Umesh Puri
Nepal is one of the most seismically active countries. It ranks the 11th most vulnerable country to earthquakes in the world, and Kathmandu is the most at-risk city. Nepal has witnessed several devastating prominent earthquakes in the past of magnitude greater than 7.5 (in 1255, 1408, 1505, 1833, 1934 and 2015). Not to mention, Nepal has also witnessed earthquakes of lesser magnitudes in 1980, 1988 and 1994.
During the 2015 Gorkha earthquake, Nepal suffered about 9,000 casualties and 22,000 injuries. This caused Nepal an economic loss of about $7 billion, half of which was contributed by the housing sector damage. This was almost 50% of the GDP of Nepal. Although it was a severe blow to the Nepal economy, on the positive side, Nepal could take this as an opportunity to brace itself for any future earthquake.
Countries of high seismicity like Japan, USA, and Chile always update their building code frequently pertinent to the latest findings in earthquake and structural engineering. Present code and standard shows that countries with the highest level of seismic hazard have the most robust and up-to-date building code to their credit. Unfortunately, that is not the case for Nepal.
Nepal’s Building Code History
Despite being a seismically active country, Nepal was deprived of any regulations or documents of its own setting out either requirements or good practice for achieving satisfactory strength in buildings for so long. As a result, housing construction in Nepal was replete with haphazard construction. The first attempt for development of any code for seismic design in Nepal only started in the nineties following the Udayapur earthquake of 1988.
The Udayapur earthquake and the resulting deaths and damage to both housing and schools, again drew sharp attention to the need for changes and improvement in current building construction and design methods. In 1988, the Ministry of Housing and Physical Planning (MHPP), with the help from the United Nations Centre for Human Settlements (UNCHS) drafted the Nepal National Building Code and ultimately promulgated in 1994 as the first official Building Code of Nepal (widely known as NBC 205: 1994).
Since then, the Nepal Building Code was neither updated nor strictly enforced in the country. Mostly, countries of high seismicity update their code every three years.
The Department of Urban Development and Building Construction (DUDBC), Ministry of Urban Development (MoUD) had initiated the preparation for the updating of the NBC 105: 1994 Seismic Design of Buildings in Nepal, however, the process was formally started only after the Gorkha Earthquake and the following aftershocks.
The process of updating the NBC 105: 1994 Seismic Design of Buildings in Nepal was started under the initiative of the Central Level Project Implementation Unit (CLPIU) of the ADB financed Earthquake Emergency Assistance Project (EEAP) under the Ministry of Urban Development (MoUD).
The revision of the NBC 105: 1994 as the major code guiding seismic design of buildings in Nepal is one of the important activities to be implemented by the EEAP. The first revision of the NBC 105: 1994 Seismic Design of Buildings in Nepal was prepared by CLPIU and later transferred to the purview of the National Reconstruction Authority (NRA), the Government of Nepal.
In August 2020, the Council of Ministers approved the updated standard, the NBC 105:2020, which would replace the earlier 1994 edition of the NBC 105 Seismic Design of Buildings Code in Nepal. The NBC 105:2020 code is the current official Building Code standard of Nepal. It is said that the updated NBC provides guidelines to prepare engineering design of all types of buildings to make them earthquake-resistant through the use of various construction materials. However, this standard has so many shortcomings and limitations compared to the prevailing design standard in earthquake-resistant buildings practiced worldwide.
NBC 105:2020 Code:Limitations
Seismic design provisions for building and non-building structures have undergone substantial changes over the past half century. It began more than a hundred years ago, when static analysis with lateral loads of about 10% of the weight of the structure was adopted in seismic regulations. For a long time, seismic loads of this size remained in the majority of seismic codes worldwide.
After the 1940 El Centro earthquake in the US, engineers quantified this lateral load being inversely proportional to the natural period of a structure. Through revisions over time, a very compressive seismic code was proposed in 1994 called the Uniform Building Code (UBC 1994). Being a very comprehensive code, the UBC code was adopted throughout the world as a model code.
