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11th International Research/Expert Conference
”Trends in the Development of Machinery and Associated Technology”
TMT 2007, Hammamet, Tunisia, 05-09 September, 2007.
ANALYSIS OF PROPERTIES OF PLYWOOD
FOR SOME AREAS OF USAGE IN WOODEN CONSTRUCTIONS
Prof. Dr. sc. Salah Eldien Omer
SAG CONSALTING
Vramčeva 17, Zagreb
Croatia
Mr. Sc. Minka Ćehić
Technical faculty in Bihać
University of Bihać
Bosnia & Herzegovina
Mr. Sc. Atif Hodžić
Technical faculty in Bihać
University of Bihać
Bosnia & Herzegovina
ABSTRACT
For production wooden floors, walls, roofs and concrete panels use plywood, that different in their
properties. Choice property of plywood depends of area use.
In this paper show analysis of properties of plywood for some characteristic case of use in wooden
constructions.
Key words: properties of plywood, wooden construction, usage, analysis
1. INTRODUCTION
The most important raw material for plywood is a renewable natural resource - wood. For production
plywood use deciduous (beech, poplar, birch and other) and conifer (for example spruce, pine)
veneers. The standard plywood is made up of thin multiple cross-banded veneers. In addition to
standard cross-banded construction a range of orientated special constructions, aimed at specific end
uses are available.
There are several types of plywood: deciduous or conifer plywood – deciduous or conifer veneers
throughout the construction; combi plywood - two deciduous veneers (for example birch) on each
face and alternate inner veneers of conifer and deciduous (birch); combi mirror plywood - one
deciduous (birch) veneer on each face and alternate inner veneers of conifer and deciduous (birch).
The vast majority of plywood is of cross-banded construction bonded with phenol resin adhesive.
Normal gluing quality is suitable for use in exterior (service class 3) situations when properly
protected. A small part cross-banded plywood production is bonded with urea formaldehyde glue.
These boards are suitable for use in dry (service class 1) or humid (service class 2) conditions. The
phenol formaldehyde gluing fulfils the requirements of EN 314-2 class 3 exterior. Phenol
formaldehyde glued plywood products exhibit very low levels of formaldehyde emissions. Urea
formaldehyde glued products have slightly higher values but they still fulfill the requirements of the
most demanding European standards relating to formaldehyde emission and content.
Deciduous, combi, combi mirror and conifer plywood panels can all be supplied overlaid or coated to
meet specific user requirements. The main types of surfaced panels manufactured by the plywood
industry are as follows: phenolic film faced, smooth or textured, painting film faced, melamine film
faced, special products: painted and stained plywood, veneered plywood, CPL or HPL laminate faced
plywood, polypropylene plastic foil coated plywood, glass fiber reinforced surfaces, metal and
mineral aggregate faced plywood and plywood provided with sound insulation.
Plywood produced by nominal thickness at 4 – 50 mm and standard size 1200/1200 mm to 3050/3660
(4000) mm.
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2. SOME PRINCIPLES OF DESIGN OF PLYWOOD
Criteria for the structural design and appraisal of structures or structural elements made from wood or
wood products are provided in European Standards. Technical properties of plywood are determined
by EN 310 to EN 13986. The design guidance given is based on the limit state design principles of
Eurocode 5 (ENV 1995-1-1) published in 1993. The design is carried out for strength limit states to
assure that the effect of factored loads, determined by a structural analysis of the effect of the
applicable types of loads and load factors, does not exceed the factored resistances calculated from
specified strengths of materials adjusted by the appropriate factors affecting the specified strengths.
For serviceability limit states (such as deflection), design ensures that the effect of specified loads
results in structural behavior that falls within the specified limits. The limit state design approach is to
provide adequate resistance to certain limit states, namely the ultimate limit state and the
serviceability limit state. Ultimate limit state refers to the maximum load carrying capacity of the
construction while serviceability limit state refers to the normal use of the construction. In ultimate
limit state design it shall be verified that the design stress σd is less than the design strength fd . The
design stress σd is calculated using the design value of the load Fd . For design situations with only
one variable load, for example snow or impose load, the design load is given by
Fd = 1.35 Fk,perm + 1.5 Fk,var. (1)
For design situations with two or more variable loads the design load is given by
Fd = 1.35Fk,perm + Σ1.35 Fk,var (2)
where Fk,perm is the characteristic value of the permanent load and Fk,var is the characteristic value of
the variable load. The most unfavourable design load shall be used. The design strength fd is given by
fd = kmod · fk / γm (3)
where fk is the characteristic value of strength and γm is the partial safety factor for the material. For
plywood as for other wood and wood based materials the value of γm is 1,3 kmod is a factor taking into
account the effect of duration of load and moisture content (service class). The partial safety factors
for loads γq given in equations may be reduced from 1,35 to 1,.20 and from 1,.5 to 1,35 for one-storey
constructions with moderate spans that are only occasionally occupied.
