Confined Masonry Construction System.

A constructioin system where a plain masonry walls are confined on all four sides by RC members or reinforced masonry is called confined masonry. The major imrovements in the performance of the confined masonry building over the plain masonry building are as follows:

Enhances greatly the connection between structural walls Improves the stability of masonry walls Improves the strength of masonry walls Provides ductility under earthquake loading Improves the integrity and containment of earthquake damaged masonry walls

The RC confining elements are horizontal members called bond-beams and vertical members called tie-columns (Figure 3). In the case of confined masonry floors should be reinforced concrete cast in-situ. Good floor to wall connection is achieved by horizontal bond beams cast just below slabs.

Figure 1- Solid units masonry confined with tie-columns and bond beams

RC confining elements are mostly used for solid masonry units construction. When masonry units from group 2a or 2b are used the horizontal and vertical confining elements are formed by reinforced concrete block masonry units which are subsequently grouted. For appropriate details for these refer the Confined block masonry section of this website. The confining elements are not intended nor designed to perform as a moment-resisting frames. When such frames are constructed to resist lateral and vertical loads the purpose of the masonry walls is only for space partitioning, and the construction system is called masonry-infilled frames. In the masonry-infilled frames type of housing the reinforced concrete frame structure is constructed first. The masonry is constructed later between the RC members. In the case of confined masonry, the masonry walls are load-bearing and are constructed to carry all of the gravity loads as well as lateral loads. Therefore the load-bearing masonry walls are constructed first. Then the vertical and horizontal confining elements are cast simultaneously with the floors, which are constructed as RC slab.

In order to achieve effective confinement of walls, vertical confining elements( tie-columns) should be located at all corners and changes of wall contour, and at all joints, wall intersections and free ends of structural walls. Vertical confining members are also necessary at both sides of any opening which according to EC 8 has an area more than 1.5 m2. According to [TOMASEVIC REFERENCE] this requirement can be relaxed for openings area up to 2.5 m2. The distance between tie-columns should not exceed 4.0 m. Figure 4, below shows typical distribution of vertical confining elements in the plan of the building.

figure 2. Typical distribution of vertical confining elements in the plan of a building

Normally the tie-columns fit into the thickness of masonry wall and the minimum tie-column cross section is 150x150 mm. The concrete for the confining members should be min grade C15. According to EC 6, the contribution of the tie-columns and bond-beams to the lateral resistance of the masonry house should not be taken into account. Consequently specific design calculations for confining elements are not required. The amount of reinforcement in vertical and horizontal confining elements is determined on empirical basis.

The min steel tie-columns reinforcement for construction in seismic zones is specified in EC 8. According to this code the min reinforcement area for tie-columns is 240 mm². For tie-columns at the house corners and wall intersections, it is recommended that, at least 4 f10 mild steel bars are used for reinforcement. In this case the total steel area is 314 mm². Mild steel stirrups f10 are placed uniformly distributed at 200 mm offsets.

Although the tie-columns and bond beams do not provide frame system adequate splicing and anchoring of rebars is required at all joints. Sixty rebar diameters splices are required according to EC 8.