Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints

Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.14 Placement of transducer in a typical specimen (Left: transducer on the top side of chord, Right: transducer on the rotating saddle) The load-displacement relationship of the trunnion brace provides an indication of the yield and ultimate load capacity of the trunnion. The load-displacement response of the trunnion brace to shear indicated the elastic, inelastic and failure behaviour of the plate trunnions under shear loading. The load-displacement relationship was obtained by plotting the readings obtained from the transducers. 3.3 Governing failure mode of trunnion In the following section, a description of the failure modes of trunnions is made. It is observed that the ultimate failure tests produces a distinct load deformation path in all the specimens and it is possible to ascertain the failure mode, ranging from chord plastification effect to fracture of the shear plate. 3.3.1 Pure pipe trunnion This set of specimens consists of trunnions with attached pipe only. It is observed that there are two distinct failure mode of this type of trunnions, namely chord tension pull 90 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints out failure and brace shear failure through fracture and chord plastification of the chord resulting in the indentation of the chord. Figures 3.15 to 3.19 show the effect of direct shear failure of the brace. The deformation of the brace and fracture line on the brace can be seen clearly from the diagrams shown. Figure 3.15 Deformation and governing failure mode of specimen CT1 Figure 3.16 Deformation and governing failure mode of specimen CT2 91 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.17 Deformation and governing failure mode of specimen CT3 Figure 3.18 Deformation and governing failure mode of specimen CT4 Figure 2.9 below shows the deformation and fracture on the chord. Due to the thinner chord wall used in this specimen, it is possible for the chord to fail by fracture on its chord wall. 92 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.19 Deformation and governing failure mode of specimen CT5 Only specimen CT3 shows that there is shear brace failure, all the other four specimens indicated chord tension pull out failure. The latter failure mode is usually accompanied by large chord indentationd as seen in the diagram. The load deformation curves for specimens CT1 through CT5 are shown in Figure 3.20. The plots show a distinct elastic range, onset of strain hardening, ultimate load and the final fracture point. The ultimate load reached for specimen CT1, CT2, CT3, CT4 and CT5 are respectively 4,800kN at a maximum deflection of 27mm, 2,175kN at a maximum deflection of 20mm, 5,417kN at a maximum deflection of 50mm, 2,940kN at a maximum deflection of 17mm, and 4,568kN at a maximum deflection of 25mm. All the specimens show high ductility prior to the ultimate failure load indicating that is there is a lot of reserve strength in the trunnions. As the grommet are placed about 200mm away from the face of the chord wall, the load on the brace is predominantly shear and it is beneficial in the design of trunnions that there is high ductility. Thus there is a less likely chance for the trunnion to suffer premature failure due to sudden 93 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints fracture at the limit load. Table 3.20 gives a summary of the ultimate loads and displacement for specimens CT1 to CT5 6000 Total Load, P (kN) 5000 4000 3000 P 2000   Specimen CT1 Specimen CT2 Specimen CT3 Specimen CT4 Specimen CT5 1000 0 0 10 20 30 40 Displacement, ' (mm) 50 60 Figure 3.20 Load deformation curves for specimens CT1 to C5 Table 3.2 Summary of the ultimate loads and displacement for specimens CT1 to CT5 d0 t0 d1 t1 Fu,test ' mm mm mm mm kN mm CT1 508.0 20.5 324.0 17.6 4800 27 CT2 508.0 12.5 324.0 12.4 2175 20 CT3 508.0 20.5 406.4 12.5 5417 50 CT4 508.0 12.5 406.4 12.5 2940 17 CT5 508.0 15.2 406.4 17.0 4568 25 Specimen 94 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 3.3.2 Through pipe trunnion This set of specimens consists of trunnions with through pipes only. In this case, the pipe is slotted though the chord wall and welded together, thereby providing greater strength in