Saturday, April 25, 2015

Earthquakes Highlight Need To Review Masonry Practices In Developing Countries

Dharahara or Bhimsen Tower, a famous 62m high historical structure in Kathmandu, has been toppled today by a 7.9 Magnitude Earthquake. For an average fee of a dollar, visitors could climb its spiral staircase and view the skies over Kathmandu. Given it's popularity, tourists from several countries might have been there today. Any number of guesses can be taken at the casualties from the disaster. 

I borrowed a pic floating on Twitter taken in the aftermath of the quake which show nothing much but 1/8th of the Tower left and a diagonal cut terminating at its end.

Masonry is poor in tensile strength but great in compression. The diagonal cut is characteristic of masonry when the earth suddenly moves sideways but the inertia of the structure has no "give" to react. This causes the upper parts of the structure to slide over lower ones, and the shearing force eventually cuts a crack at this interface. Repeated shaking of the earth back and forth opens up the diagonal crack in both directions, somewhat like an X and the structure eventually crumbles. 

An example of X shaped crack formation can be seen below from another residential building in Kathmandu. The pic was posted by CNN-IBN.

How could masonry structures be improved in developing parts of the world?

This paper published in Future Trends in Structural, Civil, Environmental and Mechanical Engineering, estimated in a lab test that for a mortar thickness of 3/4 inch and shear loading rate of 1/2 ton, an average deformation of 0.140 inch was noticed before the sample failed in shear.  The corresponding shear force was around 14000 lbs and the failure strain was 0.375 inch/inch. Another sample, with only 1/2 inch of mortar in between the bricks showed slightly more deflection but failed at approximately 13000 lbs of shear force at something like 570 inch/inch of strain. In general, the study shows that bricks with higher mortar thickness and higher cement ratios in the mortar show lower tensile stress on loading.

Certainly this sets stage for several questions. Might studies like the above indicate a need to review and control these parameters in masonry? Are more standards required for contractors to meet exacting specifications so structures could be more resilient?  Should renovation of heritage masonry structures be the perfect time to revise the underlying structural makeup?

Indeed it is surprising to learn that the Bhimen Tower is no stranger to earthquakes, having been extensively damaged in an earlier quake and rebuilt in the 1930's. I have no technical details of the structure since the renovation, however one wonders if improved construction practices may have changed the outcome of today?

Interestingly, there's a story of Bristish naturalist Brian Houghton Hodgson, who on horse ride ride with Bhimsen Thapa past a rebuilt 9 story Bhimsen Tower, was to remark on the "necessity of applying either practical knowledge or improved scientific practices in the constructions of this type". I hate to take that out of context but it may offer some clues as to what Hodgson thought of structural practices followed by the Nepalese.

Let me know if you know anything more. Meanwhile I'll update as I see technically relevant things.

Bhimsen Tower after the 1934 Earthquake

View from the 8th floor of the Bhimsen Tower, pic taken by Emily Sharples on Nov 22, 2009

The Tower after the 7.9 magnitude earthquake in Nepal on April 25, 2015. Notice the diagonal cut in the structure roughly 1/8th of the way up from the base.

 Another angle of the destruction, from the Washington Post.  It indicates lot of brickwork in the debris. 

An interesting video shown below is from a 2011 upload by Al-Jazeera, where the Joint Secretary of the Home Ministry is seen decrying the local non-compliance with building codes.

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