Review of the DBR/NRC studies on control of smoke from a fire in high buildings

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ASHRAE Transactions, 89, 1B, pp. 341-361, 1983

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Review of the DBR/NRC studies on control of smoke from a fire in high

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Tamura, G. T.

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National Research Conseil national

I

*

Council Canada

de recherches Canada

REVIEW OF THE DBRINRC STUDIES ON CONTROL

OF SMOKE FROM A FIRE I N HIGH BUILDINGS

by

G.T. Tamura

Reprinted from

ASHRAE Transactions 1983

Vol. 89, Part 1B, p. 341 - 361

DBR Paper No. 1164

Division of Building Research

En c a s d ' i n c e n d i e , l a p r o p a g a t i o n d e s f m 6 e s t o x i q u e s r i s q u e d e menacer l a v i e d e s occupants d e b a t i m e n t s d e grande hauteur. Les r e c h e r c h e s pr'esentes e t p a s s k s s u r les mouvements de l a fumse e t l e dBsenfumage B t a i e n t b a s k s s u r d e s mod3les math'ematiques e t d e s e t u d e s dans les b l t i m e n t s pour d v a l u e r l e s r l s q u e s que pr'esente l a fumse dans d l v e r s e s c o n d i t i o n s d ' l n c e n d i e . Des t e c h n i q u e s d e dBsenfmage o n t 'et6 mises au p o i n t pour s ' a s s u r e r que

les

i s s u e s d e s e c o u r s e t l e s a i r e s d e r e f u g e s o n t

B

l ' a b r i de l a fumse. Des t r a v a u x d e r e c h e r c h e a d d i t i o n n e l s dans l a t o u r e x p k i m e n t a l e de l a D i v i s i o n d e s r e c h e r c h e s e n batiment p o r t e n t s u r l e s e f f e t s d e l a temp'erature d e l ' i n c e n d i e s u r l e s mouvements d e l a fumee e t s u r l 1 e f f i c a c l t 6 des t e c h n i q u e s de dssenfumage.

Review of the DBRlNRC Studies on Control

of Smoke from

a

Fire in High Buildings

G.T. Tamura

ASHRA E Member

ABSTRACT

Occupants of h i g h r i s e b u i l d i n g s a r e s e r i o u s l y endangered by t h e spread of t o x i c smoke i n t h e e v e n t of a f i r e . Both p a s t and c o n t i n u i n g r e s e a r c h s t u d i e s on smoke movement and c o n t r o l used mathematical models and f i e l d s t u d i e s conducted i n b u i l d i n g s t o a s s e s s t h e smoke hazard under v a r i o u s f i r e c o n d i t i o n s . Techniques of smoke c o n t r o l have been developed t o p r o v i d e s a f e escape r o u t e s and refuge a r e a s . F u r t h e r i n v e s t i g a t i o n a t t h e DBR experimental f i r e tower w i l l examine t h e e f f e c t of f i r e temperature on smoke movement and t h e e f f e c t i v e n e s s of smoke c o n t r o l techniques.

INTRODUCTION

Smoke r a t h e r than h e a t i s t h e main cause of c a s u a l t i e s d u r i n g a f i r e , because smoke i s t o x i c both w i t h i n and beyond t h e f i r e region, and c a n t r a p occupants due t o l o s s of v i s i b i l i t y . The time a v a i l a b l e f o r t o t a l evacuation b e f o r e escape r o u t e s a r e f i l l e d with smoke i s , t h e r e f o r e , c r i t i c a l t o l i f e s a f e t y .l *2 I n t h i s c o n t e x t , a high b u i l d i n g can be d e f i n e d a s any b u i l d i n g i n which e v a c u a t i o n t i m e , which depends on t h e n a t u r e of occupancy and b u i l d i n g h e i g h t , i s considered excessive. L i f e s a f e t y depends n o t o n l y on s a f e t y p r o v i s i o n s i n a b u i l d i n g b u t a l s o on how quickly t h e f i r e i s brought under c o n t r o l . F i r e f i g h t i n g i s more d i f f i c u l t i n high b u i l d i n g s than i n low b u i l d i n g s , because a d d i t i o n a l time i s r e q u i r e d t o reach t h e f i r e r e g i o n , occupancy l o a d i s g r e a t e r , and v e n t i n g of h e a t and smoke i s more d i f f i c u l t t o achieve i n a s a f e and e f f e c t i v e manner.

Because of t h e s e f a c t o r s , t o g e t h e r with t h e i n c r e a s i n g number of m u l t i s t o r y b u i l d i n g s being c o n s t r u c t e d i n Canadian c i t i e s , a d d i t i o n a l requirements f o r t h e p r o v i s i o n of safeguards a g a i n s t smoke hazard i n high b u i l d i n g s were introduced i n t h e 1970 e d i t i o n of t h e N a t i o n a l Building Code of Canada. They have been reviewed and r e v i s e d i n subsequent e d i t i o n s , t h e l a t e s t i n 1980.j Recommended measures f o r smoke c o n t r o l a r e d e s c r i b e d i n c h a p t e r 3 of

The

Supplement t o t h e National Building Code of Canada 1980 .4

Mathematical models were used and f i e l d s t u d i e s i n b u i l d i n g s were conducted t o study t h e causes and n a t u r e of smoke movement i n b u i l d i n g s and t o d e v i s e and develop v a r i o u s techniques of c o n t r o l l i n g smoke movement. Recently, an experimental f i r e tower was c o n s t r u c t e d t o permit f u r t h e r i n v e s t i g a t i o n i n t o t h e movement of smoke and t h e o p e r a t i o n of smoke c o n t r o l systems under r e a l i s t i c f i r e c o n d i t i o n s . This paper p r e s e n t s t h e r e s u l t s of p a s t r e s e a r c h s t u d i e s conducted by DBR/NRC w i t h mathematical models and i n m u l t i s t o r y b u i l d i n g s , followed by a b r i e f d e s c r i p t i o n of t h e f a c i l i t i e s i n t h e experimental tower and t h e p r o j e c t e d r e s e a r c h program t o be conducted there.

SMOKE FLOW PHENOMENON

Although a f i r e may be confined w i t h i n a f i r e - r e s i s t a n t compartment, smoke generated by i t can e a s i l y spread t o a d j a c e n t a r e a s through such openings i n t h e e n c l o s u r e a s c r a c k s i n t h e w a l l s , f l o o r , and c e i l i n g and around p i p e s , d u c t s , and doors. These leakage openings permit a s u b s t a n t i a l flow of smoke when p r e s s u r e i n t h e f i r e compartment i s g r e a t e r t h a n i n a d j a c e n t spaces.

George T. Tamura, S e n i o r Research O f f i c e r , D i v i s i o n of B u i l d i n g Research, N a t i o n a l Research Council of Canada, Ottawa, O n t a r i o , Canada KIA ORh.

The p h y s i c a l p r o p e r t i e s of a smoky atmosphere a r e s i m i l a r t o t h o s e of normal

atmosphere

-

t h e main c o n s t i t u e n t f o r both i s nitrogen. Oxygen and carbon d i o x i d e may vary about 10 p e r c e n t from normal amounts w i t h o u t s u b s t a n t i a l l y a f f e c t i n g t h e p h y s i c a l p r o p e r t i e s of t h e atmosphere. The c o n c e n t r a t i o n of p a r t i c u l a t e smoke, even under c o n d i t i o n s of low v i s i b i l i t y , does n o t a l t e r t h e c h a r a c t e r i s t i c s of a n atmosphere enough t o a f f e c t i t s

movement. It can be assumed, t h e r e f o r e , t h a t smoke w i l l f o l l o w t h e o v e r a l l a i r movement w i t h i n a building.

The following equation r e p r e s e n t s t h e r a t e of mass flow a t s t a n d a r d c o n d i t i o n s through a b u i l d i n g element. where F = mass flow r a t e C = f l w c o e f f i c i e n t 6P = p r e s s u r e d i f f e r e n c e n = f l w exponent

Eq 1 i s t h e b a s i c equation from which a i r f l o w and smoke c o n c e n t r a t i o n p a t t e r n s of a b u i l d i n g can be determined and smoke c o n t r o l measures i n v o l v i n g p r e s s u r i z a t i o n and smoke exhaust can be designed. A number of compartments formed by b u i l d i n g elements c o n t a i n leakage openings and, hence, a number of f l w e q u a t i o n s a r e involved i n d e s c r i b i n g t h e a i r 'and smoke flow p a t t e r n i n a building. Evaluation of Eq 1 r e q u i r e s a knowledge of 6P, which

i s

t h e p r e s a u r e d i f f e r e n c e a c r o s s t h e b u i l d i n g element, and of C and n, which d e s c r i b e t h e air-leakage c h a r a c t e r i s t i c s of t h e element.