shear and prevents less chord tension pull-out failure where the chord wall is not thick. It is observed that there is one distinct failure mode of the trunnion here, namely brace shear failure through fracture of the brace. Figures 3.21 and 3.22 show the effect of direct shear failure of the brace. The deformation of the brace and fracture line on the brace can be seen clearly from the diagram shown. The shear failure occurs just at the intersection between the weld and the brace. Figure 3.21 Deformation and governing failure mode of specimen CT6 The load deformation curves for specimens CT6 and CT7 are shown in Figure 3.23. The plots show a distinct elastic range, onset of strain hardening, ultimate load and the final fracture point. The ultimate load reached for specimen CT6, and CT7 are 95 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints respectively 4,136kN at a maximum deflection of 34mm and 5,150kN at a maximum deflection of 41mm. All the specimens show high ductility prior to the ultimate failure load indicating that is there is a lot of reserve strength. Table 3.3 provides a summary of the ultimate loads and displacement for specimens CT6 and CT7. Figure 3.23 is the load deformation curves for specimens CT6 and CT7. As the grommet are placed about 200mm away from the face of the chord wall, the load on the brace is predominantly shear and it is beneficial in the design of trunnion when there is high ductility. Thus there is a less likely chance for trunnions to suffer premature failure due to sudden fracture at the limit load. Figure 3.22 Deformation and governing failure mode of specimen CT7 Table 3.3 Summary of the ultimate loads and displacement for specimens CT6 and CT7 d0 t0 d1 t1 Fu,test ' mm mm mm mm kN mm CT6 508.0 12.4 324.0 12.4 4136 34 CT7 508.0 12.5 406.4 12.5 5150 41 Specimen 96 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 6000 5000 Total Load (kN) 4000 3000 P 2000 ✁ 1000 Specimen CT6 Specimen CT7 0 0 10 20 30 Displacement (mm) 40 50 Figure 3.23 Load deformation curves for specimens CT6 and CT7 3.3.3 Combined shear plate and pipe trunnion This set of specimens consists of trunnions with shear plates slotted through the chord walls and welded together. The brace are welded to the shear plate inside the core to provide the circumference for the grommet during loading and they are also welded onto the chord wall. That is the static strength of these specimens utilises the strength of the brace as well as the shear capacity, effectively increasing the shear and bending carrying capacity for consideration in the trunnion design. This combined trunnion provides clues to the effectiveness of the brace and shear plate when combined together compared to the isolated cases as described in the earlier sections. It is 97 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints observed that the governing failure mode for this specimen is through chord tension pull out failure for all the four specimens. Figures 3.24 to 3.27 shows the deformation and fracture of the brace and shear plate. The inserts in the pictures show the buckling mode of the shear plate inside the chord wall. The buckling of the shear plates effectively reduces the shear carrying capacity of the trunnion. Due to the width of the outer diameter of the chord used, the shear plate used for such pipe trunnions must be sufficiently thick to reduce the effect of buckling of the shear plate. Figure 3.24 Deformation and governing failure mode of specimen CT8 Figure 3.25 Deformation and governing failure mode of specimen CT9 98 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.26 Deformation and governing failure mode of specimen CT10 Figure 3.27 Deformation and governing failure mode of specimen CT11 The load deformation curves for specimens CT8 to CT11 are shown in Figure 3.28. The plots show a distinct elastic range, onset of strain hardening, ultimate load and the final buckling of the shear plate. The ultimate loads reached for specimen CT8 to CT11 are respectively 5,482kN at a maximum deflection of 14mm, 2,862kN at a maximum deflection of 9mm, 7,596kN at a maximum deflection of 30mm and 3,500kN at a maximum deflection of 16mm. These combined pipe and shear plate specimens also show high ductility prior to the ultimate failure load indicating that is