Drift-Driven Design of Buildings

Mete Sozen's Works on Earthquake Engineering

Santiago Pujol, Ayhan Irfanoglu, Aishwarya Puranam

• Publisher: CRC Press
• Year: 2022
• ISBN:
  9781000574388
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This book summarizes the most essential concepts that every engineer designing a new building or evaluating an existing structure should consider in order to control the damage caused by drift (deformation) induced by earthquakes. It presents the work on earthquake engineering done by Dr. Mete Sozen and dozens of his collaborators and students over decades of experimentation, analysis, and reconnaissance. Many of the concepts produced through this work are integral part of earthquake engineering today. Nevertheless, the connection between the concepts in use today and the original sources is not always explained. Drift-Driven Design of Buildings summarizes Sozen's research, provides common language and notation from subject to subject, provides examples and supporting data, and adds historical context as well as class notes that were the result of Sozen's dedication to teaching. It distills reinforced concrete building design to resist earthquake demands to its essence in a way that no other available book does. The recommendations provided are not only essential but also of the utmost simplicity which is not the result of uninformed neglect of relevant parameters but rather the result of careful consideration and selection of parameters to retain only those that are most critical.

Features:

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• Provides the reader with a clear understanding of the essential features that control the seismic response of RC buildings

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• Describes a simple (perhaps the simplest) seismic design method available

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• Includes the underlying hard data to support and explain the methods described

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• Presents decades of work by one of the most prolific and brilliant civil engineers in the United States in the second half of the 20th century

Drift-Driven Design of Buildings serves as a useful guide for civil and structural engineering students for self-study or in-class learning, as well as instructors and practicing engineers.

Part I Earthquake Demand

1 General Description of Earthquake Demand

DOI: 10.1201/9781003281931-2

Bare Essentials

Be ready to expect surprises in every aspect of ground motion records.

Pay attention to the velocity and displacement of the ground, not to acceleration only. The zero-crossing rate is also informative and easy to obtain.

[Before reading this chapter, the reader who may not be familiar with the history of earthquake engineering and the nature of earthquakes may want to read the class notes in Chapters 17 and 18. In this chapter, it is assumed the reader knows why ground motion occurs and how earthquakes are measured. It looks instead at how the ground moves and how to quantify that motion in terms useful in the design of buildings.]

Before committing to one or more of the accepted definitions of earthquake demand for structural design, it is not a waste of time to stop and think what the best description of the demand could be. Is it a lateral force as in wind? Is it a one-dimensional base motion? Is it the acceleration, velocity, or displacement of the ground?

The answer is certainly not as obvious as it is for gravity-load demand. We should remember that buildings of many types were constructed for centuries, often with positive results, without an explicit definition of gravity-load demand. In the western world, the first known attempt at using mechanics for construction occurred in the 18th century in relation to the repair of St. Peter’s Dome. Three mathematicians (T. Leseur, F. Jacquier, and R. G. Boscovich) used virtual-work to understand and repair the problems with the cracking of the dome related to inadequacy of the transverse reinforcement.¹

The first formulation of a procedure for earthquake-resistant design was made by the Italian engineering community in response to the serious damages observed in buildings after the Messina Earthquake in 1908. Even though it was appreciated that the solution of the problem included dynamic-response analysis, it was thought that such an approach would be confusing for most members of the design profession. Therefore, it was decided that a lie would work better and elected to represent the demand by lateral forces at each floor level adding up to approximately W/12 (where W is the estimated total weight of the building) in each perpendicular horizontal direction. Considering that most if not all of the construction at that time did not exceed three stories, the lie worked better than the truth might have. In Century 21 it is still included in building codes.

1 Hermann Schlimme, “Construction Knowledge in Comparisons: Architects, Mathematicians and Natural Philosophers Discuss the Damage to St. Peter’s Dome in 1743,” Proceedings of the 2nd International Congress on Construction History, Cambridge 2006, pp. 2853–2867.

Given that background how should we approach the problem? The obvious fact is that building damage occurs because of the movements of the ground that shake the building. How does the ground move? We can separate the linear motion of the ground into three components, two perpendicular directions in the horizontal plane and one vertical. Of course, the ground could also rotate around those three axes. In each case, the motion can be expressed in terms of variation of displacement, velocity, and acceleration with time. We end up with 18 options.

Considering that the design of the building is completed before the earthquake occurs and the estimate of the earthquake can easily be off by 50% or even more, the choice of all 18 options would appear to be inconsistent with good engineering judgment that the ratio of result to related labor should be high.

At this point of the discussion, the reader ought to consider carefully what path should be taken to determine the earthquake demand and not necessarily follow the choices described below.

In trying to simplify the steps in the calculation process for design, first we consider the rotations about the three main axes. Admittedly based on limited evidence and projections of that evidence, we decide we can ignore the effect of the rotations in proportioning the structure. We do not deny the existence of such rotations, but we assume that their effects are not dominant issues for the safety of a structure. Then we consider the axial motion in the vertical direction. The structure is designed for gravity loading amplified by a factor of safety. For the usual building, we may ignore dynamic motion in the vertical direction. Can we consider the demands for the motions in each perpendicular horizontal direction independently? We take the risk and assume that we can do that but will consider combining their effects in special cases. In effect, we ignore science and take risks that we hope will be tolerable.

We still have the option to choose among ground displacement, velocity, or acceleration as the critical issue. We can guess that if the structure is extremely stiff, almost rigid, and attached so that it moves with the ground, we should use acceleration as the governing demand. But we are in no position to eliminate the velocity and displacement in all cases. We shall consider those issues in the next two chapters that deal with a design tool called “spectrum” in reference to the range of cases we may face.

In the following section, we shall consider a few examples of recorded ground motion. Our goal is not to develop a general description of earthquakes from this exercise but to get a perspective of the ranges of its possible variations. It is interesting to mention here that the profession started with the belief that the earthquake demand should be related to the maximum ground acceleration expected. In the 1940s, the choice for the peak ground acceleration (PGA) in design was approximately g/3, where g is the acceleration of gravity. In the 1960s, it was revised to g/2. During those years, the scientific community asserted that it would not exceed 1.0g. In 1971, the measured PGA exceeded that value. In 2011, the measured PGA in the horizontal direction reached 2.7g. That sequence of events should give us humility in choosing the earthquake demand for design [Chapter 17 offers a historical perspective on the human experience with earthquakes].

1.1 Four Examples of Recorded Ground Motion

As listed in Table 1.1, four different recorded ground motions were selected f...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Acknowledgments
• Authors
• Introduction
• Part I Earthquake Demand
• Part II Selected Works
• Part III Class Notes
• Conclusion
• Appendix 1: Does Strength Control?
• Appendix 2: Report on Drift
• Appendix 3: Richter on Magnitude
• Appendix 4: Review of Structural Dynamics
• References
• Index