Structural Analysis on the 2016 Taiwanese Earthquake: Possible Causes of Structural Failure and The Main Culprits of Structural Collapse
The Tainan earthquake of 6.4 magnitude occurred on February 6, 2016. The earthquake led to the collapse a 17 storey building, Weiguan Golden Dragon. Strangely, this building was reported to be the only high rise building to completely crumble and to cause the majority of the 59 total deaths during the earthquake. Why is that?
Surrounding Buildings Still Standing Upright? Source: Taipei Times
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The building was built in 1989. It used the old code and was never improved accordingly to the new code. The new Taiwanese building code has more stringent earthquake requirements, since it was updated after the 921 earthquake (The 921 earthquake occurred on 1999 with a 7.6 magnitude earthquake, which caused over 2300 deaths). Since the 17 storey building was still based on the old code, the collapse explains the fact that surrounding buildings, which might have used the new code for construction or retrofit, are still standing. Therefore, the lack of adapting to the new code made the 17 storey building more susceptible to seismic forces.
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Another possible cause of collapse was the possibility that the building was not designed to withstand the seismic force intensity. Hence, we now have poor engineering design to blame. The 6.4 magnitude earthquake occurred at a depth of approximately 23 km with a maximum intensity on the Mercalli intensity scale. Taiwan is in a seismic active zone, known as the Pacific Ring of Fire. Indeed, the building was still intact after the 921 earthquake, which was a 7.6 magnitude earthquake. However, the amount of damage an earthquake does on a structure depends on several factors. Factors include the frequency of ground motions, the magnitude of the seismic loads, the soil conditions underneath the structure, the stiffness of the structure, the location of the epicenter of the earthquake relative to the building, etc. All these factors might have combined to cause a localized effect on the building during the 2016 earthquake. Just because the building can withstand a 7.6 magnitude earthquake, that does not mean we can rule out poor engineering design as a possible cause. Earthquake loads are dynamic and often cannot be predicted accurately. It might just be pure luck that this building can withstand the 7.6 earthquake. But we can never gamble on sheer luck on our designs. Heck no!
Overturning Source: Wall Street Journal
Not only might have the building lack poor engineering design on seismic forces, it can be seen in pictures that the building has collapsed sideways. The structure failed by overturning, which is caused by poor foundation design. Due to the soft soil conditions throughout Tainan, it is possible that the foundation designed was not strong or deep enough to set into bedrock, in order to support the lateral pressures induced by the seismic forces. Due to the soft soil, earthquake ground motions were also amplified, causing soil liquefaction. The rapid seismic loads caused the water pressure in the low density saturated soil to increase, which caused soil particles to move around easily. Soil movement then decreased the strength and the stiffness of the soil, which led to the reduced soil support for the foundation.
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Another engineering design issue was in the superstructure. The building was taller than other buildings surrounding it. The taller the building, the higher the moment will be when lateral seismic inertial forces are acted onto higher levels. Starter bars connecting the reinforced concrete columns and shear walls to the foundation must be designed for that additional stress. The building is also U-shaped and does not have a fourth shear wall “to hold the ends of the “U””. The asymmetrical layout of the shear walls can possibly induce torsional effects on the overall structure. However, torsional failure was not the main culprit as seen in these pictures. The whole building just overturned and collapsed. There was no twisting effect. But it is not to say that missing a shear wall is structurally acceptable. In the pictures, we can see that there are horizontal cracks on the walls. This is caused by lateral stresses from the seismic forces being localized in these walls. If we have that missing wall in the building, stresses can be distributed more evenly.
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Source: MDPI - Open Access Publishing
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Source: The Guardian
The connections between the beams and the vertical elements were also an issue. The pictures showed steel rebars being pulled out from the concrete, indicating poor detailing of ductility reinforcement. Sources say that the steel anchorage was not bent in 135 degree stirrup angles, but in 90 degree angles. Beams in seismic regions should have both ends of vertical stirrups designed to be at least 135 degrees, so that stirrups do not open up during an earthquake. Adequate stirrups in beams can:
- Resist brittle failure, in particular vertical shear forces and diagonal shear cracks
- Prevent concrete aggregates from bursting out of the beam due to flexure
- Prevent flexural buckling of main longitudinal bars
Metal Tin Cans Source: American Geophysical Union
Finally, I have saved the best to the last. Photos from local media showed cooking tin cans used as fillers in decorative columns in the 17 storey building. Yes, the tin cans are only in decorative columns and not in load bearing ones. Yes, it was common practice before it was outlawed in Taiwan after the 921 earthquake. But, a real engineer would never use tin cans for construction material, even as filler! Better to use some durable adhesive filler with some sort of function.
And something is fishy here… Why would anyone want to enlarge pillars for decorative purposes? Enlarging pillars means reducing floor space, which most clients and architects hate. I rarely hear a client or architect request engineers to enlarge column sizes. It is usually the other way around for load bearing functions.
Although Taiwan has learned to put more stringent requirements for buildings after the 921 earthquake, it is not enough to make a law. They need to enforce it for each and every single suspicious building. Structural inspections should be consistent. Safety awareness should be promoted for earthquake damage prevention. Upgrades should be done to buildings to adhere to the current code, in order to retrofit the older buildings. Seismic retrofitting can include post-tensioning of high steel tendons, installing dampers, or slosh tanks. They are expensive but safety is always first!
And something is fishy here… Why would anyone want to enlarge pillars for decorative purposes? Enlarging pillars means reducing floor space, which most clients and architects hate. I rarely hear a client or architect request engineers to enlarge column sizes. It is usually the other way around for load bearing functions.
Although Taiwan has learned to put more stringent requirements for buildings after the 921 earthquake, it is not enough to make a law. They need to enforce it for each and every single suspicious building. Structural inspections should be consistent. Safety awareness should be promoted for earthquake damage prevention. Upgrades should be done to buildings to adhere to the current code, in order to retrofit the older buildings. Seismic retrofitting can include post-tensioning of high steel tendons, installing dampers, or slosh tanks. They are expensive but safety is always first!
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