to see a diagram of some sheer legs; this should be helpful.
The erection of a timber framed building involves the
lifting of large components into place. The oak trusses on our house
weigh about half a ton. They must be assembled and then lifted 12' from
ground level and placed on top of the supporting posts. It would be
easy to do this with a crane but cranes are very expensive to hire so
we used sheerlegs. These comprise a triangular structure made of 5"
X 5" timber, each side of which is about 22' long. One side is
rested on the ground and the other two sides are elevated until they
are at an angle of 70° to 80° to the horizontal. They are secured
in this position with a block and tackle attached to the top of the
triangle and then lead to a strong point some 30' away. The angle of
the sheerlegs to the horizontal can be varied by shortening or lengthening
on the block and tackle. The top of the triangle can thus be moved over
a limited distance at a height of about 20' above the ground.
Another block and tackle is attached at the top of the
triangle and this hangs down vertically to take the load to be lifted.
This (e.g. a truss) can be lifted on this second block and tackle and
then moved horizontally until it is over the posts and lowered into
position. Unlike a crane, sheerlegs cost almost nothing as the wood
they are made of is subsequently incorporated into the building.
Originally horses would have been used to provide the
lift but we employed a hand winch attached to the back of a Range Rover
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