United States Nuclear Regulatory Commission - Protecting People and the Environment

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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                                                                 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 
Page Last Reviewed/Updated Tuesday, November 12, 2013