Fire Protection engineering Summer 2000

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FIRE PROTECTIONFIRE PROTECTION

PROVIDING PRACTICE-ORIENTED INFORMATION TO FPEs AND ALLIED PROFESSIONALS

S u m m e r 2 0 0 0 Is su e N o . 7

SMOKE MANA GE-

M ENT SYSTEM S –

DO THEY W ORK?

USING M ODELS TO

SUPPORT SM OKE

MANAGEMENT

SYSTEM DESIGN

AN OVERVIEW OF

ATRIUM SMOKE

MANAGEMENT

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D ES I G N S M A N A G EM EN T

S M O K E U N IQ U E

D ES I G N S


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Fire Protection Engineering 1

3 VIEW P O IN TSm oke control m ethods differ according to the structure, but theireffectiveness depends on on e com m on factor.April Berkol

4 S M O K E M A N AG EM EN T S Y S TEM S –D O TH EY W O R K ?M ore and m ore sophisticated sm oke con trol m anagem ent requ irem entshave developed as codes have changed , but their in-service perform ancem ay dep end on factors that have not yet received sufficient atten tion.Wi lli am A. Webb, P.E.

17 U S IN G M O D ELS TO S U P P O R T S M O K E M A N A G E M EN TS Y S T E M D E S IG NCom puter-based and physical m od els can be u sed as an aid in testingsm oke m anagem ent system s.Jam es A. Mi lke, Ph.D ., P.E.

24 A N O V ER VIEW O F ATR IU M S M O K E M A N AG E M EN TThe zone fire m od el concept and other aspects of atrium sm okem anagem ent, includ ing new inform ation prop osed for addtion toN FPA 92B, are review ed.John H. Klote, Ph.D., P.E.

36 M O D E LIN G S P O T-TY P E S M O K E D E TEC TO R R ES PO N S ECurrently available m ethods and their m erits for everyday use.Wi lli am E. Pucci, P.E.

44 C A R EE R C E N TER

46 S FP E R ES O U R C ES

52 FR O M TH E TEC H N IC A L D IR EC TO RRecent legislative ch anges in Florida prom pt an exam ination o f theen gineer’s role in sprinker design and layout.Mor gan J. Hu rl ey, P.E.

contentsS U M M E R 2 0 0 0

9COVER STORY

U N IQ U E S M O K E M A N A G E M E N T D E S IG N SThe unusual architectural designs of the Luxor H otel and Casino and the M G M M ansionpresent the need for m echanical sm oke m anagem ent sytem s that are just as unique.Dou glas Evan s, P.E.

Subscription and ad dress change co rrespondence should be sen t to: Society of Fire Protection Eng ineers,Suite 1225 W est, 7315 W isconsin Avenue, B ethesda, M D 20814 U SA . Tel: 301.718.2910. Fax: 301.718.2242.E-m ail: sfpehqtrs@ sfpe.org.

Copyrigh t © 2000, Society of Fire Protection Engineers. A ll righ ts reserved .

FIRE PROTECTIONFIRE PROTECTION

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Fire Protection Engineering (ISSN 1524-900X) ispublished quarterly by the Society of Fire ProtectionEngineers (SFPE). The mission ofFire Protection Engineering is to advance the practice of fire protectionengineering and to raise its visibility by providinginformation to fire protection engineers and alliedprofessionals. The opinions a nd positions stated arethe au thors’ and do not necessarily reflect those of SFPE.

Edit ori al Advisory Board Carl F. Baldassarra, P.E., Schirm er Engin eeringCorporation

Don Bath urst, P.E.Russell P. Fleming, P.E., National Fire SprinklerAssociation

Douglas P. Forsman , Oklahom a State UniversityMorgan J. Hurley, P.E., Society of Fire ProtectionEngineersWillia m E. Koffel, P.E., Koffel Associa tesJane I. Lataille, P.E., HSB Industrial Risk InsurersMargaret Law, M.B.E., Arup FireRonald K. Mengel, Honeywell, Inc.

Warren G. Stocker, Jr., Safeway Inc.

Beth Tubbs, P.E., Internation al Conference of BuildingOfficials

Reg iona l Ed i to r s U.S. HEARTLAND

John W. McCormick, P.E., Code Consultants, Inc.U.S. M ID -ATLANTIC

Robert F. Gagnon , P.E., Gagnon Engineering, In c.U.S. NEW E NGLAND

Robert G. Sawyer, III, University of New HavenU.S. SOUTHEAST

Jeffrey Harrington, P.E., The Harrin gton Group, In c.U.S. WES T C OAST

Scott Todd, Gage-Babcock & Associates, Inc.ASI A

Peter Bressington, P.Eng., Arup FireAUSTRALIA

Richard Custer, Custer Powell, Inc.CANADA

J. Kenneth Rich ardson, P.Eng., Ken Rich ardson FireTechn ologies, In c.NEW Z EALAND

Carol Caldwell, P.E., Caldwell ConsultingUN I T E D KI N G D O M

Dr. Louise Jackm an, Loss Prevention Coun cil

Publ i shing Advisory Board Bruce Larcom b, P.E., BOCA In terna tiona lDouglas J. Rollm an, Gage-Babcock & Associates, In c.George E. Toth, Rolf Jensen & Associates

Personnel P UBLISHER

Kathleen H. Almand, P.E., Executive Director, SFPETECHNICAL E D I T O R

Morgan J. Hurley, P.E., Technical Director, SFPEM ANAGING E D I T O R

Joe Pulizzi, Penton Custom MediaART D I R E C TO R

Pat Lang, Penton Custom MediaCOVER D ESIGN

Dave Bosak, Penton Custom Media


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view point

S U M M E R 2 0 0 0 Fire Protection Engineering 3

To the uninitiated , the idea ofsom ehow directing the sm okegenerated in a fire seem s like aw onderful solution to the problem : bysom ehow keeping sm oke aw ay fromthe m eans of egress, the exiting occu-pants are p rotected; by exhausting andcontrolling sm oke from fire, occupantsw ho stay in a building during a fire arenot exposed to the toxic products ofcom bustion. A ccording to K lote andFothergill in the docum en t en titled“D esign of Sm oke Control System s for

B uildings”(1983 from Am erican Societyof H eating, Refrigerating and A ir-Condition ing Engineers), som e form ofairflow m anagem ent has been done form ore than 40 years.

The sim plest m ethod for controllingsm oke is found in stair pressurizationsystem s. Sim ply p ut, such system s w orkby forcing air into the stair en closure,thereby creating a p ositive pressureinside the en closure. W hen the doorsto the en closure are opened by exitingoccupants, the sm oke is kep t out as theair from the stairs “pushes out”and

keeps sm oke from entering the stair.Injecting air via fans into a stairw ay orelevator shaft can be done from thetop, from the bottom , or from m ultiplesources depending on the height of thebuilding. The assum ption is that, bydoing so, the co rridors leading to thestair and the rem ainder of the buildingw ill have a negative pressure in relationto the stair, so instead of going into thestair, the sm oke is “pushed aw ay”by arush of air escaping the pressurizedstair enclosure. O f course, for all of thisto w ork as desired, the system s m usthave been designed to com pensate forone or m ore doors being p ropped openor kept op en by the occup ants stream -ing into the stairw ay, have b een testedand inspected on a regular basis, andno t be depend ent on an unp rotectedpow er source.

Sm oke control for the building prop-er can involve a com bination of differ-ent m ethods depending on w hat isactually desired . The sim plest approach

is to “san dw ich”the fire floor by creat-ing positive pressure on the floorsabove and below the fire floor.M ost often this is done by usingthe existing heating, ven tilating,and air condition ing system com -po nen ts (H VA C). Fansare activated, dam persare opened andclosed, and essen tially m ore air isforced into the floors abo ve and beloww hile air is exhausted from the firefloor. In som e buildings, a dedicated

sm oke m anagem ent system m ay berequired. Such system s are there exclu-sively for m anaging sm oke from fire.Autom atic activation of the sm oke m an-agem ent system com po nen ts is not them ost com m on m ethod ; it is m ore com -m on for the responding firefigh ters totake control of the system s and activatethem m anually as they see fit.

Sm oke control for m alls, atria, andlarge areas requires the u se of a com bi-nation of different sm oke control m eth-ods. A tria are m ultistory, large, openspaces w hich com m unicate w ith m ulti-

ple floors of the buildings they arefound in. M alls are large, open spacescreated w hen ind ividual shops on m ul-tiple levels are und er on e com m onroo f. W here m alls tend to spread outhorizontally, atria tend to be m ore verti-cal in orientation. Large-volum e areasun der a single roof w hich have notbeen divided up into separate spacespresent a sim ilar challenge to sm okecontrol. Exam ples of such spaces areop en m anufacturing spaces, large w are-houses, arenas, and the like.

For such spaces, a com bination ofsm oke exhaust and a m ethod for lim it-ing the spread o f sm oke dep end ing onthe type of occup ancy is com m on . Theeffect desired is selected based on theuse of the building. In m alls, arenas,convention halls, ballroom s, exhibitionhalls, concert halls, etc., an d atria inhotels and office buildings, the desire isto protect the o ccupants and afford asafe m eans of egress for them . W hereoccupants are lim ited in num ber, the

objective m igh t beto prevent sm oke dam age tosensitive m anufacturing equip-m ent.

N ow adays, the design o fsm oke control system s for m ost struc-tures frequently involves the u se ofcom puter m odeling of different scenar-ios in order to com e up w ith the m ostdesirable m ethod or com bination ofm etho ds. Com puter m od els can fairlyrealistically sim ulate different types offires in different parts of a structure.

U sing these program s, the design engi-neer can sim ulate the effects of variousm etho ds of sm oke control, singly andin com bination , to com e up w ith thebest m ethod to m anage sm oke in thespace.

U ltim ately, the success or failure ofthe installed system s depends on thebu ilding ow ners and how they test andm aintain these system s. The com po-nents of sm oke control system s aresim ple: fans, ducts, vents, dam pers, etc.K eep ing tho se devices in top w orkingorder is som ething else. O nce installed ,

such system s are o ften not tested regu-larly. Changes to the structure w ill alsocreate changes in how sm oke m ovesw ithin them . M inor changes don’t usu-ally trigger a review of the sm oke con-trol system and w hether there is a needto m ake changes to it as w ell. O vertim e, the building interior m ay changesubstantially, w hile the fixed com po-nents of the sm oke control system areeither com prom ised or rendered u se-less. Perhaps if building m aintenancepeople and building ow ners w ere m oreknow ledgeable of how these system sare m eant to w ork, they w ou ld bem ore conscientious about testing, m ain-taining, and including them in p lansw hen changes are m ade to buildings.

