Introduction

Earthquakes can be the single most devastating natural event, with many lives claimed due to the failure of residential buildings. Whilst there are many building codes and guidelines for building back better to create new, seismic resistant buildings, this option may not be affordable to all whose houses remain standing, but are still at risk of experiencing an earthquake.

Damage types in unreinforced masonry

Unreinforced masonry, whether it is made of stone, adobe bricks, or fired bricks, is a widely used method of building in many developing countries. The methods of retrofitting will focus on these types of buildings as they are most commonly the homes of people who would require affordable retrofit solutions. However, slightly more intrusive, and therefore potentially expensive methods will also be included to give an idea of the possibilities available.

Walls will experience different modes of failure depending on their orientation to the earthquake movement. Parallel to the ground movement, walls will experience shear and cracks will form in a diagonal fashion. The cracks form an X-shape because shear will be experienced in both directions to follow the ground movement. Diagonal cracks also form from the corners of openings since there stresses are highly concentrated here. Vertical cracks are formed at the middle of walls perpendicular to the ground movement, as this is the location of high bending stresses, as are ends where adjacent walls are attached. Cracking here can lead to separation of the walls at corners. Cracks can propagate and result in sections of the wall falling away and partially collapsing. In some instances, corners, sections of wall or entire walls can fall out of plumb. Prolonged shaking can also lead to delamination, in which a layer of masonry may fall away from the wall, or bulging, where the wall face separates and creates an area of thick wall. Depending on the earthquake intensity and duration, extensive damage can lead to total collapse. It is imperative that inhabitants are able to escape before collapse happens.

Reinforcing masonry

Through Stones

The use of ‘through stones’ ensure withes are interlocked with each other, preventing them from falling away (delamination) or separating in sections (bulging). The placement of ‘through stones’ can be achieved on an existing wall by using reinforced concrete elements. This involves gently removing stones to create a 75mm (3 inch) hole and inserting concrete reinforced with a hooked bar the length of the wall thickness. The concrete is then cured for a minimum of 10 days. The type of element can be varied depending on the type of bar used, depicted in the figures below.

Note that the length of the bar should be 50mm shorter than the width of the wall, giving at least 25mm of concrete cover either side. The reinforcement must be completely covered to protect it from rust. The concrete should also be mixed with a polymer additive to prevent shrinkage.

A step-by-step manual of how to install these elements can be found in Chapter 6 of ‘’Manual for Restoration and Retrofitting of Rural Structures in Kashmir’’, pages 59-61 [1]. Rubble masonry can also be strengthened with seismic belts, which be discussed in the following section. Seismic belts refer to a retrofit that involves attaching a continuous reinforced cement strip around the perimeter of the building and similarly this method can be used to create vertical reinforcement.

Seismic belts

Masonry walls tend to fail due to in-plane tensile forces and particularly where adjacent walls meet. When masonry fails in shear it manifests in diagonal cracking, often propagating from corners of openings where stresses are concentrated. Horizontal belts can provide continuity between adjacent walls by placing them around the perimeter on both sides of the wall at plinth level, while vertical belts can be applied to corners, wall junctions and to strengthen damaged piers of openings. This provides a restraint for walls that experience bending as they are perpendicular to seismic movement and provide tensile strength for walls parallel to seismic movement. The belts are made of welded wire mesh and covered with a cement plaster. Mild steel bars are used to anchor the belt into the wall. A similar method has also been used on adobe masonry in Peru using small diameter mesh which has been shown to survive earthquakes.

A continuous seismic belt should be placed:

  • Below eave level
  • Just above lintels of doors and windows if there is a significant gap between lintel and eaves (>900mm)
  • Below floor level
  • Below top edge of gable walls

If reinforced concrete has been used in the floor or roof, or are constructed such that they can act as a diaphragm, then a seismic band will not be needed at these levels. Instead, look if connections between walls and the floor or roof should be improved. (See page 82 of ‘’Manual for Restoration and Retrofitting of Rural Structures in Kashmir.’’ Cite error: Closing </ref> missing for <ref> tag. Rafters should be tied to the seismic belt and rafters opposite each other should be tied with cross ties at half the height of the roof, or with collar beam ties at 2/3 of the roof height [1].

References and further reading

  1. 1.0 1.1 UNDP India (2007) Manual for restoration and Retrofitting of Rural Structures in Kashmir: How to Reduce Vulnerability of Existing Structures in Earthquake Affected Areas of Jammu and Kashmir http://unesdoc.unesco.org/images/0015/001593/159333E.pdf
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