ML20040C957
| ML20040C957 | |
| Person / Time | |
|---|---|
| Site: | Midland |
| Issue date: | 01/31/1982 |
| From: | Corley W, Fiorato A CONSOLIDATED TESTING LABORATORIES, INC. |
| To: | |
| Shared Package | |
| ML20040C953 | List: |
| References | |
| NUDOCS 8201290408 | |
| Download: ML20040C957 (49) | |
Text
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CCLOSURE 1 o
i Report to CONSUMERS POWEP COMPANY Jackson, Michigan EVALUATION OF FEEDWATER ISOLATION VALVE PITS AT MIDLAND PLANT s
by W.
G Corley a
A. E. Florato -
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Submitted 'oy l
CONSTRUCTION TECHNOLOGY LABORATORIES A Division of the Portland Cement Association 5420 Old Orchard Road Skokie, Illinois 60077 January 1982 8201290408 820125 PDR ADOCK 05000329 A
PDR i
t e
v TABLE OF CONTENTS Page INTRODUCTION
.1 DESCRIPTION OF STRUCTURES 1
EVALUATION OF. CRACKING 8
Feedwater Isolation Valve Pit Unit 1 (West Unit) 9 Feedwater Isolation Valve Pit Unit 2 (East Unit) 10 SIGNIFICANCE OF CRACKS 12 RECOMMENDED PROGRAM FOR MONITORING STRUCTURAL INTEGRITY DURING IMPLEMENTATION OF REMEDIAL MEASURES 15 Displacement Monitoring 17 Crack Monitoring 20
SUMMARY
AND CONCLUSIONS 23 REFERENCES 24 APPENDIX A - STRENGTH OF CRACKED REINFORCED CONCRETE MEMBERS constsuction technology laboratories
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s EVALUATION OF FEEDWATER ISOLATION VALVE PITS AT MIDLAND PLANT by W. G. Corley and A. E. Fiorato*
INTRODUCTION This report presents an evaluation of the significance of cracks observed in the Feedwater Isolation Valve Pits located at Midland Nuclear Power Plant Units 1 and 2.
Observed cracks in these structures are described and significance of the cracks with regard to future load carrying capacity is discussed.
In addition, a program for monitoring structural integrity during implementation of remedial measures is described.
Remedial measures are being undertaken to underpin selected structures.
DESCRIPTION OF STRUCTURES A site plan for the Midland Plant is shown in Fig. l. I ) *
- Feedwater Isolation Valve Pits are located at the ends of Elec-trical Penetration Areas for Reactor Building Units 1 and 2.
These penetration areas are located on either side of the Auxiliary Building Control Tower.
The plan of the Auxiliary Bailding, shewn in Fig. 2,I gives the location of the Feedwater Isolation Valve Pits.
As can be seen in the figure,,
the pits are bounded by the Electrical Penetration Area, the
- Respectively, Divisional Director, Engineering Development Division, and Manager, Construction Methods Section, Construc-tion Technology Laboratories, a Division of the Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.
- Superscript numbers in parentheses refer to references listed at the end of this report. construction technology laboratories
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The function of the Feedwater Isolation Valve Pits is to enclose Seismic Category I feedwater pipe isolation valves.
Each pit is C-shaped with the open end toward the containment build ing.
The pits are structurally isolated from surrounding structures, are constructed of reinforced concrete, and are supported on backfill soil.
Figures 3, 4,
and 5 show the general reinforcement arrange-monts for the walls, floor, and roof of the Feedwater Isolation Valve Pits.
These figures are based on Bechtel Construction Drawing C-429, Revision 4, 10/1/79.
Additional reinfcrcement details are given on Drawing C-429 as well as Drawing C-442, Revision 1, 4/6/77.
Feedwater Isolation Valve Pit walls adjacent to the Buttress Access Shaf t and the Electrical Penetration Area are 2-f t 6-in.
thick.
