Development of a probability based load criterion for American National Standard A58

236 Pages·2014·11 MB·English

by  Ellingwood, Bruce

577
NBS SPECIAL PUBLICATION
U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards
Development of a
Probability Based Load Criterion for
American National Standard A58
Building Code Requirements for
Minimum
Design Loads in Buildings
and Other Structures
.
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or sTavcakds
UBKARY
Development of a Probability Based Load Criterion jun i 9 seo
for American National Standard A58
Building Code Requirements for Minimum Design Loads
in Buildings and Other Structures
Bruce Ellingwood
Center for Building Technology
National Engineering Laboratory
National Bureau of Standards
Washington, D.C. 20234
Theodore V. Galambos
Department ofCivil Engineering
Washington University
St. Louis, MO. 63130
James G. MacGregor
Department ofCivil Engineering
University of Alberta
Edmonton, Alberta, Canada
C. Allin Cornell
Department ofCivil Engineering
Massachusetts Institute of Technology
MA
Cambridge, 02139
ion
U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary
Luther H. Hodges, Jr., Deputy Secretary
Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation
NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
(
Issued June 1980
Library of Congress Catalog Card Number: 80-600067
National Bureau of Standards Special Publication 577
Nat. Bur.Stand.(U.S.),Spec. Publ. 577,228pages(June 1980)
CODEN: XNBSAV
U.S. GOVERNMENT PRINTING OFFICE
WASHINGTON:
1980
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ABSTRACT
Recommended load factors and load combinations are presented which are compatible
with the loads in the proposed 1980 version of American National Standard A58, Building
Code Requirements for Minimum Design Loads in Buildings and Other Structures. The load
effects considered are due to dead, occupancy live, snow, wind and earthquake loads. The
load factors were developed using concepts of probabilistic limit states design which
incorporate state-of-the-art load and resistance models and available statistical information.
Reliabilities associated with representative structural members and elements designed
according to current (1979) structural specifications were calculated for reinforced and
prestressed concrete, structural steel, cold-formed steel, aluminum, masonry and glued-
laminated timber construction. The report presents the rationale for selecting the criterion
format and load factors and describes the methodology to be followed by material specification
groups for determining resistance factors consistent with the implied level of reliability
and the statistical data. The load factors are intended to apply to all types of structural
materials used in building construction.
Key words: Aluminum; buildings (codes); design (buildings); concrete (prestressed);
concrete (reinforced); limit states; loads (forces); masonry; probability theory;
reliability; safety; specifications; standards; statistical analysis; steel;
structural engineering; timber.
iii
TABLE OF CONTENTS
Abstract iii
Executive Summary 1
Objectives 1
Rationale 1
Summary of Procedure 5
Some Particular Critical Issues . 6
Future Action 7
1. Probability-Based Limit States Design „ 8
1.1 Limit States Design 8
1.2 Methods of Establishing Safety Levels 10
.1 Allowable Stress or Working Stress Design 10
.2 Strength Design 11
.3 Probability-Based Limit States Design 13
2. Probabilistic Bases of Structural Reliability 14
2.1 Historical Development 14
2.2 Analysis of Reliability of Structures 15
.....
2.3 First-Order, Second-Moment Methods 16
.1 Mean-Value Methods 16
.2 Advanced Methods 19
2.4 Approximate Methods for Including Information on Distributions 22
2.5 Load Combinations 28
3. Load and Resistance Distributions and Parameter Values 34
3.1 General 34
3.2 Characterization of Load and Resistance Variables 34
3.3 Specifications and Standards 37
3.4 Load Distributions and Parameters 37
3.5 Resistance Distributions and Parameters 39
.
.1 Resistance Statistics for Reinforced and Prestressed Concrete Structures 40
.2 Resistance Statistics for Metal Structural Members 41
.3 Resistance of Engineered Brick and Concrete Masonry 42
.4 Glulam Members in Bending, Tension and Compression 42
iv
:
4. Calibration With Existing Standards 44
4.1 General Considerations 44
4.2 Gravity Loads 44
4.3 Gravity and Environmental Loads 48
5. Development of Design Criteria 55
5.1 Scope 55
5.2 Selection of Format 55
.1 Load Factors 55
.2 Resistance Factors 58
5.3 Target Reliability Indices 59
5.4 Reliability-Based Design 61
5.5 Selection of Load Factors 68
5.6 Recommendations to Material Specification Groups 75
5.7 Resistance Factors Compatible With Selected Load Factors 85
.1 Metal Structures 85
.2 Reinforced and Prestressed Concrete Structures 86
.3 Glulam and Other Heavy Timber Structures 91
.4 Masonry Structures 93
6. Summary and Conclusions 96
7. Acknowledgements 99
8. References 100
9. Glossary 102
10. Nomenclature 104
Appendices
A. Analysis of Structural Loads 107
B. Reinforced and Prestressed Concrete Members 127
C. Data for Metal Members and Components 147
D. Masonry Structures 171
H
w
E. Glued-Laminated and Other Heavy Timber Structures 194
F. Computer Program 210
v

