The National Building Code smoke control measures: an overview

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The National Building Code smoke control measures: an overview

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National Research Council of Canada Conseil national de recherches du Canada . .

no.

872 cop. 2 6:,";' usf3 \ 1'

The National Building Code Smoke Control

Measures

-An

Overview

by

J. H.

McGuire and G. T . Tamura

Reprinted from

Engineering Digest

Vol. 25, No. 9, October 1979

p.

35-38

DBR Paper No. 872

Division of Building Research

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I

SOHMAIRE

Le Code national du bitiment

du

Canada contieat depuis 1970 dm rnesures pour cont&ler

la

f& dans les hdifices en hauteur, mais aucun document ptllilli6 n'a encore corn@ la valeur de ces diffBrentes

a e -

sures. Les auteurs btudient cette question en insistant sur

la

fiabilitk relative des mesures,

\

- - -. -

--

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The

National Building Code smoke control measures

-an

overview

by

J. H. McGuire and G. T . Tamura

Measures to control smoke in high buildings have been included in the National Building Code of Canada since 1970, but no document has been produced that compares the meriis of the various possible approaches listed. It is the intention of this paper to discuss this topic, with emphasis on the relative relia- bilities of the measure. The need for such comment is illustrated by two undesirable trends that have developed: the wide- spread use of the fully pressurized building concept (measure H) in buildings for which it is not appropriate; and the unwise use of smoke shafts intended to serve any one of a considerable number of floors.

THE list of contents relating to Chapter 2 in the Associate Committee publication "Measures for Fire Safety in High Buildings 1977" (1) constitutes a good summary of the measures referenced by the Code. In order to discuss their various merits, however, measures D&E, I and K must be further sub-divided to give the following (alphabetical) list:

Measure A Measures B & C Measures D & E Measures D & E Measures F & G Measure H Measure I Measure I Measure J Measure K Measure L Measure M

Fully sprinklered buildings Open corridor access to stairs and elevators

Vented vestibule access to stairs and elevators

Pressurized vestibule access to stairs and elevators

Pressurized stair and elevator shafts

Fully pressurized buildings Pressurized core (vented fire area) Pressurized core (exhaust system) Pressurized core

Spatially divided buildings (vented vestibules)

Areas of refuge

Residential buildings with balcon- ies

Reference to Measure N has been excluded from the above list because its objective is to eliminate the flow of smoke from one building to another connected to it, and not to control smoke migration in a single building.

Measure M will also not be compared with the remainder because it differs from them in not specifi- cally permitting the occupants of a building to vacate it. Provision of balconies can nevertheless prove quite an effective life safety measure although some expo-

sure to smoke may prevail and, in the depths of a Canadian winter, exposure to exterior temperatures may be significant hazard.

In the opinion of many Measure A-complete sprinklering, would rank as the most effective and reliable. Sprinklers have, in fact, a very good record of reliability in controlling fires and, by limiting the amount of combustible involved in fire, also restrict smoke production. As it constitutes both a crude detection system and a fire fighting measure that is automatically initiated early in a fire, a sprinkler system should enhance the effectivenss of fire depart- ment operations.

A sprinkler system minimizes smoke problems by limiting the generation of smoke. All the other measures achieve this objective by influencing the choice of flow path followed by the smoke. For this reason no further attempt will be made to compare the merits of sprinklers with the other smoke control measures listed in the Code.

Reliability Merit Sequence

The objective of the smoke control measures under discussion is to avoid widespread dispersal of dense smoke throughout a building by way of vertical shafts, which should constitute the only substantial path if floor assemblies are reasonably tight. A most interesting comparison of the measures is in terms of the reliability with which they can be expected to perform this function when required. Table 1 lists a reliability merit sequence, somewhat subjectively developed by the writers. Such a generic list omits consideration of many critical features peculiar to an individual building, and it is up to a designer to decide whether certain features relating to his building ne- cessitate a different listing.

