Opinion
8 — Build 146 — February/March 2015
DESIGNERS of fire-rated boundary walls for simple
residential structures are often asked to show
that the wall can resist ‘a uniformly distributed
horizontal face load of 0.5 kPa in any direction’.
Is this really necessary?
Getting the definitions clearFor fire resistance and fire protection, the Building
Code Acceptable Solutions C/AS1–C/AS7 define:
● structural adequacy as the time (in minutes)
that a prototype specimen carries its applied
load (within defined deflection limits) during
the standard test for fire resistance
● stability as the support provided to a building
element having a fire resistance rating (FRR).
This is intended to avoid premature failure
due to structural collapse as a result of any
additional loads caused by the fire.
The Verification Method C/VM2 has the same
definition for structural adequacy but does not
define stability.
Structural adequacy
Structural adequacy is determined by testing
and is represented by the first number in a fire
resistance rating. It is achieved when a specimen
successfully withstands the vertical (axial) loading
applied for the standard furnace test duration, for
example, 60/xx/xx.
A non-loadbearing element has no vertical
load applied in the fire test and does not have a
structural adequacy rating, for example, –/xx/xx.
Stability
Stability is not the outcome of a standard test
but a function of building design and how various
elements interact. Designing for stability requires
understanding of how a structure might collapse
following a fire.
What does the Building Code require?The Building Code requirements for stability fol-
lowing fire are in clause B1 Structure and clause C
Protection from fire.
Clause B1 Structure onerous
Clause B1 Structure requires buildings and building
elements to withstand the combination of loads
likely to be experienced during construction or
alteration and throughout their lives, considering
conditions likely to affect stability, including fire.
B1/VM1 provides criteria for checking post-fire
strength and stability and modifies AS/NZS 1170
Structural design actions. AS/NZS 1170 refers to
‘remaining walls that may collapse outwards
after fire’, seemingly permitting collapse inwards.
B1/VM1 requires stability ‘in any direction’, simply
needing buildings to remain standing during and
after fire until either repaired or demolished.
Clause B1 and B1/VM1 do not single out
elements having an FRR but appear to simply
require all buildings and all structural elements in
buildings to survive fire without loss of stability. I
consider complying with these requirements to be
unrealistically onerous, most likely unintended,
and designers prefer to comply with clause C.
Clause C6 Structural stability
Building Code clause C6 Structural stability
requires structural systems in buildings to
maintain stability during fire to ensure a low
probability of injury to occupants and Fire Service
personnel and to protect other property.
C6, again, appears to require post-fire stability
for all structural systems in buildings. However,
the performance requirements refer to ‘systems
necessary for structural stability in fire’. The most
likely intended interpretation is that stability
provisions only apply to elements required to
have an FRR. Text within Acceptable Solutions
C/AS2 to C/AS6 (paragraph 4.3) supports this
interpretation but is not included in C/AS1 and
C/AS7.
BY HANS GERLICH, CPENG, INTPE, MIPENZ, FI-ST CONSULTING LTD, PARAPARAUMU BEACH
Boundary wallsCURRENT NEW ZEALAND BUILDING CODE PROVISIONS FOR THE STABILITY OF BOUNDARY WALLS IN HOUSES FOLLOWING FIRE ARE AMBIGUOUS. IS THERE A
SIMPLER ALTERNATIVE?
Build 146 — February/March 2015 — 9
Common (and convenient) interpretation
appears to be that two-way stability against
a B1/VM1-nominated 0.5 kPa face loading ‘in
any direction’ is required for fire-rated boundary
walls, including those of simple residential
structures.
Let’s consider an alternative for single family homesI suggest an alternative compliance path for
residential boundary walls could be as follows:
● Clause B1 Structure calls up NZS 3604:2011
Timber-framed buildings and NZS 4229:2013
Concrete masonry buildings not requiring
specific engineering design as Acceptable
Solutions. Clause B1 is complied with if
buildings are constructed in accordance with
these standards.
● C/AS1 covers single household units and small
multi-unit dwellings. Structural stability
provisions (paragraph 4.3 in other C/AS
documents) do not exist in C/AS1. We could
thus, fairly assume that structural stability is
automatically satisfied, and therefore NZBC
clause C6 is complied with.
There is an argument that the comment ‘The fire
design load should be as specified in B1/VM1’
under the structural adequacy definition in the
C/AS documents means the 0.5 kPa face load
from B1/VM1 must be resisted. However, struc-
tural adequacy is clearly defined ‘In the context
of the standard test for fire resistance’, and only
vertical (axial) loads are applied in test standards
such as AS 1530.4-2005. Standard furnace test-
ing does not deal with lateral stability.
Using the suggested alternative compliance
path means that the lateral stability of
fire-rated boundary walls is automatically
complied with when buildings are constructed
in accordance with C/AS1 and NZS 3604:2011 or
NZS 4229:2013.
This is common sense for simple structures
with a 30-minute FRR where it is most unlikely
that external walls will collapse in such a way as
to threaten fire-fighters or adjacent property.