P r e s s u r e D i f f e r e n c e s a c r o s s Building Elements

The c a l c u l a t i o n of a i r f l o w r a t e s according t o Eq 1 would r e q u i r e values of 6P, a s

w e l l

a s v a l u e s of t h e a i r - l e a k a g e c h a r a c t e r i s t i c s of b u i l d i n g elements. The p r i n c i p a l f a c t o r s t h a t cause p r e s s u r e d i f f e r e n c e s , hence, a i r and smoke movement, a c r o s s t h e b u i l d i n g s e p a r a t i o n s are:

1. E f f e c t of f i r e temperature;

2. Weather c o n d i t i o n s , p a r t i c u l a r l y wind and temperature; and

3. Mechanical air-handling systems.

Heat generated by f i r e r e s u l t s i n i n c r e a s e d gas temperature, which causes p r e s s u r e d i f f e r e n c e s by thermal expansion and by s t a c k action. The gas volume i n c r e a s e s approximately i n proportion t o t h e a b s o l u t e temperature, r e s u l t i n g i n a buildup of p r e s s u r e i n t h e f i r e compartment a s long a s t h e temperature i s r i s i n g . When t h e f i r e temperature reaches a steady value, however, t h e gas i s no longer expanding; then t h e p r i n c i p a l mechanism t h a t causes p r e s s u r e d i f f e r e n t i a l i s s t a c k a c t i o n due t o t h e temperature d i f f e r e n c e between g a s e s i n t h e f i r e compartment and i n a d j a c e n t spaces a t t h e same l e v e l . With f i r e i n a s i n g l e - f l o o r a r e a , s t a c k a c t i o n a c t s over one f l o o r height. With a f i r e i n a v e r t i c a l s h a f t , such a s a n

e l e v a t o r s h a f t o r s t a i r w e l l , s t a c k a c t i o n involves t h e whole h e i g h t of t h e s h a f t . Stack a c t i o n a s s o c i a t e d w i t h h e a t i n g d u r i n g c o l d weather i s a n o t h e r mechanism by which p r e s s u r e d i f f e r e n c e s a r e induced a c r o s s t h e b u i l d i n g s e p a r a t i o n . The t h e o r e t i c a l p r e s s u r e d i f f e r e n c e due t o s t a c k a c t i o n can be c a l c u l a t e d a s f o l l w s : where h

-

v e r t i c a l d i s t a n c e from t h e n e u t r a l p r e s s u r e l e v e l g = dimensional c o n s t a n t p i = d e n s i t y of a i r i n s i d e f i r e compartment o r b u i l d i n g p o = d e n s i t y of a i r o u t s i d e f i r e compartment o r b u i l d i n g

A s the gas d e n s i t y v a r i e s i n v e r s e l y w i t h t h e a b s o l u t e temperature, t h e value of P C depends on the i n s i d e and o u t s i d e temperature.

The p r e s s u r e caused by wind on t h e o u t s i d e s u r f a c e of a b u i l d i n g , r e f e r e n c e d t o t h a t of t h e f r e e a i r s t r e a m , i s given by where

-

Cv = wind p r e s s u r e c o e f f i c i e n t V = wind v e l o c i t y a t roof l e v e l

The d i s t r i b u t i o n of wind p r e s s u r e c o e f f i c i e n t s on t h e s u r f a c e of a b u i l d i n g f o r v a r i o u s wind a n g l e s

i s

a v a i l a b l e from wind t u n n e l t e s t data.

The supply and exhaust f a n s of a v e n t i l a t i n g system supply a i r t o and exhaust a i r from t h e v a r i o u s compartments of a building. The amount of p r e s s u r e d i f f e r e n c e induced a c r o s s t h e s e p a r a t i o n s depends on t h e a i r - l e a k a g e c h a r a c t e r i s t i c of t h e e n c l o s u r e and on t h e balance between t h e r a t e s of a i r s u p p l i e d and exhausted.

Air-Leakage C h a r a c t e r i s t i c s of B u i l d i n g Elements

F i e l d measurements were conducted t o determine t h e a i r - l e a k a g e c h a r a c t e r i s t i c s of major b u i l d i n g elements t h a t form t h e e n c l o s u r e of conpartments i n a building. They were c a r r i e d o u t i n m u l t i s t o r y b u i l d i n g s b u i l t d u r i n g t h e s i x t i e s and e a r l y s e v e n t i e s . The a i r - l e a k a g e c h a r a c t e r i s t i c s of t h e e x t e r i o r w a l l s were determined by p r e s s u r i z i n g a l l t y p i c a l f l o o r spaces between t h e ground f l o o r and t h e t o p mechanical f l o o r , u s i n g l O O X outdoor a i r f o r t h e c e n t r a l supply a i r systems, w i t h r e t u r n and e x h a u s t systems s h u t d a m (Fig. 1). Supply a i r r a t e s were v a r i e d , and t h e concomitant p r e s s u r e d i f f e r e n c e s a c r o s s t h e p r e s s u r i z e d e n c l o s u r e s were recorded. From t h e s e r e a d i n g s , a i r leakage through t h e e x t e r i o r w a l l s was s e p a r a t e d a n a l y t i c a l l y from t h e remainder of t h e e n c l o s u r e . The a i r - l e a k a g e r a t e s of e i g h t t e s t b u i l d i n g s v a r i e d c o n s i d e r a b l y ;= v a l u e s were above t h a t s p e c i f i e d by a n i n d u s t r y s t a n d a r d and t h e average value was comparable t o t h a t of a 13-inch (33 cm) p l a i n b r i c k w a l l measured i n a l a b ~ r a t o r y . ~ Three t e s t b u i l d i n g s had e x t e r i o r f a c a d e s of metal panels and t h e remainder, of p r e c a s t c o n c r e t e panels; a l l had s e a l e d double-glazing u n i t s . A s t h e w a l l m a t e r i a l s were r e l a t i v e l y impermeable t o a i r , i t i s probable t h a t t h e a i r - l e a k a g e r a t e s depended p r i m a r i l y on t h e design and c o n s t r u c t i o n of w a l l j o i n t s . Two b u i l d i n g s t h a t were c o n s t r u c t e d w i t h c l o s e s u p e r v i s i o n of workmanship i n w a l l j o i n t i n g t o minimize a i r i n f i l t r a t i o n , and where remedial measures were taken when j o i n t s e a l s appeared i n a d e q u a t e , gave t h e lowest a i r - leakage r a t e s .

The air-leakage c h a r a c t e r i s t i c s of f l o o r c o n s t r u c t i o n s were measured i n t h r e e t e s t b u i l d i n g s ( t h e t e s t arrangement i s shown i n Fig. 2). The a i r was exhausted w i t h a 50 000 cfm

(23.5 m3/s) vane a x i a l f a n from two a d j a c e n t f l o o r s , b o t h s e p a r a t e l y and i n combination. When t e s t i n g i n combination, b o t h f l o o r s were exhausted a t t h e same time by opening t h e doors of t h e s t a i r w e l l connected t o t h e exhaust fan. For a g i v e n p r e s s u r e d i f f e r e n t i a l a c r o s s t h e t e s t f l o o r e n c l o s u r e , t h e sun of t h e e x h a u s t r a t e s of t h e two f l o o r s measured i n d i v i d u a l l y

w i l l be g r e a t e r than t h a t measured i n combination, because t h e former would have two

a d d i t i o n a l f l o o r c o n s t r u c t i o n s f o r a i r t o l e a k through. One-half of t h e d i f f e r e n c e between t h e two exhaust r a t e s , t h e r e f o r e , r e p r e s e n t e d t h e average leakage r a t e through t h e one-floor c o n s t r u c t i o n . Equivalent o r i f i c e a r e a s of leakage openings i n t h e f l o o r c o n s t r u c t i o n v a r i e d from 0.65 t o 1.40 f t 2 (0.06 t o 0.13 m2) f o r t h e t h r e e t e s t buildings.7 With t h e c e n t r a l a i r - handling s y s t e m s h u t down, t h e a i r d u c t s p e n e t r a t i n g t h e f l o o r c o n s t r u c t i o n r e p r e s e n t e d a d d i t i o n a l leakage openings, which v a r i e d i n s i z e from 4.41 t o 7.64 f t 2 (0.41 t o 0.71 m2 ).

