US-style seismic codes are needed in Europe still say Zygmunt Lubkowski and Xiaonian Duan.

The loss of 44 lives in Turkey in February 2002 was a stark reminder that much of Europe is still vulnerable to earthquakes and poor design standards. Fortunately, help is on the way in the form of a comprehensive new European design code that looks set to become one of the world's leading earthquake codes. However, there are still some situations in Europe where US codes could be more appropriate.

The final version of Eurocode 8, the proposed European standard for ensuring the earthquake resistance of structures, is due to be issued in the near future. Along with the other nine Eurocodes, it will soon replace existing national standards in all 15 European Union (EU) states - making it one of the world's leading seismic design codes.

This is significant for the world's insurance and construction industries in that, for the first time, most new buildings and structures in western Europe (and other countries around the world with European-based design standards) will be designed to resist earthquakes in a consistent way. The code is similar in many respects to the provisions of the US Uniform Building Code but there are also some significant differences in approach. Like the US code, Eurocode 8 adopts a force-based (rather than performance-based or deformation-based) design philosophy. It also has a `one-level' design procedure, requiring that a structure - even though it may be damaged beyond economical repair - must not collapse and cause loss of life. There are some basic deformation limits to minimise the level of structural damage, but clients or insurers which require better seismic performance to limit overall property losses should discuss this with their design engineers.

In contrast, the seismic design codes in Japan, New Zealand and China adopt a `two-level' design procedure, in which both the life-safety objective under a rare earthquake and the damage-limitation objective under a more frequent earthquake must be satisfied explicitly.

European earthquake map
The code requires structures to be designed to resist an earthquake with a 10% chance of exceedence in 50 years, otherwise known as a 475-year return period. Each EU member state is responsible for defining an appropriate seismic hazard map - which could possibly be incorporated into a single seismic zoning map (e.g. Figure 1 taken from GSHAP on previous page). The intent of Eurocode 8 is that areas with a design ground acceleration less than 0.1g (approx. 1m/s2) are treated as regions of low seismicity, and simplified design procedures can be implemented. For areas where the design ground acceleration is less than 0.05g (approx. 0.5m/s2), the provisions of Eurocode 8 do not need to be observed.

Soil effects, known as site response, have been found to be a contributory factor to damage patterns in many earthquakes, such as 1985 Mexico City and 1989 Loma Prieta, California. Eurocode 8 uses a fairly simple classification system to account for this, using four main soil classes, compared to five in the US code.

US approach
Although the European approach accounts for soil non-linearity, it fails to account for amplitude dependence. Figure 2 on the previous page, derived from the US Uniform Building Code, indicates how site response can vary with different values of effective peak acceleration and soil classes. This clearly shows that the horizontal earthquake force (base shear) on a soft-soil site in a zone of moderate seismicity is equivalent to that on a rock site in a region of high seismicity.

The implication is that Eurocode 8 may over-estimate loads on soft soils in regions of high seismicity. More significantly, it may under-estimate loads on soft soils in regions of low-to-moderate seismicity.

For reinforced concrete and masonry buildings, Eurocode 8 explicitly requires that the uncracked cross-section stiffness be used in seismic analysis. It should be noted that this modelling approach tends to overestimate the seismic forces but underestimate the seismic movements.

Recent revisions of the US Uniform Building Code and the US International Building Code explicitly require that if cracking is expected under the design earthquake, cracked cross-section stiffness values are more appropriate.

Ductile detailing and retrofit
To ensure life safety but allow severe structural damage under the design earthquake action places great emphasis on ensuring suitable ductile detailing. The importance of ductile detailing cannot be over-emphasised given the uncertainties involved in determining seismic hazard and ground-motion input parameters, structural modelling and analysis and the estimation of member strength. The images of building damage (see photos left) from the 1999 Koceali earthquake in Turkey indicate the type of failure that occurs when detailing rules are ignored.

Unlike the other Eurocodes, Eurocode 8 covers the evaluation, strengthening and repair of existing buildings, reflecting the importance of seismic evaluation and retrofit of existing structures. But in using a force-based design approach for retrofit, it differs from the US philosophy of performance and displacement-based design.

FEMA 356 and ATC 40 both require retrofit engineers to quantify the ductility capacities of existing structural components based on laboratory component test data, and then assess the seismic deformation demands using a displacement-based methodology. In conclusion, the new European seismic design code is to be welcomed for at last providing a consistent, pan-European approach for designing all types of buildings and structures to resist earthquakes. However, for soft soils, for retrofit and for tall structures, the US codes are arguably more appropriate.

References
UBC Uniform Building Code - Volume 2 Structural Engineering Design Provisions International Conference of Building Officials, California, April 1997.

GSHAP Global Seismic Hazard Assessment Program, http://seismo.ethz.ch/gshap, 1999.

FEMA 356 Prestandard and Commentary for Seismic Rehabilitation of Buildings, Federal Emergency Management Agency, Washington DC, November 2000.

ATC 40 Seismic Evaluation and Retrofit of Concrete Buildings - Volume 1, Applied Technology Council, California, November 1996.

IBC International Building Code International Code Council, California, March 2000.

Bridges, towers and other structures
Bridges: Eurocode 8 also provides seismic design rules for bridges which are broadly similar to those provisions for buildings. Bridges are classified into three importance categories for seismic design purposes, based on the need for the bridge to maintain emergency communications after the design seismic event.

Tall and slender structures: It also covers seismic design of tall and slender structures such as towers, masts and chimneys. Under seismic action, tall and slender structures are required to behave in a ductile manner or remain essentially elastic. The seismic response of such structures is particularly sensitive to the long-period ground movements. However, compared to the US Uniform Building Code, there is no lower limit to these ground movements. Furthermore, Eurocode 8 gives significantly lower movement values at long periods.

Silos, tanks, pipelines etc.: Unlike the Uniform Building Code and International Building Code 5, Eurocode 8 also provides additional principles and rules specific for the seismic design of silos, tanks, pipelines, foundations, retaining structures and geotechnical aspects.

By Zygmunt Lubkowski and Xiaonian Duan

Zygmunt Lubkowski is an Associate in the seismic engineering group of Arup Geotechnics and Xiaonian Duan is Senior Seismic Specialist in Arup's advanced technology group. Both are based in the UK.

Tel: +44 (0)20 7755 3205;
email: zygi.lubkowski@arup.com