Agriculture Reference
In-Depth Information
This with a then common standard sized rubber would, however, leave
the top of the brick down from the set out voussoir position and length on
the working drawing. The portion that had been sawn off the bottom of that
stretcher voussoir when 'soffiting' was turned over and, with or without adjust-
ment placed to the top of that voussoir. This works perfectly as the bedding
angle to extend the voussoir and meet the horizontal cross-joint as drawn is
identical. This constructional joint was then 'blinded-out', by deliberately rub-
bing into it damp brick dust during an early rubbing-up in advance of the final
'stoning' with the hand-held float stone so as to retain the desired aesthetic
appearance of the arch face. The deceit only becomes visible when, with years
of weathering, the joint is exposed.
Concern for structural weakness of straight, or flat, arches was discussed by
Neve (1726, 10-11):
Theorem the 2d Bricks moulded in their ordinary Rectangular Form; if they
be laid one by another in a level row, between any Supporters fuftaining their two
ends then all the pieces between will neceffarily fink even by their own natural
Gravity…
Emphasising how it is strengthened if curved, an early indication of the need
to camber the soffit of a straight arch, Neve continues:
Theorem the 4th If the Materials figured Wedge-wife,…fhould be difpofed in
the Form of fome Arch, or Portion of a Circle, pointing all to the fame Center,
in this cafe, neither the pieces of the faid Arch, can fink downwards for want of
room to defcend….
Though seen earlier, by the middle of the eighteenth century, a cambered sof-
fit to a straight arch was firmly accepted good craft practice and Pain (1769,
11) gives the measurement:
The soffits of the arches … to camber an
1
/
8
of an Inch in a Foot, that is ½ an Inch
in 4 Foot &c.
This measurement of
1
/
8
inch to a linear 12-inch run of span remains the
accepted camber today, now given as 3 mm per 300 mm. Yet this was not purely
for structural reasons, if these arches were set with the soffit perfectly level,
they appeared to sag in the centre. To overcome this illusion they were given
the rise to the measurements above; and it was from this action that the term
'camber arch' arose. In some eighteenth-century topics it was also suggested
that a rise of half that to the intrados could be given to the extrados, but this
was not such a common practice.
