tensile testing of mechanical bar splices for mmfx...
TRANSCRIPT
Tensile Testing of Mechanical Bar Splices for MMFX Steel
February 2004
By Antonis Michael
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Table of Contents Acknowledgements............................................................................................................. 3 Summary ............................................................................................................................. 4 Specimens ........................................................................................................................... 4 Test Setup............................................................................................................................ 5 Results................................................................................................................................. 6 Conclusions....................................................................................................................... 15
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Acknowledgements Bar Splice Products Inc. manufactured and donated the specimens. Paul Tighe helped get the MTS loading frame ready for the test and Steve Eudy set-up the data acquisition system and run it during the tests. Marc Ansley offered advice and help during the tests. Without the help of all these individuals this reports could not be completed.
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Summary Previous testing of MMFX lap splices revealed the inability to fully develop the bars even when long lap splice lengths are used (Double the length required for standard Grade 60 steel). To address this situation mechanical splices were investigated as an alternative. According to current codes a mechanical splice has to be able to develop at least 125% of the yield strength of steel bars. The codes allow the use of steel bars with yield strengths of no more than 75 to 80 ksi. Mechanical splices are designed around this requirement. MMFX steel, on the other hand, is a high strength steel that does not exhibit distinct yield point and therefore applying the 125% of yield strength criteria is inappropriate. It was decided that a mechanical splice should be stronger than the MMFX steel bar itself. Two types of commercially available mechanical splices for #6 bars were tested. Both splice types exceeded the capacity of the MMFX bar and failure occurred in the steel bar. The average stress in the bars at failure was 173.6 ksi. Specimens The company that manufactures the bar splices prepared the specimens. Approximately 2 feet long pieces of MMFX #6 bar were attached on each side of the mechanical splice. The total length of each specimen was approximately 4 feet. The first type of mechanical splice is known by the trade name BPI-Grip and it is a one-piece mechanical splice. For the #6 bar the length of the splice is approximately 8 inches. Four inches of each of the two bar pieces to be spliced are inserted into the splice on each side. After insertion, a hydraulic press pushes and deforms the splice onto the steel bar. A specimen with the BPI-Grip splice is shown in Fig. 1 (a). A close up of the mechanical splice is shown in Fig. 1 (b). The second type of mechanical splice is known by the trade name Grip-Twist and it is a 2 piece mechanical splice with tapered threads. The total length of the splice, when in place for the #6 bar, is approximately 9 inches. Three inches of each of the two pieces to be spliced are inserted into each one of the two pieces of the splice. A hydraulic press pushes and deforms the splice onto the steel bar in the same manner as the BPI-Grip. Then the 2 pieces are connected by tightly screwing one piece onto the other. A specimen with the taper threaded Grip-Twist splice before it is screwed together is shown in Fig. 2 (a). The specimen with the mechanical splice fastened is shown in Fig. 2 (b).
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(a) (b)
Figure 1 – (a) BPI-Grip Mechanical Splice Connecting 2 #6 MMFX Bars, and (b) Close-
Up of the Mechanical Splice
(a) (b)
Figure 2 – (a) Two Piece Taper Threaded Grip-Twist Mechanical Splice, and (b) Close-
Up of the Mechanical Splice Connecting 2 #6 MMFX Bars Test Setup The MMFX mechanically spliced bars were tested in tension in an MTS loading frame. The two ends of each specimen were inserted into the MTS machine grips for approximately 8 inches and hydraulic pressure was applied. Specimens in the MTS
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loading frame while applying hydraulic pressure to the grips are shown in Fig. 3 (a) and (b). For the first three BPI-Grip specimens a pressure of approximately 1200 psi was applied. Because of some slippage observed during testing of the third BPI-Grip specimen the applied grip pressure was increased to approximately 1400 psi for the remaining specimens.
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Figure 3 – (a) A Specimen Held in Place prior to the Application of Hydraulic Pressure to
the Grips and (b) Application of Hydraulic Pressure to Grips All specimens were tested in load control at a rate of 50 ksi per minute, which for a #6 bar resulted in a loading rate of approximately 22 kips per minute (367 lbf per second) in compliance with ASTM A 370-02. Results Both types of mechanical splices had similar behavior and load capacity. No significant disparity in the results was observed between the two different types of mechanical splices. The results from the BPI-Grip specimens are presented in Table 1. The peak stress is calculated by dividing the peak load recorded during the test by the area of the MMFX steel bar, 0.44 square inches.
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Table 1 – Results from BPI-Grip Mechanical Splice Specimens
Splice Type Specimen # Peak Load (kips) Peak Stress (ksi) 1 76.3 173.4 2 75.5 171.6 3 76.1 173.0 4 77.3 175.7
BPI-Grip
5 75.6 171.8 Failure occurred in the steel bar in all specimens. Therefore, the peak stress reported in Table 1 represents the ultimate strength of the MMFX bars. The average peak stress was 173.1 ksi and the standard deviation 1.64 ksi. The coefficient of variance (COV) for this group of specimens was small amounting to approximately 0.9 %, which is an indication of a satisfactory testing method. The low variation can also be attributed to good material quality control. Typical load deflection results from the BPI-Grip specimens are presented in Fig. 4 and 5. Figure 6 depicts the load deflection curves of all 5 BPI-Grip specimens. As can be seen the behavior of all the specimens is similar with no significant variation.
