United States Nuclear Regulatory Commission - Protecting People and the Environment

Guidelines for SEP Soil Structure Interaction Reviews (Generic Letter 80-109)


                                DEC 15 1980 



Enclosed for your information are guidelines for performing soil-structure 
interaction reviews for SEP facilities. Also included is a simplified 
analytical approach for evaluating the effects of soil-structure interaction 
using a lumped parameter model. The simplified approach presented does not 
preclude the use of other procedures which would be reviewed and approved on
a case-by-case basis. 


                                        Dennis N. Crutchfield, Chief 
                                        Operating Reactors Branch #5 
                                        Division of Licensing 

Enclosure:  SSRT Guidelines for SEP Soil-Structure Interaction Review

cc:  See next page 

Docket Nos:    50-155         TERA           RTedesco
               50-10 & 237    OI&E (3)       TNovak  
               50-213         ACRS (16)      SEP File  
               50-409         Heltemes, AEOD JRoe
               50-245         NRR Reading    RDiggs
               50-219         SEPB Reading
               50-255         WRussell  
               50-244         DCrutchfield  
               50-206         HSmith  
               50-29          Project Managers
NRC PDR                       GCwalina 
Local PDR                     RHermann 
OELD                          DEisenhut
TERA                          RPurple
NSIC, JBuchanan               GLainas 

                                                  URBANA, ILLINOIS 61801 
                                                       8 December 1980 

Mr. William T. Russell, Chief
Systematic Evaluation Program Branch
Division of Licensing
Office of Nuclear Reactor Regulation
U. S. Nuclear Regulatory Commission
Washington, D.C.  20555 (Mail Stop 516) 

     Re:  SSRT Guidelines for SEP Soil-Structure 
          Interaction Review 
          Contract NRC-03-78-150 

Dear Mr. Russell: 

     The Guidelines for SEP Soil-Structure Interaction Review, as prepared 
by the Senior Seismic Review Team, are transmitted herewith with signature 

     We are appreciative of the help of the many individuals who contributed
to the preparation of these guidelines. 

                                        Sincerely yours, 

                                        N. M. Newmark 
                                        Chairman, SSRT 


W. T. Russell - 2
T. Cheng - 1
N. M. Newmark - 2 
W. J. Hall - 1 
R. P. Kennedy - 1
R. Murray - 1
J. D. Stevenson - 1

                                                          December 8, 1980 



     When a structure is founded within or on a base of soil, it interacts 
with its foundation. The forces and displacements transmitted to the 
structure and the feedback to the foundation regions are complex in nature; 
the interactions that take place modify the free-field motions. Many methods
for dealing with soil-structure interaction have been proposed by a number 
of writers. These methods can be classified in various ways and involve 
generally: (1) procedures similar to those applicable to a rigid block on an
elastic half-space; (2) finite element or finite difference procedures 
corresponding to various forcing functions acting on the combined structure 
soil complex; and (3) substructure modeling techniques that may or may not 
include use of the direct finite element method. Another, and perhaps more 
convenient, classification of soil-structure interaction analysis procedures
is that of (a) direct solution techniques and (b) substructure solution 
techniques as described in the report entitled "Recommended Revisions to 
Nuclear Regulatory Commission Seismic Design Criteria", Report 
NUREG/CR-1161, May 1980. 

     The elastic half-space theory considers a foundation plate resting on 
an elastic medium with harmonic oscillation applied to the plate; the few 
test results available to date in general have been obtained for this type 
of model in this excitation condition. This concept is the basis for the 
first of the three procedures described above, although for seismic 
excitation the problem is the inverse of the original problem formulation. 


in that the excitation originates in the earth. The other two methods noted 
also involve modeling of the structure-soil system; as such the system has 
intrinsic properties reflecting the make-up of the modeled system, physical 
properties, and especially the boundaries (for example, as they affect 
motion input, and reflection). 

     These analysis methods represent major advances in computational 
ability, but unfortunately all the techniques have limitations, and in many 
cases are not well understood.  At present their use involves a great deal 
of interpretive judgment. 

     One principal difficulty with all of the techniques is associated with 
the handling of the ground input. Except for special long period waves, in 
most cases the ground motion is noncoherent and nonuniform.  Thus far it 
appears that the analysis models may not be able to handle a broad spectrum 
of complex wave motions. None of the techniques adequately handle nonlinear 
effects, which are known to be of importance. As yet no good confirmatory 
comparison basis exists between field observations and computations made 
prior to an earthquake. 