The a i r - l e a k a g e c h a r a c t e r i s t i c s of t h e e l e v a t o r s h a f t and s t a i r w e l l were measured i n e i g h t m l t i s t o r y buildings.8 The same t e s t f a n a s f o r t h e f l o o r l e a k a g e t e s t s was used t o conduct t h e s h a f t a i r - l e a k a g e t e s t s (Fig. 3). The d i s c h a r g e s i d e of t h i s f a n was connected t o t h e door opening of t h e s h a f t t o be t e s t e d by 3.0 f t (0.914 m) diameter aluminum ducts. The air-leakage measurements of each s h a f t were conducted i n two s t e p s ; f i r s t w i t h a l l door c r a c k s s e a l e d w i t h t a p e and t h e n w i t h a l l t a p e removed. Each s t e p involved p r e s s u r i z i n g t h e s h a f t w i t h t h e t e s t f a n and r e c o r d i n g t h e r e s u l t a n t p r e s s u r e d i f f e r e n c e s a c r o s s t h e s h a f t w a l l a t s e v e r a l l e v e l s , t o g e t h e r w i t h t h e concomitant supply a i r r a t e s . The a i r - l e a k a g e measurements w.ith a l l door cracks s e a l e d gave t h e a i r - l e a k a g e c h a r a c t e r i s t i c s of t h e s h a f t w a l l c o n s t r u c t i o n and, w i t h t h e door s e a l s removed, gave t h e o v e r a l l a i r l e a k a g e

c h a r a c t e r i s t i c s of t h e s h a f t enclosure. The d i f f e r e n c e between t h e two r e p r e s e n t e d t h e leakage r a t e of t h e doors.

The a i r l e a k a g e v a l u e s f o r t h e e l e v a t o r s h a f t and s t a i r w e l l a r e g i v e n i n Ref 8. It was found t h a t leakage values f o r e l e v a t o r s h a f t w a l l s c o n s t r u c t e d of masonry u n i t s a r e

considerably g r e a t e r t h a n t h o s e of cast-in-place c o n c r e t e ; leakage v a l u e s of s t a i r w e l l w a l l s a r e s i m i l a r t o t h o s e of e l e v a t o r s h a f t s c o n s t r u c t e d of cast-in-place c o n c r e t e , a s masonry w a l l s of t h e t e s t s t a i r w e l l s were u s u a l l y e i t h e r parged o r p l a s t e r e d . The v a r i a t i o n i n t h e air-leakage r a t e s of t h e w a l l s of t h e s t a i r w e l l s could n o t be r e l a t e d t o t h e type of w a l l construction. It probably depended on workmanship i n s e a l i n g crack openings around door frames, l i g h t f i x t u r e s , and s e r v i c e p a n e l s i n t h e w a l l s . The air-leakage r a t e s of s t a i r and e l e v a t o r doors c o r r e l a t e d w i t h t h e average crack width between door and door frame.

MATHEMATICAL

MODEL

P r e s s u r e s imposed by wind, s t a c k a c t i o n , and air-handling systems a r e d i s t r i b u t e d i n s i d e a b u i l d i n g i n such a way t h a t a t steady s t a t e , a i r inflow and outflow f o r i n d i v i d u a l

compartments a r e always equal. Hence, f o r a given c o n d i t i o n , t h e p a t t e r n s of p r e s s u r e d i f f e r e n t i a l and a i r leakage depend on t h e air-leakage c h a r a c t e r i s t i c s of a l l s e p a r a t i o n s i n a building. Airflow i n t o and o u t of a s i n g l e compartment cannot be c a l c u l a t e d i n i s o l a t i o n b u t rmst be considered w i t h t h e e f f e c t s of a l l o t h e r compartments. The d e t e r m i n a t i o n of t h e r e s u l t a n t p r e s s u r e d i f f e r e n t i a l s a c r o s s a l l t h e major s e p a r a t i o n s involves s e t t i n g up a mass flow balance equation, u s i n g Eq 1, f o r a l l compartments and s o l v i n g f o r p r e s s u r e s i n each compartment. Because t h e mass flow balance equations a r e n o n l i n e a r , an i t e r a t i v e technique i s r e q u i r e d t o s o l v e f o r t h e p r e s s u r e d i f f e r e n t i a l s and mass flow r a t e s a c r o s s a l l

s e p a r a t i o n s .

Once the v a l u e s of mass flow r a t e s and smoke c o n c e n t r a t i o n i n t h e f i r e compartment a r e known, smoke c o n c e n t r a t i o n i n a l l o t h e r coupartments can be c a l c u l a t e d by:

where p = a i r d e n s i t y V = volume of compartment C = smoke c o n c e n t r a t i o n t = time N a a l l compartments a d j a c e n t t o compartment i

Fji = flow r a t e from space j i n t o space i ( i =

l....N);

when t h i s i s n e g a t i v e , Cj i s replaced by Ci

M = a l l compartments excluding t h e f i r e compartment

This equation assumes i n s t a n t a n e o u s and complete mixing of smoke with a i r i n a compartment. Each v e r t i c a l s h a f t i s t r e a t e d a s i f i t were d i v i d e d i n t o s e p a r a t e compartments one-floor i n h e i g h t . The system of simultaneous d i f f e r e n t i a l e q u a t i o n s can be solved u s i n g a numerical method t o o b t a i n t h e v a l u e s of smoke c o n c e n t r a t i o n i n each compartment a t a s p e c i f i e d time i n t e r v a l .

Based on t h e above mathematical model, a computer program was prepared t o c a l c u l a t e p r e s s u r e d i f f e r e n t i a l s , mass flow r a t e s , and smoke c o n c e n t r a t i o n s i n a l l p a r t s of a

building.9 The computer models a r e shown s c h e m a t i c a l l y i n Figs. 4 and 5. The i n p u t s t o t h e computer program a r e a s follows:

1. Dimensions of a building:

number of f l o o r s , f l o o r h e i g h t s , compartment volumes, and a i r l e a k a g e v a l u e s f o r a l l b u i l d i n g s e p a r a t i o n s .

2. Motive f o r c e s :

s t a c k a c t i o n

-

i n s i d e temperature, o u t s i d e temperature, and temperature of f i r e compartment;

wind a c t i o n

-

wind p r e s s u r e s on t h e outdoor s u r f a c e s of a b u i l d i n g ;

air-handling system

-

balance of a i r f l o w r a t e s i n t o and out of each compartment. 3. F i r e f l o o r :

Smoke Movement

Assuming t y p i c a l air-leakage v a l u e s f o r t h e b u i l d i n g elements determined from f i e l d d a t a , computer s t u d i e s based on t h e mathematical models were conducted on a 20-story open plan o f f i c e building.10 The r e s u l t s i n d i c a t e d t h e r e l a t i v e i n f l u e n c e of v a r i o u s f a c t o r s on smoke movement i n b u i l d i n g s .

--For t h e Canadian c l i m a t e , b u i l d i n g s t a c k a c t i o n i s t h e p r i n c i p a l mechanism by which smoke i s t r a n s f e r r e d from a f i r e f l o o r t o t h e f l o o r s above.

--With f i r e on a lower f l o o r d u r i n g cold weather, smoke c o n c e n t r a t i o n s i n e l e v a t o r s h a f t s and s t a i r w e l l s and on upper f l o o r s r e a c h c r i t i c a l l e v e l s i n a very s h o r t t i m e . --Smoke moves mainly v e r t i c a l l y , i n e l e v a t o r s h a f t s and a i r d u c t s and, t o a l e s s e r

e x t e n t , i n s t a i r w e l l s .

--The v e r t i c a l a i r movement, and hence t h e r a t e of smoke movement, caused by wind a c t i o n a l o n e i s s u b s t a n t i a l l y l e s s t h a n t h a t caused by s t a c k a c t i o n .

-A l a r g e opening i n t h e o u t s i d e w a l l of a f i r e f l o o r a t a lower l e v e l r e s u l t s i n a g r e a t e r r a t e of v e r t i c a l smoke movement caused by s t a c k a c t i o n .

--With wind a c t i o n a l o n e , i t i s expected t h a t a l a r g e opening i n t h e windward w a l l of a f i r e f l o o r would a l s o cause a n i n c r e a s e i n t h e r a t e of v e r t i c a l smoke movement. --If f i r e i s p r e s e n t i n a v e r t i c a l s h a f t , t h e r e s u l t i n g high temperature causes a n

upward movement of a i r i n t h e s h a f t and a downward movement i n a l l o t h e r s h a f t s . T h i s can r e s u l t i n a r e c i r c u l a t i o n of a i r through t h e h e a t e d s h a f t s o t h a t smoke w i l l spread throughout t h e building.

