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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 slabs, typically between 100 and 500 mm thick, are most often used to construct floors and ...
made of reinforced concrete
Reinforced concrete, also called ferroconcrete or ferro-concrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ...
with concrete
Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...
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 , because, for a given mass of concrete, 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 (construction), framing to span an open space, often between Beam (structure), beams that subsequently transfer loads to vertical members. When incorporated into a floor framing system, joi ...
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 science, scientific or technological research, experiments, and measurement may be performed. Laboratories are found in a variety of settings such as s ...
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 the
secondary sector of the economy. The term may refer ...
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 to present experiences of a real or imagined event before a live audience in a specific place, often a Stage (theatre), stage. The performe ...
s or train station
A train station, railroad station, or railway station 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 suc ...
s. Waffle slabs are composed by intricate formwork, 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 generic formwork, forms 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#Civil engineering, In situ: Formwork construction and pouring of concrete occur on site, then the slab is assembled (if required). * Precast concrete, Precast: The slabs are made somewhere else and then brought to the site and assembled. * Prefabrication, 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 Concrete slab#Two-way slabs, 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, Mechanical, electrical, and plumbing, building services 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, London, United Kingdom *Washington Metro 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 * Centro de Bellas Artes de Caguas Parking Garage, Caguas, Puerto RicoSee also
* Waffle slab foundation * Reinforced concrete * Concrete slab * FormworkReferences
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