there is a lot of reserve strength in such trunnions. The grommets are placed about 200mm away from the face of the chord wall, thus the load on the trunnion is 99 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints predominantly shear. Table 3.4 provides a summary of the ultimate loads and corresponding displacements for specimens CT8 to CT11 9000 8000 Total Load, P (kN) 7000 P 6000 ✂ 5000 4000 3000 2000 Specimen CT8 Specimen CT9 1000 Specimen CT10 Specimen CT11 0 0 10 20 30 40 Displacement, ' (mm) 50 60 Figure 3.28 Load deformation curves for specimens CT8 to CT11 Table 3.4 Summary of the ultimate loads and displacement for specimens CT8 to CT11 d0 t0 d1 t1 Fu,test ' mm mm mm mm kN mm CT8 508.0 20.5 324.0 17.5 5482 14 CT9 508.0 12.5 324.0 12.4 2862 9 CT10 508.0 20.7 406.4 12.5 7596 30 CT11 508.0 12.5 406.4 12.5 3500 16 Specimen 100 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 3.3.4 Tubular X-joints This set of specimen consists of trunnions with only attached pipes but with the length of brace extended. The length of brace used for specimens CT12 to CT17 are respectively 400mm, 800mm, 1300mm 400mm, 800mm, 1600mm. It is observed that there are two distinct failure modes of the trunnions here, namely chord tension pull out failure and brace shear failure through fracture and chord plastification of the chord resulting in the indentation of the chord. Figures 3.29 to 3.34 show the effect of direct shear failure of the brace and chord indentation. The deformation of the brace and fracture line on the brace can be seen clearly from the diagrams shown. Figure 3.29 Deformation and governing failure mode of specimen CT12 Figure 3.30 Deformation and governing failure mode of specimen CT13 101 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.31 Deformation and governing failure mode of specimen CT14 Figure 3.32 Deformation and governing failure mode of specimen CT15 Figure 3.33 Deformation and governing failure mode of specimen CT16 102 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.34 Deformation and governing failure mode of specimen CT17 The load deformation curves for specimens CT12 through CT17 are shown in Figure 3.35. The plots show a distinct elastic range, onset of strain hardening, ultimate load and the final fracture point. The ultimate load reached for specimen CT12 to CT17 are respectively 2,780kN at a maximum deflection of 50mm, 1,512kN at a maximum deflection of 90mm, 966kN at a maximum deflection of 180mm, 3,780kN at a maximum deflection of 60mm, 2,126kN at a maximum deflection of 80mm, and 1,051kN at a maximum deflection of 240mm. All the specimens show high ductility prior to the ultimate failure load indicating that there is a lot of reserve strength for such trunnions. As the grommet is placed at lengths ranging from 400mm through 1600mm away from the face of the chord wall, the load on the brace is predominantly via bending moment. The results are later used in the comparison of the amount of shear that gives rise to bending moment as the loading arm of the brace becomes longer. 103 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 4000 Specimen CT12 Specimen CT13 Specimen CT14 Specimen CT15 Specimen CT16 Specimen CT17 3500 Total Load, P (kN) 3000 P ✄ 2500 2000 1500 1000 500 0 0 50 100 Displacement, ' (mm) 150 200 Figure 3.35 Load deformation curves for specimens CT12 to CT17 Overall, it is observed from the experimental tests of all the four different types of trunnion specimens and X-joints tested that there is high level of ductility, due to strain hardening, in the trunnions regardless of the configuration type. There is also distinct yielding behaviour under load and the load deformation plots show clearly defined ultimate failure load. The failure modes of these specimens are predominantly shear failure through fracture of the shear-loaded arm of the trunnion except for tubular X-joints where the loads gives rise to bending moments. Table 3.5 shows a summary of the ultimate loads and displacement for CT12 to CT17. The next section will focus on the static strength and how they compare with the current practice. 