Full Scale Load Testing of Selected RG4 Supporting Beam

Transcription

1 Full Sale Load Testing of Seleted RG4 Supporting Beam BPEX PROJECT Maptaphut, Rayong Final Report by Civil and Environmental Engineering Design and Consulting Servie Mahidol University Applied and Tehnologial Servie Center (MAT) Department of Civil Engineering, Faulty of Engineering for SKC/ Daewoo Teh Thailand Deember 2008

2 Full Sale Load Testing of Seleted RG4 Supporting Beam BPEX PROJECT Maptaphut, Rayong Final Report by Asst. Prof. Piya Rattanasuwan Dr. Praveen Chompreda Dr. Wonsiri Punurai Civil and Environmental Engineering Design and Consulting Servie, Mahidol University Applied and Tehnologial Servie Center (MAT) Department of Civil Engineering, Faulty of Engineering, Mahidol University ii

3 EXECUTIVE SUMMARY This report provides the full sale load testing results of RG4 beam arried out on BPEX projet site in the premise of Daewoo Teh (Thailand), loated at Maptaphut, Rayong on 27 November 2008 to resolve the questions and onerns about the loadarrying apaity of the beam. ACI Building Code was used for testing protools and aeptane riteria. Suessfully load tests were performed on site. Displaement transduers and gages were used to monitor the displaement at beam midspan, 1/3 of the span, and olumn support throughout the duration of the test. Additional minor rakings found during the test did not indiate impending failure of the beam. It was onluded that the RG4 beam met the aeptane riteria of Chapter 20 of ACI at both maximum defletion and defletion reovery, demonstrating the safety of the beam. iii

4 CONTENTS Exeutive Summary... iii Contents... iv List of Figures... v List of Tables... vi 1 Introdution Bakground Test Objetives Load Testing Program Testing Protool and Aeptane Criteria Load Testing Program Test Load Configuration Load Appliation Method Loading Proedures Instrumentations and Measurements of Load Test Defletion Measurement Crak Monitoring Evaluation of Member Capaity Load Testing Results Defletions Damages Flexural Craks Flexural Shear Craks Interfae Shear Crak between Slab and Beam Conlusion Reommendations Referened Douments Appendix: Moment Curvature Analysis of RG4 Beam First Craking At First Yielding At Crushing of Conrete iv

5 LIST OF FIGURES Figure 1. Test struture... 4 Figure 2. RG4 beam under Investigation... 4 Figure 3. Testing of Conrete Strength Using Rebound Hammer... 5 Figure 4. Effeftive Area of Test Load... 6 Figure 5. Steel Load Platform... 7 Figure 6. Loading of Steel Bar Bundles... 7 Figure 7. Loading Proedure... 8 Figure 8. Displaement Measurement Setup (a) GagEs and LVDT (B) Hook Details... 9 Figure 9. Instrumentation Setup Figure 10. Data Aquisition System Figure 11. Monitoring of Crak by Survey Telesope Figure 12. Finite Element Model of Test Struture Figure 13. Moment Curvature Relationship of Test Beam Figure 14. Load Displaement Relationship Figure 15. Displaement at Various Positions during the Test Figure 16. Flexural Crak Figure 17. Flexural Shear Crak Figure 18. Interfae Shear Crak v

6 LIST OF TABLES Table 1. Compressive Strength from Rebound Hammer Test... 5 Table 2. Summary of Fores under Various Load Cases Table 3. Defletions at Various Points of Test Beam Table 4. Moment and Curvature at Various Stages of Test Beam Table 5. Displaement at Various Stages vi