Personally, I am skeptical about theeffectiven ess of the m ore com plicatedsystem s over the life of a building.

April Berkol i s wi th Starw ood Hotels an d Resorts wor ldw id e.

SM OKE CONTROL


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4 Fire Protection Engineering N U M B E R 7

By Wil li am A. Webb, P.E.

B A C K G R O U N D

The developm ent of m odern sm oke m anagem ent system sbegan in the 1960s. A fter a series o f high-rise fires, there w as anincreased interest in sm oke m anagem ent system s, w hich led toresearch on the tim e to evacu ate h igh -rise buildings and sm okespread caused by stack effect. A few notable fires that occurredin buildings containing atria caused a reexam ination of the p ro-vision s provided for sm oke m anagem ent in atria and in coveredm alls. A ll of the m odel codes have had changes to the require-m ent for sm oke con trol or sm oke m anagem ent in each editionsince at least 1970. The Intern ational Buildi ng Code (IB C)1 hascontinued the trend.

A significant body o f research influenced the code changes. Thecom bined effect of the code changes and the research has beenthat sm oke control requirem ents have been continually changing.

W ith all of the effort in code w riting and research , it is fair toask, “H as the effort been w orth the cost; and do these system sw ork?”

This article w ill present inform ation concerning how w ellthese system s perform after they have been in service an d toencourage d iscussion and research to determ ine their effective-ness and to im prove their m aintenance.

BRIEF HISTORY

In 1968, the N ational Research C ou ncil of Canad a (N RCC)conducted a survey o f exit facilities in 20- to 40-story officebuildings. 2, 3, 4 The results revealed dangerously excessive exit

DOTHEYWORK?

S M O K E M A N A G EM EN T S Y S TEM S

© 2000SFPE. All rights rese rved.


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tim es. A little earlier, N RCC discovereda serious sm oke control problem in tallbuildings, partly due to stack effect andpartly due to deficiencies in H VA C sys-tem s and other elem ents of construction.

O n Janu ary 24, 1969, Chicago new s-papers reported “Four D ie in ChicagoApartm ent Fire.”O n T hursday, August 6,1970, the headlines read , “SkyscraperFire K ills Tw o”w ith a subhead ing thatread, “31 Injured in B laze on W allStreet.”O n D ecem ber 4, 1970, a front-page story told of a fire on the fifthfloor of a 47-story b uilding in N ewYork C ity that killed three w orkm en.The m ost disturbing p art of these firesis that they all happened in m od ernfire-resistive, high-rise office buildings;all invo lved loss of life, poor elevatoroperation, substantial sm oke spreadthrough the building, and problem sw ith exiting.

The fire protection industry reactedto these events w ith a series of confer-ences. Tw o notable ones w ere spo n-sored by the Fire Protection EngineeringD ep artm en t of the Illinois Institute ofTechn ology in 1970 and by the U .S.G eneral Services A dm inistration in1971.5

W hile code changes w ere beingdeveloped using the conclusions fromthe conferences, there w ere three h igh-rise fires of national prom inence inN ovem ber 1972. The first w as a fire onthe 95 th floor of Chicago’s John H ancockCenter. The p ho tographs w ere as spec-tacular as the fire loss, though therew ere no fatalities. The second and thirdcam e back to back in a N ew O rleanshigh-rise w ith a restaurant on the 19 thfloor and in an A tlanta retirem ent hom ew here the fire started on the 12 th floor.

The fires causing co ncern inChicago w ere in ap artm ent buildings;tho se in N ew York w ere in officebuildings. A lthough the structures inthe tw o cities had m uch in com m on ,they d iffered to a m ajor extent in com -

partm entation. This drove differentsolutions in the requirem ent initiallyadopted in each city for high-rise firesafety. N ew York’s requ irem en ts,know n as Local Law 5, 6 applied tooffice buildings. Either com partm en ta-tion and stair pressurization or auto-m atic sprinkler protection w as required,am ong other features such as voiceevacu ation, firefighters’com m unica-tion, an d elevator recall.

Chicago’s requirem ents, adopted in1975, applied to all high-rise buildings.They required either com partm entationand sm oke control or sprinkler protec-tion. The sm oke con trol system w as tobe designed to p ressurize nonfire com -partm ents on the fire floor and exhaustthe fire com partm ent. A sm okeproo ftow er w as required for nonsprinkleredbuildings. The voice evacuation andfirefighters’com m unication require-m ents w ere sim ilar to those in LocalLaw 5.

Fire safety requirem en ts for high-risebuildings w ere subsequently adopted inthe m odel codes and those of the m ajorcities in N orth Am erica. A m ong featuresintended to p revent sm oke spreadbetw een floors or groups of floorsincluded stair pressurization, H VA C sys-tem shu tdow n, fire-floor venting orexhausting, and autom atic sprinklerprotection. The specific features,required or allow ed, varied am on g thecodes. A s the popularity of atriumhotels and covered m alls increased, sodid the concern for sm oke spread inthese occupancies. This caused addi-tional sm oke m anagem ent considera-tions to be adopted in the codes.

Each year, the m od el codes haverevised sm oke m anagem ent requ ire-m ents. A s a result, m ore and m oresop histicated m etho ds of sm oke con-trol and sm oke m anagem ent havebeen needed to com ply w ith the code.The term s I use in this article for“sm oke co ntrol system ”and “sm okem anagem ent system ”are those definedon page 24.

EXPERIENCE

A lthough there have been relativelyfew high-rise fires, those that haveoccurred are often spectacular. Thesam e is true of atrium fires. Even intho se cases w here p rotection w asinadequate, a com m on thread sepa-

rates the “failures”from the “successes.”That com m on thread is inspection ,testing, m aintenance, and enforcem ent.

Let us consider a few examples.Th e January 1982, N FPA Fire

Jour na l reported on a fire at the M G MG rand. This incident clearly w as aninstance of inadequate p rotection andnum erous deficiencies. M atters such assteel straps bolted across dam pers,

H VA C m ixing room s used for storageor as offices, fusible links replaced bysteel w ire, inadequate enclosure ofexit passagew ays, and b locking ofsm okeproof tow er sm oke vents couldhave been discovered by inspectionand prevented by proper m aintenance.W hile this tragedy m ay not have beenaverted had the deficiencies been cor-rected , the deficiencies m ay have beencontributing factors.

In another inciden t in a h igh-risebuilding in the Southw est, a fireoccurred in the service elevator lobbyon the 22 nd floor of a hotel. The sm okedetector in the lobby detected the fireand released the lobby doors.Recently installed carpeting, how ever,kept the d oors from closing. In addi-tion, stair doors had been blockedopen . A s a result, sm oke infiltratedstairs and the elevator shaft, and fivefloors of the hotel had to be evacuated.

In this instance, the active portion ofthe sm oke control system , i.e., thedetection and fans, w orked properly.The passive p ortion, nam ely the doors,failed because they w ere blockedopen . This is another instance inw hich inspection and m aintenancew ould have disclosed the deficiencies.

In another instance in the sam e city,the H VA C system in a portion of thebuilding continued to operate, circulat-ing sm oke from a fire. The duct detec-tors had been disabled during m ainte-nance. Inspection by those chargedw ith fire safety could have discoveredthe deactivation of the sm oke detec-tors so that alternative p rotectioncould have been provided during theshu tdow n period .

A Fir e Jour na l 10 article rep orted on anatrium fire w hich occurred on N ovem -ber 19, 1973, at the H yatt RegencyO ’H are. The article states “. . . it w asfound that the atrium sm oke exhaustsystem had failed to operate. O n check-ing, it w as found that the sw itch con-

necting the sm oke detection system tothe exhaust system had been turned off.The fans w ere then turned on , and theatrium w as cleared of sm oke.”

A n arson fire in the B lue M axN igh tclub of the hotel spread fire an dflam es into the hotel atrium . The n igh t-club w as on the secon d floor of the10-story high atrium . The atrium w as145 feet (44 m ) square, topped by arevolving restaurant. The article states,


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6 Fire P rotection Engineering N U M B E R 7

“Although 1,000 guests w ere exposedto the fire conditions, only onerequired hospital treatm ent, because ofa heart condition. O ne firefigh ter w astreated for sm oke inhalation.”It shouldalso be n oted that the n igh tclub andguestroom s w ere no t protected bysprinklers. Firefigh ters’prom pt responseand action prevented a tragedy. It isclear that inspection and m aintenancew ould have disclosed the deficiency sothat it could have been corrected .

The January 1979 edition of Fire Jour na l rep orted that a fire w hichstarted in a sm all office on the 10 thfloor of a 13-story office buildingbroke a w indow w hich open ed to theatrium and allow ed sm oke to enter theatrium . 11 U pon firefighter arrival, “. . .thick b lack sm oke w as pouring fromthe fire floor and b anked dow n fromthe roof to below the 10 th floor.A ltho ugh the sm oke detector operated,on ly tw o of the six sm oke ven tsop ened. The other fou r released; ho w -ever, m aintenance p ersonnel reportedthat the springs had apparen tly lostsufficien t strength to open them fully.”The article goes on to state that m ain-tenance personn el did op en the vents,but sm oke continued to bank do w n.The failure of all of the ven ts to oper-ate does, how ever, clearly dem onstratethe need for inspection an d p roperm aintenance. In this case, m aintenancew ou ld have replaced the springs orthe ven ts.

C O D E S

The changes that have occurred tothe m odel building codes since the1970s included an excep tion to allowstair pressurization w ithout a vestibulein buildings protected by au tom aticsprinklers in lieu of sm okeproof tow -ers having vestibules w ith natural orm echanical ventilation. Fo r m alls andatria, the requirem en ts have m igratedfrom four or six air changes per hour(A CH ) of the interconnected volum e tofire p lum e calculations based on NFPA92B . In each of the revisions, m ost ofthe inform ation has concerned installa-tion for new buildings and for accep -tan ce testing. U ntil recently, few , ifany, requirem ents or guidance for rou-tine inspections and m aintenance havebeen adopted in building o r firecodes. It is, therefore, no w onder that

regulatory au thorities frequen tly statethat the system s function properlyw hen new , but m ay perform poorlyafter a few m onths or years in service.