104 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Table 3.5 Summary of the ultimate loads and displacement for specimen CT12 to CT17 d0 t0 d1 t1 Fu,test ' mm mm mm mm kN mm CT12 508.0 20.7 324.0 17.5 2780 50 CT13 508.0 20.7 324.0 17.6 1512 90 CT14 508.0 20.7 324.0 17.6 966 180 CT15 508.0 20.7 406.4 12.5 3780 60 CT16 508.0 20.7 406.4 12.5 2126 80 CT17 508.0 20.7 406.4 12.5 1051 240 Specimen 3.4 Discussions on the test results The specimens in the tests were carefully selected so that different types of failure mechanisms of the trunnions can be observed. The behaviour and strength of these specimens are discussed below. 3.4.1 Design strength of pure pipe trunnions Based on the joint shear resistance for the pipe trunnions as described in the previous chapter, the same formulation is used in this case for the large pipe trunnions. This series of tests have been calibrated and uses more advanced technology and resources which reflects more accurately vis-a-vis the first set of tests that grommets are more prone to unpredictable behaviour with better control of the experimental results obtained. Hence this series of tests serves to enhance the previous series of tests and also the larger dimensions used and the higher shear loads generated help to reduce any error due to sizing effects. 105 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints The test results for specimens CT1 to CT5 are given in Figures 3.36 and 3.37, which shows the various observed behaviour including first yield (indication of flaking of white wash on the chord or brace), initial crack, ultimate load reached and the fracture point. These points have been painstakingly recorded in order to fully understand the actual behaviour of the pipe trunnion during the various loading stages. The first yield has been observed through the white wash flaking, and this should not be misconstrued as the elastic yield strength. However, it serves as an important indication of the final failure mode of the pipe trunnion and where failure occurs. 106 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 2500 6000 Ultimate Load @4830kN Fracture @4460kN Initial Crack @4200kN Load (kN) 4000 2000 Initial Crack @2000kN Load (kN) 5000 Ultimate Load @2300kN Fracture @1970kN 1500 3000 1000 First Yield (Chord ) @2000kN 2000 First Yield (Chord ) @730kN 500 1000 Specimen CT2 Specimen CT1 0 0 0 10 20 30 40 Displacement (mm) 0 50 20 40 Displacement (mm) 60 4000 6000 Ultimate Load @5420kN 5000 Fracture @5350kN Initial Crack @4620kN 3000 Load (kN) Load (kN) 4000 3000 2000 First Yield (Brace ) @1890kN Ultimate Load @2950kN Initial Crack @2470kN 2000 Fracture @2460kN First Yield (Chord ) @1080kN 1000 1000 Specimen CT3 Specimen CT4 0 0 0 20 40 Displacement (mm) 60 0 20 40 Displacement (mm) 60 Figure 3.36 Design load for specimens CT1 to CT4 107 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 6000 Ultimate Load @4580kN 5000 Load (kN) 4000 Initial Crack @3780kN Fracture @4070kN 3000 2000 First Yield (Chord ) @1290kN 1000 Specimen CT5 0 0 20 40 Displacement (mm) 60 Figure 3.37 Design load for specimen CT5 The first yield occurred on the chord surface for specimens CT1, CT2, CT4 and CT5 with these specimens ultimately failing through chord tension pull out. First yield indication for specimen CT3 started on the brace and the ultimate failure mode was through shear fracture. Thus once the initial deformation starts, the load deformation generally follows the weakest link and the ultimate failure path is thus determined. Table 3.6 is a summary and comparison of the loads and formulations used to calculate the pipe trunnion design strength. The following formulations, Equations 3.1 to 3.3, are used to compare with existing recommendations on shear and bending moment effects on the specimens. Mu f y 0 ˜ t0 ˜ d1 5.1 ˜ J 1.04 ☎ ✆ 0.43 ☎ (1  0.4E )  (1  0.4E ) 2  2 2  (0.4 E ) 2 (3.1) J2 108 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints V1 M1 S ˜ d m1 ˜ t1 ˜ 0.4 ˜ f y1 (3.2) V1 ˜ w1 (3.3) For the failure mode caused by chord indentation, M1 is very close to Mu or even higher. This is because the full effective shear capacity of the brace can be mobilised before chord indentation sets in and this path is continued to the ultimate failure. When Mu is low, the joint design resistance for bending moment is low and this generally results in chord indentation when M1 increases. On the other hand, for specimen CT3 where the failure mode is due to brace shear failure, the effective shear area is