Agriculture Reference
In-Depth Information
to be able to mark accurately the centre of the three of the four key positions
(Fig. 174).
Figure 174
Setting out and
dry-bonding of the
oculus, establishing
the trammel to cut
the invert of the
brickwork and the 'U'
template upon which
the positions of the
voussoirs were marked
for both the lower and
upper half. (Courtesy of
Jeff Day)
With the use of a line radiating from the striking point, we marked onto the
hardboard all the positions of the dry-bonded voussoirs. As we had marked
everything from centre lines we were able to rotate the hardboard template to
be standing over the arch and thus accommodate the turning of the other half
of the oculus.
We prepared the brickwork to receive the oculus, which meant we had to
fix a horizontal beam with the centre line running through it upon which we
accurately positioned the striking point and fixed the two trammels. 9 mm was
added to the larger one, to accommodate the thickness of the mortar joint
around the extrados of the oculus. With the trammel rotating on our centre
nail at the striking point we cut and checked the invert. That done we were
ready to turn the lower half of the oculus. We fixed the U template to the
set brickwork, following the same procedure as when dry-bonding. The cen-
tre lines and template were aligned and with the smaller trammel fixed at the
striking point as our radiating line.
The string line was strained from the striking point to the relevant marks on
the template as the trammel governed the inner radius (Fig. 175). Each voussoir
was laid in this manner until the halfway point was reached. Then the middle
section of the oculus was bricked in with standard work to bond, checked for
curvature with the same small trammel. Then the upper half of the oculus was
constructed by simply turning the U template over and relocating it on the
established horizontal and vertical centre line, passing through the striking
point. We then proceeded to turn the upper half checking each voussoir to the