Each of the cu rren t ed itions of them odel building codes and the recentlyadopted Intern ational Buil din g Code state that system s required by the codeare to be m aintained in accordance w iththe code. There are no specific m ainte-nance requirem ents for sm oke con trolsystem s. This can be expected because,gen erally, m aintenance requirem ents arecontained in fire codes.

The three m odel fire codes requiresm oke control and sm oke m anagem entsystem s to be inspected and operatedor tested . The frequency is quarterlyfor the Uniform Fir e Code 12 and sem i-annually b y the BOCA National Fir e Preventi on Code 13 and the Standa rd Fir e Code 14. The Intern ational Fire Code 15 has the m ost com prehensiverequirem ents for m aintaining sm okecontrol and sm oke m anagem ent sys-tem s. It states that req uired system sshall be m aintained in accordance w iththe m anufacturers’instructions and thecode. It requires a w ritten schedule forroutine m aintenance and o perationaltesting be established. D ed icated sys-tem s are to be operated sem iannuallyand nond edicated system s op eratedannually.

In addition to the m odel codes, alogical place to look for the frequency

of testing and m aintenance of sm okecontrol system s w ould b e N FPA docu-m en ts. Turning first to NFPA 1 ,16 thereare no specific req uirem en ts for testingand m aintenance o f sm oke con trol sys-tem s. There is a general requirem entthat required fire safety system s are tobe m aintained . In this respect, NFPA 1 is sim ilar to the m odel fire codes.

NFPA 92A suggests that ded icatedsystem s shou ld b e operated sem i-ann ually and n on dedicated system ssho uld be operated annually. It states,“D edicated sm oke-control system s areintended for the purpose of sm okecontrol only. They are sep arate sys-tem s of air-m oving and distributionequipm ent that do no t fun ction und ernorm al building operating conditions.U pon activation, these system s operatespecifically to perform the sm oke-con-trol function. N ondedicated system sare those that share com ponents w ithsom e other system (s) such as thebuilding H VA C system . A ctivationcauses the system to change its m od eof operation to ach ieve the sm oke-control objectives.”In each case, thesystem s sho uld be op erated un derstandby p ow er. These tests are to bedocum ented and a log m ade availablefor inspection. The p urpose of the testis to determ ine that the correct outputis attained for each input.

NFPA 92 B suggests sem iannual testsw ith the results docum ented in a logavailable for inspection. A s w ith 92A ,the tests are to determ ine w hethercorrect outputs occu r for each input.The p urpose is to dem onstrate thatthe installed system s w ill continue tooperate in accordance w ith theapproved design . It is recom m endedthat the tests include both m easure-m en ts of airflow quantities and pres-sure d ifferentials.

NFPA 90A 17 recom m ends thatdam pers be exam ined every tw o years.It also recom m ends that fans andm otors should be inspected at leastquarterly and that fan controls be exam -ined and activated at least annually.

NFPA 101 18 contains a sim ilar require-m ent to those of the m odel codes tom aintain required system s. It alsoreq uires that m echanical stair ven tilationsystem s have their operating p arts test-ed sem iannually and the results logged.The recently adop ted perform anceoption chapter of NFPA 101 requires


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that system s necessary to achieve thedesign perform ance be m aintained forthe life o f the building. N o specificsm oke control or sm oke m anagem entrequirem ents are m entioned.

ASHRAE Gui deli ne 5 19 provides m eth-ods for verifying and docum enting thatthe perform ance of sm oke m anagem entsystem s conform s to the design intent. Itis critical that the designer and ow neruse a system such as Guid elin e 5 if theyexpect the sm oke control or sm okem anagem ent system to p erform asintended over the life of the building.The Guideline states, “. . . throughoutthe useful life of the building, there w illbe a need to recom m ission these sys-tem s periodically.”A key com ponent torecom m issioning is the post-acceptancephase. It requires “as-built”or recorddocum ents be review ed so that theyreflect m odifications m ade to the systemduring construction and throughout thelife of the building. The Guideline spec-ifies how docum ents are to be m ain-tained so that they reflect current sys-tem perform ance. The com m ission ingdocum ents contain records of all theoriginal tests. These are useful in m ain-tenance tests to see how the systemperform ance has changed and to deter-m ine w hat m aintenance or replacem entis needed to restore the system to itsoriginal state.

It is un necessary for codes to speci-fy the details of how inspection, test-ing , and m aintenance are to be per-form ed; how ever, they sho uld specifyw hat and w hen these tasks are to beperform ed.

C O N C L U S I O N

Returning to the question posed atthe beginning of this article, there isinsufficien t inform ation on fire experi-ence and cost im pact to jud ge w hetherthese system s are w orth the cost. It isclear that the system s generally dow ork w hen initially installed . There isanecdotal evidence to question theirin-service perform ance.

A s w e em bark on perform ance-baseddesign in the Life Safety Code and theICC Cod es, em phasis con tinues to beplaced on evaluating the initial design.The Interna tional Fir e Code has, how -ever, taken steps to determ ine that thedesign continues to m eet the objectiveover the life o f the building. Perhaps it

is to be exp ected that it has taken solong to address m aintenance, given thelack of atten tion that testing and m ain-tenance have been given in the currentcodes. If the perform ance of sm okecontrol and sm oke m anagem ent sys-tem s is questionable for those installedun der prescriptive codes, w hat can w eexpect w hen they are an integral partof a fire p rotection solution using a per-form ance code? Perhap s the reason w ehave not recorded m ore incidents ofcatastrophic failures of these system s isthat the fire suppression system s andfire detection and alarm system s onw hich the sm oke m anagem ent system sdepend are inspected and m aintainedm ore tho rou ghly. It seem s obviou s thata perform ance solution m ust include adetailed m aintenance and testing proto-col that regulatory officials m ust be pre-pared to en force and ow ners and op er-ators prepared to im plem ent. If they areunw illing to expend the effort toinspect, test, and m aintain the system sand to enforce the testing and inspec-tion p rotocol, w hy should the system sbe required at all? The cost m igh t betterbe used to enhance the reliability of thesystem s, w hich they are prepared toinspect, test, and m aintain.

This article is intended to be a call tocollect inform ation on how w ell exist-ing sm oke m anagem ent system salready installed are being inspected ,tested , and m aintained . It is encourag-ing that the Interna tional Fir e Code hasincluded requirem ents for inspectionand testing of in-service system s. Them aintenance requirem ents sho uld beinterpreted to be sim ilar to those con-tained in ASHRAE Gui deline 5 w hichstates that a m aintenance p rogramshould include develop ing and m ain-taining a standard m ethod of recordingm ainten ance tests and their results. Itm akes little difference to fire safety todevelop good requ irem ents based onsound engineering if the system s arenot tested and the requirem ents are n otenforced. N ow that w e have goodsm oke m anagem ent requ irem ents inthe codes for installation, let us be surethat the system s are inspected andm aintained properly.

Willi am Webb is with Perfor man ce Techn ology Consulti ng, LTD.

REFERENCES1 International Bui ldin g Code ,

International Code C ouncil, Inc., FallsChurch, VA , 2000.

2 G albreath, M . “Fire in H igh B uildings,”Fire Study N o. 21., N ational ResearchCouncil of Canada, O ttaw a, O N , 1968.

3 M cG uire, J.H . “Sm oke M ovem ent inBuilding Fires.” Fir e Techn ology 3 (3):1968, pp. 163-174.

4 M cG uire, J.H . “Control of Sm oke inBuilding Fires.” Fir e Techn ology 3 (4):1967, pp. 281-290.

5 Jen sen, R.H . “H igh -Rise Fire Protection.W here D o W e Stand? W here D o W e G o?”Proceedi ngs, Chicago Comm it tee on Hi gh-Rise Bui ldi ngs , Septem ber ReportN o. 2: 1972, pp. 5-17.

6 N ew Yo rk C ity, Local Law N o. 5-1973.

7 NFPA 92A , Recomm en ded Practi ce for Smok e Con tr ol Systems . N ational Fire

Protection A ssociation. Q uincy, M A , 1999.8 NFPA 92B, Gu id e for Smoke

Man agement Systems in Mal ls, Atr ia ,and Large Areas . N ational Fire ProtectionAssociation . Q uincy, M A , 1995.

9 Fire at the M G M G rand. Fire Jour na l 76(1): 1982, pp. 19-37.

10 Sharry, J.A . 1973. A n Atrium Fire. Fire Jour na l 67 (6): pp. 34-41.

11 Lathrop , J.K . A trium Fire Proves D ifficultto V entilate. Fire Jour nal 73 (1): 1979,pp. 30-31.

12 Uniform Fire Code . International Fire

Code Institute. W hittier, CA , 1997.13 The BOCA Nati onal Fir e Preventi on

Code . B uilding O fficials and CodeA dm inistrators International, Inc.Country C lub H ills, IL, 1999.

14 Stand ard Fire Preventi on Code . StandardB uilding C ode Congress International,Inc. B irm ingham , A L, 1997.

15 International Fire Code . InternationalCode Council. Falls Church, VA , 2000.

16 NFPA 1. Fir e Prevent ion Code . N ationalFire P rotection A ssociation. Q uincy, M A ,1997.

17 NFPA 90A, Stan dar d for the Installati on of Air Conditioni ng and Ventila ting Systems . N ational Fire ProtectionA ssociation . Q uincy, M A, 1999.

18 NFPA 101, Li fe Safety Code . N ational FireProtection Association. Q uincy, M A, 1999.

19 ASHRAE Guidelin e 5, Commi ssion in g Smoke Man agement Systems . A m ericanSociety o f H eating, Refrigerating and A ir-Conditioning Engineers, Inc. A tlanta, G A ,1994.


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By Dou glas Evans, P.E.

INTRODUCTION

Several of the m ost unique buildings in the w orld arelocated in Las Vegas, N evad a. B ecause of the unusualarchitectural designs incorporated into these buildings, them echanical sm oke m anagem ent system s m ust also be justas unusual. This article p rovides an overview of tw oun ique sm oke m anagem ent designs and dem onstrates thatstructures need not be co m pletely unique to w arrant dif-ferent w ays of think ing abo ut lim iting sm oke m ovem ent.

THE LUXOR PYRAMID

The 30-story Lu xor H otel and Casino is certainly o ne o f

the m ost unique structures in the w orld. Its principal fea-ture is its pyram idal shape. The interior con tains an atriumexceed ing 595,000 m 3 (21,000,000 ft 3) in vo lum e. Interiordim ension s are approxim ately 150 m by 150 m (500 ft. by500 ft.) at the base and 37 m by 37 m (120 ft. by 120 ft.)at the upperm ost level, w hich is 61 m (200 ft.) up.

