FRICTION SLIPPING BEHAVIOR BETWEEN CONCRETE AND STEEL -AIMING THE DEVELOPMENT OF BOLTED FRICTION- SLIPPING JOINT - Tomokazu Yoshioka, Masamichi Ohkubo Kyushu Institute of Design, Japan Abstract The authors are developing the connecting seismic shear walls to the surrounding structural frame through the bolted friction-slipping joints that function as energy dissipation dampers during an earthquake. This friction-slipping joint is composed of a steel plate, concrete plate, and the bolts to joint both together. This paper presents the outline of the dynamic loading tests to investigate the friction coefficient of the joint during slipping, changing displacement amplitude, loading velocity, concrete compression strength, thickness of concrete plate and the initial tension given into the bolts. And this paper also presents a simplified equation to predict the friction-slipping behavior of the joint. 1. Introduction In order to use a building continuously after a great earthquake, it is desirable to keep the earthquake response of the building into the elastic range. The concept called damage control design has been proposed as one of the seismic design methodology to archive such the strategy. In the building designed through the concept, seismic dampers as adjuncts are often applied to the main structure to dissipate earthquake vibration energy, and it is expected that the earthquake response displacement of the building is consequently minimized. The authors are developing the seismic shear wall installed into the structural frame through the bolted friction-slipping joints that function as energy dissipation dampers during an earthquake. At the joint, the surfaces between the steel plate attached to the structural frame and the reinforced concrete walls are rigidity tightened by steel bolts in general. However, slipping with a constant friction is allowed between the jointing surfaces, if the joint is subjected to the force that exceeds the friction force provided by the bolts. The concept of the shear wall system is illustrated in Fig.1. This joint system can dissipate the induced earthquake energy during slipping. In addition, the lateral 1281
BEAM FRICTION-SLIPPING JOINTS CLEARANCE CLEARANCE LATERAL FORCE REINFORCED CONCRETE WALLS SLIP HYSTERESIS LOOP FRICTION JOINTS NOT ALLOWED SLIPPING BEAM COLUMN Figure 1: Concept of Shear Wall System COLUMN force transmitted to the shear wall through the joint can be controlled by the setting adequately friction force due to the tightened bolt tension, so that the installed reinforced concrete wall can be prevented from earthquake damage as well. In this paper, the dynamic loading tests focussed on the friction-slipping joint were conducted to get the fundamental information regarding the slipping characteristics between the steel plate and concrete surface tightened by the bolts. And a simplified equation to predict the friction coefficient during slipping is presented on the basis of the statistical analysis for the test results. 2. Specimens Fig.2 shows the overall view of the assembled joint model specimen given in this test. The same two concrete blocks, which correspond a part of the concrete wall at the friction-slipping joint shown in Fig.1, sandwich a steel plate and the two concrete blocks and a steel plate are tightened together through a 19mm diameter high-tension steel bolt. In the loading test, the concrete blocks were rigidly fixed to the reaction steel frame and the sandwiched steel plate was loaded with a servo-actuator. To make the slipping displacement possible, the 26mm wide and 100mm long slot was provided in the steel plate as shown in Fig.3. Two slots are provided in the steel plate to use the same plate twice turning reversely and both side slots are applied to the one loading test. A double surfaces friction that consisted of two concrete plates and one steel plate was adopted in this test. However, another double surfaces friction technique such as the combination of two steel plates and one concrete plate can be actually considered. 1282
19mm DIA.BOLT 600 FRICTION-SLIPPING SURFACES REACTION REACTION BLOCKS 100 26 26 100 250 LOADING STEEL PLATE REACTION REACTION BLOCKS CONCRETE PLATES REACTIONS Figure 3: Steel Plate THICKNESS=200/100 16 LOADING SLOT STEEL PLATE CONCRETE PLATE Figure 2: Assembled Joint Model SLIPPING DIRECTION FRICTION-SLIPPING SURFACE 200 180 Figure 4: Concrete Block Table 1: Summary of Testing Condition Bolt Thickness fc Maximum Cyclic Number of Series Tension (*1) (*2) Velocity Amplitude Patterns Specimen (kn) (mm) (MPa) (cm/s) (mm) CS1 56.8 40 1 4 CS2 51.6 80 3 200 CS3 4 120 56.8 1 2 CS4 35.9 CS5 100 57.0 40 4 1 CS61 90 1 200 51.6 CS62 60 *1:thickness of concrete block,*2:concrete compression strength The size of the steel plate is 250mm in width, 600mm in length and 16mm in thickness as shown in Fig.3. The steel plate is a mild steel of the Grade 400MPa tensile strength. The mill scale covered on the surface of the steel plate wasn t eliminated in the tests. The size of the concrete block, which corresponds to the friction surface, is 180mm x 200mm rectangle as shown in Fig.4. The friction surface of the concrete block was provided with the condition after removing plywood. However, the tow corners of the concrete block intersected to the slipping axis were removed with about 10mm width to prevent stumbling. In the experiments, six test series were planned to compare the differences of the loading amplitude (40mm and 80mm), the loading velocity (4cm/sec and 1cm/sec), the concrete compression strength (50MPa and 30MPa), the concrete thickness 1283
