Reinforced concrete wall is designed as a compression member. Reinforced concrete wall is used in case where beam is not provided and load from the slab is heavy or when the masonry wall thickness is restricted.
Reinforced concrete wall is classified as:
- Plain concrete wall, when reinforcement < 0.4%
- Reinforced concrete wall, when reinforcement > 0.4%
Load from slab is transferred as axial load to wall. When depth is large, it is called RC wall. Design is similar to a RC column, breadth equal to thickness of wall and depth equal to 1m. RCC Wall is designed as:
- Axially loaded wall
- Axially loaded with uniaxial bending
Classification of Concrete Walls
- Plain concrete wall
- Reinforced concrete wall
In plain concrete wall, the reinforcement provided is less than 0.4% of c/s. In reinforced concrete wall, the percentage of steel provided is greater than 0.4% and is designed similar to reinforced concrete columns.
Slenderness ratio is equal to least of (l/t or h/t), where, l is effective length of wall, h is effective height of wall, t is thickness of wall If < 12, the wall is short and if > 12, the wall is slender.
Braced and Unbraced Concrete Walls
Braced: When cross walls are provided for the walls such that they can take lateral load and 2.5% of vertical load, then the wall is braced. Otherwise, the wall is known as unbraced wall.
Note: Other walls under special cases are,
i) Cantilever wall
ii) Shear walls – To take lateral loads [Take care of flexure developed due to lateral loading on the structure, depth is provided along the transverse direction]
Guidelines for Design of Reinforced Concrete Wall
1. The limiting slenderness () if any for unbraced wall is 30 and for braced wall is 45.
2. For short braced RC wall (< 12),
Pu = 0.4 x fck x Ac+ 0.67 x fy x Ast
3. For short unbraced RC wall, along with the above axial load Pu, the moment due to minimum eccentricity is checked for emin= t/20 or 20mm, where, M = P x e.
For the above axial load and moment, the RC wall is designed similar to RC column subjected to axial load and uniaxial moment.
4. Slender braced wall (< 45):
The additional moment due to additional eccentricity as per Table 1 of SP16 is considered. Where the additional eccentricity,
The additional moment due to eccentricity is added with the moment on the column and moment on the wall. The wall is designed for axial load with uniaxial moment.
5. For slender unbraced wall [limited to 30]: Similar procedure as in case 4 is adopted.
6. Detailing of Reinforcement [IS 456 Guidelines]:
- For plain concrete wall, minimum vertical steel is 0.12% for HYSD bars and 0.15% for mild steel bar.
- For RC wall, minimum vertical reinforcement is 0.4% of c/s
- In plain concrete wall, transverse steel is not required
- In RC wall, transverse steel is not required (not less than 0.4%)
- Maximum spacing of bars is 450mm or 3t, whichever lesser
- The thickness of wall in no case should be less than 100mm
- If thickness is greater than 200mm, double grid reinforcement is provided along both the faces.
7. Detailing of Reinforcement (BS 8110 guidelines):
- Horizontal reinforcement same as IS456
- Vertical reinforcement not to be greater than 4%
- When compression steel is greater than 2% of vertical reinforcement, horizontal reinforcement of 0.25% for HYSD bars or 0.3% of MS bars are provided. [As per IS456, it is 0.2% for HYSD bars and 0.3% for mild steel bars].
- The diameter of transverse bars (horizontal) should not be less than 6mm or .
- Links are provided when the compression steel is greater than 2%. Horizontal links are provided for thickness less than 220mm. Diagonal links are provided when thickness is greater than 220mm. The spacing of links should be less than 2t and diameter of links not less than 6mm or .
The support conditions for effective length of wall:
- Both ends fixed (Restrained against rotation and displacement)
- Both ends hinged
- One end fixed and other end
- One end fixed and other end hinged