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Is SMRSF permitted in Seismic Zone 2?

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Rizwan Mirza, P.E.
Lahore, Pakistan
amirr(--nospam--at)paknet1.ptc.pk

I request opinion on the following issue:

I feel that it is not the intent of the Uniform Building Code (UBC), 1994 Edition, that SMRSF be used for buildings located in the zone of moderate seismic-risk (Zone 2). Is my view correct? I rely, inter alia, on the following arguments:

(a) CODE WORDING. I begin with quoting the most direct sections of the code, in this regard:

Section 1631.2.7 of UBC 1994 states that Concrete frame members required to be a part of the lateral-force resisting system "In Seismic Zone 2 ... , shall as a minimum, be intermediate moment resisting frames"

Section 1921.2.1.3 of UBC 1994 states that "In Seismic Zone 2, reinforced concrete frames resisting forces induced by earthquake motions shall be intermediate moment resisting frames..."

If we consider IMRSF to be the only frame permitted in Zone 2, the apparent conflict of the two sections is resolved. If we consider IMRSF as a minimum requirement only, we violate Section 1921.2.1.3.

(b) SUBSTANTIAL INELASTIC ACTION NOT CONTEMPLATED IN ZONE 2. The high Rw factor associated with SMRSF clearly suggests greater reliance on ductility (inelastic action). It appears to be the very reason that the code provisions relating to the requirements of special confinement details are located in the sections of the code (Sections 1921.2 through 1921.7) that are applicable to high seismic-risk zones (Zones 3 and 4), as stated by the code in section 1921.2.1.4. Otherwise, how would someone choosing SMRSF in Zone 2, enter the relevant section of the code for provisions relating to confinement?

(c) SECTIONS RELATING TO MEMBERS NOT PART OF LATERAL FORCE RESISTING SYSTEM. The code does not require the most stringent confinement details for "members not part of the lateral load resisting system". It does not appear plausible that the code intends the engineer to pick SMRSF (a system relying heavily on inealstic action, for which large force reduction is allowed, through a higher Rw) for a structure falling in Zone 2 and then avail the facility of not meeting the requirement of the stringent special details for "members not part of the lateral load resisting system". After all, is it not true that the said confinement details are required because of the degree by which reliance is placed on inelastc action, and not for any other reason.

(d) SERVICEABILITY. The selection of lateral load, in earthquake resistant reinforced concrete design, is a trade-off between strength and ductility. Greater the reliance on ductility, greater would be cost in terms of impaired serviceability in the event of strong ground-shaking. It does not appear plausible that the trade-off is to be made by each individual structural engineer, on a case-to-case basis. It is more plausible to believe that the fact that the code restricts you to certain type of moment resisting frame in Zone 2, clearly reflects the code policy that, within a given zone, each design should be made on the basis of a uniform level of serviceability.

I live and work in Pakistan with very little help in this regard. I have written to ICBO but have not received any response, so far. Would one of you be so kind as to educate me on this issue?.

Is there a way to obtain official opinion from ICBO, in this regard?


Sincerely,


Rizwan Mirza, P.E.
Lahore, Pakistan
amirr(--nospam--at)paknet1.ptc.pk