- Determine the function of the slab: The first requirement is to determine what the slab will be used for. Will it be used for a driveway, walkway, patio, or other purpose? This will help determine the slab thickness, material, and other considerations.
- Design the size and shape of the slab: Once the function is determined, the size and shape of the slab should be designed. The size should be based on the size of the area it will cover and any other requirements, such as access points or other features.
- Consider the environment: Consider the environment the slab will be placed in. This includes the soil type, moisture levels, and climate. This will help determine the type of material needed for the slab and its thickness.
- Choose the material: Slabs can be made from concrete, stone, or a combination of both. The type of material chosen will depend on the function of the slab, the environment it will be in, and the desired aesthetics.
- Consider drainage: Slab design should take into consideration drainage, as the slab should have a slope and any necessary drainage holes or channels.
- Reinforcement: The slab should be reinforced with steel bars, mesh, or fibers to increase strength and durability.
- Consider additional features: Consider any additional features such as expansion joints, control joints, or crack control measures.
- Consider the cost: Consider the cost of the project, as this will help determine the type of material, thickness, and other considerations.
- Finalize the design: Once the design has been finalized, it should be drawn up and presented to the contractor for review. The contractor will be able to provide details on the cost, materials, and other factors.
what is L/B ?
L/B is the ratio of the length (L) of a one-way slab to its width (B). It is used to determine the most appropriate design for the slab.
If ????/???? >1.5, then the one-way slab should be designed as a flat slab, not a conventional slab. Flat slabs are thinner and lighter, and can easily span large distances with fewer columns and less reinforcement. Flat slabs have better seismic performance, and are less expensive to construct.
it is a common practice that if ????/???? > 1.5, design it one way slab.
If ????/???? ≤ 1.5, then the one-way slab should be designed as a conventional slab. Conventional slabs are thicker and heavier, but they can span shorter distances and require more reinforcement. Conventional slabs are more structurally sound, and are often more cost effective than flat slabs in short-span applications.
If ???????? > 3 ????: it is advisable to provide a two-way slab.
Common depth of slabs = 5”, 6” (125 mm, 150 mm); max=200 mm (for load = 5 ????????/????????2 ).
The depth of a slab is an important consideration when designing a slab. Generally, the depth of a slab should be kept to a minimum to reduce construction costs and to ensure the slab is able to support the desired loads. For a slab with a load of 5 kN/mm2, the maximum recommended depth is 200 mm (8 inches). Common depths for slabs are 5 inches (125 mm) and 6 inches (150 mm).
L/D ratios:
The L/D ratio (length to depth ratio) is used to determine the appropriate depth of a slab. Generally, the L/D ratio should be between 2 and 4. This means that the length of the slab should be two to four times greater than the depth of the slab. For example, if the length of the slab is 6 m, then the depth of the slab should be between 1.5 m and 3 m.
One way S.S slab ????/???? = 20 × ????. ???? = 20 × 1.4 = 28 ???????????? ???????? = 0.3%, ????. ???? = 1.4) . Adopt ???? ???? = 25.
For a one-way slab with an L/D ratio of 20, the required flexural strength should be 28 f or a pt (punching shear) of 0.3%. To achieve this strength, the length should be 25 times the depth. Therefore, the length of the slab should be 25 times the depth of the slab.
One way continuous, above value (25) shall be multiplied by 26/20 = 1.3 = 25 × 1.3 = 32.5. Adopt ????/???? = 32.5
For a one-way continuous slab with an L/D ratio of 20, the required flexural strength should be 28 f or a pt (punching shear) of 0.3%. To achieve this strength, the length should be multiplied by 1.3, giving a length of 32.5 times the depth. Therefore, the length of the slab should be 32.5 times the depth of the slab.
Two way S.S slab ????/???? = 28 (for ???????? ≤ 3.5???? ???????????? ????. ???? ≤ 3 ???????? ????2 )
For a two-way slab with an L/D ratio of 28, the required flexural strength should be 28 f or a pt (punching shear) of 0.3%. This ratio applies when the length of the slab (Lx) is less than or equal to 3.5 m and the load (L) is less than or equal to 3 kN/m2. Therefore, the length of the slab should be 28 times the depth of the slab.
Two way continuous slab ????/???? = 32 (for two-way slab, shorter of the two spans be used to calculate ????/???? ratio).
