Seismic Codes

Seismic codes are regulations which have been developed with the scope to provide a first guideline to the engineer for earthquake resistant design of ordinary buildings. Common standard response spectra like the US building code or the EC 8 (see Fig. 5) account in some way for the seismicity at or in the neighborhood of a site and for the subsurface geological conditions. Standard response spectra are typically fixed by few parameters. In the acceleration response spectrum shown in Fig. 5 we note an ascending part up to a certain frequency followed by a more or less flat part up to a further limiting frequency. In high frequency range we note an asymptotical decay to a value ag which corresponds theoretically to the peak ground acceleration. In order to adjust the standard response spectrum to the individual conditions of the site the seismic regulations provide rules concerning the reference value ag and the form of the spectrum itself.

Firstly, the reference value ag is controlled by the seismicity of the zone. Its choice reflects the strength of the design earthquake, whose parameters may vary with respect to the importance assigned the structure. In the EC 8 ag is additionally modified by a soil parameter. Both the EC 8 and the US building code prescribe that the flat part of the spectrum is 2.5 times the value of ag.

The form of the spectrum, i. e., the position of the upper and lower boundary frequencies of the flat part of the spectrum, depend on the soil conditions. The 5% elastic pseudoacceleration response spectrum of the EC 8 (preliminary version [5])is constructed with the relations:

And 1 lett seismic code is the amplification of the response spectrum with respect to peak acceleration with a proposed value of 2.5. 2 lett seism code depends on the damping 3 structure response after

and is 1 for 3 structure respo = 5%. The soil classes A, B, C in the EC 8 (see Tab. 4.1) are defined with respect to their S-wave velocity and their lithological description. Roughly speaking class A represents material of relatively high velocities (800 m/s and higher) and a favorable lithology in the sense of stiffness, class B corresponds to an intermediate situation, class C stands for weak material and an S-wave velocity of 200 m/s or less in the uppermost 20 m. For more details with respect to the definition of soil classes see the original text of the EC 8.

In the US building code only formulae (4.18a) to (4.18c) are used. Its main difference resides in the parameter definition in the table. Similarly to the EC 8 there are 3 soil classes, defined with respect to the S-wave velocities and stiffness. Type 1 (see Tab. 4.2) corresponds to “Rock and stiff soils”, Type 2 to “deep cohesionless or stiff clay soils” and Type 3 represent “soft soil to medium clays and sands”. At a fist glance the US building code looks more conservative than the EC 8, particularly with respect to the treatment of unfavorable soil conditions. On the other hand in the EC 8 the definition of ag is left to the analyst whereas the US building code gives strict rules how to obtain ag. The values indicated in table for the EC 8, moreover, represent proposals which are subject to possible modifications by national authorities. The EC 8, reflecting the differing necessities of the EU members, may be understood in some way as a strategy of how to construct a norm spectrum rather than a rigid regulation as it is the US building code.

A more sophisticated rule is given by the seismic code of Japan. First of all it should be noted that the Japanese regulation is based on two families of four response spectra related to the soil categories “A” (“tertiary or older or rock”), “B” (“Dilluvium”), “C” ( . Contrary to the US building code the shape of the response spectra depends not only on the soil class but has to be adjusted also with respect to the seismic zone where the site belongs to (see Tab. 4.3 and 4.4). This renders it rather difficult to apply the Japanese regulation outside Japan, because one had to make sure that both soil conditions and seismotectonic setting are in agreement with the corresponding situation in Japan.

From a geophysical point of view, however, it seems to be the best founded one out of the three presented here. Note that in the Japanese code the amplification of the response with respect to peak ground acceleration is assumed to depend on the soil conditions. This is certainly realistic and can be easily checked carrying out simulations with SHAKYGROUND. It is also easy to understand that the shape of the response spectra should depend on the seismotectonic setting, in other words the type earthquakes occurring close to the site. The definition of the shape of the response spectra in the Japanese regulation differs particularly in the ascending part. First of all, the spectra are flat between Tn = 0 and 0.05 and has a value corresponding to ag * S. Beyond the period of 0.05 s the spectra can be calculated after

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[5] The actual version of the EC 8 has been accepted by the responsible institutions of the EU, but is still subject to appeal.

[6] Besides the parameter S all values in Tab. 4.3 and 4.4 have been picked from the graphical representation of the code which may introduce a slight degree of inexactness during the calculation of the response spectra with the formulae 4.19