The UBC 1994 code was later updated in 1997 (UBC 1997) and used until 1999. However, seismic design philosophy was radically changed with the promulgation of ASCE 7 and the International Building Code (IBC) in 2000 making the UBC code obsolete. With minor modifications every three years, the IBC design methodology is currently in use since 2000, and the current code in practice is the IBC 2021. Since its adoption in the US in 2000, many countries around the world have modified their building codes to adapt to the IBC code.
One of the major limitations of the NBC 105:2020 code is that it still follows the UBC 1997 approach for seismic design. For example, the NBC code uses the old UBC 1997 approach to calculate the base shear needed for the equivalent lateral force procedure of seismic design.
This base shear calculation is based on seismic zone defined by a hazard level parameter called the Peak Ground Acceleration (PGA) of an area. This is the UBC 1997 way of characterizing seismic hazard level. Since the IBC 2000, seismic design philosophy has shifted from the PGA seismic zone-based approach to a new approach called the spectral acceleration distribution-based approach.
In this new approach, there are two hazard parameters, short period spectral (0.2 sec) acceleration (Ss) and one second spectral acceleration(S1). These two parameters come from the Probabilistic Seismic Hazard Assessment (PSHA) of a region. Many researchers and practicing engineers around the world have unanimously decided that the spectral acceleration is a better hazard indicator than the seismic zone.
In plain language, the PGA tells us the peak ground acceleration that the ground will experience whereas the spectral acceleration tells us the peak response experienced by a single degree of freedom (SDOF) structure with a given time period and a specified damping ratio. So, the spectral acceleration is far more meaningful than PGA. Moreover, in the IBC 2009 and onwards, the risk level for Ss and S1 has even been elevated to the risk-targeted seismic hazard, which is 1% probability of exceedance in 50 years.
Another limitation of the NBC code is that there is no mention of seismic design category. In the IBC approach, before starting off the seismic design of a structure, it first assigns a seismic design category to the structure. There are six design categories: A-F. Design category A entails minimum seismic risk whereas design category F entails high
seismic risk. Seismic design category is decided from two hazard parameters (Ss and S1) and occupancy category. The seismic design category determines:
Permissible structural system allowed.
Limitation on height and irregularity allowed. Certain vertical irregularities are not allowed in seismic design category E and F.
Detailing requirement needed.
Those components of the structure that must be designed for seismic loads, and types of analysis required.
Load combination. For category C, D, E and F, lateral force must be applied 100% and 30% in the orthogonal directions simultaneously.
Minimum and maximum base shear
Minimum and maximum diaphragm force and diaphragm design (chord, collectors)
Connection design for steel structures.
Redundancy check.
Torsional stability and torsional amplification Wind force design.
In Nepal, many masonry buildings are constructed with walls made of sun-dried/fired bricks or stone with mud mortar, and the building frame is made of wood. These types of buildings generally have flexible floors and roofs and are prevalent in rural areas. In the Gorkha earthquake, many masonry constructions failed mainly due to a lack of wall anchorage to the floor, shear and joint failure, load path discontinuity; the provision of which the NBC code clearly lacked.
Latest Research
To tap into the latest development in research and practice, it is very prudent to turn to the IBC code. Culminated through research/development and practice over the years, the IBC code is a very comprehensive design code, which addresses all materials, concrete, steel, masonry, precast/pre-stressed, timber, metal, and all loading conditions. For example, ACI 318 for concrete, ANSI/AISC 341 for steel, NDS for wood, ASCE 7 for minimum loads, etc. Not following the latest design methodology, the NBC code has seriously constricted itself in its scope and applicability.
Last but not least, Nepal needs code provisions for seismic design of non-building structures, non-structural components, retrofitting guidelines for existing non-engineered constructions, seismic safety/damage assessment, diaphragm design (rigid, semi-rigid, and flexible), etc. Currently, due to the absence of these provisions in the NBC code, Nepali engineers must turn to foreign codes willy-nilly.