Vertical reinforcement in these walls is No. 10 bars spaced at 12 in. on centers at each face.
Horizontal rein-forcement consists of No. 11 bars sphded at 12 in. on centers at each face.
Concrete compressive strength is specified at 5000 psi for the entire structure.
.The Feedwater Isolation Valve Pit wall adjacent to the Turbine Building is 3-ft 6-in. thick.
Vertical and horizontal reinforcement in this wall consists of No. 11 bars spaced at 12 in. on centers at each face.
The " exposed" wall of the Feedwater Isolation Valve Pit is 3-ft 6-in. thick.
This wall runs between the Buttress Access construction technology laboratories
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Vertical reinforcement consists of No. 10 bars at 12 in. on centers at each face.
Horizontal reinforcement consists of No. 11 bars at 12 in. on centers at each face.
The roof of the Feedwater Isolation Valve Pits is 2-f t thick.
Bottom reinforcement in the roof slab is No. 8 bars spaced at 12 in. on centers in each direction.
Top reinforce-ment is made up from No. 10 or No. 11 vertical wall bars bent at 90 into the slab.
This steel is supplemented by No. 8 bars spaced at 12 in. on centers.
The floor slab of the Feedwater Isolation Valve Pits is 4-ft th i ck.
Primary reinforcement consists of No. 11 bars spaced at 12 in. on centers in each direction at top and bottom of the slab.
Dowel bars for vertical reinforcement are also anchored in the base slab.
The floor slab is thickened along the wall adjacent to the Electrical Penetration Area.
EVALUATION OF CRACKING On January 12, 1982, personnel of the Construction Technology Laboratories inspected the Feedwatt. Isolation Valve Pits, Units 1 and 2 (west and east units).
The inspection included a visual survey of interior wall, floor, and roof surfaces.
Except for a small portion of one wall in each valve pit, exterior surfaces were not accessible for inspection.
In addition to visual observation, widths of selected cracks were measured using a 50 power crack microscope with a manufacturer's rated sensitivity of 0.001 in.
Approximate crack locations were measured using commercial quality steel
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tape measures.
Because of difficult access to many wall areas,
" exact" crack locations could not always be obtained.
- However, the accuracy of the measurements is well within that required to draw conclusions based on the results.
Weather on the day of the site visit was cold with tempera-tures ranging from approximately 15 to 20 F.
Sky conditions were mostly cloudy with intermittent snow flurries.
Feedwater Isolation Valve Pit Unit 1 (West Unit)
Although access was not ideal because of congested construc-tion scaffolding and piping, most wall areas in Unit 1 could be inspected.
Some areas were blocked by temporary supports put in place prior to start of remedial foundation work.
Natural light into the pit through the top hatch was blocked by con-struction scaffolding.
Therefore, primary light for inspection was provided by portable electric lights and hand held flashlights.
Interior wall and roof surfaces in Unit 1 were covered with a glossy clear coating.
This coating was sufficiently trans-parent to permit observation of formed surfaces.
Most formed wall surfaces contained craze crackr which are fine random cracks that commonly occur as a result of surface drying of concrete.
Craze cracks were also observed on interior roof surfaces.
Because the floor was covered with construction equipment, dirt and debris, there was only limited access for visual inspection.
The clear coating observed on walls and roof was not seen on floor surfaces.
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Figure 6 shows cracks mapped on interior f3cor and roof surfaces in Unit 1.
Primary access to all areas was from construction scaffolding located in the unit, Upper portions of the wall adjacent to the Buttress Access Shaft (Wall 4) and parts of the exposed wall (Wal] 3) were inspected from a ladder.
Cracks observed in Unit 1, shown in Fig. 6, are indica-tive of cracking that occurs as a result of restrained volume changes.
Maximum measured crack width was 0.006 in.
Vertical cracks in walls near the floor are attributed to volume changes caused by temperature and shrinkage of wall concrete combined with the restraining effect of the floor slab.
