Download Structural Timber Design PDF

TitleStructural Timber Design
File Size29.1 MB
Total Pages281
Table of Contents
1  Timber as a Structural Material
	1.1  Introduction
	1.2  The structure of timber
	1.3  Defects in timber
		1.3.1  Natural defects
		1.3.2  Chemical defects
		1.3.3  Conversion defects
		1.3.4  Seasoning defects
	1.4  Types of timber
		1.4.1  Softwoods
		1.4.2  Hardwoods
	1.5  Physical properties of timber
		1.5.1  Moisture content
		1.5.2  Density
		1.5.3  Slope of grain
		1.5.4  Timber defects
	1.6  References
2  Introduction to BS 5268: Part 2: 1996
	2.1  Introduction
	2.2  Design Philosophy
	2.3  Stress grading of timber
		2.3.1  Visual grading
		2.3.2  Machine grading
	2.4  Strength classes
	2.5  Design considerations (factors affecting timber strength)
		2.5.1  Loading
		2.5.2  Service classes
		2.5.3  Moisture content
		2.5.4  Duration of loading
		2.5.5  Section size
		2.5.6  Load-sharing systems
		2.5.7  Additional properties
	2.6  Symbols
	2.7  References
3  Using Mathcad® for Design Calculations
	3.1  Introduction
	3.2  What is Mathcad?
	3.3  What does Mathcad do?
		3.3.1  A simple calculation
		3.2.2  Definitions and variables
		3.3.3  Entering text
		3.3.4  Working with units
	3.4  Summary
	3.5  References
4  Design of Flexural Members (Beams)
	4.1  Introduction
	4.2  Design considerations
	4.3  Bending stress and prevention of lateral buckling
		4.3.1  Effective span, L [sub (e)]
		4.3.2  Form factor, K [sub (6)]
		4.3.3  Depth factor, K [sub (7)]
		4.3.4  Selection of a suitable section size
		4.3.5  Lateral stability
		4.3.6  An illustrative example
	4.4  Deflection
		4.4.1  Deflection limits
		4.4.2  Precamber
		4.4.3  Bending deflection
		4.4.4  Shear deflection
	4.5  Bearing stress
		4.5.1  Length and position of bearings
	4.6  Shear stress
		4.6.1  Shear at notched ends
	4.7  Suspended timber flooring
	4.8  References
	4.9  Design examples
		Example 4.1
		Example 4.2
		Example 4.3
		Example 4.4
5  Design of Axially Loaded Members
	5.1  Introduction
	5.2  Design of compression members
		5.2.1  Design considerations
		5.2.2  Slenderness ratio, [lambda]
		5.2.3  Modification factor for compression members, K [sub (12)]
		5.2.4  Members subjected to axial compression only (Clause 2.11.5)
		5.2.5  Members subjected to axial compression and bending (Clause 2.11.6)
		5.2.6  Design of load-bearing stud walls
	5.3  Design of tension members (Clause 2.12)
		5.3.1  Design considerations
		5.3.2  Width factor, K [sub (14)]
		5.3.3  Members subjected to axial tension only
		5.3.4  Combined bending and tensile stresses
	5.4  Design examples
		Example 5.1
		Example 5.2
		Example 5.3
		Example 5.4
		Example 5.5
6  Design of Glued Laminated Members
	6.1  Introduction
	6.2  Design considerations
	6.3  Grade stresses for horizontally glued laminated members
		6.3.1  Single-grade members
		6.3.2  Combined-grade members
		6.3.3  Permissible stresses for horizontally glued laminated members
