[Four years ago, on this day, Rana Plaza collapsed. In a short article, I explained the possible reasons for the collapse of Rana Plaza. The article was published in The New Nation as a Post-Editorial. The submitted version is given below. You can read it if interested.]

Rana Plaza, an eight-floor building (excluding basement) collapsed on April 24 in Savar near Dhaka City, Bangladesh. The building housed garment factories from third floor to seventh floor; the second floor was used for shopping center and there was a bank in the first floor; the basement was being used for parking. Cracking in a column in the third floor was observed earlier and reported before building collapse. That was a warning sign of future danger. Unfortunately, it was ignored and the garment laborers were allowed to work that has caused the death of more than four hundred lives. How this disaster occurred? What factors made Rana Plaza to collapse. This article discusses the possible causes of Rana Plaza collapse.
1. Were the floors adequate to bear the loads? It seems the floors were transferring the loads to the beams without showing signs of excessive distress (cracking, slab sagging, slab warping, concrete spalling, etc.) although the live load was more than that generally considered for residential building. Generally, some safety factors are considered in design; this could be the reason for no visible floor distress due to more live load. Moreover, the floors normally do not bear highly concentrated load similar to columns.
2. Were the beams adequate to bear the loads? The loads transferred from the floors were carried by the beams without showing any warning signs of excessive distress (cracking, beam deflection, concrete spalling, etc.), as no such distress was observed and reported. This is perhaps for the same reasons as discussed in the case of floor slabs.
3. Why cracking occurred in a column? This can be explained mainly based on materials perspectives, particularly concrete. The concrete properties for column should not be the same as those for slabs and beams depending on the floor levels. For example, the concrete strength for the basement columns must be higher than that for the uppermost columns. This is because the loads imposed on basement columns become significantly greater than the loads on the upper columns. It depends on the number of floors supported by the columns. Therefore, a 3500-psi concrete used for slabs and beams may not be sufficient for columns in some floor levels. Also, the reinforcement amount in column will vary from one floor level to another floor level. More reinforcements are needed for the lower-level columns because of higher loads. Furthermore, the sectional size of the column is an important factor in design. The column size can vary depending on the concrete strength. If concrete, reinforcement, and column size are not designed and used properly, the load carrying capacity of the columns will be significantly affected, particularly in the lower level columns. Perhaps, negligence of considering these parameters properly in design and/or construction was a major cause for column cracking in Rana Plaza. There can be other factors; for example poor construction quality (poor mixing, poor workability, poor placement, poor compaction, poor curing, etc.) and poor materials with inadequate properties. All of these factors affect achieving the target compressive strength, which is much more critical for columns than slabs and beams.
4. Were concrete mix proportions adequate? Concrete mix proportions directly affect achieving the target compressive strength. In particular, the water to cement ratio is a critical factor for compressive strength of concrete. If the proportions of concrete constituent materials are not appropriate, the target compressive strength will not be achieved. The concrete mix proportions might not be designed properly in the case of Rana Plaza. Another possibility is that the designed mix proportions might not be maintained properly due to poor workmanship or for saving some materials cost. For example, most often the normal construction laborers are inclined to add more water when they see the concrete is less workable. They may not think that they are increasing the water to cement ratio by adding more water; the increased water to cement ratio significantly decreases the compressive strength of concrete. In addition, the owner may lead the workers to use less cement for reducing cost. This increases the water to cement ratio and thus decreases compressive strength of concrete.
5. Did differential settlement cause cracking? Differential settlement can occur depending on the soil condition underneath the foundation of building. It can cause cracking in column and other structural elements. It has been reported that a portion of the land was marsh land before constructing Rana Plaza. Hence, differential settlement was likely to occur.
6. Were heavy equipment and generators responsible for cracking? Vibration from heavy equipment and generators can also be a factor that might accelerate the initiation of cracking in column. This can cause fatigue loading (cyclic loading) on column. Fatigue can occur when structural components are subjected to repeated loading and unloading. Under fatigue, structural materials generally fail before reaching the maximum stress level. Hence, fatigue can be considered as a cause for column cracking in Rana Plaza.
7. Did the whole building collapse when a single column failed? The entire structure can fail if a single column fails. It depends on how much additional loads the neighbouring columns have to carry and whether they have that capacity based on their design details and materials properties. It seems that the neighbouring columns were not capable of bearing the additional load when the cracked column failed in the case of Rana Plaza. In addition, more damage occurred due to progressive collapse.
In summary, columns are generally the most critical elements to transfer the building loads to the ground (soil) through foundation. Perhaps, columns did not receive more care and attention than slabs and beams during the design and construction of Rana Plaza. Poorly designed and constructed reinforced concrete columns in a building can be devastating, as proven by the collapse of Rana Plaza.