Considerations in the design of smoke management systems for atriums

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Considerations in the design of smoke management systems for

atriums

Lougheed, G. D.

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C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 4 8

En gin eered sm oke m an agem en t system s often p lay a role in exten d in g th e u se of an atriu m sp ace by d em on stratin g th at th e basic

requ irem en ts of th e ap p licable bu ild in g an d fire cod es can be m et or in p rovid in g ad d i-tion al p roteci-tion for occu p an ts an d p rop erty. Typ ically, th is in volves th e u se of m ech an ical exh au st system s to lim it th e accu m u lation of sm oke in th e atriu m an d its m igration in to evacu ation rou tes an d com m u n icatin g sp aces.2

Som e m od el cod es in th e Un ited States an d oth er cou n tries h ave in trod u ced requ irem en ts for sm oke m an agem en t system s based on th e d esign criteria fou n d in en gin eerin g d esign gu id es su ch as NFPA 92B (p u blish ed by th e Nation al Fire Protection Association ), “Gu id e for Sm oke Man agem en t System s in Malls, Atria an d Large Areas.” Th ese gu id es in clu d e atriu m sm oke m an agem en t con sid eration s an d d esign criteria.[1,2,3]

Atriu m sm oke m an agem en t d esign is based on th e p rin cip les u sed in m u lti-zon e fire m od els: Th e fire p rod u ces h ot gases, w h ich rise above th e fire, form in g a p lu m e. As th is p lu m e rises, it en train s air, in creasin g its diameter and mass flow rate (rate of smoke bu ild -u p ) w ith elevation — an exp an d in g con e. At th e ceilin g, th e h ot gases form a layer of sm oke (see Figu re 1). As sm oke accu m u lates, th e layer th icken s an d

d escen d s, an d can u ltim ately fill th e atriu m (resu ltin g in red u ced visibility an d in creased d istribu tion of gases to oth er areas of th e

b y G.D. Loughe e d

This Update discusses the use of an engineered approach to the design of smoke

management systems for atrium buildings, based on the principles of smoke

management in atriums outlined in Construction Technology Update No. 47.

Considerations in the Design

of Smoke Management

Systems for Atriums

2

bu ild in g). Th e p lu m e, th e sm oke layer, an d th e am bien t or cold layer of air n ot en train ed by th e risin g p lu m e con stitu te th ree sep arate zon es w ith in th e room (see Figu re 1).

Equ ation s based on exp erim en tal d ata h ave been d evelop ed for estim atin g th e p rop erties of th e sm oke p lu m e, th e th ickn ess of th e sm oke layer an d its average p rop erties su ch as tem p eratu re, gas con cen tration s an d sm oke op tical d en sity. Th e balan ce of m ass an d en ergy am on g th e th ree zon es in th e atriu m is also reflected in th e d esign equ ation s. Th ese em p irical equ ation s form th e basis for sm oke m an agem en t system d esign u sin g th e en gin eerin g d esign gu id es.[1,2,3]

De sign Proc e ss

Th e basic step s in d esign in g an atriu m sm oke exh au st system are as follow s: 1. Design criteria. Determ in e th e sp ecific

d esign objectives th at m u st be m et by th e sm oke m an agem en t system an d d evelop su itable criteria.

2. Design fire. Determ in e th e size an d location of th e fire(s) for u se in th e calcu lation of sm oke p rod u ction . 3. Mechanical exhaust considerations.

Determ in e w h eth er th e d esign criteria can be m et by allow in g sm oke to fill th e atriu m sp ace w ith ou t p rovision for sm oke exh au st. If th is is n ot th e case, calcu late the requirements of the mechanical exhaust system th at can m eet th e d esign criteria.

The various approaches used in codes and en gin eerin g gu id es for each of th ese step s are su m m arized in th e follow in g section s.