mobilised and shows that the existing shear strength estimate is very conservative. Here, it is noted that M1 is much lower, indicating the high capacity of the joint to resist chord indentation. There is also a lot of reserve strength from the ductility. Through determining the amount of joint design resistance against bending moments, the trunnion can be designed against such failure modes, such that the full effective shear capacity of the brace is mobilised. Table 3.6 Summary of the ultimate and design strength for CT1 to CT5 Specimen d0 t0 d1 t1 Fu,test kN V1 kN M1 kNm CT1 CT2 CT3 CT4 CT5 508.0 508.0 508.0 508.0 508.0 20.5 12.5 20.5 12.5 15.2 324.0 324.0 406.4 406.4 406.4 17.6 12.4 12.5 12.5 17.0 4830 2300 5420 2950 4580 3368 1879 2326 2326 3128 337 188 233 233 313 (All other units in mm) Mu Failure kNm 330 132 539 217 393 chord chord shear chord chord 109 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Thus it is concluded that the existing design formulation under predicts the static yield strength of the pipe trunnion by a wide margin. Further, it is observed that the high reserve strength enjoyed by the pipe trunnion makes the configuration suitable as a lifting point as the ductility is very high. Due to this under estimate of the static yield strength of the pipe trunnion, there is an opportunity to use this as a basis to further optimise the basis of recommendations later. This phenomenon is further investigated in the numerical analysis where the tubular X-joints were used to determine the design load where the level of chord plastification occurs so that indentation of the chord will not reduce the shear capacity. 3.4.2 Design strength of through pipe trunnions Since chord wall thickness is an important component in the design of pipe trunnions, there is potential benefit when the brace, instead of being attached to the outside of the chord, is slotted through the chord wall and welded, similar to the configuration when the shear plate is slotted through the chord. The main aim of fabricating a pipe trunnion with through pipe is to prevent chord indentation and these are suitable for situations where the chord wall is thin and no replacement can be found. Since the configuration is similar to shear plate pipe trunnions, this gives the designer greater confidence in utilising the full shear capacity of the pipe, even though this may not be necessary considering that a well-designed pipe trunnion can easily handle this task. This method of pipe trunnion design is investigated here and tested experimentally to determine whether there is any advantage in designing through pipe trunnions. 110 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints The test results for specimens CT6 and CT7 are given in Figure 3.38, which shows the various observed behaviour including first yield (indication of flaking of white wash on chord or brace), initial crack, ultimate load reached and the fracture point. The first yield has been observed through the white wash flaking, and this should not be misconstrued as the elastic yield strength. However, it serves as an important indication of where the failure mode of the pipe trunnion is likely to. 5000 6000 Ultimate Load @5160kN Ultimate Load @4140kN 5000 Initial Crack @3560kN 3000 Initial Crack @4520kN 4000 Fracture @3510kN Fracture @4830kN Load (kN) Load (kN) 4000 3000 2000 1000 First Yield (Brace ) @1940kN 2000 First Yield (Brace ) @1540kN 1000 Specimen CT7 Specimen CT6 0 0 0 10 20 30 40 Displacement (mm) 50 0 20 40 Displacement (mm) 60 Figure 3.38 Design load for specimens CT6 and CT7 The first yield occurred on the brace surface for specimen CT6 and CT7 and these specimens ultimately failed through shear fracture. Thus once the initial deformation starts, the load deformation generally follows the weakest link and continues until ultimate failure is reached. 