Designing for near-field earthquakes, using the NSCP 2001

If you are familiar with the NSCP 2001 earthquake loading provisions (ELPs), you know it is based on UBC 1997.  Some people think it is "copied" from UBC 1997, but copied "poorly."  It is not copied!  It is based on UBC 1997 but it had to be suited to "local" conditions.

Anyway some of the features of the NSCP 2001 ELPs that are in contrast with similar ones in the UBC 1997 are as follows:
  1. Near-source factors for distances less than 5 km are the same as that for a 5 km distance.  In the UBC 1997, 2 to 5 km distance would have higher near-source factors than for a 5 km distance; near-source factors for distances less than 2 km are the same as that for a 2 km distance.
  2. The definitions for seismic source types are based only on maximum magnitude.  In the UBC 1997, there are alternate criteria based on slip rate.
The two features above are intended to be used together, as the NSCP 2001 has been intended to be used as THE governing earthquake design code here in the Philippines.  Therefore, legally if you are designing here in the Philippines, you should use the NSCP 2001.

For one example building that is only 1 km away from the Valley Fault System (VFS) which is listed as Seismic Source Type A according to the NSCP 2001, the 1st mode spectral acceleration would be around 0.75g if using the NSCP 2001, the same as when the distance-to-source is 5 km.  If we used the UBC 1997, that is considering less than 2 km distance, but also considering the slip rate of the VFS listed in some literature as corresponding to Seismic Source Type B, the 1st mode spectral acceleration would also be around 0.75g.

This means that in this case wherein we have a site very near a "controversial" seismic source that is legally Type A, whereas some references suggest it is Type B, using the NSCP 2001 or the UBC 1997 should yield equivalent results.

The problem is, some structural designers trying to be "smart" and trying to suck up to their clients by "saving them money," would use the NSCP 2001 provision for near-source factors (i.e. in the case above, near-source factors equivalent to a 5 km distance) but the UBC 1997 for seismic source type definitions (i.e. in the case above, the VFS is classified as Type B).  The result is a 1st mode spectral acceleration of only around 0.56g.  This is a 25% reduction in the design seismic forces, and probably a total of around 10% to 15% reduction in structural construction costs.

Legally this should be a problem!  The designer does not fully comply with NSCP 2001 provisions!  But clients love this because a 10% savings in a $100 million project is $10 million - enough to build at least 500 low-income homes.

Clients do not know it, but they are actually owners of sub-standard buildings, at least legally.  The question is, are these buildings really sub-standard?

To answer that question, a seismic hazard analysis needs to be carried out.  Actually, a performance-based seismic structural evaluation could be carried out to further ascertain the actual building performance when the design earthquake event occurs.  The seismic hazard analysis is part of the performance-based evaluation.

Alternatively, the IBC could be used as design basis.  The IBC uses spectral acceleration maps which are based on a deterministic seismic hazard analysis.  Unfortunately there are none such maps yet available for the Philippines, although a number of consultants have carried out such seismic hazard analysis for specific project sites.

There are other near-field earthquake effects that need to be accounted for, namely directivity effects, vertical accelerations, surface fault ruptures, effects of weaker but more frequent earthquakes, and collapse prevention under the maximum credible earthquake.  A properly carried out performance-based evaluation would take these into consideration.

In the end, the question is, if a seismic hazard analysis is carried out, how will the 1st mode spectral acceleration (as a design comparison point) compare with the values listed above?  Is the building safe?  Has the designer really saved the client some money, or has he only delayed the cash outflow of the client (i.e. building repair/replacement costs in the future when the design earthquake occurs)?  Performance-based design could answer this question.  Or at least using the IBC ELPs.  For the specific case above, we previously came up with a 0.42g 1st mode spectral accelaration, and approximately 0.50g with directivity effects considered - less than the 0.56g assumed by the designer.  Other effects though are not yet included here.  Anyway, the designer may have been "correct for the wrong reasons."

I guess either way, it is still a win-win for the client, if saving money is a more important concern than structural safety.  Legally though, and professionally, the designer may have been lacking.

Nonetheless this again illustrates another justification why we should move from the UBC to the IBC as basis for updating the NSCP provisions.

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