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A waffle slab or two-way joist slab is a 
* Slab depth is typically to thick. As a rule of thumb, the depth should be of the span.
* The width of the ribs is typically to , and ribs usually have steel rod reinforcements.
* The distance between ribs is typically .
* The height of the ribs and beams should be of the span between columns.
* The width of the solid area around the column should be of the span between columns. Its height should be the same as the ribs. 
A waffle slab or two-way joist slab is a concrete slab
A concrete slab is a common structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete. Steel-Reinforced concrete, reinforced slabs, typically between 100 and 500 mm thick, are most often used to ...
made of reinforced concrete with concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
ribs running in two directions on its underside. The name ''waffle'' comes from the grid pattern created by the reinforcing ribs. Waffle slabs are preferred for spans greater than , as they are much stronger than flat slabs, flat slabs with drop panels, two-way slabs, one-way slabs, and one-way joist slabs.
Description
A waffle slab is flat on top, whilejoist
A joist is a horizontal structural member used in framing to span an open space, often between beams that subsequently transfer loads to vertical members. When incorporated into a floor framing system, joists serve to provide stiffness to the s ...
s create a grid like surface on the bottom. The grid is formed by the removal of molds after the concrete sets. This structure was designed to be more solid when used on longer spans and with heavier loads. This type of structure, because of its rigidity, is recommended for buildings that require minimal vibration, like laboratories
A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physici ...
and manufacturing
Manufacturing is the creation or production of goods with the help of equipment, labor, machines, tools, and chemical or biological processing or formulation. It is the essence of secondary sector of the economy. The term may refer to a ...
facilities. It is also used in buildings that require big open spaces, like theatre
Theatre or theater is a collaborative form of performing art that uses live performers, usually actors or actresses, to present the experience of a real or imagined event before a live audience in a specific place, often a stage. The perfor ...
s or train station
A train station, railway station, railroad station or depot is a railway facility where trains stop to load or unload passengers, freight or both. It generally consists of at least one platform, one track and a station building providing ...
s. Waffle slabs are composed by intricate formwork
Formwork is molds into which concrete or similar materials are either precast or cast-in-place. In the context of concrete construction, the falsework supports the shuttering molds. In specialty applications formwork may be permanently ...
, and may be more expensive than other types of slabs, but depending on the project and the quantity of concrete needed it may be cheaper to build.
There are two types of waffle slab system:
# One way waffle slab system
# Two way waffle slab system
Construction process
A waffle slab can be made in different ways but genericforms
Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens.
Form also refers to:
*Form (document), a document (printed or electronic) with spaces in which to write or enter data
* ...
are needed to give the waffle shape to the slab. The formwork is made up of many elements: ''waffle pods'', ''horizontal supports'', ''vertical supports'', ''cube junctions'', ''hole plates'', ''clits'' and ''steel bars''. First the supports are built, then the pods are arranged in place, and finally the concrete is poured.
This process may occur in three different approaches, however the basic method is the same in each:
* In situ: Formwork construction and pouring of concrete occur on site, then the slab is assembled (if required).
* Precast: The slabs are made somewhere else and then brought to the site and assembled.
* Pre-fabricated: The reinforcements are integrated into the slab while being manufactured, without needing to reinforce the assembly on site. This is the most expensive option.
Waffle slab design
Different guides have been made for architects and engineers to determine various parameters of waffle slabs, primarily the overall thickness and rib dimensions. The following are rules of thumb, which are explained further in the accompanying diagrams:
* Slab depth is typically to thick. As a rule of thumb, the depth should be of the span.
* The width of the ribs is typically to , and ribs usually have steel rod reinforcements.
* The distance between ribs is typically .
* The height of the ribs and beams should be of the span between columns.
* The width of the solid area around the column should be of the span between columns. Its height should be the same as the ribs.
Advantages
The waffle slab floor system has several advantages: * Better for buildings that require less vibrationsthis is managed by the two way joist reinforcements that form the grid. * Bigger spans can be achieved with less material, being more economical and environmentally friendly * Some people find the waffle pattern aesthetically pleasing * Greater load capacity than traditional one-way slabs * Forms can be implemented with wood, concrete or steel * If holes are provided between the ribs,building services
Building services engineering is a professional engineering discipline that strives to achieve a safe and comfortable indoor environment whilst minimizing the environmental impact of a building.
Alternative titles are "building services engineerin ...
can be run through them. One proprietary implementation of this system is called Holedeck.
Disadvantages
* Greater quantities of formwork materials are needed, which can be very costly * Waffle slabs are thicker than flat slabs, so the height between each floor must be greater to have enough space for the slab system and other building services * Waffle slabs are preferred for flat topographical areas not sloped sitesExamples
*Royal National Theatre
The Royal National Theatre in London, commonly known as the National Theatre (NT), is one of the United Kingdom's three most prominent publicly funded performing arts venues, alongside the Royal Shakespeare Company and the Royal Opera House. I ...
, London, United Kingdom
*Washington Metro
The Washington Metro (or simply Metro), formally the Metrorail,Google Books search/preview
Building
* Logistic and Telecommunication SL, Madrid, Spain
* Barangaroo House, Sydney, Australia
* GS1 Portugal, Lisboa, Portugal
* Galbraith Hall, UC San Diego, California
* odD House, Quito, Ecuador
See also
* Waffle slab foundation * Reinforced concrete *Concrete slab
A concrete slab is a common structural element of modern buildings, consisting of a flat, horizontal surface made of cast concrete. Steel-Reinforced concrete, reinforced slabs, typically between 100 and 500 mm thick, are most often used to ...
* Formwork
Formwork is molds into which concrete or similar materials are either precast or cast-in-place. In the context of concrete construction, the falsework supports the shuttering molds. In specialty applications formwork may be permanently ...
References
{{Concrete navbox Floors Concrete buildings and structures Concrete Reinforced concrete Structural system