Download timber framed buildings and nzs 3604

TIMBER FRAMED BUILDINGS AND NZS 3604
Roger Shelton1, Graeme Beattie2
ABSTRACT: This paper briefly charts the history of the New Zealand Timber Framing Standard (NZS 3604), gives
some of the background to the recent revision, and presents the initial findings of a survey conducted to establish the
performance of 3604 type buildings during the recent series of earthquakes in Canterbury.
KEYWORDS: Light timber framed buildings, NZS 3604, Canterbury earthquake
1 INTRODUCTION 123
2 BACKGROUND TO NZS 3604
Stick framed timber buildings have been used for
residential and low rise building structures in New
Zealand for over 100 years. The essence of stick
framing is the use of many, small section, timber
elements of low structural capacity distributing the
applied loads in conjunction with “non-structural”
elements such as linings and claddings. This approach
builds redundancy into the structure in two ways:
 Failure of one member allows re-distribution of
loads through alternative load paths
 Practical framing arrangements provide more
structure than is strictly necessary (ie walls
which are ignored for bracing purposes).
NZS 3604 “Timber framed buildings” [1] sets out the
construction requirements in means of compliance
format for light timber framed buildings in
New Zealand. A companion document “BRANZ P21
Test method” [2] provides a versatile calculation basis
and test method for lateral load bracing elements.
In applying engineering rationale to a well developed,
“sorted” building system, the standard was ground
breaking when it was first published in 1978. By placing
limits on the size and scope of buildings covered, safe
but not unduly conservative solutions are provided for a
wide range of low-rise timber buildings without the need
for the involvement of structural engineer designers.
Thus the cost of the inherent conservatism is balanced
against the savings in structural design fees, where the
simplicity of the design does not warrant such close
attention. Those limits govern the building size, height
and roof slope, floor loadings, and snow loadings. Wind
and earthquake loads are also limited by restricting the
zone or area in which the building may be situated. For
buildings outside these limits, a specific structural design
is required for each building.
Both documents have been revised and updated several
times since they were first written but remain at the core
of tiber framed building construction in new Zealand.
Over the years since 1978 most critical elements of the
building structure covered in NZS 3604 have been
verified by calculation where possible, or by test where
appropriate. The latter is particularly true of fixings. As
changes in building technology occur and new systems
and techniques are developed, this testing and
verification process continues.
Redundancy goes a long way to providing resilient
buildings which are able to cope well with
This is in contrast to the philosophy behind typical
“engineered” structures which use fewer, larger and
stronger members, each carrying much heavier loads.
The stick frame approach has evolved to suit New
Zealand’s timber resource and the carpentry skills of its
construction workforce. Thus, much of the development
and fine tuning of the system over the years has evolved
from the ground up.
Timber framed buildings now represent about 95% of
structural timber usage (by volume) in New Zealand.
They also constitute about 95% of the existing
residential building stock, although the proportion of
new buildings is slightly lower as new systems (such as
light gauge steel framing) are introduced.
1
Roger Shelton, BRANZ, 1222 Moonshine Road, Judgeford,
New Zealand. Email: [email protected]
2
Graeme Beattie, BRANZ, 1222 Moonshine Road, Judgeford,
New Zealand. Email: [email protected]
2.1 CHANGES IN 2011
The current revision of the Standard was conceived as a
"Limited technical review", and focuses on five main
areas:
i. Updating to accommodate the change in
the cited Loading Standard from
NZS 4203 to the AS/NZS 1170 series
[3].
ii. Migration of the cladding provisions and
details from NZS 3604, into the New
Zealand Building Code [4] clause
E2/AS1, the Acceptable Solution for
weathertightness.
iii. General improvement of readability and
clarification
iv.
Updating of durability requirements with
other Standards
v.
Updating to reflect new building
techniques and materials, in particular
engineered timber products.
The change of loading standard from NZS 4203 to
AS/NZS 1170 meant revisiting all the selection tables in
NZS 3604. However the net effect for users is generally
quite minor. Two areas where changes were more
significant were in earthquake and snow loads. Greater
understanding of New Zealand’s seismicity and the
effects of soil types resulted in quite major changes to
the earthquake loading standard. These were treated in
NZS 3604 by picking a default demand level (z = 0.46,
and soil class E) and providing a table for other
conditions.