This study emphasized t h e need f o r measures t o c o n t r o l smoke movement i n o r d e r t o p r o t e c t occupants and minimize smoke damage i n t h e e v e n t of f i r e .

C e n t r a l a i r - h a n d l i n g systems a r e commonly used t o d i s t r i b u t e conditioned a i r w i t h i n a building. The network of d u c t s necessary f o r t h e d i s t r i b u t i o n of a i r i n t e r c o n n e c t s v a r i o u s spaces of a b u i l d i n g . I n t h e event of a f i r e , t h e a i r - h a n d l i n g systems can, t h e r e f o r e , a c t a s a v e h i c l e f o r t h e s p r e a d of f i r e and smoke f a r beyond t h e o r i g i n of t h e f i r e . To p r e v e n t r e c i r c u l a t i o n of smoke by t h e a i r - h a n d l i n g system, most s t a n d a r d s s p e c i f y p r o v i s i o n s f o r shutdown of a f a n when e x c e s s i v e smoke i s sensed i n s i d e a n a i r duct. To e x p l o r e t h e behavior of an air-handling system r e l a t i v e t o smoke movement, computer s t u d i e s were conducted with systems p a s s i v e o r a c t i v e , w i t h t h e supply a i r system o p e r a t e d w i t h 1004 outdoor a i r and t h e r e t u r n a i r system o p e r a t e d on t o t a l exhaust. Various c a s e s were i n v e s t i g a t e d f o r a 20-story b u i l d i n g w i t h a f i r e on t h e second f l o o r . l l The r e s u l t s from t h i s study were a s follows:

-Vertical a i r d u c t s without dampers provide a passageway f o r smoke t o migrate t o upper p a r t s of a b u i l d i n g i f t h e a i r h a n d l i n g systems a r e s h u t down when t h e f i r e occurs. --Even r e l a t i v e l y leaky dampers a r e of g r e a t b e n e f i t i n reducing smoke movement through

t h e v e r t i c a l a i r ducts.

--If t h e supply and r e t u r n a i r systems a r e i n o p e r a t i o n and i f t h e r e a r e no dampers i n t h e f i r e compartment, t h e smoke problem may be i n c r e a s e d i n t h e s t a i r w e l l s and e l e v a t o r s h a f t s , but smoke c o n c e n t r a t i o n s i n t h e upper s t o r i e s w i l l be g r e a t l y reduced.

--If t h e supply a i r system i s o f f and t h e r e t u r n a i r system i s operated t o exhaust from a l l f l o o r s , t h e smoke problem may be worse t h a n w i t h both systems on o r both systems o f f , when t h e r e i s a l a r g e opening i n t h e e x t e r i o r w a l l s of t h e f i r e f l o o r .

--If both t h e supply and r e t u r n a i r systems a r e on and t h e damper i n t h e supply a i r d u c t t o t h e f i r e compartment i s closed, smoke i s c o n f i n e d t o t h e f i r e zone a s long a s t h e e x t e r i o r w a l l s of t h e f i r e f l o o r remain i n t a c t . With a l a r g e opening i n t h e e x t e r i o r w a l l s of t h e f i r e f l o o r , however, t h i s method i s u n l i k e l y t o be e f f e c t i v e i n

preventing smoke contamination.

This study i n d i c a t e d t h a t o p e r a t i n g t h e e x i s t i n g v e n t i l a t i o n systems without a d d i t i o n a l measures does not provide adequate smoke c o n t r o l .

Computer a n a l y s e s have i n d i c a t e d t h a t t h e w o r s t smoke c o n d i t i o n c a n occur d u r i n g c o l d weather w i t h a f i r e on a lower f l o o r and w i t h l a r g e openings i n t h e e x t e r i o r w a l l s of t h e f i r e f l o o r caused by w i n d w breakage due t o t h e f i r e temperature. Fig. 6 i l l u s t r a t e s p r e s s u r e and flow p a t t e r n s caused by b u i l d i n g s t a c k a c t i o n when outdoor temperature i s s i g n i f i c a n t l y lower t h a n b u i l d i n g temperature. The r e l a t i v e p o s i t i o n s of t h e p r e s s u r e l i n e s f o r v e r t i c a l s h a f t s , f l o o r spaces, and outdoor a i r a r e i n d i c a t e d . The flow p a t t e r n , which c a n be deduced from t h e p r e s s u r e diagram, s h w s a g e n e r a l upward movement of a i r ; i n t h e event of f i r e i n a lower f l o o r , smoke w i l l migrate t o t h e upper f l o o r s through v e r t i c a l s h a f t s and openings i n t h e f l o o r c o n s t r u c t i o n . With window breakages on t h e f i r e f l o o r , t h e f l o o r p r e s s u r e approaches t h e o u t s i d e p r e s s u r e . This change i n t h e f l o o r p r e s s u r e i s i n d i c a t e d by t h e dashed l i n e i n Fig. 6 ; t h e f i r e i s assumed t o be on t h e f i r s t f l o o r . The p r e s s u r e d i f f e r e n c e a c r o s s t h e w a l l s of t h e v e r t i c a l s h a f t s on t h e f i r s t f l o o r , r e p r e s e n t e d by t h e h o r i z o n t a l d i s t a n c e between t h e f l o o r s p a c e and t h e v e r t i c a l s h a f t p r e s s u r e l i n e s , i n c r e a s e d s u b s t a n t i a l l y , which would cause a corresponding i n c r e a s e i n t h e r a t e of smoke flow from t h e f i r e r e g i o n i n t o and up t h e v e r t i c a l s h a f t s and from t h e r e i n t o t h e upper f l o o r s . This i s one of t h e c o n d i t i o n s f o r which a smoke c o n t r o l system i s r e q u i r e d t o prevent a b u i l d i n g from b e i n g contaminated w i t h smoke.

Smoke Control

The mathematical model was used t o develop v a r i o u s smoke c o n t r o l measures w i t h t h e o b j e c t i v e of m a i n t a i n i n g t h e e s c a p e r o u t e s , f i r e f i g h t e r s ' e l e v a t o r , and f l o o r s p a c e s o r r e f u g e a r e a s smoke-free. These measures were developed f o r w i n t e r c o n d i t i o n s , when b u i l d i n g s t a c k a c t i o n

i s

t h e p r i n c i p a l mechanism of smoke migration. It was asstnned t h a t t h e f i r e i s contained i n t h e f i r e compartment. The b a s i c concept of smoke c o n t r o l i s t o maintain l e s s p r e s s u r e i n t h e f i r e compartment t h a n i n t h e surrounding spaces.

A smoke s h a f t , w i t h an opening t o t h e f i r e f l o o r and a l a r g e r opening t o t h e outdoors a t t h e top, c a n reduce t h e p r e s s u r e s i n t h e f i r e compartment.12 I n w i n t e r , t h e l a r g e opening a t t h e t o p of t h e smoke s h a f t , communicating t o t h e outdoors, m a i n t a i n s p r e s s u r e s i n t h e smoke s h a f t belaw t h o s e of a d j a c e n t f l o o r s p a c e s a t a l l l e v e l s of t h e s h a f t . By r e d u c i n g t h e p r e s s u r e on t h e f i r e f l o o r r e l a t i v e t o a d j a c e n t s p a c e s , t h e smoke s h a f t can induce a i r f l o w from t h e s e s p a c e s i n t o t h e f i r e f l o o r and o u t through t h e smoke s h a f t . I n t h i s way, t h e spread of smoke i n t o v e r t i c a l s h a f t s and upper s t o r i e s i s prevented. I n summer, w i t h l i t t l e o r no s t a c k a c t i o n , a smoke s h a f t i s n o t e f f e c t i v e f o r v e n t i n g t h e compartment w i t h a low- temperature f i r e . An exhaust f a n a t t h e t o p of t h e smoke s h a f t would make i t e f f e c t i v e year- round.