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Figure 4 – Load-Deflection Curve for BPI-Grip Specimen #1
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Figure 5 – Load-Deflection Curve for BPI-Grip Specimen #2
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Figure 6 – Load-Deflection Curve for all BPI-Grip Specimens
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All specimens failed by rupture of the MMFX steel bar after significant elongation. No failures were observed in the grips or at the edges of the mechanical splices. MMFX steel bar failure was observed on either side of the mechanical splice and no signs of any tendency for failure at specific locations was demonstrated. Ruptured specimens can be seen in Fig. 7 through 9.
Figure 7 – Ruptured Steel Bar in the Bottom Part a PBI-Grip the Specimen
Figure 8 – Ruptured Steel Bar in the Top Part of a BPI-Grip Specimen
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Figure 9 – Ruptured Steel Bar in the Bottom Part of a BPI-Grip Specimen Close to the Grip
The results from the Tapered Threaded Grip-Twist specimens are presented in Table 2. The peak stress is calculated by dividing the peak load recorded during the test by the area of the MMFX steel bar, 0.44 square inches.
Table 2 – Results from Tapered Threaded Grip-Twist Mechanical Splice Specimens
Splice Type Specimen # Peak Load (kips) Peak Stress (ksi) 1 76.1 173.0 2 75.8 172.3 3 76.7 174.3 4 77.0 175.0
Tapered Threaded Grip-Twist
5 77.3 175.7 Failure occurred in the steel bar in all specimens and therefore, as in the case of BPI-Grip specimens, the peak stress reported in Table 2 represents the ultimate strength of the MMFX bars. The average peak stress was 174 ksi and the standard deviation of the group 1.41 ksi. The coefficient of variance (COV) was small amounting to approximately 0.8 %, which is again an indication of a satisfactory test protocol and good quality control of the material. Typical load deflection results from the Tapered Threaded Grip-Twist specimens are presented in Fig. 10 and 11. The load-deflection curves of all the Tapered Threaded Grip-Twist specimens are shown in Fig. 6. As can be seen the behavior of all the specimens is similar with no significant variation.
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Figure 10 – Load-Deflection Curve for Grip-Twist Specimen #1
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Figure 11 – Load-Deflection Curve for Grip-Twist Specimen #2
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Figure 12 – Load-Deflection Curve for all Grip-Twist Specimens All specimens failed by rupture of the MMFX steel bar after significant elongation. No failures were observed in the grips or at the edges of the mechanical splices. MMFX steel bar failure was observed on both sides of the mechanically spliced specimens and no signs of any tendency for failure at specific locations was demonstrated. Ruptured specimens can be seen in Fig. 13 and 14. The small sudden change in the slope of the load-deflection curve that is observed in all specimens at a load value of approximately 5 kips is attributed to the MTS loading machine since it was observed during testing of bar specimens made from MMFX, stainless and Grade 60 regular steel. It is therefore a property of neither the MMFX steel bars nor the mechanical splice. Comparing two typical specimens from the two types of mechanical splices tested it can be concluded that their behavioral and strength properties are similar. As can be seen in Fig. 15 the load-deflection curves of BPI-Grip 1 and Grip-Twist 1 specimens are very similar.
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Figure 13 – Ruptured Steel Bar in the Bottom Part of a Grip-Twist Specimen
Figure 14 – Ruptured Steel Bar in the Top Part of a Grip-Twist Specimen
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Figure 15 – Load-Deflection Curves for Typical Specimens from Both Types of Mechanical Splices
When all mechanically spliced specimens are examined as a whole the average peak stress is 173.6 ksi and the standard deviation 1.53 ksi. The coefficient of variance (COV) is approximately 0.9 %, an indication of a successful test protocol. The behavior of the specimens is the same for both types of mechanical splices. Although is obvious that both types of mechanical splices did fully develop the MMFX steel bars it is of interest to compare the behavior of spliced bars with results from tension tests conducted on straight MMFX bars. This can be seen in Fig. 16 where results from typical mechanical splice specimens and MMFX bars are plotted together. The gage length over which elongation was measured was 8 inches for the MMFX bars, 32 inches for the BPI-Grip specimens and 35 inches for the Grip-Twist specimens. Significant differences in the behavior of the plain bars when compared to the spliced bars are not observed. The spliced specimens had the same stiffness as the MMFX plain bars but a lower ductility because they did not elongate as much as the plain bars prior to rupture. However, this difference can be attributed to the different material that the mechanical splices are made of and the different length of the plain MMFX bar specimens. The MMFX bars used to make the spliced specimens are from a different batch that could have contributed to the difference in the behavior. It is also possible the mechanical splices to have induced stress concentrations on the specimens that contributed to rupture with less elongation.
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Mechanical bar splices made from MMFX steel are currently under development and their introduction should bring the behavior of spliced MMFX bars even closer to the behavior of straight MMFX bar. It will also ensure that the mechanical splice will have the same corrosion resistance the bars have and will not present a weak link in highly corrosive environments.
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Figure 16 – Stress-Strain Curves for Typical Specimens of Mechanical Splices and
MMFX Steel Bars Conclusions From the results presented in this report the following conclusions can be drawn:
1. Both types of mechanical splices tested have the ability to fully develop the strength of MMFX steel.
2. The behavior of mechanically spliced MMFX bars is similar to the behavior of plain MMFX bars with the only difference been lower elongation at rupture.
3. For the two splice types tested the behavior of the spliced bars is independent of the type of mechanical splice used. This comment only applies to the two types tested. The use of another type of mechanical splice could alter the behavior of spliced bars.
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