Changes to the snow loading standard prompted fine
tuning of the treatment of snow loads in NZS 3604, and
by adjusting the altitude boundaries of the snow zones
and basic ground snow loads, much great coverage
around New Zealand was possible with negligible
penalty in terms of allowable member spans.
More detailed information on these changes is described
in The Engineering Basis of NZS 3604 [5].
The provisions for durability have changed to better
align with other Standards, in particular the corrosion
zone classifications used in ISO 9223 [6].
The
aggressive nature of timber treatments high in copper
towards galvanised fasteners has also been recognised,
with greater requirements for stainless steel fixings in
exposed situations.
Use of engineered wood products (LVL and Glulam) and
proprietary roof truss systems has been extended and
clarified.
Engineered wood products may be used either as direct
substitutes for sawn timber, or as an equivalent
proprietary system, provided the system has been
engineered in accordance with NZBC B1/VM1, and the
load reactions do not exceed 16 kN to avoid overloading
the rest of the timber framed structure. The procedure
for design checking and construction of trussed roofs
were developed in conjunction with the Frame and Truss
Manufacturers Association [7] and are consistent with
their Code of Practice. Girder truss loads and reactions
are also limited to 16 kN.
3 THE PLACE OF NZS 3604 IN THE
NEW ZEALAND BUILDING
INDUSTRY
NZS 3604 is a technical document, written and
developed by the industry to suit New Zealand’s
building environment.
It is aimed primarily at
“designers” although there is a lot of information
relevant to builders, particularly in the area of fixings
and connections.
With the introduction of the NZ Building Act in 1992 [8]
and the NZ Building Code (NZBC) shortly after,
NZS 3604 slotted neatly into the building control
hierarchy as a series of pre-engineered solutions for the
structural safety, durability and weathertightness aspects
of timber framed building construction. It thus became
referenced under the NZBC as an “Acceptable Solution”
for timber framed buildings. This means that it is the
preferred, but not the only, way of constructing timber
buildings in New Zealand. Buildings, or parts of
buildings, which fall outside its scope require specific
structural engineering design to verify that they meet the
intent of the NZBC. This has become common practise,
with the basic building covered by NZS 3604 but an
increasing number of specifically designed components
such as engineered wood products, bracing panels,
timber connectors and so on.
NZS 3604 provides a framework for all participants of
the building industry, for example the wind zones are
used by component manufacturers (such as windows),
and have also been translated into the Building Code
compliance documents for weathertightness.
The
Standard is also used as a benchmark for suppliers and
manufacturers to design and produce their own
proprietary products and systems, which are designed
and tailored to fit within its design philosophy.
To aid this process, the background, engineering basis,
and critical parameters are set out in [5].
Over the 34 years of NZS 3604’s existence, New
Zealand building styles have changed dramatically, and
this has proved to be NZS 3604’s greatest challenge.
The 100 square metre rectangular box has disappeared
and today’s houses are larger, have fewer walls, more
windows, and are built in more exposed locations, thus
placing much greater demands on the structure. There
are continual requests from many quarters to include
more situations, materials and systems, and as a result
the document has increased in size from a modest A5
pamphlet, into a large ring binder with over 400 pages.
4 CANTERBURY EARTHQUAKES
4.1 INTRODUCION
The revision was almost complete late in 2010 when the
Standard was put its greatest test, the series of
earthquakes in Canterbury lasting, with aftershocks, until
the end of 2011.
The majority of residential buildings in Christchurch are
timber framed structures, and up to half of them would
have been influenced, either wholly or in part, by
NZS 3604. Thus, its provisions and design philosophy
got a good road test.
4.2 SEPTEMBER 2010
The majority of building damage after the September
2010 event resulted from liquefaction and lateral
spreading of the foundation sub-soils, as shown in Figure
1. This resulted in widespread consequential damage to
timber superstructures as shown in the figure. However,
buildings experiencing only shaking damage fared well
in general, and no collapses of timber framed buildings
occurred.
