NISTIR 6557

Strength Evaluation of Connectors

for

Intermodal Containers

H. S. Lew

Fahim Sadek

Erik D. Anderson

United States Department of Commerce

Technology Administration

National Institute of Standards and Technology
NISTIR 6557

Strength Evaluation of Connectors

for

Intermodal Containers

H. S. Lew

Fahim Sadek

Erik D. Anderson

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899

Prepared for

Office of Maritime Safety Standards

Occupational Safety and Health Administration

Department of Labor

August 2000

U.S. Department of Commerce

Norman Y. Mineta, Secretary

Technology Administration

Dr. Cheryl L. Shavers, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Raymond G. Kammer, Director
iii

PREFACE

This report presents the results of an experimental study of semi-automatic twistlocks and

latch locks that are used to interconnect marine cargo containers. Although these

interbox connectors are primarily used to secure containers during shipping, they are also

used in lifting two empty containers, one on top of the other. In this process, known as

vertical tandem lift (VTL), the bottom container’s weight is transmitted to the upper

container by means of the interbox connectors. The Occupational Safety and Health

Administration (OSHA), the Department of Labor, is concerned with the safety of

workers during a VTL where disengagement or failure of one or more connectors could

create a hazardous condition. Under an interagency agreement with OSHA, the National

Institute of Standards and Technology (NIST), the Department of Commerce, carried out

an experimental program to assess the load carrying capacity of interbox connectors. Mr.

Paul Rossi of the Office of Maritime Safety Standards of OSHA provided technical

guidance throughout the study.

Any opinions, findings, conclusions and recommendations expressed in this report are

those of the authors and do not necessarily reflect the views of NIST or OSHA.

Certain trade names and company products are mentioned in the text or identified in

illustrations in order to adequately describe test specimens. In no case does such

identification imply recommendation or endorsement by NIST and by OSHA, nor does it

imply that the products are necessarily the best available for the purpose.
iv
v

ABSTRACT

Intermodal containers are used widely in marine cargo handling. In order to facilitate

unloading of empty containers from ship to shore, two containers are frequently lifted in

tandem. This operation is referred to as “vertical tandem lifting.” In this lifting process,

the lower container is attached to the upper container by means of interbox connectors at

the four corners of the container. During the vertical tandem lifting operation, if one or

more connectors are disengaged, the bottom container may be separated from the upper

container, thereby creating an unsafe condition to workers below. This safety concern

prompted the Occupational Safety and Health Administration (OSHA) to request the

National Institute of Standards and Technology (NIST) to study the suitability of interbox

connectors for vertical tandem lifting.

The NIST study included site visits to port facilities and laboratory tests of interbox

connectors. Test specimens included semi-automatic twistlocks and latch locks. The

experimental program included testing of twistlocks in tension, twistlock and latch lock

assemblies with corner fittings in tension and compression, and shafts of twistlocks in

tension to obtain the stress-strain relationship. In addition, measurements were made of

the bearing area of the top and bottom cones of twistlocks and the latch on the inner

surface of the corner fitting.

The study indicated that the tensile capacity of the connection is governed by the strength

of the corner fitting. The study also shows that the bearing area of latch locks on the

corner fitting is less than that required by the ISO standards.

Keywords: container; corner fittings; interbox connectors; latch locks; twistlocks;

vertical tandem lifts.
vi
vii

TABLE OF CONTENTS

PREFACE ..........................................................................................................................iii

ABSTRACT........................................................................................................................ v

