Conceptual resulted in 502 deaths and 937 non-fatal injuries,

Conceptual
approach to final solution to the problem

Two years before the collapse, several cracks started appearing in
the ceiling of the south wing’s fifth floor and the only action taken was to
remove merchandise and machinery from the top floor down to basement. The
decision of dragging the AC units rather than lifting them caused severe damage
to the slabs and resulted in the initiation of crackings as well.

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Figure 1 – Pressure caused by AC’s on columns

On the day of the collapse, a dramatic increase in the number of
cracks occurred, which forced managers to close the entire top floor and shut
down air conditioners. Regardless of this dangerous development and warnings
from executives to immediately evacuate the building, the store manager refused
to issue formal evacuation order due to the high number of customers that day
and not wanting to lose a potentially high revenue.

Knowing that this collapse resulted in 502 deaths and 937
non-fatal injuries, the store manager could have easily saved the lost lives
and prevent the injury of the rest by simply listening to the executives.
Letting go of the greed and eagerness to earn more money despite being warned
was a selfish decision by Lee Jon.

Additionally, the entire building was designed to only sustain
four floors and not five. Despite the construction company refusing to give
their consent for the addition of an extra floor, the store hired another
company to execute his plan. The fourth floor columns were 20 cm shy of
withstanding an additional top floor (80 cm) although the building remained
standing for 5 years.

On top of that, the reinforced bars were misplaced 10 cm for the
slab edge, meaning 5 cm off the required position. This misplacement results in
a 20% strength loss for the slab and thus become a strong contributor the
reduced strength of the RC frame. Floor heating systems installed on the fifth
floor resulted in thicker a thicker slab and therefore more stress on the 4th
floors RC columns.

Despite all the structure weakening factors stated above, they
were not the main cause of the collapse surprisingly.

Reasons

The main reasons of the collapse were simply not following the
building plans. An additional floor was built without the approval of the
building company due to risk of collapse, yet it was built regardless. Also,
the additional floor was initially meant to be a skating rink but was became
instead a traditional Korean restaurant. A traditional Korean restaurant
doesn’t use for the customers, instead they sit on the floor and for their
comfort, heated floor were implemented. The heated floors added four extra feet
of thickness to the fifth floor slab. It was also found that the top floor
columns were not aligned with the columns of the floor beneath, leading to bad
weight distribution (column to slab and then to another column, rather than
directly from column to column).

(McLean, et al., n.d.)

Recommended
solutions

As the collapse was caused due to punching shear, it is vital to
use punching shear reinforcement when using the flat slab construction method.
This is due to the lack of slab supporting beams and thus making the slabs rely
only on the columns to hold them up.

However, by using alternative methods like the one or two way slab
methods and implementing drop panels, the outcome could have been different.

One Way Slab

The one way slab only has supports on two opposite sides only,
which results in a one directional structural action. If the ratio between the
longer and shorter span is greater than two, then it is a one way slab. Only
the longer spans have supports which will help transfer their loads to the
opposite walls. This being said, the actual support of the structure of any one
way slab is always located on the shortest spans. (CivilEngineeringBasic, n.d.)

Figure 2 – One Way slab construction (BSBG, 2017)

Advantages

Disadvantages

Simple
and easy construction

Compromise
flexibility of partition

Easy
to manage penetrations in slabs

Greater
storey height required for beams

(BSBG, 2017)

Two Way Slab

On the other side, a two slab construction carries the main load
in both directions. The ratio between the long and short spans is less than
two. The difference between this approach compared to the one way slab method,
is that the load is transferred on every single wall, making each wall act as
support. (CivilEngineeringBasic, n.d.)

Figure 3 – Two Way slab construction (BSBG, 2017)

 

Advantages

Disadvantages

Economical
when having high loads and longer spans

Compromise
flexibility of partition

 

Greater
storey height required for beams

 

Regular
column layout required

(BSBG, 2017)

Flat slab with drop panels

This method increases the shear capacity and stiffness of the
slab, which means that thinner slabs can be used and therefore reducing their
weight. (BSBG, 2017)

Figure 4 – Flat slab with drop panels construction (BSBG, 2017)

Advantages

Disadvantages

Higher
efficiency compared to flat plates due to lower stress concentration at
column locations

More
complicated formwork

Thinner
slabs

Higher
level of coordination required for drop panels

Lower
storey heights are possible due to absence of beams

 

(BSBG, 2017)

 

Using one of these methods means that beams will be used as
support for the slabs, which is way more efficient than the single column
approach. Reinforced steel bars are necessary for the concrete slabs in order
to strengthen their tensile strength as a crack through concrete can propagate
to the next one and so forth. Therefore, having steel bars in place can half
any propagation if it occurs.

By utilising beams and placing them on top of the columns, more
bending will occur on the beams which will hold the slabs and increase the
bending resistance. Consequently, punching shear can be reduced or even
overcome unlike the flat slab method. (Ramadhan, 2015)