De sign Crit e ria

En gin eered atriu m sm oke m an agem en t system s are typ ically d esign ed to m eet on e of th e objectives of th e NBC, w h ich is to p rotect h u m an life. For d esign p u rp oses, th e area of th e bu ild in g th at m u st be kep t smoke free is established, usually by assuming th at th e base of th e sm oke layer sh ou ld rem ain above a p re-d eterm in ed h eigh t (i.e, th e d esign h eigh t, w h ich is m easu red from th e floor of th e atriu m ) for a sp ecified p eriod of tim e. Tw o exam p les of d esign criteria for m eetin g th is objective are as follow s: • Th e sm oke m an agem en t system m u st be

able to m ain tain th e base of th e sm oke layer above th e h igh est u n p rotected op en in g to ad join in g sp aces, or

• Th e sm oke m an agem en t system m u st be able to maintain the base of the smoke layer at 1.8-3 m above th e h igh est floor level in the atrium used for exit purposes for at least 20 m in u tes follow in g ign ition (Figu re 2).

An oth er p ossible d esign ap p roach is to en su re th at bu ild in g occu p an ts are n ot su b-jected to u n ten able con d ition s. How ever, m ost en gin eers are relu ctan t to d esign system s th at cou ld exp ose occu p an ts to an y sm oke at all, even if th at exp osu re is n ot leth al. En gin eerin g gu id es su ch as NFPA 92B in clu d e m eth od s for calcu latin g sm oke p aram eters, w h ich can be u sed in a h azard an alysis for th e d esign of a system based on ten ability criteria.[1]

De sign Fire (Size a nd Loc a t ion) Th e d esign fire selected for sm oke m an agem en t d esign calcu lation s agem u st be rep resen -tative of th e m ost realistic, or exp ected , fire th at is likely to occu r in th e atriu m based on the specific design of the building and its use, w h ich togeth er d eterm in e th e m aterials u sed to con stru ct th e bu ild in g an d th ose con tain ed in it. Th e d egree of in tercon n ection or openness between the atrium, or atriums, and th e ad jacen t sp aces is also a critical factor in th e d esign of th e system . An op en or p artially op en atriu m requ ires m ore com p lex fire p rotection th an a closed atriu m to lim it sm oke m ovem en t an d p rotect evacu ation rou tes — th e greater th e in tercon n ected n ess, th e greater th e p ossibility of sm oke sp read . An u n d erlyin g assu m p tion in th e selec-tion of a d esign fire, or fire scen ario, is th at it is su fficien tly large, w ith little p robability of a larger fire occu rrin g, th u s en su rin g an accep table level of safety as stip u lated by th e ap p licable bu ild in g cod e or au th ority h avin g ju risd iction .

Figure 2. The smoke management system must maintain the base of the smoke layer above the design height.

Fire Size

Th e size of th e d esign fire d ep en d s on th e exp ected am ou n t of com bu stible m aterial (fu el load in g) in th e atriu m , w h ich in tu rn d ep en d s on th e occu p an cy — a com m ercial occu p an cy, for exam p le, is assu m ed to h ave a h eavier fu el load in g p er u n it floor area th an an office bu ild in g. (Som e exam p les of d esign fires [4] are sh ow n in Table 1, p . 4.) In gen eral, th e size an d rate of grow th of d esign fires are based on th e an alysis of fire statistics for a sp ecific typ e of occu p an cy or from exp erim en ts on th e com bu stible m ate-rials (fu els) typ ically fou n d in th e occu p an cy. Th e location of th e fire in th e atriu m also h as a sign ifican t im p act on fire size an d rate of grow th (see “Fire Location ” below ).

Th e m ajor fire p aram e-ters to be con sid ered are th e h eat release rate an d th e resu ltin g sm oke m ass flow rate. Over th e years, variou s d esign fires rep re-sen tin g d ifferen t fire scen arios, w ith sp ecified h eat release rates, h ave been d evelop ed . Sm oke m ass flow rates are based on estim ated h eat release rates for u se in th e d esign of a sm oke exh au st system .[1,2,3] For com p lex atriu m d esign s, n u m erical fire m od els m ay be requ ired to assess th e ability of th e sm oke m an agem en t system to m eet d esign objec-tives (see “Nu m erical Mod ellin g for Sm oke Man agem en t Design ” below ).[5]

Fire Location

Th ere are tw o m ain location s for d esign fires:

• on th e atriu m floor

• in a com m u n icatin g sp ace.

