Timber. Its Mechanical Properties and Factors Affecting Its Structural Use

163 Pages·1967·4.28 MB·English

by  Frederick D. Silvester and Jack Kape (Auth.)

PERGAMON SERIES OF MONOGRAPHS ON
FURNITURE AND TIMBER
VOLUME 1 Brown —An Introduction to the Seasoning of Timber
VOLUME 2 Parker — Costing in the Furniture Industry
VOLUME 3 Clark —Veneering and Wood Bending in the Furniture
Industry
VOLUME 4 Akers — Particle Board and Hardboard
VOLUME 5 Findlay —Timber Pests and Diseases
VOLUME 6 Collier — Woodfinishing
VOLUME 7 Oliver —The Development and Structure
of the Furniture Industry
Timber
Its Mechanical Properties and Factors
Affecting its Structural Use
Frederick D. SILVESTER,
C.G.I.A., F.B.I.O.G., A.I.W.Sc.
PERGAMON PRESS
OXFORD · LONDON · EDINBURGH · NEW YORK
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Copyright © 1967
Pergamon Press Ltd.
First edition 1967
Library of Congress Catalog Card No. 66-17813
2612/67
Foreword
THIS book is intended as an introductory textbook to the
subject of the mechanical properties of timber and as such
will be useful to the student, architect, builder and others
requiring a knowledge of the technical properties of wood.
The strength of wood varies almost with every species and
factors affecting this in relation to working stresses are fully
indicated in the text. The very considerable postwar develop-
ments in timber engineering in the construction industries
have brought about a greater appreciation of the architectural
and design possibilities of this very versatile material. On a
smaller scale the change-over of the furniture and joinery
industries from machine assisted hand production to a com-
pletely mechanized basis has meant that rule-of-thumb methods
can no longer be applied, particularly to the newer African
timbers. The furniture designer and technician must now
have a knowledge of all the possibilities of his materials, among
which wood and wood derivatives are still pre-eminent.
The examples mentioned are but a few of the many industries
in which timber is the basic, or an important, material, and
to nearly all of which this book will be an indispensable ally.
The very drawbacks in the use of timber, such as its moisture
content, structural defects and susceptibility to pest attack,
present a challenge to the engineer and scientist. This book
may not provide the whole answer, but it indicates ways in
which the challenge can be accepted and the fundamental
difficulties can be overcome.
JACK KAPE
Editor
vii
Author's Preface
THE MECHANICS of how a tree grows and develops have provided
endless discussion in the past and this has resulted in a number
of different schools of thought on the topic.
While some of the theories which have been expounded may
be quite sound, I have found considerable difficulty at times in
following the reasoning of some of these and in trying to justify
them from a practical or structural engineering standpoint.
For this reason I have endeavoured to formulate my own
ideas on the subject after careful study of what has already
been written in order to assist others who find themselves
faced with the same difficulties with which I have had to
contend.
In doing this I have drawn freely on available sources of
information discarding that which appears unnecessary for my
purpose and introducing theories of my own where I think
these are justified.
If in dealing with the subject I have made what may be
considered serious omissions or have erred on the side of
technical inaccuracy by over-simplification, I crave the indul-
gence of the experts in my attempt to help those who desire
to know more about timber as a structural material but owing
to the complex nature of the subject find themselves in the
position of not being able "to see the wood for the trees".
"Chaffords" F.D.S.
IX
Acknowledgements
MY THANKS are due to the Timber Research and Development
Association for permission to publish this book, a considerable
amount of the information which it contains being the direct
result of my day-to-day work during the past eighteen years.
I am also indebted to many colleagues, associates and friends
for the advice and assistance which they have given me; in
checking the text, and especially with photographs, without
which the book could not have been produced.
In particular, I should like to record my appreciation of the
encouragement to write about wood and the interest and
constant help in all my activities connected with timber, which
I received from my friend, the late Bernard Alwyn Jay, during
that period of his lifetime when I had the privilege of working
with him as a colleague.
Acknowledgement is also made for permission given by the
former Department of Scientific and Industrial Research to
publish certain photographs included in the illustrations.
The views expressed in this book are those of the author and
not necessarily those of the persons and bodies who have in any
way assisted, nor of the Timber Research and Development
Association.
F. D. SILVESTER
XI
List of Illustrations
FIG. 1. Growth of the tree.
