Residential Slab and Footing Design

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Table of Contents

Introduction
Structural comparison
Construction methods analysis
Waffle slab analysis
Stiffened rib slab analysis
Conclusion
References

Introduction

The rising cost of building materials has brought about the need for innovation and the generation of new building techniques and methods. A building’s foundation is a major consumer of cement and other building materials. To reduce the cost both in terms of materials and labor the waffle pod footing system was developed. This system was developed in the 1980s and has proved to be a very economical way of producing concrete slabs to serve as building’s base. The polystyrene waffles serve as moulds in which concrete is poured into to form a slab over the entire building area. This method makes construction easier as it avoids the need for labor intensive activities such as those encountered when digging trenches for the laying of underground foundations. Instead this method implements the use of over the ground slab preparation (Mahl, 2007).

This method is said to be environmentally friendly as it uses ‘environmentally friendly polystyrene’ and minimizes on the amount of expended wastes (Iano, 2007).

Stiffened raft slabs are another common construction technique used in the Australian construction industry. This technique involves the digging of trenches on the planned site. Voids are later formed on the building’s site by the use of steel meshes, sand and stone dust. A stiffened raft slab is made up of a concrete slab of an average of 10mm in diameter, steel meshes, internal beams and edge beams. This type of footing is required to be above the ground level to prevent degradation and damage from flood water and other natural elements. According to the AS2870, a stiffened rib slab is meant to be 150mm above the normal ground level.

Structural comparison

There are pronounced structural differences between the waffle and the stiffened rib slabs. For comparison purposes these two techniques will be assessed based on a class H site case study. The depth of the stiffened rib slab is always more than that of the waffle slab. For the same structure, if the depth of the waffle is 300 mm the stiffened rib footing’s is usually 500 mm. This translates to more excavation and site preparation work making the stiffened footing more labor and time intensive. This eventually translates to a differentiable difference between the two methods (MacGinley, 1978).

The stiffened rib slab also requires bottom reinforcement. The bottom reinforcement is usually a beam made of steel mesh and concrete. The beam acts as a supporting mechanism to the slab structure. This element makes the stiffened rib slab more durable and stronger in comparison to the waffle slab. The waffle slab is usually reinforced by steel bars of Y16 gauge but does not make use of any composite beams in its structure. The waffle slab technique relies on polystyrene voids and the ribbed concrete structure for its strength. Both techniques make use of the F72 and F82 gauge steel meshes in their structure. The structural strength of the stiffened rib slabs is consequently higher as it integrates the use of steel beams, steel meshes and steel bars unlike the waffle slab technique that only involves the use of steel meshes and bars (Newlands, 2002).

The waffle footing has been designed for use in single story buildings as it is unable of supporting very large weights. Another distinct difference between the two is the spacing of the ribs as they tend to be further placed in the waffle slab compared to the stiffened rib. This results in the use of a lower amount of concrete and steel meshes in the waffle type of slab. The stiffened rib structure does not have any void packets but is controllably filled with sand and stone dust. The waffle slab on the other hand has the polystyrene pockets below the slab’s surface. These voids are said to contribute to the strength and resistance to shear and tensional forces of the building (Fine Homebuilding, 2003).

The waffle slab has been found to exhibit greater strength and resistance to cracking and failure as compared to the stiffened rib slab. The polystyrene voids have also been designed with the structural and functional aspects of the slab in mind. The voids have also been designed in a way that will facilitate the construction process of the slab; designed to be able to carry human weight and resist collapse when subjected to the weight of wet concrete and construction traffic. The strength of the voids is very critical as they are required to withstand the heavy loads subjected on them when wet concrete is poured over the setup (Newlands, 2002).

The stiffened rib slab is constructed into the ground and this makes it unsuitable for delicate soils due to its tendency to fail under such circumstances. In unstable soils such as reactive clay the waffle slab is very suitable and durable as it is laid shallow onto the ground (Cope, 1984).

Construction methods analysis

The two footing techniques also vary in their construction procedure and steps. In the waffle type of slabs, the site is first excavated and flattened to create an even ground for the laying of the slab. Uneven ground may result in a weak structure and the flattening of the soil is paramount for any strong waffle slab. In loose soils the soil can also be compacted to provide a firm platform for the slab. The piping and sewerage work is usually done before the slabs are laid down. A vapor barrier is first laid down on the construction site; this barrier serves to prevent dampening of the slab due to movement of water from the soil into the slabs. A ribbed structure that will accommodate the waffles is then prepared. The structure has a rib between each polystyrene waffle and this allows for flow of concrete into the ribs. The polystyrene waffles are then placed strategically onto the site forming a slab of pods. Once the ribbed waffles are in place concrete is then poured over the structure and allowed to dry gradually. The result is a concrete slab that is capable of supporting the intended structure (Park, 2000).

The stiffened rib slab formation process is also started by the excavation of soil from the site. The excavation is done to give an allowance of a beam of depth 300 mm to 1200 mm (depending on the class of site). Stone and sand dust are then used to create ribs within the excavation. The ribs form the basis on which the stiffened rib slab is formed from and this calls for care and concentration in their preparation. A water proofing sheet usually made of treated polythene is placed over the structure. This serves to prevent the dampening of the slabs by water from the ground. To form beams, steel meshes are strategically placed into the trenches. Concrete is then prepared and poured over the trenches to form a stiffened rib structure. The standard dimensions for the beams are 300 mm in width. The beams provide most of the slab’s strength and thus they must be well constructed. It is recommended to pour the stiffened rib slabs on the same day as pouring in different times promotes the formation of crevices and weak points in the structure that would compromise the building’s strength (Neville, 2006).