Cracke observed around the wall penetration and in the roof around the hatch opening are indicative of types of volume change cracking that often occur at discontinuities in concrete members.
The hori-zontal crack in Wall 3 did not penetrate through the clear coating.
Feedwater Isolation Valve Pit Unit 2 (East Unit)
Lighting conditions for inspection of Unit 2 were essentially the same as those encountered in Unit 1.
Primary lighting was provided by portable electric lights and hand held flashlights.
Since construction scaffolding was not available in all areas of Unit 2, access to most walls above eye level was obtained using ladders.
As was the case in Unit 1, all interior wall and ceiling surfaces were covered with a glossy clear coating.
Some crazing was observed on all surfaces of the walls and the roof.
Although the floor area in Unit 2 was cove red with some construction technology Isboratories
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No clear coating was visible on the floor surface, nor were any cracks seen.
Figure 7 shows cracks mapped on interior wall and roof surfaces of Feedwater Isolation Valve Pit Unit 2.
Maximum measured crack width was 0.007 in.
As was the case for Unit 1, observed cracks are attributed to restrained volume changes.
Wall cracks were observed near penetrations.
A vertical crack was seen at the intersection of Walls 2 and 3.
Vertical cracks were also observed in Wall 1.
The horizontal crack seen in Wall 3 did not reflect through the clear coating.
SIGNIFICANCE OF CRACKS Cracks observed on January 12, 1982 in Feedwater Isolation Valve Pit Units 1 and 2 are attributed to volume changes that occur in concrete during curing and subsequent drying.
No evidence of structural distress was observed.
As a measure of significance of observed cracks relative to future integrity of the structure,* the tensile stress that uncracked concrete is assumed to carry was compared to avail-able tensile capacity provided by structural reinforcement crossing the cracks.
Available structural reinforcement was determined from Bechtel Drawings No. C-429, Revision 4, 10/1/79 and C-442, Revision 1, 4/6/77.
Table 1 summarizes results of this comparison for members in which cracks were observed.
In the calculations, concrete
- A general discussion of strength of cracked reinforced concrete members is given in Appendix A.
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Af (kips)
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" Exposed" Wall 142.6 152.4 Wall 4 Wall Adjacent to Buttress Access Shaft 101.8 152.4 Roof Roof 81.5 94.8
- Minimum value when different reinforcement areas used in orthogonal directions.
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isassumedtocarryaprincipa1tensi]estressof4/-[where
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This assumption is consistent with Section 11.4.2.2 of the ACI Building Code. (2)
For vertical and horizontal directions, where cracks were observed in the walls and roof, resistance of reinforcement was calculated as A f, where A
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As indicated in Table 1, resistance provided by available reinforcement in the walls and roofs of the Feedwater Isolation Valve Pits exceeds tensile stress assumed to be carried by the concrete.
RECOMMENDED PROGRAM FOR MONITORING STRUCTURAL INTEGRITY DURING IMPLEMENTATION OF REMEDIAL MEASURES As' part of remedial measures to eliminate the possibility of unsatisfactory foundation conditions, selected areas of the Auxiliary Building will be underpinned.(1)
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The location of the west access shaft is shown in Fig. 8.
The east access shaft will be symmetrically located.
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that may be affected by underpinning operations.
Feedwater Icolation Valve Pit Units 1 and 2 should be monitored.
Figure 9 shous temporary supports that have been constructed for the Feedwater Isolation Valve Pits.I }
These supports will remain during underpinning operations.
The temporary supports are used to hang the Feedwater Isolation Valve Pits from the Buttress Access Shaft and the Turbine Building walls.
Temporary supports were in place at the time of the inspection on January 12, 1982.
During underpinning operations, structural integrity of the Feedwater Isolation Valve Pits should be monitored by continuous measurement of structural displacements and by regular visual inspection for cracking.
Displacement Monitoring A continuous time history of displacements of the Feedwater Isolation Valve Pits should be maintained during underpinning operations.
