Confined Masonry Construction: An Overview

Confined masonry construction has been accomplished through an unceremonious process based on its optimal performance in past earthquakes. Confined masonry construction was commenced in Mexico in the 1940s to limit the wall cracking caused by massive differential settlements under the soft soil conditions. Several years later, this system became popular in other regions of Mexico due to its worthy earthquake performance. Confined masonry construction consists of masonry walls along with horizontal and vertical RC confining members, which are construct on all sides of a masonry wall panel. Vertical members are usually called tie-columns & resemble columns in RC frame construction except that they tend to be of far smaller in cross-section. Horizontal elements are called tie-beams, resemble beams in RC frame construction. (BRZEV, December, 2007)

The confining members are usually operative in Improving the stability of masonry walls for in-plane and out-of-plane earthquake forces. Besides that confining members also help to improve the strength of masonry walls under lateral earthquake loads and help to reduce the brittleness of masonry walls under earthquake loads and hence improving their earthquake performance. (BRZEV, December, 2007)

Now it’s time to discuss the components of a confined masonry building.

•  Masonry walls transmit the gravity load from the slabs to the foundation & the walls resist horizontal earthquake forces. The walls must be confined by concrete tie-beams and tie-columns to ensure satisfactory performance. 

•         Confining elements provide restraint to masonry walls and protect them from complete disintegration even in some major earthquakes. These elements resist gravity loads and have an important role in ensuring the vertical stability of a building during an earthquake.

•         Floor and roof slabs emit both gravity and lateral loads to the walls. During an earthquake, slabs behave like horizontal beams.

•         The Plinth band transmits the load from the walls down to the foundation. It also revives the walls at the GF from the excessive settlement in tender soil conditions.

•        Foundation transmits the loads from the superstructure to the ground.

The conformation of a complete Confined masonry construction and an RC frame construction might be looked similar to the people at first but these two construction systems are substantially different. The main differences are related to the construction sequence, as well as their behaviour under seismic conditions. In CM construction masonry walls are the main load-bearing components & expected to resist both gravity and lateral loads. On the other hand, RC frames not only resist gravity loads but also resist lateral loads through their comparatively broad beams, columns, and their connections. Masonry infills are not load-bearing walls. Hence CM construction has Strip footing beneath the wall and the RC plinth band and RC frame construction have Isolated footing beneath each column. In CM Construction at first masonry walls are constructed subsequently, tie-columns are cast in place. At last, tie-beams are constructed on top of the walls along with the roof slab construction.  And in RC frame construction the frame is constructed first and then masonry walls are constructed at a later stage and are not bonded to the frame members; these are non-structural, that is, non-load-bearing walls. (BRZEV, December, 2007)

A confined masonry building is subjected can be modelled as a vertical truss. Masonry walls act as oblique struts subjected to compression, while RC confining members act in tension and compression (depending on the lateral forces). This particular model is appropriate before the cracking in the walls takes place.  Subsequently, the cracking is concentrated at the GF level and significant lateral deformations take place.  Under intense earthquakes, the collapse of confined masonry buildings may occur due to a soft story effect similar to the one observed in RC frames with masonry infills. This behavior was ensured by experimental studies (Alcocer, 2004). It was reported after the (Tecomán, 2003), Colima, Mexico earthquake, that a three-story confined masonry apartment building in Colima experienced significant damage at the ground floor level (ERRI, 2006). A feasible way to eliminate the fragile behavior related to the soft story effect is to provide horizontal reinforcement in masonry walls to improve their shear resistance (Schultz, 1994).

Figure 1: Confined masonry building: vertical truss model (left) and collapse at the ground floor level (right) (BRZEV, December, 2007)

According to some research studies it’s known that concentrated on lateral load inhibition of confined masonry walls (Klemenc, 1997); (Tomazevic, 1999); (Yoshimura, 2004) recognised the following failure strait characteristic of confined masonry walls: 

• Shear failure strait;
• Flexural failure strait.

 Shear failure strait is characterised by distributed oblique cracking in the wall. These cracks propagate into the tie-columns at superior load levels. Initially, a masonry wall resists the effects of lateral earthquake loads by itself on the other hand the tie-columns don’t play a significant induction. However, once the cracking pass-off, the wall pushes the tie-columns sideways. Then, vertical reinforcement in tie-columns becomes assigned in obstructing tension and compression stresses (Klemenc, 1997). Depreciation in the tie-columns at the ultimate load level is concentrated at the top and the quadrant of the panel. These locations, characterised by comprehensive crushing of concrete and yielding of steel reinforcement are called plastic hinges. Shear failure can lead to severe depreciation in the masonry wall and the top and quadrant of the tie-columns. 

              

                                      Figure 2: Shear Failure in Confined Masonry Structure

Flexural failure strait is characterised by horizontal cracking in the mortar bed joints on the tension side of the wall (Yoshimura, 2004). Severance of tie-columns from the wall was regarded in some cases. Comprehensive horizontal cracking usually takes place in tie-columns. (Meli, 1994)  

                              Figure 3: Flexural Failure Confined Masonry Structure.

There are some key factors which are Influencing Seismic Resistance of Confined Masonry Structures as-

• Wall Density;
• Masonry Units & mortar;
• Tie columns;
• Horizontal wall reinforcement;
• Openings.