Where t h e e x t e r i o r w a l l can be expected t o remain i n t a c t , a s i n t h e c a s e of a windowless b u i l d i n g , a smoke s h a f t c a n be e f f e c t i v e i n l i m i t i n g smoke t r a n s f e r t o upper s t o r i e s . I f windows on t h e f i r e f l o o r a r e broken, however, t h e p r e s s u r e i n t h e f i r e f l o o r approaches t h a t of t h e outdoors and t h e smoke s h a f t , w i t h o r w i t h o u t a n e x h a u s t f a n , i s no l o n g e r e f f e c t i v e i n preventing t h e spread of smoke. This can be c o u n t e r a c t e d by r a i s i n g t h e b u i l d i n g

p r e s s u r e s above outdoor p r e s s u r e s a t a l l l e v e l s by o p e r a t i n g t h e c e n t r a l a i r s u p p l y systems a t 100% outdoor a i r . The f i r e compartment can be vented e i t h e r w i t h a smoke s h a f t o r e x t e r i o r w a l l vents. T h i s approach i s commonly c a l l e d t h e b u i l d i n g p r e s s u r i z a t i o n method of smoke c o n t r o l . Other approaches t o p r o t e c t i n g t h e escape r o u t e s a r e p r e s s u r i z e d s t a i r w e l l s and e l e v a t o r s h a f t s and p r o t e c t e d v e s t i b u l e a c c e s s t o s t a i r s and e l e v a t o r s .

Computer a n a l y s i s of smoke movement has i n d i c a t e d t h a t v e r t i c a l s h a f t s c o n s t i t u t e t h e p r i n c i p a l p a t h of smoke s p r e a d through a building. An obvious approach t o d e a l i n g w i t h t h i s mechanism i s t o s e p a r a t e t h e v e r t i c a l s h a f t s p h y s i c a l l y from t h e main f l o o r a r e a s , e i t h e r by a n open balcony o r h e a t e d l o b b i e s t h a t c a n be h e a v i l y vented t o t h e e x t e r i o r i n t h e e v e n t of f i r e . Another approach i n t h e same c a t e g o r y i s t o d i v i d e a b u i l d i n g v e r t i c a l l y i n two, w i t h both p a r t s s e p a r a t e d s p a t i a l l y o r by a p a r t i t i o n , w i t h connunication through vented o r p r e s s u r i z e d v e s t i b u l e s .

The c o n t r o l of smoke, a p a r t from t h e p h y s i c a l arrangement of a b u i l d i n g , a s above, e n t a i l s t h e c o n t r o l of p r e s s u r e s , p a r t i c u l a r l y t h o s e i n t h e v e r t i c a l s h a f t s . It can be achieved e i t h e r by n a t u r a l v e n t i n g of s h a f t s a t t h e t o p o r t h e bottom, o r by mechanical p r e s s u r i z a t i o n of exhaust. A smoke s h a f t i s a s p e c i a l c a s e of top v e n t i n g of a v e r t i c a l s h a f t . Natural v e n t i n g of s h a f t s u t i l i z e s b u i l d i n g s t a c k a c t i o n f o r i t s o p e r a t i o n , and hence, i t i s e f f e c t i v e only d u r i n g c o l d weather. V e r t i c a l s h a f t s i n g e n e r a l c a n be prevented from t r a n s f e r r i n g smoke from lower t o upper f l o o r s i f t h e s h a f t i s vented t o t h e e x t e r i o r a t t h e bottom. The flow of outdoor a i r a t t h e bottom of t h e s h a f t causes t h e s h a f t p r e s s u r e s t o i n c r e a s e above t h o s e of t h e f l o o r space p r e s s u r e s , a s shown i n Fig. 6, where t h e v e r t i c a l

s h a f t p r e s s u r e l i n e i s moved e n t i r e l y t o t h e r i g h t of t h e f l o o r s p a c e p r e s s u r e l i n e and t h e n e u t r a l p r e s s u r e l e v e l is l o c a t e d a t t h e bottom of t h e s h a f t . On t h e o t h e r hand, i f a s h a f t i s vented t o t h e e x t e r i o r a t t h e t o p , t h e s h a f t p r e s s u r e l i n e i s moved e n t i r e l y t o t h e l e f t of t h e f l o o r space p r e s s u r e l i n e , s o t h a t t h e n e u t r a l p r e s s u r e l e v e l

is

l o c a t e d a t t h e t o p of t h e s h a f t . Smoke flowing i n t o t h e s h a f t a t low l e v e l s w i l l n o t r e t u r n t o any o t h e r f l o o r space, a s t h e vent a t t h e top w i l l c o n s t i t u t e t h e o n l y o u t l e t from t h e s h a f t .

Computer a n a l y s i s i n d i c a t e d t h a t optimum v e n t s i z e i s e s s e n t i a l l y independent of t h e outdoor temperature;13 i t i n c r e a s e s , however, w i t h t h e h e i g h t of t h e b u i l d i n g and t h e e f f e c t i v e a r e a through which leakage can t a k e p l a c e between t h e s h a f t and t h e r e s t of t h e building. I f v e n t i n g i s r e s t r i c t e d t o e i t h e r t h e t o p o r bottom l o c a t i o n s , t h e o p t i m m v e n t s i z e i n c r e a s e s w i t h t h e number of vented s h a f t s . Top v e n t i n g some s h a f t s and bottom v e n t i n g o t h e r s w i l l reduce t h e o p t i m m v e n t s i z e s f o r both. Top v e n t i n g i n c r e a s e s smoke

contamination of t h e s h a f t and cannot, t h e r e f o r e , be used f o r s h a f t s , such a s s t a i r w e l l s , i n which smoke contamination m s t be r e s t r i c t e d . Bottom v e n t i n g i n h i b i t s t h e flow of smoke i n t o

t h e s h a f t from a d j a c e n t contaminated spaces. The flow of a i r i n t o t h e s h a f t from o u t s i d e , however, tends t o lower t h e s h a f t a i r temperature, which a d v e r s e l y a f f e c t s t h e performance of t h e vent. These f a c t o r s , and t h e p r a c t i c a l r e s t r i c t i o n on v e n t s i z e s f o r h i g h b u i l d i n g s , l i m i t t h e c o n d i t i o n s under which v e n t i n g c a n be f u l l y e f f e c t i v e a s a smoke c o n t r o l measure.

Top v e n t i n g can a s s i s t i n t h e e v a c u a t i o n of smoke o r i g i n a t i n g from a f i r e i n a s h a f t . The v e n t s i z e r e q u i r e d , however, t o p r e v e n t a flow of a i r and smoke from a s h a f t a t a n e l e v a t e d temperature t o a d j a c e n t i n t e r i o r spaces i s much g r e a t e r than r e q u i r e d f o r s h a f t s a t n o n m l b u i l d i n g temperature. Because of t h e d i f f i c u l t y of p r e v e n t i n g smoke contamination of a b u i l d i n g from f i r e s i n s h a f t s , s p e c i a l p r e c a u t i o n s a r e r e q u i r e d t o minimize t h e p o s s i b i l i t y of t h e i r occurrence.

Whereas n a t u r a l v e n t i n g is e f f e c t i v e f o r smoke c o n t r o l d u r i n g cold weather, mechanical p r e s s u r i z a t i o n , o r e x h a u s t of v e r t i c a l s h a f t s , i s e f f e c t i v e y e a r r o u n d . The s t a t e m e n t s made w i t h regard t o t h e s i z e of t o p and bottom v e n t s f o r n a t u r a l v e n t i n g a l s o a p p l y t o t h e r a t e of outdoor a i r supply r e q u i r e d f o r mechanical p r e s s u r i z a t i o n of a s h a f t and t h e r a t e of

mechanical exhaust of a i r i n t h e s h a f t t o outdoors. The maximum flow of outdoor a i r f o r mechanical p r e s s u r i z a t i o n i s t h a t necessary t o c o u n t e r a c t b u i l d i n g s t a c k a c t i o n a t t h e w i n t e r design temperature. A combination of bottom v e n t i n g and mechanical p r e s s u r i z a t i o n can be used f o r t h e p r o t e c t i o n of a s t a i r w e l l from smoke contamination; t h e e x i t door a t ground l e v e l , which i s l i k e l y t o be open f o r e v a c u a t i o n , can s e r v e a s a bottom vent.

