Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
April 25, 1997
Information Notice No. 97-22: FAILURE OF WELDED-STEEL MOMENT-
RESISTING FRAMES DURING THE
NORTHRIDGE EARTHQUAKE
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the factors contributing to the failure of
welded-steel moment-resisting frames (WSMFs) during the Northridge earthquake.
It is expected that recipients will review the information for applicability
to their facilities and consider actions, as appropriate, to avoid similar
problems. However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances
On January 17, 1994, at 4:31 a.m. Pacific Standard Time, a magnitude 6.7
earthquake occurred in the Northridge area of metropolitan Los Angeles,
California. This earthquake caused considerable damage to industrial
facilities, lifelines, commercial centers, and industrial buildings located
within 40 km [25 miles] of the epicenter. San Onofre Nuclear Generating
Station, located about 130 km [80.8 miles] from the epicenter, is estimated to
have experienced a peak horizontal ground acceleration (PHGA) of less than
0.02g, and Diablo Canyon Nuclear Power Plant, located about 239 km [149 miles]
from the epicenter, is estimated to have experienced a PHGA of less than
0.01g. The earthquake caused no damage to these plants. Reference 1 is a
comprehensive assessment of the effects of the Northridge earthquake on
various facilities.
The post-earthquake investigations of many (more than 100), otherwise intact
buildings indicated considerable structural damage to WSMFs. The frames were
designed to withstand large seismic forces on the basis of the assumption that
they are capable of extensive yielding and plastic deformation. The intended
plastic deformation consisted of plastic hinges forming in the beams, at their
connections to columns. Damage was expected to consist of moderate yielding
at the connections and localized buckling of the steel elements. Instead, the
WSMF failures were brittle fractures with unanticipated deformations in
girders, cracking in column panel zones, and fractures in beam-to-column weld
connections. Federal Emergency Management Agency (FEMA) Publication 267
(Reference 2) provides a detailed discussion of the WSFM damage and provides
interim guidelines for the evaluation, repair, and modification of WSMFs.
9704230013. IN 97-22
April 25, 1997
Page 2 of 4
Discussion
A number of factors related to seismic analysis and design, materials,
fabrication and construction are identified as contributing to the failure of
WSMFs and are the focus of FEMA-sponsored research projects. Although the
steel structures in nuclear power plants are fabricated and constructed using
the same national standards [e.g., the American Institute of Steel
Construction (AISC) specifications and the American Welding Society (AWS)
welding code] as were used in the construction of WSMF structures, the method
of computing seismic loads, combination with other loads, acceptance criteria,
and quality assurance requirements are significantly different from those for
non-nuclear buildings designed using national building codes, such as the
Uniform Building Code and the Building Officials and Code Administrators
International Code. The following paragraphs discuss the extent of
applicability of the factors contributing to the failure of WSMF, as they
relate to steel structures in nuclear power plants.
1. Seismic Analysis and Design: Two levels of ground motion have been
defined for designing the safety-related structures, systems, and
components in the operating nuclear power plants. For the first-level
earthquake, the Operating-Basis Earthquake (OBE), the load factors and
acceptable allowable stresses ensure that the stresses in plant
structures remain at least 40 percent below the yield stress of the
material. For the second-level earthquake, the Safe-Shutdown Earthquake
(SSE�whose vibratory motion is usually twice that of the OBE), the
associated load factors and allowable stresses ensure that the stresses
in steel structures remain close to the yield stress of the material; a
small excursion in the inelastic range is allowed when the SSE load is
combined with accident loads. The design requirements, promulgated by
Standard Review Plan provisions, prohibit the use of significant
inelastic deformation of any steel member or connection (that is allowed
in the design of WSMFs) in nuclear power plants under design-basis
seismic events. Also, the use of broadband response spectra,
conservatively defined structural damping values, consideration of
amplified forces at higher elevations in the plants, and consideration
of all three components of the design-basis earthquakes ensure that the
loads and load paths of the design-basis seismic events are properly
considered in the design, as opposed to the use of static base shear
forces in non-nuclear structures.
Localized inelastic deformations of steel structures are allowed for
impactive and impulsive forces associated with high-energy pipe
ruptures, chemical explosions, and tornado- and turbine-generated
missiles. However, even under the deformed conditions, designers are
required to assess the overall stability of the structure.
2. Materials: Three distinct factors related to the steel material used in
WSMFs were identified: (1) higher-than-specified yield strength of
American Society of Testing and Materials (ASTM) A36 steel, (2) lack of
adequate through-thickness strength of thick-column flanges, and (3)
inadequate notch toughness of the base metal.
The post-earthquake investigations (Ref. 2) indicated that consistently
higher yield strength (25 to 35 percent higher than the minimum
specified yield strength) restricted. IN 97-22
April 25, 1997
Page 3 of 4
the girder rotation at the design moment. Thus, the restrained
connections were required to dissipate the large amount of energy
associated with the seismic event by fracturing. It was the inability
of the girder to rotate that induced large unaccounted-for through-
thickness forces in the thick-column flanges of the WSMFs. American
National Standards Institute/AISC (ANSI/AISC) N690 (Ref. 3) requires
through-thickness testing and ultrasonic examination when high-heat
input welds and/or highly restrained conditions are encountered to
alleviate the possibility of lamellar tearing. For Classes 1, 2, 3,
and MC component supports, Subsection NF of Section III of the American
Society of Mechanical Engineers Boiler and Pressure Vessel Code (the
ASME code) (Ref. 4) requires through-thickness testing for plates (which
could be part of a rolled shape) thicker than 2.5 cm (1 in), if they are
subjected to through-thickness loading. However, for nuclear power
plant steel structures, both these requirements are relatively recent
(promulgated after 1984) and would not have been used in a majority of
the operating nuclear power plants designed and built before 1984. Some
architect-engineers and utility engineers may have utilized similar
requirements in their project specifications.