TABLE OF CONTENTS..................................................................................................vii

1.0 INTRODUCTION................................................................................................... 1

1.1 Objectives.................................................................................................... 2

1.2 Outline of report.......................................................................................... 2

2.0 SCOPE OF STUDY................................................................................................ 3

2.1 Site visits ..................................................................................................... 3

2.2 Laboratory study ......................................................................................... 3

3.0 DESCRIPTION OF TEST SPECIMENS ............................................................... 5

3.1 Specimen identification............................................................................... 6

4.0 TEST METHODS................................................................................................. 11

4.1 Tension capacity of twistlocks .................................................................. 11

4.2 Tension capacity of twistlock and latch lock assemblies.......................... 11

4.3 Compression capacity of twistlock and latch lock assemblies.................. 12

4.4 Tensile strength of twistlock shaft materials............................................. 12

5.0 TEST RESULTS................................................................................................... 17

5.1 Tension capacity of twistlock shafts ......................................................... 17

5.2 Tension capacity of twistlock and latch lock assemblies.......................... 17

5.3 Compression capacity of twistlock and latch lock assemblies.................. 18

5.4 Tensile strength of twistlock shafts........................................................... 19

5.5 Measurement of load bearing areas........................................................... 19

6.0 CONCLUSIONS AND RECOMMENDATIONS................................................ 35

6.1 Conclusions ............................................................................................... 35

6.2 Recommendations ..................................................................................... 36

7.0 REFERENCES...................................................................................................... 37
1

1.0 INTRODUCTION

Intermodal containers are widely used in marine cargo handling. The containers are

usually stacked more than four tiers high on the ship deck during sea transport. To secure

containers to the ship deck during transportation, they are interconnected by interbox

connectors. In addition, diagonal bracings are used for the lower two tiers of

interconnected containers to provide lateral stability against racking. When empty

containers are unloaded from ship to shore, two containers are frequently lifted in

tandem, one on top of the other. This operation is referred to as “vertical tandem lift

(VTL).” In this lifting process, the weight of the lower container is transmitted to the

upper container by means of the interbox connectors at the four corners. Thus, the upper

container and connectors together become a lifting device for the lower container.

In general, an intermodal container is comprised of four parts. They are: (1) a boxshaped steel frame including four corner posts and corner fittings (or corner castings),

which are welded to the top and bottom of the four corner posts; (2) end and side walls;

(3) a door opening at one end; and (4) floor and roof. The connectors are engaged

directly into the corner fittings, and thus, all lifting loads are transferred to the corner

posts through the corner fittings.

Newly manufactured containers are designed, constructed, and tested according to

standards promulgated by the International Organization for Standardization (ISO). The

relevant ISO standards are ISO 1161, the general specification for corner fittings [1] and

ISO 1496, the standard for design and testing of containers [2]. Because all corner

fittings are manufactured according to the ISO standard, connectors of all types fit into

the corner fittings.

The shipping industry uses a variety of interbox connectors, broadly grouped into four

categories: (1) conventional twistlocks; (2) semi-automatic twistlocks; (3) fully automatic

twistlocks; and (4) latch locks. The primary components of these connectors are made of

either forged or cast steel. Although the locking mechanisms of twistlocks and latch

locks differ, they all function the same way. They lock into the corner fittings of the top

and bottom containers, thereby interconnecting them.

During the vertical tandem lifting operation, if one or more connectors are disengaged

from corner fittings, the bottom box may be separated partially or totally from the upper

container, thereby creating an unsafe condition for workers below. This safety concern

prompted the Occupational Safety and Health Administration (OSHA) to request the

National Institute of Standards and Technology (NIST) to study the structural

performance of interbox connectors.

At present, there is no standard for design, manufacturing, testing, or use of interbox

connectors in lifting operations. The manufacturers of such connectors provide

information on the tensile and shear strengths and safe-working capacities of new
2

connectors based on their test data. Without a testing standard to follow, it is expected

that disparities exist in manufacturers’ test data.

In the course of many repeated uses, connectors may sustain varying degrees of damage

due to overstressing and corrosion. No systematic investigation has been carried out to

determine whether there is a significant strength reduction or malfunctioning of

connectors with use. Thus, it is not possible at present to establish safe working strengths

of connectors relative to their use history and age. In absence of the strength data of

connectors in service, no guidelines have been established to modify the safe working

strength of connectors as a function of their use history and age.

For this study, OSHA selected semi-automatic twistlocks and latch locks for strength

evaluation. The semi-automatic twistlocks were selected because of their wide use while

the latch locks were chosen because of their unique operational feature.

