Conceptual years before the collapse, several cracks started appearing

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)