Comparative Study on Standard No. 2800 Design Spectra with Derived Uniform Hazard Spectra for Tehran Megacity

Design spectrums are used to compute seismic demand in structural design process. Therefore, seismic design
codes try to provide safety margin in performance of structures by developing appropriate design spectra. The
design spectra in Iranian seismic design code (Standard No. 2800) were developed via the analysis of different
earthquake response spectra. In Standard 2800, a mapped PGA value for 10% in 50 years used as an anchor point to
produce the design spectrum at a desired point. Meanwhile, this approach fails to provide a quantitative safety
margin as claimed in design code. Furthermore, their accuracy have not yet been examined by conventional
approach such as uniform hazard spectra (UHS), which is conventionally employed by many design codes such as
Eurocode 8 (1998). Spectral ordinates of UHS have equal probability of exceedance (PE) across all periods of
vibration. Therefore, designed structures with recent design spectrums have the same imposed seismic hazard level.
An attempt was made in this study to directly examine the accuracy of proposed design spectra in Standard 2800
by those of UHS derived in Tehran Megacity as a case study. For this purpose, the area under study is divided into
632 grids with a 1 km × 1 km area. The hazard curve was calculated by considering the soil type at each grid point
as defined in Standard 2800 from hard to soft (I to IV). The number of grids fall at each soil type are 10, 465, 150
and 7 points for I, II, III and IV soil types, respectively. Hazard curves at the grid points were calculated and were
used to compute uniform hazard spectra for 2% and 10% PE in 50 years (return periods of 2475 and 475 years,
respectively). For each soil type, the mean of uniform hazard spectra at different grid points with the same soil type
were calculated, which were used as a representative uniform hazard spectrum for that soil type. The derived
representative uniform hazard spectra for 10% PE in 50 years were compared with those proposed in Standard 2800
for different soil types. Furthermore, spectral acceleration ratio of uniform hazard with respect to those from
Standard 2800 at periods of 0.2 s and 1.0 s were computed and were compared for the whole points according to
their soil types. The outcomes reveal that the spectral acceleration in design spectra of Standard 2800 are larger than
those in UHS for all the periods, especially for the periods larger than 0.5 s for soil types I and II. Meanwhile, the
spectral accelerations of Standard 2800 become almost the same at periods lower than 0.5 s, and again become
larger than UHS at periods larger than 0.5 s for soil types III and IV. In general, it is found that the proposed design
spectra in Standard 2800 are conservative. This may need further examine by more recent approach such as uniform
risk, and then being considered in future revision of Standard 2800.