1.1 Objectives

The objectives of this experimental study were to (1) assess the load capacity of selected

semi-automatic interbox connectors in tension and compression, and thereby assess their

suitability for vertical tandem lifts, and (2) identify the weakest link in the connectorcorner fitting assembly.

1.2 Outline of report

The report is organized into seven sections. Section 2 describes the scope of the study.

Section 3 presents the description of the twistlocks and latch locks that were tested.

Section 4 describes the test methods and fixtures used for each type of loading. Section 5

presents the results of the tests and other pertinent measurements. Section 6 provides the

conclusions and recommendations from this study. Section 7 lists the references cited in

the report.

Tables and figures are contained at the end of each section. Tables and figures are

numbered by section and by order of occurrence within the section.
3

2.0 SCOPE OF STUDY

This study included (1) site visits to view loading and unloading operations of containers,

and (2) laboratory tests of both new and used twistlocks and latch locks.

The study focused on testing selected types of interbox connectors that included

twistlocks and latch locks. The results presented in this report should not be extrapolated

to other types of connectors not included in this study. During the course of this study,

no attempt was made to conduct a statistically rigorous testing program, but only to

assess in broad terms the structural performance of the connectors and identify their

failure mechanism and the weakest link. The objective of this study was not to qualify or

disqualify a certain product or to suggest preference of one product over another.

The study was limited to investigating interbox connectors and connector-corner fitting

assemblies. No attention was given to the overall structural integrity of the container.

For example, the welded connection between the corner fitting and the corner post may

present a weaker connection than the interbox connector-corner fitting assembly. Further

studies are required to address this issue.

2.1 Site visits

Prior to initiating the laboratory study, port facilities in Baltimore, Maryland and

Hampton, Virginia were visited.

At the port of Baltimore, a demonstration was made using a gantry crane to simulate

vertical tandem lifts (Fig. 2.1), and to show how the sway motion is imparted to the

containers due to sudden stopping of the gantry crane (Fig. 2.2). When the containers are

blocked by a fixed object, the sway motion may cause shear force in the connectors. At

the port of Hampton, actual operations of unloading of containers from ship to shore were

viewed.

2.2 Laboratory study

Four different load tests were performed. These included:

- Tension capacity of twistlocks,

- Tension capacity of twistlock and latch lock assemblies with corner fittings,

- Compression capacity of twistlock and latch lock assemblies with corner fittings,

and

- Tensile strength of twistlock shaft materials.

In addition, measurements were made of the bearing area of the top and bottom cones of

twistlocks and the latch of latch locks on the inner surface of the corner fitting.
4

Figure 2.1 Lifting of Containers by Gantry Crane

Figure 2.2 Sway Motion of Containers
5

3.0 DESCRIPTION OF TEST SPECIMENS

Semi-automatic twistlocks are manufactured worldwide. At the time of this study, about

twelve manufacturers produced most of the twistlocks used by the shipping industry.

They are manufactured in Australia, Denmark, Germany, Japan, Sweden and the United

States. The latch locks are manufactured only in Japan. NIST contacted U.S.

representatives of eight manufacturers, and four responded with immediate availability of

twistlocks. NIST purchased test specimens from the following manufacturers.

Manufacturer Model Number Figure No.

All Set Marine C5AM 3.1

MacGregor-Conver CV-20/2 (dual function) 3.2

Peck & Hale F738-M-1 3.3

Taiyo Seiki NA-3 3.4

Taiyo Seiki NA-7 (latch lock) 3.5

In addition to new twistlocks, NIST obtained two types of used twistlocks from a

terminal operator.

Manufacturer Model Number Figure No.

All Set Marine C5AM-DF (dual function) 3.6

MacGregor-Conver CV-14 3.7

The semi-automatic twistlocks are comprised of a single-piece steel shaft encased in a

steel housing. The shaft has an upper and a lower cone that are oriented in different

directions and fit inside the corner fittings. Some of the non-dual-function twistlocks

have either a flat top upper cone (Figs. 3.1 and 3.7) or a pointed upper cone (Figs. 3.3 and

3.4). All dual-function twistlocks have pointed top and bottom cones (Figs. 3.2 and 3.6).

