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DESIGN OF TALL BUILDINGS IN

HIGH-SEISMIC REGIONS

MARSHALL LEW* MACTEC Engineering and Consulting, Inc., Los Angeles, California, USA

1. THE RESURGENCE OF TALL BUILDINGS AROUND THE WORLD

In the aftermath of the events of 11 September 2001, there was a feeling that tall buildings were a

relic of an age that had passed into oblivion because the destruction of the World Trade Center Towers

in New York exposed the USA and whole world to the new reality of terrorism on a massive scale

that had never been expected or experienced before. The World Trade Center had been a target of an

earlier terrorist attack on 26 February 1993 when a car bomb was detonated in the parking structure

beneath Tower One. The World Trade Center was targeted because it was a symbol of the USA andthe financial system that underpins the nation. In 2001, the terrorists succeeded on the second try with

the help of two airliners that crashed into the two towers. The events of September 11 brought a chill

on the future of tall buildings.

However, after some time of reflection, there is a new rush to be the tallest building in the world.

The Petronas Towers in Kuala Lumpur lost its title to the Taipei Financial Center (Taipei 101) (Figure

1). The Burj Dubai Tower in the country of Dubai promises to be 300 metres higher at a height of a

staggering 808 metres (about one-half-mile high) when completed in 2008 (Figure 2). Many super tall

buildings are being planned in the USA (New York, Chicago and Las Vegas), Russia, South Korea,

China, Taiwan, Germany, Kuwait and other countries. Time will tell if all of these buildings will actu-

ally be built, but the tall building has regained and even enhanced its position as a symbol of promi-

nence, prestige, wealth and accomplishment.

1. THE RESURGENCE OF TALL BUILDINGS IN THE USA

Although there is some discussion about changing the design of the proposed Freedom Tower to be

constructed on the site of the World Trade Center in New York City to be taller than the Burj Dubai

Tower, the resurgence in the interest to construct tall buildings in the USA is being driven more by

market forces and the reality of living in the urban environment. The value of time and the high cost

of gasoline may be part of the economic drivers that have sparked renewed interest in urban living

and a return to the central city or downtown areas of many cities, which is a reverse trend from living

in the suburbs as in the past. The convenience of having all of the services one needs in a single

building is now becoming a reality with mixed-use buildings; some of these buildings may also bring

the prospect of being able to live and work without leaving the building. Thus, the increased interestin tall buildings in the USA is due more to a utilitarian nature as the projects still must also be

economically viable.

Copyright © 2007 John Wiley & Sons, Ltd.

* Correspondence to: Marshall Lew, MACTEC Engineering and Consulting, Inc., 5628 E. Slauson Ave., Los Angeles, CA90040, USA. E-mail: [email protected]

THE STRUCTURAL DESIGN OF TALL AND SPECIAL BUILDINGSStruct. Design Tall Spec. Build. 16, 537–541 (2007)Published online in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/tal.436

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538 M. LEW

Copyright © 2007 John Wiley & Sons, Ltd. Struct. Design Tall Spec. Build. 16, 537–541 (2007) DOI: 10.1002/tal

Figure 1. Taipei Financial Center (aka Taipei 101)

Figure 2. Burj Dubai Tower (Skidmore, Owings & Merrill)

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DESIGN OF TALL BUILDINGS IN HIGH-SEISMIC REGIONS 539


1. THE RESURGENCE OF TALL BUILDINGS IN CALIFORNIA AND OTHER

HIGH-SEISMIC REGIONS

The state of California and the west coast of the USA have also seen a renewed interest in the devel-

opment of tall buildings to meet the new urban needs. Driven primarily by development in the resi-

dential market, many of the new tall buildings consist of condominiums and/or apartments. Some of these developments may have a mixed use, including residential, office and commercial development.

After nearly a decade of inactivity in tall building construction, there are many tall building projects

in construction or in planning in Los Angeles, San Francisco, San Diego and Seattle. Although the

numbers change quite often, it is believed that some 50 tall buildings are either planned or in construc-

tion in Los Angeles at the present time; there are about 30 tall buildings planned for San Francisco.