Figure 1. House severely distorted by ground movement
and lateral spreading.
4.3 FEBRUARY 2011
The February 2011 event produced ground shaking up to
2 times the ULS design levels, and damage to timber
framed houses was much more widespread, particularly
on the hillside suburbs near the epicentre to the southeast of the city. Houses on concrete floor slabs suffered
again from liquefaction and lateral spreading of the
foundations, although houses built on suspended timber
floors with perimeter concrete foundations generally
performed well.
Damage that did occur was
concentrated on brittle elements such as brick chimneys,
veneers and roof tiles, and timber elements such as
weatherboards fared particularly well. There was no
loss of life in residential buildings, but there were a few
building collapses, and the cost of repairs and rebuilding will be a significant drain on New Zealand’s
economy for many years to come.
4.4 JUNE 2011
The effects of the major aftershocks in June were similar
to those of the February event. In particular, many
hillside houses in the Sumner/Redcliffs area were further
damaged as this was very close to the epicentre.
4.5 BRANZ DAMAGE SURVEYS
BRANZ undertook a comprehensive survey of the
performance of residential buildings after both the
September and February events.
After the September event, the BRANZ surveyors
accompanied the EQC Insurance Assessors, in an effort
to minimise disturbance to the home owners, many of
who were badly traumatised by the event. Unfortunately
this gave little control over the properties visited and
meant that the sample was badly skewed. This survey
was almost completed when the 22 February 2011
earthquake struck.
Following the February event, BRANZ undertook a
another survey, this time of over 300 houses, randomly
selected from within the boundaries of Christchurch city.
The process involved randomly selecting a little more
than 50 mesh blocks from the Statistics New Zealand
database. Within each mesh block, six adjacent houses
were selected for surveying at the southeast corner of
each mesh block. Each property was visited by a team
of two BRANZ representatives with a comprehensive
survey form to gather observations about the site and its
hazards (eg liquefied, rock-fall susceptible), house age,
house style, construction materials and then estimates
were made of the extent of damage sustained by the
various elements of the structure.
Of the 314 fully completed surveys 93 houses were
identified as having been built during the period of
existence of NZS 3604.
The data collection process was designed to fully
describe the property and its construction, and to
quantify the damage experienced in the two earthquakes.
4.6 SUMMARY OF FINDINGS
4.6.1 Foundations
The predominant types of foundations used in
Christchurch houses were either a suspended timber
floor on concrete piles with a concrete perimeter
foundation, or in the more modern houses, a concrete
floor slab.
The effects of liquefaction and associated lateral
spreading on the concrete floor slabs has been well
documented.
However if there was no ground
movement on the property, the slab performed well.
Timber suspended floors performed relatively well, even
on soils affected by liquefaction. This performance can
be attributed to the presence of the concrete perimeter
foundation which provided a squat lateral force resisting
element. In buildings where ground movement or lateral
spreading did occur, at least the piled foundations
allowed access beneath for repairs or re-levelling.
Where problems did occur was where the foundation
wall supporting the veneer and the piled floor foundation
were separate or not well connected together. This detail
was unable to resist ground movement, as can be seen in
Figure 2.
function and held the upper part of the house together.
This can be seen in the severely deformed house in
Figure 4. Valleys were one exception to this, and proved
to be a weak point in many houses due to the loss of
continuity (see Figure 5). The consequential damage can
be seen in Figure 5.
Figure 2. Independent wall and floor foundations have
separated.
4.6.2 Walls
Timber framed walls generally performed well where the
site was not affected by liquefaction. Timber claddings
such as weatherboards were virtually undamaged and
because of their elemental nature didn’t show the effects
of minor ground distortion. Sheet cladding such as
plywood or fibre-cement were also relatively
undamaged.
They clearly provided a bracing or
stiffening function to the building even if not specifically
designated as bracing walls.
Plasterboard was the predominant type of wall lining in
the houses surveyed, with lath and plaster, and fibrous
plaster the next most common systems. 85% of wall
linings had some extent of cracking to at least the sheet
joints. Diagonal cracking to sheets, usually emanating
from internal corners (see Figure 3) was much less
common, and complete sheet detachment was rare.
