SEISMIC RESISTANT
RETROFITTING FOR BUILDINGS
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. Seismic retrofitting is defined by Arya (2005) as:
“...actions for upgrading the seismic resistance of an existing building so that it becomes safer under the occurrence of probable future earthquakes”.
Figure 1: Girl in a place of risk from the damaged building, Peru. Photo: Soluciones Prácticas.
This brief will look at some methods of retrofitting traditional and ‘non-engineered’ housing, with suggestions of how to decide which method is appropriate.
Damage types in unreinforced masonry
Separation of adjacent walls Out of plane bending crack In-plane shear cracks Diagonal cracks at openings
Ground movement
Figure 2: Damage typologies 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.
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Seismic resistant retrofitting for buildings
Practical Action
The diagram above describes where cracks may first appear. Walls will experience different modes of failure depending on their orientation to the eart hquake 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 ar e 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. Random Rubble Masonry The predominant type of building in Bhuj, India is random rubble stone masonry. During the 2001 earthquake many walls made of random rubble failed, mostly due to separation of withes and lack of interlock (Madabushi, 2005). The stones used are usually undressed (or uncut) granite and units can vary in the amount of weathering they have been subjected to. This means a variation in surface characteristics and hence a variation in the bonding with mortar. The mortar itself could also be weak in some cases . 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.
(a) Stitching element: stitches withes together. Length is 50mm shorter than wall width.
(b) Seismic belt shear connector: anchors seismic belt to wall. Anchor is 150mm in length.
(c) Vertical reinforcement shear connector: anchors vertical reinforcement. Anchor is 400mm in length.
Figure 3: Variety of bond elements based on UNDP India (2007).
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.
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Seismic resistant retrofitting for buildings
Practical Action
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. 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.
Stone or brick unreinforced masonry
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. See later case study for more information.
Figure 4: Illustration sketch of welded mesh configuration based on Arya (2005).
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.)
The surface should be prepared so that the mesh has good connection with the wall; this involves removing any plaster layers and cleaning to expose the masonry underneath. The mesh should be continuous and any splices should have at least a 300mm overlap. 3
Seismic resistant retrofitting for buildings
Practical Action
The welded mesh should be attached to the longitudinal bars with binding wire. The strip should then be attached with 100-150mm (4-6”) nails with the nails driven into the mortar joints. Spacers should be provided to allow at least 15mm (1/2”) thickness between the wall and the mesh, to ensure the mesh is completely covered by the cement plaster. The ratio of cement: sand should be 1:3. For a random rubble wall, L-shaped shear connectors should be cast as depicted in Figure 3(b) made of reinforced concrete. These should be installed every 1.25-1.5m (4-5ft) and once the concrete has set, the protruding dowel bar can be attached to the welded mesh strip with binding wire. 100mm (4”) square headed nails should be used to attach the welded mesh to the wall in the fashion described previously. Step-by-step instructions including illustrations can be found on pages 64 -67 of Manual for Restoration and Retrofitting of Rural Structures in Kashmir. Recommendations for vertical mesh reinforcement are covered in pages 68-72. Table 1 - Reinforcement recommendations for horizontal seismic belts (National Disaster Management Division, Govt. of India, 2006) Length of Wall (m) <5 6 7 8 Cat. D Gauge g10 g10 g10 g10 N 8 10 10 10 H 230 280 280 280 Cat. E Gauge g10 g10 g10 g10 N 10 10 10 10 H 280 280 280 280
With 2 bars of 6 mm Ø With 2 bars of 6 mm Ø With 2 bars of 8mm Ø With 2 bars of 8 mm Ø With 3 bars of 8 mm Ø
Table 2 - Reinforcement recommendations for vertical reinforcement (National Disaster Management Division, Govt. of India, 2006) No. of storeys Cat. D Single Bar, mm One Two One Top Bottom 10 10 12 Mesh (g10) N 10 10 14 B 300 300 400 12 12 16 Cat. E Single Bar, mm Mesh (g10) N 14 14 14 B 400 400 400
Three
Top Middle
10 12
10 14
300 400
12 16
With 1 bars of 12mm Ø 14 400 14 400 With 1 bars of 12mm Ø 14 400 With 1 bars of 12mm Ø
Bottom
12
14
400
16
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Seismic resistant retrofitting for buildings
Practical Action
Notes 1. N = Number of made longitudinal wires in the belt at spacing of 25 mm. 2. H = Height of belt on wall in micro-concrete, mm. 3. The transverse wires in the mesh could be spaced up to 150 mm. 4. The mesh should be galvanized to save from corrosion.
Gauge g10
Diameter 3.25 mm
Categorisation of Buildings Seismic Zone* Ordinary Building s E D C Import ant Buildin gs E E D
g11 g12 g13 g14
2.95 mm 2.64 mm 2.34 mm 2.03 mm
V IV III
*Where seismic zone is refers to the Modified Mercalli Intensity scale.
Roof stiffening Having a stiff roof that is capable of diaphragm action is important, as this allows loads to be distributed more evenly to the walls to which they are connected. A roof that acts as a diaphragm is able to transfer lateral loads to walls that are able to take in -plane shear. Many flat roofs that are made with parallel timber joists and covered by either earth or planks cannot act as a rigid diaphragm. The roof (or floor) needs to be diagonally braced. Arya (2003) suggests that planks similar to that already used, or galvanised metal strips (1.5mm x 50mm) should be used to create an X-brace. The timber joists should also be nailed at both ends to joists below.
Brick or stone wall
Wood joist
Planks underneath joists Diagonal tie (X - brace) Wood planks
Figure 5: How to stiffen a flat roof/floor based National Disaster Management Division, Govt. of India (2006). In the case of sloping roofs that are carrying clay tiles of galvanised iron, the roof tends to push out during an earthquake (Arya, 2003). 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 (UNDP India, 2007). See page 81-85 of Manual for Restoration and Retrofitting of Rural Structures in Kashmi r.
Timber construction
Provided good connections are made and good materials are used, timber buildings usually perform well during earthquakes. Members are lighter, attracting fewer seismic forces, and nailed 5
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