Agriculture Reference
In-Depth Information
of the Arch to be; for if the Piers be a good breadth, as three or four Bricks in
length, then the Streight Arch may be defcribed (as its vulgarly faid) from the
Oxi, which being but part of a Word, is taken from the work Oxigonium fignify-
ing an Equilateral Triangle, with three fharp Angles; but if the Piers are fmall, as
fometimes they are but the length of two Bricks, and fometimes but one Brick
and an half, then the breadth of the Window or more, may be fet down upon the
middle Line for the Centre, which will give a lefs Skew-back, or Sommering, than
the centre from an Oxi.
The two common methods to determine or 'describe' a skewback are detailed.
First, by geometry, creating an equilateral triangle below the opening from
either 'springing point' to establish the 'striking point', from this the skewback
could be drawn giving a constant angle of 60º. This has always been consid-
ered to be the best angle for this arch as it gives a perfect counter-thrust to the
reciprocal equilateral triangular area of direct load above the opening.
The second method creates a less acute skewback achieved either by geometry
or a mathematical formula known as the 'One-Third Rule' (see page 220).
This gives Moxon's skewback on the left-hand (see Fig. 85) side a drawn angle
of 65º, whereby 70º is usual. In Moxon's example on the right-hand side, he
establishes the striking point a vertical distance down the centre line from the
springing line that equals the span of the relevant arch.
Moxon gives as the need for this reduction the narrow piers between open-
ings - especially on narrow-fronted terraced houses - unable to accommodate
the 60º angle of skewbacks as, for example, with a one and a half-brick wide
pier the opposing springing voussoirs would collide. In reality the angles of
skewbacks on straight arches could and did vary, as discussed in Chapter 4.
The straight arch is generally, though not always, found to be set-out to a
vertical face height of four standard courses of brickwork which, in the seven-
teenth century gives:
12 Inches; but moft commonly thefe fort of Arches are but 11 Inches in the height,
or thereabouts, which anfwers to four Courfes of Bricks, but you may make them
more or lefs in height according as occafion requires…. (Moxon, 1703, 274)
Having drawn the outline of a straight arch, Moxon details the techniques of
dividing the arch face into the number of courses the arch would contain. The
first involves setting out, or more correctly 'pricking out', the voussoirs on the
extrados with the aid of:
…Compaffes the thicknefs that a Brick will contain, which I fuppofe to be two
Inches when it is rub'd. (Moxon, 1703, 281)
This method of setting out creates a flat arch with voussoirs of regular widths
at the extrados. Curiously, though Moxon correctly provides for the arch being
worked to a 'key brick', which is best practice, he shows a joint at the centre in
his drawing.