Another approach t o p r o t e c t i n g t h e v e r t i c a l s h a f t s i s t o provide p r e s s u r i z e d o r

n a t u r a l l y vented v e s t i b u l e s around s t a i r s and e l e v a t o r doors. N a t u r a l v e n t i n g i s p o s s i b l e i f a v e s t i b u l e i s a s s o c i a t e d w i t h a v e r t i c a l s h a f t a d j a c e n t t o t h e e x t e r i o r w a l l of t h e

v e s t i b u l e , and comaunicating t o t h e e x t e r i o r . With l a r g e v e n t openings, t h e p r e s s u r e s i n t h e v e s t i b u l e s a r e c l o s e t o t h o s e of outdoors a t a l l l e v e l s . This h a s t h e e f f e c t of i s o l a t i n g t h e v e r t i c a l s h a f t from t h e remainder of t h e b u i l d i n g s o t h a t i t no l o n g e r s e r v e s a s a v e h i c l e f o r t h e spread of smoke from f l o o r t o f l o o r . The v e s t i b u l e p r o t e c t e d by mechanical p r e s s u r i z a t i o n , which i s a n a l t e r n a t i v e t o n a t u r a l v e n t i n g , i s n o t r e s t r i c t e d by t h e l o c a t i o n of t h e v e r t i c a l s h a f t s . The p r e s s u r e s i n t h e v e s t i b u l e s p a c e s a r e i n c r e a s e d s o t h e d i r e c t i o n of flow i s from t h e v e s t i b u l e i n t o t h e v e r t i c a l s h a f t and f l o o r s p a c e s , and t h e s h a f t i s p r o t e c t e d from smoke contamination. P r e s s u r i z e d v e s t i b u l e s a s s o c i a t e d w i t h a s t a i r w e l l can s e r v e a s a r e a s of refuge i f they a r e l a r g e enough t o accommodate a l l occupants of a building.

Various smoke c o n t r o l measures t h a t a r e a p p l i c a b l e t o h i g h b u i l d i n g s a r e s e t o u t i n c h a p t e r 3 of The Supplement t o t h e National ~ u i i d i n ~ Code of Canada, 1 9 8 0 . ~ The requirements f o r vent s i z e s and mechanical supply a i r r a t e s were based mainly on t h e d a t a o b t a i n e d from

t h e mathematical model of a b u i l d i n g , w i t h leakage v a l u e s o b t a i n e d from t e s t s i n m u l t i s t o r y b u i l d i n g s . An overview of t h e NRC smoke c o n t r o l measures i s given i n Ref 14.

SIMPLE ANALYSIS

Whether d i s c u s s i n g smoke movement i n a n unprotected b u i l d i n g o r t h e development of smoke c o n t r o l measures, t h e flow p a t t e r n s found i n b u i l d i n g s a r e o f t e n s o cooplex t h a t a d e t a i l e d a n a l y s i s of a l l t h e f e a t u r e s on which q u a n t i t a t i v e i n f o r m a t i o n i s d e s i r a b l e u s u a l l y

n e c e s s i t a t e s a computer study. A number of r e l e v a n t p r e s s u r e and a i r f l o w p a t t e r n s can be analyzed t o a c l o s e approximation, both q u a n t i t a t i v e l y and q u a l i t a t i v e l y , without t h e use of a computer. 15

For a n open p l a n b u i l d i n g , a s shawn i n Fig. 6, t h e p r e s s u r e d i f f e r e n t i a l a c r o s s t h e v e r t i c a l s h a f t w a l l s and t h e o u t s i d e w a l l s can be c a l c u l a t e d by i n t r o d u c i n g a v a l u e f o r t h e t h e r m a l d r a f t c o e f f i c i e n t , y , which depends on t h e t i g h t n e s s of f l o o r s e p a r a t i o n r e l a t i v e t o t h a t of t h e e x t e r i o r w a l l s . It is t h e r a t i o of t h e p r e s s u r e d i f f e r e n c e a c r o s s t h e o u t s i d e w a l l s t o t h e t h e o r e t i c a l p r e s s u r e d i f f e r e n c e . For most b u i l d i n g s , t h e i n t e r i o r a i r t i g h t n e s s w i t h r e g a r d t o a i r f l o w due t o s t a c k a c t i o n i s governed by t h e a i r - l e a k a g e v a l u e s of t h e v e r t i c a l s h a f t w a l l s r a t h e r t h a n t h o s e of t h e f l o o r c o n s t r u c t i o n . Knowing t h e a i r - l e a k a g e v a l u e s f o r t h e e x t e r i o r and v e r t i c a l s h a f t w a l l s , and assuming a n a i r t i g h t f l o o r

c o n s t r u c t i o n , t h e v a l u e f o r y can be approximated by c o n s i d e r i n g s e r i e s flow a t any l e v e l from outdoors t o t h e f l o o r space and from t h e f l o o r s p a c e t o t h e s h a f t s .

where

PC = p r e s s u r e d i f f e r e n c e due t o s t a c k a c t i o n Pw = p r e s s u r e d i f f e r e n c e a c r o s s o u t s i d e w a l l s

$

= a i r l e a k a g e v a l u e f o r t h e o u t s i d e w a l l of a t y p i c a l f l o o r

As = a g g r e g a t e air-leakage v a l u e f o r a l l v e r t i c a l s h a f t w a l l s of a t y p i c a l f l o o r The e x p e r i m e n t a l l y determined v a l u e of y v a r i e d from 0.63 t o 0.82 based on measurements on s e v e r a l buildings.16 For o u t s i d e w a l l s t h a t a r e r e l a t i v e l y a i r t i g h t , t h e v a l u e of y i s a l s o a f f e c t e d by t h e o p e r a t i o n of t h e a i r - h a n d l i n g system. With t h e a i r - h a n d l i n g system s h u t down, a s may be t h e c a s e i n t h e e v e n t of a f i r e , t h e a i r d i s t r i b u t i o n d u c t s a c t a s

i n t e r c o n n e c t i o n s between f l o o r s , which lowers t h e i n t e r i o r a i r t i g h t n e s s and r e s u l t s i n a g r e a t e r v a l u e of y t h a n w i t h t h e a i r h a n d l i n g system o p e r a t i n g . T h i s e f f e c t would be m c h g r e a t e r f o r b u i l d i n g s w i t h c e n t r a l WAC s y s t e m , compared t o b u i l d i n g s w i t h systems

compartmented on e a c h f l o o r .

Knowing o r assuming a v a l u e f o r y , t h e v a r i o u s p r e s s u r e d i f f e r e n c e s caused by s t a c k a c t i o n can be c a l c u l a t e d a t a n y ' l e v e l i n a b u i l d i n g (Fig. 6 ) .

P r e s s u r e d i f f e r e n c e a c r o s s o u t s i d e w a l l s :

P r e s s u r e d i f f e r e n c e a c r o s s v e r t i c a l s h a f t w a l l s :

P r e s s u r e d i f f e r e n c e a c r o s s f l o o r c o n s t r u c t i o n of f l o o r i:

The l o c a t i o n of t h e n e u t r a l p r e s s u r e l e v e l , r e q u i r e d f o r c a l c u l a t i n g PC of Eq 2 , can be determined f o r e x t e r i o r w a l l s and s h a f t s w i t h uniform l e a k a g e from t h e f o l l o w i n g e q u a t i o n :

where

h = n e u t r a l p r e s s u r e l e v e l measured from t h e bottom

H

= h e i g h t of v e r t i c a l s h a f t o r b u i l d i n g Ti a b s o l u t e i n s i d e a i r temperature To a a b s o l u t e o u t s i d e a i r t e m p e r a t u r e

I n Eq 9 , Ti > Tot If Ti < To, t h e r a t i o Ti/To i s i n v e r t e d .

For an e n c l o s u r e with two unequal openings w i t h known a i r f l o w c h a r a c t e r i s t i c s , t h e l o c a t i o n of t h e n e u t r a l p r e s s u r e l e v e l i s g i v e n by t h e f o l l o w i n g equation:

where

h = n e u t r a l p r e s s u r e l e v e l measured from t h e lower opening

A

= v e r t i c a l h e i g h t between openings A, = lower opening

%

= upper opening

I n Eq 10, Ti > To. I f Ti < To, t h e r a t i o Ti/To i s i n v e r t e d .

The s t e a d y - s t a t e c o n c e n t r a t i o n , C, t o be expected i n t h e upper h a l f of a simple h i g h b u i l d i n g , a s a r e s u l t of a f i r e a t a low l e v e l and of s t a c k a c t i o n a s s o c i a t e d w i t h b u i l d i n g h e a t i n g , is given t o an approximation by

where

Cf = t h e s t e a d y - s t a t e smoke c o n c e n t r a t i o n on t h e f i r e f l o o r N = t h e number of s t o r i e s i n t h e b u i l d i n g

n = t h e number of s i m i l a r compartments i n t o which t h e f i r e f l o o r i s d i v i d e d

The e x p r e s s i o n r e l a t e s t o t h e s i t u a t i o n where a l l windows a r e i n t a c t and t h e doors t o a l l compartments on t h e f i r e f l o o r a r e closed. V i s i b i l i t y and carbon monoxide c o n s i d e r a t i o n s suggest t h a t smoke c o n c e n t r a t i o n s more t h a n one p e r c e n t of t h o s e i n t h e f i r e a r e a a r e l i k e l y t o be unacceptable.