Table 1 also includes a statement as to whether the method protects the local escape route in the region of the fire, e.g., the escape route of the occupants of hotel rooms adjacent to the one in which a fire originated. In general, development of a natural or forced air movement system to achieve this secondary objective can involve considerable sophistication, which in turn can adversely affect reliability.

The protection of occupants in areas directly adjacent to the fire is often best considered separately. A well engineered fire detection and alarm system (re-

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sulting in evacuation) may be the most convenient option.

As can be seen from Table 1, the measures ranked highest do not depend on hydro power.

If

the electrical power requirement is minimal, battery back-up is possible and for a greater requirement emergency generator back-up offers a fairly reliable alternate power source. For massive air injection, however, emergency generator back-up will usually be regarded

as prohibitively expensive.

Where air injection is called for another important factor governing reliability is the sophistication of the pressure differential pattern intended to be estab- lished. One condition to be avoided is asymmetry of air deliveries to the extent that flow reversals occur. Such a situation can exist, for example, if stair shafts and elevator shafts are to be protected and flows to the latter are excessive. Pressures in the floor spaces, as a result of flows from the elevator shafts,

can

rise to values higher than those in a stairshaft, giving flow reversal between floor space and stairshaft.

Another factor is responsible for the pressurized building concept being ranked the poorest. I t is the need, as a system is activated, for positive identification of the area of origin of the fire. Measure H essentially involves the venting of the fire area to ensure that flows are from and now to all adjacent areas.

Superficially, identification of the fire region might seem very simple. In practice this might not be so. Identification by means of heat detectors might be the most reliably accurate automated approach. Re- sponse would often, however, be too slow, particularly with the advent of furnishings such as the "overstuf- fed" settee capable of smouldering and of itself pro- ducing sufficient smoke to foul quite a large building. Smoke detection is therefore desirable and with the current emphasis on mechanical ventilation, design of a system satisfying a requirement of accurate spatial identification is very difficult. A particular air flow pattern could well cause a detector in an adjacent compartment to operate first. If the smoke control system were then initiated with the second compartment being declared the fire area, correction of the

Merit Measure Ranking Local Escape Route Protected -

B 8 C Open corridor access

K Divided building (Spatial separation and vented vestibules)

K Divided building (others) D (L E Vented vestibules F 8 G Pressurized shafts

I Pressurized core (vented fire area) I Pressurized core (exhaust system) D 8 E Pressurized vestibules J Pressurized core L Areas of refuge H Pressurized building Yes No N o N o N o Yes Yes? N o N o No N o -

Table 1

-

- Reliability merit sequence

error even manually would probably be impossible. As smoke became distributed by the "smoke control system", various other areas would shortly be declared fire areas. Even if one of them were the true (original) fire area, the venting of several areas would result in failure of the system to be one hundred per cent effective.

A feature of Table 1 that may be inappropriate is the listing of Measures

C,

E and

G

with the corre- sponding basic Measures B,

D

and

F.

Their operating reliability should be slightly superior to that of their parent measures because they have much more re- stricted objectives. The magnitude of this negative feature (restricting the objectives) has, however, not been evaluated.

Perhaps a further complication should also have been introduced, the subdivision of basic Measures

H

and I depending on whether the venting of fire areas involves the use of smoke shafts. The problems created by smoke shafts are discussed in a later section.

Individual Measures

There is no question that Measure B-open corridor access-is the most reliable of the measures listed, clearly satisfying all the criteria just discussed. The concept should also give an acceptable level of effec- tiveness. Unfortunately, however, the measure will

I

almost invariably be unacceptable for the Canadian climate because it does not permit the heating of corridors. For very high buildings, the heating of shafts would also be unwise if doorlpressure problems are to be avoided.

Measure K-buildings divided by a spatial separation with vented vestibule communication, will achieve an almost comparable reliability, but life safety will involve movement from one side of the building to the other before the development of intol- erable levels of smoke. The next category of divided building could in turn be subdivided into the following three classifications (a) spatial separation and pressurized vestibules, (b) firewall and vented vestibules and (c) firewall and pressurized vestibules. Types (a) and (b) are probably of comparable merit and both will be superior to (c).

