Information Notice No. 97-22: Failure of Welded-Steel Moment-Resisting Frames During the Northridge Earthquake

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C. 20555

                                April 25, 1997

Information Notice No. 97-22:  FAILURE OF WELDED-STEEL MOMENT-
                                   RESISTING FRAMES DURING THE                 
                                   NORTHRIDGE EARTHQUAKE


All holders of operating licenses or construction permits for nuclear power


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
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


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

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:       E-mail:

1.  References
.                                                               Attachment 1
                                                               IN 97-22
                                                               April 25, 1997
                                                               Page 1 of 1


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

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.


Page Last Reviewed/Updated Thursday, March 25, 2021