Confined masonry construction is a robust building technique that combines masonry walls with reinforced concrete (RC) members to improve structural performance. This system uses masonry walls made from clay bricks or concrete blocks and surrounds them with horizontal and vertical RC confining members (tie-columns and tie-beams). Unlike traditional RC frame construction, the confining elements in confined masonry are smaller and specifically designed to enhance the integrity of the masonry walls. This article explores the structural components, differences, advantages, and applications of confined masonry construction.
Structural Components of Confined Masonry
1. Masonry Walls
Masonry walls play a crucial role in confined masonry construction, serving as the primary load-bearing elements. They successfully transfer vertical loads from upper slabs to the foundation. Additionally, these walls act as bracing panels that resist horizontal earthquake forces. To ensure their performance during seismic events, masonry walls are enclosed by RC tie-beams and tie-columns, providing enhanced stability and resilience.
2. Confining Elements
RC confining elements are integral to confined masonry. These tie-columns and tie-beams restrain masonry walls, preventing disintegration even during major earthquakes. They provide vertical stability and help resist gravity loads, ensuring the structural integrity of the building.
3. Slabs (Floor and Roof)
The slabs in confined masonry structures serve dual purposes: they transmit both gravity and lateral loads to the walls and act as diaphragms during earthquakes. These slabs, functioning like horizontal beams, distribute forces effectively, contributing to overall stability.
4. Plinth Beam or Band
A plinth beam or band ensures effective load transfer from walls to the foundation. It also safeguards ground-floor walls against excessive settlement in soft soil conditions, maintaining the structure’s integrity.
5. Foundation
The foundation in confined masonry structures functions like that of conventional masonry, transferring the load from the superstructure to the ground. It forms the base upon which the rest of the structure is built.
Comparison: Confined Masonry vs. RCC Frame Construction
Load-Resisting System
- Confined Masonry: Masonry walls act as the primary load-bearing components, resisting both gravity and lateral loads. The RC confining members are smaller compared to those in RCC frame systems.
- RCC Frames: Large RC beams, columns, and connections resist gravity and lateral loads. Masonry infills are non-structural and do not bear loads.
Foundation Construction
- Confined Masonry: Utilizes strip footings beneath walls, combined with RC plinth bands for load distribution.
- RCC Frames: Features isolated footings beneath individual columns.
Construction Sequence
- Confined Masonry: Walls are constructed first, followed by tie-columns, tie-beams, and finally the floor or roof slabs.
- RCC Frames: The frame is built first, and non-structural walls are added later, disconnected from the structural members.
Factors Influencing Seismic Resistance
Several factors impact the seismic performance of confined masonry structures:
1. Wall Density
Studies show that higher wall density leads to better earthquake performance. Wall density is calculated as the ratio of wall area in a given direction to the total floor area. Structures with higher wall density suffer less damage during seismic events. For example, during the 1985 Llolleo earthquake, confined masonry buildings with 1.15% wall density experienced significantly less damage than those with 0.5% density.
2. Masonry Units and Mortar
The strength of masonry units and mortar directly influences the lateral load resistance of walls. Solid units or grouted hollow blocks provide better performance than low-strength or ungrouted blocks, making material choice critical for seismic resistance.
3. Tie-Columns
Tie-columns with closely spaced transverse reinforcements at their ends improve wall stability and ductility during the post-cracking stage. These components are vital for enhancing the overall durability of confined masonry.
4. Horizontal Wall Reinforcement
Horizontal reinforcement is particularly beneficial in taller buildings with more than four stories. It improves wall ductility and results in a more uniform distribution of shear cracks, enhancing the structure’s ability to withstand seismic forces.
Advantages of Confined Masonry
Confined masonry offers several benefits:
- Enhanced Stability: Improves the integrity of masonry walls against in-plane and out-of-plane earthquake loads.
- Increased Strength: Strengthens masonry walls under lateral earthquake forces.
- Reduced Brittleness: Decreases the susceptibility of walls to brittle failure during seismic events, ensuring better earthquake performance.
Applications of Confined Masonry
The practice of confined masonry began in Chile during the 1930s after the devastating Talca earthquake of 1928 (magnitude 8.0). This event highlighted the vulnerability of unreinforced masonry buildings, leading to the development of confined masonry techniques. The 1939 earthquake (magnitude 7.8) further demonstrated the resilience of confined masonry buildings, cementing its popularity in Chile.
In the 1940s, confined masonry was introduced in Mexico City to address wall cracking caused by large differential settlements in soft soil conditions. The method proved highly effective, showcasing excellent earthquake performance. Colombia also adopted confined masonry in the 1930s for housing construction, and today it is widely used in buildings ranging from single-story dwellings to five-story apartment complexes.
Conclusion
Confined masonry construction has proven to be a reliable and resilient building method, particularly in seismic regions. Its combination of masonry walls with RC confining elements ensures structural integrity and stability during earthquakes. With advantages such as improved wall strength and reduced brittleness, confined masonry continues to be a preferred choice for housing and medium-rise buildings worldwide. Its applications in countries like Chile, Mexico, and Colombia highlight its effectiveness in addressing both structural and seismic challenges.