It is recommended that readings be taken on a daily basis with a maximum interval of one week.
Additional readings should be taken at selected construction milestones.
Displacement measurements will be made to monitor both absolute movement and relative distortions of structural ele-ments.
Figure 10 shows approximate locations of recommended displacement measurement points.
As a minimum, vertical dis-placements of the base slab of the structure should be measured at each of these points.
Relative horizontal displacements between the Feedwater Isolation Valve Pits and adjacent struc-tures may also be measured.
Displacement measurements of the construction technology laboratories
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base slab can be supplemented with measurements at the roof level.
Displacement measurements should he recorded as a function of time for the duration of underpinning operations.
Signifi-cant construction milestones should be marked at appropriate time intervals.
Prior to start of underpinning, limiting dis-tortion criteria should be selected so that critical deformation limits of the structure are not exceeded.
In this way, the distortion versus time plot will provide a warning of impending structural distress.
If distortion limits are reached, con-struction should be stopped until remedial measures are evaluated.
It is also recommended that the time history of distortions be submitted on a regular basis to a consultant familiar with reinforced concrete behavior and design.
The consultant could then provide recommendations on trends observed in the data.
Prior to start of construction and distortion monitoring, the consultant should review details of the monitoring plan.
Crack Monitoring As a supplement to the displacement monitoring program, periodic visual inspections of the Feedwater Isolation Valve Pits should be made to determine if new cracking has developed or if existing cracks have changed in width or length.
Crack inspections should be conducted on a continuing basis by qualified personnel.
In addition, a consultant knowledgeable in reinforced concrete design and behavict should inspect the valve pits at significant construction milestones.
Personnel
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who monitor cracking should be instructed in crack mapping techniques by the consultant prior to start of operations.
The following criteria should be used for evaluation of observed crack widths:
1.
If a new crack develops that is wider than 0.010 in.,
a consultant should evaluate significance of the new cracking.
Within two hours after observation of the crack, the consultant should provide a verbal report recommending whether construction should stop or con-tinue.
The verbal report should be confirmed with a written report within five days.
2.
If any crack exceeds 0.030 in. in width, a consultant should evaluate significance of the cracking.
Within two hours af ter observation of the crack, the con-sultant should provide a verbal report recommending whether construction should stop or continue.
The verbal report should be confirmed with a written report within five days.
3.
If development of yield strain in the reinforcement is I
inferred from any observed crack, construction should be stopped immediately.
Individual criteria will be recommended by the consultant for each structure.
If criteria are exceeded, a consultant should evaluate significance of the cracking.
Within two hours after observation of the crack, the consultant should provide a verbal report recommending whether construction should continue.
The verbal report should be confirmed by a written report within five days.
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The following criteria should be used in evaluation of significance of cracks that develop in the Feedwater Isolation Valve Pits:
1.
Geometry of member 2.
Amount and distribution of reinforcement in the member 3.
Material properties of the member 4.
Function of the member 5.
Magnitude and distribution of loads on the member 6.
Construction technique 7.
Sequence of construction 8.
Crack location and distributi~on 9.
Crack size 10.
Interaction of multiple cracks.
Basically these criteria define a procedure that requires the function and load carrying mechanism of the member or structure to first be defined.
Then the influence of cracks on the path of load distribution is determined.
In this way, the-cause of cracking is defined and the influence of cracking on future load capacity of the structure can be evaluated.
In evaluating cracks in reinforced concrete structures, it is not sufficient to base conclusions on a single criteria such as c, rack width.
The overall crack pattern including location and direction of cracks, length and width of cracks, and inter-relationship between multiple cracks must be considered.
The pattern of cracking provides significant clues with regard to causes of cracks and their effects on future performance.
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SUMMARY
AND CONCLUSIONS This report presents an evaluation of the significance of cracking observed in the Feedwater Isolation Valve Pits located at Midland Plant Units 1 and 2.