7 Full Sale Load Testing of Seleted RG4 Supporting Beam BPEX PROJECT Maptaphut, Rayong 1 INTRODUCTION 1.1 BACKGROUND Several flexural raks were observed in a RG4 reinfored onrete beam at line M8 M10/ PE PF of level TOS under servie dead load ondition. A number of questions asked with regard to the adequay of the beam s load arrying apaity. In response to these questions, a full sale load test was arried out on BPEX projet site in the premise of Daewoo Teh (Thailand), loated at Maptaphut, Rayong on 27 November 2008 to resolve the questions and onerns about the load arrying apaity of the beam in a reliable and definitive manner. This report provides the test results and their interpretation with regard to the performane of the beam in omparison with the ACI 318 ode aeptane riteria. 1.2 TEST OBJECTIVES The objetives of arrying out full sale load test on the RG4 beam are: To proof testing that the beam an resist working design loads in a servieable fashion for whih it had been designed. To asertain that the maximum defletions and rakings fall within limits onsidered aeptable by ACI 318. To aomplish the objetives, the RG4 beam was load tested. ACI Building Code was used for testing protools and aeptane riteria. 1

8 2 LOAD TESTING PROGRAM 2.1 TESTING PROTOCOL AND ACCEPTANCE CRITERIA The proedure used for onduting load tests in onrete strutures is desribed in hapter 20 of ACI 318 Strength Evaluation of Existing Strutures. In summary, the hapter provides guidane on establishing the magnitude of the test load and evaluating of the results. It suggests that the test load be plaed in stages, to eliminate impat and to allow the struture some time to deform under the load. After reording of the base reading (whih shall be taken not more than 1 hr before loading), the load is typially applied in inrements of 25% of the full test load (four or more equal inrements approximately) every hour. The defletions are measured after eah load inrement and after the full test load is in plae. At the end of a 24 hr holding period, another set of defletion measurements are taken and the test load is then removed. The defletion reovery or the residual defletion is measured after a 24 hour rest period. This is to permit at least some time dependent effets suh as reep and load redistribution within the strutural system to our and if they are signifiant to beome apparent and measurable. Aording to ACI , the total test load should not be less than 1.15D+1.5L 1.0D = 0.15D+1.5L where D is the dead load and L is the live load on the struture Load tests for both uniform and onentrated loading an be performed. The test an be onduted using piled up sandbags, steel bundles or onrete bloks. ACI 318 deems the test satisfatory if the struture does not show any evidene of failure (spalling and rushing of ompression zone onrete) and if the measured defletions of the member satisfy any of the following two limits Δ Δ 2 l t / 20,000h Δ / 4 max r max where Δ max is the maximum displaement measured during appliation of the test load relative to the initial position no more than 1 hour prior to the appliation of the test load, l t is the member s span length under the test load and h is its overall thikness or height of the member being tested. Δ r is the residual defletion the maximum displaement measured 24 hour after removal of the test load relative to the initial position. If the test fails to meet these two riteria, the ode allows a repetition of the test, whih an be performed not earlier than 72 hr after the initial load of the first test is 2

9 removed. The seond test may be onsidered aeptable if the maximum residual defletion does not exeed 20 perent of the maximum defletion during the seond test, measured from the level at the beginning of the seond test. Any raking that ours during load testing is a ause for onern and should be investigated. Craks that indiate an imminent shear failure of the tested members are of speial importane and should be viewed with alarm. The ode speifially mentioned long raks (those with a horizontal projetion longer than the depth of the member) ourring in regions where transverse reinforement is absent. Similarly, horizontal or short inlined raks in the regions of probable anhorage or lap splies of reinforement should be investigated, beause suh raks ould be preursors of brittle failure. 2.2 LOAD TESTING PROGRAM A number of items were onsidered prior to the load test, they inluded test load onfiguration, the means by whih the test load would be applied, the proedure for appliation of the test load, and the duration of appliation of the test load. These items are disussed in this report TEST LOAD CONFIGURATION A site meeting was arranged to review the field onditions prior to the load test. The first task faing engineers and tehniians onduting a field investigation was to determine the type of struture and to hek it against any information available drawings. During a preliminary walk through, the team visited an area above and below the tested member. There, the dimensions of the beam ross setion measured of 500x900 mm. The beam spanned 9 m on enter arrying the slab, supported by reinfored onrete olumns. The slab thikness was estimated as 150 mm. Based on measurements onduted, the onrete beam seemed to be in good ondition, with little damage observed. Figure 1 shows the typial PE and PF views of the test struture. Figure 2 shows the ondition of the RG4 beam investigated. A series of in situ non destrutive tests was performed with the purpose of investigating 28 day onrete ompressive strength. Figure 3 shows the rebound hammer test and Table 1 summarizes the test result. Based on the test, the average ompressive strength was above the speified ompressive strength of 280 kg/m 2 as speified in the drawing. 3

