Concrete Slabs in Concrete Frame Buildings

framed structure

The choice of the slab type and form is very crucial at the conceptual design stage of any project. Many factors must be considered  some of which are within and others outside the control of the designer. The economy of construction, the expertise of the contractors can best be described as geographical variables outside the control of the designer. The design loads, span configuration, serviceability and strength requirements are equally important factors.  Thus, when developing a structure’s scheme design, the chosen form of the floor slab must be given adequate and thorough consideration.

In this post, the basic forms of floor slabs that are available and currently used is presented. Each has its unique features, advantages and disadvantages, which this post attempts to identify and explain. Floor slabs are continually evolving, thus, it would be impossible to present all slab types in this post. However, note that new floor slab solutions are only an extension of the basic slab forms described in this post.

Solid Slabs

Solid slabs are the most common and basic forms of concrete slabs in reinforced concrete building structures. These slabs could be either one way or two ways spanning slabs, depending on the supporting beams configuration. One-way slabs are supported by beams only in one direction and have a aspect ratio greater than two. While two way slabs have beams along all four sides and have a aspect ratio less-equal to two. The supporting beams can be made either deep or shallow depending on the architectural and structure’s requirement.

describes solid slabs
Figure 1: Solid Slab

This form of slab construction is very suitable for office buildings, residential units warehouses and similar buildings. Generally, this form of construction is used where gridlines are irregular such that other slab types are unsuitable.

See: One Way Continous Slab-Worked Example

Advantages

Solid slabs analysis and design is well covered in most codes of practice and well understood by most designers. Also, being a common form of slab construction, all concrete frame contractors and builders can handle this type of slab. Importantly, they are very useful in transfer structures. Where the loads from a wall/column above a slab are transferred via beams to columns/wall below the slab.

Disadvantages

It is not a fast method of slab construction as work tends to be tedious and labour intensive. For example, a lot of time is often wasted in preparing the slab formwork due to the presence of drop beams.

It might not be an effective solution in highrise building, where the dead loads of slabs are desired to be minima. If solid slabs are used, the cumulative weight of the slabs will lead to a much larger structural elements which would significantly impact on the foundations.

Flat Slabs

Next to solid slabs, flat slabs are the most popular slab form. Flats slabs are supported directly on columns without supporting beams. They are a highly versatile form of slab constructions offering reduced depth, fast construction and flexible layout. They are sometimes thickened at the supports to form a drop panel or the columns flared to form a column head. The purposes of which is to increase the efficiency of the concrete section.

describes flat slabs
Figure 2: Flat Slabs

Flat slabs can be designed using coefficients, equivalent frame method, yield line methods and finite element methods. The use of coefficient and equivalent frame method is only possible where the grid layout is regular. Where the column layout is irregular, the slab can only be analyzed and designed using yield line analysis or finite element methods.

Punching shear and deflection are usually the governing factors in determining the depth of the section. Shear reinforcement may be required and are generally provided by links, shear rails etc.

See: Designing a Flat Slab-Worked Example

Advantages

Flat slab is one of the fastest method if not the fastest method of slab construction. This is due to the simplification of the formwork due to the absence of supporting beams.

Flats slabs are very useful and economic in long-span structures generally for spans up to 12m. They can be constructed to give a good surface finish, thereby allowing the exposed soffits to be used. Also, flat slab buildings offer reduced storey heights thus, the overall building height is greatly reduced.

Disadvantages

Holes from building services affect the punching shear design of the slab. This could lead to reinforcement congestion along columns which is generally undesirable.

Ribbed and Waffle Slabs

Ribbed and waffle slabs construction introduces voids to the soffits of slab or replaces voids with lighter materials. This form of slabs is based on one of the key assumption in the design of reinforced concrete members i.e all tensile stresses are fully carried by the reinforcements. This is because the tensile strength of concrete is negligible and not effective in tension. It, therefore, follows that the volume of concrete in the tensile zone will be a waste. Thus, introducing voids to the soffits of slabs reduces the weight of slabs and increases the efficiency of the concrete section.