Cracks observed in these 1
structures by Construction Technology Laboratories' personnel on January 12, 1982 are attributed to restrained volumetric changes that occur during curing and drying of concrete.
No l
indications of structural distress were observed during the site visit.
Calculations based on section geometry indicate that structura.i reinforcement provided in the walls and roofs provides a capacity in excess of the tensile cracking stress attributed to the concrete.
A program for monitoring structural integrity of the Feedwater Isolation Valve Pits during implementation of remedial' measures to underpin the structure is also outlined.
It is recommended that measured displacements be used as the primary means of monitoring behavior of the structures.
It is also recommended that continuous displacement measurements be sup-plemented with visual inspections to monitor cracking in the structures.
Displacement and crack monitoring should be reviewed by a consultant knowledgeable in reinforced concrete 1
behavior. and design. construcilon technology laboratories
REFERENCES 1.
" Testimony of.Edmund M.
Burke, W. Gene Corley, James P.
Gould, Theodore E. Johnson, and Mete Sozen, on Behalf of i
the Applicant Regarding Remedial Measures for the Midland Plant Auxiliary Building and Feedwater Isolation Valve j
Pits," linited States of America Nuclear Regulatory Com-mission, Atomic Safety and Licensing Board, Public Hearing Testimony, Docket Nos. 50-3290M, 50-3300M, 50-3290L, and 50-3300L, Vol'. 1-Text and Vol. 2-Figures.
2.
ACI Committee 318, " Building Code Requirements for Reinforced Concrete (ACI 318-77)," American Concrete Institute, Detroit, 1977.
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APPENDIX A STRENGTH OF CRACKED REINFORCED CONCRETE MEMBERS l
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APPENDIX A
_ TABLE OF CONTENTS Page No.
INTRODUCTION A-1 TESTS OF STRUCTURAL WALLS A-1 Tests of " Low-Rise" Structural Walls A-2 Tests of "High-Rise" Structural Walls A-4 TESTS OF BEAMS A-13 TESTS OF CONTAINMENT ELEMENTS A-15
SUMMARY
AND CONCLUSIONS A-16 REFERENCES A-23 L
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APPENDIX A STRENGTH OF CRACKED REINFORCED CONCRETE MEMBERS by A. E. Fiorato and W. G.
Corley*
INTRODUCTION Cracking is an inherent characteristic of reinforced con-crete structures.
The e,xistence of cracks is not necessarily indicative of structural distress. The objective of this report is to clarify the relationship between cracking and strength of reinforced concrete memters.
The relationship will be demon-strated by examining the response of selected structural members that have been loaded to destruction in the laboratory.
To provide a cross-section of data, results from tests on struc-tural walls, beams, and containment elements will be considered.
TESTS OF STRUCTURAL WALLS Reinforced concrete structural walls are commonly used as lateral load resisting elements in buildings.
Both " low-rise" walls, which act as deep beams, and "high-rise" walls, which undergo significant flexural yielding, have been tested in the laboratory.
- Respectively, Manager, Construction Methods Section and Divisional Director, Engineering Development Division, Construction Technology Laboratories, a Division of the Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.
A-1 construction technology laboratories
Tests of " Low-Rise" Structural Walls Figure 1 shows the test setup used to apply reversing loads to eight specimens representing " low-rise" structural walls with boundary elements. (l
- Principal variables in this test program included amount of flexural reinforcement, amount of horizontal wall reinforcement, amount of vertical wall reinforcement, and height-to-horizontal leng th ratio of the wall.
Flexural reinforcement was varied from 1.8 to 6.4% of the' boundary element area.
Horizontal and vertical wall reinforcement were varied from 0 to 0.5% of the wall area.
Height-to-horizontal length ratio of the wall was varied from 1:4 to 1:1.
The test program was designed to deter-mine effects of load reversals.
Data obtained also provided information on the relationship between cracking and strength.
