05000334/LER-1980-008-01, /01T-1:on 800225,during in-plant Insp,Potential Overstress Condition of Cable Spreading Room Floor Indicated Due to Postulated Seismic Loading.Caused by Utilizing Underside of Floor to Higher Degree than Original Design

/01T-1:on 800225,during in-plant Insp,Potential Overstress Condition of Cable Spreading Room Floor Indicated Due to Postulated Seismic Loading.Caused by Utilizing Underside of Floor to Higher Degree than Original Design

ML19332A569
Person / Time
Site:	Beaver Valley
Issue date:	09/11/1980
From:	Lacey W DUQUESNE LIGHT CO.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
Shared Package
ML19332A564	List: 05000334/LER-1980-008, Forwards LER 80-008/01T-1 05000334/LER-1980-008-01, /01T-1:on 800225,during in-plant Insp,Potential Overstress Condition of Cable Spreading Room Floor Indicated Due to Postulated Seismic Loading.Caused by Utilizing Underside of Floor to Higher Degree than Original Design
References
LER-80-008-01T, LER-80-8-1T, NUDOCS 8009160411
Download: ML19332A569 (3)

LER-1980-008, /01T-1:on 800225,during in-plant Insp,Potential Overstress Condition of Cable Spreading Room Floor Indicated Due to Postulated Seismic Loading.Caused by Utilizing Underside of Floor to Higher Degree than Original Design

Event date:
Report date:
3341980008R01 - NRC Website

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?S R E'CR T OATE dJ EVENT DESCRIPTION AND PR08A8LE CONSEQUENCES h ITTT1 IBased on in-plant inspections, a review was conducted on the as-built confieuration i m lof cable trays and conduits that are attached to the underside of the cable spreading l loiRi lrcom floor at elevation 725 ft.

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4 Attachment To LER 80-08/01T-l' Beaver Valley Power Station Duquesne Light Company Docket No. 50-334 During an in-plant inspection, it was noted that the underside of the service building floor at elevation 725 ft. 6 in, was utilized to provide support for conduit and cable trays to a higher degree than the original design anticipated.

As a result, an on-site reivew was made to determine the worst-case loading condition for the purpose of providing the basis of reanalysis. An area was investigated which was the most congested, and in which conduits and cable trays were supported in a manner which raised questions as to the adequacy of the light steel framing members. A preliminary simplified analysis indicated that, during a DBE event, the loads imposed by acceleration of cable trays and conduits

- would overstress the structural steel beams when forces due to horizontal and vertical bending were combined together algebraically.

The floor support system was checked for the normal dead load, live load, and vertical earthquake component and was found satisfactory.

It was the imposition of forces due to the DBE horizontal acceleration of the conduits and tray supports which were attached to the lower flanges of the steel beams which caused beam local overstresses.

The analysis for the OBE condition indicated that although some beams were slightly over allowable stress, no major structural damage would occur during the shutdown condition.

This problem is limited to the cable spreading floor because no other, Category I area of the plant has this configuration of light framing beams (B Series) with similar imposed loading from conduits and cable trays.

Based on the initial analyses that were performed, Duquesne Light Company was advised of this problem by the architect-engineer on February 15, 1980.

LER 80-08/01T was submitted to the NRC, in accordance with 10 CFR 50.36, to report this condition. The approach taken to resolve this problem is as follows:

Due to the difficulties encountered in analytical modeling of the geometry of the cable tray, conduit, and their supports, it was decided to establish a conservative design criteria and. proceed with modifications rather than attempting further reanalysis. The effort was limited to ensuring the structural adequacy of steel beams supporting the floor, conduits, and cable trays below the floor. The N'

as-built configuration of cable trays, conduits, and supports was developed to provide input to the analysis which determined the necessary modifications required to limit-the combined stresses within allowable limits for the design conditions.'

The original design criteria, i.e., BVM-52, " Structural Design Criteria", governed the analysis and design.

Seismic forces imposed by cable trays and conduits were computed as static acceleration loads equal to 1.5 times the applicable peak

- amplified resonant response, based on soil structure interaction, for the support elevation, using a value of four percent damping.

Four percent damping is acceptable according to Regulatory Guide 1.61 for bolted steel or reinforced concrete structures.

It should be noted that the 1.5 x p+ak is conservative and j

obviates the need for determining natural frequency characteristics of the system.

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Attachment To LER 80-03/OlT Page 2 Beaver Valley Power Station Duquesne Light Company Docket No. 50-334 4

It was assumed that the top flanges of the beams are continuously supported and horizontal seismic loads in the slab and the top flanges are directly resisted by diaphragm action of the concrete slab.

Stresses due to horizontal and vertical bending of beams were combined algebraically.

Significant torsional stresses were taken into account. Any loadings from the floor above were uncoupled by modifying the connections of elements which frame to both floors.

Resultant combined stresses were limited to the allowable limits.

The modifications to the floor framing system consisted of providing lateral bracing to the bottom flanges of the floor framing beams where stresses were exceeding the allowable limits. The lateral loads were transferred to the nearest column or wall. The loads from cable trays on the floor above were disconnected and fully supported from elevation 735 ft. 6 in.

Necessary drawings and calculations for beam modifications were developed.

The service building floor at elevation 725 ft. 6 in. has been modified so that the load carrying capacity is satisfactory for the design seismic loadings.

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