Each time an earthquake strikes, we be taught one thing new from it. As a structural engineer it's all the time troublesome to clarify folks as why a constructing collapsed. The one motive is we can not predict earthquakes. However wait I'm not accomplished right here, but..!! Allow us to take a deep breath and soar into the probabilities of failure of buildings within the metropolis of Tainan which is positioned simply at a distance of 48km (30 miles) from the epicenter. Allow us to check out USGS map of the affected zone.
Now the earthquake was a really sturdy earthquake of class 7 which describes the wave velocity of 20 cm/s and a peak acceleration of 0.22g. Another factor to remember is that the earthquake was a shallow earthquake which implies that the depth of the earthquake may need amplified.
Speaking in regards to the constructing collapses in Tainan, we can not instantly blame at design engineers and the one motive I'm saying it's because Taiwan has been well-known for earthquakes and never small depth however the massive depth ones. Earthquakes between the vary of 5.5 to 7.7 have been fairly frequent on this a part of the world. They're designing the buildings on the idea of response spectrum methodology which, so far as I do know, can also be included of their design codes and was revised in 2011.
Now allow us to check out pseudo spectral accelerations (PSA) for 0.Three sec - Three sec durations obtained from USGS web site:
Now the tallest constructing collapsed within the metropolis of Tainan was a 16-17 story constructing. So far as I do know the constructing may have a pure interval of 1.5s-2s which implies it would the pseudo spectral acceleration of the constructing will lie someplace in between 0.15g to 0.11g. This implies the bottom shear within the constructing might be within the area of 15% of the constructing weight to 11% of the constructing weight. to grasp the kind of failure that the constructing suffered, allow us to first check out the constructing collapse.
Sure, the constructing toppled over aka overturning. This appears bit uncommon to me as there are only a few causes for such a failure to occur. The constructing has remained intact even after falling on the bottom, the one failure that I can see within the pictures is close to the bottom the place bars have been pulled off from the concrete pointing in the direction of inadequate improvement lengths however we can not blame a bar splice for a constructing failure as a result of the failure is overturning and never a neighborhood failure.
Overturning factors us to a motive that the axial forces weren't predicted to this extent which factors to drifts of the constructing and which in flip factors to quantity of base shear a constructing is experiencing. Overturning additionally factors to the consequences of P-Delta results and let me share a picture exhibiting an identical failure which occurred in northridge earthquake.
This failure reveals the impact of P-delta in columns. On this case it was only a Three story parking construction and in case of the overturning of a constructing it was a 17 story construction. Now I'm simply guessing as what may need occurred as a result of I have no idea what the lateral load resisting system of the construction but it surely ought to be concrete shear wall and I additionally do not know what the design spectrum would have seemed like. However I do know for certain, if we simply do response spectrum evaluation then we certainly don't see P-delta results on structural instability. However there may be additionally another analysis exhibiting that the impact of P-Delta in a stiff system like core partitions isn't so distinguished. So sure it is vitally early to foretell as what may need occurred however we will not less than slender all the way down to few causes to what may need occurred.
Another uncommon factor that occurred is the collapse of multistory buildings. Now constructing of heights round 20-30 ft weren't collapsed and as a substitute all of the tall constructing has collapsed which provides another signal of earthquake and that's low frequency waves. A taller constructing has larger time interval and a decrease frequency whereas a shorter constructing is precisely the other. Thus a low frequency wave can create a form of resonating impact which pumps increasingly more power into the constructing whereas a shorter constructing is doing all okay. This makes taller constructing to fall and crumble because it can not resist the speed at which the earthquake power is getting into into the system. Let me offer you an instance of resonance:
Allow us to take a look at one other constructing which failed:
This reveals shear failure in columns of the construction.
Now, as USGS defined that because it was a shallow earthquake, the depth of the earthquake may need magnified and it created extra harm that anticipated however why simply multistory buildings and never small constructions? For that we should wait however we all know that it has to do with the Tainan basin which could be a softer basin as it's a separate basin towards the remainder of the Taiwan.
We hope that every one the folks have been rescued and are protected now. We are going to get into extra depth as quickly as we get some extra evidences and extra correct knowledge.