Case study · concrete

Case Study: Mantri Square Mall Cantilever projection collapse Report

Preamble:

The commercial building Mantri mall, is a conventional RC framed structure with two basements, ground and three upper floors. The building was built during 2006 to 2008. It was reported that part of this commercial building located at Malleshwaram, Bengaluru was collapsed at about 1.30PM on Monday, 16th Jan 2017.

The scope of this study is limited to the examination of the collapsed region, arriving at reasons for collapse and recommending strengthening measures.

Observations:

  1. The cantilever slab of span 2m are provided in the second and third roof slab levels at the rear portion of the building. They intent to serve as fire exit pathway and access to services.
  2. The collapse of a portion of cantilever roof slab and a portion of second floor balcony slab had occurred at the northwest corner of Mantri Mall Building.
  3. The debris had fallen on ground at rear region of the building. Some portions of the slab, along with service cables were hanging in the collapsed region,
  4. In the cantilever portion of the roof, two chiller pipes of 600mm diameter are placed on regular pedestal, carrying water.
  5. In the northwest region, cantilever roof slab to a length of about 17m had collapsed, along with the RC parapet. The collapsed had created distress in the form of cracks in the remaining cantilever slab to a length of about 8m. These cracks were along the cantilever support.
  6. Cracks were also observed in the RCC parapet wall which is existing near to the collapsed region.
  7. It appears that the failure is first triggered in the roof and the debris fell on the third floor cantilever slab. This gave way due to the impact of the falling debris. However, the second floor cantilever slab suffered only the damage to the parapet and no visible stress was observed in the cantilever slab region.
  8. The fractured section of the cantilevered roof slab shows the top layer of membrane waterproofing followed by screed concrete layer. A bituminious water proof layer was seen between the screed and RCC slab. The thickness of RCC slab was measured as 200mm and the thickness of screed concrete was measured as 250-275mm.
  9. The fractures surface indicated brownish stain in the upper portion of RCC slab. The reinforcement at the bottom was found to be hanging down. In order to see the top reinforcement, the fractured surface was gently chipped to expose the top bars. the top bars were snapped at the fractured surface. They were found to be positioned at about 100mm from top (mid depth) of the RCC slab.
  10. The left over bars in the fractured region and the snapped bars indicate that the bars and corroded exhibiting brown patches and formation of scales.
  11. Wide cracks were observed in the peripheral wall in the third floor of scary room area of the building.

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Test Results:

mantri-mall-full-report-page-032

mantri-mall-full-report-page-034

Sketch:

Mantri Mall full report-page-044.jpg

 

 

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