	6.4  Grade stresses for vertically glued laminated beams
		6.4.1  Permissible stresses for vertically glued laminated members
	6.5  Deformation criteria for glued laminated beams
	6.6  Curved glued laminated beams
	6.7  Bibliography
	6.8  Design examples
		Example 6.1
		Example 6.2
		Example 6.3
		Example 6.4
		Example 6.5
7  Design of Ply-webbed Beams
	7.1  Introduction
	7.2  Transformed (effective) geometrical properties
	7.3  Plywood
	7.4  Design condsiderations
		7.4.1  Bending
		7.4.2  Deflection
		7.4.3  Panel shear
		7.4.4  Rolling shear
		7.4.5  Lateral stability
		7.4.6  Web-stiffeners
	7.5  References
	7.6  Design Examples
		Example 7.1
		Example 7.2
8  Design of Built-up (Spaced) Columns
	8.1  Introduction
	8.2  Spaced Columns
	8.3  Design considerations
		8.3.1  Geometrical requirements
		8.3.2  Modes of failure and permissible loads
		8.3.3  Shear capacity of spacer blocks
	8.4  Compression members in triangulated frameworks
	8.5  Reference
	8.6  Design examples
		Example 8.1
		Example 8.2
9  Design of Timber Connections
	9.1  Introduction
	9.2  General design considerations
	9.3  Joint slip
	9.4  Effective cross-section
	9.5  Spacing rules
	9.6  Multiple shear lateral loads
	9.7  Nailed joints
		9.7.1  Improved nails
		9.7.2  Pre-drilling
		9.7.3  Basic single shear lateral loads
		9.7.4  Axially loaded nails (withdrawal loads)
		9.7.5  Permissible load for a nailed joint
	9.8  Screwed joints
		9.8.1  Basic single shear lateral loads
		9.8.2  Axially loaded screws (withdrawal loads)
		9.8.3  Permissible load for a screwed joint
	9.9  Bolted and dowelled joints
		9.9.1  Basic single shear lateral loads
		9.9.2  Permissible load for a bolted or dowelled joint
	9.10  Moment capacity of dowel-type fastener joints
	9.11  Connectored joints
		9.11.1 Toothed-plate connectors
		9.11.2  Split-ring and shear-plate connectors
		9.11.3  Metal-plate connectors
	9.12  Glued joints
		9.12.1  Durability classification
		9.12.2  Design considerations for glued joints
	9.13  References
	9.14  Design examples
		Example  9.1
		Example 9.2
		Example 9.3
		Example 9.4
		Example 9.5
		Example 9.6
		Example 9.7
		Example 9.8
10  Design to Eurocode 5
	10.1  Introduction
	10.2  Design philosophy
	10.3  Actions
	10.4  Material properties
		10.4.1  Design values
	10.5  Ultimate limit states
		10.5.1  Bending
		10.5.2  Shear
		10.5.3  Compression perpendicular to grain (bearing)
		10.5.4  Compression or tension parallel to grain
		10.5.5  Members subjected to combined bending and axial tension
		10.5.6  Columns subjected to combined bending and axial compression
		10.5.7  Dowel-type fastener joints
	10.6  Serviceability limit states
		10.6.1  Deflections
		10.6.2  Vibrations
		10.6.2  Joint slip
	10.7  Reference
	10.8  Bibliography
	10.9  Design examples
		Example 10.1
		Example 10.2
		Example 10.3
Appendix A: Section Sizes for Softwood Timber
Appendix B: Weights of Building Materials
Appendix C: Related British Standards for Timber Engineering
Document Text Contents
Page 2