The casino level is actually the ground floor and islocated directly b elow the low est level of the atrium(Attractions Level). The A ttractions Level contains severalinterior structures, including restau rants and three theaters,that are occupiable. Several additional structures are strictlyfacades and essentially unoccupiable.

B alconies, w hich are open to the atrium on 27 floors,provide access to m ore than 2,500 gu est roo m s. A roo m atthe apex o f the pyram id contains m echan ical equipm ent.

S M O K E M A N A G E M E N T A P PR O A C H

A t the tim e the facility w as being designed , the 1988U niform B uilding C ode requ ired a m inim um m echanicalexhaust capacity o f four air changes p er hour (A CH ). In

Attractions Level. Notice slots in “ obelisk” for beamdetectors.

UNIQUE UNIQUE SM OKEMANAGEMDESIGNS

© 2000SFPE. All rights reserved .


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10 Fire Protection Eng ineering N U M B E R 7

atria, the co de also required one-halfof the air being exh austed to bem echanically injected upw ard at thebase of the atrium . This translated toan exhaust capacity o f approxim ately635 m 3/s (1,350,000 cfm ) and 320 m 3/s(675,000 cfm ) of supply.

These prescriptive requirem entscreated several problem areas. Them echanical room at the top of the pyr-am id w ou ld h ave to be designed toaccom m odate this num ber and w eightof fans. A significant w all area of exte-rior grills w ould be necessary. Sincethe atrium narrow s as it approachesthe top, the upper several levels ofexit balconies could have excessive airvelocity upon activation o f the sm okem anagem en t system . Stratification ofsm oke could lim it the ability toexhaust sm oke and cause interm ediatelevels to becom e un tenable.

O ne of the m ain concerns w assm oke obscuring exit balconies w hileit w as being d raw n upw ard. Sm oke isexpected to rise until it contacts oneof the exit balconies. That balcony andbalcon ies above can be exp ected tobecom e un tenable.

It appeared that strict code com pli-ance m ight not solve these concernsand could actually increase the h azardto occup ants on upper levels. It w asagreed that a perform ance-basedapproach m ay be better able toachieve the desired go als than theprescriptive requirem en ts. The basicfire p rotection go als agreed to w ere:

• M aintain a tenable en vironm entfor occupants no t intim ate w iththe fire.

• Lim it tem perature and sm okegenerated by a fire to m aintainexit balconies ten able forevacuation purposes.

• Reduce the im pact of stratificationof sm oke at an interm ed iate level.

The engineer w orking w ith thedesign team prop osed a radical depar-ture from the p rescriptive code. H esuggested a series of fans and ductssup plying air to the low est level opento the atrium , orien ted such that theentire volum e o f air w ould rotate (asview ed from abo ve). This rotation w asto w ork in con jun ction w ith exhaustfans located in the m echanical room atthe top of the p yram id. H e theorizedthat this co unterclockw ise rotation

w ou ld cause sm oke to b e draw n intothe atrium void, aw ay from exit bal-conies, w hile it w as being exhaustedout the ap ex of the atrium .

Th e C lark C ounty review team (con-sisting of C lark C ounty B uilding andFire D epartm ents and third-party peerreview ers) w as skeptical that this pro-posal w ould achieve the desired goalsbu t agreed that the concep t w ou ld becon sidered if adequate docum entationcould be subm itted to substantiate theproposed design.

The architectural design of theA ttractions Level also n eeded to betaken into consideration. D ue to theheight of the space, the A ttractionsLevel is essentially unprotected byautom atic sprinklers. A greem ents w erereached that significantly restricted thecom bustible load in nonsprinkleredportions of the A ttractions Level. It w asagreed that the fire size expectedw ithin the atrium w ou ld no t exceed2110 kW (2000 B TU /s) m axim um heatrelease rate.

Fo r estim ating sm oke quantity, car-bon m onoxide levels, direction of airm ovem ent, and tem perature, the

design team used several recognizedreferences. 1, 2, 3, 4, 5, 6

The FloV EN T com puter m odel 1 indi-cated that this counterclockw ise rota-tion in con junction w ith the m echani-cal exh aust, w ould d raw the air m asstow ard the center of the atrium andup. A lternate calculations w ere p er-form ed to provide a com fort level thatsm oke w ou ld be diluted to tenableconcen trations, as w ell as reduce heatbuildup o n exit balconies. Calculationsindicated that tem perature and carbonm on oxide levels w ou ld be w ithin ten-able lim its 23 m (75 ft.) abo ve the n oz-zle of the vortex sup ply fans.

The Clark C ou nty review team tho r-ou ghly analyzed the prop osed design.Several m eetings and iterations of thedesign w ere necessary to ach ieve con-currence, w hich allow ed this uniquesm oke control design to be condition-ally ap proved . Final accep tance w asbased on the system ’s perform ancedu ring com m ission ing .

Eigh t sup ply fans w ere installed out-side the building at the base o f thepyram id. Their associated ductw orkw as routed to the interior perim eter

Attractions Level. East interior view taken shortly after Luxor’s grand opening inthe mid-’90s.


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portions at the base o f the atrium .Each of these fans is capable of pro-viding up to 14 m 3/s (30,000 cfm ) at16.0 m /s (3,150 ft. per m inute). Thedischarge w as oriented approxim ately22 degrees from the horizontal and 11degrees from guestroom balcon ies. Atotal atrium exhaust rate o f 188 m 3/s(400,000 cfm ) w as provided at theapex of the atrium . This com binationof exhaust and injection ports createsa high-velocity air stream parallel toguest roo m balcon ies, w hich causes alow -pressure area that draw s sm okeinto the atrium to be exhausted outthrough the apex. This design actuallyprovides less than one air change perhour.

Th e atrium sm oke m anagem ent sys-tem activates w henever any o f the fol-low ing events occur: autom atic sprin-kler w ater flow anyw here in the atri-um (including any exit balcon y); oper-ation of any tw o of the m ore than2,000 area sm oke detectors installedon the exit balconies; or activation ofany one o f 24 beam sm oke detectorslocated in a structure in the cen ter ofthe A ttractions Level. M anual overridesare also p rovided in a protected roomspecifically designated for fire dep art-m ent em ergency response (FC C).

ACC EPTANC E TESTING / COMMISSIONING

A fter the contractors and design ershave confirm ed fire p rotection system sfunction as intended and prior to grant-ing occupancy for any m ajor facility,Clark County B uilding and FireD ep artm en ts w itness “all system s”tests.These series of tests are inten ded tosim ulate reasonable fire scenarios. Asignificant portion of the facility ism ethod ically stepped through to con -firm proper functioning, as w ell ascoordination of all fire protection sys-tem s.

D uring testing o f the atrium sm oke-con trol system , a 2,110 kW (2,000B TU /s), 3 m (10 ft.) diam eter, propanebu rner w as m oved on to the A ttraction sLevel. Theatrical sm oke w as injectedinto the heat plum e generated by thepropane burner to visually verify airm ovem ent. Through visual verification,as w ell as review of the com puter out-put from carbo n dioxide m on itors andtherm ocou ples placed at tw elve loca-tions on the exit balconies, it w asdeterm ined that the atrium systemfunctioned as indicated by the m odels.Proper configu ring of all dam pers, fans,and other op erating equ ipm ent w as

also con firm ed. Status and m anualoverrides w ere confirm ed from the FCC.

To sim ulate a guestroom fire w iththe do or blocked op en, the engineerof reco rd injected theatrical sm okeonto one interm ediate-level balcony.W hile the air m ass w as rotating, the

“floor of origin”w as relatively clear.The theatrical sm oke did n ot adverselyim pact alternate floors. Sm oke w asdraw n into the atrium and exhaustedout through the ap ex. W ithout the airm ass rotating, the theatrical sm oke notonly im pacted the floor of origin, butalso the balcon ies above and below .

D uring an alternate test, theatricalsm oke w as injected o nto o ne o f thecorners of an exit balcony w here theexit stairs and elevator lobbies create apartial enclosure. Initially, the sm okew as so thick that visibility w as reduced

to approxim ately 1 m (3 ft.). W ithin acouple o f m inutes, the rotational airm ass drew sm oke from the enclosedportion of the balcony into the atriumand tow ard the exhaust fans to theexten t that visibility w as increasedsignificantly.

T H E M G M M A N S IO N

The M G M H otel and C asino boastsm ore gu estroom s than any other hotelin the w orld (m ore than 5,000). A fewyears after opening, the o w ners decid-ed to add a four-story atrium w ith adom ed skyligh t above a garden -likecentral court. This portion of the facili-ty w as to contain 25 guest suites forthe w orld’s elite, each ranging in sizefrom 740 m 2 (8,000 ft 3) to 1100 m 2(12,000 ft 3). Each suite w as to be a sin-gle level w ith w indow s and privatebalcon ies op ening o nto the cen tralcourt.

To m ake these “villas”as com fort-able as possible, the design ers decidedon op erable w indow s and doors fromthe suites into the atrium on all fourlevels. Fu rtherm ore, these doors andw indow s w ere prop osed to be neitherfire-rated nor self- or autom atic-clos-ing. The p rescriptive codes adopted atthat tim e w ere the 1994 U niformCodes, w hich specifically required theinterface betw een guestroo m s and theatrium to act as a sm oke barrier and ,therefore, did not allow this proposedarrangem ent.

First Floor. Main entrance into the casino complex.


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S U M M E R 2 0 0 0 Fire P rotection Engineering 13

S M O K E M A N A G E M E N TAPPROACH

To justify this arrangem en t, the fireprotection engineer of reco rd p ro-posed m itigating m easures to providethe level of protection intended by the

prescriptive requirem en ts. Since allexits above the atrium floor w ere inde-pendent of the atrium , it w as pro-posed to use the atrium void as asm oke reservoir. A pproxim ately one-half of the m echanically sup plied airw ould be injected into the guest suitesw ith the rem ainder injected at lowvelocity near the atrium floor. A llexhaust fans w ere located at the top ofthe atrium .

This design concept w as dep endenton the area of fire origin. The sm okem anagem ent scenario w ould be differ-

en t if the fire w ere in the atrium or ina guest suite. To com pensate for this,the autom atic sprinklers protecting thesuites w ere zoned independently fromthose protecting the atrium space.B eam -type sm oke detectors w ereinstalled at tw o levels w ithin the atri-um to help com pensate for stratifica-tion. In addition, area-typ e sm okedetectors w ere installed in the suites atopenings into the atrium , as w ell asw ithin each sleep ing room . U sing

these initiating devices, the supply aircould be deactivated w ithin the suiteson the floor of origin if it w ere deter-m ined that the fire originated w ithinone of the suites.

