Nonbuilding Structures Seismic Design Code ...

EARTHQUAKE SPECTRA The Professional Journal of the Earthquake Engineering Research Institute THEME ISSUE: SEISMIC DESIGN PROVISIONS AND GUIDELINES

CONTENTS VOLUME 16 • NUMBER I • FEBRUARY 2000

FOREWORD

V

PREFACE

vii

THEME ]SSUE EDITORIAL REVIEW BOARD

XV

MANUSCRIPTS

USGS National Seismic Hazard Maps

1

A. D. Frankel, C. S. Mueller, T. P Barnhard, E. V. Leyendecker, R. L. Wesson, S. C. Harmsen, F. W Klein, D. M. Perkins, N. C. Dickman, S. L. Hanson, and M. G. Hopper

Development of Maximum Considered Earthquake Ground Motion Maps

21

Edgar V. Leyendecker, R. Joe Hunt, Arthur D. Frankel, and Kenneth S. Rukstales

New Site Coefficients and Site Classification System Used in Recent Building Seismic Code Provisions

41

R. Dobry, R. D. Borcherdt, C. B. Crouse, I. M. Idriss, W B. Joyner, G. R. Martin, M. S. Power, E. E. Rinne, and R. B: Seed

Active Fault Near-Source Zones Within and Bordering the State of California for the 1997 Uniform Building Code

69

Mark D. Petersen, Tousson R. Toppozada, Tianqing Cao, Chris H. Cramer, Michael S. Reichle, and William A. Bryant

The Seismic Provisions of the 1997 Uniform Building Code

85

Robert E. Bachman and David R. Bonneville

The 1997 NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures

101

William T. Holmes

Model Code Design Force Provisions for Elements of Structures and Nonstructural Components

115

Richard M. Drake and Leo J Bragagnolo

Nonbuilding Structures Seismic Design Code Developments Harold 0. Sprague and Nicholas A. Legatos

iii

127

Nonbuilding Structures Seismic Design Code Developments Harold 0. Sprague,

M.EERI,

and Nicholas A. Legatos,

M.EERI

The building code development process has traditionally given little effort to developing the seismic design process of nonbuilding structures. This has created some unique problems and challenges for the structural engineers that design these types of structures. The intended seismic performance requirements for "building" design are based on life safety and collapse prevention. Structural elements in buildings are allowed to yield as a method of seismic energy dissipation. The seismic performance of nonbuilding structures varies depending on the specific type of nonbuilding structure. Nonlinear behavior in some nonbuilding structures is unacceptable while other nonbuilding structures may be allowed to yield during an earthquake. Nonbuilding structures comprise a vast myriad of structures constructed of all types of materials, with markedly different dynamic characteristics, and with a wide range of performance requirements. This paper .discusses the development of codes, design practices, and future of the seismic design criteria for nonbuilding structures. INTRODUCTION

Nonbuilding structures are a general category of structures that are distinct from buildings. Key features that differentiate nonbuilding structures from buildings include human occupancy, function, dynamic response, and risk to society. Human occupancy, which is incidental to most nonbuilding structures, is the primary purpose of most buildings. The primary purpose and function of nonbuilding structures can be incidental to society or the purpose and function can be critical for society. This paper will examine some of the history, discuss the current status, and develop recommendations for future code development and seismic structural design criteria for nonbuilding structures. The development of seismic structural design criteria for nonbuilding structures has traditionally been a small part of building code development. But the building code is often the only tool that the structural engineer has had to predict seismic behavior of nonbuilding structures. The historic lack of guidance within the building code has created problems that become apparent for the designers of nonbuilding structures when compliance to a building code is required by an owner or building official. Some building code provisions are not applicable to nonbuilding structures, and impose unintended consequences when compliance is forced. These consequences have resulted in confusion, delays, unnecessary structural features, and in some cases designs that were not conservative. Some of the commonly referenced building codes include the 1997 Uniform Building Code (UBC), the 1999 BOCA (Building Officials and Codes Administrators) National Building Code, the 1999 Standard Building Code, and the American Society of Civil Engineers' (ASCE) Minimum Design Loads for Buildings and Other Structures (ASCE 7-95). (HOS) Black & Veatch, P.O. Box 8405, Kansas City, MO 64114-0405 (NAL) Preload Inc., 839 Stewart Ave., Garden City, NY 11530

