ML20150C454

Safety Evaluation Concluding That Seismic Analysis Methods for Bldg 10 & Walkover Structure Conservative.Gaps Provided Adequate to Accommodate Relative Motions Which Occur Between Subj Structures & Walkover Structure & Turbine Bldg

ML20150C454
Person / Time
Site:	Fort Saint Vrain
Issue date:	03/10/1988
From:	Office of Nuclear Reactor Regulation
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ML20150C448	List: ML20150C444 ML20150C454
References
TAC-55287, NUDOCS 8803180258
Download: ML20150C454 (3)

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION STRUCTURAL AND GEOSCIENCES BRANCH SEISMIC ANALYSIS OF BUILDING 10 AND WALKOVER STRUCTURE FORT ST. VRAIN NUCLEAR GENERATING STATION DOCKET NO. 50-767

1.0 INTRODUCTION

Public Service Company of Colorado (PSC) added two naw seismic safety-related structures to the Fort St. Vrain Nuclear Generating Station. The Walkover Structure (designed by PSC)gned by Stone and Webster) and the new structures are Building 10 (desi which connects Building 10 and the Turbine Building. Since the methnds used for the seismic analyses perfomed on these structures are different from those contained in the FSAR, a review of the methods used is required.

2.0 EVALUATION Building 10 is a reinforced concrete frame structure 34 feet by 45 feet in plan and about 60 feet high. The building is founded on 6 caissons, each 4 feet in diameter, and extending about 60 feet to bedrock. The soil con-sists of 4 layers each having different shear wsve velocities. The struc-tural model used to perfom the seismic analysis was a stick model with masses lumped at each of the six floor elevations. Soil-structure interaction effects were modeled with springs attached to the foundaton of the structure.

The spring constants were evaluated based upon the stiffnass charactaristics of the caissons. Tha methods in Ref. I ware used to evaluate the stiffnasses of a unifom soil layer. The PILAY2 computer program (Ref. 2) was used to detemine horizontal interaction spring constants using the actual layered soil configuration. Damping values of 7% for the SSE* and 4% for the OBE ware used for all modes. The input to the model consisted of a Reg. Guide 1.60 spectrum anchored at 0.05g and 0.109 for tha OBE and SSE respectively.

Spectral plots are made to compare the ST (damping) OBE R.G.I 60 spectrum for Building 10 with the ?% OBE FSAR spectrum and the 7% SSE R.G.I.60 spectrum for Butiding 10 with the 5% DBE FSAR spectrum. The Building 10 spectra envelope the FSAR spectra in both cases. A time history analysis is perfomed and the results are used to evaluate stresses in the struc-tural members of Butiding 10 and to detemine the response spectra for each mass point in the Building 10 model.

• Safe Shutdown Earthquake (SSE) and Design Basis Earthquake (DBE) have the same meaning. The latter tam has been used in the FSAR of Fort St. Vrain.

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2 The Walkover Structure is a braced steel frame structure providing a l

walkway between the various levels of Building 10 and the Turbine Buildir.g.

It is about 19 feet by 6 feet in plan aer' 54 feet tall.

It is founded I

on a reinforced concrete footing varying between 4 feet and 6 feet in thickness. A lumped mass cantilever model was used to evaluate the seismic response of the Walkover Structure. Since the structura is vary light, teil-structura interaction (SSI) effects are emitted and the same seismic input rntions used for Buildirp 10 are input directl" to tha foundation. Reg. Guide 1.60 spectra were used as input to the structural model. These spectra are conservative ralativa to the FSAR desigr spectra and therefore considered to be acceptable.

The affect of structure-structure interaction were specifically considerad.

Since the masses of the new buildings are ima11 and their fundamantal fre-quencias are high as compared with those of the Turbine Puilding, the motion of the Turbine Building would not be affected by the new buildings. However, on the other hand, the Turbine Building response may affect the horizontal motion of Building 10 and the WalkcVer Structure.

Inputting the Turbine Building basement motion directly to the mat of Building 10 and the Walk-over Structure would provida bounds of this effect.

In viaw nf the relative masses of tha structures this approach is acceptable.

The isolated paak E-W DBE displacements cf the three buildings are:

Turbine Puilding 0.096 inches Puilding 10 0.210 inchas Walkover Structure 0.858 inches The maxirum relative displacements between the Turbire Buildtrg/ Walkover Structure and the Building 10/Valkover Structure are 0.954 inches and 1.068 inches respectively. Both of these are less than the available 2 inch gap. Thare is a i inch gap betwean the checker plates and the Turbine Building, and it is shown that the bolts holding the checker plates will fail at ar app 1ted load of 334 pourds per foot which will rot have a significant effect on tha impact on the Turbine Building. This is because of the fact that the load is small, and that tha nature of the bolt shear failure is <uch that once impact occurs the bolts will fail and the checker plates cannot be involved in further impacts.

There is a 1-inch gap betwaen Building 10 and the step en the Walkover Structure. This clearanca exceeds the calculated relative motion of 0.414 inches and is therefore adequate.

3.0 CONCLUSION

Tha seismic analysis methods used for Building 10 and the Walkover Struc-ture have been found to ba conservative relative to the FSAR. The gaps

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j provided are found to adaquately accomodate relative motions which occur between these two structures and between the Walkover Structure and the

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Turbine Building.

References

• 1.

J. P. Singh, N.C. Donovan, and A.C. Jobsis, "Dasign of Machine Foundation on Piles" ASCE Journals of the Geotechnical 1

Engine **ing Division, August 1977.

2.

Systems Analysis Control and Design Activity (SACDA), itser Manual, PILAY2, A Cortuter Program for Calculation of Stiffness and Damping of Piles in Layered Media. The University of Wettern Ontario, London, Ontario, Canada, January 1981.

Dated: March 10,1988 Principal Contributor:

S. P. Chan 1

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