Wall Density is one of the significant parameters which influence the seismic performance of confined masonry building. Wall density can be calculated by the transverse area of walls in each major direction divided by the total floor area of the certain building. In Mexico, a procedure was originated to calculate the required wall density for each direction for buildings in which the wall seismic resistance is governed by shear effects (Meli, 1994). After the 1985 earthquake, according to the code a 40% increase in the design for seismic resistance is required, this resulted in a significant increase in wall density requirements in confined masonry construction. As an example, it can be recalled that for a five-story building in Mexico City, at this moment it’s required to provide wall density of around 6% in each direction, whereas in the areas of highest seismic risk, this value is near to 10%.

                            Figure 4: Wall density (d) vs the number of stories of CMB in Mexico.

Lateral load resistance of CM walls strongly pivots on the strength of the masonry units and the mortar used. The walls built using low strength bricks or un-grouted hollow units had the lowest strength while on the other side the ones built using solid units or grouted hollow units had more strength. Also, the dull the mortar the lesser the masonry strength. From the Test results of previous research on this topic have also shown that there is no significant difference in strength between unreinforced and CM wall specimens if both consist of the same geometry and material properties (Alcocer and Klingner, 1994).

The contribution of Tie-columns is significant in the post-cracking period. Tie-columns significantly have an influence on the ductility and stability of cracked confined masonry walls. The effect of tie-columns in increasing lateral resistance of CM structures has recently been recognized by the researchers (Alcocer, 2006). The provision of closely spaced transverse reinforcement (ties) at both top and bottom ends of tie-columns results in enhanced wall stability and ductility in the post-cracking period (Alcocer and Klingner, 1994).

In many countries where CM construction is practiced, reinforcement is usually not provided in masonry walls. However, in four-to-five story construction in some country like Peru tends to provide Horizontal joint reinforcement in the form of one or two wires laid in the mortar bed joints (Casabonne, 1994), Beside that the Horizontal rebars should be anchored into the tie-columns & the anchorage should be provided with 90° hooks at the far end of the tie-column. The bar diameter should be larger than 3.5 mm & less than ¾ the joint thickness.

                                                  Figure 5: Horizontal reinforcement in CMB.

From an experimental research study it’s known that, when the opening area is less than approximately 10% of the total wall area, the wall lateral load resistance is not significantly reduced as compared to a solid wall (Yanez et al., 2004). The walls with larger openings produce diagonal cracks, except that the cracks are formed in the piers between the openings. The study also recommends estimating the lateral strength of walls with window openings based on the net transverse wall area. Most building codes suggest the maximum permitted opening size beyond which the tie-columns need to be provided.

                                     Figure 6: Failure modes in the CM walls with openings.

There are some guidelines about Confined masonry structures that should be strictly maintained while building a certain structure.

• Architectural guidelines;
• Construction guidelines.

☑ Architectural guidelines: 

• The Building plan should be regular or symmetrical.

                                                          Figure 7: Regular building plan

•  The building’s length should not be excessively long relative to its width. Hence, the length to width ratio should not exceed 4.
•   Walls of CM structure should be built in a symmetrical manner.

                                                   Figure 8: Symmetrical wall layout.

•   Walls of the CM structure should be continued up the building height.

                                         Figure 9: Continuous walls up the building height.

• Openings (doors and windows) of the CM structures should be placed in the same position up to the building height.   

                                           Figure 10: Position of openings in a building.

•  Tie-beams have to be placed at every floor level at a vertical spacing not to exceed 3m & the Tie-columns should be placed at a maximum spacing of 4 m on each floor level.

                            Figure 11: A sample floor plan illustrating the placement of tie-columns.

•  At least two confined masonry walls have to be provided in each principal direction.

                                              Figure 12: Wall distribution in two directions.

•   Wall density of at least 2 % is required to ensure good performance during an earthquake in CM construction.
•    Confined masonry is suitable for low to medium rise building depending on the seismic zone.

☑  Construction guidelines:

• Reinforcement for the first story tie-columns should be assembled before the foundation construction takes place.

• The tie-column reinforcement should consist of four 10 mm diameter deformed bars for longitudinal reinforcement, & 6 mm ties at 200 mm spacing (6mm @200 mm). Hence, vertical bars should be lapped by a minimum of 500 mm.

•  The minimum cross-sectional dimensions of the tie-columns should be 100 mm by 100 mm.

•   It’s essential to construct a plinth band on top of the foundation.

•   The minimum wall thickness should be 100 mm & the CM structure’s wall height/thickness ratio should not exceed 30.

•   Toothed edges should be left on each side of the wall in CM structure & besides that horizontal dowels should be also provided at the wall-to-column interface.

• It’s mandatory to Pour concrete in the tie-columns upon the completion of each 1.2 m of the wall height. Apart from that Bricks should be moistened before the concrete is poured. And it’s very important to notice that the concrete needs to be vibrated thoroughly & formwork support must be provided.

• Tie-beams are constructed on top of the walls at each floor level in the CM structure. Usually in CM structure, the tie-beam reinforcement consists of four 10 mm diameter deformed bars for longitudinal reinforcement, and 6 mm stirrups at 200 mm spacing (6mm @200 mm).  And the tie-beam reinforcement needs to be continuous along with the longitudinal reinforcement bars lapped by at least 500 mm.

•  The minimum cross-sectional dimensions of tie-beams are 100 mm by 100 mm.

•  Special lintel beams may be provided across the larger openings to ensure safety.

Confined masonry buildings have inured well in several earthquakes globally. This type of construction has great potential for protecting lives and belongings. However, like any other construction practice, good earthquake performance is based on the subsequent premises such as the use of good quality materials, and simple architectural design. (BRZEV, December, 2007)

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