What is waffle slab? -A General Overview

Waffle slab or ribbed ceiling is a concrete slab which has a beautiful appearance and many economical and technical advantages. Waffle formworks are used to construct these types of slabs. These formworks reduce the dead load of the structure and makes a great view in the lower part of the roof. In Zagros Bana’s website you will get information about the history and how to design and construct waffle slabs. Also advantages and disadvantages of waffle slabs will be explained. In the following first we will answer the question “what is waffle slab?”

History

To answer the question of “What is waffle slab?” we must refer to its history of emersion. The idea of two-way joist system with holes in the bottom is refer back to about 2500 years ago in Achaemenid period. According to the archaeologists’ analysis, orthogonal wood beams have been used in the roof of Perspolis. This type of slabs recently are known as coffer slabs which is the origin of waffle slabs. Various types of this slab have been used in in the last century. Ancient Roman Pantheon temple is another example of utilizing coffer slabs. In this temple hollow slab is used to cover a 43 meter span.

Waffle slabs in its modern sense is referred back to 20^th century as the construction industry have been expanded and architectural issues became more important. During this period, the need for wider spans with less columns for implementing irregular architectural designs has been become a challenging issue. But it is not clear that which country or which building used the waffle slab for the first time.

Waffle slab construction procedure

In the first step vertical and horizontal supports should be arranged. Then waffle formworks are placed in the specified areas according to the structure plan. In this case, there isn’t any need for precast concrete moulds because waffle formworks are enough to prevent concrete from leaking. The joists are commonly formed by using standard square dome forms.

In the third step reinforcement is placed according to the plan. Due to the high strength of waffle formworks, it will be easy to move and work on these moulds. To increase punching shear resistance solid heads considered at the columns (this part constructed by ommiting waffle moulds around the columns). After checking the reinforcement concrete pouring step is started. When the concrete gets hardened enough, Waffle formworks will be removed. Thus, the waffle like shape appears at the bottom surface and a strong slab is ready to operation.

Waffle slab designing

Waffle slab designing could be done with the help of software and regulations as designing other slabs. Safe software and special standards and regulations for designing structures with waffle slab.

Parameters in designing this type of roof includes top slab thickness, web thickness in joists, width of joists, and spacing between each joist. As you know precast joists would not be used in waffle slab and the width of the joists will be determined based on the width of the two edges of waffle formworks. Characteristics of waffle slabs which could be considered in its designing are as follow:

• Top slab thickness is between 75 and 130 millimeter (ratio of slab thickness to slab span is 1 to 24)
• Joists width are between 130 and 150 millimeter depending on the width of formworks edges.
• Joists are spaced every 915 millimeter (maximum) on center.  Spacing depends on the size of waffle formworks used.
• Ratio of web thickness in joists to slab span is 1 to 1 to 25.

The above mentioned values may vary depending on the regulations used in different countries.

Advantages of waffle slab

In this section we will mention some advantages of waffle slab floor system. The most important advantage of waffle slab floor system is the elimination of unnecessary columns. In addition to the negative impacts of these columns on the architecture of the building, they also increase the structures weight and cost. This system provides bigger spans with less material which makes the construction more economical and environmentally friendly. Besides two way joist system leads to have greater carrying capacity than traditional one-way slabs. Waffle system also provides an architecturally desirable appearance in the ceiling which could be more attractive using lighting ideas. Beside this system provides less vibration due to the existence of two-way joists reinforcements that form the grids.

As mentioned before one of the advantages of waffle slab system is elimination of the precast joists which cost more with difficult transportation and storage. Also the materials of the moulds result in less water absorption of concrete and higher concrete quality. In this case the vibration process is don better and easily. Building services can be run through them, if soles provided between the ribs. One proprietary implementation of this system is called Holedeck.

Important structures with implementation of waffle slab floor system

Due to technical, economical and aesthetic advantages of waffle ceiling, this system has been used in many important structures in the last decades. Waffle slabs are mostly used slab in structures such as airport, industrial halls, multilevel parking, commercial structures, etc. Some of them are mentioned as below:

 The Royal National Theatre in London, commonly known as the National Theatre utilized waffle system. It is one of the UK’s three most prominent publicly funded performing arts venues which hase more than 2000 seats capacity.

The Washington D.C Metro is one of the most frequented Brutalist architectural structures. Waffle roof system is utilized in this building. Consistent designs of concrete structures, bronze and granite fixtures, inset lighting, and signage aesthetically unify these stations.

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Other examples are: Logistic and Telecommunication SL, Madrid, Spain; Barangaroo House, Sydney, Australia, GS1 Portugal, Lisboa, Portugal; Galbraith Hall, UC San Diego, California.