10 (A) PE View (B) PF View FIGURE 1. TEST STRUCTURE FIGURE 2. RG4 BEAM UNDER INVESTIGATION 4

11 TABLE 1. COMPRESSIVE STRENGTH FROM REBOUND HAMMER TEST Rebound Number Test Number Compressive POSITION Average strength kg/m FIGURE 3. TESTING OF CONCRETE STRENGTH USING REBOUND HAMMER After the site visit, the team onfirmed that onduting a load test as shown in the following layout (Figure 4) was the best method to produe a ritial flexural response in the ritial region of the beam under investigation. SKC and Daewoo onsultant engineers asked for the magnitude of the test uniform load, whih was omputed by our engineer as shown below. The total existing dead load was omputed from the weight of slabs, beams and finishing and it was estimated as 450 kg/m 2 Using a live load of 500 kg/m 2, the total required uniform test load was 5

12 1.15D+1.5L 1.0D = 0.15D+1.5L = 1268 kg/m 2 Using the effetive area of 51 m 2, this uniform distributed load is equivalent to the total load of 40 tons as shown in Figure 4. The moment and shear demand vs. apaity of the beam subjeted to this load were heked numerially by both parties to ensure safety prior to the test. FIGURE 4. EFFEFCTIVE AREA OF TEST LOAD LOAD APPLICATION METHOD Engineers from both parties had deided to use steel reinforement bundles whih were readily available at site for the test load. The test load would be entered over the speial designed supporting platform, transferring the test load as two point loads over the third points of the test beam. The platform was made up of H setion 300 X 300 X 10 X 15 mm with a total length of 27,000 mm. Using a unit mass of 94 kg/m, the total self weight of the platform was alulated as tons. The required loading after subtrating the self weight of the loading platform would be equal to 19 bundles of DB25s. Figure 5 and Figure 6 shows the loading platform and the plaement of steel bar bundles on the platform, respetively. 6

13 FIGURE 5. STEEL LOAD PLATFORM FIGURE 6. LOADING OF STEEL BAR BUNDLES 7

14 2.3 LOADING PROCEDURES As shown in Figure 7, the loading proedure was divided into 4 stages: 1. Monotonially loaded the beam from 0 to 100% of the full test load. The test load was applied every hour in inrements of 25% of the full test load with one hour rest between inrements. At every interval, defletion readings were taken. 2. Sustain the full test load in plae for 24 hours. At the end of the 24 hours sustaining period, another set of defletion measurements were taken and the test load was then removed. 3. Monotonially unloaded the beam from 100% of the full test load, in derements of 25%, until the test load was ompletely removed. This was to omplete within 2 hours. At every interval, defletion readings were taken. 4. Monitor the beam under no load for 24 hour period. The defletion reovery or the residual defletion was measured at the end of that period. FIGURE 7. LOADING PROCEDURE 8

15 3 INSTRUMENTATIONS AND MEASUREMENTS OF LOAD TEST 3.1 DEFLECTION MEASUREMENT To determine defletions (vertial displaements) of the beam under the test loads, speial able mounted dial gages and linear variable differential transduers [LVDTs] were used. Measurements were taken at the midspan, 1/3 of the span (loation of the framing beam), and over olumn support. Figure 8 and Figure 9 shows the instrument setup. The measurement auray of the displaement transduers is ±0.01 mm. The instruments were onneted eletrially to a data logger. Data olletion was aquired automatially. The sanning rate of the data logger is approximately 1 minute per hannels. (A) (B) FIGURE 8. DISPLACEMENT MEASUREMENT SETUP (A) GAGES AND LVDT (B) HOOK DETAILS 9

