Earthquake-resistant construction is crucial for ensuring the stability and safety of buildings in regions prone to seismic activity. Among various structural components, masonry walls play a pivotal role in resisting lateral forces generated by earthquakes. However, traditional masonry walls may not be enough to withstand the intense shaking during an earthquake. Special types of masonry walls have been designed and adapted to improve their seismic performance. This article will explore several types of earthquake-resistant masonry walls, focusing on their design principles, behavior under seismic loads, and considerations for construction.
I. Cantilever Masonry Wall
A cantilever masonry wall is a single, isolated wall that can be built on either a flexible or rigid foundation. These walls are often used in low-rise buildings or as external walls of larger structures. The key feature of a cantilever wall is that it is not connected to other walls through rigid beams, allowing it to act independently to resist seismic forces.
Design Considerations:
- Foundation and Floor Slabs: Cantilever masonry walls can be constructed on flexible foundations to allow for more movement during seismic activity. It is important to use flexible floor slabs to connect different cantilever walls, rather than rigid coupling beams, as this helps to reduce the amount of seismic moment transferred from one wall to another. This also minimizes the risk of wall failure under high seismic loads.
- Minimizing Openings: Openings, such as windows or doors, should be kept to a minimum in cantilever walls. Openings weaken the wall’s ability to resist vertical seismic loads, which could affect the wall’s overall performance during an earthquake.
- Columns for Vertical Load Support: Columns may be introduced to help the wall bear vertical loads, ensuring that the cantilever wall can adequately support the building’s structure during seismic shaking.
- Plastic Hinges for Energy Dissipation: To enhance energy absorption, plastic hinges are carefully designed at the base of the wall. These hinges absorb and dissipate the energy generated by the earthquake, reducing the forces transferred to the rest of the structure.
- Displacement Control: The displacement of a cantilever wall during an earthquake is controlled by the plastic rotation capacity of the plastic hinges at the base. This ensures that the wall remains intact and stable despite lateral forces.
In summary, cantilever masonry walls can effectively resist seismic forces if designed with flexibility, minimal openings, and energy-absorbing mechanisms such as plastic hinges.
II. Coupled Masonry Wall with Pier Hanging
A coupled masonry wall consists of two or more walls connected by a coupling beam. These walls are usually perforated with openings such as doors or windows, making them vulnerable to seismic damage. When seismic forces act on a coupled masonry wall, hinges can form in either the piers (vertical sections of the wall) or the spandrels (horizontal sections between openings).
Behavior Under Seismic Loads:
- Hinges in Piers: In most cases, seismic forces cause hinges to form in the piers. The piers bear most of the seismic load, and they need to have sufficient ductility (ability to deform without breaking) to absorb these forces. This is particularly important for the piers on the lower stories of buildings, as they experience the most stress during an earthquake.
- Deformation and Ductility: The piers must be designed to resist both flexural and shear deformations. In lower stories, the demand for ductility is very high, and the piers need to be carefully designed to handle significant deformation without failure.
Coupled masonry walls with piers that have high ductility can perform well in seismic conditions, but their design must account for significant deformation, especially in the lower levels of the building.
III. Coupled Masonry Wall with Spandrel Hinging
In some cases, the spandrels (the horizontal sections between openings) may be weaker than the piers. When seismic forces act on the wall, hinges may form in the spandrels instead of the piers. This type of behavior is less common but can still occur in masonry walls where the spandrels have lower compressive strength compared to the piers.
Behavior Under Seismic Loads:
- Hinges in Spandrels: If the spandrels are weaker than the piers, seismic forces will cause cracks and hinge formation in the spandrels. The resulting damage can compromise the structural integrity of the wall, and the wall may not perform as well during an earthquake.
- Design Challenges: The performance of masonry walls with spandrel hinging is generally not as favorable as those with piers. Masonry coupled walls with spandrel hinges face significant challenges due to the low compression strain capacity of the masonry. This means that the wall’s ability to withstand seismic forces is limited.
- Improving Seismic Resistance: To improve the seismic performance of coupled masonry walls with spandrel hinges, engineers recommend several measures:
- Designing for lower displacement ductility to limit the movement of the wall.
- Introducing joints between spandrels and the wall to prevent spandrel damage caused by excessive rotation of the wall.
- Avoiding the use of coupled masonry walls with spandrel hinges in highly seismic regions if possible.
Due to the limitations of masonry in terms of ductility and compression strain, this type of wall is generally not recommended for earthquake-prone areas.
IV. Selection of Primary and Secondary Lateral Force Resisting Systems
In certain masonry wall configurations, a rational analysis of seismic forces may be difficult due to the complexity of the shape or the number of bearing walls. In such cases, it is more practical to treat the wall as consisting of a primary system and a secondary system.
Primary System:
- The primary system carries both the gravity loads (the building’s weight) and the lateral seismic forces. It is designed to resist the main forces during an earthquake.
Secondary System:
- The secondary system supports gravity loads and face loads (such as inertial forces), but it does not resist lateral seismic forces directly. While the secondary system is not designed for lateral loads, it will still experience some seismic forces.
Design Considerations:
- Stiffness of Secondary Walls: To prevent the secondary system from becoming overstressed, its stiffness should not exceed one fourth of the primary system’s stiffness. This ensures that the primary system takes on most of the seismic load, preventing plastic deformation in the secondary system.
- Center of Rigidity: To minimize torsional effects during an earthquake, the center of rigidity for both the primary and secondary systems should be as close as possible. Torsional motion can destabilize the structure, so this consideration is crucial for effective seismic resistance.
By properly differentiating between primary and secondary systems, engineers can improve the overall performance of the building during an earthquake.
V. Face Loaded Masonry Walls
Masonry walls must also be capable of withstanding face loads—forces that act out of the plane of the wall. Face loads occur due to seismic earth pressure on retaining walls or the inertial response of walls to seismic excitation.
Design Considerations:
- Dual Load Resistance: In earthquake-prone areas, masonry walls must be designed not only to resist in-plane seismic forces (horizontal shaking) but also to endure out-of-plane forces (vertical shaking). This requires a more robust design to ensure that the wall can handle both types of loads simultaneously.
- Structural Requirements: The wall needs to resist out-of-plane bending moments caused by seismic forces. Engineers must consider the overall strength of the masonry to ensure it can withstand the combined stresses from both in-plane and out-of-plane forces.
VI. Conclusion
The design of earthquake-resistant masonry walls is a critical component of building safety in regions prone to seismic activity. The various types of masonry walls—such as cantilever, coupled walls with pier or spandrel hinging, and face-loaded walls—each have their own advantages and challenges. Proper design and construction techniques, including the use of flexible slabs, minimizing openings, and incorporating plastic hinges, can significantly improve the seismic performance of masonry walls.
When selecting the appropriate type of wall, engineers must consider factors like ductility, load-bearing capacity, and the potential for seismic damage. Additionally, understanding the interaction between primary and secondary lateral force-resisting systems, as well as the ability to resist face loads, is essential to ensuring the safety and stability of the structure during an earthquake.