Proposed design chart of mechanical joints on steel-PHC ...

ORIGINAL ARTICLE

Proposed design chart of mechanical joints on steel-PHCcomposite piles

Yunsup Shin • Myoungmo Kim • Junyoung Ko •

Sangseom Jeong

Received: 18 December 2012 / Accepted: 9 June 2013 / Published online: 20 June 2013

� RILEM 2013

Abstract A fundamental study of a mechanical joint

in a steel-PHC composite pile subjected to combined

loads was done using three-point bending tests and

3D finite element analyses. The three-point bending

tests were conducted to evaluate load-deformation

response, strain distribution on the pile, ultimate

bending moment and failure mode of the mechanical

joint on steel-PHC composite piles. In addition, 3D

finite element analysis for the mechanical joint was

performed and then, the stress distributions and the

maximum load resistances of each parts of the joint

were estimated by comparing the calculated stresses to

the yielding stresses of the joint materials. The 3D

numerical methodology in the present study represents

a realistic mechanism of mechanical joints. Through

detailed numerical analysis, it is found that the

behaviour of mechanical joint of composite piles

shows safe side under working load. Based on these

results, the design chart for steel-PHC piles has been

proposed to be convenient for preliminary design

stage which can be used to evaluate the safety of

mechanical joints.

Keywords Composite pile � Mechanical joint �Three-point bending test � Three-dimensional finite

element analysis � Design chart

1 Introduction

Steel piles are frequently used in civil engineering

structures because they have many advantages such as

high load-bearing capacity, light weight and outstand-

ing workability. However, steel piles are more expen-

sive than other materials, and the cost of steel has been

rising recently in South Korea. The pretensioned spun

high strength concrete pile (PHC pile) is frequently

used in engineering practice in Asia. The PHC pile is

similar to ICP pile in North America [4]. PHC pile is

widely used in various constructions such as plants,

buildings, bridges and marine structures. The quality

and strength of the PHC pile are generally reliable,

because it is prefabricated in factories. Due to

economic considerations, PHC pile has been applied

in most parts of civil structures. PHC pile overcomes

concrete’s weakness in tension compared to the

concrete pile. It is used to apply floors, beams or

bridges with a longer span than is practical with

ordinary concrete pile.

Y. Shin

Technical Division, GS Engineering and Construction,

Seoul 100-722, Republic of Korea

M. Kim

Department of Civil and Environmental Engineering,

Seoul National University, Seoul 151-742,

Republic of Korea

J. Ko � S. Jeong (&)

Department of Civil and Environmental Engineering,

Yonsei University, Seoul 120-749, Republic of Korea

e-mail: [email protected]

Materials and Structures (2014) 47:1221–1238

DOI 10.1617/s11527-013-0124-3

Steel-PHC composite piles are often used as a

viable replacement for steel piles or PHC piles because

of their lower cost and their excellent load-bearing

capacity. No research has been conducted to study

steel-PHC composite piles.

In general, composite piles are defined as piles

consisting of two or more materials. Composite piles

were first used in the United States in the late 1980s as

replacements for timber fender piles at the port of Los

Angeles [6]. A prototype composite pile first used in

1987 was composed of a composite steel pipe encased

in recycled plastic [7]. Since the first application of

composite piles, several types of composite piles, such

as fiber-reinforced plastic (FRP) piles, concrete-filled

steel tube pile (CFST) and steel–concrete (SC) piles,

have been used in many structures.

Composite piles have been studied by many

researchers. Several empirical and numerical methods

for the analyzing the behaviour of composite piles

have been investigated. Although previous studies

have considered several types of composite piles, the

types of piles considered can generally be classified

into three groups: (1) concrete-filled FRP piles [1–3, 8,

11–13, 18], (2) CFST piles [14, 16, 17, 20], and (3)

steel–concrete piles [10]. However, less is known

about the steel-PHC composite pile.

Steel-PHC composite piles, also called hybrid

composite piles, have been used in some construction

projects in South Korea. A hybrid composite pile is

composed of steel and PHC pile. To investigate the

behaviour of steel-PHC composite piles, several pile

load tests and numerical analyses of their welded

joints have been performed [10]. However, welding

methods have some limitations, such as dependence

on weather, wind speed and skilled workers. On the

other hand, many engineers have been working in the

past few years to develop various types of non-welded

joints based on assembly mechanisms that overcome

the weaknesses of welded joints.

A rigorous numerical approach of the mechanical

joint is computationally expensive and time consum-

ing because of the 3D nature of the problem.

Therefore, a simplified design method is more suitable

for the preliminary design stage.

The overall objective of this study was to investi-

gate the behaviour of steel-PHC composite pile under

combined loadings. A series of three-point bending

tests and 3D FE analyses of the behaviour of

mechanical joints in steel-PHC composite piles were

performed. The design chart for mechanical joint has

been proposed to be convenient for preliminary design

stage. The design chart for mechanical joint on the

basis of both experimental test results and detailed

numerical analysis is proposed. The proposed chart is

shown to be capable of predicting the behaviour of

mechanical joint under combined loadings.