Table 2: Detail of Cyclic Patterns Cyclic Amplitude (mm) 10 20 40 20 10 Pattern Frequency (Hz) 2 1 0.5 1 2 No.1 Num. of Cycs. 1 1 10 1 1 Cyclic Amplitude (mm) 10 20 40 20 10 Pattern Frequency (Hz) 0.5 0.25 0.125 0.25 0.5 No.2 Num. of Cycs. 1 1 10 1 1 Cyclic Amplitude (mm) 10 20 40 80 40 20 10 Pattern Frequency (Hz) 2 1 0.5 0.25 0.5 1 2 No.2 Num. of Cycs. 1 1 1 5 1 1 1 (200mm and 100mm) and the initial tension given into the bolt (60kN, 90kN and 120kN). Table 1 summarizes the entire scheme. For the CS1 to CS5 series four specimens were prepared under the same testing condition, while for the CS61 and CS62 only one specimen was prepared 3. Testing setup The load that enforced slipping at the joint surfaces was applied to the sandwiched steel plate by a 200kN servo actuator while the concrete blocks were fixed to the steel reaction frame as shown in Fig.2. Three different cyclic patterns were planed to make the time history of the enforced displacement. Table 2 shows the detail of the cyclic patterns that are arranged by the amplitude, the frequency and the number of cycles with a sinusoidal wave. The friction force, the relative slipping displacement between the steel plate and the concrete blocks and the bolt tension were measured. The intervals of the data sampling were set to 6 milliseconds, which was the maximum speed of the measuring equipment, for CS1, CS2, CS4 and CS6, 7 milliseconds for CS3, and 22 milliseconds for CS2. 4. Test Results Fig.5 shows the relations between the friction coefficient and the slipping displacement obtained by the experiments of CS1-1 and CS3-1. Here, the friction coefficient is that the load applied to the steel plate was divided by the tension force, which was introduced into the steel bolt before loading, considering the number of friction surfaces. Fig.6 shows the relations between the friction coefficient and the total slipping displacement in the specimens CS1-1 and CS3-1. Here, the total slipping displacement is the summation of slipping displacement experienced by the time from the beginning of the test. Both the specimens, which are the first one of specimens in the standard CS1 series and the CS3 series with the lower velocity than the standard series respectively, represent the common friction slipping characteristics to all the tests. In Fig.6, one characteristic behavior is observed regarding the relations between 1284
FRICTION COEFFICIENT 0.5-0.5 CS1-1:V=4cm/sec CS3-1:V=1cm/sec - -25-20 -15-10 -5 0 5 10 15 20 25 SLIPPING DISPLACEMENT (mm) -25-20 -15-10 -5 0 5 10 15 20 25 SLIPPING DISPLACEMENT (mm) Figure 5: Friction Coefficient and Slipping Displacement Relations CS1-1:V=4cm/sec CS3-1:V=1cm/sec FRICTION COEFFICIENT 0.8 0.6 0.4 0.2 Figure 6: Friction Coefficient and Total Slipping Displacement Relations the friction coefficient and the total slipping displacement. It is that if an envelope curve is drawn on the relations between the friction coefficient and the total slipping displacement, the curve can be represent by two lines which consist of the first characteristic that the peak value of friction coefficients increases gradually as the total slipping displacement increases and the second characteristic that the peak value of the friction coefficients is almost stable after the total slip displacement reached a certain amount. This first characteristic is observed in all the tests although they were under the different testing conditions. However, the second one was not observed in only the CS5 series in which the thin of concrete blocks were used. In the cycle, while the stable friction slipping is kept, the relation between the friction coefficient and the slipping displacement approximately shows a rigid-plastic pattern as shown in Fig.5. 1285
