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Perhaps the earthquake is the most fearful natural phenomenon in one’s life. It is more so, because it is unpredictable and arises without notice or without announcing its vigour and strength. Earthquake is the shaking of the ground caused by the sudden breaking and movement of large sections (tectonic plates) of the earth's rocky outermost crust. The edges of the tectonic plates are marked by faults (or fractures). Most earthquakes occur along the fault lines when the plates slide past each other or collide against each other.
Earthquakes may cause devastating destruction to the infrastructure, but they may not kill the people directly. What kills the people during earthquake is the collapse of building structures in which they are living!
Our traditional construction practices of un-reinforced load bearing brick masonry and poorly detailed RCC framed building structures are the root causes for the death toll of thousands and lakhs of people during earthquakes in our country.
Though it is not feasible and economical to construct earthquake-proof buildings, it is possible to design and construct cost effective and structurally efficient earthquake-resistant buildings of shear wall structural system.
Framed construction :- for multistoried and industrial buildings
Box type construction :- for residential buildings
In FRAMED CONSTRUCTION people get the opinion of the experts of the various fields, Architects for the plan, Soil investigators for soil investigation, Structural Consultants for structural design and Civil Engineers for the proper implementation of the project.
But in BOX TYPE CONSTRUCTION generally people get the plan from the consultant and engage a contractor for the execution of the work. On the other hand if one gets the technical opinion of the expert, right from planning, selection of materials, structural design and Project Management, definitely the construction cost will be less without affecting stability and durability of the structure.
As per clause 8.4.1 of IS 4326 1993 all masonry buildings shall be strengthened by the methods, as specified for various categories of buildings as listed in the table below.
Strengthening arrangements recommended for masonry buildings (Rectangular masonry units.)
Note: In case of four storey building of category B, the requirements of vertical steel may be checked through a seismic analysis using a design seismic coefficient equal to four times the one given in (a) 3.4.2.3 of IS 1893 : 1984. (This is because the brittle behavior of masonry in the absence of a vertical steel results in much higher effective seismic force than that envisaged in the seismic coefficient, provided in the code). If this analysis shows that vertical steel is not required the designer may take the decision accordingly..
From the table above it can be noticed that the strengthening arrangements depends on the category of the buildings. The buildings have been categorized in 5 categories A to E based on the value of ah given by:
From the above, the measures to be adopted also depends on 'I' the important factor of the building.
In our opinion each and every house is important for the owner irrespective of the size. So one should take all the precautions as envisaged in the IS 4326; 1993 during the construction.
The design and construction of masonry walls using rectangular masonry units in general shall be governed by IS 1905:1987 and IS 2212:1991.
Well burnt bricks, solid concrete bricks and hollow concrete bricks having a strength not less than 35Mpa shall be used. (see IS 1905: 1987 and IS 1597(part 2): 1992.
Cement mortar of ratio not less than 1:6 shall be used as specified in IS 4326 : 1993.
Masonry bearing walls built in mortar, unless rationally designed as reinforced masonry, shall not be built of greater height than 15m subject to a maximum of four storey’s when measured from the mean ground level to the roof slab or ridge level.
The masonry bearing walls shall be reinforced in accordance with 8.41 of IS 4326: 1993,to make it a sheer wall. The overall strengthening arrangements to be adopted consists of horizontal bands at plinth, bottom sill of windows, below the roof or floors, and vertical reinforcement bars at corners, junctions of walls and jams of doors windows and openings.
The design and construction of masonry walls using rectangular masonry units in general shall be governed by IS 1905:1987 and IS 2212:1991.
Well burnt bricks, solid concrete bricks and hollow concrete bricks having a strength not less than 35Mpa shall be used. (see IS 1905: 1987 and IS 1597(part 2): 1992.
Cement mortar of ratio not less than 1:6 shall be used as specified in IS 4326 : 1993.
Masonry bearing walls built in mortar, unless rationally designed as reinforced masonry, shall not be built of greater height than 15m subject to a maximum of four storey’s when measured from the mean ground level to the roof slab or ridge level.
The masonry bearing walls shall be reinforced in accordance with 8.41 of IS 4326: 1993,to make it a sheer wall. The overall strengthening arrangements to be adopted consists of horizontal bands at plinth, bottom sill of windows, below the roof or floors, and vertical reinforcement bars at corners, junctions of walls and jams of doors windows and openings.
Steel reinforcing bars shall be embedded with adequate cover in cement sand mortar not leaner than 1:3 with minimum clear cover of 10mm.The bearing walls in both direction shall be straight and symmetrical in plan as far as possible.
Door and window openings in walls reduce their lateral load resistance and hence, should preferably be small and more centrally located. The guidelines on the size and position of opening are given in table of IS 4326:1993.
Providing vertical reinforcement as in Fig (a) & (b) shown above is difficult and labor oriented with conventional bricks (clay).
by C.V.R.Murty of IIT Kanpur and Sponsored by BMTPC (Building Materials and Technology Promotion Council) New Delhi.
Role of Horizontal Bands
Horizontal bands are the most important earthquake-resistant feature in masonry buildings. The bands are provided to hold a masonry building as a single unit by tying all the walls together, and are similar to a closed belt provided around cardboard boxes. There are four types of bands in a typical masonry building, namely gable band, roof band, lintel band and plinth band (Figure 1), named after their location in the building. The lintel band is the most important of all, and needs to be provided in almost all buildings. The gable band is employed only in buildings with pitched or sloped roofs. In buildings with flat reinforced concrete or reinforced brick roofs, the roof band is not required, because the roof slab also plays the role of a band. However, in buildings with flat timber or CGI sheet roof, roof band needs to be provided. In buildings with pitched or sloped roof, the roof band is very important. Plinth bands are primarily used when there is concern about uneven settlement of foundation soil. The lintel band ties the walls together and creates a support for walls loaded along weak direction from walls loaded in strong direction. This band also reduces the unsupported height of the walls and thereby improves their stability in the weak direction. During the 1993 Latur earthquake (Central India), the intensity of shaking in Killari village was IX on MSK scale. Most masonry houses sustained partial or complete collapse (Figure 2a). On the other hand, there was one masonry building in the village, which had a lintel band and it sustained the shaking very well with hardly any damage (Figure 2b).
