Experimental Study of the Behavior of Cable Trays Used in Nuclear Power Plant Applications

Transcription

1 Missouri University of Science and Technology Scholars' Mine International Specialty Conference on Cold- Formed Steel Structures (1980) - 5th International Specialty Conference on Cold-Formed Steel Structures Oct 18th Experimental Study of the Behavior of Cable Trays Used in Nuclear Power Plant Applications Suresh G. Pinjarkar Follow this and additional works at: Part of the Structural Engineering Commons Recommended Citation Pinjarkar, Suresh G., "Experimental Study of the Behavior of Cable Trays Used in Nuclear Power Plant Applications" (1980). International Specialty Conference on Cold-Formed Steel Structures This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Specialty Conference on Cold-Formed Steel Structures by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu.

2 EXPERIMENTAL STUDY OF '!HE BEHAVIOR OF CABLE TRAYS USED IN NUCLEAR POWER PLANT APPLICATIONS by SURESH G. PINJARKAR,l Ph.D. INTRODUCTION The cable trays used in nuclear power plant applications are required to withstand high earthquake forces. They are classified as Seismic Category I components and are expected to remain functional during and after the severe earthquake. The seismic qualification (performance evaluation) of the trays is accomplished by analysis, testing, or combination of analysis and testing. The seismic qualification analysis of the cable trays is performed using the AlSI Specification for Cold-Formed Steel Members. (1) For Seismic Category I structures, maximum allowable stresses to be used in design are limited to 90 to 100 percent of the specified minimum yield strength of the material. The design strength is often governed by buckling rather than by yielding resulting in much lower allowable stresses. An accurate analysis to determine the actual ultimate strength of the structure is complex. Method described in AlSI specification referr ing to laterally unbraced compression flanges (Section 3 Part III) is conservative and does not adequately take into account the post-buckling strength of the members. An experimental test program was undertaken to determine vertical load carrying capacity of the cable trays and study their behavior under extreme loads. This paper discusses vertical load tests to failure of solid bottom cable trays. Tests were performed in accordance with NEMA standards, (2) Sections lproject Manager, Wiss, Janney, Elstner, and Associates, Inc., Northbrook, Illinois. 189

3 190 FIFTH SPECIALTY CONFERENCE VEl-3.01 and VEl The results of tests were used to supplement the seismic qualification requirements of the cable trays. DESCRIPTION OF TESTS Test Setu..E. The vertical load tests to failure were performed on two representative 12-in. wide, 115-in. long solid bottom tray straight section specimens. A roller support near each end of the tray was used to create a simple beam span of a ft, with unrestrained ends. The dead load of the cables was simulated by means of a-ft long steel strips. An additional vertical downward uniform load was applied to the bottom flange of the tray by creating a vacuum under it. A test setup to apply vacuum loading is shown schematically in Fig. 1 and the photographs in Fig. 2. The vacuum pressure was regulated by means of a vent and control valve. The actual load was measured with a water-filled manometer. Instrumentation Strains and deflections at the center of the tray were measured during the test. The location of instrumentation is shown in Fig. 1. Electrical resistance strain gages were attached to the top flange of the tray to measure compressive strains. The vertical deflection at each flange was measured by means of dial gages. Test Procedure An initial set of readings was taken when the tray was empty. Next, the steel strips simulating a cable load of approximately 52 Ibs/ft were placed in the tray, and another set of readings was taken. The vacuum load was applied in increments of approximately 12 inches of water, which is equivalent to 62 lbsjft of vertical load on the t~ay. The deflection and strain data was recorded at each increment. The vacuum load was increased until tne capacity of the compressor was reached at approximately 9 ft of water. For the purpose of inducing the

4 BEHAVIOR OF CABLE TRAYS " 1 CABLE TRAY s PLASTIC FILM WOOD FRAME METAL STRIPS INSIDE SUCTION PIPE DEFLECTION DIAL AND STRAIN GAGE ELEVATION I ~ T 0.5".083'-"-' -...,..- PLASTIC FILM VACUUM TEST FLOOR CROSS SECTION AT CENTER Fig. 1 - Test 3etup for Vacuum Loading and Location of Instrumentation

