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ASHIYANA-INTERIM POST DISASTER SHELTER
   
   
SHOCK TABLE TEST
 
 

Shock Table Program for Earthquake Resistant Building Technologies

 

A program of learning and confidence building in Earthquake Resistant Building Technologies through testing of half scaled models was carried out by NCPDP-CEDAP at Nirma University during year 2005. The tests focused on (a) commonly followed practices, and (b) new technical features for increasing earthquake resistance.

A new facility was set up for this purpose. This included (a) a shock table consisting of 3m X 6.5m reinforced concrete platform on rollers to build the test models on, (b) a steel stopper at one end to restrict the movement of the platform to a maximum of 9”, and (c) a 1.5 ton pendulum of 2m height at the other end for giving desired shocks to the platform. This facility is the biggest of its kind in the country.

Four tests were conducted and each test was a public event in which building professionals, engineering and architecture students and faculty, masons and contractors, and general public were present to witness the test. During the test pendulum was raised through various angles and released to give the desired impact. The damage to the models was documented. The acceleration caused in the platform and the models was monitored with the help of accelerometers installed on them and a laptop attached to the accelerometers. The event was documented on video and digital photographs.

All test models were built of either burnt bricks or concrete block masonry in mud mortar. The reason to use mud mortar was to ensure that the effect of the earthquake features gets highlighted and, hence, become easily visible. The following tests were conducted.

   
  Test 1 : The purpose of the first test was to observe the effect of the containment reinforcement on the performance of the walls subjected to earthquake like out of plane (lateral) forces. The containment reinforcement consisted of simple 2.5mm galvanized iron wires stretched from the top of the wall to its base on both faces of the wall at 0.5m spacing and either side of door openings. The test clearly showed that the containment reinforcement clearly slows down the collapse of the masonry wall subjected to high lateral forces. It is observed that even though the walls may be severely damaged, they do not collapse readily. Another interesting learning of the test was that even if an earthquake-resisting feature is installed in only some of the walls, the stronger walls do reduce the damage in the adjacent walls.
   
 

Test 2 : The purpose of the second test was to observe the difference in performance of a structure with column like RC elements with four 6mm bars in every corner against a structure with the single rod vertical reinforcement distributed in solid concrete block wall. Today in the country this type of load-bearing masonry structures with corner “columns” are being built in a big way in the name of earthquake safety. In the test the structure with such columns collapsed with the RC columns separating from the masonry. The test clearly showed that the use of RC “column like” elements (which could be more appropriately termed as “pseudo columns”) in the absence of RC band at lintel level in a masonry structure does not offer any earthquake resistance contrary to the popular belief in many parts of the country. The same amount of steel and concrete if used in the form of reinforced masonry offer significant earthquake resistance.

   
 

Test 3 : The purpose of the third test was to observe the performance of two retrofitted structures of different types – one being a commonly built brick structures with doubly pitched Manglore pattern tiles roof and the other being a similar structure with column like RC elements in each corner. At the end of the test both the structures suffered fairly severe damage. The test clearly showed that the structure with Pseudo RC Columns even when retrofitted with a seismic belt suffers more damage than the retrofitted traditional structure. Hence, the use of Pseudo RC Columns in masonry structure is simply not recommended. It also showed that the vertical reinforcing for retrofitting if done using welded wire mesh strapping along the outer face of the corners appears to be more effective than a single bar installed to both the walls in the inside face of corner. In case of Manglore Pattern tile roofing the existing roof could be best anchored down to the wall using MS flats rather than the twisted GI wires stretched from rafters to eave belt.

   
 

Test 4 : The purpose of the fourth test was to observe the difference in performance of a structure with walls having multiple bands but no vertical reinforcement against a structure with a band only at lintle level but also having vertical reinforcement embedded within the masonry as per BIS code. At the end of the test the structure with just one band collapsed while the other structure stood up with fair amount of damage. The test clearly demonstrated that the presence of mere lintel band may prevent the damage/cracking in the portion of that wall above the band. But the portion below the band may get severely damaged. In other words if bands are installed at one or more levels below the lintel then the crack propagation would be quickly checked. The old concept from the traditional construction of Himachal Pradesh and Uttaranchal, as well as from the post Quetta Earthquake rehabilitation (around 1934) has been demonstrated effectively through this test. Multiple bands will significantly reduce the possibility of collapse of the structure. These bands used in conjunction with the vertical reinforcement will further improve the performance, and greatly reduce the possibility of severe damage.

   
 

Summary : All the learning has practical significance and direct application possibility. The lessons clearly point at what the right practice should be for earthquake safety for new construction as well as for the retrofitting of the existing structures. The program also effectively demonstrated that the shock table set up made by us at Nirma University is a simple and cost effective tool for engineers as well as ordinary people.

 

 

Prepared By:
Rajendra Desai, NCPDP 

 

February 25, 2006

   
 

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