127 ©Earthquake Spectra, Volume 16, No. I, February 2000

128

H. 0. SPRAGUE AND N.A. LEGATOS

Traditionally, the individual industries have developed their own design guidelines or consensus rules and standards for the seismic design of structures other than buildings. This tradition has created a disconnect between standards for nonbuilding structures and standards for buildings. This disconnect is due to the fact that there has been no single entity that could coordinate all of the various industry standards and practices, and the building code. This disconnect was exemplified by the changes in the 1997 edition of the NEHRP (National Earthquake Hazard Reduction Program) Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (1997 NEHRP Provisions). The 1997 NEHRP Provisions departed from the familiar "seismic zones" and moved to seismic spectral maps as a means to determine site seismicity and the maximum considered earthquake (MCE). The most current seismicity information available was of little use to engineers for many types of nonbuilding structures. In 1995 the Building Seismic Safety Council (BSSC) Board of Direction added a technical subcommittee to the existing 12 technical subcommittees developing the 1997 NEHRP Provisions. The new technical subcommittee was Technical Subcommittee 13Nonbuilding Structures (TS-13). The task of TS-13 was to develop design guidelines for nonbuilding structures. TS-13 was established from a collection of experts in various industries that covered a cross section of the structures comprising as many nonbuilding structure types as possible. The BSSC and TS-13 were concerned that if complete seismic regulations for each type of nonbuilding structure were included directly in the 199 7 NEHRP Provisions, the chapter on nonbuilding structures would be exceptionally large. In order to avoid this and still address the seismic design issues of nonbuilding structures, many standards were included by reference. Of some concern was the traditional methodology employed in the design of nonbuilding structures. Some nonbuilding structure design guidelines were American National Standards Institute (ANSI) consensus standards, some were industry standards, some were company standards, and some were merely generally accepted references in various books. A large section of this paper is devoted to nonbuilding structures that are classified as "not similar to buildings," and tanks and vessels in particular. This is due to the unique dynamic characteristics of this class of nonbuilding structures. These types of nonbuilding structures used seismic design methodologies that were traditionally linked to the coefficients for seismic zones. In order to use the newly developed seismic spectral maps, a bridge was developed to link industrial standards to the maps contained in the 1997 NEHRP Provisions. The bridge proved to be extensive, as is the breadth of the types of structures that tanks and vessels encompass. There was a concerted effort to coordinate TS-13 and Technical Subcommittee 8, Mechanical/Electrical Systems and Building Equipment and Architectural Elements (TS-8). For instance, the fundamental methodology of calculating seismic induced fluid forces was intended to be similar if the tank is elevated in a building or supported on the ground. The same was true for storage racks and other nonbuilding structures that could become nonstructural components. CATEGORIES AND DEFINITIONS OF NONBUILDING STRUCTURES

One unique challenge for the development of seismic provisions for nonbuilding structures was the varied nature of these structures. Each type of nonbuilding structure has its

NON BUILDING STRUCTURES SEISMIC DESIGN CODE DEVELOPMENTS

129

own performance criteria, functional requirements, structural characteristics, dynamic responses, and potential hazards. TS-13 and TS-8 addressed the distinction in the 1997 NEHRP Provisions of nonbuilding structures (as opposed to nonstructural components). Nonbuilding structures were related to nonstructural components and were differentiated according to the relative weights. The distinction was presented in Section 14.1.2 ofthe 1997 NEHRP Provisions. "14.1.2 Nonbuilding Structures Supported by Other Structures: If a nonbuilding structure is supported above the base by another structure and the weight of the nonbuilding structure is less than 25 percent of the combined weight of the nonbuilding structure and the supporting structure, the design seismic forces of the supported nonbuilding structure shall be determined in accordance with the requirements of Sec. 6.1.3. •

If the weight of a nonbuilding structure is 25 percent or more of the combined weight of the nonbuilding structure and the supporting structure, the design seismic forces of the nonbuilding structure shall be determined based on the combined nonbuilding structure and supporting structural system .... "