Adopting t h e one percent c r i t e r i o n , t h e e x p r e s s i o n s u g g e s t s t h a t , under t h e c o n d i t i o n s s p e c i f i e d , s e r i o u s smoke c o n d i t i o n s should n o t a r i s e i n t h e upper s t o r i e s of a high b u i l d i n g provided n.N

.

300. Thus, f o r an apartment b u i l d i n g having t e n apartments p e r f l o o r , upper f l o o r s would remain t e n a b l e i n t h e e v e n t of f i r e on a lower f l o o r , provided t h e b u i l d i n g were h i g h e r than 30 s t o r i e s . F r a c t u r e of windows i n t h e f i r e a r e a (when t h i s i s a t a low l e v e l i n a b u i l d i n g ) w i l l , i n g e n e r a l , i n c r e a s e smoke d e n s i t i e s i n t h e upper l e v e l s by a s u b s t a n t i a l f a c t o r , a s a r e s u l t of s t a c k a c t i o n . I f t h e door t o t h e compartment involved i n f i r e i s open, t h e n t h e advantage of coupartmentation, a t l e a s t a s r e g a r d s i t s i n f l u e n c e on smoke l e v e l s i n remote p a r t s of t h e b u i l d i n g , i s n u l l i f i e d . Providing automatic c l o s u r e t o t h e room doors can g r e a t l y reduce t h e p o s s i b i l i t y of a n i n t o l e r a b l e l e v e l of smoke i n t h e c o r r i d o r .

So f a r , p r e s s u r e d i f f e r e n c e s were considered i n terms of g a s flow r a t e and flow d i r e c t i o n . Another c o n s i d e r a t i o n i s t h e f o r c e imposed on t h e s t a i r doors by t h e s t a c k p r e s s u r e d i f f e r e n c e s , which can i n t e r f e r e w i t h t h e i r o p e r a t i o n d u r i n g e v a c u a t i o n . The l i m i t i n g p r e s s u r e d i f f e r e n c e may be t a k e n t o be 0.40 i n c h of w a t e r (LOO Pa) based on t h e r e s u l t s of door t e s t s conducted w i t h handicapped people.17 The maximum s t a c k p r e s s u r e d i f f e r e n c e s a c r o s s a s t a i r door, which o c c u r a t t h e bottom and top f l o o r s , w i t h a l a r g e opening i n t h e e x t e r i o r w a l l s caused by a f i r e on t h e s e f l o o r s , can be c a l c u l a t e d from Eq 2.

A s f o r t h e b u i l d i n g p r e s s u r i z a t i o n method, w i t h an a c t i v e smoke c o n t r o l system, t h e n e u t r a l p r e s s u r e l e v e l of t h e b u i l d i n g i s lawered t o t h e ground f l o o r l e v e l . The maximm p r e s s u r e d i f f e r e n c e occurs on t h e t o p f l o o r only and i s twice t h e v a l u e i n t h e previous case.

Both c a s e s a r e p l o t t e d i n Fig. 7 , which g i v e s t h e b u i l d i n g h e i g h t a t which t h e maximum p r e s s u r e d i f f e r e n c e s a r e 0.40 i n c h of w a t e r (100 Pa) f o r a g i v e n outdoor temperature. For b u i l d i n g s w i t h e x t e r i o r w a l l s i n t a c t and w i t h a thermal d r a f t c o e f f i c i e n t of 0.80, t h e h e i g h t o f t h e b u i l d i n g w i t h no a c t i v e smoke c o n t r o l would be f i v e times t h a t g i v e n i n Fig. 7. With a c t i v e smoke c o n t r o l t h e h e i g h t would depend upon t h e type of system used. Where a p r e s s u r e d i f f e r e n c e exceeding 0.40 i n c h of w a t e r (100 Pa) i s a n t i c i p a t e d , a n o f f s e t hinge door o r a power-assisted door can be used. Another a l t e r n a t i v e is t o provide a f u l l s p r i n k l e r system t o t h e b u i l d i n g , s o t h a t t h e p r o b a b i l i t y of window breakage i s minimized.

The amount of a i r r e q u i r e d t o d i l u t e a contaminated atmosphere t o a t e n a b l e l e v e l a t a g i v e n time c a n be c a l c u l a t e d approximately. Assuming p e r f e c t mixing of t h e c o n t a d n a t e d and d i l u e n t a i r :

where

C = f i n a l c o n c e n t r a t i o n of contaminant Co = i n i t i a l c o n c e n t r a t i o n of contaminant

e = 2.718

a r a t e of d i l u e n t a i r flow ( a i r changes p e r u n i t time) t = time between i n i t i a l and f i n a l c o n c e n t r a t i o n

I f the l e v e l of contamination i n a compartment i s e q u i v a l e n t t o t h a t of t h e f i r e compartment, and no more smoke i s e n t e r i n g , t h e amount of c l e a n a i r needed t o c r e a t e t h e one percent t e n a b l e atmosphere would be f i v e times t h e volume of t h e compartment. I f c l e a n a i r i s s u p p l i e d a t t h e r a t e of one volume every two minutes, t h e atmosphere i n t h e c o q a r t m e n t would be a c c e p t a b l e i n about t e n minutes.

FIELD

TESTS

F i e l d study c o n s t i t u t e s a necessary a r e a of r e s e a r c h i n a s s e s s i n g smoke hazards i n m u l t i s t o r y b u i l d i n g s and i n developing smoke c o n t r o l measures. It n o t only couplements t h e work

conducted with mathematical models, but h a s proven e s s e n t i a l i n improving t h e models i n some a s p e c t s previously omitted o r underestimated.

F i e l d t e s t s were i n i t i a l l y conducted t o develop d a t a on t h e air-leakage c h a r a c t e r i s t i c s of b u i l d i n g elements and t o o b t a i n t h e p r e s s u r e p a t t e r n caused by s t a c k a c t i o n f o r a i r i n f i l t r a t i o n and smoke movement s t u d i e s . Subsequent s t u d i e s sought t o determine t h e flow r e s i s t a n c e s of s t a i r w e l l s and t o a s s e s s t h e performance of v a r i o u s smoke c o n t r o l techniques and t h e i n s t a l l e d smoke c o n t r o l system. Another important a r e a of r e s e a r c h i n v o l v i n g f i e l d s t u d i e s conducted by DBRINRC i s on t r a d i t i o n a l t o t a l and s e l e c t e d s e q u e n t i a l e v a c u a t i o n s of b u i l d i n g s . l s 2 T h i s a s p e c t of f i r e s t u d i e s a f f e c t s t h e d e s i g n of smoke c o n t r o l systems, because opening s t a i r doors by occupants during evacuation and use of s t a i r s and e l e v a t o r s by f i r e f i g h t e r s can change t h e a i r and smoke flow p a t t e r n i n a building.

The parameters of Eq 1 t h a t were measured were p r e s s u r e d i f f e r e n c e s a c r o s s b u i l d i n g elements, and a i r v e l o c i t i e s a t i n t a k e and exhaust g r i l l e s , i n s i d e ductwork f o r supply and exhaust of a i r , and a t open doors and e x t e r i o r w a l l vents. The p r e s s u r e d i f f e r e n c e l a i r v e l o c i t y method i s simple, s a f e and inexpensive; however, i t does n o t t a k e i n t o account t h e e f f e c t of f i r e temperature and i t does not provide v i s u a l o b s e r v a t i o n of t h e movement of smoke. Chemical smoke was used on a few occasions t o demonstrate t h e e f f e c t i v e n e s s of a smoke c o n t r o l system.

P r o t e c t i o n of S t a i r w e l l s

I n i t i a l l y , t e s t s on t h e s t a i r w e l l s were conducted t o determine the e f f e c t i v e n e s s of n a t u r a l v e n t i n g a t t h e bottom. Bottom v e n t i n g of s t a i r w e l l s can be r e a d i l y achieved by opening a s t a i r w e l l door l e a d i n d i r e c t 1 outdoors a t grade l e v e l . A t y p i c a l s t a i r door r e p r e s e n t s a vent a r e a of 20 f

b

(1.86

f

).

