Information Notice No. 88-25: Minimum Edge Distance for Expansion Anchor Bolts
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
May 16, 1988
Information Notice No. 88-25: MINIMUM EDGE DISTANCE FOR EXPANSION
ANCHOR BOLTS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to test results
indicating that the recommended minimum edge distance for the design and
installation of expansion anchor bolts may be unconservative with respect to
effectively resisting the shear forces imposed on anchor bolts during a
design-basis event such as a safe-shutdown 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 do not constitute NRC
requirements; therefore, no specific action or written response is required.
Description of Circumstances:
On March 18, 1988, Hilti, Inc., informed the Nuclear Regulatory Commission
(NRC), in accordance with 10 CFR Part 21, of recent tests of Hilti Kwik-Bolts
that indicate that the previously recommended minimum distance from an anchor
bolt to an unsupported concrete edge may be insufficient to develop 100% of
the recommended anchor capacity. In view of the foregoing, the NRC staff met
with representatives of Hilti, Inc. (Hilti) to discuss the notification.
During the meeting, the NRC staff was informed that prior to November 1985,
the edge-distance guidelines provided by Hilti for the installation of Hilti
Kwik-Bolts were those of the Expansion Anchor Manufacturers Institute (EAMI).
The EAMI guidelines are used throughout the United States and recommend an
edge distance, the distance from the center of an expansion anchor bolt to an
unsupported edge of a concrete support, of five anchor diameters to achieve
100% of the recommended anchor capacity. In November 1985, Hilti revised its
design and installation guidelines and recommended a greater edge distance
than the EAMI guidelines. These conservative guidelines were adopted based on
information and broader technical knowledge and experience gained as a result
of marketing Hilti bolts to foreign customers. Consequently, Hilti initiated
a testing program in 1986 to support their revised edge-distance guidelines.
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. IN 88-25
May 16, 1988
Page 2 of 3
The 10 CFR Part 21 notification documents the results of this testing program
and indicates that Hilti Kwik-Bolts installed in accordance with the EAMI
guidelines have only 30% of the recommended anchor shear capacity and 78% of
the recommended tensile capacity. Hilti further indicated that the test
results are believed to be applicable to other expansion anchor bolts that use
the same type of anchor mechanism as Hilti Kwik-Bolts.
Discussion:
The NRC staff considers the test results indicating a 70% reduction in shear
capacity to be significant and to indicate that there is a potential safety
concern that will require further NRC review. However, the test results do
not indicate an imminent safety concern for expansion anchor bolts installed
in nuclear power facilities for the reasons given below.
(1) The purpose of the Hilti edge-distance test program was to establish
design recommendations for the resistance capacity of a single bolt as a
function of edge distance. The test anchorage consisted of only one bolt
anchored to a concrete block that was not reinforced. Equipment or
system anchorage arrangements at nuclear power facilities usually use
more than one anchor bolt and a steel bearing plate or base assembly.
Installation pretensioning torque loads applied to the anchor bolt nuts
create compressive loads and frictional resistance forces between the
bearing plate and the concrete that tend to make the anchorage and con-
crete support act as a single unit and mitigate the severity of the
single bolt edge-distance problem. If the frictional resistance forces
between the concrete support and the bearing plate are reduced due to the
creep of concrete and the relaxation of anchor bolts, the bearing plate
can still redistribute the additional shear forces, which cannot be
resisted by the edge bolts with reduced capacity, to other bolts that
have no capacity reduction due to the edge-distance problems.
(2) The test results are an acceptable basis for establishing design
recommendations; however, they should be viewed as conservative for the
installation of bolts. The presence of steel reinforcement between the
bolt and an unsupported concrete edge should increase the bolt shear
capacity, depending on the amount of reinforcement and the type of
placement, and also mitigate the consequences of the edge-distance
problem. This is particularly true for large diameter expansion bolts
(greater than 3/4-inch bolts).
Based on the NRC staff assessment of the information that has been provided
since the submittal of the 10 CFR Part 21 notification, it appears that 1)
most anchor bolts in nuclear power facilities were installed using the EAMI
recommended guidelines and 2) the potential failure of these bolts to perform
their intended function during a design-basis event, such as a seismic event,
could jeopardize the integrity of safety-related systems and equipment.
Therefore, the NRC will continue to evaluate this issue and take appropriate
regulatory action if it is determined that a significant safety hazard exists
at nuclear power facilities. However, addressees may wish to review the
information and consider appropriate actions if this information indicates
that a significant safety hazard exists at their facility.
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May 16, 1988
Page 3 of 3
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the techni-
cal contacts listed below or the Regional Administrator of the appropriate
regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: John Ma, NRR
(301) 492-1376
Jaime Guillen, NRR
(301) 492-1153
Attachment: List of Recently Issued NRC Information Notices
. Attachment
IN 88-25
May 16, 1988
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
88-24 Failures of Air-Operated 5/13/88 All holders of OLs
Valves Affecting Safety- or CPs for nuclear
Related Systems power reactors.
88-23 Potential for Gas Binding 5/12/88 All holders of OLs
of High-Pressure Safety or CPs for PWRs.
Injection Pumps During a
Loss-of-Coolant Accident
88-22 Disposal of Sludge from 5/12/88 All holders of OLs
Onsite Sewage Treatment or CPs for nuclear
Facilities at Nuclear power reactors.
Power Stations
88-21 Inadvertent Criticality 5/9/88 All holders of OLs
Events at Oskarshamn or CPs for nuclear
and at U.S. Nuclear power reactors.
Power Plants
88-20 Unauthorized Individuals 5/5/88 All holders of OLs
Manipulating Controls and or CPs for nuclear
Performing Control Room power, test and
Activities research reactors,
and all licensed
operators.
88-19 Questionable Certification 4/26/88 All holders of OLs
of Class 1E Components or CPs for nuclear
power reactors.
88-18 Malfunction of Lockbox on 4/25/88 All NRC licensees
Radiography Device authorized to
manufacture,
distribute, and/or
operate radio-
graphic exposure
devices.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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