ntering._--Centering is temporary timber or framing erected so as to
carry concrete floors or arches of brick or stone, &c.; when the work
has set the centering is removed gradually. The centering for concrete
floors is usually composed of scaffold boards resting on wood bearers
(fig 20). One wood bearer rests along on top of the steel joists;
through this bearer long bolts are suspended, and to the bottom of these
bolts a second bearer is fixed, and on the bottom bearer the scaffold
boards rest. Another method, not much used now, is to fit the boards to
the size of the floor and prop them up on legs, but among other
disadvantages this process takes up much space and is more costly.

[Illustration: FIG. 21.]

Turning piece is a name given to centering required for turning an arch
over (fig. 21); it is only 4-1/2 in. wide on the soffit or bed, and is
generally cut out of a piece of 3 or 4 in. stuff, the top edge being
made circular to the shape of the arch. It is kept in position whilst
the arch is setting with struts from ground or sills and is nailed to
the reveals, a couple of cross traces being wedged between. In the case
of a semicircular or elliptical arch with 4-1/2 in. soffit this turning
piece would be constructed of ribs cut out of 4 in. stuff with ties and
braces. Or the ribs could be cut out of 1 in. stuff, in which case there
must be one set of ribs outside and one inside secured with ties and
braces; each set of ribs when formed of thin stuff is made of two
thicknesses nailed together so as to lap the joints. For spans up to 15
ft. the thin ribs would be used, and for spans above 15 ft. ribs out of
4 in. stuff and upwards. For arches with 9 in. soffit and upwards,
whether segmental or semicircular or elliptical, the centres are formed
with the thin ribs and laggings up to 15 ft. span; above 15 ft. with 4
in. ribs and upwards (fig. 22). The lower member of centres is called
the tie, and is fixed so as to tie the extremities together and to keep
the centre from spreading. Where the span is great, these ties, instead
of being fixed straight, are given a rise so as to allow for access or
traffic underneath. Braces are necessary to support the ribs from
buckling in, and must be strong enough and so arranged as to withstand
all stresses. Laggings are small pieces or strips of wood nailed on the
ribs to form the surface on which to build the arch, and are spaced 1
in. apart for ordinary arches; for gauged arches t