Figure 4: The roof framing has held the upper part of the
building together.
This is a simple detail to rectify and has already received
a positive response from practitioners spoken to by the
authors.
Figure 5 Ceiling damage consequent to failure at the
valley
Figure 3. Cracking of plasterboard frequently began at
internal corners.
An observation was made by several owners that their
houses now seemed more “flexible” as a result of the
earthquakes and the numerous aftershocks. This was
evidenced by vibrations and creaking during strong
winds. BRANZ is currently investigating the causes of
this phenomenon and possible remedial measures.
4.6.3 Roofs
Roof structures (either stick framed or nail plate trussed)
were rarely damaged, even when the house was severely
distorted. The continuity provided by purlins or tile
battens and ceiling battens provided a diaphragm
4.6.4 General
Taking an overview of the performance of light timber
framed buildings in the Canterbury earthquakes, while it
is true that they performed well in general, there are
some lessons to be learnt from the experience.
Structures on hillsides, complex shapes, and horizontal
and vertical irregularity will always provide challenges
for light timber framing systems. Multiple foundation
levels can make dynamic behaviour unpredictable, and
take the structure outside the simple dynamic models
that both AS/NZS 1170.5 and NZS 3604 are based upon.
Additionally, complex plans and elevations often result
in connection details that are difficult to achieve in the
context of light timber framing.
Buildings with large window openings may result in
structures with low local stiffness, and there were many
examples in the hill suburbs in particular where lack of
stiffness resulted in damage to windows and large
glazing elements (see
Figure 6).
allowed surveyors access, at a time of real anguish and
upheaval in their own lives.
REFERENCES
[1]
[2]
[3]
[4]
[5]
Figure 6. The window was unable to accommodate the
in-plane displacement of the timber structure.
One result of the Canterbury earthquake series was that
the seismic hazard factor, z, for Christchurch has been
increased from 0.22 to 0.3 to account for the greater
seismicity of the Canterbury region for at least for the
next few years. Fortuitously, the seismic zone of
NZS 3604 that encompasses Christchurch (zone 2)
already included a z factor of 0.3 so no alteration was
required.
5 CONCLUSIONS
The initial conclusion from observations of the
performance of light timber framed buildings in the
Canterbury earthquakes was that they performed well if
they were within the scope of NZS 3604, and there was
no liquefaction at the site. This is in spite of the fact that
many experienced levels of ground shaking that were
beyond what they would have been designed to resist.
However damage was very costly, and thought should be
directed towards ways to minimise this for the future.
There are already a number of lessons to be learnt from
the experience which should improve performance in
future events.
The results of two surveys of timber framed building
performance are being analysed at the moment, and
more detailed findings will be available towards the end
of the year.
ACKNOWLDEGEMENTS
The survey was funded by the Building Research Levy.
The authors are grateful for assistance in conducting the
surveys provided by contractors, and staff and students
from Auckland and Canterbury Universities. They
would also like to pay a special tribute to the people of
Christchurch who willingly opened their homes and
[6]
[7]
[8]
Standards New Zealand. 2011. NZS 3604 Timber
framed buildings. SNZ, Wellington, New Zealand.
Shelton RH. 2011. A wall bracing test and
evaluation procedure. BRANZ Technical Paper
P21. BRANZ Ltd, Judgeford, New Zealand.
Standards Australia/Standards New Zealand.
2002. AS/NZS 1170. Structural design actions,
Parts 1 to 5. Standards Australia Sydney,
Australia.
New Zealand Government. The Building
Regulations 1992. First Schedule, The Building
Code. NZ Government, Wellington, New
Zealand.
Shelton RH. 2007. The engineering basis of
NZS 3604. BRANZ Study Report 168. BRANZ
Ltd, Judgeford, New Zealand.
International Standards Organisation, 1992.
Corrosion of metals and alloys – Corrosivity of
Atmospheres – Classification. ISO 9223. ISO
Geneva, Switzerland.
Frame and Truss Manufacturers Association of
New Zealand (FTMA). 2010, Code of Practice.
FTMA, Wellington, New Zealand.
New Zealand Government. The Building Act
1992. NZ Government, Wellington, New
Zealand.