ABSTRACT:
This
paper focus on study of worldwide changes in seismic design codes after large
earthquakes especially in design ground motion. Kathmandu is moving towards one
of mega cities in Asia, so for urban development in future we have to consider
all perspectives of design methodologies to have resilient city not only
Kathmandu but also all other major cities. High rise buildings and major
lifeline structures within city and gateway to cities are very important to be
safe. Design methodologies are major factors of control for the disaster
resilient structures alongside with regulation during construction. Here we
discuss about consideration of phase for design ground motion for safer
structural design.
ABSTRACT: This paper focus on study of worldwide changes in seismic design codes after large earthquakes especially in design ground motion. Kathmandu is moving towards one of mega cities in Asia, so for urban development in future we have to consider all perspectives of design methodologies to have resilient city not only Kathmandu but also all other major cities. High rise buildings and major lifeline structures within city and gateway to cities are very important to be safe. Design methodologies are major factors of control for the disaster resilient structures alongside with regulation during construction. Here we discuss about consideration of phase for design ground motion for safer structural design.
Key
Words: Urban
development, seismic design, phase spectra, design ground motion
INTRODUCTION
Nepal
lies in active seismic zone, several minor tremors are feeling on eastern and
western part of country. Himalaya ranges where Indo-Australian and Tibetan
plates sub duct each other, which are the main source of large earthquake for
Nepal. The Bureau of Crisis Prevention and Recovery of United Nations
Development Program ranks Nepal in 11th for earthquake vulnerability,
in 2004 Report. From historical records Nepal has major catastrophes due to
earthquake within some interval of decades. Developing phase of Nepal is just
trying to move from benchmark, hence we have to consider for the future
possible quakes, while preparing design codes and its implementation. All
countries who had experienced large earthquakes, then they tend to change codal
provisions but they had already lagged socially and economically. Hence we have
to learn from other countries experiences and prepare to reduce future damages.
JSCE
and other research institutes are concerned to develop codes and also to make
technical policies in Japan, which is the most vulnerable country and also
having all possible structural facilities. They are even doing many researches
towards resilient structures. Other major countries in the development of
earthquake research are USA, some European countries, New Zealand etc. USA has
different codes and provisions by states, AASHTO (American Association of State
Highway and Transportation Officials) and Caltrans (California Department of
Transportation) are the leading organizations for research and preparing codes.
European countries designs have mostly guided by EU code. New Zealand is
another country for advancement of seismic design code. Other countries having
frequent occurrence of earthquake such as Indonesia, Chili, Philippines, Taiwan,
Iran etc. also practice for their own in developing phase.
Vulnerability
of buildings in Nepal with current design and construction practices with NBC
are high in comparison with ATC-40 and FEMA-356 .In
Nepal we are starting to develop our own design code, which is in its
preliminary stage so we mostly follow Indian code. For maturity of codes we
have to do several researches and also have to record performance of existing
structures. Nepal is developing country and has several mega projects in
pipeline; just build any project will not be healthy for future economy, if we
are not considering several factors that will affect life of the structure.
Development
phase in which, most of the people in rural and remote areas will migrate to
convenient cities and then planners will get challenges to facilitate the
population. Only the residential small buildings cannot host all population so
we have to move for construction of several high rise buildings and other
structures for management of space. We do not have to consider only residential
buildings for importance during design and planning within city; other life
line structures which are facilitating to the people such as road networks and
bridges, electric transmission lines, water supply conduits, hospitals etc.
also have to put in priority during planning and design phase. Especially after
large earthquake if such facilities will halt in operation for long time, then
death toll will increase rapidly.
WORLDWIDE TREND OF CHANGE IN DESIGN CODE
Japan is
one of the most earthquake occurring countries. It has many records of
earthquake disasters and also from those events they move ahead on preparing
earthquake resilient structures. Kanto earthquake of 1923, having estimated fatalities
of about 142,800, is the base to prepare seismic design code and they take
seismic coefficient of 0.2. Japan used seismic design as include as just one
chapter in building code but after 1995 Kobe earthquake, one most devastating
tremor cause collapse of several structures, they propose “second proposal on
earthquake resistance for civil engineering structures” which is an independent
document for seismic design criteria . In 1997
researchers propose for the design ground motions of each level of earthquake
and performance and place in codal provision, separate for highways and
railways. Japanese code mainly categorize for the level one and level 2
earthquakes consideration and having performance level of 1 to 3 in each,
depends on importance of structures.
San
Fernando earthquake 1971 is the milestone for seismic design codes development
in USA, for Caltrans. Later on Loma Prieta earthquake 1989 was second major
event to change in seismic design codes. In New Zealand Specially they prepare
separate codes for building in 1992 and for bridges in 1994 for seismic design.
After Christchurch earthquake 2010 they are now on the process to change the
design codes.
Indonesia
is another developing country facing severe damages by earthquake in high
frequency. They have their design codes on main islands from 1970, they are
also following the same trend as developed countries did in past for changing
in codes. West Sumatra has design response spectrum 0.1g for working stress
design later on 1987 they divide the zones in level 1 to 6. For one historical
city Padang they specify 0.28g and 0.36g for different part considering soil
condition. In 2002 after Sumatra earthquake they change code and taken as 0.8g
for the same city. During Padang earthquake M7.9, most of the buildings having
short period were collapsed that spectral acceleration 2.15g exceeds the design
spectra 0.8g (After this catastrophe they change codal provision to 1.9g for government
buildings and other important buildings within Padang city despite of some
researchers suggest for 0.5, 1.5 and 2.1g respectively to hard, medium and soft
soil.