The stiffened rib is an older method of building slabs and is steadily being replaced by the waffle slab. The waffles slab is much easier and simpler to construct and promises better results compared to the stiffened rib.

Waffle slab analysis

In modern construction the need for a simple and effective way of constructing buildings is on the forefront of engineers’ minds. The waffle technique is the best of its time. Below are the strengths and weaknesses of the waffle technique:

Advantages (Mahl, 2007)

It uses polystyrene voids that are environmentally friendly and safe for human use
The polystyrene voids used also serve as insulator mechanisms for the building and this helps in reducing the long term energy expenses of the building
It can be used on unstable soils such as reactive clay sites, thus has a wider range of applications and operating environments
It is easier to build
It is faster to build
Requires less man hours and machinery to construct
It is much cheaper and cost effective compared to the stiffened rib slab
The amount of concrete that is used in the structure is greatly reduced
The waffle slab has improved strength and durability properties
The construction process can take place in a wide range of weather conditions as the polystyrene voids are resistant to weathering
Waffle slabs can be constructed on rocks and surfaces that are difficult to penetrate/excavate

Disadvantages

Cannot be used on sloping ground. In such instances the ground has to be levelized
Is not suitable for complex designs and structures

Stiffened rib slab analysis

This slab construction technique was a very popular method in the past. Newer and more advanced methods have been found but this does not mean that the stiffened ribbed slab does not have a few strengths. Below are the advantages and the disadvantages of the stiffened slab technique:

Advantages (Neville, 2006)

Can be used to construct building slabs in sloping and uneven ground
Is most suitable for complex and technical construction processes
Has great flexibility and strength even in unsuitable ground
Very suitable for class M and H sites

Disadvantages (Mahl, 2007)

Uses more concrete in comparison to the waffle raft
The concrete amounts used in the slab cannot be accurately controlled and monitored
Cannot be used on unstable soils such as reactive clay due to its tendency to have its beams embedded in the clay
It is labor intensive as it requires extensive digging of trenches to support the rib structure
It is much harder and technical to build as it requires elaborate planning and equipment to execute
It is expensive in comparison to the waffle raft slab
It has lesser strength compared to the waffle slab, coupled with higher costs making it uneconomical to implement
The construction process of the stiffened rib slab can only take place in suitable weather conditions unlike the waffle slab that can be done even in wet weather.
Cannot be constructed on rock surfaces that are difficult to excavate and dig.

Conclusion

This study has shown that each and every slab construction technique has its advantages and disadvantages. For engineers to come up with the best type of slab there is a need for the assessment of the prevailing conditions and of the unique needs prevailing at the time. One major determinant is the type of soil and the orientation of the ground. These two factors greatly influence the type of slab to be used and even surpass the estimated cost as a determinant of the method to be used.

The stiffened slab is generally more expensive in comparison to the waffle slab. This is because of the high amounts of concrete used, high labor cost and the technical construction activities needed. Therefore for a home builder who is cost conscious and is trying to cut on his / her expenses, the waffle raft would be the best option as it is substantially cheaper when compared to stiffened raft technique (Sinha, 1996).

From the study it has generally been found that the waffle technique is the best option for a home builder. This is because this method is suitable for structures that are not very complex. It is also very simple to construct and provides just enough of what is needed to support a residential house. The design of the waffle raft will be based upon the plans of the house that have been done by the architect. The engineer will then be required to assess the soil properties of the construction site and decide on whether the waffle technique would be suitable. If the soil and orientation of the ground cannot support the waffle slab, then the stiffened rib slab will have to be used regardless of the cost as it would be the only reasonable option in such an instance.

The waffle slab is growing in popularity in Australia and many other countries. The future could bring better and improved means of constructing waffle slabs.

References

Cope, Robert. Concrete slabs: analysis and design. Essex: Elsevier Science Publishing., 1984. Print.

Fine Homebuilding. Foundations and concrete work. New York: Taunton Press., 2003. Print.

Hurd, Mary. Formwork for concrete. Michigan: American Concrete Institute., 2005. Print.

Iano, Joseph. The architect’s studio companion: rules of thumb for preliminary design. New Jersey: John Wiley and Sons., 2007. Print.

MacGinley, Thomas. Reinforced concrete: design theory and examples. New York: Taylor and Francis., 1978. Print.

Mahl, Iver. Building anatomy: an illustrated guide to how structures work. New York: McGraw Hill., 2007. Print.

Newlands, Moray. Concrete floors and slabs. London: Thomas Telford., 2002. Print.

Neville, Adam. Concrete: Neville’s insights and issues. London: Thomas Telford., 2006. Print.

Park, Robert. Reinforced concrete slabs. New Jersey: John Wiley and Sons., 2000. Print.

Sinha, Shah. Handbook of reinforced concrete design. New Delhi: Tata McGraw Hill., 1996. Print.

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