16 FIGURE 9. INSTRUMENTATION SETUP FIGURE 10. DATA ACQUISITION SYSTEM 10

17 3.2 CRACK MONITORING A survey telesope was utilized to monitor any possible raks that ourred in the tested member during load testing, as shown in Figure 11. FIGURE 11. MONITORING OF CRACK BY SURVEY TELESCOPE 4 EVALUATION OF MEMBER CAPACITY Prior to the load testing, 3 dimensional finite element analysis of the struture, as shown in Figure 12, was onduted to evaluate the fores in various members and ompared with the member s apaity. Two models were onsidered, one is the model with uniformly distributed load ating on the slabs as typially assumed during the design stage. The aim is to evaluate whether the members were properly designed aording to the ACI gravity load ombinations. The other model is the one with the same load onfigurations as tested to evaluate the fores ourring during the test and to predit the struture s response under test. The moment and shear fores at ritial setions for different load ombinations are summarized in Table 2. Due to the onfiguration of the test load, the moment and shear produed by the load test at ritial setion were slightly larger than those produed by having the design live load (500 kg/m 2 ) ating uniformly on the slab. 11

18 Test Beam FIGURE 12. FINITE ELEMENT MODEL OF TEST STRUCTURE TABLE 2. SUMMARY OF FORCES UNDER VARIOUS LOAD CASES Fore Finite Element Analysis Uniform Load As Tested Positive Moment at Midspan (kn m) Dead Load Dead Load Live Load 1.2 Dead Load Live Load Negative Moment over Support (kn m) Dead Load Dead Load Live Load 1.2 Dead Load Live Load Shear at fae of support (kn) Dead Load Dead Load Live Load 1.2 Dead Load Live Load * Using ACI methods, with strength redution fators per ACI Nominal Capaity*

19 The predited defletion at various point under test load is shown in Table 3. TABLE 3. DEFLECTIONS AT VARIOUS POINTS OF TEST BEAM Load Condition Midspan 1/3 of Span Column Shortening Dead Load Dead Load Live Load Expeted Defletion under Test Beause the flexural failure is the most likely mode of failure, a Moment Curvature analysis of the test beam, as well as the beams framing to it, was onduted in order to gain more insight into the behavior of the member at various stages. The moment urvature of the beam is shown in Figure 13 and summarized in Table 4. Details of alulation are in the Appendix. It an be seen from the moment urvature analysis that the raking moment (225 kn m) is smaller than the dead load moment (322 kn m). Thus, under the dead load, the beam an be expeted to rak as seen during the preliminary site investigation. However, the analysis annot predit the extent of the rak nor whether the rak found was not due to understrength of the member. The ultimate moment apaity of the beam (1210 kn m) is greater than the fatored ultimate moment (769 kn m). Thus, the beam appears adequate for the design gravity loads. The urvature at failure is about 8 times of that at yielding, so the failure behavior (under overloading) is expeted to be dutile. Moreover, the shear apaity of the beam (867 kn) is also greater than the fatored ultimate shear fore (324 kn m) so shear failure is unlikely. Under the test load, the moment aused by the test load (795 kn m) is well below the moment at first yielding of the steel (1147 kn m) so the beam is expeted to behave elastially during the test. 13

20 Moment (kn m) B 500x E E E E E E 05 Curvature (/mm) FIGURE 13. MOMENT CURVATURE RELATIONSHIP OF TEST BEAM TABLE 4. MOMENT AND CURVATURE AT VARIOUS STAGES OF TEST BEAM Stage Moment (kn m) Curvature (/mm) Craking E 07 First Yielding of Steel E 06 Crushing of Conrete E 05 14