Principal test results for the eight walls are shown in Table 1.
For all specimens, except B5-4, the maximum nominal shear stress in the wall exceeded the stress at first observed shear cracking by a facter of at least 2.4.
For Specimen B5-4, which contained no vertical reinforcement in the diaphram, the maximum nominal shear stress exceeded the stress at first shear cracking by a factor of 1.5.
The ratio of maximum nominal shear force to first shear cracking even exceeded 2.5 for Spe~cimen B4-3 which contained no horizontal reinforcment.
For each of the " low-rise" walls tested, measured capacity exceeded
- The superscript numbers in parentheses refer to references listed at the end of this report.
A copy of each reference is attached.
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(2) Spec 6 mens subjected to static loading All other specimens subiected to load reversais.
Note 1 in. = 25 4 mm; 1,000 psi = 70.3 hg per square centimeter A-3
that calculated by American Concrete Institute Building Code Requirements for Reinforced Concrete.
Figure 2 shows crack patterns in the " low-rise" walls at the ultimate load levels listed in Table 1.
The inclined cracks are indicative of shear stresses that predominate in short cantilever members.
It is. apparent that the presence of cracks does not necessarily indicate loss of structura) capa-city.
Even with the extensive cracking shown in Fig. 2, the walls were carrying maximum applied loads.
For a particular section geometry and applied loading, structural capacity is a function of the amount and distribution of reinforcement.
There was no evidence that reversing loads caused residual stresses that reduced strength of the walls.
Additional data on these tests are given in Reference 1.
Tests of "High-Rise" Structural Walls Tests reported in References 2, 3, and 4 were conducted to obtain data on strength and deformation capacity of structural walls subjected to significant numbers of inelastic load reversals.
Effects of load history, section shape, vertical and horizontal reinforcement, confinement reinforcement, moment-to-shear ratio, axial compressive stress, and concrete strength were considered.
Figure 3 shows the setup used for tests of "high-rise" walls.
The walls were tested as vertical cantilever members with forces applied through the top slab.
The behavior of one of the test specimens is described in detail in the following A-4 construction technology laboratories
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Thi s behavior illustrates the influence of cracks that developed during the tests.
Additional data or other specimens can be obtained in Ref erences 2, 3, and 4.
Figure 4 shows the measured load vs deflection relationship for Specimen B3.
This was a barbell shaped specimen unich represented a wall with column boundary elements at each end.
As can be seen in Fig. 4, the wall was subjected to increasing levels of load reversals.
The test consisted of 42 complete load cycles.
Initial cracking was observed in the fourth cycle at a load of 28 kips.
First yielding in the vert.i. cal flexural reinf orce-ment occurred in Cycle 10 at a load of 45 kips.
Maximum measured crack widths were 0.012 in. in the tension boundary element and 0.025 in, across a diagonal crack in the web.
Figure 5 is a photograph of Specimen B3 at Load Stage 112.
This load stage, which is mar ked on Fig. 4, represents a point in the test when the specimen was unloaded.
There were no applied in-plane horizontal forces.
Figure 5 shcws the inter-secting pattern of cracks in tF: lower six feet of the wall af ter the first 21 load cycles.
From Load Stage 112, loads were increased in a positive direction until Load Stage 117 was reached.
Figure 6 shows the condi tion of the specimen at Load S tage 117.
At Load Stage 117, maximum measured crack width in the tension boundary element was 0.07 in. and maximum measured crack width in the wall web was approximately 0.16 in.
It should be noted that, at this load stage, the wall had been pushed to a lateral deflection of more than three times its yield deflection.
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Figure 7 shows the condition of Specimen B3 at Load Stage 123.
At this load stage, the maximum crack width measured in the tension column was approximately 0.07 in. and the maximum measured crack width in the wall web was 0.16 in.
When the wall was again unloaded, to Load Stage 125, the crack pattern shown in Fig. 8 resulted.