111 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.39 is a comparison of the load deformation plots for CT2 & CT6 and CT4 & CT7. It is noted from the earlier discussions that specimens CT2 and CT4 both fail through chord indentation, that is the full effective shear area of the braces were not fully mobilised before chord plastification sets in. This reduces considerably the joint shear resistance of specimens CT2 and CT4. The figure shows the sharp contrast between pipe trunnions that are designed with and without through pipe configuration. Both specimens CT2 and CT4 have lower stiffness and the ultimate loads reached were much lower than that of specimens CT6 and CT7 respectively although they have identical dimensions. Thus it is possible for pipe trunnions, to have a lower shear capacity due to the thin chord. These should be re-designed to carry much higher shear capacity without changing any configuration, by merely inserting the brace through the chord wall. The calculation for the design would be exactly the same as that for normal pipe trunnions. CT2 CT6 CT4 6000 CT7 6000 5000 4000 Load (kN) Load (kN) 4000 3000 2000 2000 1000 0 0 0 20 40 Displacement (mm) 60 0 20 40 Displacement (mm) 60 Figure 3.39 Comparison load deformation plots for CT2 & CT6 and CT4 & CT7 112 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints This configuration is further investigated in the numerical analysis where a series of through pipe were used to determine the applicable range of parameters that are useable and effective. 3.4.3 Design strength of combined shear plate and pipe trunnions Combining both the effective shear area of pipe and shear plate based on the formulation as discussed above, the load deformation curves are plotted in Figure 3.40. The table presents the values calculated using Equation 3.4 to 3.6. V1 = S dm1 t1 (0.4 fy1) (3.4) Vs = 2 ds ts (0.4fys) (3.5) V1 + Vs = 0.4 (2 ds ts fys + S dm1 t1 fys) (3.6) The plot shows the various observed behaviour including first yield (indication of flaking of white wash on chord or brace), initial crack, ultimate load reached and the fracture point. The first yield has been observed through the white wash flaking, which should not be misconstrued as the elastic yield strength. However, it serves as an important indication of where the failure mode of the pipe trunnion would be. The first yield occurred on the chord surface of specimens CT8 and CT10 while in the case of specimens CT9 and CT11 it occurred on the brace. Table 3.7 provides .a summary of the design strength for CT8 to CT11. 113 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 7000 3500 Ultimate Load @5510kN Load (kN) 5000 3000 2500 Initial Crack @5060kN 4000 Fracture @5100kN Load (kN) 6000 Ultimate Load @2870kN 3000 2000 Initial Crack @2610kN 1500 First Yield (Brace ) @1025kN 1000 First Yield (Chord ) @1575kN 1000 Fracture @2400kN 2000 500 Specimen CT8 Specimen CT9 0 0 0 9000 20 40 Displacement (mm) 0 4000 Ultimate Load @7600kN 8000 7000 Initial Crack @6320kN 5000 4000 20 30 40 Displacement (mm) 50 Initial Crack @3200kN Fracture @2860kN 2000 First Yield (Brace ) @1485kN First Yield (Chord ) @3300kN 3000 10 Ultimate Load @3500kN 3000 Fracture @6980kN Load (kN) 6000 Load (kN) 60 1000 2000 1000 Specimen CT11 Specimen CT10 0 0 0 20 40 Displacement (mm) 60 0 20 40 Displacement (mm) 60 Figure 3.40 Load deformation curves for specimens CT8 to CT11 114 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Table 3.7 Summary of the design strength for CT8 to CT11 d0 t0 d1 t1 ds ts V1 Vs V1 + Vs Fu,test mm mm mm mm mm mm kN kN kN kN CT8 508 20.5 324 17.5 364 16.4 3505 1681 5186 5620 CT9 508 12.5 324 12.4 364 16.4 1879 1681 3560 3220 CT10 508 20.7 406.4 12.5 446.4 16.4 2326 2062 4388 7950 CT11 508 12.5 406.4 12.5 446.4 12.4 2326 1320 3646 3660 Specimen In the earlier chapter, it was shown that the static strength with attached pipe and shear combined is still conservative. Here, the experimental results show that whether in designing pipe trunnions with or without shear plates, there is a need to ensure that the chord wall thickness is sufficient to sustain the full development of the shear load. In all these four specimens, the full effective shear strength was not fully mobilised, hence the failure through chord indentation as the shear plate provided in these specimens buckled internally. As a result, even though the combined shear strength for shear plate and pipe is high, the full shear strength was not achieved because of chord indentation, in this case, through the buckling of the shear plate internally. Another important factor to consider is in the designing the shear plate beyond the outer diameter of the brace. The four specimens were intentionally fabricated with the shear plate within the internal diameter of the brace. As a result, the extent of shear mobilisation on the trunnion stub becomes dependent on the joint design resistance against bending moment which caused chord plastification to occur. The combined static strength of pipe and shear plate is much larger than the joint resistance against bending moment, thereby causing the chord wall to indent with resulting failure in the chord rather than brace shear. 