Nonbuilding structures generally fall into two categories, as designated in the 1997 NEHRP Provisions. Nonbuilding structures that are constructed and have dynamic characteristics similar to buildings are building-like structures. Nonbuilding structures that have little performance and construction similarity to buildings are nonbuilding-like structures. This classification and the separate chapter in the 1997 NEHRP Provisions allows the engineer to use appropriately selected portions of the "building" provisions by reference and without duplication. TS-13 categorized nonbuilding structures as either ''Nonbuilding Structures Similar to Buildings" or "Nonbuilding Structures Not Similar to Buildings," as indicated in Table 1. The intent was to include several specific types of structures into the scope of Chapter 14 of the 1997 NEHRP Provisions. It was recognized that some nonbuilding structures do not lend themselves well to categorization. Amusement structures are an example of a large group of structures that could have elements that would be similar to buildings. Amusement structures also have elements that could be categorized as not similar to buildings. TS-13 agreed to the categorization recognizing that there would be exceptions.

STANDARDS AND DESIGN METHODOLOGY During development of the 1997 NEHRP Provisions, TS-13 observed that the industry standard's reliance on seismic coefficients was at odds with using the seismic spectral ordinate maps generated for the NEHRP Provisions. It was also observed that designers are not always aware of the numerous accepted standards within an industry or if the accepted standards are adequate. TS-13 developed a bridge for the 1997 NEHRP Provisions for nonbuilding structures, for the observed gap between building code and existing industry standards. One of TS-13 's goals was to review and list appropriate industry standards to serve as a resource. The subcommittee developed links so that accepted industry standards could be used with the seismic ground motions established in the 1997 NEHRP Provisions. Some non-

130

H. 0. SPRAGUE AND N. A. LEGATOS

Table 1. Table of categories of nonbuilding structures

NONBUILDING STRUCTURES SIMILAR TO BUILDINGS

NONBUILDING STRUCTURES NOT SIMILAR TO BUILDINGS

Pipe Racks

Earth-retaining Structures

Steel Storage Racks

Tanks and Vessels

Electrical Power Generating Facilities

Electrical Transmission, Substation, and Distribution Structures

Structural Towers supporting Tanks and Vessels

Telecommunication Towers

Piers and Wharves

Stacks and Chimneys

Fixed cranes

Amusement Structure

Pedestrian bridges

Special Hydraulic Structures, such as separation walls, baffle walls, weirs, and other similar structures

Ore processing plants

Buried Structures

Nuclear fuel reprocessing plants

Inverted Pendulums

Material conveying structures building structures are very similar to a building and can be designed employing sections of the NEHRP Provisions directly with minor modifications. Other nonbuilding structures require special analysis unique to the particular type of nonbuilding structure. Industry standards and references, and their applications to nonbuilding structures are summarized in Table 2. Some of the referenced standards are consensus documents while others are not. Another entity evaluating design standards is the ASCE Technical Council on Lifeline Earthquake Engineering (TCLEE). TCLEE and TS-13 began working together during the 2000 NEHRP Provisions development cycle in an effort to coordinate activities as they pertain to lifeline design and construction. In past building codes there have been problems with industry standards conflicting with building codes. An example of the problem is in the design of multi-legged water towers. Historically, such towers have performed well when properly designed per American Water Works Association (AWWA) standards, but these standards differ from the 1997 NEHRP Provisions because tension-only rods are used and the connection forces are not amplified. However, industry practice requires upset rods that are preloaded at the time of installation, and the towers tend to perform well in earthquake areas. The nature of the design practice is for ductile yielding to occur during a seismic event along the length of the tension brace, which intuitively results in good nonlinear performance. Another issue effecting industry standards is the American Institute of Steel Construction (AISC) and building code development. The AISC supports the development of building codes based on limit state design. Most of the current standards in the design of many types of steel nonbuilding structures are predicated on allowable stress design. A method of modifying the loads calculated for nonbuilding structures for application in allowable stress design is included in Chapter 14 of the 1997 NEHRP Provisions. The trend in the industry is

s


Ill

Table 2. Standards, Industry Standards, and References (from the 1997 NEHRP Provisions)

Application

Standard or Reference

Steel Storage Racks

RMI

Piers and Wharves

NCEL R-939, NAVFAC DM-25.1

Welded Steel Tanks for Water Storage

AWWADlOO

Welded Steel Tanks for Petroleum and Petrochemical Storage

API 650, API 620

Bolted Steel Tanks for Water Storage

AWWAD103

Concrete Tanks for Water Storage

AWWAD115, AWWADllO, ACI 350.3

Pressure Vessels

ASMEPV

I

I

'

Refrigerated Liquids Storage: Liquid Oxygen, Nitrogen and Argon

NFPA59

Liquefied Natural Gas (LNG)

NFPA 59 A, DOT 49CFR, NFP A 30

LPG (Propane, Butane, etC.)