It is clearly evident from the behavior of Specimen B3 (and from other specimens tested) that the presence of cracks did not prevent the walls from main-taining their structural integrity and developing their nominal s treng th.
Figure 9 shows Specimen B3 at Load Stage 196.
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The cracking pattern in Fig. 9 is indicative of severe distress in the member, yet at this stage the wall carried its maximum load which corresponded toapproximately3.lvf[.
For purposes of comparison, the design strength this member calculated in accordance with the American ConcreteInstituteBuildingCodeis2.3vf[.
A question that occurs in evaluating cracked reinforced concrete structures is whether residual st resses associated with the occurrence of cracks influence strength of the member.
It is evident from the behavior of Specimen B3 that internally balanced residual stresses, such as those existing when the specimen was unloaded, did not influence streng th.
A-10 construction technology laboratories
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A-12
o TESTS OF BEAMS Background data on strength of cracked reinforced concrete members can also be obtained from tests on reinforced concrete beams.
Data from tests reported by Scribner and Wight are shown in Figs. 10 and 11.(5)
Figure 10 shows the load vs displacement curve for a reinforced concrete beam element that contained positive and negative steel.
The beam was subjected to increasing levels of ful'y reversed load cyc es.
Yielding occurred in the first load cycle as indicated in Fig, 10.
Figure 11 illustrates crack patterns that developed during the first inelastic loading and during subsequent load rever-sals.
As increasing numbers of load cycles wert spplied, the entire beam moment at the face of the column was carried by a force couple between the top and bottom layers of longitudinal steel.
Thus, applied moments were primarily resisted by the positive and negative longitudinal reinforcement.
Under load reversals a complete crack plane, labeled A-B-C in Fig. 11, formed through the beam.
This crack plane did not prevent the beam from transferring load.
During the final stages of the test, increasing numbers of inelastic load rever-sals caused concrete near the face of the column to abrade and eventually disintegrate.
This resulted in a " slip plane" along the beam at the face of the column.
The significance of such a slip plane is related to the number of inelastic load reversals and the level of shear stress on the beam.
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the crack plane did not become significant until repeated num-bers of inelastic cycles were applied.
Additional data on beam tests can be obtained from References 6 and 7.
In addition, tests of beam-column joints reported in Reference 8 also provide useful information.
Results shown in Fig. 10 indicate that beams can transfer flexural and shear loads even with the presence of cracks through their entire depth.
Tests conducted at the University of Washington have shown? that the effectiveness of web rein-forcement in resisting shear in reinforced concrete beams is not affected by axial force in the beam.59)
These tests were conducted on beams subjected to combined axial tension, bending, and shear.
Results indicated that ef fectiveness of web rein-forcement is not reduced by the presence of axial tension.
In the tests, applied axial load was sufficient to cause cracking prior to the application of transverse load.
For all beams with web reinforcement, measured load capacity of the precracked beams exceeded values calculated in accordance with the American Concrete Institute Building Code.
TESTS OF CONTAINMENT ELEMENTS Another series of tests that can be used to demonstrate the strength of cracked reinforced concrete members is reported in an experimental program to investigate shear transfer in cracked containments without diagonal reinforcement. (10)
The test setup was designed and constructed to simulate boundary conditions of a wall element of a pressurized containment sub-jected to tangential shear stresses.
Forces on an element in A-15 construction technology laboratories
a containment wall.are illustrated in Fig. 12.
Figures 13 and 14 show the test setup used for the experiments.
The experimental program included monotonic and reversing load tests on large-scale specimens subjected to biaxial tension and shear.
Specimens were 5-ft square and 2-ft thick with No. 14 and No. 18 reinforcement.
This discussion includes a description of one of the test specimens.
Additional data are available in Reference 10.
Figure 15 shows the' crack pattern observed in Specimen' Mal after reinforcement in the element was loaded to obtain a ten-sion stress of 54 ksi in the oteel.