A utom atic sprinklers protecting theatrium w ere installed just below theatrium skyligh t, approxim ately 34 m(110 ft.) high. A t this height, the au to-m atic sprinklers are not expected toadequately control a fire o n the floor.Therefore, a thorough analysis w as

conducted of reasonable fire scenarioson the atrium floor. D ue to the lim itedcom bustible load, it w as determ inedthat the m axim um expected fire sizew ou ld not exceed the m inim um firesize requ ired by the U niform B uildingCode of 5,275 kW (5,000 B TU /s).D oubling this fire size for a factor ofsafety, the sm oke p lum e dyn am icsw ere estim ated, and the exhaust fansw ere sized to provide at least 141 m 3/s(300,000 cfm ). This design is expected

Three-dimensional view of Luxor interior, showing approximate locations ofvortex injection fans.

Luxor exterior, southeast exposure.


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to contain sm oke from the m axim umdesign fire at a reasonable level abovethe atrium floor, but it is not sufficientto m aintain sm oke below the upp erguestroom levels.

The quantity of sup ply air into thesuites w as prop osed to be lim ited to am axim um , based on all the d oo rs andw ind ow s op en. To restrict sm okem igration from the atrium into thesuites, the m inim um velocity of sup plyair needed from the suites throughthese o penings w as estim ated to be0.66 m /s (130 ft./m in), w hich w as alsoless than the 1 m /s (200 ft./m in) lim itbelieved to im pact plum e dynam ics. Ifall the doors and w indow s w ereclosed, the pressure in the atriumw ould be negative relative to the guestsuites, w hich w ould also restrictsm oke m igration into an un involvedsuite.

This arrangem ent is exp ected toprotect occupants w hether on the atri-um floor or in an uninvolved suite bym aintaining sm oke ab ove the atriumfloor and restricting sm oke m igrationinto an uninvo lved suite. Thisapproach allow s occup ants to rem ainin uninvo lved suites or safely evacuatethe building.

ACC EPTANC E TESTING / COMMISSIONING

A irflow and pressure differencesw ere m easured at various locationsthroughout the addition. Initial testinguncovered aspects of the system thatw ere not perform ing as exp ected.U pon exam ination , the prob lem areasw ere determ ined, and correcting m ea-sures w ere im plem ented. In this case,it w as no t only n ecessary to m odifythe system , but it w as actually neces-sary to revise the design concept inorder to determ ine if the system actu-ally w as able to provide the level ofprotection intended by code.

The system as described abo ve isthe final design that w as accep ted forthis facility; bu t this exam ple not onlyillustrates a unique ap proach for m an-aging sm oke, it is also a rem inder thatw e m ust not lose sight of our initialgo al. In this case, ou r principal go alw as to reduce the potential for sm okem igration into an uninvolved suite andm aintain the atrium floor safe for evac-uation. If it ap pears that our firstattem pt to m eet our go al has failed ,m aybe w e only need to change thew ay w e think abo ut achieving that

goal in order to realize the sim plicityof the solution.

Dou glas Evan s is with the Clar k Coun ty, Nevada, D epar tment of Building.

REFERENCES

1 FloV EN T, B uilding Services Research andInform ation A ssociation and Flom erics,U .K .

2 AH SRAE,Han dbook of Fundam entals ,1989, Chapter 31, A m erican Society forH eating, Refrigeration and A irConditioning Engineers, A tlanta, G A .

3 Purser, D .A . “Toxicity A ssessm ent ofCom bustion Products,”Section 1,Chapter 14, SFPE Ha nd book of Fir e Protection Engin eeri ng , N FPA , Q uincy,M A , M ay 1988.

4 K lote, J.H ., “Sm oke Control,”Chap ter 3,Section 9, SFPE Ha nd book of Fir e Protection Engin eeri ng , N FPA , Q uincy,M A , M ay 1988.

5 NFPA 92A , Recomm en ded Practi ce for Smok e Con tr ol Systems , N FPA 1988.

6 NFPA 92B , Gui de for Smoke Man agement Systems in Mal ls, Atr ia ,an d Large Areas, N FPA, 1991.

Luxor facility model showing new twin towers along with existing p yramid and sphinx. Viewed from northeast side.


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An en gineering analysis isneeded to assess the ability ofa sm oke m anagem ent systemto satisfy stipulated perform ance crite-ria. This analysis can be conducted toverify acceptab ility, num erically test, ortroubleshoot problem s associated w iththe system .

The pressure difference betw eenspaces is the focus of analyzing stairpressurization and zoned sm oke con-trol system s. For sm oke m anagem entsystem s in atria, the analysis consistsof assessing the residual hazard p osedby sm oke in term s of the extent ofsm oke spread, sm oke layer depth, orsm oke layer properties. In m any cases,a sim plified analysis invo lving theapplication of algeb raic equations 1, 2 issuitable to assess the perform ance ofsm oke m anagem ent system s.

H ow ever, in som e cases, theassum ptions associated w ith the alge-braic equations are unacceptable. Fo rstairw ell pressurization system s, thealgeb raic equations neglect vertical

leakage and w ind , and requ ire sym m e-try if m ore than one p ressurized stairis provided. Lim itations of algebraicequation m etho ds for atrium sm okem anagem ent are:

• stead y o r t 2 fires only;• uniform horizontal cross-sectional

area for all levels of the atrium ;• uniform conditions through out the

upper layer/zone, even in spacesw ith large h orizontal areas; and

•analysis of the pre-venting, sm oke-filling period or steady, eq uilibriumconditions during venting.

A lgebraic equation m etho ds cannotaddress the interaction betw een m ulti-ple sm oke m anagem ent system s, suchas stair pressurization and atriumsm oke exhau st, in the sam e b uilding.

Several typ es of m odels are availableto assist design professionals either inlieu of or as a supplem en t to the alge-braic eq uations. These m odels includesm all-scale physical m odels, com puter-based zone m odels, netw ork flowm odels, and CFD m odels. W hile thisarticle p rovides an overview of all ofthese m odels, the em phasis is onsm all-scale m odels and netw ork m od-els, given the extensive treatm en t ofzone and C FD m odels elsew here.

SMALL-SCALE MODELS

Sm all-scale m odels provide p hysicalrep resentations of a space, though in areduced scale. Scale m odels are espe-cially useful in exam ining atria w ithnum erous projections or irregularshapes. M ilke and K lote review theapplication of scale m odels as a designaid for sm oke m anagem ent system s 3.Q uintiere an d D illon developed a scalem odel to assess the perform ance of asm oke m anagem ent system in a fireincident in a covered m all. 4

A sm all-scale m odel m ay bedesigned follow ing the p rinciples of

Froude m odeling. Q uintiere 5 provideda review of scaling relationships topreserve the Frou de nu m ber. The scal-ing relationships seek to preserve thefollow ing ratios:

• fire energy/flow energy;• fan flow /buoyan t flow ; and• convection heat transfer/w all heat

transfer.

The scaling relationships are:

Tem perature:T m = T F

(1)

Position: (2)

Pressure: (3)

Velocity: (4)

Tim e: (5)

Convective H eat Release:

(6)

Vo lum etric Flow Rate:

(7)

The subscripts m and F correspo nd tom odel and full-scale, respectively.

x x l

lm F m

F

=

∆ ∆ p p llm F m

F

=

v v l

lm F m

F

=

1 2 /

t t l

lm F m

F

=

1 2 /

Q Q l

lc m c F m

F , ,

/

=

5 2

˙ ˙, ,

/

V V l

l fan m fan F m

F

=

5 2

U S IN G M O D ELSTO S U P P O R T

SmokeManagementSystem Design

By Jam es A. Mi lk e, Ph.D., P.E.



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M any of the param eters in equation s(1) to (7) are functions of tim e. Thusthe scaled param eters should also befunctions of tim e. Froude m odelinghas the advantage o f conductingexperim en ts in air at atm osph eric pres-sure. W hile Froude m od eling do es not

preserve the Reynolds num ber, achiev-ing fully develop ed flow by m akingthe critical dim ensions of the m odel atleast 0.3 m m inim izes this shortcom -ing. Fu lly developed flow only need sto b e achieved in those areas w herethe sm oke b ehavior is of interest. Thecritical dim ension for a m odel of ashopping center and atria could be thedistance from the floor to the under-side o f a balcony.

In add ition , Frou de m od eling do esnot preserve the d im ensionless heattransfer param eters. O ften , this lim ita-

tion has little effect because the tem -perature is the sam e for the scalem odel and the full-scale facility. W hileFroude m odeling is inapplicable inhigh-tem perature locations, e.g., nearthe flam e, Frou de m odeling still pro-vides useful inform ation about sm oketransport aw ay from the fire.

Som e surface effects can be p re-served by scaling the therm al proper-ties of the construction m aterials forthe m odel. The therm al properties canbe scaled by:

Therm al properties:

(8)

H ow ever, selection of enclosurem aterials m ay be acceptably based on

flow visualization needs, rather thanscaling of therm al properties, given thesecondary effect of the therm al proper-ties on fluid flow .

EXAM PLE 1

A scale m odel is proposed to deter-m ine the equilibrium sm oke layerposition for the atrium dep icted inFigu re 1. B ecause the h orizontal cross-sectional area varies w ith heigh t, alge-braic eq uation and com puter-basedzon e m odels are of lim ited value. The

atrium heigh t is 30.5 m , and the designfire is a 5 M W stead y fire. The p ro-posed exhaust fan capacity is 142 m 3/s.B y ap plying the scaling relationships,the basic param eters for the scalem odel are:

• H eight: 3.8 m tall m odel (1/8 scale)• Fire size: 28 kW• Fan capacity: 0.78 m 3/sG iven the 1/8 scale, the w idth of the

scaled spill plum e w ou ld need to be1/8 of that at full scale.

COMPUTER-BASED ZONEM O D E L S

O verview s of num erous zone m od-els are availab le. 6, 7 Q uintiere sum m a-rizes the assum ption s of zone m od els 8.The principal advantage of com puter-

based zone m odels is their ab ility toaddress transien t effects invo lvingsm oke spread , delays in fan startup,effects of environm ental conditions,and a variety of fire-grow th profiles.Som e com pu ter-based zone m odelsare ap plicable to spaces w here theceiling is sloped or the h orizontalcross-sectional area varies w ith heigh t.In ad dition, som e of the com puter-based zone m od els sim ulate cond ition sin m ultiple room s or levels, w here thealgebraic equations are lim ited to asingle com partm ent.

