definition be deleted. These para-graphs ruled materials of low flammability as noncombustible, and their deletion returns noncombustible to the original dictionary meaning of the term.

Now a new problem has occurred. Certain materials such as noncombustible materials held together with combustible glues or binder s are excluded, even though these materials would present no appreciable increases in fire hazard.

In formulating a solution to this problem:

1. the consequences for industry of tampering with the recognized con-cept of noncombustibility must be appreciated; the meaning of non-combustible must remain constant; it cannot be linked, as it is in the 1960 Code, with what is acceptable in a noncombustible building because what is acceptable will vary from time to time;

2. the difference between noncombustibility in the structural frame and noncombustibility in the non-bearing wall boards and panel materials must be recognized; the CSA test procedure may provide a satis-factory answer for the fir st but it is too severe for the second; in fact, the CSA test is probably not adequate for the first; the develop-ment of materials such as cinder block for use in load-bearing

situations is quite possible.

The real issue now is the prospect of using almost immediately combustible materials that are able to perform as well as the traditional noncombustible materials in a load-bearing or non load-bearing capacity.

Ifthe materials serve the purpose there are two possibilities for control. 1. The definition of noncombustible can be written (as in the 1960 National

Building Code of Canada) so that the materials in question become noncombustible.

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-2. The definition of noncombustible can remain in the CSA Specification, but the concept of the traditional noncombustible building can be changed to one of limited combustibility.

Of these two possibilities the second is preferable. Noncombustibility is an elemental concept, but noncombustible construction is only a standard that has proved satisfactory for tall buildings and some other situations. When construction using combustible materials is developed that satisfies the

conditions, the standard should be changed to permit it.

The reader will undoubtedly appreciate the significance of this suggestion. It reveals that there is no preamble to the present regulations outlining the hazards they are intended to mitigate or the conditions they are intended to satisfy. What are these conditions? They may be known, but it is not general knowledge. Lacking a satisfactory diagnosis of the problem, responsible authorities place their trust in noncombustibility. Where one can truly eliminate all combustibles there can be no fire.

This solution cannot be justified much longer. Attention should be directed to the hazards that size, and particularly height, introduce. Agreement on what the hazards are and the degree of risk acceptable will

pave the way for alternate solutions "deemed to satisfy" tlie'se conditions. Non-combustibility will always be one, but not necessarily the only, solution.

If eventually a performance criterion for fire safety in large or tall buildings is accepted, materials and components will probably have to be judged on the basis of fuel contributed, in addition to stability under fire conditions, heat transfer, flame spread, and smoke and toxic contributions as well as performance criteria to satisfy structural and health hazards.

The determination of performance criteria for materials for buildings of different sizes would be a major step ahead in the regulation of heights and areas of buildings".

Suggested Interim Measure

Meanwhile one might classify as low-combustible all buildings that are now required to be noncombustible. Materials for these buildings could be accepted as follows:

1. any material that passes the CSA Specification B-54-1960;

2. (a) any material that has a surface flame spread rating not higher than 25 without evidence of continued progressive combustion, and of such composition that surfaces exposed by cutting through the material in any way would not have a flame spread rating higher than 25 without evidence of continued progressive com-bustion; and

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-(b) has a fuel contribution of not more than 5000 Btu/sq ft/hr.

The fuel contribution might be measured by preparing a sample construction assembly - say 30 by 30 inches containing the material as it would be used in practice and subjecting it to the time -temperature con-ditions of ASTM E1l9. The fuel contribution would be determined by

measuring in some simple practical way the proportion of the material con-sumed during the time period and multiplying this quantity by the heat of combustion.

The limit of 5000 Btu would be the total permissible for all the materials forming a square foot section of any building component such as a wall, floor or roof.

Attached is a listing of materials showing flame spread rating and fuel contributions (Appendix B).

Noncombustible as applied to a building construction material means a material that falls in one of the following groups (a) through (c).

(a) Materials that are classed as noncombustible when tested in accordance with CSA specification B54. 1-1960, "Determination of Non-Combustibility of Building Materials. "

(b) Materials having a structural base of noncombustible material, as defined in (a), with a surfacing not over 1/8 -in. thick which has a flame spread rating not higher than 50.

(c) Materials, other than as described in (a) or (b), having a sur-face flame spread rating not higher than 25 without evidence of continued progressive combustion and of such composition that surfaces that would be exposed by cutting through the material in an y way would not have a flame spr ead rating higher than 25 without evidence of continued progressive combustion.

Note - "Noncombustible" does not apply to surface finish materials nor to the determination of whether a material is noncombustible from the standpoint of clearances to heat-producing units, venting equipment, pipes and ducts regulated in Part 6: Building Services.

No material shall be classed as noncombustible which is subject to increase in combustibility or flame spread rating beyond the limits herein established, through the effects of age, moisture or other atmospheric conditions. Flame spread rating as used herein refers to ratings obtained according to the "Tentative Method of Surface/Burning Characteristics of Building Materials, " ASTM E84-59T.

APPENDIX B

FUEL CONTRIBUTION OF VARIOUS MATERIALS

Thickness Flame Spread Fuel Contribution

Material (inch) Index Btu/ft 2

Roof insulation boar d 1 61 2,925

Cellulose mineral 2 1.3 18,000

board

VAPOUR BARRIERS

Asphalt coated paper 0.015 275-i900

Polyethylene film 0.002 188 CONCRETE Cinder aggregate (60% solid, l6-x8-x 8 -in. block) 8 0 114,400 Slag aggregate (60% solid, l6-x8-x 8 -in. block) 8 0 3,560 Shale aggregate (6G% solid, 16- x 8-x 8 -in. block) 8 0 200 Calcareous gravel aggregate 8 0

<

<

Asbestos cement board 3/16 0.0 144

Asbestos cement board

+

O.

GYPSUM

.,

.

B-3

Thickness Flame Spread Fuel Contribution

Material (inch) Index Btu/ft 2

Gypsum board

+

Gyp sum boar d with

paper removed 3/8 0

<

LATH

Metal diamond me sh 0.025 0 1,370

Metal diamond mesh,

(paint removed) 0.019 0 420 METALS Structural steel, unpainted 0.060 0 190 Aluminum 0.004 0 0.8 Magnesium O. 128 0 5,400