See: Designing a Troughed Floor-Worked Example

Ribbed and waffle slabs provide lighter and stiffer slabs. They are essentially very useful in long-span floors, typically up to 12m. The lightweight of the slab means that the extent of their foundations is reduced relative to other slab forms.

Although ribbed and waffle slabs are based on the same design principle, there’s a distinction between the two slab forms. Ribbed slabs are one-way slab systems and have ribs running only in one orthogonal direction. While waffle slabs are two-way slab system having ribs spanning both directions. Overall, they tend to be dipper than their equivalent flat and solid slabs counterparts. Even though waffle slabs tend to be dipper than an equivalent ribbed slab.

While ribbed slabs are normally orientated so that the ribs span the longer distance, and the band beams the shorter distance, waffle slabs work best with a pure square grid.

See: Designing a Waffle Slab-Worked Examples

describes waffle slabs
Figure 3: Waffle Slabs
Advantages

They are particularly useful in long-span floors, typically up to 12m and provides a lighter and stiffer slab. Thus they are suitable in high-rise buildings where reducing the cumulative weight of slabs is an issue.

Ribbed and waffle slabs provides good surface finish just like flat slabs. Thus the slab profile may be exposed in the final building for passive cooling or for aesthetics. They also provide a very good form where slab vibration is an issue, such as laboratories and hospitals.

Disadvantages

This is a very slow form of slab construction, this is because the ribs are constructed using table forms/moulds. Thus, the time required to set-up the formwork can be time-consuming.

Voided Slabs

The voided slabs are sometimes called bi-axial slabs. This is an extension of the principle of the ribbed and waffle slabs form. Since it is agreed that the bulk of the work in reinforced concrete is done by the reinforcement and not the concrete. It follows that the volume of concrete in the middle of the slab is also useless and can be replaced with lightweight materials.

describes voided slabs
Figure 4: Voided Slabs

Thus voided slabs incorporate lighter weight materials in the middle of reinforced concrete slabs. The sole purpose is to reduce the weight of the slabs while retaining the efficiency of the concrete section. Some of these materials include polystyrene, plastics, rubber etc.

Advantages

The major advantage of the voided slabs is its reduction in slab weight compared with regular solid decks. Up to 50% of the slab volume may be removed in voids resulting in less load on the structural members. This also allows increased imposed loads and/or span, since the self-weight of the slab contributes less to the overall load.

Similarly, since the amount of concrete needed is greatly reduced, the amount of carbon emitted into the air during construction is reduced. Also, since most of these lightweight materials are purely recycled waste, it poses a good environmental benefit.

Disadvantages

The presence of holes within the slab poses design challenges, as with flat slabs.

Post-tensioned Slabs

Post-tensioned slabs are slabs in which the volume of concrete is mobilized to resist the forces via pre-stressing. This involves keeping the concrete mainly in compression by using steel cables known as tendons. This is achieved through the application of tension to the tendons during the curing process of the concrete with strand jacks at the perimeter of the slab. On releasing the tendons, compressive forces are transferred by the tendons to the slabs.

describes post-tensioned slabs
Figure 5: Post-tensioned Slabs

Normal reinforcement is required wherever prestress is not present. This includes the edges of the slab and in any closure or infill strips. It is also needed at anchorages. Where there are large bursting stresses due to high local forces.

Advantages

Postensioned slabs offer the thinnest slab type as the concrete is effectively used, mostly by keeping it in compression.

Longer spans can be achieved as compressive force applied to the slabs effectively stiffens the prestressed slabs and beams.

Post-tensioned slabs can be cheaper than the equivalent RC frame particularly for spans exceeding 7.5m/8m. Their construction also tends to be faster due to the reduction in steel fixing and the formwork can also be struck earlier.

Disadvantages

Post-tensioned slabs require the services of specialist contractors. Not every concrete frame contractors have the capacity to carry out prestressing.

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Omotoriogun Victor
About Omotoriogun Victor 66 Articles
A dedicated, passion-driven and highly skilled engineer with extensive knowledge in research, construction and structural design of civil engineering structures to several codes of practices

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