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272 Pages·2005·15.046 MB·English

Practical design of
timber structures to
Eurocode 5
Hans Larsen and Vahik Enjily
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TranslatedandupdatedfromtheoriginalDanishpublication
TrÆOGtrækonstruktioner1and2#TOP2007
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Copyright © ICE Publishing, all rights reserved.
Contents
Introduction 1
1 Basis of design 8
1.1 Overview of UK and European design principles 8
2 Construction products 32
2.1 The tree 32
2.2 Structural timber 52
2.3 Glulam 59
2.4 Wood-based panel products 67
2.5 Joints and fasteners 80
3 Structural examples 96
3.1 Introduction 96
3.2 Main and secondary members 96
4 Straight members and beams with varying depth 106
4.1 Tension and compression 106
4.2 Tapered beams 122
4.3 Shear 127
5 Columns 134
5.1 Introduction 134
5.2 Axially loaded columns 134
5.3 Laterally loaded columns 138
5.4 Lateral torsional buckling of laterally loaded columns 143
iii
Copyright © ICE Publishing, all rights reserved.
6 Curved beams and frames 144
6.1 Curved beam 144
6.2 Pitched cambered beam 148
6.3 Arches and frames 154
7 Trusses and bracings 161
7.1 Introduction 161
7.2 Structural design of trusses 162
7.3 Bracings 169
8 I, T and box-beams 174
8.1 Introduction and background theories 174
8.2 Transformed (composite) cross-sections 174
8.3 Beams with thin webs 177
8.4 Beams with thin flanges (stress skin panels) 185
9 Connections and fasteners 194
9.1 Design of multi-fastener joints 194
9.2 Load-carrying capacity of dowel-type fasteners 199
9.3 Nailed connections 210
9.4 Connections with staples 223
9.5 Bolted connections 225
9.6 Connections with dowels 230
9.7 Connections with screws 231
9.8 Joints with connectors 237
10 Diaphragms 247
10.1 Introduction 247
10.2 Roof diaphragm (simply supported diaphragm) 248
10.3 Wall diaphragm (cantilever diaphragm) 249
References 255
Index 259
iv
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Introduction
Design basis for [BS 5268]
[BS5268],Structuraluseoftimber,isbasedonpermissiblestressdesign,
and this phrase appears in the title of Part 2. However, it does not
include a description of the design principles equivalent to those in
[EN 1990]. Grade stress values for properties of materials, most of
which are given in tabular form in [BS 5268-2] itself, incorporate
safetyfactorssothattheyrepresentthestressthatthematerialisconsid-
ered able to bear over the life of a building with a reasonable level of
safety.Thegradestressesareconsideredfor50—60years’loadduration.
A series of modification factors are then applied to the mechanical
properties, generally increasing the load-bearing capacity for shorter
load durations. The permissible stress in service for a particular type
and duration of load to the element is thus established, and verified
against the stress applied by the design loads. These design loads are
normally obtained from the parts of [BS 6399].
In addition to supporting normal loads, [BS 5268-2]requires timber
structures to withstand accidental damage without catastrophic
collapse. The Approved Document A gives guidance for the require-
ments of disproportionate (progressive) collapse design.
[BS5268-2]hasbeenthekeycodeintheseries,coveringstructural
designoftimberingeneral.Theotherpartsprovidedsubsidiaryinforma-
tion. [BS 5268-2] specified how timber structures may be designed to
withstand applied loads. [BS 5268] includes several parts as listed
below:
. Part 2: 2002, Code of practice for permissible stress design, materials
and workmanship
. Part 3: 1998, Code of practice for trussed rafter roofs
. Part 4.1: 1978, Fire resistance of timber structures. Recommendations
for calculating fire resistance of timber members
. Part4.2:1990,Fireresistanceoftimberstructures.Recommendationsfor
calculatingfireresistanceoftimberstudwallsandjoistedfloorconstructions
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Copyright © ICE Publishing, all rights reserved.
Practical design oftimberstructures toEurocode5
. Part 5: 1989, Code of practice for the preservative treatment of struc-
tural timber
. Part 6.1: 1996, Code of practice for timber frame walls. Dwellings not
exceeding seven storeys
. Part6.2:2001,Codeofpracticefortimberframewalls.Buildingsother
than dwellings not exceeding seven storeys
. Parts 7.1 to 7.7: Recommendations for the calculation basis for span
tables for various elements
The appropriate loads to be used for design purposes were given in
[BS 6399] with three parts covering dead, imposed and wind loads.
Definitions
Element and component
There is great confusion in definition of elements and components at
European level. The UK’s definition of elements and components is
reversed in the Eurocode and the rest of Europe. For clarity the
following are definitions which the UK must adopt:
Element is a structure or part of the structure which consists of
members or components. For example, a trussed rafter is an ‘element’
because it includes members/components which are rafters, internal
web,ceilingtieandpunchedmetalplateconnectors.Anotherexample
would be a wall panel which is termed ‘element’ because it consists of
elements or components such as studs and sheathing.
Component or member is a part of an element.
