Experimental Investigation of Large Pipe Trunnions and Tubular X-Joints

It isobserved that the ultimate failure tests produces a distinct load deformation path in allthe specimens and it is possible to ascertain the failure mode, ranging from chordplastifica

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 theyield and ultimate load capacity of the trunnion The load-displacement response ofthe trunnion brace to shear indicated the elastic, inelastic and failure behaviour of theplate 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 isobserved that the ultimate failure tests produces a distinct load deformation path in allthe specimens and it is possible to ascertain the failure mode, ranging from chordplastification 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 thatthere are two distinct failure mode of this type of trunnions, namely chord tension pull

out failure and brace shear failure through fracture and chord plastification of thechord resulting in the indentation of the chord Figures 3.15 to 3.19 show the effect ofdirect shear failure of the brace The deformation of the brace and fracture line on thebrace 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

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 thinnerchord wall used in this specimen, it is possible for the chord to fail by fracture on itschord wall

Figure 3.19 Deformation and governing failure mode of specimen CT5

Only specimen CT3 shows that there is shear brace failure, all the other fourspecimens indicated chord tension pull out failure The latter failure mode is usuallyaccompanied by large chord indentationd as seen in the diagram The loaddeformation curves for specimens CT1 through CT5 are shown in Figure 3.20 Theplots show a distinct elastic range, onset of strain hardening, ultimate load and thefinal fracture point The ultimate load reached for specimen CT1, CT2, CT3, CT4 andCT5 are respectively 4,800kN at a maximum deflection of 27mm, 2,175kN at amaximum 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 about200mm away from the face of the chord wall, the load on the brace is predominantlyshear and it is beneficial in the design of trunnions that there is high ductility Thusthere is a less likely chance for the trunnion to suffer premature failure due to sudden

0 1000

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

3.3.2 Through pipe trunnion

This set of specimens consists of trunnions with through pipes only In this case, thepipe is slotted though the chord wall and welded together, thereby providing greaterstrength 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 Thedeformation of the brace and fracture line on the brace can be seen clearly from thediagram shown The shear failure occurs just at the intersection between the weld andthe 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 thefinal fracture point The ultimate load reached for specimen CT6, and CT7 are

of the ultimate loads and displacement for specimens CT6 and CT7 Figure 3.23 is theload deformation curves for specimens CT6 and CT7.

As the grommet are placed about 200mm away from the face of the chord wall, theload on the brace is predominantly shear and it is beneficial in the design of trunnionwhen there is high ductility Thus there is a less likely chance for trunnions to sufferpremature 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

0 1000

P

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 chordwalls and welded together The brace are welded to the shear plate inside the core toprovide the circumference for the grommet during loading and they are also weldedonto 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 bendingcarrying capacity for consideration in the trunnion design This combined trunnionprovides clues to the effectiveness of the brace and shear plate when combinedtogether compared to the isolated cases as described in the earlier sections It is

observed that the governing failure mode for this specimen is through chord tensionpull out failure for all the four specimens Figures 3.24 to 3.27 shows the deformationand fracture of the brace and shear plate The inserts in the pictures show the bucklingmode of the shear plate inside the chord wall The buckling of the shear plateseffectively reduces the shear carrying capacity of the trunnion Due to the width of theouter diameter of the chord used, the shear plate used for such pipe trunnions must besufficiently 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

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 thefinal buckling of the shear plate The ultimate loads reached for specimen CT8 toCT11 are respectively 5,482kN at a maximum deflection of 14mm, 2,862kN at amaximum deflection of 9mm, 7,596kN at a maximum deflection of 30mm and3,500kN at a maximum deflection of 16mm These combined pipe and shear platespecimens also show high ductility prior to the ultimate failure load indicating that isthere is a lot of reserve strength in such trunnions The grommets are placed about200mm away from the face of the chord wall, thus the load on the trunnion is

0 1000

P

predominantly shear Table 3.4 provides a summary of the ultimate loads andcorresponding displacements for specimens CT8 to CT11

Figure 3.28 Load deformation curves for specimens CT8 to CT11

Table 3.4 Summary of the ultimate loads and displacement

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 arerespectively 400mm, 800mm, 1300mm 400mm, 800mm, 1600mm It is observed thatthere are two distinct failure modes of the trunnions here, namely chord tension pullout failure and brace shear failure through fracture and chord plastification of thechord resulting in the indentation of the chord Figures 3.29 to 3.34 show the effect ofdirect shear failure of the brace and chord indentation The deformation of the braceand 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

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

Figure 3.34 Deformation and governing failure mode of specimen CT17

The load deformation curves for specimens CT12 through CT17 are shown in Figure3.35 The plots show a distinct elastic range, onset of strain hardening, ultimate loadand the final fracture point The ultimate load reached for specimen CT12 to CT17 arerespectively 2,780kN at a maximum deflection of 50mm, 1,512kN at a maximumdeflection of 90mm, 966kN at a maximum deflection of 180mm, 3,780kN at amaximum deflection of 60mm, 2,126kN at a maximum deflection of 80mm, and1,051kN at a maximum deflection of 240mm