Dual-function twistlocks facilitates the fitting and removal of twistlocks without affecting

the container loading and unloading operations.

A latch lock has a locking flange on one side of the housing and a latch mechanism on

the other side that works like a door latch (Fig. 3.5). When a latch lock is inserted into

the corner fitting, the horizontal sliding latch is extended inside the corner fitting, thereby

locking the latch lock to the corner fitting.

Peck & Hale Model F463-1R and F463-2R corner fittings were used to make the test

fixtures for tension testing of twistlocks and latch locks, see Fig. 4.2.
6

3.1 Specimen identification

The following designations are used to identify test specimens.

Manufacturer Designation Age

All Set Marine/C5AM ASM-T-N New

All Set Marine/C5AM-DF ASM-T-U Used

MacGregor-Conver/CV-20/2 CON-T-N New

MacGregor-Conver/CV-14 CON-T-U Used

Peck & Hale/F738-M-1 P&H-T-N New

Taiyo Seiki/NA-3 TYS-T-N New

Taiyo Seiki/NA-7 TYS-L-N New

The letters “T” and “L” indicate that the connector is either a twistlock or a latch lock,

respectively.
7

Figure 3.1 All Set Marine C5AM (Twistlock)

Figure 3.2 MacGregor-Conver CV-20/2 (Twistlock)
8

Figure 3.3 Peck & Hale F738-M-1 (Twistlock)

Figure 3.4 Taiyo Seiki NA-3 (Twistlock)
9

Figure 3.5 Taiyo Seiki NA-7 (Latch Lock)

(a) Front View

(b) Side View
10

Figure 3.7 MacGregor-Conver CV-14

Figure 3.6 All Set Marine C5AM-DF
11

4.0 TEST METHODS

4.1 Tension capacity of twistlocks

The purpose of this set of tests was to determine the tensile capacity of twistlock shafts.

For this purpose, a special tension test fixture was fabricated as shown in Fig. 4.1. The

fixture was placed into a 1775 kN (400 000 lbf) Tinius-Olsen universal testing machine.

The accuracy of the testing machine in tension is 0.0034 %. The dimensions of the

apertures in the top and bottom cylindrical fittings were the same as those specified in the

ISO 1161 Standard [1] for corner fittings. Heat treating process (quenching and

tempering) was used to harden the cylindrical fittings.

The capacity of a twistlock is limited by the capacity of the shaft to which the top and

bottom cones are attached. To observe the mode and location of failure, the shaft was

removed from the twistlock housing, and tested. Since the cones at the shaft ends are not

oriented in the same direction and to avoid damage to the test fixture, the cylindrical

fittings were rotated with respect to each other so that a maximum bearing area could be

attained between the surface of the cone and the fitting. This was accomplished by

rotating the fittings so that both the top and bottom cones were perpendicular (90°) to the

long axis of the opening in the fitting. It should be noted that in actual use, the bottom

cone, which locks automatically into the top corner fitting of a container, rotates less than

90°, which results in less than the maximum attainable bearing area.

The load was applied at a rate of 444 kN/min (100 000 lbf/min) until the specimen (shaft)

fractured.

4.2 Tension capacity of twistlock and latch lock assemblies

The purpose of these tests was to determine the capacity and to identify the weakest link

of the interbox connector-corner fitting assembly in tension. To simulate field

conditions, twistlocks and latch locks were tested together with corner fittings. The test

setup and fixture are shown in Fig. 4.2. High strength nuts were welded to 38 mm (1 1/2

in) thick steel plates, which were in turn welded to the top and bottom corner fittings.

This was done to ensure proper transfer of the load to the walls of the corner fitting

without breaking its top plate. High strength bolts, which were screwed into the nuts,

were fastened to the moving and fixed heads of the Tinius-Olsen testing machine.

Because the corner fittings were severely deformed or damaged during each test, a new

set of corner fittings was used for each test. This required fabrication of many sets of test

fixtures.

For this series of assembly tests, the corner fittings were not rotated to attain a maximum

bearing area between the cone and the inner surface of the corner fitting as was done for

the twistlock tension capacity test. A twistlock or latch lock was placed between the top

and bottom corner fittings similar to the field condition, see Fig. 4.2 (b).
12

A loading rate of 444 kN/min (100 000 lbf/min) was maintained until failure occurred.