Fig. 1 The steel-PHC composite pile

1222 Materials and Structures (2014) 47:1221–1238

2 Background theory of steel-PHC composite piles

2.1 Composite piles

Steel-PHC composite piles are representative of

typical composite piles that are used in South Korea.

Steel-PHC composite piles are composed of different

pile materials, the upper part being steel pile and the

lower part being PHC pile. A schematic representation

of a steel-PHC composite pile is shown in Fig. 1.

The connection methods used with composite piles

can generally be classified into two categories: weld-

ing methods and non-welding methods. In general,

steel-PHC composite piles are connected using weld-

ing methods [10]. Pile load tests and 3D FE analysis of

the welding joint have been performed to verify the

pile behaviour by some researchers. In the case of a

steel-PHC composite pile, the drivability and integrity

of the pile should be confirmed.

The dynamic behaviour of steel-PHC composite

piles embedded in weathered rock has been tested

using a pile driving analyzer. Lateral pile load tests

have been carried out to analyze the response of a

steel-PHC pile to lateral loads. The instrumented pile

used in those tests was 500 mm in diameter and

23.2 m length. The pile consisted of a 4.2 m steel pile

and a 19.0 m PHC pile connected by typical welding.

Along with this steel-PHC pile, a steel pile 46.0 m in

length, was also tested at a nearby site to compare the

results for the two piles [10].

Although several investigators have examined the

behaviour of the longitudinal composite piles, very

Fig. 2 Mechanical joint of

the steel-PHC composite

pile

Fig. 3 Schematic structure

diagram of three-point

bending test pile

Materials and Structures (2014) 47:1221–1238 1223

little information and test data exist on the driveability,

and axial and lateral behaviour of composite piles,

such as steel-PHC composite piles. More research

should be conducted to confirm the findings of

previous studies concerning the effect of the different

thicknesses of steel and PHC piles on the axial and

lateral behaviour of the joint between the two piles.

The composite pile behaviour might also depend on

the location of the joint and the method of connection.

In the early 1990s, some countries developed a

special type of pile-connecting method for use without

welding. However, no design methods or specifica-

tions have been developed for steel-PHC composite

piles.

2.2 Mechanical joints

Mechanical joints, also referred to as non-welded

composite pile joints, are composed of a three-piece

side plate and 12 bolts fitting both the upper pile toe

and the lower pile top. To install the joint, the upper

pile is placed on top of the lower pile. These are

connected to the three-piece side plate with bolt holes

to make a single connected pile. As shown in Fig. 2,

the mechanical joints consist of four parts: the

connection rib, the three-piece side plate, the 12 bolts,

and the PHC pile bend.

The mechanical joint transmits bending moments

and shear stresses through the upper pile bottom to the

side connecting panel at the joint, and downward to the

lower pile head. Because of this loading-bearing

mechanism, the mechanical joint becomes capable of

producing a bearing capacity against the bending

moments and shear stresses that is greater than that of

the original pile body. In addition, mechanical joints

offer more consistent quality and easier installation

and maintenance than welding joints. Mechanical

joints have good workability in all weather and offer

the economic benefits of composite piles, especially

for large-diameter piles. However, there are few case

histories and little research data available for mechan-

ical joints of composite piles, so verification of the

behaviour of mechanical joints of composite piles,

using field pile load tests and numerical analyses is

warranted.

3 Experimental program

3.1 Three-point bending test

The bending moments and shear stresses imposed by

lateral loads on piles can be estimated using the three-

point bending test. In this study, three-point bending

tests were conducted to investigate the behaviour of

mechanical joints subjected to lateral loads. The

behaviour of pile under lateral loading is generally

dominated by the bending moment. Therefore, the

testing in a flexural mode is performed to reflect the

behaviour of pile under lateral loading. The testing

was conducted based on the Korean Industrial Stan-

dards (KS) F 4306 ‘‘Pretensioned spun high strength

concrete piles’’. The simply supported test method was

Table 1 Configuration of three-point bending test pile

Test pile no. Pile type Diameter

(mm)

Thickness

(mm)

Section area

(m2)

Crack bending

moment (kN-m)

Failure bending

moment (kN-m)

Strain gage

Pile no. 1 Steel pile 500 12 0.0184 – – No

PHC pile 500 80 0.1056 103.0 155.0

Pile no. 2 Steel pile 500 12 0.0184 – – Yes

PHC pile 500 80 0.1056 103.0 155.0

0

50

100

150

200

250

300

0 10 20 30 40 50

Deformation at midspan (mm)

App

lied

load

(kN

)

Cycle1 : No crackCycle2 : No crackCycle3 : No crackCycle4 : CrackCycle5 : FailureCycle6 : Failure

Cycle 5 (140kN)

Cycle 4 (120kN)

Cycle 6 (160kN)

Fig. 4 Load-deformation response of test pile No. 1


selected to simplify the experimental setup. A steel-

PHC composite pile was supported at the butt and the

tip, and a load was applied at the centre. The span

length (Ls) between the two end supports was 6.0 m,

and the total length (L) of the pile was 10.0 m. Each

steel support had a roller mounted on a triangular

moving block, which provided enough space under the

pile to accommodate deflection. Cyclic loads were

applied with a hydraulic actuator. Figure 3 shows a

schematic diagram of the three-point bending test. The

configuration of a three-point bending test pile is

summarized in Table 1.