NFPA 59, NFPA 30, API 2510

Ammonia

ANSI K61.1

Concrete silos and stacking tubes

ACI 313

Petrochemical structures

ASCE Design of Secondary Containment in Petrochemical Facilities

Impoundment dikes and walls: Hazardous Materials

ANSI K61.1

Flammable Materials

NFPA30 NFPA 59A, DOT 49CFR

Liquefied Natural Gas Cast-in-place concrete stacks and chimneys

ACI 307

Steel stacks and chimneys

ASME STS

Guyed steel stacks and chinmeys

ASME STS, Troitsky 1990

Brick masonry liners for stacks and chimneys

ASTM C1298

Amusement structures

ASTMF1159

toward limit state design. In the future when the industry standards are converted to limit state design, the allowable stress design provisions will be eliminated. BUILDING-LIKE STRUCTURES Building-like structures are structurally similar to buildings in seismic dynamic behavior. Their primary difference lies in function. Typical building-like structures are listed in Table 1. Many nonbuilding structures often house large machines that require only a minimum degree of human occupancy for the function and maintenance of the plant. Building seismic design focuses on collapse prevention and life safety. The focus is on the safety of building occupants, and an economical design. History has indicated that this

132


level of performance can be successfully achieved, but the building may have to be demolished because the earthquake damage has degraded the habitable function of the building. The intended seismic performance of electrical power plants and other cited examples generally include some expectation of a degree of continued performance. Life safety and collapse prevention should be considered minimum performance criteria. Building codes have also included some rules that encumber the design of particular types of building-like structures. One example was the height limitation for particular framing types. Several years ago it was agreed that a height limit should be employed for buildings with concentric-braced steel frames. The height limit that was established was present elsewhere in the code and had its roots in fire-fighting apparatus limitations. The typical framing scheme for electrical power generation plants is a steel concentric-braced frame. Boiler buildings for electric power generation plants often exceed 160 feet in height. The result was a conflict with a rule that was originally intended to apply to buildings. TS-13 included guidance to the designer that would allow the general use of the 1997 NEHRP Provisions that were applicable for buildings while addressing the elements of the 1997 NEHRP Provisions that were not appropriate for nonbuilding structures. This was one of the main reasons that a separate table for determining response factors (R values) and height limitations was developed for nonbuilding structures. Structural designs with height limitation constraints included several types of industrial structures that tended to use concentric-braced frames. The R values were not viewed as a constraint and remained unchanged, but the height limits were modified. NONBUILDING-LIKE STRUCTURES The nonbuilding-like structure category is very broad and consists of structures that do not lend themselves well to the vast majority of the 1997 NEHRP Provisions that were developed for buildings. The reasons that they don't lend themselves to building type designs are due to materials contained, the dynamic response of the structure itself, or the intended performance. Examples ofnonbuilding-like structures are listed in Table 1. TS-13 developed a separate category ofnonbuilding structures to guide the designer to an industry acceptable solution. TS-13 also developed links from the various industry standards to the new seismicity maps contained in the 1997 NEHRP Provisions. BRIDGING THE GAP BETWEEN THE 1997 NEHRP PROVISIONS AND INDUSTRY STANDARDS

There were significant changes ushered in by the 1997 NEHRP Provisions, and the consolidation of the national building codes into the 2000 International Building Code (IBC). TS-13 undertook the task of addressing the issues of nonbuilding structures. Specifically, the immediate objective of TS-13 in regards to the IBC was twofold: 1. To partially expand the code's coverage of structures containing liquids as well as other types ofnonbuilding structures. 2. To provide comprehensive references to all the applicable industry standards.