5. Influence of Various Testing Conditions to the Friction Coefficients In order to actually apply the concept of bolted friction-slipping joints to the structural design of a building, the variation and the stability regarding friction coefficient (a) MAXIMUM AMPLITUDE (b)t.s.d.=100mm (c)t.s.d.=800mm FRICTION COEFFICIENT 0.8 0.6 0.4 0.2 CS1(40mm) CS2(80mm) CS1 CS2 0 20 40 60 20 20 40 60 80 AMPLITUDE (mm) AMPLITUDE (mm) Figure 7: Influence of Slipping Amplitude CS1 CS2 (a)cs1 (b)cs2 SLIPPING WEAR SCARS Figure 8: Conditions of Slipping Wear Scar during the slipping should be investigated under the various different conditions. However, it is difficult to discuss the friction coefficient by adopting the concept of tribology, because the friction-wear mechanism that occurred on the joint surfaces between steel and concrete are extremely complex. Therefore, several series of experiments under the various conditions were carried out, and the influences of the testing conditions to the friction-slipping behavior are investigated here. Firstly, we discuss the influence of the slipping amplitude comparing the result of the CS1 and CS2 series whose maximum amplitudes in the cyclic loading were 40 mm and 80 mm, respectively. Fig.7 shows the friction coefficients obtained from both CS1 and CS2. Here, the values of the friction coefficient plotted in these figures are represented at the time when the slipping displacement goes across the zero axis in each half cycle. As seen in Fig.7 (a), the friction coefficients in the CS1 series are generally higher than those in the CS2 series. According to Fig.7 (b), the difference between both the series is approximately 18 percents regarding the average friction coefficient in the range where the friction coefficient increases as the total slipping displacement increases. The 1286
coefficient of variation is approximately the same 10 percents in both the series. According to Fig.7(c), the difference of the friction coefficient between both the series is approximately 23 percents in the range where the friction-slipping behavior has stabilized, and it is little larger than those in Fig.7 (b). Fig. 8 shows the conditions (a) SLIPPING VELOCITY (b) CONCRETE STRENGTH FRICTION COEFFICIENT 0.8 0.6 0.4 0.2 CS1(4cm/sec) CS3(1cm/sec) CS1(56.8MPa) CS4(35.9MPa) (c) CONCRETE THICKNESS (d) BOLT TENSION FRICTION COEFFICIENT 0.8 0.6 0.4 0.2 CS1(200mm) CS5(100mm) CS1(N=120kN) CS61(N=90kN) CS62(N=60kN) Figure 9: Influence of Several Testing Conditions of the slipping wear scars observed on the surfaces of the steel plates after the testing. The slipping wear scar of the CS1 widely distributed along the slot, while the scars in the CS2 concentrated only in the diagonal corner areas on the plate. This difference of the scar distribution between both the series may cause the difference of friction coefficient. However, the reason why the difference of displacement amplitude causes the difference of the scars distribution is not cleared up in the tests. Secondary, in order to investigate the influence of other testing conditions, the comparisons of the friction coefficients between the CS1 and the other series are shown in Fig. 9(a) through (d). As seen in Fig. 9(a) and (b), the approximately same trends were observed among the three series of CS1, CS3, and CS4, regarding the relations between the friction coefficients and the total slipping displacement. This indicates that the loading velocity and the concrete compression strength little influence to the friction coefficient. In the comparison of the CS1 and CS5 series which identify except for the concrete block thickness, however, it was observed that the friction coefficients in the CS5 using thinner concrete blocks slightly decrease in the slipping after about 400mm of 1287
the total slipping displacement as shown in Fig.9(c). The decreasing of the friction coefficient wasn t caused by the applicable decreasing of the bolt tension. Finally, the friction coefficients of the CS1 series, CS61 and CS62 were compared to investigate the influence of the initial bolt tension. As seen in Fig.9 (d), the friction coefficients in the range of the stable slipping of the CS61 and CS62 whose initial bolt tensions were 60kN and 90kN were smaller than those of the CS1 series with the higher 120kN bolt tension. The behaviors of the friction coefficient of the CS61 and CS62 resembled those of the CS2 shown in Fig.7, and the slipping wear scar distributions on the steel surface observed in the CS61 and CS62 also resembled those of the CS2 series shown in Fig.8. Therefore, smaller friction coefficients obtained in the CS61 and CS62 may be caused by the narrow contact area on the friction surfaces as mentioned before. However, the reason why the difference of the initial bolt tension caused the smaller contact area on the friction surfaces is not cleared up in the tests. 