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Daily Digest Mon Dec 6 23:00:04 2004

 
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ibarua
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PostPosted: Tue Dec 07, 2004 2:44 am    Post subject: Daily Digest Mon Dec 6 23:00:04 2004 Reply with quote

07 December, 2004

1. Re.: Bond between "cracked" concrete & rebars

Engr. V Urala's explanation is very good and is the answer to a question that may have been plaguing many of us. I have personal experience of bond failure. In 1974-75, concrete in a folded plate structure just fell down after removal of the formwork. This happened in the second set of plates cast, the first set remaining intact. The rebars remained in position (of course somewhat sagged); the concrete just fell through the meshes in the rebars. It was later found that the contractor, which, by the way, was a government agency, had used very old cement including sweepings lying on the floor of the godown. It was a perfect case of bond failure.

2.  Re.: Shear failure

Shear failure is sudden and violent, and therefore is very dangereous. Adequate care therfore should be given in designing and detailing shear rebars, even in columns, particularly in seismic zones IV and V.

3.  Re.: Living with earthquakes

Unlike many of you, I have first hand experience of a few major earthquakes, including the Great Assam Earthquake of 1950.

I was at that time a schoolboy in Shillong about 350 km. away from the epicentre, living in a single storied, timber framed, ‘Assam type’ house. The first shock was at about 8:15 PM on 15 August – a night of black skies and heavy rains. The house shook like an old lady with the ague. There were creaking sounds from the timber frames supporting the roof. Mercifully, after a few violent heaves which had us all standing outside in the pouring rain, the shaking abated, gradually at first, and then died away altogether, leaving us panic stricken, but relieved. It was our custom to be in bed by 9:30 P.M. and the next shock came about then, and it started to rain heavily again. The house shook with tremendous creaking sounds and with a distinct twisting and rocking motion with the rain beating down on the tin roof in a cacophony of noise. We thought that the end of the world had come. The shaking went on in differing degrees of
ically throughout that dreadful night.

That earthquake had a magnitude of 8.5 on the Richter scale. In 1950, Assam had only single or at the most a few double storied buildings, and the damage to property was not very high by today’s standards. This writer remembers that a tea planter’s three storied building in Tinsukia built a decade earlier developed some cracks after this earthquake.

The English botanist, Capt. F. Kingdon Ward, who happened to be camping at Rima, the very epicentre of the 1950 earthquake on that fateful day, describes it graphically:

“ it was felt as though a powerful ram were hitting against the earth beneath us with the persistence of a kettledrum. I had exactly the sensation that a thin crust at the bottom of the basin, on which we lay was breaking up like an ice floe and we were all going down together through an immense hole, into the interior of the earth.”

Regards to all,

Indrajit Barua
Guwahati, Assam.

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