This endeavor has brought about a standardization and consistency of design practices for the benefit of both the practicing engineer and the public at large. In the case of the seismic design of nonbuilding structures, standardization necessitates certain adjustments on the part of current industry standards to minimize existing

S

e


133

inconsistencies among them. At the same time, however, this process must be cognizant of the fact that structures designed and built over the years in accordance with these standards have performed well in earthquakes of varying severity. The most important inconsistencies among current standards that need to be addressed in any standardization/update process relate primarily to differences in the derivation of the terms that make up the traditional base shear equation V = ZIS C W . An examination of those

Rw

terms as currently used in the different industry standards reveals the following:

•

ZS: The "Seismic zone coefficient" Z has been rather consistent among all these standards by virtue of the fact that it has traditionally been obtained from the seismic zone designations and maps of the national building codes. On the other hand, "Soil Profile Coefficient" S varies from one standard to another. In some standards these two terms are combined.

•

Importance Factor I has also varied from one standard to another, but this variation is unavoidable owing to the multitude of uses and degrees of importance of liquid-containing structures.

•

Coefficient C represents the dynamic amplification factor that defines the shape of the design response spectrum for any given maximum ground acceleration. Since coefficient C is primarily a function of the frequency of vibration, inconsistencies in its derivation from one standard to another stem from at least two sources: differences in the equations for the determination of the natural frequency of vibration; and differences in the equation for the coefficient C itself. For example, for the shelVimpulsive liquid component of lateral force, the steel tank standards use a constant design spectral acceleration (namely, a constant C) that is independent of the "impulsive" period T. In addition, the value of C will vary depending on the damping ratio assumed for the vibrating structure (2%- 7%). Where a site-specific response spectrum is available, calculation of coefficient C is not necessary-except in the case of the convective component (coefficient Cc) which is assumed to oscillate with 0.5% of critical damping, and whose period of oscillation is usually high (>2.5 seconds). Since site-specific spectra are usually constructed for high damping values (3%-7%), and since the site-specific spectral profile may not be well defined in the high-period range, an equation for Cc applicable to 0.5% damping ratio is necessary in order to calculate the convective component of the seismic force.

•

The Response Modification Factor Rw is perhaps the most difficult to quantify, for a number of reasons. While Rw is a compound coefficient that is supposed to reflect the ductility, energy-dissipating capacity, and structural redundancy of the structure, it is also influenced by serviceability considerations, particularly in the case ofliquid-containing structures.

In the 1997 NEHRP Provisions and the IBC, the base shear equation for most structures has been reduced to V = C5 W, where the Seismic Response Coefficient Cs replaces the product ZSC . Cs is determined from the Design Spectral Response Accelerations Sos or S 01

Rw

(at short periods, or at 1 second period respectively) which, in tum, are obtained from the mapped MCE (maximum considered earthquake) spectral accelerations Ss and S 1 obtained from the new seismic maps. As in the case of the prevailing industry standards, where a site-

134


specific response spectrum is available, Cs is calculated using the actual spectral values of that spectrum. EARTH-RETAINING STRUCTURES

The 1997 NEHRP Provisions are currently intentionally vague regarding earth-retaining structures. The development of seismic design standards for earth-retaining structures proved to be a particularly difficult effort. Complicating the effort was the lack of consensus practices for determining seismic-induced soil loads. For example, some geotechnical engineers contend that there are no seismic-induced soil loads on earth-retaining structures. Fortunately, the lack of consensus guidance is in the process of being addressed, and the commentary of the 1997 NEHRP Provisions describes current state-of-the-practice methodologies for determining soil seismic effects. After the nature of seismic-induced soil loads is agreed upon by the geotechnical engineering community, TS-13 will be able to develop design criteria such as sliding and overturning safety factors and load factors for the structural design of the walls. Further complicating the effort is the very nature of soils and earth-retaining structures. Soils can be cohesive and non-cohesive in the broad sense. Soil-retaining walls can also be subject to impact loads such as avalanches, and they can be subject to liquefaction in the supporting soils or in the retained soils. Retaining walls can be tied back and rigid or they can be flexible cantilevered walls. The very nature of the soil restraint has a marked effect on the applied soil pressures. The effort to develop guidelines for the structural design of earthretaining structures is currently in the very early stages of development. Inclusion of provisions for earth-retaining structures is planned for the 2003 NEHRP Provisions development cycle. TANKS AND VESSELS