Sc, MSc, PhD, FIVVSc
~ a ~ i e r ~niver~ity, Edinbur~~


Page 140

Design of Ply-webbed Beams 'l 27

Table 7.1 Modification factor K36 for the grade stresses and moduli of plywood
(Table 33 of BS 5268 :Part 2)

Duration of loading Value of K36

Service classes 1 and 2 Service class 3

Stress Modulus Stress Modulus

Long term 1 '00 1 .oo 0.83 0.57
Medium term 1.33 1 S4 l .08 1.06
Short and very short term 1.50 2.00 1.17 1.43

BS 5268 : Part 2 : 1996, Section 4 deals with plywood for structural use.
Dimensions and section properties of plywoods are given in Tables 25-32 of
the code. They are based on the minimum thicknesses permitted by the
relevant product standards and apply to all service classes.

The grade stresses and moduli for plywoods are given in Tables 34-47 of
the code. These apply to long-duration loading in service classes 1 and 2,
and should be used in conjunction with the corresponding section properties
given in Tables 25-32. For other durations of load andlor service class 3
condition, the stresses and moduli should be multiplied by the modifica-
tion factor K36 given in Table 33 of the code. A summary of this table is
reproduced here as Table 7.1.

It is important to note that the bending stresses and moduli given in
Tables 34-47 of the code apply when the bending is perpendicular to the
plane of the plywood panel (i.e. case (1) above). In situations where plywood
is subject to bending about an axis perpendicular to the plane of the board
(i.e. with the edge loaded, as in a ply-webbed beam) the tensile and com-
pressive stresses induced by the bending moment should be considered
individually, and the tension and compression stresses and moduli for the
appropriate face grain orientation should be used.

The following criteria should be considered when designing ply-web beams:

(1) Bending.
(2) Deflection.
(3) Panel shear.
(4) Rolling shear.
(5) Lateral stability.
(6) Web-stiffeners.

Page 141

128 Structural Timber Design

BS 5268 :Part 2, Clause 4.6 deals with the use of plywood in flexural
members. The permissible stresses for plywood in flexural members are
governed by the particular conditions of service and loading described in
Clauses 2.8 and 2.9 of the code (which relate to load duration and load
sharing respectively) and should be taken as the product of the grade
stresses given in Tables 34-47 and the modification factor from Table 33
of the code (Table 7.1).

BS 5268 : Part 2, Clause 2.10.10 specifies that the modification factors K27,
K28 and K29 (for glulam members) given in Table 22 may also be used for the
flanges of glued built-up beams such as I and box ply-webbed beams. The
number of pieces of timber in each flange should be taken as the number of
laminations, irrespective of their orientation, to determine the value of the
stress modification factors K27, and K29 for that flange.

As mentioned earlier, the plywood web(s) of a ply-webbed beam will be
subjected to bending about an axis perpendicular to the plane of the board;
and it is therefore necessary to check that the maximum applied bending
stress in the transformed section is not greater than the lesser of:

(1) the permissible tensile stress of the timber flange, Ufimber,t,adm,/
(2) the permissible compressive stress of the timber flange, U t i ~ b ~ r , c , u d m , /
(3) the permissible transformed tensile stress of the plywood web,


=p lywood , t , a~ , / -"-.
Ef imber

(4) the permissible transformed compressive stress of the plywood web,

uplywood,c,adm,/ ' p


Therefore the applied bending stress in the transformed section, Om,u , / ,
should not be greater than the lesser of the above permissible stresses, where,

the permissible tensile stress for timber flanges is

the permissible compressive stress for timber flanges is

=tjmber,c,adm,/ = grade Stress (Table 719 oc,g,/ x K$3K&28 (7.2b)

and for plywood web(s), the permissible tensile or compressive stress is


X"----- K8K36 (7 .2~ & d)

Page 280

m orksheet IS

All design examples given in this book are produced in the form of
worksheets using Mathcad computer software and are available on 3 p' disks
to run under Mathcad Version 6, or higher, in either one of its editions:
Student, Standard, Plus or Professional. Mathcad runs under Windows
Operating System on any IBM compatible Personal Computer.

The worksheet files are labelled where m refers to the
chapter number and n to the example number in that chapter. For example refers to example 2 in Chapter 9.

It is recommended that the user makes a backup copy of the worksheets.
This way he or she is free to experiment with the files. When a worksheet is
loaded it should be saved under a new name so that when modified the
original disk file is not altered.

Although every care has been taken to ensure the accuracy of the example
worksheets, it remains the responsibility of the user to check their results.

Please copy and complete the form at the back of the book, enclosing a
cheque or postal order for E20 which includes handling and postage and
packing, made payable to A. Kermani, and send to:

A. Kermani, 4 Mid Steil, Glenlockhart, Edinburgh EH10 5XB, 1JK
email: [email protected]


Page 281

268 Structural Timber Design

Structural Timber Design
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