Little needs to be said about the fourth listed measure, D, except that doorlpressure problems will make it unacceptable for very tall buildings in winter unless the heating of the shafts is reduced.

Measure F. heads the list of those that are de- pendent for their implementation on the use of forced air supply fans. The principal negative features of it are the two mentioned earlier that are common to all measures using forced-air supplies, viz., dependence on electrical power and the possibility of reversed flows resulting from asymmetrical air distribution.

The subdivided Measures I-the pressurized core concept, may have been inappropriately listed as sixth and seventh in the merit sequence and perhaps should follow the pressurized vestibule Measure D. The negative feature of the first listed is that the fire area must be identified in order to vent it. This has not, however, been accorded as much significance as with Measure H because small compartments in a Measure I

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building lead to greater reliability in identification and to the generation of smaller quantities of smoke. The latter factor, in fact, contributed

to

the Associate Committee's decision

to

accept the Measure I concept in the absence of venting of the fire suite, designating it Measure

J.

In a Measure

J

building slight asymme- tries (e.g. open windows) could give rise to some con- tamination of the central core.

Measures D and E-pressurized vestibule concept appears low on the list partly because of summerlwinter balancing problems. Thus in summer similar vestibules at different levels in a building should have the same pressure differential with respect to the exterior. In winter, however, as a result of stack action, differentials at the top of the building are likely to be much higher than the minimal or possibly even negative values at lower levels. As the air supply must, of course, originate from the exterior and not the floor spaces, control problems arise. One solution is to use high pressure constant volume fans and it must then be ensured that flows are not excessive lest unaccepta-

ble door pressures develop.

A

second possible criticism of the pressurized vestibule concept is that considerable ductwork might be

necessary, some of it passing through areas that might

be involved in fire. Theoretically this should not create a hazard, but in actual practice this might not be the case.

It is because the reliability of the air supply is of even more importance in the area of refuge concept that Measure L has been listed even lower.

Measure H, the pressutized building approach, is the most unreliable, principally because its operation depends essentially on the accurate identification of the area in which the fire originated. Should the wrong area be identified as the fire area, the initiation of the control measure is likely to accentuate rather than reduce hazard. Identification by automatic smoke sensing is a popular approach and, although this ques- tion has already been discussed earlier in this note, it

can hardly be overemphasized that smoke detection is

quite likely to give false fire area identification. Air

handling systems, in particular, can _maintain_a_clean

atmosphere around a detector in the fire area, so that a detector elsewhere might be the first to report a fire.

Another unfortunate feature of Measure H, when applied to a very high building, is the

need

to modu-

late air supplies with exterior temperature. Where leakage characteristics are uniform up the height of

the building, air distribution in summertime should also be uniform with height (i.e., independent of height). In winter, when stack action differentials out- weigh other considerations, distribution with height for such a building, needs to follow a

fi

law

r2)

(although this latter is not a Code requirement).

It

is not feasible to achieve modulation by controlling systems and dampers. A more practical approach for appropriate air distribution is to rely on adequate communication between floor spaces and common shafts such as elevator shafts. Appropriate leakage characteristics are usually only to be found in buildings that are fairly symmetrical with height. Measure

H should not be regarded as suitable for buildings such as those involving a tower communicating with a large plaza or basement complex. Such buildings are,

of course, the most common so Measure h should usually be regarded as inappropriate.

Even with careful summerlwinter modulation ot air supplies in a Measure H building stairshafts, and to some extent elevator shafts, can become contami. nated when doors communicating with the exterioi are opened. To ease this problem injection of air intc: the shafts is called for (2). In fact as more consideration is given to the problem, diversion of a greater proportion of the air to elevator and stairshafts be- comes more appealing until the method virtually be- comes Measure

F,

the most reliable of the measures essentially depending on forced air supplies.

A

designer will often use a smoke shaft as a means of venting the fire area in a Measure H building. Some of the problems that can arise are discussed in the next section.