21 5 LOAD TESTING RESULTS 5.1 DEFLECTIONS The load defletion relationship of the test beam is as shown in Figure 14 and the defletions at various stages are summarized in Table Load (ton) Displaement (mm) 2 Midspan Third Point Column FIGURE 14. LOAD DISPLACEMENT RELATIONSHIP TABLE 5. DISPLACEMENT AT VARIOUS STAGES Stage Defletion (mm) Column Third Point Midspan ACI Exp. FEM Exp. FEM Exp. FEM Limit Full Test Load Max During Sustained Load 4.5 Full Test Load after hours Initial Reovery Hours Reovery

22 The maximum defletion at midspan was found to be within the limit of L 2 /20000h = 4.5 mm aording to ACI Therefore, the beam is onsidered satisfatory for the load test and the reovery rate need not be heked. Nonetheless, the maximum reovery at midspan is 0.66 mm, whih is 24.5% of the maximum defletion. This reovery rate is also smaller than the limit of 25% required by ACI Thus, the beam is also onsidered satisfatory using the reovery rate riteria. Note that a member that has maximum defletion below the limit of L 2 /20000h is onsidered very stiff by ACI. This limit is muh more stringent than the limit of maximum defletion under live load of L/360 = 25 mm. Figure 15 shows the defletion at olumn support, 1/3 of the span, and at midspan of the test beam at different loading stages. The gage for olumn was installed on the tension side due to spae onstraints. During the initial loading, the defletion measured at the olumn inreased beause the bending of the olumn has greater effet than the shortening of the olumn. However, after sustaining the full test load for 24 hours, negative (shortened) value was observed. The reoveries of the members were almost immediate as very little hange in the initial and 24 hours reovery was observed. However, some small residual deformations were observed, espeially at midspan due to additional rakings of the member during the appliation of test loads Defletion (mm) Full Test Load Full Test Load after 24 Hours Initial Reovery 24 Hours Reovery 3.0 Column 1/3 Span Midspan Position FIGURE 15. DISPLACEMENT AT VARIOUS POSITIONS DURING THE TEST 16

23 5.2 DAMAGES During the testing, existing and new raks were losely monitored. Three types of raks were found on the test struture during the test, namely, flexural raks, flexural shear raks, and Interfae shear rak between slab and beams. No rushing of onrete was observed FLEXURAL CRACKS Before the testing, there exist some flexural raks in the middle portion of the beam. Craking of onrete beam near the midspan is to be expeted as the moment urvature analysis of the beam setion showed that the beam would rak under the dead load moment alone. During the testing, there were several new flexural raks formed in the middle third portion of the beam as shown in Figure 16. However, the rak widths remained very small (less than 0.5 mm) and no spalling of onrete was observed. FIGURE 16. FLEXURAL CRACK 17

24 5.2.2 FLEXURAL SHEAR CRACKS Flexural shear raks were observed at the east side of the beam next to the framing beam as shown in Figure 17. During testing, the rak propagates into the ompression zone of the member but the rak width remained small. No spalling of onrete was observed. The rak partially losed upon reloading. FIGURE 17. FLEXURAL SHEAR CRACK INTERFACE SHEAR CRACK BETWEEN SLAB AND BEAM During the last loading stage, raks were observed at the interfae of slab and beam on the east side of the struture near the olumn support as shown in Figure 18. No suh rak was observed on the west side of the beam. This rak ourred due to large shear transfer at the beam and slab interfae. Calulation has shown that the onrete frition between the interfae alone is not suffiient to transfer suh shear fore. Thus, steel reinforement was required to transfer the interfae shear. Aording to the drawing and alulation, this beam fulfill this interfae reinforement by extending the beam s stirrup reinforement into the slab; Therefore raking of onrete at the interfae may be allowed. The rak width remained less than 1 mm even after sustained loading and partially losed upon unloading. 18

