Coffered ceilings and slabs are rigid, planar structures that use a series of intersecting ribs to distribute loads across a space. The document discusses the history and architectural uses of coffered ceilings. It also describes different types of coffered slab structures like waffle slabs and drop slabs that are used for their load bearing capacities in long span structures like schools and hospitals. Various coffered slab construction techniques are outlined, including the use of precast elements and how services can be run through the coffered spaces.

1. COFFERED
SLAB

2. Gilded coffering on a barrel vault
of the apsis in Nazaré, Portugal
Giuliano da Sangallo's flat caisson ceiling,
Basilica di Santa Maria Maggiore, Rome
DECORATIVE COFFERED SLABS IN HISTORICAL STRUCTURES
Coffering on the Arch of Septimius Severus, Rome
A series of these sunken panels were used as
decoration for a ceiling or a vault, also
called caissons ('boxes"), or lacunaria
("spaces, openings")

3. Late 16th century coffered ceiling
of Santa Maria in Aracoeli, Rome
Coffering on the ceiling of
the Pantheon,Rome

4. WORKING
• Coffered structures are rigid,
planar, often monolithic
structures that disperse applied
loads in multi-directional pattern,
with the loads generally following
the shortest stiffest routes to the
supports.
• A coffer can be envisioned as a
series of adjacent beam ribs,
inter-connected continuously
along there lengths.
• As an applied load is transmitted
to the supports through bending
of one beam ribs, the load is
distributed over the entire
coffered slab by vertical shear
transmitted from the deflected rib
to adjacent ribs.

5. WORKING
• The bending of one beam rib also
causes twisting of transverse ribs,
whose torsional resistance increases
the overall stiffness of the slab.
Therefore, while bending and shear
transfer and applied load in a
direction of the loaded beam rib,
shear and twisting transfer the load
at right angles to the loaded rib.
• A coffer should be square or nearly
square to ensure that it behaves as a
two-way structure. As a coffer
becomes more rectangular, the two-
way action decreases and a one-way
system spanning the shorter
direction develops because the
shorter coffer ribs are stiffer and
carry a greater portion of the load.
• Greater the depth of the ribs more
resistance to the bending, hence
larger the span.

6. • If a column grid is increased from 6
to 12 square or near square, it
becomes economical to use a floor
with intermediate cross beams
supporting thin floor slabs.
• The intermediate cross beams are
cast on a regular square grid that
gives the underside of the floor the
appearance of a waffle, hence the
name.
• The advantage of intermediate
beams of the waffle is that they
support a thin floor slab and so
reduce the dead weight of the floor
as compared to a flush slab of
similar span.
• This type of floor is used where
widely spaced square column grid is
necessary and the floor support
comparatively heavy loads.
• The economic span of the floor slabs
between intermediate beams lies
between 0.9 to 3.5m.
• The waffle grid form of the floor may
be cast around plastic or metal
formers laid on timber centering, so
that the smooth finish of the soffit
may be left exposed.

7. • For greater shear strength
and moment resisting
capacity, solid heads at
column supports are formed.
Size depends on and load
conditions.
• Suitable for up to 16m long
spans, longer spans may be
possible with post tensioning.
• For the maximum efficiency,
coffers should be square or
nearly square as possible.
• Waffle slab can be efficiently
cantilevered in two directions
up to 1/3 of a main span.
when no cantilevered is
present, a perimeter slab
band is formed by omitting
dome forms.
• Coffer underside is usually left
exposed.

8. • A waffle slab is a two-way concrete slab
reinforced by ribs in two directions.
• Waffle slabs are able to carry heavier
loads and span longer distances than flat
slab.
• 3’’ to 4-1/2’’ (75 to 115mm) slab depth,
rule of thumb for total depth – Span / 24
• 5’’or 6’’ (125 or 150) rib width
• Square metal or fiberglass dome forms
available in – 19’’ and 30’’ (485 and 760)
widths and form 8’’ to 20’’ (205 to 510)
depths I 2’’ (51) increments. Larger sizes
are also available. Tapered sides allow
easy removal
• 19’’ (485) domes and 5’’(125) ribs create
a 2’(610) module & 30’’(760) domes and
6’’(150) ribs produce 3’(915) module.

12. DIAGONAL GRIDS
* The egg-crate construction is not efficient as
diagonal system.
* Particularly if it is required to support the grid at four
points only, the diagonal grid has the greater tortional
rigidity.
PLAN

13. 3-WAY GRIDS FOR
TRIANGULAR &
HEXAGONAL AREAS
* With the beams in three
direction, the grid becomes 3-way,
and this system is stiffer than 2-
way diagonal grid.

14. COMPARISON:
LONG SPAN
STRUCTURES

15. PRECAST CONCRETE TEE BEAMS
• Precast prestressed
concrete tee beam floors
are mostly used for long
span floors in building such
as stores, supermarkets,
swimming pools and multi-
storey car parks where
there is a need for wide
span floors.
• The floors units are cast in
the form of a double Tee.
• The strength of this units is
in the depth of the ribs
which supports and act with
the comparatively thin top
web.
• A structural reinforced
concrete topping is cast on
top of the floor units.

16. LENS LIGHT CONCRETE ROOF
• Lens lights –to provide
diffused daylight
through concrete roofs
• Lens lights are used in a
concrete roof as roof
lights to provide
resistance to fire , for
reasons of security and
to reduce sound
transmission.
• Square or round glass
blocks or lenses those
are cast into reinforced
concrete ribs
• The lens lights can be
pre-cast and bedded in
place on site or in-situ
cast in a concrete roof

17. • This floor construction consists of a
floor slab which is thickened between
columns in the form of a shallow but
wide beam.
• A drop slab floor is of about the same
dead weight and cost as a comparable
slab and beam floor.
• It will have up to half the depth of
floor construction from the top of slab
to the soffit of beams.
• e.g. On a 12.0 square column grid the
overall depth of a slab and beam floor
would be about 1.2 where the depth
of a drop slab floor would be about
600.This difference would cost a
significant reduction in overall height
of a construction of a multi-storey
building with appreciable saving in
cost.

18. SERVICES IN
COFFERED SLAB

20. * The coffer blocks used in the
Coffer Slab are manufactured
from plastic, making them light
and easy to handle.
* The assembly of the coffer
blocks is done on the ground
before being lifted into position
between the supporting ribs.
The result is less labor and
quick and easy installation.

21. Museum of the Park
South America - Brazil - Fortaleza

22. • Spanish architects
Alarcon Asociados have
developed a new
construction product that
allows a six-storey
building to fit into a five-
storey volume
• Developed for buildings
with large construction
spans such as schools and
hospitals, Holedeck is a
concrete waffle slab
system that can
accommodate electrical
cables, plumbing and
ventilation ducts within
the floor structure rather
than hung below. This
prevents the need for
suspended ceilings,
which are installed to
hide these services.