The testing was conducted in controlled-load mode

at a constant loading rate. The peak or maximum load

was anticipated based on the standard for pile capac-

ity. Loading was applied in cycles of increasing

amplitude to assess residual deformation. Load cycles

that represented 40, 60 and 80 % of the expected

failure load were applied to each pile. The pile was

loaded up to 200 % of the standard crack bending

moment of pile capacity. Finally, the pile was loaded

to failure, which is defined by the peak load.

The bending moment and shear force of a simply

supported beam with a concentrated load at the mid

span can be illustrated by a diagram equivalent to that

for a cantilever beam with one-half the span length and

one-half the applied tip load. Therefore, the three-

point bending test procedure can be adopted to test the

(a) Cycle 1 (60kN) (b) Cycle 2 (80kN)

(c) Cycle 3 (100kN) (d) Cycle 4 (120kN)

(e) Cycle 5 (140kN) (f) Cycle 6 (over 160kN)

Fig. 5 Failure mode of test

pile No. 1 with load

increments


response of a pile subjected to lateral loads. Two types

of tests were performed: (1) simple load-deformation

tests; and (2) load-deformation tests with strain

gauges. The load-deformation response, strain distri-

bution, ultimate bending moment capacity and mode

of failure of the mechanical joint were evaluated.

Because the ultimate bending moment of a PHC

pile is smaller than that of a steel pile in a composite

pile, the material capacity of the PHC pile should be a

criterion in the stability analysis of a composite pile.

The loading at the point when cracks are created in a

PHC pile is calculated from the following equation. In

a conservative analysis, bending moments generated

by the self-weight of the pile can be ignored. When the

0

50

100

150

200

250

0 10 20 30 40 50

Deformation at midspan (mm)

App

lied

load

(kN

)

Cycle1 : No crackCycle2 : No crackCycle3 : No crackCycle4 : CrackCycle5 : FailureCycle6 : Failure

Cycle 4 (100kN)

Cycle 5 (120kN)

Cycle 6 (140kN)

Fig. 6 Load-deformation response of test pile No. 2

(a) Cycle 1 (60kN) (b) Cycle 2 (80kN)

(c) Cycle 3 (82.6N) (d) Cycle 4 (100kN)

(e) Cycle 5 (120kN) (f) Cycle 6 (140kN)

Fig. 7 Failure mode of test

pile No. 2 with load

increments


external load is 82.4 kN, cracking in the PHC pile can

be predicted based on the Eq. (1):

Mload ¼P

2� x ¼ 82:4

2� 2:5

¼ 103 kN � m� 103 kN � mðcrack bending

moment of the PHC pile) ð1Þ

where Mload is the bending moments due to an external

load (kN m), P is the external load (kN), and x is the

distance from the mid span to the support (m).

3.2 Test results and discussion

In the case of pile No. 1, three-point bending tests

were conducted to evaluate the load-deformation

response, ultimate bending moment and failure mode

to verify the intactness of the mechanical joint of the

steel-PHC composite pile. Figures 4 and 5 illustrate

the deformation and failure mode, respectively, of the

test pile as a function of the load increment. The

results indicated no signs of cracking in the PHC pile

for loads of 60–100 kN, applied in increments. Under

a loading of 120 kN, however, a partial crack was

observed in the reinforcing band of the PHC pile.

Under a loading of 140 kN, a 100 mm-long tensile

crack was generated in the PHC pile. When loads of

160 kN and more were applied, a tensile crack

approximately 350 mm-long and a diagonal crack

nearly 150 mm-long were created, at which point the

joint finally failed.

In the case of pile No. 2, three-point bending tests

were conducted to evaluate the load-deformation

response, ultimate bending moment, failure mode and

strain distribution. Figures 6 and 7 show the deformation

and failure mode, respectively, of the test pile as a

function of the load increment. The results indicated no

signs of cracking in the PHC pile for loads of 60–82.6

kN, applied in increments. Under a loading of 100 kN,

however, a partial crack was observed in the reinforcing

band of the PHC pile. Under a loading of 120 kN,

a 150 mm-long tensile crack was generated in the PHC

Fig. 8 Strain gauges

location for three-point

bending test

Fig. 9 Strain-time response

of three-point bending test


pile. When loads of 140 kN and more were applied,

tensile cracks approximately 300 mm-long cracks were

generated in PHC pile. When an external load is 82.4 kN

equivalent to the crack bending moment of PHC pile, no

cracks were generated in the mechanical joint. When

loads of 140 kN and more were applied, cracking was

generated in the PHC pile, while no cracks were

generated in the mechanical joint. Therefore, the

0

20

40

60

80

100

120

140

-1400 -1200 -1000 -800 -600 -400 -200 0 200 400 600

Strain (micro strain)

App

lied

Loa

d (k

N)

Strain gage-1Strain gage-2Strain gage-3Strain gage-4Strain gage-5Strain gage-6Strain gage-7Strain gage-8Strain gage-9Strain gage-10

Strain gauge-1Strain gauge-2Strain gauge-3Strain gauge-4Strain gauge-5Strain gauge-6Strain gauge-7Strain gauge-8Strain gauge-9Strain gauge-10Strain gauge-11

Fig. 10 Applied load–

strain relation of three-point

bending test

(a) 3D view (b) Detailed modeling

Fig. 11 Typical 3D model for FE analysis


mechanical joint was not damaged before the pile

materials of piles were damaged. Test pile No. 2 also

exhibited an ultimate bending moment higher than the

standard bending moment specification of PHC piles.

