Research timber

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RESEARCH 1

DEVELOPMENT OF A RELIABILITY-BASED DURABILITY DESIGN METHOD

FOR TIMBER CONSTRUCTION

Agonos, Kim Nio!"

B#n$ig#n, M#%& C!"o

C#m'os#no, R#("! Ann

C#s)i!!o, K#("# *o(#&n"

I+ INTRODUCTION

Durability is one of the most important considerations in the use of wood in

construction. While many timber durability guidelines and design aids are available to

engineers and designers, the current approach to timber durability design is still very much

an art; design solutions vary from person to person and control of performance depend

almost exclusively on compliance to good building practice. This tends to inhibit innovation

and optimization of building design, and is not useful for assessing the impact of rapid

changes enforced by legislation (e.g., banning of chemicals.

!f the factors affecting durability and the mechanisms of degradation, including

inherent uncertainties, are "uantified, design procedures can be developed so that a product

or building can be designed to perform satisfactorily with a given level of confidence, much

li#e the way it is currently done in structural design. $ut while the technology of structural

engineering is well developed and applied with confidence in design, the technology re"uired

for an engineering approach to durability is far behind.

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II+ OB*ECTIVE OF THE STUDY

The long%term goal is to develop tools and methods to enable designers to&

'. redict the durability of any type of timber construction located anywhere in

)ustralia;

*. +a#e informed decisions on important variables affecting timber durability and

their influence on performance;

. Design and specify a durable system for any use or application that will achieve

its performance goals.

III+ SCOPE AND LIMITION

This paper presents an overview and some preliminary results of a ma-or

multidisciplinary timber durability research effort in )ustralia, focused on the development

of a reliability%based durability design method for timber construction. ne factor that helps

us appreciate the complexity of this underta#ing is the fact that since the inception of the

pro-ect, about /00 related references have been collected and reviewed.

The general prediction models can also be used to assess the performance of new and

innovative products and the impact of rapid changes enforced by legislation (e.g., banning of

chemicals. !n con-unction with a non%destructive testing method, the models can be used to

predict the residual life of a timber element and to develop an optimized inspection and

maintenance program. Thus, the models are #ey components of an improved asset

management system for timber structures.

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IV+ DEPENDENT AND INDEPENDENT VARIABLES

V+ SUMMARY OF METHODOLOGY

A+ P%o"$%"s

• 1ub%program '& +odel development and calibration

2igure ' illustrates the #ey steps in the development of the attac# models and

the design procedure. !n the case of decay, for example, we start with classical models

of moisture movement in porous media, using fundamental material parameters.

3limate, soil and other environmental data are used to identify the context of

durability performance, and as input to predicting performance. ) simplified

degradation or attac# model, with new material parameter inputs that can be easily

and "uic#ly measured and with reasonable prediction capability (compared to the

fundamental model, is then developed and calibrated using all available data

including historical durability index properties, new field and laboratory data

generated in the present pro-ect (e.g., from 1ub%programs * and and, when

necessary, expert opinion. Then, the durability design procedure is developed from a

composite of all the attac# models.

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• 1ub%program *& 4nvironmental agents 5 maps and databases

Detailed climate data have been collected and are being related to

microclimate within the house, and eventually to local element condition (2igs. * and

. This process re"uires climate sub%models that relate global climate data to a

specific site, from external climate to microclimate within the house, and from

microclimate in the cavity to moisture conditions of the timber (2ig. *. 3limate data

that have been collected include rainfall, temperature, relative humidity, wind,

evaporation, cloud cover and various components of solar radiation. +icroclimate

data being monitored in cavities of selected houses in locations with tropical,

subtropical and temperate climates are temperature and relative humidity (2ig. ;

also being monitored are 1implified Degradation6)ttac# +odel 4ngineering

rocedures 2undamental 4"uations 3limate Data ther 4nvironmental Data 7se

fundamental parameters of materials. 3onditions that are conducive to degradation

(initiation and6or acceleration are being investigated.