Algebraic equ ation s for estim atin g th e sm oke m ass flow for each of th ese location s are p rovid ed in th e d esign gu id es.[1,2,3]

Wh en th e fire is on th e atriu m floor, th e p lu m e of h ot gases rises to th e ceilin g of th e atriu m , con tin u ally en train in g air from th e su rrou n d in gs, th u s in creasin g th e volu m e of sm oke. In h igh atriu m s, th e am ou n t of air en train ed in th e p lu m e can be sign ifican t an d can h ave a m ajor im p act on th e requ ire-m en ts of th e sire-m oke ven tin g systeire-m .

In p rin cip le, an atriu m fire cou ld be located an yw h ere on th e floor — n ear a w all or in a corn er. In both cases, th e ad ja-cen t w alls lim it air en train m en t in to th e p lu m e. For m ost d esign ap p lication s, it is

gen erally assu m ed th at th e fire w ill be in th e cen tre of th e atriu m w h ere th e sm oke p lu m e w ill be sym m etrical abou t th e cen tral axis an d w ill en train air on all sid es, p ro-d u cin g th e greatest volu m e of sm oke — th e w orst-case scen ario (see Figu re 1).

Algebraic equ ation s are also available for estim atin g th e sm oke m ass flow rates for fires located in com m u n icatin g sp aces. Th is in clu d es sm oke flow th rou gh an op en in g, su ch as a w in d ow, located in th e w all of an atriu m . It also in clu d es sm oke flow from a con n ectin g sp ace th at accu m u lates u n d er a balcon y before en terin g th e atriu m . Th e latter situ ation is kn ow n as a balcon y sp ill p lu m e, w h ich can resu lt in con sid erable air en train m en t in to th e sm oke flow, lead in g to h igh sm oke p rod u ction even for sm all fires. De sign Approa c he s Ba se d on Fire Size a nd Grow t h

On e of th e m ost com m on ly u sed ap p roach es to th e d esign of atriu m sm oke m an agem en t system s assu m es a stead y-state d esign fire (Table 1, p . 4) — i.e., th e fire is alw ays th e sam e (largest p red icted ) size. A system d esign ed to d eal w ith th is fire w ill also be able to h an d le th e fire as it grow s.[1,2,3] Stead y-state d esign fires, w ith th eir sim p le algebraic equ ation s, are relatively easy to u se to estim ate sm oke p aram eters, in clu d in g sm oke m ass flow rate based on th e size of th e fire (h eat release rate). How ever, becau se th is ap p roach assu m es th e largest p ossible fire from th e tim e of ign ition , th ere is a large factor of safety resu ltin g in a h igh er cost.

Maxim u m h eat release rates for som e p rod u cts are given in NFPA 92B.[1] For exam p le, th e p eak h eat release rate from a p lastic trash bag filled w ith p ap er is 120-350 kW an d from a sm all (6.5-7.4 kg) d ry Ch ristm as tree it is 500-650 kW.

A secon d ap p roach , w h ich u ses a fire th at grow s over tim e to m od el actu al grow th , can resu lt in d esign criteria th at are less on erou s. Frequ en tly, th e fire grow th is assu m ed to be p rop ortion al to tim e squ ared (t2_{). Th ere are fou r basic classification s for}

fire d evelop m en t, or grow th rate: slow, m ed iu m , fast an d u ltra-fast. How ever, as in th e case of fire size, fire d evelop m en t is d ep en d en t on a n u m ber of factors; th u s, th ere is a ran ge of p ossible fire grow th rates. Th e d esign er selects th e d esign fire grow th rate th at m ost closely rep resen ts w h at is likely to occu r for th e occu p an cy an d exp ected fu el load s, an d d esign s th e system to p rovid e ad equ ate p erform an ce.

Heat release rateis a m ea-su re of th e size of th e fire. Th e h igh er th e h eat release rate, th e bigger th e fire.