Lim itations of these m odels resultfrom their assum ptions. For exam ple:

• the sm oke layer form s im m ediate-ly, neglecting transport lag;

• the p lum e is unaffected by w indor m echanical ventilation;

• the u pper layer/zone is un iform ,independent of the area involved ;and

• as sm oke enters a tall room froma short room , entrainm ent isdeterm ined based on a newaxisym m etric plum e rather thanfrom a balcony spill or lineplum e.

FIELD MODELS

Com putation al fluid d ynam ics (CFD )m odels sim ulate fluid flow at a level ofdetail im possible w ith other m ethodsof com pu ter m odeling. 9, 10, 11 C FD m o d-els divide the fluid flow field intonum erous sm all cells and num ericallysolve the conservation equations ofm ass, m om entum , and en ergy for eachcell. B oundary conditions are estab-lished at the room boundaries, open-ings to the outside, and exhaust inletsby specifying velocities.

W hile generalizations concerning thenum ber and size of cells are d ifficultto m ake, given the w ide range o f fea-tures and capabilities of CFD m odels,generally the sm allest cells are nearthe fire and at the ceiling. The govern-ing equations cann ot account for tur-

k c k c l

l p w m p w F m

F

ρ ρ ( ) = ( )

, ,.0 9

Figure 1. Small-Scale Model of Atrium


15/32


bulence on a scale sm aller than thecells. Further, it is im portan t that thecell size and tim e step be co ordinatedso that cells are not “skipped”fromon e tim e step to the next by the m ov-ing fluid.

N E TW O R K F LO W M O D E LS

N etw ork flow m odels such asC O N TA M 9612 can be applied to evalu-ate pressure differences betw een com -partm ents, direction of m ass flow s,and m ovem ent of contam inants.CO N TA M 96 is often described as thesuccessor to A SC O S, an early, w idelyused netw ork flow m odel 13. N etw orkm odels sim ulate a building as a net-w ork of airflow paths com prised ofdo orw ays, w ind ow s, vents, and leaksin building assem blies.

The principle advantage o f netw orkflow m odels is their ability to consider:

• m ech anical and natural ventilation;• environm en tal conditions (includ-

ing w ind );

• interacting sm oke m anagem entsystem s;

• bu ildings w ith com plex geom e-tries; an d

• leakage paths betw een b uildingspaces.

Lim itations of netw ork flow m odelssuch as CO N TAM 96 are:

• uniform conditions (tem peratureand co ncentration of contam i-nan ts) are assum ed througho uteach “zone”, w here a “zone”is atleast one room .

• transient fire conditions (e.g., tem -perature o r m ass flow in thesm oke p lum e or tem perature inthe fire zone) resulting from agrow ing fire are no t considered.

H ow ever, fire conditions can be

incorporated into the m odel by analo-gy. The volum etric flow in a sm okeplum e can be sim ulated as a shaft, w itha fan supplying air at each level of theshaft. The air en trained at each levelcan be estim ated using H eskestad’splum e entrainm ent equation. 14

(9)

w here:= volum etric en trainm ent rate (m 3/s)

ρ = density of sm oke (kg/m 3)= convective p ortion o f heat release

rate (kW )

z = clear height (m )

The entrainm ent for a p articularlevel needs to be determ ined based onthe am ount of air entrained only w ith-in that increm ent of heigh t. A s such,the am ount of air en trained for a par-ticu lar level of the building is the d if-ference in the am ount of air en trainedup to the top of the level w ith thatentrained up to the bottom of thatlevel. B uoyan cy effects can be includ-ed by setting the tem perature at eachlevel of the “shaft”using H eskestad’s

plum e cen terline correlation. 14

(10)

w here:T c = plum e centerline tem perature (°C)

= convective p ortion of heat releaserate o f fire (kW )

z = height abo ve top of fuel (m )

W hile the centerline tem perature ofthe plum e overestim ates the bu oyancyof the overall plum e, generally thisapproach is adequate for design pur-poses.

The m ass release rate of con tam i-nant can b e estim ated as:

(11)

w here:= generation rate of con tam inan t

(kg/s)f c = yield fraction (kg con tam inant/kg

fuel) [the yield fraction dep endson the fuel, burning m ode (flam -ing, sm oldering) and the avail-able oxygen concentration]

= heat release rate o f fire (kW )= heat of com bustion (kJ/kg)

CO N TA M 96 can conduct a steady orunstead y an alysis of the flow of conta-m inants (sm oke). Inp ut for CO N TA M 96includes:

• A rea and h eigh t of spaces• Shaft characteristics• H VA C system

Q̇

Hotel rooms

Atrium

Figure 2. Typical floor of five-story building.

˙˙

m f Q

H c c c= ∆

Q̇∆ H c

∆T Q zc c= −25 2 3 5 3˙ / /

Q̇

ρ ˙ . ˙ . ˙ / / V Q z Qc c= +071 0 00131 3 5 3

V̇

ṁc


16/32


• Fans: constant volum e, m ass, orcurve

• Environm ental conditions: w ind,tem perature

• Connection of spaces via leakagepaths

• Release rate o f contam inant(kg/s): unsteady or steady

O utput from CO N TAM 96 includes:• Pressure difference betw een zo nes• A irflow betw een zones• Contam inant concen tration in

zone

A n exam ple ap plication ofCO N TA M 96 is provided for a five-storybuilding (see Figu re 2). U singCO N TA M 96, the interaction betw eenthe atrium sm oke m anagem ent andstairw ell pressurization system s isinvestigated . The cap acity o f the stair-w ell pressurization fans is 2.83 m 3/s.The exh aust fan capacity in the atriumis 160 m 3/s, w ith tw o sim ulations con-ducted w ith the capacity of the m ake-up air fans being either 76 o r 94 m 3/s.The resulting pressure d ifferences areprovided in T able 1. The pressure dif-feren ces for the stairw ell pressurizationsystem s acting alone o r together w iththe atrium sm oke m anagem ent systemare appreciably different.

C om pu ter-based and physical m od-els are applicable as aids for sm okem anagem ent design. B ecause each ofthe m odels provides sim plifications ofactual behavior, m odels can be used

as an aid in estab lishing or testing thedesign o f a sm oke m anagem ent sys-tem . T he approp riateness of assum p-tions should be confirm ed , either bycom paring p redictions to data or con-ducting a sensitivity analysis.

Jam es Mi lke is with the Uni versi ty of Maryland.

REFERENCES

1 K lote, J.H ., and M ilke, J.A ., Design of Smok e Management Systems , A m ericanSociety of H eating, Refrigerating and A ir-Conditioning Engineers, A tlanta, G A .1992.

2 NFPA 92B , Gui de for Smoke Man agement Systems in Mal ls, Atr ia , and Lar ge Ar eas ,Q uincy: N FPA , 1995.

3 M ilke, J.A ., and K lote, J.H ., “Sm okeM anagem ent in Large Spaces inBuildings,”B uilding C ontrol Com m issionof V ictoria, Victoria, Australia, 1998.

4 Q uintiere, J.G ., and D illon, M . E., “ScaleM odel Reconstruction of Fire in anAtrium ,” Proceedi n gs of the Intl . Conf.On Fir e Resear ch an d Engin eeri ng , B FRLand SFPE, 1995, pp. 397-402.

5 Q uintiere, J.G ., “Scaling Applications inFire Research,” Fir e Safety Jour n al , 15,1989, pp. 3-29.

6 Friedm an, R., “An International Survey ofCom pu ter M odels for Fire and Sm oke,”Jour na l of Fir e Protection Engineeri ng , 4,3, 1992.

7 W alton , W .D ., and Bud nick, E.K .,“D eterm inistic Com puter Fire M odels,”Fire Protection H an dbook , 18th Ed., J.L.Linville (ed.), N FPA , Q uincy, M A, 1997.

8 Q uintiere, J.G ., “Com partm ent FireM od eling,” SFPE Ha n dbook of Fir e Protection Engin eeri ng , 2n d Ed., P.J.

D iN enno (ed.), N FPA , Q uincy, M A , 1995.9 Chow , W .K ., “A Com parison of the U se

of Fire Z on e and Field M od els forSim ulating A trium Sm oke-FillingProcesses,” Fir e Safety Jour na l , 25, 4,1995, pp. 337-354.

10 M cG rattan, K .B ., Baum , H .R., and Reh m ,R.G ., “Large Eddy Sim ulations of Sm okeM ovem ent,” Fir e Safety Jour n al , 30, 2,1998, pp. 161-178.

11 R ho, J.S., and Ryou, H .S., “A N um ericalStudy of A trium Fries U singD eterm inistic M odels,” Fir e Safety Jour na l , 33, 3, 1999, pp. 213-230.

12 W alton , G ., CO N TA M 96, N ationalInstitute of Standards and Technology,G aithersburg, M D , 1997.

13 K lote, J.H ., “A Com puter Program forA nalysis of Pressurized Stairw ells andPressurized Elevator Shafts,”N B SIR 82-2512, N ational Bureau of Standards,G aithersburg, 1982.

14 H eskestad, G ., “Engineering R elationsfor Fire Plum es,”SFP E T R 82-8, SFP E,B oston , M A , 1982.

Table 1.Pressure Difference a t Stairwel l (Pa) from CONTAM96 Analysis

No Atrium 160 m3/s exhaust 160 m3/s exhaustFloor Exhaust 76 m3/s supply 94 m3/s supply

1 54.8 65.2 61.0

2 55.0 65.7 61.5

3 55.3 66.2 62.0

4 55.8 67.0 62.7

5 56.8 68.0 64.0

6 58.0 69.5 65.2


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By Joh n H . Klo te, Ph.D., P.E.

Afew years ago , m ost codes inthe U nited States m andated theair change m etho d that basedthe sm oke exhau st flow rate of anatrium on the volum e of the atrium .W hile this m ethod is sim ple to apply,it alm ost alw ays provides the w rongansw er. To day, m ost codes prescribeatrium sm oke p rotection that is basedon the zone fire m odel con cep t that isdiscussed later in this pap er.