If most of these buildings are ultimately constructed, the skylines of these cities will be altered

significantly.

1. CHALLENGES IN THE DESIGN OF TALL BUILDINGS IN HIGH-SEISMIC AREAS

Many designers in the USA have found that there is a difficulty in designing tall buildings in

high-seismic regions using the present provisions of the current building codes, which are either

based on the International Building Code or the Uniform Building Code (in California). These

codes are very prescriptive by the general nature of all codes and have many restrictions on materials,

building systems, building heights and performance limits. There is a general feeling that the

codes are sufficient for the large majority of buildings that are not classified as tall buildings;

however, when it comes to tall buildings, the codes are not well suited to tall buildings in high-seismic

areas and are very restrictive. Some engineers even consider that the codes may even be

dangerous and may not meet the objective of providing a design that is safe. Certainly, from

experience, it appears that tall buildings designed in strict conformance to the codes are not

economical and, thus, the codes have become a deterrent to the development of this class of

construction.In response to this challenge, many designers have gone the route of designing tall buildings using

alternative procedures as allowed in the building codes. Alternative lateral-force procedures are defined

as procedures using rational analyses based on well-established principles of mechanics, which may

be used in lieu of those prescribed in the provisions of the building code. Using alternative procedures,

designers are using performance-based approaches that will enable construction using new framing

systems that extend to heights beyond the limitations that are in the prescriptive building code

provisions.

There has been an active debate, particularly in Los Angeles and San Francisco, regarding what

would be the acceptable alternative procedures and what basic code requirements must still be main-

tained to ensure the safety and sustainability of tall buildings following future earthquakes. In par-

ticular, there has been much discussion about attaining equivalent performance using performance-based

design methods. The Los Angeles Tall Buildings Structural Design Council is actively involved in thedevelopment of a new framework for performance-based design for tall buildings in Los Angeles

through sponsoring workshops, holding meetings presenting the latest information and research, and

development of the Council’s own criteria document. The Pacific Earthquake Engineering Research

Center (PEER) is undertaking the Tall Buildings Initiative to develop a framework for seismic design

of tall buildings in California and similar environments. The goal of the Tall Buildings Initiative is to

develop a guideline document containing principles and specific criteria for the seismic design of tall

buildings in the next 2 years.

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1. ISSUES RELATED TO EARTHQUAKE GROUND MOTIONS FOR THE DESIGN

OF TALL BUILDINGS IN HIGH-SEISMIC AREAS

It is very apparent that the specification of the earthquake ground motions is very important in the

process of designing a tall building. The development of the design criteria for performance-based

seismic design may include provisions to ensure that a tall building will not collapse under a very rare

event such as the maximum considered earthquake (MCE) ground motion (usually defined as those

ground motions having a 2% probability of being exceeded in 50 years, or having a return period of

about 2400 years).

The MCE ground motion is usually represented initially in the form of uniform hazard response

spectra that is the result of a probabilistic seismic hazard analysis (PSHA) that considers all of the

seismic sources in the surrounding region of the site, given the estimated activity of the sources and

the site characteristics for that given level of risk. As the performance-based seismic design will gen-

erally dictate that a nonlinear dynamic analysis should be performed for the tall building system, a

suite of acceleration-time histories is needed to represent the MCE ground motions.

As the MCE design spectra are estimated from the PSHA for a very long return period, the uncer-

tainty included in the analysis is large. First, there is an uncertainty coming from the reality that thereare very few recorded ground motions at close distances for large-magnitude events; thus, there is

little guidance to constrain ground motion attenuation relations that attempt to model ground motions

at these distances and magnitudes. Second, the long return period results in more uncertainty in the

events, and the dispersion of the results increases the ground motion estimates. Third, most current

ground motion attenuation relations do not extend to more than 2–5 s. As many of the new proposed

tall buildings are well over 30 storeys to over 70 storeys, the fundamental period may be as high as

8–10 s. Thus, there is even a greater uncertainty as to the characteristics of the spectral ordinates of

the ground motion spectra at these long periods. The PEER Next Generation Attenuation of Ground

Motions Project provides a promise in the development of attenuation relations that extend to longer

periods that are of most important interest in the design of tall buildings.