5 192 FIFTH SPECIALTY CONFERENCE Fig. 2(a) - General Vacuum Test Setup Fig. 2(b) Location of Instrumentation

6 BEHAVIOR OF CABLE TRAYS 193 failure, a concentrated load was applied at the center of the tray by means of a hydraulic ram. The concentrated load was applied to a steel plate placed directly over the steel strips. For Test No.1, the concentrated load was applied in the absence of vacuum loading and was increased until the failure occurred. For Test No.2, the vacuum load was maintained at 9 ft of water, and the concentrated load was increased until failure occurred. Results for Test No.1 The results of the test using only the vacuum loading are shown in Table 1. A load-strain curve is shown in Fig. 2. The results indicate total recovery when the vacuum load was removed. The maximum vacuum load in this case was 551 Ibs/ft with the deflection of 0.50 in. The results of the test using concentrated load are shown in Table 2. Fig. 2 also shows a load-strain curve indicating the equivalent uniform load when the concentrated load is distributed over an 8-ft length. The concentrated load was assumed to be distributed uniformly over the span length in order to obtain better correlation between concentrated load test and vacuum load test. At the equivalent load of 627 Ibs/ft, the top flanges started undergoing large lateral deformations. The failure occurred at a load of 696 Ibs/ft due to buckling of web immediately below the top flanges. The tray continued to carry the load after failure without additional deformations. At the center of the tray both the flanges moved inward and at the end the flanges moved outward. Results for Test No.2 The results of the test are shown in Table 3. The load-strain curve is shown in Fig. 3. When the capacity of the vacuum pump was reached, the load was held constant at 9 ft of water and a concentrated load was applied at the center. Fig. 3 shows the load-strain curve with the concentrated load assumed

7 194 FIFTH SPECIALTY CONFERENCE TABLE 1 - RFSULTS OF VACUUM LOAD TEST: TEST NO. 1 Water Head Uniform Load (lbs/ft) Deflection Compressive Strain (in.) (micro in./in.) Dial 1 Dial 2 Gage 1 Gage 2 Remarks 0' ' '-0" '-0" '-0" '-0" '-0" '-6" '-0" '-6" '-0" ' Full recovery after load was removed TABLE 2 - RFSULTS OF CONCENTRATED LOAD TEST: TEST NO.1 Eq. Uniform Compressive Strain Concen tr a ted (micro in./in.) Average Load Load (lbs) Strain (lbs/ft) Gage 1 Gage , , , , , , ,325 1,592 1,459

8 BEHAVIOR OF CABLE TRA YS r ~----~ ~----+_----~----~----_+----_, 1000 ~ Ii. "- VI III...J 0 c( 0...J ~ --- _... _... -_ '.-;,.:;.:~' '/ ;" ;" / / /" ;" 't' :i : ~; j, ' YIELDING OF TOP FLANGE : APPROXIMATE FAILURE LOAD a 100 lilt I It, CURVE FOR VACUUM LOAD CURVE FOR CONCENTRATE D LOAD ASSUMED DISTRIBUTE D OVER 8 FT, o STRAIN (MICRO IN/IN) Fig. 2 - Load-Strain Curve for Test No r , 1000 ~ Ii. "- VI III...J 0 c( 0...J BOO APPROX IMATE FAILURE LOAD' 1300 Ibt/fl '4--- YIELDING OF TOP FLANGE I 200 o o STRAIN (MICRO IN/IN) Fig. 3 - Load-Strain Curve for Test No. 2

9 196 FIFTH SPECIALTY CONFERENCE TABLE 3 - RESULTS OF VACUUM PLUS (x)ncentrated LOAD TEST: TEST NO. 2 Water Head Uniform Load (lbs/ft) Deflection Compressive Strain (in. ) (micro in./in.) Dial 1 Dial 2 Gage 1 Gage 2 Remarks 0'-0" '-0" '-0" '-0" '-0" '-0" '-0" '-0" '-0" '-0" Full recovery when load was removed. 460 Ibs* Apply concentrated load at the center. 920 lbs" ,840 lbs* ,300 Ibs* , Start yield 2,760 lbs" ,430 1,100 Large deformations 3,680 Ibs* 1, ,510 1,565 Approximately 3" vertical deflection. 5,520 lbs* 1, ,070 - Final deflection equal to 1/2" after load removal. *Additional concentrated load at the center.