The development of standards for the seismic design of liquid-containing structures was a particular area of concern and focus. Tanks are generally designed according to material from which the tank is constructed and the material contained in the tank. The response of liquidcontaining structures is markedly different than that of building structures. Long-period ground motion is of particular concern to tank designers. Tanks and vessels presented several unique challenges. The basic design of liquidcontaining structures is in the process of changing. The American Petroleum Institute and the American Water Works Association have recently revised the methodology to compute the forces due to earthquakes. The American Concrete Institute (ACI) is in the process of making similar revisions to ACI 350 (ACI 1999). The various entities that develop design standards have traditionally related the site seismicity to the zones as defined in the UBC. In order to use the newly developed seismic hazard maps contained in the 1997 NEHRP Provisions, a "bridge" had to be developed. Methods of seismic design of tanks, currently adopted by a number of industry standards have evolved from earlier analytical work by Jacobsen, Housner, Veletsos, Haroun, and others. These methods entail three fundamental steps: 1. The dynamic modeling of the structure and its contents. When a liquid-filled tank is

subjected to a ground acceleration, the lower portion of the contained liquid, identified as the impulsive component of mass W" acts as if it were a solid mass rigidly attached to the tank wall. As this mass accelerates, it exerts a horizontal force,


135

S

f

g

d s

u

;.

e e il

0

e

;.

e e

n

e

I-

f s

a n

ld l-

e e g :e

c ls d IS

i, ;s

.,

Pr, against the wall that is directly proportional to the maximum acceleration of the tank base. This force is superimposed on the inertia force of the accelerating wall itself, Pw. Under the influence of the same ground acceleration, the upper portion of the contained liquid responds as if it were a solid liquid mass flexibly attached to the tank wall. This portion, which oscillates at its own natural frequency, is identified as the convective component We, and exerts a force Pc on the wall. The convective component oscillations are characterized by the phenomenon of sloshing, whereby the liquid surface rises above the static level on one side of the tank, and drops below that level on the other. 2. The determination of the frequency of vibration, ffir, of the tank structure and the impulsive component; and the natural frequency of oscillation (sloshing), ffic, of the convective component. 3. The selection of the design response spectrum. The response spectrum may be sitespecific; or it may be constructed deterministically on the basis of seismic coefficients given in national codes and standards. Once the design response spectrum is constructed, the spectral accelerations corresponding to ffir and ffic are obtained and are used to calculate the dynamic forces Pr, P w, and P c· Until now, detailed guidelines for the seismic design of circular tanks, incorporating these concepts to varying degrees, have been the province of at least three industry standards: AWWA D100 (AWWA 1996)for steel tanks (since 1964); AWWA DJJO (AWWA 1995a) for prestressed, wire-wrapped tanks (since 1986); and AWWA D115 (AWWA 1995b)for prestressed concrete tanks stressed with tendons (since 1995). In addition, API 650 and API 2510 (API 1998 and 1995b), contain provisions for oil storage tanks and cryogenic storage tanks, respectively. The detail and rigor of analysis employed by these standards has evolved from a semi-static approach in the early editions to a more rigorous approach at the present reflecting the need to consider dynamic properties of these structures. More recently, ACI Committee 350 drafted a document, entitled ACI Practice for the Seismic Design of Liquid-Containing Structures (ACI 350.3). This document, which covers all types of concrete tanks (prestressed and non-prestressed, circular and rectilinear), is currently being revised to conform to the seismic hazard criteria of the 1997 NEHRP Provisions and the IBC. This ACI "practice" will serve as a practical "how-to"-and yet, rigorous-guide to supplement Chapter 21 ("Special Provisions for Seismic Design") of ACI 350. A common characteristic of all these standards (particularly the ACI and A WWA standards) is that, owing to the dynamics of liquid-containing structures, the seismic design guidelines in these documents are detailed and extensive. As such, they do not lend themselves to inclusion in national codes that are primarily concerned with "building structures" as opposed to "nonbuilding structures." Consequently, national building codes; specifically the UBC, BOCA, SBCI, and ASCE-7, have either refrained from covering the seismic design of liquid-containing structures, or have provided only simple, static-force equations for calculating base shears. These equations make no allowance for the dynamic properties of the structure and its contents as outlined above. Moreover, until recently, the national building codes have not made reference to the aforementioned industry standards to · guide the practicing engineer to "structure-specific" guidelines. This in effect represents an unnecessary and undesirable "disconnect" between the national codes and industry standards. As part of its task, TS-13 has introduced a number of provisions, each designed to bring the design criteria of a particular industry standard in line with the latest 1997 NEHRP