Smoke mass flow rate is th e rate at w h ich th e vol-u m e/ m ass of th e sm oke increases in the smoke layer (the rate of smoke build-up).

Smoke Management w ith Mechanical Exhaust

Wh en it is d eem ed th at th ere is in su fficien t tim e for occu p an ts to resp on d an d evacu ate before th e sm oke layer d escen d s below th e d esign h eigh t, a m ech an ical exh au st system can be u sed .

Typ ically, th e m ech an ical exh au st system is d esign ed to m ain tain th e sm oke above th e d esign h eigh t for a sp ecified d esign fire. By assu m in g a fire size an d a h eigh t for th e sm oke layer, algebraic equ ation s p rovid ed in th e en gin eerin g d esign gu id es can be u sed to estim ate th e m ass flow rate of sm oke in to th e sm oke layer above th e d esign h eigh t.[1] In ord er to m ain tain th e sm oke layer at or above th e d esign h eigh t, th e fan for th e sm oke exh au st system m u st h ave th e cap acity to rem ove a su fficien t qu an tity of sm oke from th e sm oke layer (see Figu re 3). Th is m ean s th at system d esign ers m u st carefu lly con sid er p h en om en a th at m igh t com p rom ise th e effectiven ess of th e sm oke exh au st system , su ch as p lu gh olin g an d ceilin g jets.

Plugholing. Th e p ossibility of fresh air from below th e sm oke layer bein g p u lled in to th e exh au st in lets m u st be con sid ered for d esign s in w h ich th e d istan ce betw een th e d esign h eigh t an d th e exh au st in lets is lim ited . Th is p h en om en on in w h ich th e exh au st is m ad e u p n ot on ly of sm oke bu t also of air from in sid e th e atriu m is kn ow n as p lu gh olin g. With p lu gh olin g, som e of th e cap acity of th e exh au st system is

exp en d ed in rem ovin g air rath er th an sm oke. Th is d im in ish ed efficien cy of th e system n eed s to be con sid ered w h en d eterm in in g th e d esign h eigh t an d th e fan cap acity (see Figu re 4).

In a recen tly com p leted ASHRAEsp on -sored research p roject, IRC fire research ers u sed fu llscale p h ysical m od el stu d ies com -bin ed w ith n u m erical m od ellin g to in vesti-gate th is issu e.[6,7,8] Th ey fou n d th at a d esign ap p roach sim ilar to on e u sed in th e Un ited Kin gd om to p reven t p lu gh olin g in gravity ven tin g system s (op en ven ts, n o fan s) cou ld also be ap p lied to an atriu m m ech an ical sm oke exh au st system . Design m eth od s for ad d ressin g p lu gh olin g are p ro-vid ed in th e 2000 ed ition of NFPA 92B.[1] Th e m ain ap p roach u sed to m in im ize p lu gh olin g in volves p rovid in g m u ltip le in lets for th e exh au st system in ord er to red u ce th e am ou n t of exh au st th at h as to p ass th rou gh a sin gle in let.

4 Construction Technology Update No. 48

In th is ap p roach , th e m axim u m fire size m u st be stated , th e assu m p tion bein g th at th e fire w ill on ly grow to a certain size becau se sp rin klers are p resen t to con trol th e fire.

A th ird ap p roach for d eterm in in g th e ap p rop riate d esign fire u ses in form ation obtain ed from fu ll-scale fire tests in w h ich th e fu el load corresp on d in g to a sp ecific occu p an cy h as been recreated . Th ese d ata can be u sed by d esign ers to p red ict th e con sequ en ces of th e sam e fire (fu el load ) in bu ild in gs of d ifferen t geom etries. Th is ap p roach is u sefu l w h en th e an ticip ated fu el load is exp ected to be very sim ilar to th at u sed in th e test arran gem en t.

At p resen t, real fire test d ata are n ot read ily available for u se in th e d esign of sm oke m an agem en t system s. Existin g d atabases are qu ite lim ited , alth ou gh th ere are som e d ata in d esign gu id es su ch as NFPA 92B, w h ich is p robably th e m ost w id ely u sed tool.[1] Resu lts for both sp rin -klered an d n on -sp rin -klered fire scen arios can be fou n d in th e literatu re.