The strains at the mid span of the steel-PHC

composite pile were monitored during the three-point

bending tests for pile No. 2. The purpose of monitoring

using strain gauges was to confirm the stability and

strength of the mechanical joint.

As Fig. 8 shows, strain gauges were bonded to the

top and bottom of the pile. Gauges 1 and 2 were

bonded inside of the steel pile and the PHC pile, and

the others were attached outside the pile. The load-

strain distributions for the steel-PHC composite pile

)dnebelipCHP(2-traP(b))birnoitcennoc(1-traP(a)

)elipCHP(4-traP(d))etalpedis(3-traP(c)

)tlob(6-traP(f))elipleets(5-traP(e)

Fig. 12 Detailed modeling of mechanical joint of steel-PHC composite pile


are presented in Figs. 9 and 10. The compressive and

tensile strains of the PHC pile were monitored by the

gauges 3, 6 and 7. The strains in the steel pile were

recorded by gauges 5, 9 and 10. In addition, the steel

side plate was monitored by gauges 4, 11 and 8.

Loading was applied in six cycles at amplitudes that

increased from 60 to 140 kN. Until the third cycle at

82.6 kN, there was no sign of cracks or any damages to

the pile, while ?280 microstrain was measured at the

bottom and -413 micro strain was measured at the top

of the pile. During the fourth loading cycle, at 100 kN,

the axial strains reached peak values of ?320 micro-

strain (gauge 7) at the bottom of the PHC pile and

?208 micro strain (gauge 9) at the bottom of the steel

pile. During this fourth loading cycle, a partial crack

was observed in the reinforcing band of the PHC pile.

During the fifth and sixth loading cycles, after cracks

and damages had occurred in the pile, the tension

value did not increase any more, and the compression

value increased rapidly, reaching a peak value of

-1,580 microstrain (gauge 3) at the top of the PHC

pile. Thus, the PHC pile subjected to bending failed in

tension at the mid span location where the load was

applied. The standard ultimate strain of the PHC and

steel pile can be calculated as follows;

econ ¼ r=E ¼ 80=35; 000 ¼ 2; 300� 10�6

concrete compressionð Þ ð2Þ

econ ¼ r=E ¼ 5:66=35; 000 ¼ 160� 10�6

concrete bendingð Þ ð3Þ

esteel ¼ r=E ¼ 240=200; 000 ¼ 1; 200� 10�6

steel compressionð Þ ð4Þ

Through tests, the tested pile satisfied the standard

of design criteria in material capacity. As the results, it

is found that the mechanical joint body is relatively

stronger than any other parts of steel-PHC piles, and

thus represents to confirm safety of mechanical joint.

4 Numerical analysis

4.1 3D finite-element modeling

This section describes the ABAQUS-based numerical

analyses performed on the mechanical joint of the

steel-PHC composite pile. The analyses included

examining stresses generated in the joint under axial,

lateral, and tensile loading for the purpose of evalu-

ating the level of stability of the joint. In the numerical

analyses conducted using the ABAQUS program,

stresses generated in the joint under a specific type of

loading were investigated, and the maximum loads

experienced by the joint were estimated.

The typical 3D FE mesh used to analyze the

mechanical joint of the steel-PHC composite pile is

shown in Fig. 11. The outer boundary of the mesh was

fixed against displacements. As for the boundary

conditions of the joint, only the bottom surface of the

Table 2 Material properties used for FE analysis

Part no. Type Model c(kN/m3)

Elastic

modulus

(MPa)

l

Part 1 Connection

rib

L.E.* 72.0 210,000 0.20

Part 2 PHC bend L.E. 72.0 210,000 0.20

Part 3 Side plate L.E. 72.0 210,000 0.20

Part 4 PHC pile L.E. 27.4 35,000 0.25

Part 5 Steel pile L.E. 72.0 210,000 0.20

Part 6 Bolts L.E. 72.0 210,000 0.20

* L.E. linear-elastic model

Table 3 Loading conditions for numerical analysis (Unit: kN)

Load types 1 step 2 step 3 step 4 step 5 step 6 step 7 step 8 step

Axial load 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

Lateral load 60 80 100 120 140 160 180 200

Tensile load 100 150 200 250 300 350 400 500

Axial ? lateral load aA-250 A-500 A-750 A-1,000 A-1,250 A-1,500 A-1,750 A-2,000bL-60-200 L-60-200 L-60-200 L-60-200 L-60-200 L-60-200 L-60-200 L-60-200

a A axial loadb L lateral load


PHC pile was fixed with respect to the x, y, and z axes,

while the joint, the steel pile, and the PHC pile were

free to move with respect to all axes, which allowed

the displacement and rotation. As Fig. 12 shows, the

mechanical joint is composed of (a) connection rib,

(b) the PHC pile bend, (c) side plates (three units),

(d) the PHC pile, (e) the steel pile, and (f) bolts (12

units). The mechanical joint can be divided into three

sections: the top, the bottom, and the side. The top

includes the steel pile and the connection rib. The

bottom includes the PHC pile and the PHC pile band.