System is a structure which consists of elements.
Notation
Introduction
The symbols and notations are very vital and important as the UK is
used to different symbols and notations than those given in Eurocode.
Many major errors can occur if these notations are mixed when used
in design.
Member axes
TheUKhasbeenusedtox—x,y—yandz—zbeingthemajor,minorand
out-of-planeaxesrespectively.Thesehavebeenchangedintoy—y,z—z
and x—x respectively in the Eurocode (see Fig. 1).
2
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Introduction
y z
x x y y
y z
z–z being out-of-plane axis x–x being out-of-plane axis
In [BS 5268] In [EN 1995-1-1]
Fig.1 Differencesbetweenaxesaccordingto[BS5268]andEurocode5
Decimal symbols
The UK uses a full point (.) to denote a decimal place, whereas in the
Eurocodes this is seen as a comma (,). For example:
In [BS 5268]
1,000.00 means one thousand; 1.000 means one
In [EN 1995-1-1]
1,000 means one; 1.000,00 means one thousand
Symbols
General
Symbols usually consist of a main symbol with one or more indices
separated by commas. For example f which is the characteristic
c;0;k
compression strength parallel to the grain (where ‘f’ is strength, ‘c’ is
compression, 0 is parallel to the grain and ‘k’ is characteristic).
In accordance with Eurocode 5, the following general symbols are
used.Symbolsnotmentionedaredefinedwhereusedandtheirmeaning
may vary.
Main symbols
A cross-sectional area
E modulus of elasticity
F force, action
G permanent action; or
shear modulus
I second moment of area, sometimes called moment of inertia
M bending moment
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Practical design oftimberstructures toEurocode5
N axial force
Q variable action
V shear force or also sometimes used as volume
W section modulus
a distance
b width
d diameter or also sometimes used as side length (nails)
e eccentricity
f material strength
h height
i radius of gyration
k constant
l length
r radius
t thickness
u, v, w deflections parallel to the axes
x, y, z coordinates
(cid:1) angle
(cid:2) partial coefficient
(cid:3) slenderness ratio
(cid:4) density
(cid:5) normal stress
(cid:6) shear stress
Indices
apex apex
mean mean or average
c compression
cr critical
d design
def deformation (deflection)
eff effective
fin final
h embedment
inst instantaneous
k characteristic
m bending
ser serviceability
t tension
4
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Introduction
tor torsion
ult ultimate
v shear
vol volume
x, y, z axes
y yield
(+ Symbol for fibre direction
Axes, forces and moments
For beams a coordinate system as shown in Fig. 2 is used.
Combined stress index (verification)
Inordertoverifytheadequatestrengthofastructuralmember,allthe
appliedstressesshouldbecombined.Forexample,ifastructuraltimber
memberisunderbending,compressionandtension,forstrengthverifi-
cationthesumoftheratiosofstresstostrengthforparticularloadsmust
be less than or equal to 1.0. For example, if a timber beam is in both
tension and bending, then:
(cid:5)=f þ(cid:5) =f (cid:1) 1 ð1Þ
t t m m
where (cid:5) is the tensile stress and f is the tensile strength, (cid:5) is the
t t m
bending stress and f the bending strength.
m
Construction products
Building materials and products are covered by the EU Construction
Products Directive (Council Directive 89/106/EEC). The purpose of
y x
z
N
T
y
My Vz
M
z
Fig.2 Coordinatesystem(axes,forcesandmoments)
5
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Practical design oftimberstructures toEurocode5
Fig.3 CEmark
thedirectiveistoensurethefreemarketinthebuildingsectorthathas
traditionally been one of the most nationally regulated.
For building products, compliance with the Construction Products
Directivemustbedemonstratedwhentheyaremarketedforpermanent
incorporationinbuildingsorcivilengineeringworksandhaveinfluence
on:
1. mechanical resistance and stability
2. safety in case of fire
3. hygiene, health and the environment
4. safety in use
5. protection against noise
6. energy economy and heat retention.
Nationalbuildingauthoritiesareonlypermittedtoputrequirementsin
relationtothesesixso-called‘EssentialRequirements’.Thismeansthat
it cannot be requested that ‘convenience standards’ are followed.
Example of a convenience standard can be a standard for timber sizes.
Products meetingthe requirements of the harmonised standard (see
below) may be marked with the CE mark, see Fig. 3.
Most European countries, but excluding the UK and a few other
European countries, will require manufacturers to mark their product
with the CE mark. However, it is prudent for UK users to specify and
requestCEmarkingwhichwillbeproofofcompliancewiththeHarmo-
nised Standard, thereby complying with the Construction Products
Directive.
TheCEmarkallegedlytellstheconsumerthattheproductmeetsall
legal essential requirements in all EU member states.
TheBuildingProductsDirectiveappliestoallproductswhetherthey
areproducedwithinoroutsidetheEU.ThebasisfortheCEmarkingis
either:
. aharmonisedEuropeanstandard,i.e.astandardproducedbyCEN
(Comit´e Europ´een de Normalisation), the European Organisation
for Standardisation, or
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