Centering ppt

CONSTRUCTION TECHNOLOGY FOR CRAFTMEN

1. CENTERING
PRESENTED BY
AYINI RICHARD (SENIOR INST.)

2. DEFINITION
Is a wood member or frame used
as a temporary support for an arch
during its construction.

3. BASIC DEFINITIONS
(TERMONOLOGIES)
Springing line: a horizontal reference or datum line at the base of
an arch (where the arch springs from).
Span: the distance between the reveals (sides) of the opening.
Centre line: a vertical setting-out line equal to half the span.
Rise: a measurement on the centre line between springing and
intrados.
Intrados or soffit: the underside of the arch.
Extrados: the topside of the arch.
Crown: the highest point on the extrados.

4. Voussoirs:wedge-shaped units in the arch.
Key/Keystone:the central voussoirat the crown (i.e. the final insert
that locks the arch structurally).
Centre:the pivoting or compass point of the radius.
Radius:the geometrical distance of the centrepoint from the
concave of a segment or circle.
Abutment:This is the end support of an arches.
Pier: This is support an intermediate of an arch.
Haunch of an arch:This is the portion of arch situated centrally
between the key and skew backs.
Spandrel: This is the triangular walling enclosed by the extrados of
the arch, a horizontal line from the crown of the arch and
perpendicular line from the springing of the outer curves.
Skew back: This the surface of the abutment on which the arch
rests.

5. PARTS OF AN ARCH

6. TYPES OF CENTRES
TURNING PIECE
CENTRES FOR CAMBER ARCH
SEGMENTAL ARCH
CENTRES FOR CIRCULAR OR BULL’S-EYE ARCH
SEMI-CIRCULAR ARCH
CENTRE FOR SEMI-ELLIPTICAL ARCH

7. REQUIREMENTS FOR A CENTRE
The main points for consideration in the design of centres are as follows:
The upper surface must conform accurately with the shape of the soffit or
“intrados” of the arch required.
It must be strong enough to carry the often heavy load of brickwork or masonry
without movement or distortion, bearing in mind that the structure may be required
for use several times.
It must be so designed that the upper surface or laggings adequately support the
type of masonry or brick (the “voussoirs”) which are to form the arch.
It must be so supported during use that can eventually be removed gently i.e.
eased if necessary with of jarring or vibration.

8. TURNING PIECE

10. CENTRES FOR CAMBER ARCH

12. SEGMENTAL ARCH

13. CENTRES FOR CIRCULAR OR BULL’S-EYE
ARCH

14. SEMI-CIRCULAR ARCH
( FOR BRICKWORK)

15. SEMI-CIRCULAR ARCH (FOR MASONARY WORK)

16. CENTRE FOR SEMI-ELLIPTICAL ARCH

17. LAGGINGS
The type of laggings to be used on a
centre is governed by the kind of
voussoirused in building the arch;
Open lagging for a rough brick arch
Close lagging for a gauged brick
arch

18. OPEN LAGGINGS
Open lagging for a rough brick arch

19. CLOSE LAGGINGS
Close lagging for a gauged brick arch

20. THE END
THANKS