Floors are usually designed to service class 1 and load duration class medium-term. The design
strength fd is given values of kmod = 0,80 , kdef = 0,25 , γq = 1,5 and γm = 1,3
Properties of plywood depend on load cases, for example: a uniformly distributed load or a
concentrated load over an area of 50 x 50 mm on a continuous plate strip with one and two equal span
lengths, a uniformly distributed load or a concentrated load over an area of 50 x 50 mm on a simply
supported plate. If there are high loads over a small contact area, compression perpendicular to face of
plywood could be critical. In most practical cases the following values can be used: birch plywood 9
[N/mm2], combi plywood 5 [N/mm2], spruce plywood 4 [N/mm2] for bearing on face
Roofs are usually designed to service class 2 and load duration class medium-term. Consequently, the
same load resistance values given for floors can be used. Furthermore, the deflection values shall be
multiplied by kdef, corr = 1,04.
Vehicle floors are designed to service class 2 and load duration class shortterm. Based on general
design principles, tabulated load resistance values for floors exposed to loads from wheels of different
spans and thicknesses are given. Design the load resistances and deflections were calculated
according to the following assumptions: γq = 1,0 , γm = 1,.0 , kmod = 0, 90 , kdef = 0,00 .
The majority of plywood used in concrete formwork is phenol film surfaced. The strength of the
formwork board depends on the type of plywood used. Based on general design principles, tabulated
load resistance values for continuous plate strips with equal spans used as concrete formwork. The
load resistances and deflections were calculated according to the following assumptions: γq = 1.2 ,
γm = 1.3 , kmod = 0.70 , kdef = 0.40 . The plywood used in concrete formwork are designed to service
class 3 and load duration class short-term
3. ANALYSIS OF PROPERTIES OF PLYWOOD
In addition to strength, modulus of elasticity and shear modulus the density and section properties are
needed as input values in the design process.
The mass of plywood primarily depends on the wood species, but is also affected by the in service
moisture content. For practical design purposes, typical values range from 500 to 600 [kg/m3] for
Douglas Fir plywood, and 400 to 500 [kg/m3] for Canadian Softwood plywood, 680 [kg/m3] for ,
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Birch plywood, 620 [kg/m3] for combi plywood, 520 kg/m3 for conifer plywood (thin veneers) and
460 kg/m3 for conifer plywood (thick veneers).
The moisture content of plywood is normally 7-12 % when leaving the mill. After delivery the
moisture content of plywood may change (usually increasing) during transportation, storage and
further processing. Like all other wood-based materials, plywood is a hygroscopic product and
exhibits viscous-elastic mechanical behavior. For these reasons, it is necessary to take moisture
conditions into consideration when loading plywood. Plywood has balanced moisture content under
given conditions of relative humidity (RH) and air temperature (T). In the basic condition defined in
Eurocode 5: with T = 20°C and RH = 65 %, the equilibrium moisture content of thin-veneer plywood
(birch, combi and conifer) is around 12 % and thick-veneer conifer plywood 10 %. An increase in
moisture content will result in a decrease in the strength, modulus of elasticity and shear modulus
values. Unlike some other wood-based panel products, exterior quality plywoods will normally revert
to their original strengths and modules when returned to their original moisture content.
The dimensional changes in and across the face grain direction of exterior plywood averages 0,015 %
increase per 1 % increase of moisture level of plywood, throughout the working range of moisture
content of 10 - 27 %. Changes in board thickness over the same working range of moisture content
will average 0.3-0.4 % increase per 1 % increase of moisture level. The moisture permeability of
panels is important in, for example the design of composite external walls and roofs of buildings.
Transmission rate for combi plywood thickness 6,5 – 21 mm is g/(m2·24h) 7,0 – 16,4 for film-faced
combi plywood thickness 6,5 – 21 mm is 2,9 – 3,5 and average 14,8 for conifer plywood thickness 9
mm. Value of transmission rate increase with thickness reduction of panels. The vapour permeability
of plywood is dependent on its moisture content. When the moisture content of plywood increases, the
vapour permeability is also greater.
The thermal conductivity of plywood is dependent on its moisture content. Plywood has excellent
dimensional stability under heat, far superior to that of metals and plastics. In practice, the thermal
deformation of plywood is so small, that it can generally be disregarded. Standard plywood and most
coated plywood products are suitable for use at temperatures of 100°C and many up to 120°C.
Plywood has an optimal dimensional stability under heat and a low rate of combustion, better than
solid wood. The temperature at which plywood will ignite when exposed to a naked flame is about
270°C whilst a temperature of over 400°C is needed to cause spontaneous combustion. When exposed
to a fully developed fire, plywood chars at a slow and predictable linear rate (about 0.6 mm per
minute), which enables it to be used in certain fire resisting constructions. This property can be
improved by impregnation or coating the plywood with proprietary formulations or by facing with
non-combustible foils. Plywood is a good insulating material in relation to its weight. The sound
insulation of plywood can be improved by using sandwich construction and by avoiding gaps between
elements.
