Difference between One way slab and Two way slab

Difference between One way slab and Two way slab

One-way slabs and two-way slabs are both used in construction according to requirements and load-carrying capacity. Here are some important Difference between One way slab and Two way slab.

S.NOne-Way SlabTwo-Way Slab
1.The bending moment occurs in only one direction onlyThe bending moment occurs in only one direction only
2.Its Longer span to shorter span > 2Its Longer span to shorter span < 2
3.Main Reinforcement is provided in one direction onlyReinforcement provided perpendicular to the main reinforcement
4.Distribution reinforcement provided perpendicular to the main reinforcementDistribution reinforcement is not provided
5.It is less expensive compared to two-way slabsDeflection Shape occurs in Dish-shaped
6.Support remains of beams on two opposite sides onlySupport remains of beams on all four sides
7.Economic Span Limit remains up to approximately 3.6 metersDeflection Shape occurs in a parabolic shape
8.It is designed generally for Low-rise buildingsIt is designed generally high-rise buildings
9.It is Less expensive compared to two-way slabsIt is generally more expensive than one-way slabs
10.It is more susceptible to deflectionIt is lesser susceptibility to deflection
11.Load-Carrying Capacity is less as compared to two-way slabsThe economic Span Limit remains for Panel sizes up to 6m x 6m

One way slab reinforcement Details

A one-way slab is a type of concrete slab that is supported on two sides and bends primarily in one direction.

  1. The ratio of the long span to the short span of the slab is greater than 2 i.e.>2.
  2. The shorter span of a one-way slab is provided with main rebars while the longer span is provided with distribution rebars.
  3. The main reinforcement is provided to oppose the tensile forces developing in the slab due to the bending moment and distribution reinforcement in the slab is provided to distribute the load evenly across the slab and prevent it from cracking.
  4. The main rebars are usually made of a high-yield steel bar (HYSD bar) with a diameter of 8 mm, 10 mm, 12 mm, or 16 mm.
  5. Distribution rebars are usually made of mild steel bars with a diameter of 6 mm or 8 mm.
  6. The main reinforcement is placed to the maximum spacing between center to center of rebars is 3 times the effective depth of the slab or 300 mm, whichever is less.
  7. The distribution reinforcement is placed at a maximum spacing of 200 mm.

Factors affecting designing one-way slab reinforcement:

  • Maximum Loads on the Slab
  • Dimensions of the slab.
  • Strength of concrete and steel.
  • Type of slab (Residential, Commercial, Industrial).
  • Climate (hot, cold, humid).
  • Seismic zone

The main reinforcement is bent at the ends to provide anchorage and make a strong bond between concrete and rebar. The length of the anchorage should not be less than or equal to 40 times the diameter of the rebar.

Two way slab reinforcement Details

Difference between One way slab and Two way slab

a) Main reinforcement:

Main reinforcement in the slab is provided to oppose the tensile forces developed due to the bending moment.

It is usually made of a high-yield steel bar (HYSD bar) with a diameter of 8 mm, 10 mm, 12 mm, or 16 mm. The main reinforcement is placed to a maximum spacing of 3 times the effective depth of the slab or 300 mm, whichever is less.

b) Distribution Reinforcement:

Distribution reinforcement in the slab is provided to distribute the load evenly across the slab and prevent cracking. It is usually made of mild steel bars with a diameter of 6 mm, 8 mm, or 10 mm. The distribution reinforcement is placed at a maximum spacing of 200 mm.

c) Anchorage:

The main rebars and distribution rebars are bent at the ends to provide anchorage to make a strong bond between rebar and concrete. The length of the anchorage is usually equal to 40 times the diameter of the bar.

d) Torsional reinforcement:

Torsional reinforcement is required at the corners of the slab when the slab is restrained against uplift.

Torsional reinforcement is usually made of mild steel bars with a diameter of 6 mm or 8 mm or sometimes 10 mm according to design. The torsional reinforcement is placed at a maximum spacing of 200 mm.

Factors affecting designing  a two-way slab reinforcement:

  1. Loads to which the slab will be subjected.
  2. Dimensions of the slab.
  3. Strength of concrete and steel.
  4. Type of slab (Residential, Commercial, Industrial).
  5. Climate (hot, cold, humid).
  6. Seismic zone.

Two way slab reinforcement 3d

Difference between One way slab and Two way slab

Procedure for one way slab design and two way slab design

Here are the steps to be followed to design one way as well as a two-way slab.

Step1:- Calculate the total load acting on the slab

First of all calculate the loads acting on the slab including dead load, live load, and wind load. Dead load is the weight of the slab itself, beam, joist, and any other permanent load connected with the slab.

Live load is the weight of people, furniture, and other movable objects on the slab. Wind load is the force of the wind on the slab laterally or horizontally.

Step 2:- Set the dimension of the slab.

The span of the slab is the distance between the two supports. The span of the slab affects the amount of reinforcement required.

Step 3:- Determine the effective depth of the slab.

The effective depth of a slab is the distance from the top of the slab to the centroid of the reinforcement. The effective depth of the slab affects the amount of reinforcement required.

Step 4:- Determine the amount of reinforcement required.

The amount of reinforcement required depends on the total load acting on the slab, the span of the slab, and the effective depth of the slab.

Step 5:- Determine the size and spacing of the reinforcement.

The size and spacing of reinforcement will depend on the amount of reinforcement required and the load per unit area on the slab.

Step 6:-  Grade of concrete and reinforcement.

The grade of concrete and reinforcement should be decided according to the load-bearing requirement of the slab. The type of reinforcement depends on material availability and cost.

Step7:- Prepare details of the reinforcement

Finally, The reinforcement should be detailed on the drawing paper. The reinforcement drawing should be able to show the location, size, shape, and spacing of the reinforcement in the slab.

As a Structural engineer, I am providing you with some more bonus tips to get a safe, durable, and strong slab design.

  • Hire a qualified structural engineer to design the slab.
  • Use high-quality materials and workmanship.
  • Follow the manufacturer’s instructions for reinforcement.
  • Inspect reinforcement before covering the slab with concrete.
  • Provide proper clear cover to the slab while casting.

Finally, the main Difference between One way slab and Two way slab is the ratio of longer span to shorter span. If the ratio is greater than 2, the the slab is one way slab but if the ratio is less than 2 then the slab is two way slab.

Read Also,

Bar bending schedule (BBS) of two way slab Practically

Rebar Calculator for Slab

Concrete slab calculator | Concrete slab cost calculator

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"Structural Engineer" with over 5 years of experience in estimation, structural design, and surveying. I am passionate about using his skills to create safe and sustainable structures. I am also a keen writer, and I enjoy sharing my knowledge and experiences with others.