Design Guide of Earthquake Resistant Masonry building

Nowadays there is much talk about the earthquake-resistant design of buildings and we should know the fundamental of the subject. For the disaster-resistant design of buildings (against earthquakes or cyclones), we divide the structures into three categories as follows:

 

1. Fully resistant structures which will behave elastically under the worst loads, for example, hospitals, electrical stations, etc. which should survive the event.

 

2. Partially resistant structures which may undergo damages that can be subsequently repaired, for example, other important buildings.

 

3. Survival structures may get very much damaged during the disaster but there should be features which will exclude any loss of life, for example, temporary structures and low-cost buildings.

 

Again, depending on the design and construction of buildings in India, we divide buildings into another three categories as follows:

1. Engineered buildings: These buildings are fully designed by engineers and architects like tall multi-storey buildings which have to be designed by engineers.

 

2. non-engineered buildings: These buildings are fully conceived and constructed by artisans such as carpenters and masons. (A great part of residential houses built in India are built on the basis of empirical knowledge.)

 

3. Semi-engineered buildings: These buildings are not designed fully, but unlike the non-engineered buildings, the codal provisions of IS 4326 (COP of Earthquake-Resistant Design of Buildings) and IS 13828 (Improving Earthquake-Resistant Design of Low Strength Masonry Buildings) are incorporated in their construction. (The necessary features of these buildings are fully explained in a pamphlet "Guidelines for Improving Earthquake Resistance of Housing" published by Building Materials and Technology Promotion Council, New Delhi.) IS 13828, 1993, "Indian Standards for Improving Earthquake Resistance of Low Strength Masonry Buildings", also explains the features to be incorporated in such buildings.

 

DESIGN AND CONSTRUCTION OF EARTHQUAKE-RESISTANT BUILDINGS

Multi-storey buildings have to be built as engineered buildings (framed structures). We should strictly adhere to codes in their planning (provision of shear walls and other features with frame construction), in their analysis and design (by providing ductile detailing) as well as in their construction. However, most of the one- or three-storey buildings can be built as semi-engineered buildings to provide seismic resistance.

As unreinforced or unconfined masonry will crack under large earthquake forces, earthquake resistance can be provided by confining masonry with RC bands at the plinth, lintel and roof level according to the codes already given in the article Design of Brick Masonry Walls. The provision of lintel bands for the various zones is given in Table 8.1 in Chapter 8. The amount of reinforcement will depend on the probable intensity of the earthquake or the zone in which the building is built.

 

NEED FOR DUCTILE DETAILING

We must have a clear idea about the need of ductile detailing of structures for earthquake resistance. We must remember that it is extremely difficult to predict the magnitude of the earthquake forces. In the case of cyclonic wind forces, we can estimate the full force of the wind and we assume these full forces as static forces for analysis. However, in the estimation of the seismic forces, what the codes give is an estimate of the probable forces, and the actual earthquake forces are dynamic and can be much larger. The objectives of ductile detailing are as follows:

(a) The structure should resist the moderate intensity of earthquakes without any damage.

 

(b) The structure should resist the exceptionally large intensities of earthquakes which can occur without collapse.

 

We have to incorporate the ductile behaviour of the structure so that it can take a large amount of overloading beyond the design load and does not collapse, thus ensuring no loss of lives during a large earthquake. This is carried by the following rules laid down for ductile detailing by the code IS 13920.

 

RECOMMENDED METHODS OF DESIGN

From the above discussion, we can arrive at the following conclusions:

1. In regions of low earthquake intensity zone 2, normal design according to the general codes IS 456-2000 for RCC and IS 1905 for masonry will give enough built-in safety.

 

2. If we have to get an absolutely safe building in other zones, we should adopt a framed fully engineered building according to the codes IS 1893 (1984) and IS 13920 (1993).

 

3. Otherwise we can have semi-engineered masonry buildings having not more than 4 storeys in zones 3 and 4 and not more than 3 storeys in zone 5. In such buildings, we must use the prescribed guidelines published in the references given in above which are given in below

 

BRIEF REVIEW OF RECOMMENDATIONS FOR SEMI-ENGINEERED BUILDINGS

The main recommendations for keeping a building intact are as follows:

1. Provision of good foundations

2. Provision of plinth beams to tie together the foundation

3. Provision of lintel band

4. Provision of roof/floor band

5. Vertical bar at corners

6. Reinforcing around door and window openings

The details of the above recommendations can be obtained from the IS codes and references given in above

Key Components: Confined Masonry Building

Conclusion

Tall and other buildings which should be fully resistant to earthquake forces should be designed and built as fully engineered buildings. However, the ordinary masonry low-rise buildings, which are the majority of houses built in India, can be built as semi-engineered buildings to be detailed according to the recommendations of IS codes and "Guidelines for Improving Earthquake Resistance of Buildings" published by the Building Materials and Technology Promotion Council, Government of India, referred in Section above

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