This paper is an overview of sm okecontrol technology, including n ewinform ation that has been proposedfor addition to NFPA 92B . M oredetailed design inform ation on thissub ject can be found in the follow ingpublications: NFPA 9 2B, 1 theA SH RA E/SFPE sm oke control book byK lote and M ilke 2, and NISTIR 5516 byK lote. 3

TERMINOLOGY

Readers of the ab ove publicationsare caution ed abo ut the m eaning ofthe term s smoke contr ol and smoke management . Th e term sm oke controlis reserved for system s that providesm oke protection by u se of pressuriza-tion, such as a pressurized stairw ell.System s that use any technique includ-ing co m partm entation, pressurization,airflow , and bu oyancy o f ho t sm okeare referred to as sm oke m anagem entsystem s. U sing this term inology, atriumexhaust system s are sm oke m anage-m ent system s because they rely uponthe bu oyancy of ho t sm oke. This alsoholds for the other typ es of atriumsm oke p rotection discussed below .

For sm oke m anagem ent pu rposes,smoke is considered to consist of theairborne products of com bustion p lusthe air that is m ixed w ith them . Theairborne products are com bustiongases, solid particu lates, an d liquidparticu lates. Inclusion of air in the def-inition of sm oke allow s us to considersm oke protection system s w here thesm oke being generated, exhausted, orvented is actually air m ixed w ith rela-tively sm all quan tities of particu latesand com bu stion gases. B ecause the

concentrations of these other quantitiesare relatively sm all, en gineering designanalysis for these sm oke m anagem entsystem s considers the specific heat, gasconstant, and other properties ofsm oke to be the sam e as those of air.

A n atrium can b e considered a largespace o f tw o or m ore stories. O therlarge open spaces include enclosedshopping m alls, arcades, spo rts arenas,exhibition halls, and airplane hangars.The m ethods of this paper also apply

to these spaces. For sim plicity, theterm atrium is used in this paper in ageneric sense to m ean any of theselarge spaces.

ZONE FIRE MO DEL CO NCEPT

A ll conventional approaches to atri-um sm oke m anagem ent are based onthe zone fire m odel con cep t. This con-cep t is also the basis of several com -puter m od els. 4, 5, 6, 7, 8 This section is abrief synopsis of zone fire m odeling,but for m ore inform ation about thissub ject, readers are referred to othersources. 9, 10, 11, 12

B ecause zone fire m od els w ere orig-inally developed for room fires, thisdiscussion w ill start w ith a room fire.In a room fire, hot gases rise abovethe fire, and these gases form a p lum e.Since the p lum e entrains air, the d iam -eter and m ass flow rate of the p lum eincrease w ith elevation. A ccordingly,the plum e tem perature decreases w ith

A n O verview of

Atrium SmokeManagement

Smoke layer

Doorjet Doorjet

Plume Plume

Fire Fire

Smoke layer

Figure 1. Room fire (a) sketch and (b) zone model idealization

(a) Sketch of an atrium fire (b) Zone model idealization of atrium fire



18/32


elevation. The fire gases of the p lum eflow upw ard and form a hot stratifiedlayer under the ceiling. H ot gases inthis sm oke layer can flow throughopenings in w alls to other spaces, andsuch flow is referred to as a doorjet.The doorjet is sim ilar to a p lum e in

that air is en trained w ith sim ilar effectson m ass flow and tem perature. Figure1(a) is a sketch of a room fire.

The concep t of zone m od eling is anidealization of the room fire conditionsas illustrated in Figure 1(b). For thisidealization, the tem perature of the hotupper layer of the room is uniform ,and the tem perature of the low er layeris also uniform . The h eigh t of the dis-continuity betw een the tw o layers isthe sam e everyw here in the roo m . Th edynam ic effect on pressure is consid-ered negligible, so that pressure is

treated as hyd rostatic. O ther propertiesare considered uniform for each layer.A lgebraic equations are used to calcu-late the m ass flow s due to plum es anddoorjets.

Som e com pu ter zone fire m odelsallow exh aust from the upper layer,and this capab ility is essential for sim -ulation of atrium sm oke exhaust sys-tem s. M any zone fire m odels estim ateheat transfer by m ethods ranging froma sim ple allow ance as a fraction of theheat release of the fire to m ore com -plicated sim ulations of conduction,convection, and radiation . Zon e m od elapplication to atrium sm oke exhaust isillustrated in Figures 2(a) and (b).

AXISYMMETRIC PLUME MODELS

M orton, Taylor, and T urner 13 devel-oped the classic analysis of the tim e-averaged flow of plum es. For a height

abo ve the plum e source, they consid-ered the air en trained at the p lum eedge to be prop ortion al to som e char-acteristic velocity o f the p lum e at thatheigh t. They considered the p lum e tobe com ing from a point sour ce thatm ay be either abo ve or below the sur-

face o f the fuel. Figure 3 is an illustra-tion of a p lum e next to the idealizedm od el of an axisym m etric plum e.

Researchers have extended the w orkof M orton, Taylor, and Turner to devel-op m odels of turbulent plum es due tofires in building spaces. 14, 15, 16 N oexhaustive study has been conductedto evaluate these plum e m odels forvarious applications. B ased on the lim -ited inform ation available, it seem s thatthe H eskestad m odel m ay be the m ostappropriate for atrium applications.

Th e axisym m etric p lum e equations

of the design publications m entionedabo ve are tho se of H eskestad excep tthat the virtual origin correction hasbeen neglected. The justification forthis sim plification is that the virtual ori-gin correction, z o , is sm all com pared tothe plum e heights of interest in atriumap plications. Further, for a generaldesign fire of unknow n fuel, the virtu-al origin co rrection, z o , can be eitherpositive or negative. A dditional infor-m ation about the virtual origin correc-tion is provided by K lote. 3

Th e m ass flow of a plum e dependson the h eigh t, z , and heat release rate,Q . B ecause of an im proved u nder-standing of plum e p hysics, the p ro-posed NFPA 92B has changed z tom ean the height abo ve the base of thefuel rather than the h eigh t above thetop of the fuel. This results in som e-w hat larger values of z and correspond-ingly h igher sm oke exh aust flow rates.

The tem perature drop du e to eleva-tion is dram atic as can be seen fromFigu re 4, and the concern w ith sprin-kler perform ance for high ceilingspaces becom es apparent.

H eskestad develop ed his equationsfor strongly buoyan t plum es, and itfollow s that these equations are notapplicable for sm all plum e tem pera-ture rises above am bien t. W hile littleresearch h as been condu cted on thesetem perature lim its, this au thor suggeststhat the axisym m etric plum e equationsof NFPA 92B no t be used for tem pera-ture rises less than 2 °C (4 °F).

It should be noted that com puterzone fire m odels do no t use the plum eequations of NFPA 92 B . This isbecause of num erical difficulties associ-ated w ith a discontinuity in these equa-tions at the m ean flam e heigh t. Readersshould exp ect som e differencesbetw een calculations m ade w ith theequations of NFPA 92 B and com puterzone fire m odels.

BALCONY SPILL PLUMES

NFPA 92B refers to balcony spillplum es and w indo w plum es. Thesetw o types of plum e are very differentand shou ld no t be con fused. A bal-cony spill plum e can be from any sizefire such that sm oke flow s under abalcony ceiling an d into an atrium ,and a w indow plum e is from a post-flasho ver fire such that the sm okeflow s from the fire room through anop ening to an atrium .

C l e a r

h e i g h t

Exhaust fan

Smokelayer

Plume

Fire

Exhaust

Smokelayer

Plume

Fire

Flame

SmokeAir Air

Z0 Virtualorigin

Velocitydistribu

Figure 2. Atrium fire (a) sketch and (b) zone model idealization

(a) Sketch of an atrium fire (b) Zone model idealization of atrium fire

(b) Idealization of plumodel

Figure 3. Axisymmetric, point source plume(a) sketch and (b) idealized model

(a) Sketch of plume

Q̇


19/32


The balcony spill plum e equations inNFPA 92B are for a fire in a room thatopens to a balcony (see Figu re 5). Itshould be noted that the balcony spillplum e figu re in the 1995 edition ofNFPA 92B incorrectly sho w s the fire onthe b alcony, but this w ill be corrected .

The balcon y spill plum e equationsof NFPA 92B on ly apply w hen there isa doorw ay lintel betw een the fireroom and the balcony. This lintel m ustbe enou gh below the ceiling so thatthe m om entum of the ceiling jet in thefire room does not directly contributeto the flow out the open ing. A ceilingjet consisting of sm oke flow ing radiallyunder the ceiling form s w here a fireplum e im pacts the ceiling. The dep thof the ceiling jet is about 10 percent to20 percent of the height from the baseof the fire to the ceiling.

The balcon y spill plum e equationsof NFPA 92B also are used in theU nited K ingdom . 17 M organ et al. 18 pre-sent a nu m ber of approaches that gobeyond the constraints of the NFPA92B equations. In general, theseapproaches are based on the perim eterof the fire rather than the heat releaserate, and they require som ew hat m orecum bersom e calculation s. W hen facedw ith situations that are d ifferent fromthe sp ill plum e of NFPA 92B , design ersm ay w ant to consider the app roachesof M organ, et al.

For narrow balconies, sm oke cancurl inw ard tow ard the structure andm ove into portion s of any balcon iesabove. M organ et al. indicate thatexperim ents have sho w n that suchinw ard curling sm oke can occur forbalconies less than 2 m (6.6 feet) w ide.

WINDOW PLUMES

A w indow plum e can occur from apost-flashover fire. A post-flashover fireis one in w hich every object in the fireroom that can b urn is burning. Theheat release rate o f a post-flashover firedepends on the am ount air availablefor com bustion, and the airflow intothe room dep ends on the size andshape of the opening. B ecause of thecom m on u se of sprinklers, con sidera-tion o f w indow plum es generally isreserved for unusual designs.

SMOKE MANAGEMENT SYSTEMS

The follow ing are three types ofsm oke m anagem ent system s: (1) fan-pow ered sm oke exhaust, (2) sm okefilling, and (3) gravity venting. Thesesystem s can be designed for a steadyfire o r an unsteady fire. Fo r inform a-tion about design fires, readers arereferred to NFPA 92B and theA SH RA E/SFPE sm oke control boo k.

The three system types are based onthe o bjective of preventing sm okefrom contacting occupants. A n alterna-tive objective is a system designed toprovide tenable conditions even w ithsm oke com ing into contact w ith peo-ple. Such tenability system s are recog-nized by NFPA 92B , the A SH RA E/SFPEsm oke control book, and CIB SE 17.Inform ation about tenability design isprovided by K lote 19.

