Page 71 - Build 161

Vertical re spread
BRANZ investigations into the e ect of ames projecting from building openings such as windows will result in useful guidelines for
re safety practitioners.
BY KEVIN FRANK, BRANZ FIRE RESEARCH ENGINEER
BUILDING FIRE SAFETY designers typically consider two main hazards with vertical re spread over external walls of buildings: ● Firespreadthroughunprotectedopeningssuchaswindows.
● Firespreadinvolvingcombustiblefaçadematerials.
While the contribution of combustible façades has received a lot of attention recently due to some high-pro le res, re spread via unprotected openings is also important.
How re spreads in a building
Like any re, a re in a building requires heat, oxygen and fuel. As a re starts to grow, the building will begin to a ect the re development. When the building envelope heats up, it radiates heat back to the re and elements such as windows can start to fail.
The presence of the building also restricts the ability of the re to access oxygen from the air. The heat in the building will start to produce more ammable fuel vapours than can be burned by the oxygen entering through openings in the building. If enough heat and fuel vapour are escaping with the smoke leaving the building, ames will start to burn outside of the building as it mixes with fresh air.
These ames project vertically upwards and can threaten unpro- tected openings such as non- re-rated windows above the re, with a risk of re spread back into the building on upper oors. In some speci c situations, building design must account for this type of re spread to comply with the Building Code.
More guidance needed
The New Zealand Acceptable Solutions and Veri cation Methods for protection from re both provide some guidance on preventing external re spread via unprotected openings. In the Acceptable Solutions, combinations of re-rated spandrels and horizontal projections (known as aprons) are permitted where this hazard is a concern (see Table 1).
The Veri cation Method requirements are satis ed by calculating the e ect of radiation from re plumes ( ames) projected from openings. No detailed guidance on how to evaluate the radiation from the projected re plume is given.
Table 1
C/AS permitted combinations of spandrel and apron geometry
Fire safety
SPANDREL HEIGHT (M)
APRON PROJECTION (M)
0.00
0.60
0.50
0.45
1.00
0.30
1.50
0.00
Build 161 — August/September 2017 — 69
FEATURE SECTION