Building codes for aseismic design

Building codes for aseismic design

Abstract: In this chapter, various building codes for seismic design are compared and the salient features are discussed. Various design examples have been carried out using IS1893-2002 Part 1. Using similar procedures, designs can be carried out using different codes.

Key words: capacity design, structural performance factor, zone factor, ductility, spectrum analysis.

Introduction

The purpose of building codes is to promote and protect public welfare, which includes health and safety of individual citizens as well as economic well-being of the community. This task is accomplished by the building codes by setting minimum standards for materials of construction that may be used for structures of different types of occupancies. Governments have the power to enforce these standards through the code adoption process in converting the code to a legal standard. If building codes were not specified in a unified manner, design and construction processes would vary widely and many structures would be unable to afford their occupants adequate protection against collapse.

Design loads are set by building codes at levels that have a moderate to low probability of earthquake occurrence during the life of the structure. Buildings may be designed for earthquake shaking likely to occur once every 500 years or wind load anticipated once in 100 years. Building code provisions for earthquakes are unique. They do not intend that structures be capable of resisting design loads within the elastic or near-elastic range of response, in that some level of damage is permitted.

The provisions governing design for earthquake resistance by building codes may be traced back as far as building regulations enacted in Lisbon, Portugal, following the earthquake of 1755. Early building code provisions for earthquake design focused on probability of certain type of construction. But modern codes supplement this prescribing requirement, with specifications of minimum permissible structural strength and stiffness. Although most developed countries develop and enforce their own building codes, the seismic provisions currently used throughout the world generally follow one of four basic models:

1.     NEHRP (National Earthquake Hazard Reduction Program – Recommended provisions developed by Building Science Safety Council in the USA (BSSC 1997).

2.     Building Standard of Japan.

3.     New Zealand Building Standard Code.

4.     Eurocode 8.

Although each individual code has many original requirements and provisions, in general all are based on similar concepts.

Historical development

1.     Phase I, termed as experimental basis. This phase consists of observations and behaviour of real structures in earthquakes and the development of prescriptive rules.

2.     Phase II termed as theoretical basis. It consists of the body of analytical and experimental research that has been developed over the years.

3.     Phase III termed as engineering judgement. This is based on the expertise of practising civil engineers.

The first modern code containing seismic provisions is generally acknowledged to be the first edition of the Uniform Building Code (UBC) published by Pacific Coast Building Officials in 1927 (PCBO 1927) following the 1925 Santa Barbara earthquake. The PCBO later became the International Conference of Building Officials (ICBO) and continue to publish UBC for another 70 years, the last edition being published in 1997. The seismic provisions of the UBC were based primarily on the SEAOC (Structural Engineers Association of California) recommendations and remained in a leadership role over the full 70 years.

The 1927 edition of UBC incorporates the lessons learned observing series of earthquakes in California during 1868–1925. Since there were no records of actual ground motion available in 1927 the selection of 10% distribution of lateral strength level must surely have been judgemental.

In the 1937 edition of UBC, the concept of differentiating seismic resistance by means of zonal maps was introduced. The first map divided the United States into three zones. Base shear can be given by the formula

where N is the number of stories. Short structures were designed for the most severe lateral forces equivalent to 10% of structure’s weight while the design force of taller structures can be reduced in proportion to the number of storeys.

The recommendations of Biot (1941, 1942) and, Housner (1959), and research recommendations were incorporated by SEAOC into the first edition of recommended lateral force requirements and commentary (SEAOC 1999) commonly known as the Blue Book and adopted in the 1958 UBC. Total lateral force is given by the formula

V = ZKCW                     --- --- 19.2

Z = zone coefficient related to seismicity (0 to 3) For zone 3 Z = 1

Zone 2 Z = 1/2

Zone 1 Z = 1/4

Zone 0 Z = 0 no earthquake requirement of seismicity K = structural system coefficient given by Table 19.1

C is the coefficient accounting for spectral amplification of ground motion given by

where T = fundamental mode natural period and W = total dead weight of the structure.

Once the base shear was determined, lateral forces were distributed to each level of structure proportional to mass supported at that level (assuming uniform distribution). Allowable stresses for load conditions containing earthquake were permitted to be increased by one-third relative to one of gravity load resistance. For 10 years after publication of the 1958 code seismic provisions remained stable.

The magnitude 6.6 earthquake that occurred on 9 February 1971 near Sylmar, California, was one of the most significant earthquakes of modern times. The SEAOC formed the Applied Technology Council (ATC) as a not-for-profit applied research agency and sought funding for earthquake engineering. In 1978 ATC published ATC-3-06 (1978) report in the development of seismic provisions.

In 1988 UBC were rewritten by SEAOC and some important recommendations were made:

•       Introduction of site factors to account for the effect of soils.

•       Introduction of occupancy importance factors.

•       A one-third increase in minimum design force level for all structures.

•       Introduction of inter-storey drifts.

•       Requirement to design anchorage for nonstructural components.

In 2000, three model building codes, UBC, BSSC, ASCE-7, served as the basis of building requirements in the United States. These three codes have now been replaced by a single code ‘International Building Code’ (IBC). The seismic provisions in IBC are transcribed from the 1997 edition of NEHRP provisions with some modifications.

The seismic provisions in the following codes will be discussed in the next sections (see Paz, 1994).

•       International Building Code USA–2000

•       New Zealand Standard NZS-1170-5

•       Eurocode 8

•       Uniform Building Code (UBC) 1997

•       National Building Code (NBC) of Canada 1995

•       Mexican Federal District Code (MFDC) 1993

•       Japanese Society of Civil Engineers (JSCE) 2000

•       Iranian code

•       Chinese code

•       Indian standard ‘Criteria for earthquake resistant design of structures’, IS 1893–2002