6. Equations to Predict the Friction Coefficient The variation of the friction coefficients during slipping was at maximum some 20 percents, according to the test results based on the various testing conditions. This suggests that if the differences of the slipping amplitude, the loading velocity, the concrete compression strength, the concrete plate thickness and the initial bolt tension are within the scope of the examined conditions, the friction coefficient during slipping seems to be little influenced by those conditions. In this paper, assumed that the variation and the decreasing behavior of the friction coefficient are so small that they can be ignored, the equations to predict the relations between the friction coefficient and the slipping displacement is obtained on the basis of the all test results. Equation (1) and (2) represent two characteristics that were observed in the range of the friction coefficient increasing and the range of the stable friction slipping. 0mm <= TSD <= 180 mm: FC = 0.69-1.25x10-5 (TSD-180) 2 (1) 180mm < TSD <=1000mm: FC = 0.69 (2) where FC = predicted friction coefficient TSD = total slipping displacement Equation (1) was derived by a regression analysis, which adopted a secondary polynomial model, by using all the friction coefficients measured at each end of a half cycle until 200mm of the total slipping displacement. We set the maximum friction coefficient during the stable slipping that was defined in equation (2). Table 3 shows the average friction coefficient and the standard deviation at each total Table 3: Average and Standard Deviation of Friction Coefficient Start 1Cyc. 2Cyc. 3Cyc. 4Cyc. 5Cyc. 6Cyc. 7Cyc. 8Cyc. 9Cyc. 10Cyc. 11Cyc. 12Cyc. 13Cyc. 14Cyc. T.S.D. 0 23 67 153 239 325 411 497 583 668 753 838 925 967 988 Ave. 0.36 0.38 0.54 0.65 0.68 0.69 0.70 0.70 0.70 0.69 0.69 0.69 0.69 0.69 0.68 S.D. 35 84 69 72 85 76 77 78 77 83 75 80 76 82 84 1288
FRICTION COEFFICIENT 0.5-0.5 - Expriment -25-20 -15-10 -5 0 5 10 15 20 25 SLIPPING DISPLACEMENT (mm) FRICTION COEFFICIENT 0.5-0.5 - Assumption -25-20 -15-10 -5 0 5 10 15 20 25 SLIPPING DISPLACEMENT (mm) Figure 10: Comparison of Hysteresis Loops TEST RESULT Ar(mm) 140 120 100 80 60 40 20 0 (a) 1.25 0 0.80 CS1 CS2 CS3 CS4 CS5 CS6 140 120 100 80 60 40 20 (b) Figure 11: Comparison of Hysteresis Loop Areas TEST RESULT Ar(mm) 1.25 0 0.80 CS1 CS2 CS3 CS4 CS5 CS6 0 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 PREDICTED VALUE Ap(mm) PREDICTED VALUE Ap(mm) MAX. FRICTION COEFFICIENT 0.8 0.6 0.4 0.2 CS1 CS3 CS5 CS2 CS4 CS6 Figure 12: Variation of Maximum Friction Coefficient Upper Limit (=average plus 2 x S.D.) Average of M.F.C. Obtained from Results in CS1, 3,4 and 5 1289
slipping displacement that were based on the data observed at the first slipping and the end of slipping in each cycle in the total of twenty-two specimens. The average friction coefficients obtained after 4th cycle are equal to the value of equation (2) approximately. Fig. 10 shows the comparison between the hysteresis loops of the relations between the friction coefficient and slipping displacement obtained from the experiment of CS1-3 and those predicted by Equation (1) and (2). Both the hysteresis loops approximately match. Fig.11 shows the comparison of the loop s area obtained from all the experiments and predicted by the equations, which area corresponds to the energy dissipation due to each cyclic friction slipping. Making the hysteresis loops by using Equation (1) and (2), the behavior of rigid-plastic pattern for the relations between friction coefficient and slipping displacement was assumed. As seen in Fig.11 (a), the proposed equations estimate approximately good hysteresis loop areas. The average and standard deviation regarding the ratio of the experiments to the predicted loops were 0.99 and 0.12, respectively. In addition, the comparison between the loop areas of the experiment and the equations subtracted 2 x S.D. is shown in Fig.11 (b). Here, the value of standard deviation S.D. was assumed to be equal to 85 which corresponds to the maximum standard deviation shown in Table 3. As seen in Fig.11 (b), the proposed equations that subtracted 2 x S.D. approximately estimated the lower limit for the test results. Finally, we discuss the maximum friction coefficient of the bolted friction-slipping joint. Fig.12 shows the maximum friction coefficients in every cycle obtained from all the test results. The average and the standard deviation of the maximum friction coefficients throughout the overall slipping, which are the sixteen values obtained from the results of CS1, CS3, CS4 and CS5 series, are 0.83 and 26, respectively. The maximum friction coefficient plus the two times of standard deviation equals to 0.88 and the value which estimates a upper limit of the test results is shown by a chained line in Fig.12. 7. Conclusion We presented the equations that represented the relations between friction-coefficient and slipping displacement for the bolted friction-slipping joint composed by concrete and steel plates. The equations are derived on the basis of the dynamic loading tests and the data analysis that considered the influences of the maximum slipping amplitude, the loading velocity, the concrete compression strength, the thickness of concrete plate and the initial bolt tension and the equations well predicted the friction-slipping behavior. 1290