136


Provisions and IBC practices. These provisions follow and are identified with particular types of liquid-containing structures and the corresponding standards. Underlying all these provisions is the understanding that the calculation of the periods of vibration of the impulsive and convective components is left up to the industrial standards. •

1997 NEHRP Provisions Subsection A14.3.2.1-General

The 1997 NEHRP Provisions provide that ground-supported storage tanks for liquids shall be designed in accordance with the prevailing approved industry standards, with the exception of the height of the sloshing wave, 05 , which is illustrated in Figure 1 and defined by equation (Al4.3.2.1.1-1) 55 =0.5DIS 8

(A14.3.2.1-1)

This equation utilizes a spectral response coefficient Sa= l.SS 01 for Tslosh< 4.0 sec., and Tslosh

S8 =

6

~ 01 for Tslosh > 4.0 sec. The first definition of Sa represents the constant-velocity

Tslosh

region, and the second the constant-displacement region, of the response spectrum at 0.5% damping. In practical terms, the latter is the most commonly used definition owing to the fact that, for most tanks, the fundamental period of liquid oscillation (sloshing wave period) is usually greater than 2.5 sec.

H -=:ul:::lll:::lll:::lll:::lll:::lu=-

-=•11:="·

Figure 1. Dynamic model of liquid-containing tank rigidly supported on the ground. The variables are: R = radius of the tank; D = diameter of the tank; H = normal height of liquid; 8, = height of sloshing wave.

•

1997 NEHRP Provisions Subsection A14.3.2.2-Water and Water Treatment Structures Welded Steel and Bolted Steel:

Given Tw, the first mode sloshing wave period (defined as Tslosh in Al4.3.2.1.1), (a) For Ts < Tw

~ 4.0, the term ~in the base shear and overturning moment Rw

equations of AWWA DJ00-96 is replaced by

Sos I ; and the Site 2.5(1.4R)

Amplification FactorS in those equations is set equal to 1.25Ts

NONBUILDING STRUCTURES SEISMIC DESIGN CODE DEVELOPMENTS

(b) ForT w > 4.0, _E. is replaced by Rw

Sa I

2.5(1.4R)

137

; and the Site Amplification FactorS

in those equations is set equal to 1.111 Ts; where T5 , Sa and Sns are defined in Section 4.1.2.6. Reinforced Concrete and Prestressed Concrete:

Given h the natural period of tank shell plus the confined (impulsive) liquid; and Tc (or Tw), the first-mode sloshing wave period (defined as Tslosh in A14.3.2.1.1), (a) For T1 < T0 , and T1 > Ts, the term ZIC 1 R•

in the base shear and overturning

moment equations ofAWWA DlJ0-95 andD115-95; and the term ZISC; in the R;

base shear and overturning moment equations of draft ACI 350.3 are both replaced by

Sl

_a_

1.4R

ZIC

ZISC

S

I

(b) For To s T1s Ts, - -1 and--' are replaced by ____~&_ ~ ~ UR

ZIC ZISC (c) For all values ofTc, _ _ c and _ _c are replaced by 6SDII h Ts. sa, - - 2 w ere Rc Rc (Tslosh} S1, and Sns are defined in Section 4.1.2.6, and Tslosh is defined in Section A14.3.2.1.1.