The side includes the side plate and bolts. The top

section includes part-1 (the connection rib) and part-5

(the steel pile). The bottom section includes part-2 (the

PHC pile bend) and part-4 (the PHC pile). The side

section includes part-3 (the side plates) and part-6 (the

bolts).

To obtain detailed information by 3D FE analysis, a

1.0 m-long section of steel-PHC composite pile

around the joint was selected. Each element of the

joint was partially generated and assembled to repre-

sent the complete joint which was used to analyze the

behaviour of the joint under loading. The properties

and constitutive model using analysis were summa-

rized in Table 2. ‘‘The interface element modeled by

the Coulomb’s frictional model is employed to

simulate the interface of different materials. This

model is available in ABAQUS ver 6.8 [9]. The shear

behaviour in the interface is that elastic behaviour has

going on until critical shear stress (scrit) in Eq. (5)

Table 4 Material properties

Part no. Type Size (mm) Elastic

modulus (MPa)

Yielding

stress (MPa)

Allowable

stress (MPa)

10 % increased

allowable stress (MPa)

Part 1 Connection rib – 210,000 240 140 154

Part 2 PHC bend – 210,000 240 140 154

Part 3 Side plate 14t 210,000 240 140 154

Part 4 PHC pile A-type 35,000 80 20 22

Part 5 Steel pile 500 9 12t 210,000 240 140 154

Part 6 Bolts M14 9 25 210,000 900 530 583

0

25

50

75

100

125

150

175

200

0 100 200 300 400 500

Strain (micro strains)

App

lied

Ben

ding

Mom

ent

(kN

-m)

Strain gage-6 (PHC)

Strain gage-7 (PHC)

Strain gage-6 (after crack)


ABAQUS result

Cycle4(100kN)

Cycle 1~3(0~82.6kN)

Cycle5(120kN)

Cycle6(140kN)

Strain gauge-6 (PHC)

Strain gauge-7 (PHC)

Strain gauge-6 (after crack)


ABAQUS result

Fig. 13 Comparison of moment-strain relation on PHC pile

0

25

50

75

100

125

150

175

200

-100 0 100 200 300 400 500

Strain (micro strains)

App

lied

Ben

ding

Mom

ent

(kN

-m)

Strain gage-9 (Steel pile)

Strain gage-10 (Steel pile)



ABAQUS result

Cycle4(100kN)

Cycle1~3(0~82.6kN)

Cycle5(120kN

Strain gauge-9 (Steel pile)

Strain gauge-10 (Steel pile)



ABAQUS result

Fig. 14 Comparison of moment-strain relation on steel pile


reached and after that only shear displacement

increases without increase of shear stress.

scrit ¼ l� p ð5Þ

where, l is friction coefficient and p is contact

pressure.

The interface friction coefficients (l) for steel on

steel were generally ranges from 0.70 to 0.74 [5, 19].

In addition, the interface friction coefficients (l) for

steel on concrete were generally ranges from 0.57 to

0.70 [15]. In this study, the interface friction

coefficients are adopted as 0.7 for steel–steel inter-

face and 0.6 for concrete–steel interface, respec-

tively.

To evaluate the stability of the mechanical joint,

several relevant types of loads were applied to the joint,

and the stresses and displacements generated in the joint

by each type of load were estimated. Table 3 shows the

loads applied to the joint, i.e., axial loads, lateral loads,

tensile loads and combined loads (axial and lateral load).

Each load was applied in increments for each step to

check the interaction between the joint and the stress.

Based on the results of the numerical analyses, the

stress generated in each element of the mechanical

joint was calculated and compared to the yielding or

allowable stress of each material to determine the

yielding state of the pile materials. The yielding and

allowable stresses of each element were estimated as

shown in Table 4. The magnitude of each stress was

indicated in terms of the von Mises stress which, in a

tri-axial state of stress, is defined as follows:

where, rx, ry and rz = the normal stresses in the x, y

and z axes, respectively and sxy, syz and szx = the

shear stresses in the x, y and z axes, respectively.

The results of the numerical analyses showed that

the stresses generated in each element of the mechan-

ical joint included simple tensile and shear stresses as

well as tri-axial stress. By comparing these stresses to

the yield stress of the material, the maximum stress

that each material can support without experiencing

plastic deformation can be estimated. This maximum

Table 5 Maximum stress values by axial loads at mechanical joint

Part no. Allowable

stress (MPa)

Maximum stress values by axial loads (Von Mises, MPa)

500 kN 1,000 kN 1,500 kN 2,000 kN 2,500 kN 3,000 kN 3,500 kN 4,000 kN

Part 1 154 35 68 103 138 172 207 242 301

Part 2 154 30 52 77 100 124 148 171 194

Part 3 154 18 25 37 52 68 85 103 121

Part 4 22 7 14 21 28 35 42 49 56

Part 5 154 32 62 93 124 155 185 216 246

Part 6 583 15 19 27 36 43 51 58 66

Fig. 15 Stress distribution of mechanical joint under axial

loads

rVonMises ¼1ffiffiffi

2p �

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

rx � ry

� �2þ ry � rz

� �2þ rz � rxð Þ2þ6 s2xy þ s2

yz þ s2zx

� �

r

ð6Þ


stress refers to the stress corresponding to the occur-

rence of permanent deformation in the material and

can be viewed as an approximate value of the elastic

limit.

