Protecting the structure from any kind of external forces is essential for any type of construction. Numerous external factors can damage the structure; among those, the damage caused by winds and earthquakes can completely collapse the structure. Shear walls are provided to protect the structure from any kind of lateral pressure. Here is everything you need to know about the shear wall.
What is a Shear Wall?
A shear wall is a structural member in an RCC (Reinforced Cement Concrete) structure that is designed to resist external lateral forces such as wind, hurricanes, earthquakes, and more. They are primarily used in high-rise buildings, which are subject to lateral wind and seismic forces. It provides stiffness to the structure by reducing the damage caused by lateral forces.
Importance of Shear Wall
- Shear walls provide the necessary lateral strength to withstand lateral forces such as wind and seismic forces.
- They reduce the damage caused by earthquakes and external wind.
- They provide strength and stiffness to the building against sway.
- They prevent damage to structural and non-structural members by transferring the load to the foundation.
Types of Shear Walls
Reinforced Concrete Shear Wall
These are the most widely used shear walls, which are provided in both horizontal and vertical directions. They are constructed using concrete that is embedded with steel reinforcements. They provide both compressive and tensile strength to the structure, making it robust and able to withstand lateral forces. They are commonly used in high-rise residential and commercial buildings, especially in high seismic areas. They are highly durable, have excellent load-bearing capacity, and are highly resistant against lateral forces.
Masonry Shear Wall / Concrete Block Shear Wall
Masonry Shear Walls or Concrete Block Shear Walls are constructed using concrete bricks or blocks along with steel reinforcement bars to maximise the effect of seismic loads. Reinforcements are placed in the hollow spaces and concrete is poured inside the block to embed the RCC bars and the concrete together. This type of wall can withstand both gravity loads and lateral loads. They are vital for low-rise residential buildings, small commercial structures, and boundary walls in low-seismic areas.
Steel Plate Shear Wall
It is constructed using thin steel sheets that are added to the building’s structural frame. They are highly flexible and are resistant to bending. They are well-suited for buildings where tensile strength and lateral stability are crucial. They are commonly used for multi-story commercial and industrial buildings. They can also be fitted to existing structures to improve seismic resistance.
Plywood Shear Wall
Plywood shear walls are traditional types of walls that are built to resist lateral forces such as wind and earthquakes. They consist of plywood sheets and studs, where the plywood transfers the shear force, while the studs resist the tension and compression. They are ideal for lightweight and low-rise residential buildings.
Mid-Ply Shear Wall
It is an improved version of the plywood shear wall, where an extra plywood sheet is placed in the center of the normal plywood wall and the studs are positioned on both sides. They are used for multi-story buildings.
Materials used in Shear Wall
Concrete
Reinforced Cement Concrete (RCC) is used for the construction of shear walls to provide high lateral strength and stability. These concrete walls are widely used for buildings that are medium to high-rise, from 4 to 35 floors.
Steel
Steel plates are used for the construction of steel plate shear walls. They are most commonly used in high-rise buildings in high-seismic areas, to protect the structure from earthquakes. They provide flexibility and tensile strength to the building and can be applied to existing structures.
Plywood
Plywood or timber is used for constructing the plywood shear walls and mid-ply shear walls that are designed to resist wind loads. The use of plywood along with the studs is vital for low-rise residential buildings in non-seismic areas.
Mechanism of Shear Wall
When lateral forces like heavy wind, hurricanes, or earthquakes put pressure on a building, the shear wall absorbs the force from the weaker parts of the building such as exterior walls, floors, and roofs, and transfers the load to the foundation. Although they have absorbing and transferring characteristics similar to load-bearing walls, shear walls resist the horizontal forces and protect the building from collapsing sideways.
Location of Shear Walls in Building
The location of the shear walls varies based on the architectural and structural requirements. Numerous factors decide the ideal location of the shear walls in the building. Locating the shear wall in the middle of the building helps decrease the shear force symmetrically in all parts of the building. However, it is not always possible or economical to place a shear wall in the middle. Numerous factors decide the location of shear walls and here are some of the ideal locations where shear walls are generally placed.
Each Half of the Building
Interior shear walls can be placed at the center of each half of the building. These shear walls come in the form of core walls (box-shaped columns) and help protect the building from seismic forces.
Ends of the Building
The exterior shear walls are usually located at the ends of the building. Shear walls are placed at each end of the building, creating symmetry and holding the building in place, when subjected to seismic forces.
Near Staircase and Elevator shafts
Generally, places around staircases and elevator shafts don’t need an opening. This makes it one of the ideal places to locate the shear walls for better performance.
Construction & Design Considerations for Shear Walls
Design Considerations
The shear walls should possess maximum strength and stability to effectively support the building from lateral forces. Here are some of the factors that play a crucial role in designing a well-balanced shear wall as per IS 456:1978.
- The thickness of the wall should not be less than 100 mm. The wall thickness can be anywhere between 150 mm to 400 mm.
- If the shear stress exceeds 0.25 x √fck or if the thickness of the wall exceeds 200 mm, reinforcements should be provided in two layers of the wall.
- The diameter of the reinforced bar should not exceed 1/10 of the thickness of the wall.
- The minimum reinforcement should be 0.0025 times the gross area of the wall.
- The larger the width, the greater will be the resistance against the lateral loads.
- The maximum spacing of the reinforcement bar should be less than 450 mm or 1/5th of the wall length or three times the wall thickness (whichever is less).
- Shear walls in the external structure should be relatively thicker, as they are subject to high flexural and shear strength.
- Lateral ties should be provided around lapped splice bars that are larger than 16 mm in diameter.
- The diameter of the tie bar shouldn’t be less than 5 mm or 1/4 of the spliced bar and the spacing of the tie should be less than 150 mm in the center.
Construction Process
- After the horizontal reinforcement is placed according to the design, the vertical reinforcement steels of Fe415 are placed with sizes from 10 mm or 12 mm.
- After the vertical reinforcements are placed, wooden steel or aluminum formworks are placed around the reinforcements and locked with the bolts.
- The concrete mixture should be made of M30-grade cement, with the size of the aggregate varying from 10 mm to 12 mm and using robust sand.
- The concrete mortar, after mixing, is filled into the formwork that holds the reinforced steel rods and compressed using vibrators to remove air gaps.
- After filling, allow the walls to rest for a complete day to harden them with periodical curing.
- After one complete day of curing, the formwork should be removed without any damage.
Conclusion
Shear walls are highly resistant structural elements that protect the building from external forces. They protect both the load-bearing and non-load-bearing parts of the building from wind and other seismic loads. By carefully designing, constructing, and locating the shear walls, one can reduce the damages caused by lateral forces.