10 BEHA VIOR OF CABLE TRA YS 197 distributed over the 8-ft length. At approximately 900 lbs/ft load, the top flanges started exhibiting large deformations. At approximately 1,074 lbs/ft load, the maximum vertical deflection was approximately 3 in., with one of the top flanges exhibiting signs of lateral buckling. The test was discontinued when a load of 1,304 lbs/ft was reached and held constant. After the load was removed, the cable tray recovered a major portion of its deformation. The final vertical deflection, permanent set, was approximately 1/2 in. Fig. 4 shows the deformed shape of the tray after the test. DISCUSSION The allowable bending stresses in the top flange were computed according AISI Specifications considering lateral buckling of the unrestrained compression flanges. Although, AISI Specification does not provide a method to compute ultimate load capacity, it may be computed by using the safety factors used in AISI Specification. As shown in Ref. 3, for l2-in. wide trays, the allowable compressive working stress was ksi. Using a factor 1.67 (1.0/0.6) the allowable compressive stress for severe earthquake condition would be ksi which is less than the yield strength of 33 ksi. The computed ultimate load capacity of the trays based on ksi stress would be 416 lbs/ft. The ultimate laod capacity based on a yield stress of 33 ksi would be 563 lbs/ft. The tests indicated that at yield stress (strain = 1,100 micro in./in.) the cable trays were able to sustain an equivalent load of 675 lbs/ft in Test No. 1 and 900 lbs/ft in Test No.2. Also the tests indicated that ultimate load capacity of the trays was 700 lbs/ft. in Test No.1 and 1,300 lbs/ft in Test No.2. An ultimate load capacity was defined as a maximum load the cable trays were able to sustain without collapse. In both tests, the top flanges had undergone severe deformations and yielding before failure due to lateral buckling of the compression flanges. The

11 198 FIFTH SPECIALTY CONFERENCE Fig. 4 - Deformed Shape of the Tray After Test

12 BEHAVIOR OF CABLE TRAYS 199 lateral buckling did not occur before yielding, and, thus, the maximum allowable bending stress under severe loading earthquake loading can be assumed to be equal to 33 ksi. Furthermore, the cable trays could sustain and carry the load even after apparent failure due to buckling of the top flanges. CONCLUSIONS 1. The ultimate load carrying capacity of the trays was in excess of that computed using AISI equations and safety factors. 2. Based on the tests, the ultimate load capacity of the cable trays may be computed using an allowable compressive bending stress equal to the yield strength of the material. 3. The cable trays were found to be more flexible as indicated by higher deflections ~s compared to the deflections computed by using AISI methods. At higher loads, the cable trays continued to carry the load accompanied by large deformations without buckling. The deflection recovery after failure due to buckling was approximately 80%. 4. It was evident from the tests that the cable trays could sustain high loads accompanied by large deformations, though without total collapse. This may be helpful in designing the cable trays for Safe Stutdown Earthquake (SSE) where ultimate goal is to maintain integrity of the cables during and after the earthquake. ACKNOWLEDGEMENTS Experimental work was performed at Wiss, Janney, Elstner, and Associates where the author was previously employed. The tests were sponsored by United States Gypsum Company. Appreciation is expressed to Dr. J. M. Hanson, Hugh Pincus, and the technician staff at Wiss, Janney, Elstner, and Associates. Also appreciation is expressed to Construction Technology Laboratories where the author is currently employed for providing assistance in preparing the manuscript.

13 200 FIFTH SPECIALTY CONFERENCE APPENDIX - REFERENCES 1. Specification for the Design of Cold-Formed Steel Structural Members, 1968 Edition, American Iron and Steel Institute, Washington, D.C. 2. NEMA Standards Publication No. VEl-1976, National Electrical Manufacturers Association, New York, New York Pinjarkar, S.G., "Load Tests of U.S.G. Cable Trays," WJE Report No , Wiss, Janney, Elstner and Associates, Inc., Northbrook, Illinois.