115 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints This phenomenon is further investigated in the numerical analysis where tubular Xjoints were used to determine design load at which chord plastification occurs so that such indentation of the chord will not reduce the trunnion’s shear capacity. 3.4.4 Transition of shear and bending moment Six tubular X-joint specimens were tested close to failure. Not all the specimens reached maximum failure load due to the maximum elongation constraints of the test rig. It is interesting to note the change in failure mechanism as the loading arm of the brace is increased progressively from 0.22 d1 to 4.0 d1. In one of the set of the tubular X-joints, it was observed that as the loading arm extends, the initial failure mode (brace failure dominated) slowly changes to that of chord failure. This observation reflects the interaction of the moment and shear contribution as the loading arm is slowly increased from that of a fully shear loading to one of fully moment load. Figures 3.41 and 3.42 shows the load deformation curves for specimens CT12 to CT15 and CT16 to CT17 respectively. The plot shows the various observed behaviour including first yield (indication of flaking of white wash on chord or brace), initial crack, ultimate load reached and the fracture point. The first yield has been observed through the white wash flaking, which should not be misconstrued as the elastic yield strength. However, it serves as an important indication of where the failure mode of the pipe trunnion is likely to. 116 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 3500 1800 Ultimate Load @2780kN Load (kN) 2500 Initial Crack @2420kN 2000 1500 Fracture @2730kN Fracture @1520kN 1200 Load (kN) 3000 Ultimate Load @1520kN 1500 Initial Crack @1180kN 900 600 1000 First Yield (Brace) @600kN 500 First Yield (Chord ) @380kN 300 Specimen CT13 Specimen CT12 0 0 0 20 40 60 Displacement (mm) 80 0 1200 Ultimate Load @960kN Fracture @950kN Initial Crack @3360kN 3000 Initial Crack @770kN 600 300 Fracture @3550kN 2000 First Yield (Chord ) @1180kN 1000 First Yield (Chord ) @200kN 40 60 80 100 120 140 160 Displacement (mm) Ultimate Load @3800kN 4000 Load (kN) Load (kN) 900 20 Specimen CT15 Specimen CT14 0 0 0 50 100 150 200 Displacement (mm) 250 0 20 40 60 80 100 Displacement (mm) 120 Figure 3.41 Load deformation curves for specimens CT12 to CT15 117 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 2500 1200 Ultimate Load @2130kN 1000 2000 Fracture @1980kN Initial Crack @1840kN 1500 1000 First Yield (Chord ) @710kN 500 600 400 First Yield (Chord ) @300kN 200 Specimen CT16 0 Fracture @1050kN Initial Crack @900kN 800 Load (kN) Load (kN) Ultimate Load @1050kN Specimen CT17 0 0 20 40 60 80 100 120 140 160 Displacement (mm) 0 50 100 150 200 250 Displacement (mm) 300 Figure 3.42 Load deformation curves for specimens CT16 to CT17 Figure 3.43 shows a sharp contrast between the two main series of the tubular Xjoints. The first series, specimens CT1, CT12, CT13 and CT14 consist of brace of I323.9x17.5 (Vy=520kN/mm2, Vu=550kN/mm2) and chord of I.0x19.1mm (Vy=443kN/mm2, Vu=497kN/mm2) with braces of length 200mm, 400, 800, and 1300mm respectively. The reduction in shear capacity reduces exponentially as the length of the brace increases. The shear strength of the brace no longer governs the failure load and the joint resistance against bending moment takes over. Similarly, the second series, specimens CT3, CT15, CT16 and CT17 consist of brace of I406.4x12.7 (Vy=376kN/mm2, Vu=473kN/mm2) and chord of I.0x19.1mm (Vy=443kN/mm2, Vu=497kN/mm2) with braces of length 200mm, 400, 800, and 1600mm respectively. 