Should a designer

of

a

Measure

H

building decide

to avoid the use of a smoke shaft and vent by

an

opening in the exterior wall he should bear in mind that flaming gases will be issuing under pressure from

the opening, He would be advised not to have windows immediately above. A convenient approach would be to use fire resistive vent covers and tomount them one

above the other up the side of the building.

Smoke Shafts

Chapter 3 of "Measures for Fire Safety in High Buildings" (1) and another publication (3), give some design information on smoke shafts and one apparent feature is that, unless they are very tightly construct- ed, they are likely to deliver smoke to the topmost storeys of buildings over five or six hundred feet in height. Maintaining adequate closure all the way up the shaft in fact constitutes the principal design problem and it is complicated by having to consider poten- tial exposure to very

high

temperatures.

Far from assuming that gas temperatures reduce as gases flow up a smoke shaft, vigorous burning in the

shaft can be expected as leakage of diluent air brings

previously over-rich mixtures down

to

_within_the flammability limits. The design of dampers for each floor area to compensate for an exposure of this nature

is no easy matter. The fmt obvious consideratan is

to

try to maintain tightness even in the face of tempera-

tures of approximately 1000°C (1800°F).

If leakage precludes compliance with the design

data given in references (I) and (3), a useful approach is

to

avoid serving the top-most storeys by the smoke shaft under consideration, having it permanently

and

tightly sealed as it passes through those levels. A lower neutral pressure plane (between the smoke shaft and adjacent areas) is then permissible.

A second consideration, which is too often neg- lected, is to protect combustibles in the floor area (possible in a concealed ceiling space) from heat con- ducted through the damper. currently it is not un- common for smoke shaft dampers to be of all metal construction and it is conceivable that the use of a smoke shaft to vent a fire at a low level in a high building might result in disaster by the initiation of secondary fires at every higher level.

A practice that is becoming popular, both in Meas- ure H buildings and generally, is the use of a return air

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duct or shaft as a smoke shaft. It must be borne in mind that, during normal use of such an air-handling facility, the dampers a t all floor levels will probably be open. Extreme care must be exercised to ensure that, when service as a smoke shaft is required, every one of these dampers closes tightly (with the exception of the one serving the fire floor, of course). Attention must also be paid to the design features already discussed, particularly that relating to the severity of the fire exposure that might prevail.

Conclusions

For a smoke control measure to achieve the greatest reliability the following should be avoided:

(1) Dependence on hydro power.

(2) Dependence on accurate identification of the area of origin of the fire.

(3) Sophistication.

Neglecting Measures B and C which do not have much application in Canada, Measure K, the divided building concept, heads a reliability merit list, com- plying with all the above criteria. The pressurized building concept (Measure

H)

is a t the bottom of the list and shaft pressurization (Measures F and G ) constitutes the most reliable of the measures essentially involving mechanical air movement.

The design of smoke shafts calls for careful attention. Not merely must leakage be minimized and clo- sures remain effective at very high temperatures, but dampers must have some fire resistance in order to avoid multiple ignitions in the floor spaces, above the fire floor, through which a shaft passes.

John H. McGuire, P. Eng., formerly Research Officer, Fire

Research Section, Diu. of Building Research, NRC of Can- ada-now retired; and George Toshiaki Tamura, P. Eng., Research Officer, Energy and Services Section, Diu. of Building Research, NRC of Canada, Ottawa, Ont.

References

( 1 ) "Measures for Fire Safety in High Building," Associate

Committee on the National Building Code of Canada, Division of Building Research, National Research Council of Canada, 1977. (NRCC 15764).

(2) Tamura, G.T. and J. H. McGuire, "The Pressurized

Building Method of Controlling Smoke in High Rise Buildings," Division o f Building Research, National Re-

search Council of Canada, September 1973. (NRCC 13365).

(3) Tamura, G. T. and C. Y. Shaw, "Basis for the Design of

Smoke Shafts," Fire Tech., Vol. 9, No. 3, August 1973,

pp. 209-222.

This paper is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the approval of the Director of the Diui- sion.

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