De t e rm ining t he N e e d for M e c ha nic a l Ex ha ust

Wh en th e qu an tity an d rate of sm oke p ro-d u ction h ave been ro-d eterm in ero-d u sin g ro-d esign equ ation s as d escribed above, a d ecision can be m ad e abou t w h eth er a m ech an ical exh au st system is requ ired .

Smoke Management w ithout Mechanical Exhaust

En gin eerin g equ ation s can be u sed to estim ate th e rate at w h ich sm oke w ill fill an atriu m for a given d esign fire.[1] Som e atriu m s h ave a large en ou gh volu m e to allow sm oke to accu m u late above th e d esign h eigh t for a sp ecified p eriod of tim e, p erm ittin g occu p an ts to evacu ate safely. If occu p an ts can resp on d an d evacu ate before th e atriu m fills w ith sm oke, fu rth er sm oke m an agem en t m easu res to lim it sm oke accu m u lation m ay n ot be requ ired . An atriu m th at is on ly p artially op en to th e ad join in g bu ild in g, for exam p le, m ay n ot requ ire a m ech an ical exh au st system becau se th e sm oke can be con tain ed above th e op en in g (see Figu re 2).

Table 1. Steady-state design fires for atriums.[4]

Design fire

Fuel loading (MW)

Low (m in im u m fire for fu el-restricted atriu m ) 2 Typ ical (m in im u m fire for atriu m

w ith com bu stibles) 5

Ceiling jet. In ad d ition to ad d ressin g th e p lu gh olin g p h en om en on , NFPA 92B also recom m en d s th at th e sm oke layer d ep th below th e exh au st in lets be su fficien t to accommodate the smoke flow once the smoke reach es th e atriu m ceilin g (see Figu re 5, p . 6).[1] At th is p oin t, th e sm oke flow s ou t-w ard s to th e t-w alls. Th is flot-w is kn ot-w n as th e ceilin g jet. At th e w all, th e sm oke flow w ill be red irected back in to th e atriu m .

Fire-p rotection en gin eers h ave gen erally assu m ed th at th e sm oke flow at th e ceilin g occu p ies betw een 10 an d 20% of th e h eigh t of th e atriu m , w h ich w as recen tly con -firm ed by IRC exp erim en tal w ork.[9] Th is typ e of sm oke flow lim its th e d ep th of th e sm oke layer th at can be accom m od ated u sin g a m ech an ical exh au st system an d th erefore n eed s to be con sid ered w h en d eterm in in g th e d esign h eigh t.[1]

N um e ric a l M ode lling for Sm ok e M a na ge m e nt De sign

Wh ile em p irical equ ation s are su fficien t for d esign p u rp oses in m an y ap p lication s, n u m erical fire m od els are requ ired for m ore com p lex p roblem s. Man y zon e fire m od els (n u m erical m od els based on th e d ifferen t zon es in an atriu m , as sh ow n in Figu re 1) can take in to accou n t exh au st from th e sm oke layer, an d can th erefore be u sed to sim u late atriu m sm oke m an agem en t system s. How ever, before u sin g su ch a m od el, th e d esign er sh ou ld d eterm in e w h eth er it h as been verified for th e h eigh ts typ ical of th ose fou n d in th e atriu m u n d er con sid eration .

Som e m od ern bu ild in gs w ith m u ltip le atriu m s an d in tercon n ected com m u n icatin g sp aces, or w ith very large op en sp aces, are too com p lex to be ad equ ately rep resen ted by th e available em p irical equ ation s an d / or zon e m od els. For th ese bu ild in gs, m ore exten sive n u m erical m od ellin g is w arran ted . Design ers of sm oke m an agem en t system s often u se Com p u tation al Flu id Dyn am ic (CFD) m od els for th is task becau se th ey allow for a d etailed exam in ation of sm oke flow in a bu ild in g.

