About Seismic Zone and the earthquake resistant building in Bangalore

In my several of earlier posts I have raised my concerns about the consideration of the seismic factors on the high raise apartment has been taken care or not.I just browsed to find out the seismic zone where Bangalore is placed.

Seismic Hazard of Karnataka

Source:

http://asc-india.org/seismi/seis-karnataka-goa.htm

The seismic hazard map of India was updated in 2000 (5) by the Bureau of Indian Standards (BIS). According to the new map, the state of Karnataka lies in Zones II and III. The coastal districts as well as the northern interior districts along the border with Maharashtra, lie in Zone III, where a maximum MSK intensity of VII can be expected. The remaining districts, as well as the city of Bangalore lie in Zone II. Interestingly, the Bellary region, which experienced a strong earthquake in the 1840's has been downgraded to Zone II.

The 1984 BIS Zoning map had placed it in Zone III. It must be noted that BIS estimates the hazard, based in part, on previous known earthquakes. Since the earthquake database in India is still incomplete, especially with regards to earthquakes prior to the historical period (before 1800 A.D.), these zones offer a rough guide of the earthquake hazard in any particular region and need to be regularly updated (See also: GSHAP Hazard Map for Karnataka & Goa).

See my earlier posts:

http://eastindian-mike.blogspot.com/2008/10/recent-collapse-of-high-raise-apartment.html

http://eastindian-mike.blogspot.com/2008/09/lifestyle-greenage-apartments-bangalore.html

Interestingly  an article in Hindu written by  CVR Murthy has drawn my attention( dt 10/06/2006).

Quake-resistant buildings needed now

Many unprofessional players in the real estate area exploit the common man's ignorance on this aspect

About 60 per cent of India's land area is under the threat of moderate to severe seismic hazard. Nine earthquakes in the Indian tectonic region over the past 17 years have provided constant reminders of the vulnerability of Indian construction.

It was the Bhuj earthquake of 2001 that stirred-up maximum concern on the vulnerability of Indian constructions, particularly the modern reinforced cement concrete (RCC) constructions.

The Indian public is under-informed and under-educated on earthquake resistant design and construction. Let us highlight the seismic vulnerability of the large number of a class of reinforced concrete multi-storey buildings being constructed in the urban areas.

These buildings have open ground storey to accommodate parking and 230 mm size columns in the ground storey.

The typical RCC buildings in India are made of frames consisting of vertical and horizontal members. The former are called columns while the latter members consist of (a) slabs, that is, the flat, plate-like parts of the building on which people stand, walk and conduct themselves in the building and (b) beams, that is, the stiff, rib-like parts of the building running under the slab but between columns which support the slabs.

Stilt structures

The typical buildings being built in India accommodate parking in the ground storey. Most buildings having parking in the ground storey have no masonry walls built between any columns.

While having parking in the ground storey is not the concern, having all masonry walls removed and leaving the columns bare is the crux of the matter. Such buildings are abruptly flexible and weak in the ground storey and perform poorly during earthquakes.

Many unprofessional players in the real estate development industry are doling out substandard civil engineering products and exploiting the ignorance of the common man on many technical aspects of civil engineering. For example, most RCC frame buildings in the present context use the same 230mm (9-inch) wide columns in them irrespective of the number of storeys they support above them.

Let us focus on the use of 230 mm wide columns in a special class of buildings.

A stark feature of these columns and also the most unsafe aspect of the buildings they hold is that their size is fixed - 230 mm. Irrespective of the overall height of the building (1, 3, 5, 7, 11, 14 or even 21 storeys), the location of the building (in severe seismic zone or in low seismic zone), the type of soil strata underneath the building (soft soil or hard rock) and the distance between adjacent columns (2m, 3m, 4m or even 5m), the column size is unbelievably the same: 230 mm.

The person who built your apartment may not necessarily be qualified to make earthquake-resistant buildings. This is because quake design has not been taught in any college at the under-graduate level since Independence; graduates from government colleges in Gujarat passing out in 2007 will be first set to have basic knowledge on earthquake resistant design .

Open ground storey buildings have consistently shown poor performance during past earthquakes across the world; a large number of them have collapsed.

The presence of walls in upper storeys makes them many times stiffer than the open ground storey. Thus, the upper storeys move almost together as a single block and most of the horizontal displacement of the building occurs in the soft ground storey itself. Thus, such buildings swing back and forth like inverted pendulums during earthquakes and the columns in the open ground storey are severely stressed.

If the columns are weak (do not have the required strength to resist these high stresses) or if they do not have adequate ductility they may be severely damaged or even lead to collapse of the building.

Neglected

In the current design practice, stiff masonry walls are neglected and only bare frames are used in design calculations. Thus, the inverted pendulum effect is not captured in the structural design.

There are several features associated with the open ground storey buildings with 230 mm wide columns that make them vulnerable during earthquakes. The vertical members in buildings support the horizontal members. So, vertical members must be stronger than the horizontal ones.

After the Bhuj earthquake, the Indian Seismic Code IS: 1893 (Part 1) - 2002 has included special design provisions. First, it specifies when a building becomes soft or weak. Second, it specifies higher design forces for the soft storey.

C.V.R. MURTY