Measurements were conducted i n w i n t e r on a ten-story b u i l d i n g with a heated s t a i r w e l l a d j a c e n t t o t h e e x t e r i o r wa11.13 With a l l doors closed, a i r flawed i n t o t h e s t a i r w e l l from t h e f l o o r s below t h e n e u t r a l p r e s s u r e l e v e l a t t h e e i g h t h f l o o r , and o u t of t h e s h a f t above t h i s l e v e l . With t h e e x i t door open and a l l o t h e r doors c l o s e d , p r e s s u r e measurements a c r o s s t h e s t a i r w e l l doors i n d i c a t e d t h a t a i r flowed from t h e s t a i r w e l l t o a l l f l o o r s . S i m i l a r measurements on a 38-story b u i l d i n g , a l s o w i t h a h e a t e d s t a i r w e l l a d j a c e n t t o t h e e x t e r i o r w a l l , i n d i c a t e d t h a t , with a l l doors c l o s e d , the n e u t r a l p r e s s u r e l e v e l was l o c a t e d on t h e 35th f l o o r , w i t h a i r f l o w i n t o t h e s t a i r w e l l below i t and o u t from t h e s t a i w e l l above i t . 1 3 When the e x i t door was opened, a i r flowed o u t of t h e s h a f t from t h e second t o t h e 20th s t o r y

l e v e l s and from t h e 34th t o t h e 38th s t o r y . Air flowed i n t o t h e s h a f t from t h e 2 1 s t t o t h e 33rd s t o r y , although t h e r a t e s of a i r i n f l o w were decreased, a s i n d i c a t e d by t h e r e d u c t i o n i n t h e adverse p r e s s u r e d i f f e r e n c e s .

These t e s t s confirmed t h e r e s u l t s obtained from computer a n a l y s i s t h a t when t h e a i r temperature o u t s i d e a b u i l d i n g i s belaw t h a t i n s i d e , v e n t i n g a s h a f t a t t h e bottom r e s u l t s i n a flow of o u t s i d e a i r i n t o t h e s h a f t . This causes s h a f t p r e s s u r e s t o i n c r e a s e r e l a t i v e t o t h o s e i n a d j a c e n t s p a c e s and t h u s p r e v e n t s o r d e c r e a s e s t h e flow of smoke from t h e s e spaces

i n t o t h e s h a f t . Bottom v e n t i n g can be a p p l i e d d u r i n g a f i r e t o p r o t e c t t h e s t a i r w e l l by simply opening t h e s t a i r door a t e x i t l e v e l t o connect t h e s t a i r w e l l t o e x t e r i o r pressures. I n t h i s c o n t e x t , a s t a i r w e l l t h a t e x i t s d i r e c t l y t o o u t s i d e a t grade l e v e l , r a t h e r t h a n i n t o t h e ground f l o o r lobby, is p r e f e r r e d , p a r t i c u l a r l y f o r a f i r e on t h e ground f l o o r o r t h e f l o o r below grade.

Mechanical p r e s s u r i z a t i o n of s t a i r w e l l s does n o t r e l y on b u i l d i n g s t a c k a c t i o n f o r i t s o p e r a t i o n , and i t i s e f f e c t i v e a t a l l times of t h e year. The r a t e of a i r supply r e q u i r e d t o p r e s s u r i z e a s t a i r w e l l t o a d e s i r e d l e v e l depends upon t h e a i r t i g h t n e s s of t h e s h a f t

e n c l o s u r e and on t h e number of s t a i r doors designated t o be open. It was found from t e s t s on t h e p r e s s u r i z e d s t a i r w e l l s of a 23- and a 37-story b u i l d i n g , w i t h bottom and t o p a i r

i n j e c t i o n , r e s p e c t i v e l y , t h a t t h e r e were s u b s t a n t i a l p r e s s u r e l o s s e s i n s i d e t h e s t a i w e l l caused by t h e flow r e s i s t a n c e of t h e winding s t a i r c a s e . 1 8 This caused a nonuniform p r e s s u r i z a t i o n of t h e s t a i r w e l l , w i t h t h e g r e a t e s t p r e s s u r i z a t i o n n e a r t h e p o i n t of a i r i n j e c t i o n and t h e l e a s t a t t h e f a r t h e s t p o i n t from it. T e s t s on s t a i r w e l l s of e i g h t m l t i s t o r p b u i l d i n g s i n d i c a t e d t h a t t h e flow r e s i s t a n c e s of conventional s t a i r w e l l s were about 1150 times g r e a t e r than those of a i r ducts.8

The flow r e s i s t a n c e of a s t a i r w e l l can a l s o be r e p r e s e n t e d by an o r i f i c e a t each f l o o r l e v e l , assuming no r e s i s t a n c e between f l o o r s . The s i z e of t h e o r i f i c e can be c a l c u l a t e d from t h e p r e s s u r e l o s s c o e f f i c i e n t by t h e following equation: where = o r i f i c e a r e a = c r o s s - s e c t i o n a l a r e a of a s h a f t Cd = c o e f f i c i e n t of discharge of an o r i f i c e K = p r e s s u r e l o s s c o e f f i c i e n t L 5 h e i g h t of s h a f t per f l o o r De = e q u i v a l e n t diameter

The c a l c u l a t e d values of t h e o r i f i c e s i z e from t h e measured values of p r e s s u r e l o s s c o e f f i c i e n t v a r i e d from 24% t o 32% of t h e c r o s s - s e c t i o n a l a r e a of t h e s h a f t . The i n t e r n a l flow r e s i s t a n c e of a s t a i r w e l l , because i t i s s u b s t a n t i a l , mst be taken i n t o account i n d e s i g n i n g t h e p r e s s u r i z a t i o n system. These s t u d i e s l e d t o t h e i n c o r p o r a t i o n of t h i s f a c t o r i n terms of o r i f i c e a r e a i n t o t h e computer program f o r c a l c u l a t i n g smoke c o n c e n t r a t i o n s i n m l t i s t o r y b u i l d i ~ i g s . ~ Using t h e modified computer program, t h e design of a s t a i r w e l l p r e s s u r i z a t i o n system was i n v e s t i g a t e d , with t h e e x i t door a t grade l e v e l assumed t o be l e f t open f o r evacuation and bottom venting.19 The main conclusion from t h i s study was t h a t m u l t i p l e i n j e c t i o n , r a t h e r than s i n g l e i n j e c t i o n a t e i t h e r t o p o r bottom, i s r e q u i r e d f o r providing a uniform p r e s s u r i z a t i o n i n t h e s t a i r w e l l . T h i s w i l l avoid e x c e s s i v e p r e s s u r e s on t h e s t a i r doors and ensure a uniform supply of a i r f o r d i l u t i o n of smoke t h a t may have e n t e r e d t h e s t a i w e l l w i t h u s e of s t a i r doors d u r i n g a f i r e . It was a l s o concluded t h a t venting t h e f i r e f l o o r , t o g e t h e r w i t h o p e r a t i n g t h e p r e s s u r i z a t i o n system, can g r e a t l y reduce t h e p o s s i b i l i t y of smoke m i g r a t i n g i n t o t h e s t a i r w e l l .

It is p r e f e r a b l e t o l o c a t e t h e f a n near t h e bottom, r a t h e r than a t t h e t o p , because s t a c k a c t i o n i n w i n t e r s i m p l i f i e s s t a r t - u p of t h e f a n and b r i n g s outdoor a i r i n t o t h e s t a i r w e l l through t h e f a n and duct system i n t h e event of f a n f a i l u r e . Also, because of f r i c t i o n a l r e s i s t a n c e i n t h e v e r t i c a l supply d u c t , more a i r i s l i k e l y t o be d e l i v e r e d t o t h e s t a i r w e l l a t low l e v e l s than a t high l e v e l s .

Venting of F i r e F l o o r

Venting of a f i r e compartment h e l p s t o reduce p r e s s u r e s t h e r e t o minimize o r even e l i m i n a t e t h e flow of smoke from t h e f i r e r e g i o n t o a d j a c e n t a r e a s . It a s s i s t s f i r e f i g h t e r s and h e l p s t o p r o t e c t occupants by i n h i b i t i n g t h e spread of smoke and t o x i c g a s e s from t h e f l o o r of o r i g i n t o o t h e r f l o o r s . One means of v e n t i n g t h e f i r e r e g i o n i s t o provide openable panels o r windows i n the e x t e r i o r w a l l s ; o t h e r s a r e mechanical venting and smoke s h a f t s . The former method permits d i r e c t v e n t i n g of smoke from t h e f i r e f l o o r t o t h e e x t e r i o r ; t h e l a t t e r methods allow smoke from t h e f i r e f l o o r t o flow i n t o and o u t of t h e exhaust s h a f t above t h e roof of a building.