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Discussion this modification, instead of using cranes, they

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Discussion
& Recommendations.

The investigation showed that the technique used to build the building
was used in an incorrect way. The technique used it is called Flat
Slab Structure. Normally reinforced concrete building uses columns with
beams but this technique does not use beams, so in other words, all the load
from the upper floor is being supported by the flat slab directly on the
columns. This area must be reinforced so the column under severe load do not
penetrate the slab through. The investigation that was carried showed that many
columns to reduce the cost of production, the diameter was reduce to 60cm. Even
worst, the number of steel reinforced columns were 8 instead of the 16 on the initial
blueprint. Due to these modifications, the structure was weakened by 20%. ?

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Two years before
the collapse, the management of the building received noise complaints from the
air-conditioning from the 5th roof. Due to this issue, they agreed
on moving the three AC units to the west side of the building. When they
carried out this modification, instead of using cranes, they were pulled with
rollers across the roof. The structure of the building was not design to carry
transfer-loads because the structure did not have crossing beams between the
columns. This dragging cause cracks on the roof and on the columns. Apart from
causing cracks, moving the AC units and their heavy weight made the structure
to be instable. When the AC units were on, they transmitted vibrations through
the columns which made the cracks to expand up to 10cm wide. The fatigue made
by the vibrations made the structure to suffer irreversible damage. After two
years, the columns supporting directly the load from the AC units gave away.

From the FEA analysis
shows very important and interesting results on how the roof was deformed by
the extreme high loads, which were four times higher than the design limit. Two
analysis were done, the first one was the structure without the Flat
Slab and the second analysis was with Flat Slab.

Analysis of Results.
Without Slab:
The roof
suffered from a maximum punching shear stress up to 2.926 N/mm2 at
the mid-point were the column is located. The rest of the roof suffered a
stress between 1.06 N/mm2 up to 2.128 N/mm2. The
distribution of the stress along the roof does not follow a pattern along the roof
which suggest that is not uniformly distributed. The stress on the column is
uniformly distributed suffering up to 1.862 N/mm2. Going back to the
previous page, to the first isometric view of the 5th floor roof, it
is possible to appreciate with different colours where the maximum stress is
happening. Big deformation is visible.

With Slab:
In
this case, the roof suffered a maximum punching shear stress up to 2.529 N/mm2
at the mid-point were the column is located. The remaining roof suffered
from a variation of stress from 1.124 N/mm2 up to 2.529 N/mm2.
In this case, the punching shear stress follows a pattern around the roof,
which means that is uniformly distributed which will generate better results
regarding the fatigue of the roof. In the case of the column, the stress is
uniformly distributed suffering up to 1.124 N/mm2. Going back to the
previous page, to the first isometric view of the 5th floor roof, it
is possible to appreciate with different colours where the maximum stress is
happening. The deformation is clearly less, the responsible of less deformation
is the Flat Slab on the column.

 

Comparison
Chart.

The following chart would show the difference between the structure with
Flat
Slab or with-out. The blue colour represents the stress that the
structure is suffering with-out the slab. The orange colour represents the
stress that the structure is suffering with the slab. Slab makes a big
difference. 

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