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$ased on seven years of

)ustralian data on exposed 8%-oint

specimens and the mean monthly

temperature and rainfall in )ustralia, an

above ground decay hazard map for

)ustralia has been plotted (2ig. 9a

using the 1cheffer:s index. nowing

that faster evaporation in the )ustralian

climate allows wetted timber to dry out

faster than in the northern hemisphere

(where the 1cheffer:s index was originally derived, a new decay index, referred to as

the D! index, has been derived as a function of percentage of wet days and the

average vapour pressure deficit (+. 3ause, private communication. This index fits

the updated nine%year 8%-oint data better than the 1cheffer:s index did. The

differences are seen in comparing 2igs. 9a and 9b; the D! index provides a more

refined and appropriate above ground decay hazard map for )ustralia. 2urther

refinements in the calculation of the above ground durability hazard are being

investigated.

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The termite hazard map will be based on a combination of results of a public

survey and prediction using the attac# model. The first stage of a national survey

has been completed and the second stage is on%going at the time of this writing.

+etal corrosion is a combination of atmospheric corrosion on exposed steel and

cathodic corrosion on the embedded steel. 2or the case of atmospheric corrosion,

critical attac# parameters are the effective time of wetness and concentration of air

pollutants, particularly sea%salt. $ased on analysis of available exposure test data

and modeling results, 2ig. < shows the estimated corrosion of a metal plate of mild

steel containing copper.

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• 1ub%program & +aterial resistance and test protocols

art of this sub%program is focused on developing standardised durability

assessment methods for materials, including the suitability of accelerated field

simulation laboratory techni"ues to determine resistance to termites, fungi and

corrosion. )nother focus is the determination of durability rating resistance of

common timber products for prescriptive design guidelines and of degradation rates

of treated and untreated timber for prediction models. The latter is based on both new

field data and previously collected data originally intended to obtain durability index

properties.

) considerable amount of material resistance evaluation is in progress, with

decay and termites, both in%ground and above ground, for both treated and untreated

wood. There are also above ground wood panel exposure tests and corrosion tests

with both nails and metal plates

B+ S)#)is)i#! Too!s

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The #ey to developing a design procedure is first to have predictive models for

durability. 2igure = illustrates the way the models and design procedure will be used. ) set of

input parameters is used to define the design situation, a particular hazard type is selected,

and a calculation model is used to predict the performance of the element or building system.

Typical input parameters are those that relate to the hazard, building location,

building construction, building element and maintenance programs (or user action.

erformance criteria will be classified as either structural collapse, unserviceability (such as

excessive deflections, water entry or loss of material or aesthetic deterioration. The hazards

considered include attac# by&

• 2ungi

• Termites

• 3orrosion agents

• +echanical degradation

1ince there are uncertainties in the data, models and analysis methods, it is natural to

apply probabilistic techni"ues and recognize and incorporate these uncertainties in the model

development process. 4ach attac# model is assumed to comprise a se"uence of events as

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shown in 2ig. >. The duration of each event is a random variable. This leads to a prediction in

statistical terms as shown in 2ig. ?.

VI+ RESULTS AND CONCLUSION

This pro-ect provides a framewor# and method of using fundamental degradation

models, and climate and relevant environmental factors to develop general durability

prediction models and formal design procedures for timber construction.

@esearch to date indicates that the development of formal durability design

procedures, based on prediction models, is not only feasible but is a better and more flexible

approach to ensure durable timber construction. The existence of formal timber durability

design procedures, similar in concept to structural design, will facilitate the teaching and

globalization of the technology, and hopefully lead to the day when failures related to

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durability will be regarded as failures of applied technology, and not ta#en to be an un#nown

ris# associated with the use of timber.

V+ RECOMMENDATION OF THE READER

+uch research is needed to better understand fundamental degradation mechanisms

of timber in%service, improve the attac# models and provide more reliable data to validate

and calibrate the prediction models.

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