118 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints CT1 CT13 CT12 CT14 5000 CT3 CT16 6000 CT15 CT17 5000 4000 Load (kN) Load (kN) 4000 3000 3000 2000 2000 1000 1000 0 0 0 20 40 60 80 Displacement (mm) 100 0 20 40 60 80 100 120 140 160 Displacement (mm) Figure 3.43 Comparison of the two series of load deformation plots This effect is further investigated in the numerical analysis where the tubular X-joints were used to determine the load at which the level of chord plastification occurs so that indentation of the chord will not reduce the trunnion’s shear capacity. 119 [...]... effect of direct shear failure of the brace and chord indentation The deformation of the brace and fracture line on the brace can be seen clearly from the diagrams shown Figure 3.29 Deformation and governing failure mode of specimen CT12 Figure 3.30 Deformation and governing failure mode of specimen CT13 101 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.31 Deformation and. .. the failure load and the joint resistance against bending moment takes over Similarly, the second series, specimens CT3, CT15, CT16 and CT17 consist of brace of I406.4x12.7 (Vy=376kN/mm2, Vu=473kN/mm2) and chord of I.0x19.1mm (Vy=443kN/mm2, Vu=497kN/mm2) with braces of length 200mm, 400, 800, and 1600mm respectively 118 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints CT1 CT13... X-Joints Figure 3.31 Deformation and governing failure mode of specimen CT14 Figure 3.32 Deformation and governing failure mode of specimen CT15 Figure 3.33 Deformation and governing failure mode of specimen CT16 102 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.34 Deformation and governing failure mode of specimen CT17 The load deformation curves for specimens CT12... actual behaviour of the pipe trunnion during the various loading stages The first yield has been observed through the white wash flaking, and this should not be misconstrued as the elastic yield strength However, it serves as an important indication of the final failure mode of the pipe trunnion and where failure occurs 106 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 2500 6000... trunnion design is investigated here and tested experimentally to determine whether there is any advantage in designing through pipe trunnions 110 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints The test results for specimens CT6 and CT7 are given in Figure 3.38, which shows the various observed behaviour including first yield (indication of flaking of white wash on chord or brace),... 3.4 Summary of the ultimate loads and displacement for specimens CT8 to CT11 d0 t0 d1 t1 Fu,test ' mm mm mm mm kN mm CT8 508.0 20.5 324.0 17.5 5482 14 CT9 508.0 12.5 324.0 12.4 2862 9 CT10 508.0 20.7 406.4 12.5 7596 30 CT11 508.0 12.5 406.4 12.5 3500 16 Specimen 100 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints 3.3.4 Tubular X-joints This set of specimen consists of trunnions. .. different types of failure mechanisms of the trunnions can be observed The behaviour and strength of these specimens are discussed below 3.4.1 Design strength of pure pipe trunnions Based on the joint shear resistance for the pipe trunnions as described in the previous chapter, the same formulation is used in this case for the large pipe trunnions This series of tests have been calibrated and uses more... specimens CT6 and CT7 The first yield occurred on the brace surface for specimen CT6 and CT7 and these specimens ultimately failed through shear fracture Thus once the initial deformation starts, the load deformation generally follows the weakest link and continues until ultimate failure is reached 111 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Figure 3.39 is a comparison of the... the loads gives rise to bending moments Table 3.5 shows a summary of the ultimate loads and displacement for CT12 to CT17 The next section will focus on the static strength and how they compare with the current practice 104 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints Table 3.5 Summary of the ultimate loads and displacement for specimen CT12 to CT17 d0 t0 d1 t1 Fu,test ' mm... normal pipe trunnions CT2 CT6 CT4 6000 CT7 6000 5000 4000 Load (kN) Load (kN) 4000 3000 2000 2000 1000 0 0 0 20 40 Displacement (mm) 60 0 20 40 Displacement (mm) 60 Figure 3.39 Comparison load deformation plots for CT2 & CT6 and CT4 & CT7 112 Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints This configuration is further investigated in the numerical analysis where a series of through