Major reasons behind the poor performance of a structural element that may lead to a partial or even a complete collapse!
Ageing, decay and environmental degradation
Accidental loads or third party action
Results of a forensic investigation help civil engineers to discover the reasons responsible to trigger unfortunate event and also to uncover the complete story of failure. In this way, we can reanalyze the structure to regenerate the collapse scene and mechanisms. NDT techniques can help us to assess the as-built rebar layout, concrete and steel strengths. Finally, we can piece together all gathered information and analysis results to imagine the fateful scene, identify the responsibilities. We can take lessons for prevention of incidents in future.
Polashbari Incidence: Identification of Collapse Mechanism
One 9-storeyed sweater factory building owned by Spectrum Sweaters Ltd., located at Palashbari Village under Savar Thana of Gazipur District, Bangladesh collapsed at around 1.15 am on April 12, 2005 killing more than 60 and wounding more than 100 persons. The nation was deeply shocked of the incident. The Government also took the matter with proper attention. The Hon’ble President and Prime Minister of the Government of the Peoples’ Republic of Bangladesh visited the site a short time after the incident. Soon after the collapse, the Fire Brigade, Army Engineers Corps of Bangladesh Army and different agencies started the rescue operation. The operation continued till 19th April 2005 (for 8 days) when all the debris over the fallen building was cleared and all the trapped victims were rescued and the dead bodies were recovered.
Checking the design adequacy
With a view to find out the real causes of failure, an independent design of the structure by taking into consideration the as-built column layout, adopted flat plate-column structural form, recommended material specification mentioned in the original designer’s design sketches, end use of the constructed facility and site location. To do this, a finite element model of the structure was developed. The Bangladesh National Building Code (BNBC 1993) was consulted to assign the dead load, vertical live load, wind load and earthquake loads in the process of analysis.
The design trial suggested that the structure was inadequately designed for the following aspects:
- The column sections were under designed at all story levels. The reduction of column section at upper story levels made this situation much more difficult.
- The structural system had no effective lateral load resisting system to limit the sway of the structure within acceptable limits under wind or earthquake loads. Not to mention, the so-called shear wall of the structure built at the extreme west side of the building was not structurally connected with the slabs or columns. Neither any design detail or any proposal for this shear wall was available in the designer’s design sketches.
- In general, most of the slab-column connections at upper story levels were found to be inadequate against both punching and lateral shears.
Different phases of the collapse sequence were simulated due to the removal of the north-east corner column support. The figure shows that due to this support removal, hinges start to form at the slab-column connections in a gradual fashion starting from the top story. However, formation of such hinges were found to be mostly concentrated in the top most four stories (Story 5, 6, 7, 8), except the north-east bays where hinges were found to be developed in all story levels. This largely matched very closely with the Available Information Bases for Investigation indicating the closeness of the simulation results with the real phenomena that took place at that sad hours. Based on this findings, the investigation team was able to derive the following conclusions:
Either the failure of the northeast corner column or its foundation system (or both) is (are) responsible for triggering the collapse of the structure at that fateful night.
Upon the triggering, the hinges started to form at slab-column connections at top story levels pushing the upper floors to fall in a sway motion towards east to north-east direction. A mild sway of the southern side columns to southward direction also took place.
The collapse of the upper floors caused a tremendous vertical impact on the lower stories. This action caused the lower stories to come down vertically.
North-east corner bays of the structure collapsed vertically at all floor levels.
The north-east side corner column location need to be excavated to find out what really went wrong on 12 April 2005 at 1:15 am.
The subsequent revelations through field excavation were completely in line with the simulation predictions. The fact that connection between the column and pile cap failed primarily due to very weak concrete strength associated with defective construction practice.
Failure of Shore Piles in Sadharan Bima Bhaban, Dhaka, Bangladesh
A three basement multistoried building was under construction at Motijheel Commercial Area, Dhaka, just adjacent to Bangabhaban (President’s House, Bangladesh). The bulding is to house the head office of sadharan Bima Corporation, the state owned general insurance company of Bangladesh. Due to heavy raining, the shore piles of the building under construction (2005) failed. Subsequent analysis showed inadequacy of bracings installed to brace the shore piles.