Boundary walls · 2015-02-02 · ii 8 — Build 146 — February/March 2015 DESIGNERS of fire-rated boundary walls for simple residential structures are often asked to show that the

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.