The IBC code is developed and written by the “International Code Council” (ICC). The ICC is made up of code and building officials, engineers, firefighters, builders, designers, architects, and anyone who wants to be involved from the US. The code development procedure is very dynamic.
Every three years the IBC is revised or updated to allow for new materials, technologies, products and to correct issues that have been problematic or dangerous in the past. The IBC is the code that most countries around the world use as a model code to revise their local codes. Unfortunately, the NBC code is based on the old UBC code; so, it is not possible for Nepali engineers to utilize current state-of-the-art design methodology adopted in the IBC code.
Vision for Seismic Design Code
Seismic design philosophy has evolved over time. Early constructors believed in the Stiffness-Based Design. The common conception was that earthquake can be countered by constructing a stiff and massive structure. These constructions were more intuitive and used no engineering. Their primary focus on structures was having large stiffness and mass. As a result, we would see lots of old buildings and cultural heritage sites that are full of such massive construction.
For these structures, although stiffness considerably reduces their deformation, but the bulky mass was a liability. We have seen such construction perform very poorly during earthquakes. One recent example is the destruction of cultural icons in Basantapur and Sundhara, Kathmandu in the Gorkha earthquake.
The more rational and engineered way of seismic design introduced in the 19th century was the Force-Based Design. This philosophy rests on the fact that the structure must have enough design capacity to meet the imposed demand force. This design philosophy disregarded ductility and overstrength of material. All structural members were essentially designed to remain elastic under any lateral loads.
In the late 19th century, the Capacity-Based Design was introduced to complement the Forced-Based Design. The Capacity-Based Design is a design process in which, in design phase, it is decided which members within a structural system will be permitted to yield (ductile members) and which members will remain elastic (brittle members). This is the current design philosophy of seismic design of structures around the world. As for Nepal, the NBC 105:2020 code introduces the Capacity-Based Design; however, it needs to be improved and expanded.
An alternative design procedure other than the code-prescribed procedure in use now is the Performance-Based Design. It is a design procedure for the achievement of specified results rather than adherence to particular technologies or prescribed means. Strangely enough, the NBC 105:2020 code does not mention anything about this procedure.
Some new seismic design philosophies are under research and development. The most buzz of the time is the Direct Displacement-Based Design proposed by Prof. M.J. Nigel Preistley. The Energy-Based Design, which is now still under research, is another promising philosophy which is supposed to be even better than the Direct Displacement-Based Design.
To sum up the vision for the seismic design code of Nepal, the Performance-Based Design, the Direct Displacement-Based Design, and the Energy-Based Design should be the likely future codes of practice. Yet, in the short term, improving the NBC 105:2020 to catapult it at par with the IBC code should be the top priority.
Roadmap For Earthquake Resilienc
The literal meaning of resilience is the ability to not only survive but to also thrive in the face of adversity. In this case the adversity is an earthquake. Earthquake resilience involves both pre-disaster mitigation (activities to reduce the amount of loss in an event) and the ability to mute post-event losses and rapidly recover from an event. There are four areas where we could improve to achieve earthquake resilience in Nepal.
Research and academics: Accurate characterization of seismic hazard Earthquake-resistant design of structures
Building codes, standards and enforcement mechanism: Development of improved guidelines and standards
Enforcement of codes using local regulations and bylaws
Prevailing state of field practice and design: Capacity building and training of local structural engineers for seismic analysis of structures
Public education, awareness and role of media : Magazine articles and other forms of print media. Development of awareness content through digital media
Quake Hazard Mitigation
German philosopher Friedrich Nietzsche once said, “What doesn’t kill you, makes you stronger.” Those of us who survived the Gorkha earthquake have to rise stronger, make our building code up-to-date, and strictly enforce it nationally. An up-to-date building code that reflects the latest development in science and technology is the first step towards a seismic building construction practice in the country.
Being a highly seismically active country, Nepal could learn a lot from earthquakes and be an example with state-of-the-art building code and construction practice just like other seismically active countries in the world. Let’s wake up, and be abreast of our earthquake hazard and apply mitigation measures before Mother Nature teaches us another hard lesson.
Source : TRN,