Formaldehyde emission from phenol formaldehyde and urea formaldehyde resin adhesive bonded
plywood determined according to EN 717-2, the formaldehyde emission from unsurfaced exterior
plywood and limits of EN 1084.
The majority of plywood used in concrete formwork is phenol film surfaced. The strength of the
formwork board depends on the type of plywood used. Every types of plywood have characteristic
value of properties. Table 1. show values for several properties of birch Finnish plywood.
Birch plywood is characterised by its excellent strength, stiffness and resistance to creep. It has a high
planar shear strength and impact resistance, which make it especially suitable for heavy-duty floor and
wall structures. Oriented plywood construction has a high wheel carrying capacity. Birch plywood has
excellent surface hardness, damage and wear resistance. Sanded birch plywood has a smooth and
durable surface. Its pleasant, light-coloured visual appearance offers the best base for finishing.
Properly surfaced and edge sealed birch plywood also offers excellent weather and moisture
resistance. Panel shear about 9,5 [N/mm2] and modulus of rigidity GV ll = 620 [N/mm2] . Typical end
uses of birch plywood are concrete formwork systems, floors, walls and roofs in transport vehicles,
container floors, floors subjected to heavy wear in various buildings and factories, scaffolding
materials, shelves, load bearing special structures, traffic signs, furniture and die boards.
Combi plywood is characterized by its strength and stiffness properties which are in many respects
virtually the same as those of birch plywood. The strength and stiffness properties on its major axes
are quite similar, which ensures a balanced structure. An exception to this is planar shear, where the
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strength in the cross-grain direction of the face veneer is clearly inferior to the strength in the grain
direction.
Table 1. Lay-up, thickness, area, and section modulus, second moment of area as well as bending,
tension and compression properties of cross sections of sanded Finnish birch plywood
Combi plywood has a smooth and durable birch face and surface hardness and damage resistance are
comparable to those of birch plywood. Its pleasant, light-coloured visual appearance offers a good
base for finishing. Bending about fmll = 29,9 - 50,8 [N/mm2] and fm⊥ = 29,0 – 34,6 [N/mm2],
compression fcll = 19,5 – 24,5 [N/mm2] and fc⊥ = 22,8 – 25,3 [N/mm2], tension ftll = 15,1 – 19,1
[N/mm2] and ft⊥ = 32,8 – 36,5 [N/mm2]. Panel shear about 7,0 [N/mm2] and modulus of rigidity GV =
581 – 600 [N/mm2] . Typical end uses of combi plywood are concrete formwork systems, floors,
walls and roofs in housing constructions, farm buildings and related structures, vehicle floors, walls
and roofs, furniture, fixtures and shelves, scaffolding materials and packages.
Spruce plywood is characterised by its less dense surface when compared with birch, a prominent
grain structure and a larger number of knots. The panel has a low weight and is easy to work and nail.
Strength and stiffness properties are reasonably good and dimensional changes when subjected to
moisture variations are minimal. Bending about fmll = 21,8 – 37,6 [N/mm2] and fm⊥ = 6,0 – 19,8
[N/mm2], compression fcll = 18,5 – 22,0 [N/mm2] and fc⊥ = 14,0 – 17,5 [N/mm2], tension ftll = 14,4 –
17,1 [N/mm2] and ft⊥ = 10,9 – 13,6 [N/mm2]. Panel shear about 7,0 [N/mm2] and modulus of rigidity
GV = 530 [N/mm2]. Typical end uses of spruce plywood are floors, walls and roofs in house
constructions, wind bracing panels, vehicle internal body work, packages and boxes, hoarding,
fencing and temporary works.
4. CONCLUSION
Plywood with modern construction is good used widely in constructions.
The characteristics are as follows: bending about fmll=21,8–65,9 [N/mm2] and fm⊥=6,0–34,8 [N/mm2],
compression fcll=18,5–31,8 [N/mm2] and fc⊥=14,0–25,6 [N/mm2], tension ftll=14,4–45,8 [N/mm2] and
ft⊥=10,9–36,9 [N/mm2]. Panel shear about 7,0–9,5 [N/mm2] and modulus of rigidity GV =530–620
[N/mm2].
Definite the development of plywood construction opens wide range of usage: formwork systems,
floors, walls and roofs in transport vehicles, container floors, scaffolding materials, shelves, load
bearing special structures, traffic signs, furniture and die boards
5. REFERENCES
[1] S.E.Omer, M.Ćehić: Rationalization using of wooden mass to application in civil engineering, Scientific
conference with international participation –Manufacturing and managment in 21st century., Ohrid, 2004
[2] S.E.Omer, M.Ćehić: Upotreba šperploča u građevinskim konstrukcijama, 5th international scientific
Conference on Production Engineering RIM 2005 , Bihać, 2005.
[3] Association of Finnish Plywood Industry (AFPI): Handbook of Finnish plywood, Finland, 2002.
[4] Canadian Plywood Association (CANPLY): Plywood Design Fundamentals, Canada, 2005.
[5] Javor: Vezane plošče, Javor Pivka, Slovenia, 2004.
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