Sum m a ry

Th e d esign of a sm oke m an agem en t system for an atriu m bu ild in g d ep en d s on th e u se an d th e d esign of th e bu ild in g, both of w h ich affect th e size of th e fire an d its rate of grow th , an d h en ce th e ability of occu -p an ts to evacu ate. Variou s a-p -p roach es or tools are available for th e d esign of th ese

Figure 4. The plugholing phenomenon

Figure 3. The smoke exhaust system must have adequate capacity to remove a suffiecient quantity of smoke from the smoke layer.

“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.

For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6

Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca

© 2000

Nation al Research Cou n cil of Can ad a Decem ber 2000

ISSN 1206-1220 system s, from em p irical equ ation s for

sim p ler bu ild in gs to n u m erical m od ellin g for m ore com p lex on es.[1,2,3] Th e u se of sm oke m an agem en t system s p rovid es op tion s for exten d in g th e u se of th e atriu m w h ile m ain tain in g safe egress rou tes. How ever, in ap p lyin g th ese system s, th e d esign er m u st take in to con sid eration factors th at lim it system effectiven ess, su ch as p lu gh olin g an d ceilin g jets.

Re fe re nc e s

1. NFPA 92B, Gu id e for sm oke m an agem en t system s in m alls, atria, an d large areas. Nation al Fire Protection Association , Qu in cy, MA, 2000.

2. Klote, J.H. an d Milke, J.A. Design of sm oke m an agem en t system s. Am erican Society of Heatin g, Refrigeratin g an d Air-Con d ition in g En gin eers, Atlan ta, GA, 1992.

3. Morgan , H.P., Gh osh , B.K., Garrad , G., Pam litsch ka, R., De Sm ed t, J.-C. an d Sch oon baert, L.R. Design m eth od ologies for sm oke an d h eat exh au st ven tilation . BRE 368, Con stru ction Research

Com m u n ication Ltd , Lon d on , UK, 1999.

4. Fire an d sm oke m an agem en t, h eatin g, ven tilatin g an d air-con d ition in g ap p lication s h an d book. Am erican Society of Heatin g, Refrigeratin g an d Air-Con d ition in g En gin eers, Atlan ta, GA, 1999.

5. Milke, J.A. Usin g m od els to su p p ort sm oke m an agem en t system d esign . Fire Protection En gin eerin g, Nu m ber 7, 2000, p p . 17-23.

6. Lou gh eed , G.D. an d Had jisop h ocleou s, G.V. Investigation of atrium smoke exhaust effectiven ess. ASHRAE Tran saction s, Volu m e 103, 1997, p p . 519-533.

7. Had jisop h ocleou s, G.V., Lou gh eed , G.D. an d Cao, S. Nu m erical stu d y of th e effectiven ess of atriu m sm oke exh au st system s. ASHRAE Tran saction s, Volu m e 105, 1999, p p . 699-715.

8. Lou gh eed , G.D, Had jisop h ocleou s, G.V., McCartn ey, C. an d Taber, B.C. Large-scale physical model studies for an atrium smoke exh au st system . ASHRAE Tran saction s, Volu m e 105, 1999, p p . 676-698.

9. Lou gh eed , G.D. an d Had jisop h ocleou s, G.V. Th e sm oke h azard from a fire in h igh sp aces. ASHRAE Tran saction s, Volu m e 107, 2001, p p . 720-729. Foot not e s

1. Alth ou gh th e figu res sh ow a raised roof system , th is is n ot a requ irem en t of th e Nation al Bu ild in g Cod e of Can ad a 1995.

2. “Com m u n icatin g sp aces” refers to th ose sp aces in a bu ild in g w ith an op en p ath w ay to th e atriu m su ch th at sm oke m ovem en t betw een th e sp aces an d th e atriu m is u n im p ed ed . Th is in clu d es sp aces th at op en d irectly in to th e atriu m as w ell as th ose th at con n ect th rou gh p assagew ays.

Dr. G.D. Loughe e d is a sen ior research officer in th e Fire Risk Man agem en t Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .