The Constructor

Earthquake-Resistant Confined Masonry – Components, Difference, and Advantages

Confined Masonry

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Confined masonry construction is a combination of two masonry, which consists of masonry walls (clay brick or concrete block units) and horizontal and vertical RC confining members built on all four sides of a masonry wall panel. Vertical members such as tie-columns or practical columns act as columns in RC frame construction except that they are of smaller cross-section. Horizontal elements such as tie-beams resemble beams in RC frame construction.
Fig 1: Confined masonry construction.
In this article, we discuss the structural components, differences, advantages, and application of confined masonry.

Structural Component of Confined Masonry

1. Masonry Walls

Masonry walls transfer the loads of the upper slabs to the foundation successfully. The walls in the confined masonry act as the bracing panels, which resist horizontal earthquake forces. The walls of this type of masonry are confined by concrete tie-beams and tie-columns to ensure satisfactory earthquake performance.
Fig 2: Components of Confined Masonry.

2. Confining Elements

The confining elements provide restraint to masonry walls and protect them from complete disintegration, even in the event of a major earthquake. These elements resist gravity loads and have an essential role in ensuring the vertical stability of a building in an earthquake.

3. Slabs (Floor and Roof)

The slab of the confined masonry helps in transmitting both gravity and lateral loads to the walls. In an earthquake, the piece behaves like horizontal beams and is called diaphragms.

4. Plinth Beam or Band

The plinth band transfers the load from the walls down to the foundation by protecting the ground floor walls from excessive settlement in soft soil conditions.

5. Foundation

The foundation acts typically as in conventional masonry by transferring the load from the structure to the ground.

Difference between Confined Masonry Construction and RCC Frame Construction

Table 1: Difference between confined masonry and RC frame construction.

  Confined Masonry Construction RC Frame Construction
Gravity and lateral load-resisting system Masonry walls are the main load-bearing elements and are expected to resist both gravity and lateral loads. Confining elements (tie-beams and tie-columns) are significantly smaller in size than RC beams and columns. RC frames resist both gravity and lateral loads through their relatively large beams, columns, and their connections. Masonry infills are not load-bearing walls.
Foundation construction Strip footing beneath the wall and the RC plinth band Isolated footing beneath each column
Superstructure Construction sequence 1. Masonry walls are constructed first. 2. Subsequently, tie-columns are cast in place. 3. Finally, tie-beams are constructed on top of the walls, simultaneously with the floor/roof slab construction. 1. The frame is constructed first. 2. Masonry walls are constructed at a later stage and are not bonded to the frame members; these walls are nonstructural, that is, non-load bearing walls.

Factors Influencing the Seismic Resistance of Confined Masonry

The factors or components of the confined masonry which influence the seismic resistance are detailed below:

1. Wall Density

General studies have shown that the building having more wall density suffers less damage than the ones having less wall density. The wall density is calculated as the transverse area of walls in each principal direction divided by the total floor area of the building. The above reason is why each country codes provide the minimum wall density requirement for different storied buildings, based on their traditional way of wall construction. A survey done during the Llolleo earthquake in 1985 showed that the damage that occurred to the confined masonry with 1.15% wall density was very less compared to the masonry building with 0.5% wall density.

2. Masonry Units and Mortar

The tests have shown that the lateral load resistance of confined masonry walls strongly depends on the strength of the masonry units and the mortar used. The walls built using low-strength bricks or ungrouted hollow block units had the lowest strength, while the ones made using grouted or solid units had the most significant advantage.

3. Tie-Columns

The provision of closely spaced transverse reinforcement (ties) at the top and bottom ends of tie-columns results in improved wall stability and ductility in the post-cracking stage. Tie-columns significantly influence the ductility and stability of cracked confined masonry walls.

4. Horizontal Wall Reinforcement

The provision of horizontal wall reinforcement in the building with more than four stories has a beneficial effect on wall ductility. Walls with horizontal reinforcement showed a more uniform distribution of inclined shear cracks than the unreinforced specimens. The horizontal reinforcement is provided in the form of one or two wires laid in the mortar bed joints, as shown in the figure below-
Fig 3: Horizontal Wall Reinforcement in Confined Masonry.

Advantages of Confined Masonry

  1. It enhances the stability and integrity of masonry walls for in-plane and out-of-plane earthquake loads.
  2. It enhances the strength (resistance) of masonry walls under lateral earthquake loads.
  3. It reduces the brittleness of masonry walls under earthquake loads and hence improving their earthquake performance.

Applications of Confined Masonry

  1. The practice of confined masonry construction started in Chile in the 1930s after the 1928 Talca earthquake (Magnitude 8.0) that affected a significant number of unreinforced masonry buildings.
  2. Subsequently, the 1939 earthquake (Magnitude 7.8) that struck the mid-southern region of the country revealed the excellent performance of confined masonry buildings.
  3. The confined masonry construction was introduced in Mexico City, Mexico, in the 1940s to control the wall cracking caused by large differential settlements under the soft soil conditions, which later became famous for its excellent earthquake performance.
  4. The use of confined masonry in Colombia dates from the 1930s, and it is currently widely used for housing construction, from single-story dwellings to five-story apartment buildings.
Read More: 1. What are the Four Virtues of Earthquake Resistant Buildings? 2. 10 Causes of Earthquake Damages in Unreinforced Masonry Buildings  
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