Flat slab: What is it, types advantages and disadvantages
In a conventional building, the columns and beams support the slab. The flat slab method eliminates the need for beams by directly supporting the slab with columns. Because of its practicality, versatility in interior design, and ease of building, a flat slab is a popular option for constructing asymmetrical column layouts. This article will give you all the information you need to make the right decision.
What is flat slab?
When we talk about a flat slab, we’re referring to a slab of reinforced concrete that firmly rests on columns. The behaviour of a flat slab is the same as that of a two-way slab, with its thickness generally being greater than the thickness of the concrete surface in a beam-slab system. Flat slabs must be at least 125 millimetres thick to comply with the requirements of IS 456:2000.
Flat Slab: Uses
Most constructions use flat slabs, particularly in areas that demand greater aesthetics and light dispersal or where the construction requires less formwork. It is common practice to employ flat slabs when constructing uneven column layouts, such as ramps, storeys with bent shapes, etc., in public spaces like theatres, skyscrapers, parking garages, etc.
Using flat slabs creates advantageous conditions for building flat soffits, gives a depth solution and allows for greater freedom in the design layout. Flat slabs give a great deal of pliability and can be used wherever there is a possibility that internal layouts will need to be altered in the future.
Types of flat slab
Simple flat slab
A simple flat slab is often made of concrete and does not include any beam support but is sustained by caps and columns. The simple flat slab may be constructed with little effort and needs only a small quantity of formwork.
As a result, the building’s loads are transmitted into the columns so that the structure can maintain an efficient balance. These simple flat slabs are appropriate for spans extending from 6 to 9 metres in length.
In this context, the term “span” is the distance that exists between the structure’s two discrete supports. Likewise, the flat slabs are the optimal choice for dynamic loading weighing around 7 kN/m2 and up.
Flat slab featuring drop panels
Slabs that use drop panels and column capitals are called “flat slabs with drop panels.” The slab’s increased thickness at its uppermost column is known as the drop panel. The shear resistance of the flat slab can be improved greatly with the help of these drop panels.
The flat slab with drop panels added to it also boosts the structure’s potential for absorbing negative moments. The bending moment that is created when the compressive stress from the bottom end of the beam acts in conjunction with the tensile stress from the top side is known as the negative moment capacity. These flat slabs significantly decrease deflection while enhancing the slab’s overall stiffness.
Flat slab featuring column heads
A column head is an expansion made of reinforced concrete that is placed on the top of a column and functions as an essential component of the system. For architectural applications, the angle of such column heads could be designed to be anything the architect desires. On the other hand, the concrete should be placed at an angle of 45 degrees on both faces of the column if designs are to be stamped into the concrete.
Flat slab: Problems
While designing flat slabs, punching shear failure is a major concern. The flat slab fails as a result of extreme localised stress. This occurs typically towards the column’s base or the point where the column meets the slab.
A punching shear failure occurs when a column’s focused support reaction exerts a pushing force on the slabs, and the loads cause fractures to form all over the loaded sections of the slab, leading to a truncated failure of the slab. These approaches are taken to address this issue:
Maximise the thickness of the slab as a whole, including column heads and drop panels
The shear perimeter can be concealed by increasing column size.
Provide appropriate shear reinforcement.
Flat slab: Advantages
Flat slabs, as opposed to traditional slabs, provide several advantages that will be covered in the following points.
Rapid construction
Since there is less need for formwork when constructing flat slabs, the building time for these is shortened. Additionally, the use of tabular formwork could further simplify and decrease the amount of labour needed to complete the project. Precast welded mesh shortens the time it takes to set up flat slabs, which speeds up the construction process by allowing more time to place vertical members.
Flexibility for designer
Designers can better express their visions while working with flat slabs because walls do not constrain them. The overall floor plan can be altered without impacting the building’s framework. Additionally, higher-quality management is more widely available and is regarded as superior in fire resistance. Reinforcement details for a flat slab also become less complicated, and design work is possible for a specific soffit level.