When designing a steel-PHCcomposite pile, however,

uncertainty involving the site and the safety factor of the

pile material needs to be considered. By comparing von

Mises stress generated in the material to the additional

allowable stress, the stability of the material can be

evaluated. In this study, the following design criterion

was used to evaluate the stability of the mechanical joint:

rVonMises� 1:1�1:2ð Þ � rallowable. . .:OK ð7ÞWhen evaluating the stability of a pile joint by

comparing von Mises stress to the allowable stress, an

extra 10–20 % stress can be added to the allowable

stress. In this study, 10 % increased allowable stress

was applied considering a margin of safety in the

analytical data. Table 4 shows the allowable stress for

each material or element of the joint.

4.2 Validation of the 3D FEM with three-point

bending test result

The three-point bending tests were conducted to evaluate

bending moment-strain relation with verifying the results

of ABAQUS analyses. Testing was carried out in 6 cycles

with increasing amplitude of load. As the results of strain

distribution on the PHC pile (gauge 6 and 7), the moment-

strain relation shows the linear-elastic behaviour until the

third cycles. After cracks and damages on the PHC pile,

the moment-strain curves present the characteristics of

non-linear behaviour. ABAQUS analyses are conducted

by using linear-elastic model. Therefore the moment-

strain curve analyzed by ABAQUS can be compared with

the bending test curves which are the moment-strain

relation under the third cycles with no damage. As shown

Fig. 13, the moment–strain relations resulted from the

bending test and ABAQUS analyses are presented to the

almost same results.

Lastly, as the results of strain distribution on the steel

pile (gauge 9 and 10), the moment-strain relation shows

the linear-elastic behaviour. And the slopes of the

moment-strain curves are steeper than those of PHC pile.

Until the 6th cycle of peak load with 140 kN, there was no

yielding sign or damages on the steel pile showing ?235

micro strains (gauge 9) on the bottom and -690 micro

strains (gauge 5) on the top of the pile. The moment-strain

curve analyzed by ABAQUS can be compared with the

bending test curves. As shown Fig. 14, the moment-strain

relations resulted from the bending test and ABAQUS

analyses are presented to the almost same results.

4.3 Stress transfer at the mechanical joints

For compressive stress, numerical analyses were

carried out at a total of eight axial load levels, ranging

Table 6 Maximum stress values by lateral loads at mechanical joint

Part no. Allowable

stress (MPa)


60 kN 80 kN 100 kN 120 kN 140 kN 160 kN 180 kN 200 kN

Part 1 154 152 196 241 277 312 346 380 430

Part 2 154 67 97 125 143 160 178 194 210

Part 3 154 141 184 226 253 286 324 362 396

Part 4 22 7 9 11 13 15 17 19 21

Part 5 154 24 32 40 48 56 64 72 80

Part 6 583 118 156 192 228 265 301 337 368

Fig. 16 Stress distribution of mechanical joint under lateral

loads


from 500 to 4,000 kN, in increments of 500 kN. Based

on the analytical data, the maximum von Mises stress

was estimated for each of the joint elements. The

maximum compressive stresses the joint could endure

were then calculated by comparing the maximum von

Mises stress applied at the joint to the allowable stress

of each material.

Table 5 and Fig. 15 summarize the magnitudes of

the von Mises stresses generated in each element of the

joint for axial loads of 500–4,000 kN. The ABAQUS

analysis results indicate that the largest stresses were

generated in the PHC pile (Part-4), the connection rib

(Part-1), and the steel pile (Part-5). Comparisons

between the stresses generated and the allowable

stresses of the materials showed that the stresses

generated exceeded the allowable stress in the PHC

pile (Part-4) when the maximum axial load transferred

to the joint was 1,600 kN or more.

For bending stress, numerical analyses were carried

out for a total of eight lateral load levels from 60 to

200 kN in increments of 20 kN. Based on the results,

the maximum von Mises stress was estimated for each

joint element. The maximum bending stresses that the

joint can endure were calculated by comparing the

maximum von Mises stress applied to the joint to the

allowable stress of each material.

Table 6 and Fig. 16 summarize the magnitudes of

the von Mises stresses generated in each element of the

joint for lateral loads of 60–200 kN. The ABAQUS

analysis results indicate that the largest stresses were

generated in the side plate (Part-3), and the connection

rib (Part-1). Comparisons between the stresses gener-

ated and the allowable stresses of the materials showed

that the stresses generated exceeded the allowable

stress at the connection rib (Part-1) when the maxi-

mum lateral load transferred to the joint was 62 kN or

more.

For tensile stress, numerical analyses were carried

out for a total of eight load levels from 100 to 500 kN

in increments of 50 kN. Based on the results, the

maximum von Mises stress was estimated for each of

the joint elements. The maximum tensile stresses the

joint can endure were calculated by comparing the

maximum von Mises stress applied to the joint to

the allowable stress of each material.