This stress corresponds to 90% of the yield stress of the reinforcement.
Crack width-measurements made on the specimen after biax'ial tension was applied indicated a maximum width of approximately 0.036 in.
Figures 16 and 17 show the crack pattern and~ nominal shear stress vs shear distortion relationship for Specimen.MBl.
s Shear forces were applied while constant biaxial tension was ma in ta ined.
It is evident from Fig. 17 that the reinforced concrete element was capable of transferring shear forces even though it was traversed by biaxial tension cracks through the complete thickness.
SUMMARY
AND CONCLUSIONS Test data presented in this report demonstrate that cracks in an adequately reinforced concrete member do not prevent the member from developing its expected strength.
Adequate rein-forcement for the test specimens was determined in accordance with current code provisions.
Data presented also indicate the A-16 construction technology laboratories g-*g
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A-21
d level or severity of cracking associated with severe stress in reinforced concrete members.
Obviously the presence of cracks in a reinforced concrete structure cannot be summarily dismissed as insignificant.
The pattern of cracking and crack widths should be evaluated to determine their significance.
- However, the mere presence of a crack does not necessarily indicate that the integrity of the structure is'in jeopardy, or that its load-carrying capacity has been reduced.
A-22 construction technology laboratories
o,. s RE FE RENCES 1.
Barda, F., Hanson, J.M., and Corley, W.G., " Shear S trength of Low-Rise Walls with Boundary Elements," Special Publica-tion SP-53, Reinforced Concrete Structures in Seismic Zones, American Concrete Institute, Detroit, 1977, 496 pp.
2.
Corley, W.G., Fiorato, A.E., and Oesterle, R.G., "S t r u c-tural Walls," Special Publication, C.P. Siess Symposium, American Concrete Institute, Detroit, 1979 (to be published).
3.
- Oesterle, R.G.,
Fiorato, A.E.,
and Cor ley, W.G., " Rein-forcement Details for Earthquake-Resistant Structural Walls," Concrete International, December 1980, pp. 55-66.
4.
- Oesterle, R.G, Florato, A.E., and Corley, W.G., " Effects of Reinforcement Details on Seismic Performance of Walls,"
Proceedings of a Conference on Earthquakes and Earthquake Engineering:
The Eas tern United S tates, Vol. 2, Ann Arbor Science Publishers, Inc., 1981, pp. 685-707.
5.
Scri bner, C.F.
and Wight, J.K.,
"A Method for Delaying Shear Strength Decay of RC Beams," Proceedings of a Workshop on Earthquake-Resistant Reinforced Concrete Building Construction, Vol.
3, University of California,
Berkeley, June 1978, pp. 1215-1241.
6.
Wight, J.K. and Sozen, M. A., " Strength Decay of RC Columns Under Shear Reversals," Journal of the S tructural Division, AS CE, M ay 197 5, pp. 1053-1065.
7.
Brown, R.H. and J irsa, J.O., " Reinforced Concrete Beams Under Load Reversals," Journal of the American Concrete Insti tute, Vol. 6 8, No. 5, May 1971, pp. 380-390.
8.
Hanson, N.W. and Conner, H.W.,
" Tests of Reinforced Concrete Beam-Column Joints Under Simulated Seismic Loading," Research and Development Bulletin RD012, Portland Cement Association,1972, 12 pp.
9.
Haddadin, M.J.,
- Hong, S.T.,
and Mattock, A.H.,
"S tirrup Effectiveness in Reinforced Concrete Beams with Axial Force," Journal of the Structural Division, ASCE, September 1971, pp. 2277-2297.
10.
Oesterle, R.G. and Russell, H.G.,
" Shear Transf er in Large Scale Reinforced Concrete Containment Elements," Report No.
1, NUREG/CR-1374, Construction Technology Laboratories,
a Division of the Portland Cement Association, prepared for U.S. Nuclear Regulatory Commission, Washington, D.C.,
April 1980.
A-23 s
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