COFFERED SLAB

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")

Late 16th century coffered ceiling of Santa Maria in Aracoeli, Rome

Coffering on the ceiling of the Pantheon,Rome

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.

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.

• 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.

• 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.

• 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.

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

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.

COMPARISON: LONG SPAN STRUCTURES

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.

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

• 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.

SERVICES IN COFFERED SLAB

* 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.

Museum of the ParkSouth America - Brazil - Fortaleza

• 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.