25 FIGURE 18. INTERFACE SHEAR CRACK 6 CONCLUSION Suessfully load tests were performed on site to verify the strutural apaity of the beam. During the testing, there were some new flexural raks, flexural shear raks, and interfae shear raks formed in several portions of the beam. However, these rak widths remained very small and no rushing and spalling of onrete was observed The maximum defletion was found to be within the limit of L 2 /20000h = 4.5 mm aording to ACI Therefore, the struture is onsidered satisfatory for the load test and the reovery rate need not be heked. The maximum reovery at midspan is 0.66 mm, whih is 24.5% of the maximum defletion. This reovery rate is smaller than the limit of 25% required by ACI Thus, the struture is also onsidered satisfatory using the reovery rate riteria. The struture met the aeptane riteria of Chapter 20 of ACI at both maximum defletion and defletion reovery, demonstrating the safety of the beam. 19

26 7 RECOMMENDATIONS Although the raks found in the test beam did not appears to indiate strutural inadequay and the defletion under test load was well below the maximum allowable defletion, exessive raking ould pose a risk of long term orrosion of the steel reinforement inside the onrete. Further, long term reep and shrinkage of onrete may ause widening of the rak. Sine this struture is loated in the oastal area, where the risk of steel orrosion is high, it is reommended that periodi inspetion should be arranged for signs of orrosion. 8 REFERENCED DOCUMENTS ACI Committee 318 (2008). Building Code Requirements for Strutural Conrete (ACI ) and Commentary, Amerian Conrete Institute, Farmington Hills, MI, USA. ACI Committee 437 (2003). Strength Evaluations of Existing Conrete Buildings, ACI 437R 03, Amerian Conrete Institute, Farmington Hills, MI, USA. Bungey, J. H. and Millard, S. G. (1996), Testing of Conrete in Strutures, 3rd Edition, Chapman & Hall, London. 20

27 9 APPENDIX: MOMENT CURVATURE ANALYSIS OF RG4 BEAM The moment urvature analyses were based on the following material properties: Conrete Compressive 28 MPa Steel Yield 400 MPa Strength Strength Conrete Modulus of Rupture 3.3 MPa Steel Elasti 200 GPa Modulus Conrete Elasti Modulus 25.4 GPa Steel Yield Strain Conrete Crushing Strain To be onservative, the ontribution of slab flanges and ompression reinforement were ignored. This would give slightly lower moment apaity for eah ase ompared with when taking them into aount. 9.1 FIRST CRACKING Before raking, the beam is elasti. The raking moment is obtained by equating bottom stress to the onrete modulus of rupture. M = fri Where = h = mm, 1 I = = bh x10 mm 4 Thus, M = 225 kn m r Strain in onrete s bottom fiber ε f r = = 1.311x10 E Curvature is obtained by dividing the strain by the neutral axis depth ε 7 φ = = 2.91x10 mm

28 9.2 AT FIRST YIELDING After raking, the stress strain relationship of onrete is assumed to be paraboli up to failure, as shown below: b Strain of onrete d h d -d φ d- Cs C T Strain of steel Beam Setion Strain Distribution Stress Distribution The ompression fore in onrete is obtained by integration of parabola stress blok: C f bφ φ 1 3ε ' 2 = ε 0 0 ε φ = = d At yielding, tension fore in steel is: T = A s f y Equating ompression fore in onrete and tensile fore in steel and solve for neutral axis depth to get = 318 mm. ε = d = The.g. of ompression fore is obtained from: 8ε 0 3ε x = 12ε 4ε 0 = 202 mm 22

29 C All dimension are in mm T Moment orresponding to the first yielding of steel: ( d ( ) = 1147 M n = As f y x kn m Curvature orresponding to the first yielding of steel: = ε y 6 φ = 2.36 x 10 mm 1 d 9.3 AT CRUSHING OF CONCRETE Conrete is assumed to rush in ompression when its strain reahes C f bφ φ 1 3ε ' 2 = ε 0 0 φ = T = A s f y Equating ompression fore in onrete and tensile fore in steel and solve for neutral axis depth to get = 157 mm. 8ε 0 3ε x = 12ε 4ε 0 = 98.2 mm 23

30 Nominal moment apaity of the setion orresponding to the rushing of onrete: ( d ( ) = 1223 M n = As f y x kn m Curvature at moment apaity: φ = = 1.91x10 mm 1 24