Design of Lintel Bands
During earthquake shaking, the lintel band undergoes bending and pulling actions (Figure 3). To resist these actions, the construction of lintel band requires special attention. Bands can be made of wood (including bamboo splits) or of reinforced concrete (RC) (Figure 4); the RC bands are the best. The straight lengths of the band must be properly connected at the wall corners. This will allow the band to support walls loaded in their weak direction by walls loaded in their strong direction. Small lengths of wood spacers (in wooden bands) or steel links (in RC bands) are used to make the straight lengths of wood runners or steel bars act together. In wooden bands, proper nailing of straight lengths with spacers is important. Likewise, in RC bands, adequate anchoring of steel links with steel bars is necessary.
Indian Standards
The Indian Standards IS:4326-1993 and IS:13828 (1993) provide sizes and details of the bands. When wooden bands are used, the cross-section of runners is to be at least 75mm×38mm and of spacers at least 50mm×30mm. When RC bands are used, the minimum thickness is 75mm, and at least two bars of 8mm diameter are required, tied across with steel links of at least 6mm diameter at a spacing of 150 mm centers.
Related IITK - BMTPC Earthquake Tip
Tip 5: What are the seismic effects on structures?
Tip12: How brick masonry houses behave during earthquakes?
Tip13: Why masonry buildings should have simple structural configuration?
Resource Material
Next Upcoming Tip
Why is vertical reinforcement required in masonry buildings?
Authored by: C.V.R.Murty
Indian Institute of Technology KanpurKanpur, India
Sponsored by: Building Materials and
Technology Promotion
Council, New Delhi, India
his release is a property of IIT Kanpur and BMTPC New Delhi. It may be reproduced without changing its contents and with due acknowledgement. Suggestions/comments may be sent to: eqtips@iitk.ac.in. Visit www.nicee.org or www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.
by C.V.R.Murty of IIT Kanpur and Sponsored by BMTPC (Building Materials and Technology Promotion Council) New Delhi.
Response of Masonry Walls
Horizontal bands are provided in masonry buildings to improve their earthquake performance. These bands include plinth band, lintel band and roof band. Even if horizontal bands are provided, masonry
buildings are weakened by the openings in their walls (Figure 1). During earthquake shaking, the masonry walls get grouped into three sub-units, namely spandrel masonry, wall pier masonry and sill masonry.
Consider a hipped roof building with two window openings and one door opening in a wall (Figure 2a). It has lintel and plinth bands. Since the roof is a hipped one, a roof band is also provided. When the ground shakes, the inertia force causes the small-sized masonry wall piers to disconnect from the masonry above and below. These masonry sub-units rock back and forth, developing contact only at the opposite diagonals (Figure 2b). The rocking of a masonry pier can crush the masonry at the corners. Rocking is possible when masonry piers are slender, and when weight of the structure above is small. Otherwise, the piers are more likely to develop diagonal (X-type) shear cracking (Figure 2c); this is the most common failure type in masonry buildings.
In un-reinforced masonry buildings (Figure 3), the cross-section area of the masonry wall reduces at the opening. During strong earthquake shaking, the building may slide just under the roof, below the lintel band or at the sill level. Sometimes, the building may also slide at the plinth level. The exact location of sliding depends on numerous factors including building weight, the earthquake-induced inertia force, the area of openings, and type of doorframes used.
How Vertical Reinforcement Helps
Embedding vertical reinforcement bars in the edges of the wall piers and anchoring them in the foundation at the bottom and in the roof band at the top (Figure 4), forces the slender masonry piers to undergo bending instead of rocking. In wider wall piers, the vertical bars enhance their capability to resist horizontal earthquake forces and delay the X-cracking. Adequate cross-sectional area of these vertical bars prevents the bar from yielding in tension. Further, the vertical bars also help protect the wall from sliding as well as from collapsing in the weak direction.
Protection of Openings in Walls
Sliding failure mentioned above is rare, even in unconfined masonry buildings. However, the most common damage, observed after an earthquake, is diagonal X-cracking of wall piers, and also inclined cracks at the corners of door and window openings. When a wall with an opening deforms during earthquake shaking, the shape of the opening distorts and becomes more like a rhombus - two opposite corners move away and the other two come closer. Under this type of deformation, the corners that come closer develop cracks (Figure 5a). The cracks are bigger when the opening sizes are larger. Steel bars provided in the wall masonry all around the openings restrict these cracks at the corners (Figure 5b). In summary, lintel and sill bands above and below openings, and vertical reinforcement adjacent to vertical edges, provide protection against this type of damage.
Related IITK - BMTPC Earthquake Tip
Tip 5: What are the seismic effects on structures?
Tip12: How brick masonry houses behave during earthquakes?
Tip13: Why masonry buildings should have simple structural configuration?
Tip14: Why horizontal bands are required in masonry buildings?
Resource Material
Authored by:C.V.R.Murty
Indian Institute of Technology KanpurKanpur, India
Sponsored by: Building Materials and
Technology Promotion
Council, New Delhi, India
This release is a property of IIT Kanpur and BMTPC New Delhi. It may be reproduced without changing its contents and with due acknowledgement.
Suggestions/comments may be sent to: eqtips@iitk.ac.in Visit www.nicee.org or www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.
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