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Design errors are a result of inexperience

Letters

The authorities in Gujarat are not carrying out column jacketing as described in your report 'Concrete jackets leave Gujurat's buildings unsafe' (NCE 22 February). Quite the reverse in fact. The Times of India on 19 February carried large adverts by the Government of Gujarat providing appropriate advice on building strengthening and emphasising the need to use qualified structural engineers.

Despite this, a number of builders are carrying out substandard repairs. As yet there is still no effective control of building work and construction of part-finished apartments has restarted, despite displaying all the flaws of their predecessors.

The newer outskirts of the towns are built of reinforced concrete frames, which should in principle have adequate earthquake resistance. India has had a seismic design code for many years and the current version IS 1893: 1986 is similar to the American UBC of that time.

The difficulty, as you have reported previously, is that the code has been ignored by speculative builders during the Indian development boom of the 1990s.

It is striking how many of the failed reinforced concrete buildings are less than 10 years old. Design fees for a typical five storey apartment block are roughly equivalent to one week's salary at local rates, and reputable engineers are simply unable to compete in such a market.

The nett result is that 'designs' are prepared by inexperienced individuals, often based on no more than custom and practice.

It may take some time yet to change this practice; the foyer of the building in Ahmedabad which houses the Babtie office now proudly displays a report commissioned by the building owner which states that the building has been inspected and remains safe.

The author of this comforting note is not Babtie, but the concrete cube testing company which occupies the basement!

Babtie has, needless to say, conducted its own inspection on behalf of its staff.

The classic design error, as you have reported, is a ground floor soft storey in which the area between the columns is left open for car parking. Under horizontal earthquake loading the columns simply hinge at top and bottom and the storey collapses. This probably accounted for 80% of the reinforced concrete frame failures. However, a significant number were also prompted by the universal adoption of very large roof water tanks of about 40t, sited over the stairwell, and often supported on slender columns which effectively form a soft storey.

Numerous instances were seen in which the water tank had fallen the height of the building, smashing through each stair flight in turn. In several cases fleeing occupants were crushed on the stairs in this way, while their slower neighbours survived in the apartments.

A number of builders have now begun to jacket columns in surviving buildings, and there seems to be widespread misunderstanding of the soft storey problem. The spalling of the concrete at the hinges seems to have led many to conclude that vertical load capacity is the problem, and therefore jackets the answer.

The absence of any real connection to the existing structure and tendency to simply cast on top of the paviors in the parking bays illustrates how poorly this is being executed. All the jackets do is ensure that in a future earthquake the columns will simply form particularly neat hinges at each end of the jacket.

Alan Stewart (M), divisional director, Babtie Group, 95 Bothwell Street, Glasgow G2 7HX

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