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Just how strong are our buildings?

posted Jul 7, 2016, 10:07 AM by jeffery jim   [ updated Jul 7, 2016, 11:56 AM ]



Few months ago, after the Taiwan 2016 earthquake, i took my time to review back the intensity of the earthquake in Taiwan and try to relate it in Malaysia context, specifically Kota Kinabalu City. Therefore, I took shakemaps out and make comparison between Mount Kinabalu magnitude 6.0 and Taiwan recent earthquake magnitude 6.4. That's it!

   
 Illustration 1: Taiwan 2016 Earthquake Shakemap Illustration 2: Mount Kinabalu 2015 Earthquake Shakemap 


The only difference between Kota Kinabalu and Tainan is only ONE SCALE STRONGER based on Mercalli scale force reacted toward structures at Tainan and left such horrific devastation. Kota Kinabalu endured scale 5 and 6 (a mix between Moderate and Strong in the scale where unstable objects will overturn and at point the ceiling may collapse or furniture are shifted) while Taiwan was overpowered by scale 6 and 7 (a mix between strong and very strong where ill-constructed structures may lapse their service limit and some might collapse). A step up in Modified Mercalli scale means an increase in Richter magnitude which means the intensity of the shake increases by 30 times.

Geotechnically, we have share similar features with Taiwan and Hong Kong (this is the main reason why JKR adopts Geospec 5 for rock engineering in Malaysia) but did share the same structural strength based on design code and construction quality? It is quite subjective but it is presumed that most of the buildings withstand the first shock except for a building which was built fairly inadequate and unethically in 1994 (Weiguan Jinlong in Yongkang District) and caused the death of 115 people with exception of two others from Guiren District. For readers clear understanding, it was years before the introduction of dynamic analysis (using response spectrum method) to be included in the seismic force requirements for submission purposes and submission is based on UBC. The other newer buildings have undergone through proper design since the 1999 Chi-Chi earthquake and this shall be the basis to review the building ultimate limit state for this discussion. Taiwan code adopts the ACI318 for structural concrete with different safety factors configuration in comparison to the British Standard (BS) or the Euro Code (EC).

Generally, the BS have higher safety factor comparison with 1.4 for dead load and 1.6 for live load, while ACI allows for 1.2 for dead load and 1.6 for live load. In recent advances in engineering code, the EC introduced the use of 1.35 for dead load and 1.5 for live load for Ultimate Limit State design. Another notable information about Taiwan seismic design would be the additional 10% risk of over the 50 years earthquake return period which give provision of additional 10% on vertical loading. Buildings in Taiwan were designed with ultimate limit state which are able to cater up to magnitude 6.5 Richter scale or around 0.22g. Given the similarity of geotechnical condition, structures in both Malaysia (BS standard) and Taiwan (ACI Standard) would behaves similarly except for the differences in self weight limit capacity which may affect the structure when structural members are no longer in equilibrium and enforces/imposes additional load to the structural member which propagates the load toward bearing. This can be discounted with the plasticity limit for columns under such condition where ACI dictates a higher minimum steel ratio (almost twice). Overall, the design moment capacity is almost similar and it is appropriate to assume that structures in Malaysia and Taiwan have almost the same strength.

Nevertheless, when we review the performance of the buildings exposed to earthquake shocks, we need to emphasize on retrofitting works on existing structures and rethink about ways to use both force and energy for structural design in the future. Retrofitting allows the increase rigidness for structural members or to allow damping effect when facing the seismic force. But overall, the design should also cater the ability to extend the ultimate limit state and/or to fail in a manner which ensure safe exit by allowing pockets of safe zone.
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