How do I design a two-way slab

What is a two-way plate?

RCC frame structures are very popular for almost all types of buildings these days. Ceilings, beams, pillars and foundations are the key segments of the building / house that provide strength to remain undisturbed against various loads and provide strength so that it will function well during its useful life. Here we are talking about a two-way plate. Before understanding the disposable plate or the two-way plate, one should understand the plate.

The slab is a basic flat part of a building / house used to develop floors and roofs. It is usually made of reinforced concrete. Slabs are the most common structural elements, the thickness of which is significantly less than their other dimensions. Plates can have different shapes and support conditions. They can be of solid, ribbed, and waffle types. Depending on their load-bearing behavior, they are classified as one-way or two-way panels.

The one-way slab is a slab supported by parallel walls or beams, the length-to-width ratio of which is equal to or greater than two and only flexes in one direction (tensioning direction) while loads are transferred to the walls or beams.

Read more about the disposable plate.

Here is the brief information about the two-sided plate:

Two-way panel under construction

The two-way slab is a slab that is generally supported on all sides by walls or girders, the length to width ratio of which is less than two, and rotates or flexes in either direction as the loads on the walls or carriers are transferred.

Two-way plate deflection

The two-way plate deflects in a bowl or saucer-like shape under the action of loads. If the slab is not poured monolithically with the supports (walls / beams), the corners of the two-way slab will rise. The two-way plate is designed for both directions as it flexes in both directions.

Even the panel supported on three edges or two adjacent edges can also be viewed as a two-way panel. In this case, charges are shared in two directions, which depend on the ratio of the sides.

In two-way plates, the main bars are provided in both directions and they are perpendicular to each other.

If the corner of the simply supported two-way panel is not held down, it will tend to lift off the support and this will create negative moments. Therefore, the corner reinforcement (torsion reinforcement) should be provided on a discontinuous edge of the two-way plate.

The usual thickness of these panels is in the range from 100 mm to 200 mm, depending on the span. The two-way plate is suitable and economical for plate sizes up to 6m x 6m.

Minimum depth of the two-way plate

The minimum depth of the two-way plate is mostly determined by deflection criteria. Sometimes the minimum depth is sufficient according to the bending design, but if the bending criteria are not met, the panels will vibrate even with small loads such as walking, jumping, etc. Therefore, even design reviews can result in shallower depth and always check for deflection. Here you can provide a marginally large depth and save steel to save. Please note that the vibrations do not give residents a feeling of safe construction.

Types of designs for two-way plate

The choice between these different two-way slab systems is made based on architectural, structural (height of design loads, bay lengths, and envisaged lateral load resistance systems) and design considerations.

Also Read: What Are The Building Structural Systems?

There are mainly three types of designs for two-way panels.

Two-way hard drives on bars

With a two-way solid ceiling system, the plate is supported on beams. Depending on the beam and column arrangements, it can be designed for a wide range of load conditions.

Two-way waffle plate (with bars)
Courtesy - Davidchola

The two-way waffle sheet system is used for office buildings (low building), parking garages and warehouses.

Cast in situ two-way waffle plate with integrated beams
Courtesy - Kordek

The two-way waffle plate has a larger span than the solid plate. The two-way waffle plate is economical when used in the modular or pre-made form.

We can assume "two-way panel effect" for the design of the other panels, such as the two-way panel with the hollow block, the waffle base (panel with beam supports) made from the prefabricated double-T profile, the waffle base with the hollow block or hollow core panels , Two-way flat sheets, two-way flat slabs, and two-way flat sheets.

Two-way plate with the hollow block

The reciprocal panel effect is assumed in a ribbed floor (panel with beam supports) made from the prefabricated double-T profile, in the ribbed floor with integrated beams and in hollow block or hollow core panels.

Two-way flat plate plate

The two-way flat plate is simply a plate of uniform thickness that is supported directly on pillars and is generally suitable for relatively light loads. Two-way flat plates can be used for spans up to 8 m with reinforced concrete and up to 11 m with the post-tensioning system. Due to the simplicity of the flat slab, it is the most economical flooring system as the formwork and reinforcement work can be done easily. Two-way flat panels save construction time and offer exposed flat ceilings with maximum storey height.

Two-way flat plates have a low punching shear capacity and a low stiffness for deflection. Beams are often provided around the perimeter of the slab to stiffen the free edges and support the brick walls. They are not recommended in earthquake areas. Shear walls can be provided to withstand all of the lateral loads from earthquakes so that the flat plate with the column can only withstand the vertical loads.

Two-way flat plates

The two-way flat slabs are better suited for the larger loads, and the spans are comparable to the flat slabs. It is due to the drop plates and / or column capitals that provide greater shear and flexural strength. Flat plates are used for heavier loads and longer spans. It requires less concrete and reinforcement compared to flat slabs. However, they require more formwork for capital and panels, and take more time to construct than the flat panel. They are used in fields that are square or almost square. Flat ceilings may be used for spans of up to 10 m and loads of up to 7 KN / m2.

Courtesy - tree hugger
Two way waffle flat plate

A slab supported on beams on all sides of each floor slab is commonly referred to as a two-way slab system. A flat waffle plate corresponds to a two-way beam system or can be visualized as a solid plate and it is used to reduce the weight of the plate. Flat waffle sheets have a square grid of closely spaced beams with filler sheets over the pillars. Similar to ordinary two-way waffle sheets, lightweight concrete blocks on temporary or permanent formwork can be used in their construction and arched shapes are omitted around the pillars to create massive sheets. The concrete in the ribs and slabs is poured integrally and creates aesthetic soffits of up to 750 mm 2 and pockets up to 500mm deep.

Remember that the choice of panels for a given structure depends on economy, buildability, load conditions and span. Panels can contain 15 to 20% amount of RCC in the total amount of RCC in the building. The cost of the panel can be very high in terms of the total cost of the building, rather than the cost of the column or beam. If the double-sided plate is designed for momentary only and only for shear, this results in a thinner plate thickness. So we observe vibrations on the floor, but they don't collapse. However, if the plate is designed to withstand the moments, shear and deflection, this will result in a higher thickness of the plate and you will get a vibration free plate. So always design it for the moment, shear and distraction.

Also read:

Difference between flat plate and conventional plate-jet system
Difference between column and shear wall
Posted in Build a House