•

1997 NEHRP Provisions Subsection A14.3.2.3-Petrochemical & Industrial Liquids

Given T, the natural period of first mode sloshing (defined as Tslosh in A14.3.2.1.1), (a) For T < T s 4.0, the term Zl in the overturning moment equations of API 650 (Appendix E) and API 620 (Appendix L) is replaced by 1.667Snsl; and the Site Amplification FactorS in the equation for C2 is set equal to 1.2T5 • (b) For T > 4.0, the term ZI is replaced by 1.667Snsl; and the Site Amplification

Factor S in the equation for C2 is set equal to 1.067Ts; where Ts, Sa and Sns are defined in Section 4.1.2.6. •

1997 NEHRP Provisions Subsection Al4.3.4.1.5-Welded Steel

Coefficient substitutions are the same as those shown for welded and bolted steel tanks under Subsection Al4.3.2.2. ELECTRICAL TRANSMISSION, SUBSTATION, DISTRIBUTION STRUCTURES, AND TELECOMMUNICATION STRUCTURES

The category of nonbuilding structures that consist of electrical transmission, substation, distribution, and telecommunication structures is generally quite flexible and has performed well during past seismic events. There have been exceptions and the exceptions can result in major electrical power disruptions. The biggest area of potential concern, as with other standards, is the disconnect from the seismic hazard maps contained in the 1997 NEHRP Provisions. To address the particular issues, the industry standard codes such as the National Electric Safety Code, Institute of Electrical and Electronics Engineers, Recommended Practices for Seismic Design of Substations (IEEE 693), Power Engineering Society, and the

138


Telecommunication Industry Association (TIA) are working with members of TS-13 to employ the new seismic hazard maps and to evaluate their structures relative to the 1997 NEHRP Provisions. An issue that presents a particular challenge is the role of the building official and the designers of electrical transmission structures. If building officials have jurisdiction over electrical transmission systems, the implication is that as systems cross jurisdictions, different building officials may have different interpretations and requirements. This would cause confusion, delays, and added expense for a perceived problem as opposed to a real or historic problem. Of particular concern are the structures that provide power to essential facilities. Electric power uses grids and redundant systems. The current codes and the IBC do not make allowance for redundant systems. The preference within the electric power industry is to exclude electrical transmission and distribution structures and substation structures from the purview of local municipal building officials.

CONCLUSION

The effort to coordinate and develop a code for nonbuilding structures is a relatively new and challenging enterprise. The diverse nature of nonbuilding structures, the rapidly evolving development of seismic design, and the new 1997 NEHRP Provisions design maps (though necessary) have complicated code development. The involvement of the industrial segments that design and construct non-building structures is critical to the development of a successful code. The effort to develop a code must be coordinated with the many standards used in the design ofnonbuilding structures. The importance of this effort can not be overstated. The functions of the vast majority of nonbuilding structures are critical for the continued operation of basic endeavors. Electricity, water for fire fighting, fresh water, waste-water disposal, containment of hazardous compounds, fuel-delivery systems, communications, are functions that must be protected from earthquake damage and are beyond the scope of previous building codes. The effort to protect critical assets belongs in the building code, and will require close integration with industries to assure safe and functional facilities in the event of major earthquakes. This paper represents a very broad view of the seismic design issues for nonbuilding structures. There are many types of nonbuilding structures not mentioned here that require fundamental evaluations to determine the best method of seismic design. The effort to reach consensus and develop uniform design codes will require many years of development and closer coordination with industry. Specific future goals ofTS-13 are as follows: •

Continue to work with various industries to develop industry specific consensus standards that use the knowledge base contained in the NEHRP Provisions process.

•

Work with TCLEE in an effort to identify critical needs for seismic code development in the various industries that comprise lifelines.

•

Identify and help develop quality control and quality assurance practices for nonbuilding structures.

•

Work with FEMA, BSSC, TCLEE, and the various industries to coordinate efforts to develop design and inspection standards for nonbuilding structures that are consistent, coordinated, and well developed.

NONBUILDING STRUCTURES SEISMIC DESIGN CODE DEVELOPMENTS

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ACKNOWLEDGEMENTS The authors wish to acknowledge the aid of the other members of TS-13: Victor D. Azzi, P.E., Rack Manufacturers Institute Clayton L. Clem, TVA Transmission Power Supply Ralph T. Eberts, Black & Veatch Frank J. Hsiu, Chevron Research Technology Co. Leon Kempner, Jr., Bonneville Power Administration John V. Loscheider, Loscheider Engineering Company Stephen W. Meier, Tank Industry Consultants, Inc. The authors further wish to acknowledge John Gillengerten-TS-8 Chair, John A. Martin & Associates.

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Nonbuilding Structures Seismic Design Code ...

Nonbuilding Structures Seismic Design Code ...

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