     This entire topic is one that requires the most careful consideration. 
Exercise of judgment as to the meaning of the results, in the light of the 
comments given above, is required. Reliance on any sole approach is to be 

SEP Review Guideline Recommendations 

     In keeping with the SEP approach to review existing facilities, and as 
reflected in the philosophy and criteria developed to date, it appears 


desirable to outline briefly one technical procedure for estimating 
soil-structure interaction effects. As a result of extensive discussions 
between members of the SSRT and the NRC/LLL staff, and with recognition of 
the many uncertainties and complexities of the topic under consideration, 
the general approach presented below is recommended at this time as a 
guideline. It will be appreciated that many decisions will have to be made 
as a part of the calculational procedures described below and the exercise 
of judgment obviously will be required. Justification and documentation are 
necessary parts of the final analysis product. 

     At the outset it should be noted that the simplified approach described
below is not intended to preclude the use of any other procedures. The 
structural input motions (at the foundation level), however developed and 
justified, under no conditions shall correspond to less than 75 percent of 
the defined control motions (normally taken as the free-field surface 
motions); if a reduction in translational input motion is employed, then the
rotational components of motion also should be included. If other procedures
are employed they should be reviewed on a case-by-case basis. 

     For purposes of SEP review, one simplified approach for evaluating the 
effects of soil-structure interaction, involving a lumped parameter model, 
is deemed to be acceptable when employed under the following conditions. 

     1.  The control motions are defined as the free-field surface motions 
and are input at the structure foundation level. 

     2.  The soil stiffness, as represented by springs anchored at the 


foundation level, shall be modeled as follows. 

     i)  To account for uncertainty in soil properties, the soil properties,
the soil stiffnesses (horizontal, vertical, rocking and torsional) employed 
in analysis shall include a range of soil shear moduli bounded by (a) 50 
percent of the modulus corresponding to the best estimate of the large 
strain condition and (b) 90 percent of the modulus corresponding to the best
estimate of the low strain condition. For purposes of structural analysis 
three soil modulus conditions generally will suffice corresponding to (a) 
and (b) above, and (c), a best estimated shear modulus. 

     For structural capacity review the analyst generally should employ the 
worst case condition. For equipment review the in-structure response spectra
shall be taken as a smoothed envelope of the resulting spectra from these 
three analyses. 

     ii)  When embedment is to be considered it is recommended that the soil
resistances (stiffnesses as noted above) shall correspond to 50 percent of 
the theoretical embedment effects. This reduction is intended to account for
changes in soil properties arising from backfilling, and any gap effects. 

     iii)  Where it is judged necessary to model the supporting soil media 
as layered media, the stiffnesses are to be estimated through use of 
acceptable procedures. 

     3.  The radiation and material energy dissipation (i.e., the damping 
values) are considered to be additive for computation convenience. Normally 
the material damping can be expected to be about 5 to 8 percent. 

     The geometric damping (radiation energy dissipation) is recognized to 
be frequency-dependent.  However, in order to reduce the calculational 


effort (at least initially), and to be sure that excessive damping is not 
employed, it is recommended that values of damping be estimated 
theoretically (on a frequency-independent basis) as follows. 

       i)  Horizontal to be taken as 75 percent of the theoretical value.* 

      ii)  Vertical to be taken as 75 percent of the theoretical value.* 

     iii)  Rotation (rocking and torsional) to be taken at 100 percent of 
the theoretical value.* 

     In the case of layered systems the approach employed in establishing 
these values needs to be justified. 

     4.  The following analysis approaches are considered to be acceptable. 

          i)  When all composite modal damping ratios** are less than 20 
percent, modal superposition approaches can be used without any validation 

          ii) If in investigating the use of modal superposition approaches 
it is ascertained that a composite modal damping ratio** exceeds 20 percent,
one must perform a validation analysis. To perform this validation, it is 
generally acceptable to use a time-history analysis in which the energy 
dissipation associated with the structure is included with the structural 
elements, and that associated with the soil is included with the soil 

  *As calculated by generally accepted methods, as for example given in the 
book Vibrations of Soils and Foundations, by F. E. Richart, Jr., J. R. Hall,
Jr., and R. D. Woods, Prentice-Hall Inc., 1970. 

**As defined by generally accepted methods. 


in-structure response spectra obtained from a superposition analysis 
employing composite modal damping throughout the frequency range of interest
must be similar to or more conservative than those obtained from the 
validation analyses. 

     It is emphasized that the aforementioned procedures are intended to be 
guidelines and may be subject to revision as experience is gained under the 
SEP Program in attempting to arrive at relatively economical and simplified 
techniques for estimating the possible effects of soil-structure 

Respectfully submitted by the Senior Seismic Review Team: 

                                        N. M. Newmark, Chairman 

                                        W. J. Hall 

                                        R. P. Kennedy 

                                        R. C. Murray 

                                        J. D. Stevenson 

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