From Wikipedia, the free encyclopedia Knee (construction)
Knee (construction)
In woodworking, a knee is a curved piece of load-bearing particularly for thick members. Also, not all species of
wood that is often used to connect adjacent members at wood steam bend well.
approximately right angles to one another. Knees are of-
ten used in the construction of wooden boats and ships, Laminated Knees - Laminated knees are formed by coat-
particularly as natural angle brackets to fasten the deck ing thin, flexible strips of wood with adhesive, layering
to the hull and to reinforce critical structural locations. them to achieve the required thickness, then forcing the
desired bend into the layup and securing it until the ad-
hesive sets. Laminated knees are very strong and can be
Strength Characteristics of made in shapes which would be difficult to achieve using
other methods, but they require time for the adhesive to
Knees cure, they are messier to construct, and they must use a
jig or fixture to secure them until the adhesive cures.
Wood is a highly anisotropic material (its strength varies
considerably with the direction of applied force, i.e. par-
Grown Knees - The term "grown knees" refers to any
allel, radial, or tangential to the grain). Because wood is
knee which is made from a natural crook or bend in a
strongest when loaded in tension or compression along
tree. Grown knees can be taken from several locations
the grain, the best knees are those in which the wood
within a tree, with the most common being the intersec-
grain follows the bend. For a knee with relatively little
tion of the trunk and a large branch[citation needed], crotch-
bend, it may be possible to cut the knee out of a single
es, and the roots. The roots are a particularly useful
straight-grained board and still achieve sufficient
source as the root structure of many species of trees nat-
strength. However, with increasing bend this method be-
urally spreads out laterally just beneath the ground in or-
comes problematic since more and more of the knee is
der to help anchor the tree. This provides a fairly reliable
aligned across the grain and is therefore considerably
source of approximately 90 degree crooks which may be
weaker. A knee laid out this way might easily snap in two
impossible to find in other portions of the tree. In order
under hand pressure alone, even if it is generously sized.
to obtain this raw material for knees builders may dig up
In boat joinery constantly subject to shock and fatigue
a stump in its entirety, as unlike other portions of the
loading this method is unsuitable.
tree, it is impossible to judge the quality and quantity of
To avoid this issue knees requiring sharper curves
available material in the roots as they are underground.
are made using methods which ensure that the wood
Once the stump has been dug up the knees can be sawn or
grain and direction of load are closely aligned. This can
split from suitable natural crooks. However, knees sawn
be achieved by steam bending, laminating, or selecting
from a stump can quickly dull tools used to shape and
a natural crook with matching grain - a "grown knee".
finish them - as the roots grow they envelop small par-
Grown knees are generally considered as the "best"
ticles of soil and rock, which acts as an embedded abra-
method among boatbuilders and have a strong traditions
sive and accelerates the wear of edged tools. For species
associated with their use, but they may not achieve the
of wood with appropriate splitting characteristics, such
same strength as a good laminated knee.
as oak, the stump can be split into wedges, with one large
root on each wedge; each wedge is then carved into a rib
for a small boat.
Types of Knees
Bent Knees - Bent knees are formed by plasticizing the Favored Species of Wood for
wood to make it flexible via boiling, steaming, or mi-
crowaving (for small components). While still hot, the Knees
wood can be bent into a shape suitable for the location Due to tradition, ease of workability, strength and dura-
- either on a form or by forcing and securing it directly bility characteristics, some species of wood are particu-
into the final service location. Steam-bending is a time- larly prized for making knees. Tamarack (also known as
honored method for shaping boat frames, but it does hackmatack) stumps are among the preferred softwood
weaken the wood slightly, it can leave residual stresses species for grown knees, while white oak and elm are
which may cause breakage or spring-back over time, and preferred for hardwoods for bent knees due to their ease
it is limited in the degree of bend which it can achieve, of steam bending.
1
From Wikipedia, the free encyclopedia Knee (construction)
Retrieved from "http://en.wikipedia.org/w/index.php?title=Knee_(construction)&oldid=465826355"
Categories:
• Woodworking
This page was last modified on 14 December 2011 at 15:16. Text is available under the Creative Commons Attribution-
ShareAlike License; additional terms may apply. See Terms of use for details. Wikipedia® is a registered trademark of
the Wikimedia Foundation, Inc., a non-profit organization.Contact us
Privacy policy About Wikipedia Disclaimers
2