Types of Earthquake Resistant Masonry Walls Construction
There are various types of masonry wall which may be suitable to construct in areas prone to earthquakes. Types of these seismic resistant masonry wall are discussed.
Types of Earthquake Resistant Masonry Walls Construction
- Cantilever masonry wall
- Coupled masonry wall with pier hanging
- Coupled masonry wall with spandrel hinging
- Selection of primary and secondary lateral force resisting system
- Face loaded masonry walls
Cantilever Masonry Wall
Cantilever masonry wall is an isolated wall constructed on either flexible or rigid foundation. Flexible floor slabs are commonly used to connect different cantilever masonry walls.
It can be used to resist seismic loads if necessary measures are considered. For example, it is recommended to use flexible floor slabs to joint different cantilever masonry walls rather than employing rigid coupling beams. This leads to decrease in amount of moment transferred from one wall to another.
Another consideration is keeping opening in the cantilever masonry wall as minimum as possible to prevent the influence of openings on the vertical cantilever action of the wall.
Added to that, columns may be introduced to help the wall in supporting applied vertical loads.
As far as energy absorption is concerned, plastic hinges, which are carefully designed at the base of the wall, is provided to dissipate earthquake energy.
Finally, cantilever masonry wall displacement capacity is controlled by plastic rotation capacity of the plastic hinges.
Fig.1: Cantilever Masonry Wall Linked by Flexible Floor Slabs
Coupled Masonry Wall with Pier Hanging
Generally, masonry construction composed of peripheral masonry walls penetrated by openings, for instance, windows and doors as shown in Figure-2.
Fig.2: Masonry Wall with Door and Window Openings
Fig.3: Pier and Spandrel of Masonry Wall
When lateral seismic force acts on the coupled masonry wall as can be observed in Figure-4, then hinges would form either in piers as shown Figure-5 in or in spandrel as in Figure-6.
Fig.4: Lateral Seismic Force Acts on Coupled Masonry Wall
Fig.5: Seismic Load Acts on Coupled Masonry Wall Laterally and Creates Cracks in Piers (Hinges Formed in Piers)
Fig.6: Seismic Load Acts on Coupled Masonry Wall Laterally and Generates Cracks in Spandrels (Hinges Formed in Spandrels)
By and large, the formation of hinges in piers occurs in most of cases and it is more common compared with the case where the hinge created in spandrels. In the former case, the piers must show considerable ductility if not designed to withstand elastic deformations caused by design earthquakes.
Regarding plastic displacements caused by flexure and shears, it is mostly accumulated in the piers of the lowest story of the building and eventually the ductility demand in this story would be astronomically high.
Coupled Masonry Wall with Spandrel Hinging
There are limited cases in which pier of masonry wall is stronger than spandrels and consequently hinges are formed in spandrels rather piers when seismic loads acts on the wall laterally as shown in Figure-7.
Fig.7: Spandrels are relatively weaker than piers and as a result cracks are formed in spandrels and hinging would be in spandrels as well
Moreover, the behavior of the masonry wall will be somewhat like coupled wall. This type of wall may show satisfactory performance during earthquake and substantial ductility demand generated in coupling beams specifically at upper level can be tackled if the coupled wall is designed properly and constructed from reinforced concrete.
However, masonry coupled wall does not possess such advantage since its ultimate compression strain is low. That is why masonry coupled wall is not proper option in seismic prone areas.
Furthermore, there are options which are provided to improve seismic resistance of coupled masonry wall with spandrel hinges. For example, it is recommended to design the wall for a decreased overall displacement ductility or introduced joints between spandrels and wall to avoid spandrel damages caused by the rotation of the wall.
Finally, it is clear from the above discussion that the best option is to prevent the application of coupled masonry wall with spandrel hinges completely in seismic regions.
Selection of Primary and Secondary Lateral Force Resisting System
There are certain types of masonry walls for which rational analysis under lateral seismic forces cannot be applied because of the orientation, shape complexity and number of bearing walls.
Therefore, it is more realistic to assume that the masonry wall is composed of primary system and secondary system. The former supports gravity loads and the whole lateral seismic forces whereas the latter carries gravity loads and face loads.
Fig.8: Dividing of masonry walls to primary and secondary systems, A is assumed to poorly divided and B is well divided since primary and secondary wall are eccentric in A.
It is considered in the analysis that secondary system does not support lateral seismic forces, but this assumption is not accurate because it carries unspecified amount of lateral forces.
That is why such walls should be designed to withstand displacements which might occur due to lateral seismic forces. This can be obtained by accepting minimum code requirements.
Additionally, the stiffness of any secondary wall should not be greater than one fourth of the largest stiffness of primary wall. This measure is specified to prevent considerable plastic deformation of secondary wall and keep its role which is supporting gravity load.
Finally, it necessary to make the center of rigidity of both primary and secondary wall as close as possible to decline the torsional effects which is not desirable.
Face Loaded Masonry Walls
Masonry walls should be able to withstand out of plane bending moment or face loads. This is because masonry walls are likely to be subjected to face loads. Examples of face loads are seismic earth pressure on masonry retaining wall and inertial response of walls to transverse seismic excitation.
Therefore, not only does the masonry wall need to mainly resist in plane seismic loads but also withstand the out of plane and in plane seismic effect at the same time.