Table 7 and Fig. 17 summarize the magnitudes

of the von Mises stresses generated in each element of

the joint for tensile loads from 100 to 500 kN. The

ABAQUS analysis results indicate that the largest

stress was generated in the side plate (Part-3).

Comparisons between the stresses generated and the

allowable stresses of the materials showed that the

generated stresses exceeded the allowable stress at

the side plate (Part-3) when the maximum tensile load

transferred to the joint was 245 kN or more.

In most practical situations, the loads applied to a

pile include both axial and lateral loads. Therefore,

this study analyzed combined loads as well. Under

Table 7 Maximum stress values by tensile loads at mechanical joint

Part no. Allowable

stress (MPa)



Part 1 154 43 55 67 78 90 101 113 137

Part 2 154 26 37 49 61 72 83 94 117

Part 3 154 77 109 133 156 181 212 245 305

Part 4 22 3 4 5 6 7 9 10 12

Part 5 154 6 9 12 16 19 22 25 31

Part 6 583 32 43 53 63 72 83 92 110

Fig. 17 Stress distribution of mechanical joint under tensile

loads


axial loads varying from 250 to 2,000 kN, lateral loads

were increased from 60 to 200 kN as von Mises stress

was calculated. Table 8 and Fig. 18 summarize the

magnitude of the von Mises stresses generated in each

element of the joint. The ABAQUS analysis results

indicate that the largest stress was generated in the side

plate (part-3), connection rib (part-1), and bolts (part-

6). Comparisons between the stresses generated and

the allowable stresses showed that the generated

stresses exceeded the allowable stress at the side plate

(Part-3) when the maximum lateral load transferred to

the joint was 112 kN or more.

4.4 Design chart for mechanical joints

A rigorous numerical approach of the mechanical joint

is computationally expensive and time consuming

because of the 3D nature of the problem. Therefore, a

simplified design method is more suitable for the

preliminary design stage. A design chart based on the

results of numerical analysis, in which pile joint

stability is considered, can be proposed for the design

method of steel-PHC composite piles.

The magnitude of stress that the mechanical joint

can endure under axial, lateral, tensile, and combined

loading was predicted. The analytical results can be

verified using the results of the three-point bending

tests with strain gauges measurements. Based on the

aforementioned results of the numerical analyses, the

stress transfer can be calculated for each element of

the mechanical joint under the combined loadings.

The mechanical joint is composed of six elements,

namely, the connection rib (part-1), the PHC pile bend

(part-2), the side plates (part-3), the PHC pile (part-4),

the steel pile (part-5), and the bolts (part-6).

In Fig. 19, the stress transfer and distribution at

each element of the mechanical joint are shown for

axial loads of 0–2,000 kN and lateral loads of

0–200 kN. The stress distributions at parts-1, 2, 3,

and 6 tend to increase or decrease depending on the

scale of the axial and lateral loads. Those of parts-4

and 5 have a tendency to increase almost linearly as

the axial and lateral loads increase. Analysis of the

stress distributions can be used to analyze the causes of

joint failure and to reinforce the stability of the

mechanical joint with greater ease.

Based on the stress transfer and distribution, the

maximum load resistance values of the mechanical

joint can be estimated by comparing the von Mises

stresses with the allowable stresses of the pile

material, as shown Fig. 20. In the case of steel

elements, such as parts-1, 2, 3, 5, and 6, the maximum

lateral resistances tend to increase as the maximum

axial resistance increases but start to decrease beyond

a certain point of axial resistance. However, in the case

of concrete elements, such as part-4, the maximum

lateral resistance tends to decrease as the axial

resistance increases.

Table 8 Maximum stress values by combined loads at mechanical joint (axial load = 500 kN)

Part no. Allowable

stress (MPa)



Part 1 154 59 77 103 141 186 247 311 367

Part 2 154 47 53 60 68 78 103 136 167

Part 3 154 74 86 121 170 228 293 345 393

Part 4 22 11 13 15 17 19 21 23 25

Part 5 154 43 49 56 63 70 78 85 92

Part 6 583 45 63 87 113 144 185 230 269

Fig. 18 Stress distribution of mechanical joint under combined

load (axial load 500 kN and lateral loads)


Based on the results of the detailed numerical

analyses, the design chart for mechanical joint, which

is possible to achieve the preliminary design stage,

can be developed. Figure 21 shows the proposed

design chart for mechanical joint. The design chart is

classified into four different zones such as (1) the

stable zone, (2) the unstable steel zone, (3) the unstable

concrete zone, and (4) the failure zone. The proposed

0

100

200

300

400

500

0 500 1,000 1,500 2,000 2,500

Axial Load (kN)

Stre

ss (

Von

Mis

es, M

Pa)

Lateral - 0kNLateral - 60kNLateral - 80kNLateral - 100kNLateral - 120kNLateral - 140kNLateral - 160kNLateral - 180kNLateral - 200kN

0

100

200

300

400

500

Axial Load (kN)

Stre

ss (

Von

Mis

es, M

Pa)