For a two-way continuous slab, the L/D ratio should be 32. This ratio applies when the shorter of the two spans is used to calculate the ratio. Therefore, the length of the slab should be 32 times the depth of the slab.
Common diameter of bars = 8, 10, 12 mm. (dia. ≯ ????/8)
• Approx. area of main steel in slabs, ???????????? = ????????/0.8????.???????? (???????? = 0.138????????????????????2 )
The common diameter of bars used in slabs is 8, 10, and 12 mm. The diameter should be at least one-eighth of the depth of the slab. The approximate area of main steel in slabs is ???????????? = ????????/0.8????.????????, where ???????? is the maximum moment and ???????? is the yield strength of the steel. The value of ???????? is typically 0.138 ????????????????????2.
The formula for calculating the approximate area of main steel in slabs is ???????????? = ????????/0.8????.????????, where ???????? is the maximum moment and ???????? is the yield strength of the steel.
Theoretically, calculated ???????????? is provided in the middle strip and min. ???????????? is provided in edge strip. In practice, bars are uniformly provided spaced throughout spans in both directions.
• Assume ???????????? = 0.3 − 0.45% (one way slab)
• Assume ???????????? = 0.2 − 0.3% (two way slab)
• Assume ???????????? = 0.5% (one way continuous)
• Maximum area of main steel in slab = 4% ???????? ???????? (As per ACI).
Theoretically, the calculated ???????????? should be provided in the middle strip and the minimum ???????????? should be provided in the edge strip. In practice, bars are usually uniformly spaced throughout the spans in both directions. The assumed area of main steel in one-way slabs is 0.3-0.45%, in two-way slabs is 0.2-0.3%, and in one-way continuous slabs is 0.5%. According to ACI, the maximum area of main steel in a slab should not exceed 4% of the total cross-sectional area.
All top reinforcement of slabs shall continue up to ????/3 of span at both sides of C/L of wall or Beam.
All top reinforcement of slabs should extend up to ????/3 of the span at both sides of the centerline of the wall or beam. This will provide adequate support and reinforcement to the slab near the wall or beam.
Corners free to lift: cast over walls and no parapet above it.
If a slab is cast over walls and there is no parapet above it, the corners of the slab will be free to lift. This is due to the lack of support from the walls, which can cause the slab to warp and lift at the corners. To prevent this, additional reinforcement may be needed near the corners of the slab.
Roof slab with corner held down is achieved by 230 mm parapet wall, 600 mm high and 2 m length on either side of the corner.
A roof slab with a corner held down can be achieved by constructing a 230 mm parapet wall, 600 mm high and 2 m long on either side of the corner. This will provide support to the corner of the slab, preventing it from warping or lifting.
Area of distribution steel = 0.12% ???????? ???????? (For floor slab)
Area of distribution steel = 0.24% ???????? ???????? (For roof slab)
The area of distribution steel for a floor slab should be 0.12% of the total cross-sectional area, while the area of distribution steel for a roof slab should be 0.24% of the total cross-sectional area. This additional reinforcement will help distribute the load of the slab more evenly and reduce the risk of cracking or warping.
Short span bars are provided in the bottom layer.
In slabs with short spans, additional reinforcement bars should be provided in the bottom layer. This reinforcement will help to distribute the load more evenly, reduce the risk of cracking or warping, and increase the strength of the slab.
If two different dia. bars are used, provide larger dia. bars at the bottom layer.
When two different diameter bars are used in a slab, it is best practice to provide the larger diameter bars in the bottom layer. This will ensure the slab is adequately reinforced and will help to distribute the load more evenly, reducing the risk of cracking or warping.
When openings are made in a slab, the area of steel that is interrupted should be replaced with an equivalent amount. This will ensure the slab is adequately reinforced and will help to distribute the load more evenly, reducing the risk of cracking or warping. Of the replacement steel, half should be placed along each edge of the opening.
When designing a one-way slab, there are several practical considerations to keep in mind. The spacing of the main bars should be between 150-200 mm, and the spacing of the distribution bars should be between 150-300 mm. The maximum spacing of bars should be 3???? or 300 mm, whichever is less. The clear spacing between bars should not be less than 25 mm.
Practical considerations of one-way slab
• Spacing shall be between 150-200 mm (main bars)
• Spacing shall be between 150-300 mm (distribution bars)
• Max. Spacing of bars 3???? or 300 mm, whichever is less.
• Clear spacing between bars shall not be less than 25 mm