To address factor (3), inadequate notch toughness of the base metal,
AISC conducted a statistical survey of the toughness of material
produced in structural shapes (wide flanges, tees, angles, etc.), based
on data provided by six producers for a production period of
approximately 1 year (Ref. 5). This survey showed a mean value of
Charpy V-notch (CVN) toughness for all shape groups to be in excess of
27J (20 ft-lbf) at 21 �C (70 �F) and 20J (15 ft-lbf) at 4 �C (40 �F).
For structures or structural components that are designed to withstand
impactive and impulsive loadings, Reference 3 requires the average CVN
values to vary between 20 and 40J (15 and 30 ft-lbf), at a temperature
of 17 �C (30 �F) below the lowest service metal temperature of the
structure. Reference 4 also has similar requirements for vital
component supports in nuclear power plants. Considering the normal
service metal temperatures of steel structures in nuclear power plants
and the range of CVN values as experienced in the survey, factor (3) is
probably not a concern for the steel structures in nuclear power plants.
However, this factor may be applicable for safety-related steel
structures (or non-safety steel structures that could affect the safety
function of a safety-related structure, system, or component) designed
to withstand impactive and impulsive loadings if the structures may
experience low service metal temperatures, i.e., structures located
outdoors.
3. Fabrication and Construction: For damaged WSMFs, a number of issues
related to connection detailing and weld quality, such as fracture
toughness, weld material, welding procedures, weld inspection, and
welders' qualification, were addressed.
The research project carried out at the Center for Advanced Technology
for Large Structural Systems (ATLSS) at Lehigh University examined the
effects of weld metal toughness and fabrication defects on the seismic
performance of WSMF connections. The examination and testing performed
at ATLSS revealed that the weld fractures . IN 97-22
April 25, 1997
Page 4 of 4
were initiated from porous weld roots adjacent to the back-up bar and
that the fracture toughness of welds made with E70T-4 weld electrodes
used in the connections was very low [< 14 J (10 ft-lbf) at 21 �C (70
�F)] (Ref. 6).
The arc welding process used in the steel structures could be (1)
shielded metal arc welding (SMAW), (2) flux cored arc welding (FCAW),
(3) submerged arc welding (SAW), or (4) gas metal arc welding (GMAW).
The American Welding Society's "Structural Welding Code - D1.1,"
provides the requirements for weld design, welding techniques, standards
for workmanship, procedure and personnel qualifications, and
inspections. For safety-related steel structures in nuclear power
plants, the quality assurance requirements of Appendix B to 10 CFR Part
50, as promulgated by ANSI N45 (now NQA) series standards, are also
applicable. The use of the E70T-4 electrode is associated with the FCAW
process. Its use is allowed by the AWS Code. The electrode must meet
specific physical and chemical requirements. Its minimum mechanical
properties requirements areas follows: a tensile strength of 72 ksi, a
tensile yield strength of 60 ksi, and an elongation of 22 percent.
However, the electrode need not be tested for notch toughness. It
should be recognized that there are other AWS-permissible FCAW
electrodes which are also not required to be tested for notch toughness
unless specifically called for in the project specification. They are
E60T-4, E60T-7, E60T-11, E70T-7, and E70T-11. For projects with notch
toughness requirements, use of these electrodes would not be permitted
unless a separate notch toughness qualification had been performed.
This information notice requires no specific action or written response.
However, comments and input related to the technical issues discussed are
encouraged. If you have any questions about the information in this notice or
you wish to provide additional information related to the technical issues
discussed, please contact the technical contact listed below or the
appropriate Office of Nuclear Reactor Regulation (NRR) project manager.
signed by M.M. Slosson for
Thomas T. Martin, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Hans Ashar, NRR Eric Benner, NRR
(301) 415-2851 (301) 415-1171
E-mail: hga@nrc.gov E-mail: ejb1@nrc.gov
Attachments:
1. References
. Attachment 1
IN 97-22
April 25, 1997
Page 1 of 1
REFERENCES
1. "The January 17, 1994, Northridge Earthquake: Effects on Selected
Industrial Facilities and Lifelines," prepared by Mark Eli, S. Sommer
(LLNL), and T. Retch, K. Merz (EQE International), dated February 1995.
Available from the National Technical Information Service, U. S.
Department of Commerce, 5285 Fort Royal Road, Springfield, VA 22161.
2. FEMA 267: "Interim Guidelines: Evaluation, Repair, Modification and
Design of Welded Steel Moment Frame Structures," prepared by a joint
venture of (1) Structural Engineers Association of California, (2)
Applied Technology Council, and (3) California Universities for Research
in Earthquake Engineering. Available from: Federal Emergency
Management Agency, P. O. Box 70274, Washington, DC 20024.
3. ANSI/AISC N690: "Nuclear Facilities�Steel Safety-Related Structures for
Design, Fabrication and Erection," 1984, 1995. Available from the
American Institute of Steel Construction, Inc., One East Wacker Drive,
Suite 3100, Chicago, IL 60601.
4. Subsection NF of Section III of the ASME Code: "Supports," 1995 and
earlier editions. Available from the American Society of Mechanical
Engineers, United Engineering Center, 345 E. 47th Street, New York, NY
10017.
5. AISC Report, "Statistical Analysis of Charpy V-Notch Toughness for Steel
Wide-Flange Structural Shapes," dated July 1995. Available from the
address in listed Ref. 3.
6. Kaufman, E., Xue, M., Lu, L., Fisher, J.: "Achieving Ductile Behavior
of Moment Connections," published in Modern Steel Construction, January
1996. Available from the address listed in Ref. 3.