SMOKE EXHAUST

Sm oke can be exh austed near thetop of an atrium to prevent sm oke

050

100

150

200

250 150

100

50

050 100 150 200 250 300 0 0.25 0.50 0.75 1.00 1.25 1.50

A v e r a g e

t e m p e r a

t u r e

( F )

Elevation above fuel (ft) Atrium exhaust (millions of cfm)Note: Average temperature calculated from equations ofNFPA 92B (1995). Note: Atrium exhaust calculated from equations ofNFPA 92B .

C l e a r

h e i g h t ( f t )

Heat release rate (kW):10,000 5,000 2,000

Heat release rate (kW):10,000 5,000 2,000

H

Section

Front view withdraft curtains

b

Draftcurtain

Front view withoutdraft curtains

Figure 4. Average temperature of axisymmetric plume Figure 6. Atrium exhaust needed to maintain a constantclear height with a fire in the atrium

Figure 5. Balcony spill plume

°C = (°F-32)/1.8 1 m = 3.3 ft . 1 m 3 /s = 2100 cfm


20/32


contact w ith o ccup ants during evacua-tion. There are tw o approaches todesign of smoke exh au st system s: (1)size the sm oke exhaust to m aintain aconstant clear heigh t and (2) size thesm oke exh aust so that a descendingsm oke layer do es not con tact occu-

pants during evacu ation.The first approach has the advan-

tage that the calculations are relativelysim ple, and Figure 6 show s theexhaust needed to m aintain a constantclear height w ith a fire in the atrium .Calculations for the second approachare m ore com plex, and these calcula-tions can be done by com pu ter zonem odels.

SM O KE FILLING

W hile sm oke exhaust is prob ably

the m ost com m on form of atriumsm oke m anagem ent in the U nitedStates, som e atrium s are o f such sizethat no sm oke exhau st is needed tokeep the occupants from con tactingsm oke throughout a fire evacu ation.Th is form of sm oke m anagem ent iscalled smoke fill in g and requires noexhaust capab ilities.

W itho ut sm oke exhaust, the sm okelayer that form s under the ceilinggrow s thicker, and the bottom of thatsm oke layer drop s dow nw ard.Equations can be used to calculate thetim e that it takes for the sm oke todrop to a level that is abo ve the head sof all the p eople in the atrium duringa fire evacuation. If this sm oke fillingtim e is greater than the evacuationtim e, sm oke filling is a viable form ofsm oke p rotection .

People m ovem ent calculation s areused to d eterm ine the evacuation tim e.A s w e all know , w hen a fire alarmsounds, m ost peop le have a tend encyto w ait to see if there really is a fire orto see if conditions are threaten ing.This decision tim e needs to be allow edfor in any calculation of evacuation tim e.Readers interested in p eople m ovem entcalculations should see The SFPE Han d- book of Fir e Protection Engin eeri ng. 20

GRAVITY VENTING

In the U nited K ingd om andA ustralia, gravity venti ng is often usedw here w e in the U .S. w ou ld u se fan-

pow ered sm oke exhaust. This naturalm ethod con sists of opening ven ts inthe atrium ceiling or high on the atriumw alls to let the sm oke flow out w itho utthe aid of fans. The p roposed NFPA92B addresses gravity venting system s.

The flow rate through a gravity vent

can be calculated , and it depends onthe (1) size of the vent, (2) dep th ofthe sm oke layer, and (3) tem peratureof the sm oke. W hen sm oke is detected,all the vents need to be opened at on etim e. Therm ally activated vents, likethose often used for industrial heat andsm oke venting, are inappropriate forgravity venting of atria because o f thetim e d elay in opening the vents.

The applicability of gravity ven tingdepends prim arily o n the (1) size o fthe atrium , (2) ou tside design tem pera-tures, and (3) w ind conditions. G ravity

venting is sim pler and less costly thanfan-pow ered exhausting. B ecause lossof pow er can occur during fire situa-tions, there is a significant advantageto a sm oke m anagem ent system thatrequires no pow er for fans.

Som e peop le are uncom fortablew ith gravity venting, probably becauseof the lack of positive assurance o fobtaining the desired flow . H ow ever,the reliab ility and econom ic benefits ofgravity venting are such that gravityventing w ill likely find a p lace in U .S.buildings in the future.

M AKE-UP AIR

For sm oke exhaust system s and grav-ity venting system s, air m ust be sup-plied to the atrium to m ake up for thesm oke exhaust. A few m onths ago, anengineer erroneously indicated to theauthor that he thought that m ake-up airw ould not be needed for large atria,because they already have such a largevolum e of air. This is not so. M ake-upair is essential for all sm oke exhaustsystem s and gravity venting system s.M ake-up air can be either fan p ow eredor nonpow ered. The netw ork com puterprogram , CO N TAM 96,21 can be used toanalyze nonpow ered m ake-up airflow s.

VELOC ITY LIM IT

There is a co ncern that air velocitycould disrupt the structure of theplum e resulting in failure o f the sm oke

m anagem ent system . The best currentinform ation availab le is that a velocityof 1 m /s (200 fpm ) or less w ill notcause such disruption. Fo r this reason,NFPA 92B indicates that velocitiessho uld not exceed 1 m /s (200 fpm ) inthe atrium w here there could be a

plum e. This applies to any air velocityw hether it is for m ake-up air or forsom e other pu rpose.

STRATIFIC ATION AND DETECTION

The issue of sm oke stratification isincluded in the proposed NFPA 92B .O ften a hot layer of air form s underthe ceiling o f an atrium , the result ofsolar radiation on the atrium roof.W hile stud ies have not been m ade o fthis hot air layer, m any professionalsbelieve that such layers are often in

excess of 50 °C (120 °F) (Figu re 7).W hen the tem perature of the sm okeplum e is less than that of the p restrati-fied layer, the sm oke cannot reach theceiling. In this situation, the sm okecannot activate ceiling-m ounted sm okedetectors (Figure 8).

B eam sm oke detectors can be usedto overcom e this detection difficulty,and the p rop osed NFPA 92B describesthree m etho ds of using beam detectorsfor this purpose. Tw o of the m etho dsem ploy ho rizontal beam s w ith theintent of (1) detecting the sm oke layeror (2) detecting the p lum e.

Th e third m ethod uses up w ard-angled beam s w ith the intent of detect-ing the d evelop m ent of a sm oke layerat w hatever stratification conditionsexist. In this third approach , one orm ore beam s are aim ed at an upw ardangle to intersect the sm oke layerregardless of the level of sm oke stratifi-cation . For redun dancy w hen usingthis approach, m ore than on e beamsm oke detector is recom m ended.

NUM BER OF EXHAUST INLETS

W hen the sm oke layer depth belowan exhaust inlet is relatively shallow , ahigh exhaust rate can lead to entrain-m ent of cold air from the clear layer(Figu re 9). This phenom enon is called“plugholing,”and it is addressed in theproposed NFPA 92B . To preventplugholing, m ore than one exhau stinlet point m ay be n eeded. The m axi-


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m um m ass flow rate, w hich can beefficien tly extracted using a singleexhaust inlet, is given as:

w here:= m axim um volum etric flow rate

at T s , m 3/s (cfm )T s = absolute tem perature o f the

sm oke layer, K (R)T o = absolute am bient tem perature,

K (R)d = d epth o f sm oke layer below

exhaust inlet, m (ft)β = exhaust location factor (dim en-

sionless)C 1 = 0.00887 (0.537)

The above eq uation is consistentw ith the approach o f CIBSE (1995).B ased on lim ited inform ation, suggest-ed values of β are 2.0 for a ceilingexhaust inlet near a w all, 2.0 for a w allexhaust inlet near the ceiling, and 2.8for a ceiling exhaust inlet far from anyw alls. It is suggested that d/D begreater than 2, w here D is the diam eterof the inlet. For exhaust inlets, use D = 2ab/(a + b) , w here a and b are thelength and w idth of the inlet. Theresults of experim ents conducted at theN ational Research Coun cil of Canadaare consistent w ith this approach toavoiding p lugholing. 22, 23, 24

SEPARATIO N BETW EEN INLETS

W hen the exhaust at an inlet is nearthis m axim um flow rate, adequ ate sep-aration betw een exhaust inlets needsto be m aintained to m inim ize interac-tion betw een the flow s near the inlets.This separation is also addressed inthe prop osed NFPA 92B . O ne criterionfor the sep aration betw een inlets isthat it be at least the distance from asingle inlet that w ould result in anarbitrarily sm all velocity based on sinkflow . U sing 0.2 m /s (40 fpm ) as thearbitrary velocity, the m inim um separa-tion distance for inlets located in aw all near the ceiling (or in the ceilingnear the w all) is:

Exhaust fan

Hot air layer

Temperature

E l e v a t

i o n

Exhaust fan

Plume

Hot air layer

Note: Beam smoke detectorscan be used to overcomethe detection problemcaused by a layer of hot airunder the ceiling.

Exhaust fan

Plume

Fire

Plugholing of air into smokeexhaust

Figure 7. Hot layer of air under atriumceiling (a) sketch and (b) temperatureprofile

Figure 8. Stratified smoke in an atrium with a hot layer of air under the ceiling

Figure 9. Plugholing of air into smoke exhaust inlets can result in failure of asmoke exhaust system

(a) Sketch of hot air layer (b) Temperature profile

˙maxV C d T T T o s o= −( )1 5/ 2β

˙maxV

(1)


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w here:S min = m inim um separation betw een

inlets, m (ft)= vo lum etric flow rate, m 3/s (cfm )

β = exhaust location factor (dim en -sionless)

C 2 = 0.32 (0.023)

SM OK E LAYER DEPTH

The p roposed NFPA 92B clearlyindicates that the sm oke layer m ust bedesigned to b e deep enough to allowfor a ceiling jet.

W HEN EQU ATION S D ON ’T APPLY

A s a general rule, it can be stated

that equations don’t apply w hen theassum ptions behind those equationsare not appropriate. Fo r exam ple, theequation for the m ass flow of anaxisym m etric plum e does not apply ifobstructions break up the plum e flow .A lso the basic zone m od el approachdo es not apply if the sm oke plum ecools so m uch that there is no w ell-defined sm oke layer under the ceiling.A no ther exam ple w ou ld be that thebalcony spill equations of NFPA 92B do no t apply w hen the fire is on thebalcony. It is not possible to catalog allpossible situations for w hich equationsdon’t ap ply, because of the variety andcom plexity of buildings. Practitionersneed to un derstand the assum ption sbehind the eq uations they u se to besure that the equations apply.

Documents

Fire Protection engineering Summer 2000