Information Notice 1997-22, Failure of Welded-Steel Moment Resisting Frames During Northridge Earthquake

Failure of Welded-Steel Moment Resisting Frames During Northridge Earthquake
	ML031210426
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	04/25/1997
From:	Martin T; Office of Nuclear Reactor Regulation
To:	;
References
	IN-97-022, NUDOCS 9704230013
	Download: ML031210426 (11)
	v • d • e

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 April 25, 1997 NRC 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.

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9 IN 97-22 April 25, 1997 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 Intemational 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

tomado- 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 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-bf) at

21 IC (70 OF) and 20J (15 ft-lbf) at 4 IC (40 IF). 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 OF) 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

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IN 97-22 April 25, 1997 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 OF)] (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-1 1, E70T-7, and E70T-1 1. 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.

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: ejbl@nrc.gov

Attachments:

1. References

C 2. List of Recently Issued NRC Information Notices

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Attachment 1 IN 97-22 April 25, 1997 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. 0. 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 Modem Steel Construction, January 1996. Available from

the address listed in Ref. 3.

Attachment 2 IN 97-22 April 25, 1997 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

97-21 Availability of Alternate 04/18/97 All holders of OLs

AC Power Source Designed for nuclear power

for Station Blackout Event reactors

97-20 Identification of 04/18/97 All holders of OLs

Certain Uranium for nuclear power

Hexafluoride Cylinders

that do not comply

with ANSI N14.1 Fabrication

Standards

97-19 Safety Injection 04/18/97 All holders of OLs

System Weld Flaw at or CPs for nuclear

Sequoyah Nuclear power reactors

Power Plant, Unit 2

94-14, Failure to Implement 04/14/97 All holders of OLs

Supp. 1 Requirements for or CPs for nuclear

Biennial Medical power and non-power

Examinations and reactors and all

Notification to the licensed reactor

NRC of Changes in operators and senior

Licensed Operator reactor operators

Medical Conditions

97-18 Problems Identified 04/14/97 All holders of OLs, During Maintenance CPs, and decommissioning- Rule Baseline Inspections stage licenses for

nuclear power reactors

97-17 Cracking of Vertical 04/04/97 All holders of OLs

Welds in the Core or CPs for boiling- Shroud and Degraded water reactors

Repair

OL = Operating License

CP = Construction Permit

IN 97-22 April 25, 1997 were initiated from porous weld roots adjacent to the back-up bar and that the fracture

toughness of welds made with E70T-401Xwldbbctrodes used in the connections was

very low [< 14 J (10 ft-lbf) at 21 OC (70 OF)] (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