• A waffle slab is a type of building material that has two-directional reinforcement on the outside of the material, giving it the shape of the pockets on a waffle.

• This type of reinforcement is common on concrete, wood and metal construction. A waffle slab gives a substance significantly more structural stability without using a lot of additional material. This makes a waffle slab perfect for large flat areas like foundations or floors.

• The top of a waffle slab is generally smooth, like a traditional building surface, but the underside has a shape reminiscent of a waffle. Straight lines run the entire width and length of the slab, generally raised several inches from the surface. These ridges form the namesake square pockets of the entire length and width of the slab.

• Helps insulate the floor since hot air gets trapped in the pockets.

WAFFLE SLAB

HISTORY OF WAFFLE SLABS

Pantheon dome

FIAT FACTORY, Lingotto, Italy

TORONTO INTERNATIONAL AIRPORT WASHINGTON DC SUBWAY

YALE UNIVERSITY ART GALLERY

Waffle slab has a thick solid-slab floor from which the bottom layerconcrete in tension is partially replaced by their ribs along orthogonal directions. The ribsare reinforced with steel

Detailed view of a waffle slab floor with band beams

• Single waffle column panel

• Four-waffle column panel

• Beam support* Solid column panels resist shear

WAFFLE SLAB VS

•TWO WAY SLAB•RIBBED SLAB•ONE WAY JOIST SLAB

Waffle slab• Waffle slabs are two-way

spanning slabs whose beams are at very close centers

• efficient structurally for heavier loading situations and longer spans in concrete.

• Extra depth of the slab which gives more span/load capacity does not create dead weight in the middle of the span.

• columns don’t have to line up. This gives exceptional flexibility in column layout.

• Slab thickness above the pans is usually 4 to 6 Inches

wall partitions that have to meetthe slab can really only fall on the joists. Thismakes curved partitions very hard to build

thin topping slab and narrow ribs spanning

Ribbed slab• made up of wide band beams

running between columns with equal depth narrow ribs spanning the orthogonal direction. A thick top slab completes the system.

•Longer spans with heavy loads (12-15m) • Reduction in dead loads due to voids • Electrical and mechanical installations can be placed between voids • Good resistance to vibrations

•DISADVANTAGE • Only moderate and uniformly distributed load can be accommodated

One way joist slab• Joist slabs usually can span farther and carry heavier loads because they eliminate

concrete not contributing to the slabs strength.• To address heavier loading conditions, its necessary to remove the concrete that’s

acting as dead weight - working against the slab that comes along with an increase in the uniform thickness of a slab.

•Using prefab formwork set on a plywood deck voids are formed between the joists which make the slab lighter, and stiffer.

•usually not much reinforcing in the slab,and not much in the joist, most of the steel isconcentrated in the band carrying the joists.

•The void gives places for lighting, ducts areharder to place here because it is almostimpossible to put a hole big enough for a ductthrough the joist band.

Two way slab• it’s reinforced to span two ways. This is possible because the beams are

developed within the slab and span across all columns.• used for light loading applications where it’s thin structural depth gives a

low floor to floor height.• As it reaches it’s maximum span, there is more concrete needed in the

beam strips, so the slab gets thicker.• more concrete in the center which is just acting as dead weight, working

against the slabs efficiency!

Waffle slab

deeper than the equivalent ribbed slab•Suitable for heavy loads • Only moderate and uniformly distributed load can be accommodated

the slab is less thicker for two way slabLess concrete in the center, lower dead weight

• not much reinforcing in the one way slab• lighting, ducts are harder to place here because it is almostimpossible to put a hole big enough for a ductthrough the joist band.

ADVANTAGES

• Medium spans• Lightweight• Level soffit• Profile may be expressed architecturally, or used

for heat transfer• allows a considerable reduction in dead load as

compared to conventional flat slab construction since the slab thickness can be minimized due to the short span between the joists

DISADVANTAGES

• Higher formwork costs than for standard moulds and other slab systems

• Slow. Difficult to prefabricate reinforcement• Work quality and precision not easy to control• Labour safety at risk• Tuff to shorten the construction time.

MUSEUM OF ISLAMIC ARTS .. Architect: I.M. Pei Architect

CASESTUDY

CONSTRUCTION

NEW TECHNIQUES OF CONSTRUCTION

Plastic “domes” post-tensioned waffle slab construction

The number of strands in each rib is typically limited to one or two, with bottom rebar notexceeding two (12mm). Where design requirements demand more reinforcement that isgenerally assigned to a typical interior waffle stem, solid strips along the lines of supports isused to accommodate the excess of reinforcement.

With larger loads and longer spans, such as is common in department stores a heavier solidslab band between the supports accommodates the overage of reinforcement from theindividual waffle stems in each direction

DEFLECTION AFTER POST TENSIONING