Flexibility for the proprietor
Since a flat slab allows for a great deal of interior plan flexibility, it may be easily modified if revisions are needed or if the occupier desires a scope for additional development. As a result, designers and owners benefit from the adaptability afforded by a grid that excludes beams at right angles to the square or near-square intersections.
A reduced storey height
In the beam-slab system, the beam and the slab’s combined thickness is more than the flat slab’s thickness. When the length of the storey’s vertical elements is shortened, the total height of the building is reduced, and the building’s own weight is lowered as a result. High-rise buildings benefit the most from this technique because the whole saved height and reduced self-weight can significantly reduce development costs.
Easy electrical and mechanical system installation
In buildings with flat slabs, there are no beams. Thus, there is no need to bend cables or cut through beams to provide mechanical and electrical services. Flat slabs make it easier to install things like air conditioning ducts, fire suppression ducts, electric ducts, and so on.
Aesthetics, acoustics, and dispersion of light
A flat slab can reveal a fascinating glimpse of the structure’s inside. Since there are no beams or dividing walls to get in the way, the entire slab is completely clear and open for any and all uses. Light is more diffused in all directions, and the ceiling’s lack of decoration helps to keep the room evenly lit. Most auditoriums are made up of flat slabs so that the light and sound may spread as far as possible.
Fire resistance
Flat slabs provide better fire resistance due to their uniform thickness, ensuring consistent protection for embedded reinforcements.
Cost-effectiveness
The reduced need for formwork and labor makes flat slabs a cost-efficient option for construction, especially in urban areas.
Improved aesthetics and light dispersion
The absence of beams creates smooth ceilings, enhancing aesthetics and allowing for even light distribution, which is ideal for auditoriums and malls.
Ease of mechanical and electrical installations
Flat soffits simplify the installation of HVAC systems, electrical conduits, and fire suppression systems without the need for beam penetrations.
Flat slab: Disadvantages
Before deciding to use a flat slab for your building project, you need to take into consideration not only the benefits but also the drawbacks that come along with using it.
Thickness
Compared to the slab utilised in the beam-slab system, the thickness of the flat slab is significantly greater. As a result of this, a significant quantity of concrete is utilised in the building process.
Restrictions on span
A greater bending moment value corresponds to a larger span. Effective slab depth will need to be increased to meet this challenge, and drop panels and columns may need to have a bigger diameter.
Vulnerable to lateral loading
When it comes to resisting lateral stresses, such as those caused by wind, earthquakes, and other seismic activity, brittle (masonry) partitions are not a good choice to be supported by a flat slab structure.
Impediment and failure
Restrictions should be placed on how much drop panels can interfere with mechanical ducting. Areas around columns must be free of vertical penetrations, often known as punching shear failure.
Potential interference with ducting
Drop panels and column heads can obstruct mechanical ducting and other systems, requiring careful planning during design.
Higher material requirements
Flat slabs require greater thickness than beam-slab systems, increasing concrete usage and higher material costs.
Vulnerability to lateral forces
Flat slabs are only effective at resisting lateral forces, such as those from wind or seismic activity, with additional reinforcement like edge beams.
Lifecycle maintenance of flat slabs
Flat slabs, like any structural system, require proper maintenance to ensure their longevity and structural integrity. Below is an in-depth discussion on the maintenance requirements and long-term durability considerations for flat slabs:
Maintenance requirements
Inspection for cracks and deformation: Flat slabs are prone to cracks due to load concentration and punching shear at column connections. Regular visual inspections should be conducted to identify any surface cracks, deflections, or warping.
Punching shear vulnerabilities: Frequent checks around column-slab junctions are essential. Reinforcement integrity in these areas should be examined to avoid sudden failures caused by excessive localized stress.
Waterproofing and sealing: Flat slabs exposed to weather conditions, such as rooftop slabs, need waterproofing to prevent moisture ingress. Periodic resealing of joints and cracks helps protect against corrosion of embedded reinforcement.
Corrosion control: In humid or coastal environments, flat slabs are vulnerable to reinforcement corrosion. Applying anti-corrosion coatings or cathodic protection systems can mitigate this issue.
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