A. Practices in Hong Kong
1.
WBTC No. 32/92 - The Use of Tropical Hardwood on Construction Sites
2.
PNAP ADV-5 (PNAP153) - Tropical Hardwood Timber
3.
CITES Official Documents - AFCD Endangered Species Protection
4.
The Protection of Endangered Species of Animals and Plants Ordinance (the Ordinance), Cap. 586
5.
CITES species database
B. Technical Information mainly from TRADA
1. Durability
The classifications quoted refer to the resistance to fungal decay of the heartwood of the species only. The sapwood of most species is not durable or slightly durable and should not be used in exposed situations without preservative treatment. Five natural durability to wood-destroying fungi classes are recognised in BS EN 350-1 Guide to the principles of testing and classification of the natural durability of wood:
· Class 1 very durable
· Class 2 durable
· Class 3 moderately durable
· Class 4 slightly durable
· Class 5 not durable
Information on the resistance of some timbers to insect and marine borer attack is given in BS EN 350-2 Guide to natural durability and treatability of selected wood species of importance in Europe .
Guidance on the need for preservative treatment in particular situations is given in BS 8417:2003. Note that BS 5268-5 and BS 5589 are now obsolete. Common preservative treatment include copper/chromium/arsenic (CCA), creosote, organic solvent (OS) and Boron, subject to risk categories and treatability.
2. Treatability
Refers to how easily timbers can be penetrated with preservatives applied by vacuum pressure processes. BS EN 350-2 includes four levels of treatability but recognises that the classes cannot be separated exactly from each other. Note that the ratings given here relate to the treatability of the heartwood unless specified otherwise.
The categories included in BS EN 350-2 are:
· Easy
· Moderately easy
· Difficult
· Extremely difficult
BS EN 351-1 Durability of wood and wood-based products - Preservative treated solid wood - Classification of preservative penetration and retention gives guidance on the selection of preservative treatment. Guidance is also given in BS 1282 Guide to the choice, use and application of wood preservatives and in the Draft for Development 239 Code of practice for the preservation of timber.
3. Moisture Movement
Refers to the dimensional changes that occur when dried timber is subjected to changes in atmospheric conditions. It is classed as small, medium or large and is not directly related to the shrinkage which occurs when green timber is dried to moisture contents suitable for internal use. For structural purposes, movement is not usually significant but species with small movement should be specified where stability in varying humidities is important eg decorative wood flooring.
ArchSD's General Specification cl.13.03
The maximum permissible moisture content in timber to be incorporated
into the work shall be as follows:
(a) Internal timber for use in Air Conditioned space 12%
(b) Internal timber generally 16%
(c) Timber with one face to the exterior of the building and one face to
the interior (e.g. window frames) 18%
(d) External timber (e.g. fencing etc.) 20%
4. Density
Varies depending on species and moisture content. Averages at 15% moisture content are quoted; 0.5% of the given weight can be added for every 1% increase in moisture content.
5. Source of Supply (cl. 13.012 ArchSD's GS)
It is preferably
Forest Stewardship Council (FSC) certified.
Source in Progress to Creditable Certification including
WWF Global Forest and Trade Network (GFTN) Producer Groups, the
Tropical Forest Trust, Smart Step, and
SGS Malaysia (Certification Support Programme) are considered as sources in progress to creditable certification.
A list of acceptable forest certification system according to ArchSD's GS 13.01.03 with a chain of custody system include the followings:
(a) Programme for the Endorsement of Forest Certification Schemes
(PEFC) – General
(b) PEFC – United Kingdom
(c) PEFC – Germany
(d) PEFC – Sweden
(e) Canadian Standards Association (CSA)
(f) Cerflor (Brazil)
(g) Malaysian Timber Certification Council (MTCC)
6. Endengered Species Protection (controlled by
AFCD)
Trading in some species is governed by
CITES - the Convention in International Trade in Endangered Species which lists species in three categories:
· Appendix I - Trading prohibited
· Appendix II - Trade permitted subject to export permits from the country of origin (and re-export permits as appropriate) and AFCD's import permits.
· Appendix III - Species protected within individual party states.
7. Fire Protection
(i) Timber is combustible and will easily burn out under fire. Timber shows brown colour at about 120°C–150°C, black colour around 200°C–250°C and evolves combustible vapours at about 300°C. At about 400°C to 450°C (or 300°C if a flame is present), the surface of timber will ignite and char at a steady rate. Only charred parts of a section lose all their strength whereas the remaining parts may assume to have no significant loss in strength.
(ii) Design consideration (BS5268-4.1 s5.1.2)
· residual section - making allowance for charring rate
· strength - load-bearing capacity of a flexural member using residual section and stress of 2.25 x permissbile dry stress for B>70mm or 2.0 x permissbile dry stress for B < 70mm.
· deflection - < span / 20 calculated by residual section
(ii) Some typical charring rates are recommended by IStructE and BS5268-4.1
| Species |
30min (mm)
|
60min (mm)
|
| (a) | All structural species except those in item (b) and (c) |
20
|
40
|
| (b) | Western red cedar (softwood) |
25
|
50
|
| (c) | Oak, utile, keruing, gurjun, teak, greenheart, jarrah (hardwood) |
15
|
30
|
The area of section loss due to rounding refer to BS5268-4.1 fig 1
(iii) Protection
a) Flame-Retardant Treatments (Fire-Retardant-Treated Wood and Flame-Retardant Pressure Treatments)
Fire retarded can be used to improve the surface spread of flame charateristics, with impregnation methods being preferable unless maintenance of surface treatments can be provided. The retardant does not improve the resistance, because even though the timber is relatively non-combustible the timber will still char in the event of a fully developement fire.
b) Flame-Retardant Coatings include intumescent and nonintumescent.
(iv) Established practice (Eurocode 5 and BS5268)
Temperature at an actual char-line in softwood is typically about 300°C.
Depth of char (mm): d char = b0 x t
where bo = charring rate; t = time (min)
Effective cross section: deff = d char + k0 d0
where
d0 obtained from test calibration; coefficient
k0 table in EC5.
Reference:
i.
IUCN Red List
ii.
Wikipeda - IUCN Red List endangered species (Plantae)
iii.
FSC Certificate Database
iv. American Wood Protection Association
v.
Timber species - properties and uses, Queensland Government
vi.
Timber Fender - 21.20 General Specification for Civil Engineering Works Volume 2, 2006 Edition, CEDD
vii.
Wood handbook : wood as an engineering material by US Depart. of Agriculture
viii.
Code of Practice: Raised timber deck structure, Timber Decking Association
ix.
木材防腐加工
x.
木材科學與技術