All the specimens show high ductility prior to the ultimate failure load indicating thatthere is a lot of reserve strength for such trunnions As the grommet is placed atlengths 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 theloading arm of the brace becomes longer

0 500 1000 1500 2000 2500 3000 3500 4000

P

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 oftrunnion specimens and X-joints tested that there is high level of ductility, due tostrain hardening, in the trunnions regardless of the configuration type There is alsodistinct yielding behaviour under load and the load deformation plots show clearlydefined ultimate failure load The failure modes of these specimens are predominantlyshear failure through fracture of the shear-loaded arm of the trunnion except fortubular X-joints where the loads gives rise to bending moments Table 3.5 shows asummary of the ultimate loads and displacement for CT12 to CT17 The next sectionwill focus on the static strength and how they compare with the current practice

Table 3.5 Summary of the ultimate loads and displacement

for specimen CT12 to CT17

3.4 Discussions on the test results

The specimens in the tests were carefully selected so that different types of failuremechanisms of the trunnions can be observed The behaviour and strength of thesespecimens 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 previouschapter, the same formulation is used in this case for the large pipe trunnions Thisseries of tests have been calibrated and uses more advanced technology and resourceswhich reflects more accurately vis-a-vis the first set of tests that grommets are moreprone to unpredictable behaviour with better control of the experimental resultsobtained Hence this series of tests serves to enhance the previous series of tests andalso the larger dimensions used and the higher shear loads generated help to reduceany error due to sizing effects

The test results for specimens CT1 to CT5 are given in Figures 3.36 and 3.37, whichshows the various observed behaviour including first yield (indication of flaking ofwhite wash on the chord or brace), initial crack, ultimate load reached and the fracturepoint These points have been painstakingly recorded in order to fully understand theactual behaviour of the pipe trunnion during the various loading stages The first yieldhas been observed through the white wash flaking, and this should not bemisconstrued as the elastic yield strength However, it serves as an importantindication of the final failure mode of the pipe trunnion and where failure occurs.

0 1000 2000 3000 4000 5000 6000

Figure 3.37 Design load for specimen CT5

The first yield occurred on the chord surface for specimens CT1, CT2, CT4 and CT5with these specimens ultimately failing through chord tension pull out First yieldindication for specimen CT3 started on the brace and the ultimate failure mode wasthrough shear fracture Thus once the initial deformation starts, the load deformationgenerally 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 tocalculate the pipe trunnion design strength The following formulations, Equations 3.1

to 3.3, are used to compare with existing recommendations on shear and bendingmoment effects on the specimens

2

2 2

43 0 04 1 1

0

) 4 0 1 ( ) 4 0 1 (

1

J

E E

f

M

y

1 1

1

1 1

For the failure mode caused by chord indentation, M 1 is very close to M u or evenhigher This is because the full effective shear capacity of the brace can be mobilisedbefore chord indentation sets in and this path is continued to the ultimate failure

When M u is low, the joint design resistance for bending moment is low and this

generally results in chord indentation when M 1 increases On the other hand, forspecimen CT3 where the failure mode is due to brace shear failure, the effective sheararea is mobilised and shows that the existing shear strength estimate is very

conservative Here, it is noted that M 1 is much lower, indicating the high capacity ofthe joint to resist chord indentation There is also a lot of reserve strength from theductility Through determining the amount of joint design resistance against bendingmoments, the trunnion can be designed against such failure modes, such that the fulleffective shear capacity of the brace is mobilised

Table 3.6 Summary of the ultimate and design strength for CT1 to CT5

(All other units in mm)

Thus it is concluded that the existing design formulation under predicts the static yieldstrength of the pipe trunnion by a wide margin Further, it is observed that the highreserve strength enjoyed by the pipe trunnion makes the configuration suitable as alifting point as the ductility is very high Due to this under estimate of the static yieldstrength of the pipe trunnion, there is an opportunity to use this as a basis to furtheroptimise the basis of recommendations later.

This phenomenon is further investigated in the numerical analysis where the tubularX-joints were used to determine the design load where the level of chord plastificationoccurs 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 ofthe chord, is slotted through the chord wall and welded, similar to the configurationwhen the shear plate is slotted through the chord The main aim of fabricating a pipetrunnion with through pipe is to prevent chord indentation and these are suitable forsituations where the chord wall is thin and no replacement can be found Since theconfiguration is similar to shear plate pipe trunnions, this gives the designer greaterconfidence in utilising the full shear capacity of the pipe, even though this may not benecessary considering that a well-designed pipe trunnion can easily handle this task.This method of pipe trunnion design is investigated here and tested experimentally todetermine whether there is any advantage in designing through pipe trunnions