0

100

200

300

400

500

Axial Load (kN)

Stre

ss (

Von

Mis

es, M

Pa)


0

20

40

60

80

100

Axial Load (kN)

Stre

ss (

Von

Mis

es, M

Pa)


0

50

100

150

200

250

300

Axial Load (kN)

Stre

ss (

Von

Mis

es, M

Pa)


0

100

200

300

400

500

Axial Load (kN)

Stre

ss (

Von

Mis

es, M

Pa)


(a) Part-1 (connection rib) (b) Part-2 (PHC pile bend)

(c) Part-3 (side plate) (d) Part-4 (PHC pile)

(e) Part-5 (steel pile) (f) Part-6 (bolt)

0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,5000 500 1,000 1,500 2,000 2,500

0 500 1,000 1,500 2,000 2,500

Fig. 19 Load-stress relations of mechanical joint under combine load with axial and lateral loads


design chart can be adopted and utilized for designing

and evaluating the stability of the mechanical joint.

Curve-1: y1 ¼ 66:81e0:001x ð8Þ

Curve-2: y2 ¼ �6� 10�5x2 � 0:02xþ 201:5 ð9Þ

Curve-3: y3 ¼ �5� 10�8x3 � 1� 10�4x2 � 0:113x

þ 298:45

ð10ÞFigure 21 shows that zone-1 is the stable zone in

which all of the elements of the mechanical joint are

stable. Zone-2 is the zone in which the mechanical

joint is partially unstable because the stresses in the

steel elements exceed the allowable stress of steel.

Zone-3 is also a zone of partial instability in which the

stresses in the concrete elements exceed the allowable

stress of concrete. Each zone can be indicated as

follows.

Zone-1 is the stable zone in which the area of the

maximum axial resistance x and the maximum lateral

resistance y are as follows:

0� x \ 795; y \ y1 and

795� x \ 1680; y \ y2 ð11Þ

Zone-2 is the unstable steel zone in which the area

of the maximum axial resistance x and the maximum

lateral resistance y are as follows:

0� x \ 795; y1� y \ y1 ð12ÞZone-3 is the unstable concrete zone in which the

area of the maximum axial resistance x and the

maximum lateral resistance y are as follows:

795� x\1190; y2� y\y1 and

1190� x\2200; y2� y\y3 ð13Þ

Zone-4 is the failure zone in which the remaining

area is as follows:

0� x\795; y� y2 and

795� x\1190; y� y1 and

1190� x\2200; y� y3 ð14Þ

The estimated von Mises stress and the allowable

stress of each pile material were compared to estimate

the maximum stress endured by each element. Based

on these results, design chart for the mechanical joint

is proposed, with three curve equations and four zones

(the stable zone, the unstable steel zone, the unstable

concrete zone, and the failure zone). The proposed

design chart can be used to evaluate the safety of a

mechanical joint.

5 Summary and conclusions

The main objective of this study was to propose design

chart for mechanical joints in steel-PHC composite

piles for various loading conditions. Three-point

bending tests were conducted to investigate the

behaviour of the mechanical joint. In addition, the

3D finite element analysis for the mechanical joint was

performed. Based on these results, the design chart for

mechanical joint has been proposed to be convenient

for preliminary design stage. On the basis of the

findings of this study, the following conclusions are

drawn:

0

100

200

300

400

500

0 500 1,000 1,500 2,000 2,500 3,000

Max. Axial Resistance (kN)

Max

. Lat

eral

Res

ista

nce

(kN

) Part-1 Part-2Part-3 Part-4Part-5 Part-6

1

2

3

4

5

6

Fig. 20 Evaluation of maximum axial and lateral resistance

values at each element of mechanical joint

Fig. 21 Design chart for mechanical joint


1. Three-point bending tests were conducted to

evaluate the load-deformation response, strain

distribution on the pile, ultimate bending moment

and failure mode of mechanical joints in steel-

PHC piles. The tests showed that the mechanical

joints body is relatively stronger than any other

parts of steel-PHC piles, and thus represents to

confirm safety of mechanical joint.

2. A rigorous numerical approach of the mechanical

joint is computationally expensive and time

consuming because of the 3D nature of the

problem. Therefore, a simplified design method

is more suitable for the preliminary design stage.

The design chart for mechanical joint needs to be

proposed in preliminary design stage.

3. The 3D numerical methodology in the present

study represents a realistic mechanism of mechan-

ical joints. The stress distribution and maximum

load resistance of each part of the joint were

estimated by comparing the calculated stresses to

the yield stresses of the joint materials. Through

detailed numerical analysis, it is found that a

mechanical joint of steel-PHC composite piles has

more reliable than any other parts of piles under

working load.

4. Based on the results of the detailed numerical

analyses, design chart for mechanical joints,

which is possible to achieve the preliminary

design stage, can be developed. The proposed

design chart is classified into four different zones

such as (1) the stable zone, (2) the unstable steel

zone, (3) the unstable concrete zone, and (4) the

failure zone. The proposed design chart can used

to evaluate the safety of mechanical joints.

Acknowledgments This work was supported by the National

Research Foundation of Korea (NRF) grant funded by the Korea

government (MSIP) (No. 2011-0030842).

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