Current building codes have requirements that time histories must meet to be representative of the

ground motion design spectra. Generally, the response spectra of the time histories must match thedesign spectra within a range of structural periods that may be from 0·2 T to 1·5T , where T is

the fundamental period of the building. While many feel that it is best to use actual earthquake time

histories with minimal modification or scaling for the nonlinear dynamic structural analysis, it is

virtually impossible to find any set of time histories that can be matched to the design spectra by only

adjusting the amplitude uniformly at all time steps. There are several important reasons why this is

the case. First, a uniform hazard response spectra do not represent a single earthquake event; rather,

it represents a conglomeration of the contributions of every seismic source according to its activity

rates for the given seismic risk level. Thus, the design spectra are not likely to be representative of a

single earthquake with a given moment magnitude and distance. It is more likely to be representative

of several earthquakes with differing magnitudes and distances from the source to the site. In the Los

Angeles metropolitan area, it is possible that the shorter periods of the MCE design spectra are

dominated by a large local event with a moment magnitude in the range around 7, while the longerperiods could be dominated by a large earthquake event that are more distant. It is also possible that

the MCE design spectra are dominated by just local events, but may be due to multiple sources with

different characteristics, including the style of faulting. To try to capture all of these characteristics in

a single time history by just amplitude scaling is impossible. To further try to capture these charac-

teristics in a suite of at least seven time histories by just amplitude scaling is even more impossible.

There have been methodologies developed to modify actual time histories to match the design spectra

by either manipulation in the frequency domain or by adding packets of sine waves; however, ques-

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DESIGN OF TALL BUILDINGS IN HIGH-SEISMIC REGIONS 541


tions arise as to the validity of these manipulated time histories, as the energy content is greatly

modified and in most cases does not match the real world.

Even in the development of the uniform hazard design spectra, there are additional considerations

in high-seismic areas. Tall building sites near active seismic sources that now dictate that near-source

and directivity effects should be taken into account as part of the evolution of the state of practice.

There are still some practical considerations on how these effects should be accounted for in the context

of the PSHA analysis. If these effects are to be accounted for in the uniform hazard design spectra,

finding appropriate actual acceleration-time histories with these effects embedded becomes a further

challenge.

There are some feelings among practicing professionals in the development of ground motions that

there must be changes in the code prescriptions regarding the design ground motions having to conform

to the design spectra across the wide range of structural periods. Rather than attempting to find or

manufacture time histories that meet the spectral requirements, it may be more realistic and perhaps

even more desirable to use a set of time histories that model the design spectra over different ranges

of period so that all of the periods are represented by the spectral envelope of multiple time histories.

In this manner, it may be possible to incorporate special effects such as near-source and directivity

without modifying the recordings in a manner that may compromise the integrity of the record. Asthe design spectra may also have contributions from multiple seismic sources, time histories for record-

ings for events having different styles of faulting can be included. However, to be able to do this,

changes in the current building codes would be needed.

1. OTHER ISSUES IN THE DESIGN OF TALL BUILDINGS IN HIGH-SEISMIC AREAS

Ground motion issues and other issues, including structural and non-structural issues, will need to be

addressed in an expedient manner as there are many tall buildings being planned and in construction

at the present time. These buildings will likely still be in service into the next century and may be

subjected to severe ground motions not previously experienced by tall buildings before. The boom in

construction in tall buildings has pushed the envelope in many areas including seismic-resistant design.

The public’s expectation of the performance of tall buildings may be different from what structuralengineers had been believing to be expected performance (life safety). As the occupancy and the costs

of the construction go higher, so have been the expectations. The current re-examination of the design

of tall buildings is a healthy exercise in the evolution of the design process. It is hoped that some of

those remaining issues will begin to be addressed in this volume.

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