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To follow up with my previous question on the seismic design of buildings, I would like to ask if it makes sense to support buildings with strong thick 'crisscrossed' metallic frames.

I have the impression that an 'X-structured design' provides good structural integrity, as commonly seen in construction sites. Would it be wise to construct buildings out of a metallic 'skeleton' that is formed throughout by intersecting the diagonals of squares?

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earth quake is basically random vibration of the earth in several different ways, S wave, P wave etc. which is a complex and different subject.

X bracing is a way of making the structure stiff, so in short buildings combined with other measures is mostly an effective method.

But in high risers and tall building the critical thing is to mitigate the huge shears that X bracing imparts on the foundation and also make sure the building structure vibration caused the earth quake does not resonate with the vibration of site.

So in tall buildings the strategy is to design the building for earth quake and wind fluttering in a way that the natural frequency of the building is not close to the frequency of earthquake or wind turbulence.

That is why in tall buildings they sometimes use moment frame as opposed to X bracing. These are classified as passive methods.

In some critical structures they insulate the building from the foundation by using rollers and shock absorbers.

Or in some new projects the have active smart earthquake resisting computer controlled hydraulic jacks and braces, which make the building sway, opposing and breaking the impulse of the earthquake.

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  • $\begingroup$ "Stiff" is not the same as "strong". In fact most building designs used by ethnic people living in earthquake zones are very flexible, compared with modern technology - an earthquake might cause minor damage, but not enough to make the structure useless until it was repaired or completely rebuilt. $\endgroup$ – alephzero Jul 23 '19 at 0:17
  • $\begingroup$ @alephzero, I use stiff as a "not desired characteristic" . Stiff building would not have flexibility to dance with the shaking of the ground, so a sudden 2g acceleration can easily tear 1.5 inch thick steel base plates due to large shear concentration, as I observed in the jobs I was doing trying to rehab to the extent possible, in the aftermath of the Northridge 1994 earthquake. That earthquake lead to major changes in the codes for new construction and extensive "order to comply" for the rehab of existing buildings especially under-reinforce masonry and building with soft story garages. $\endgroup$ – kamran Jul 23 '19 at 0:37
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This is indeed commonly done to perform earthquake structural retrofits on buildings which do not meet current code and which are too valuable to tear down and completely replace. It is also common to insertut huge shock absorbers into the X-bracing structure to dissipate the earthquake energy.

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  • $\begingroup$ So has the problem of earthquakes been completely solved today? Was it solved only recently? $\endgroup$ – Chong Lip Phang Jul 21 '19 at 16:29
  • $\begingroup$ this is a problem which can only be solved by successively revising the building codes based on actual earthquake experience. The Japanese are the world leaders in this science. $\endgroup$ – niels nielsen Jul 21 '19 at 20:59

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