FIG. 2. Section through trunk of tree.
FIG. 3. Growth rings in softwood and hardwood.
FIG. 4. Knot zones and clear timber zone.
FIG. 5. Structure of softwood.
FIG. 6. Structure of hardwood.
FIG. 7. Schematic diagram of structural elements of wood.
FIG. 8. Diagram of cell wall structure.
FIG. 9. Stress-strain diagram.
FIG. 10. Diagram showing direct stresses.
FIG. 11. Diagram showing flexure or compound stress.
FIG. 12. Structural axes of wood.
FIG. 13. Compression wood.
FIG. 14. Compression wood showing intercellular spaces.
FIG. 15. Compression wood fracture.
FIG. 16. Tension wood.
FIG. 17. Tension wood showing woolly texture.
FIG. 18. Effect of slope of grain on strength.
FIG. 19. Effect of irregular growth of tree on slope of grain.
FIG. 20. Effect of spiral growth on slope of grain.
FIG. 21. Interlocked grain.
FIG. 22. Wavy grain.
FIG. 23. Wild grain.
FIG. 24. Effect of grain angle on strength.
FIG. 25. Grain disturbance around knot.
FIG. 26. Knot arrangements.
FIG. 27. Knots in boxed heart.
FIG. 28. Grain disturbance caused by knot cluster.
FIG. 29. Orientation of knots.
Xlll
LIST OF ILLUSTRATIONS
FIG. 30. Stress distribution in beam of rectangular cross-section.
FIG. 31. Ingrown bark.
FIG. 32. Effects of shakes, checks or splits on shear strength.
FIG. 33. Brash fracture in softwood.
FIG. 34. Normal splintery fracture in softwood.
FIG. 35. Brittleheart fracture showing curvature at limit of
brittleheart material.
FIG. 36. Brittleheart fracture in small clear specimen showing
demarcation between brittleheart and sound timber.
FIG. 37. Formation of brittleheart in over-mature trees.
FIG. 38. Comparison of (a) normal fracture with (b) brittle-
heart fracture.
FIG. 39. Honeycombing.
FIG. 40. Comparison of sapstain fungus and wood-destroying
fungus.
FIG. 41. Effect of temperature on strength.
FIG. 42. Effect of low temperature combined with high moisture
content on strength.
FIG. 43. Measurement of rate of growth.
FIG. 44. Assessment of shakes and checks.
FIG. 45. Assessment of wane.
FIG. 46. Method of measuring wane.
FIG. 47. Edge knot and method of measuring.
FIG. 48. Splay knot and method of measuring.
FIG. 49. Arris knot and method of measuring.
FIG. 50. Face knot and method of measuring.
FIG. 51. Margin knot and method of measuring.
FIG. 52. Knot group or cluster.
FIG. 53. Using a grain detector.
FIG. 54. Measurement of slope of grain.
FIG. 55. Examination of timber in stack.
FIG. 56. Effect of moisture content on strength.
FIG. 57. Numerical cell increase in relation to growth.
FIG. 58. Casehardening test samples.
FIG. 59. Moisture content discs.
FIG. 60. Typical softwood fracture.
xiv
LIST OF ILLUSTRATIONS
FIG. 61. Typical hardwood fracture.
FIG. 62. Slenderness ratio of compression members.
FIG. 63. Effect of compression perpendicular to grain.
FIG. 64. Oblique shear failures in compression.
FIG. 65. Cleavage caused by wedge action of nail.
FIG. 66. Histogram showing distribution of results.
FIG. 67. Distribution curve showing statistical minimum.
FIG. 68. Buckling failure in strut member.
FIG. 69. Tension failure due to slope of grain.
FIG. 70. Shear failure at bolt hole.
FIG. 71. Typical bending failures.
FIG. 72. Dead loading test on plywood box girder.
FIG. 73. Field test on rigid frame barn.
FIG. 74. Laboratory test on 80 ft span bowstring roof truss.
xv
Introduction
ANY material which is a natural product is inevitably variable
in quality and in this respect timber is no exception. Soil and
climatic conditions affect the rate of growth of a tree, its
structure, shape and strength properties. In its natural state a
tree grows and develops according to the requirements needed
to ensure its survival and in any single specimen these require-
ments may vary considerably from season to season and from
year to year. A wet season generally encourages fast growth and
a tree will adapt itself to cope with the volume of water with
which it has to deal by producing a structure suitable for the
conduction of moisture, with the result that the girth of the
tree will be increased by a wide band of low density wood tissue.
The necessity for the conduction of moisture in a dry season
being almost negligible, the tree readjusts its structure accord-
ingly and instead of producing a wide band of low density
material it produces a band of high density wood tissue which,
while being narrower in width, provides greater structural
strength. Lack of light can cause a tree to grow rapidly in
height, putting on little in girth, in an endeavour to keep pace
with surrounding vegetation in the struggle for survival. This
will result in the formation of low density material in narrow
width bands during the wet season and bands of high density
material, proportionately narrower in width, during the dry
season. The result of this rapid upward growth is that close
grown material of low density is produced. Such material is
usually straight grained and easily worked but structurally
weak.
Where conditions of growth are ideally suited to the deve-
lopment of the tree the proportion of high and low density
2 MPT 1
XVI1