EXECUTIVE SUMMARY
American National Standard Committee A58 periodically issues revisions to ANSI Standard
A58 - "Building Code Requirements for Minimum Design Loads in Buildings and Other Structures."
This document defines magnitudes of dead, live, wind, snow and earthquake loads suitable
for inclusion in building codes and other regulatory documents. The A58 Standard Committee
is a broad-spectrum group of professionals from the research community, building code
groups, industry, professional organizations and trade associations. Their approval of a
proposed standard signifies that a consensus of those substantially concerned with its
scope and provisions has been reached, in that affected parties have had an opportunity to
comment on the standard prior to its implementation and opposing points of view have been
treated fairly.
The A58 Standard is concerned solely with structural loadings. The specification of
specific allowable stresses or design strengths for materials of construction is outside
its scope. The current version of the A58 Standard, ANSI A58.1-1972, is being revised,
with a tentative approval and publication date set for 1980.
This report addresses itself to changes to the A58 Standard which may occur subsequent
to the 1980 revision. Its purpose is to develop a load criterion, including load factors
and load combinations, which would be suitable for limit states design with different
materials and methods of construction. The current standard already contains a set of
load combinations and probability factors for allowable stress design. This Executive
Summary is presented to review briefly the conclusions of the main report, giving an
overview of the recommendations and a concise rationale for their development.
Objectives
:
1) To recommend a methodology and set of load factors and corresponding load definitions
for use in the A58 Standard which would be appropriate for all types of building materials
(e.g., structural steel, reinforced and prestressed concrete, heavy timber, engineered
masonry, cold-formed steel, aluminum) and, in the future, for building foundations; and
2) To provide a methodology for the various material specification groups to select
resistance factors consistent with these load factors and their own specific objectives.
(<|>)
Rationale
:
Structural design is a complex process involving iterative cycles of analyzing the
performance of idealized structures. Each analysis cycle involves the checking of subassemblies,
members, components and connections against various limit states defined in a structural
specification dealing with the particular structural material. Typically this checking
process involves satisfying a design criterion of the general form:
Factored Resistance Effect of factored loads.
>^
In the common case where the total load effect is a linear combination of individual loads,
n
d>R > E v.Q.
i=l
In this formula the left side reflects the resistance (capacity) of the structural element
under consideration, and the right side denotes the forces which the element is expected
to support during its intended life (load effects) The term R is a nominal resistance
. n
corresponding to a limit state (e.g., maximum moment which can be carried by a cross
section, buckling load, shear capacity), and is the "resistance factor," which is less
<j>
than unity and which reflects the degree of uncertainty associated with the determination
of the resistance. The sum yQ is the product of the "load effect" Q (i.e., the force on
the member or the element - bending moment, shear force, torque, axial force - or the
stress on the component) due to the loading from different structural loads (e.g., dead
load, live load due to occupancy, wind load, snow load, earthquake load) and a load factor
Y, generally larger than unity, which accounts for the degree of uncertainty inherent in
the determination of the forces Q. When nonlinearities in behavior are significant, the
load factor should be applied before performing the structural analysis.
In a more general sense may represent a number of limit states (e.g., yielding
<f>R
n
and tensile strength in a metal tension member) for each element, and E Y-Q- reflects
i=l
the largest of several load combinations. A substantial portion of this report is devoted
to the determination of values for these y^. Using as an example a metal tension member,
the following combinations might be checked:
<f>yFyAn \ / YtD,Dn L n
<j>uFuAn / I YnDDn + yt L + YTWTWn
where dy>y and dY)u are the resistance factors for the y3ield limit state, Fy', and the tensile
strength limit state, F^, respectively, A^ is the net area, D^, and are the load
ftA glossary of terms is presented in Chapter 9.