Air & Smoke Movement in and around High Rise buildings

(Air Tracks)

Mark Fishlock

2011

Understanding the dynamics of air or smoke movement is critical to safely working in

high rise buildings!!

Affecting Factors FRS Operations:

Wind EffectBuilt-in ventilation systems -HVAC

Fire pressurisation systemsFire shafts

Lift shafts (Piston affect)Stack Effect

Combustion pressureCompartmentation breeching

AND THEY CAN ALL COMBINE!!

Built in Ventilation systems

Common on newer buildings ie HVAC

Will have over-rides (either manual or automatic)

Built in smoke control and detection

Fire Pressurisation systems

Usefulness now brought into doubt by BRE bd2410 (2005)

PPV ?

Fire fighting shafts

Requirement of Approval Document B

BS 9999 / BS 5588 Part 5: 2004.

Naturally vented with AOV on roof and shutters dampers on each

floor

Research FR 11.24

Lifts and Lift Shafts

Normal lift are not/can not be smoke tight

Lift shafts can have a micro-climate

Lift movement can create a noticeable air flow

Lift shafts

Piston Effect

Lift shafts

Piston Effect

Lift shafts

Piston Effect

Lift shafts

Piston Effect

Stack Effect

(or Chimney effect)

Combustion Pressure

High Rise are designed as hermetically sealed buildings

BRE bd2410 (2005) shown pressure increases in small mixed load fire

(plastic/carbonaceous domestic) @ 1.5 MW is 35 Pascal's .00035 Bar

Applying water increases it to 190 Pascal's

Pressurised protection of buildings aims for 50 Pascal’s

WIND

• Wind is the flow of gases on a large scale.

• Measures in KPH (MPH) or Knots

• Strongest gust UK (1989) 146mph (low level)

• Strongest sustained wind UK (1971) 73MPH

Wind

Wind: Behaviour

Winds can go from still state to Hurricane force in under 5 minutes (RAF

Lossiemouth 1981)

Wind can change direction by up to 90 degrees in 20 mins (RN Boatcraft D3)

Wind: Pressure

• Wind blowing on an object creates pressure

Wind: Pressure

• Wind blowing around an object can create a reduced pressure

Wind: Pressure

The basic formula for wind pressure is:P = .00256 V2whereP = the wind pressure (pounds per square foot)V = wind speed (miles per hour)The force created by the wind on a structure is:F = P × A × CdwhereP = the wind pressureA = the flat projected area of the structure (square feet)Cd = drag coefficient for the shape of the structure’s members.The coefficient for a flat surface is 2.0.

Wind: Pressure

SO for a failed window opening

4’ x 3’ and a wind of 40mph

P = .00256 V2 = 4 psi

18PSI = 128.6% increaseEffectively 27% O2 (Dalton law of partial pressure)

F = P × A × Cd = 98 pounds

Wind: Pressure

SO for a failed balcony patio door

7’ x 6’6” and a gust of 60mph

P = .00256 V2 = 9.2 psi

23.2PSI = 166% increaseEffectively 35% O2 (Dalton law of partial pressure)

F = P × A × Cd = 839 pounds

Wind Effect

Wind effect modelling

Wind and fire compartment

Primarily has two effects:

To create wind driven fire (an air pathway) or to pressurise or depressurise a compartment.

Wind driven fires are FORCE VENTILATED

Pressurised fires are Oxygen rich

De-pressurised fires can be under ventilated

Wind driven fire

There are inlet AND outlet to compartments creating an air path

Very high energy release rates 17Mw +

Fuel used quickly

Wind driven fire

If air pathway runs FROM the fire compartment and travels inside building

then extremely hazardous to Ff’s

Wind driven fire

If air pathway runs FROM inside the building and travels to the fire compartment then

less hazardous to Ff’s

WIND DIRECTION

C

WIND DIRECTION

C

Compartment pressurisation

There is no simple air path. (An in and out)

Fire compartment has opening(s) on same face

Fire is ventilated and fed from same window

CONTAIN & Let it burn

out???

C

Compartment pressurisation

Wind blowing onto opening (+ Pressure)

Smoke bleeding into corridor

Fire could be well ventilated

Very dangerous to enter !!

Compartment pressurisation

Wind blowing across or away from opening (-Pressure)

Air being sucked from corridor.

Ensure its not a developing

Backdraft: Confirm its vented

Safer but still poses serious risk to enter

Unventilated compartment.

MAY BECOME VENTILATED AT ANY TIME (DOOR OPENING/WINDOW FAILIURE).

May be sucking or blowing depending of phase of fire.

Risk of sudden escalation or Backdraft upon Ff entry.

May be impossible to externally ventilate.

Dangerous to enter

Wind Effect sometimes called the “Blow torch effect” ??

Has the greatest bearing on abnormal fire development in High Rise Incidents

Window failure or Fire-fighters ventilating

create direct air tracts

Fire energy is rated as its energy release over a period of time (MW). One of the primary effectors of this time span is availability of O2 and wind can greatly enhance this supply

SAFETY• FIREFIGHTERS WORKING IN THE BUILDING CHANGE ITS DYNAMICS

• KNOW THE BUILDING LAYOUT AND ITS FIXED SYSTEM

• KNOW ITS VENTILATION SYSTEM AND HOW IT WORKS

• KNOW THE STATE OF THE HOUSEKEEPING

• ALWAYS BE AWARE OF UNPREDICTED SMOKE TRAVEL (Safety officers)

• MINIMISE USE OF LIFTS

• STRIVE TO MAINTAIN COMPARTMENTATION (especially to stairwells)

• BE AWARE THAT WEATHER (WIND & TEMP) WILL HAVE A SERIOUS EFFECT ON AIR/SMOKE TRAVEL

THANK YOU

Any Questions?