4.3 Compression capacity of twistlock and latch lock assemblies

This set of tests was conducted to determine the capacity of the interbox connector and

corner fitting assembly in compression. Figure 4.3 shows the setup for the compression

test. A twistlock or latch lock was inserted into a top corner fitting, and placed under the

moving head of a stiff 4448 kN (1 000 000 lbf) capacity Baldwin/Prism compression

testing machine. The accuracy of the testing machine in compression is 0.0066 %. The

load was applied at 444 kN/min (100 000 lbf/min) through a flat steel plate which was

placed between the loading head and the top of the cone.

The test specimen was first loaded to 222 kN (50 000 lbf) without stopping, and unloaded

for examination. Beyond this point, the load was increased in 44.5 kN (10 000 lbf)

increments. At each load increment level, the specimen was unloaded to examine the

functioning of the twistlock and any permanent deformation of the assembly. Testing

was terminated when the twistlock failed to function or when permanent deformation or

damage to the assembly was observed.

4.4 Tensile strength of twistlock shaft materials

These tests were performed to obtain the tensile stress-strain characteristics of the shaft

materials of twistlocks. The test specimens were prepared from the shafts of twistlocks.

The shaft was machined to a diameter of about 26 mm (1 in), and two strain gages (5 mm

gage length) were placed 180° apart at the mid height of the specimen (Fig. 4.4). Prior to

each test, the shaft diameter was measured at several places using a micrometer. The

cross sectional area of the shaft, which was computed using the average of the measured

diameters, was used to determine the tensile strength. The test setup was the same as that

described in Section 4.1. Applied loads and strains were recorded using a data

acquisition system and processed by a computer. The specimens were tested according to

ASTM Test Methods A370-99a [3].
13

Figure 4.1 Tension Test Fixture for Twistlock Shafts
14

(a) Tension Test Setup

(b) Test Assembly

Figure 4.2 Tension Test Setup for Twistlock and

Latch Lock Assembly

Twistlock or latch lock

Corner fitting

38 mm steel plate

High strength nut
15

Figure 4.3 Compression Test Setup
16

Figure 4.4 Tensile Strength Test Specimen
17

5.0 TEST RESULTS

5.1 Tension capacity of twistlock shafts

Table 5.1 lists the failure loads of twistlock shafts loaded in tension using the special test

fixture described in Section 4.1. As seen from the table, a replicate test was made for

each group. Depending upon the composition of metal, the shafts fractured either in

brittle or ductile modes. Examples of brittle, ductile, and ductile-shear modes of failure

of the shafts are shown in Figs. 5.1, 5.2, and 5.3, respectively.

Latch locks were tested using a test fixture made with corner fittings, and the test results

are reported in Section 5.2.

The ultimate tensile strengths ranged from 562 kN (126 400 lbf) to 802 kN (180 300 lbf).

Similar test results were also reported by Anderson [4], wherein the ultimate loads ranged

from 477 kN (107 000 lbf) to 797.1 kN (179 200 lbf). The ISO 1161 Standard [1]

requires that the top corner fitting be designed for a lifting load of 150 kN (33 700 lbf).

While this requirement is not directly related to vertical tandem lifts (VTL), the

magnitudes of the ultimate loads of the twistlock shafts, regardless of their modes of

failure, are substantially greater than the ISO minimum design load of the top corner

fitting.

Although a limited number of used connectors were tested, the values in Table 5.1 do not

indicate that used twistlocks are weaker than new twistlocks.

5.2 Tension capacity of twistlock and latch lock assemblies

Table 5.2 lists the maximum tensile loads of twistlock and latch lock assemblies, and

brief descriptions of their failure modes. As described in Section 4.2, in this series of

tests, each specimen was loaded with top and bottom corner fittings.

Comparing the ultimate loads in Tables 5.1 and 5.2, it can be seen that the capacity of the

assembly is limited by failure of the corner fitting. Failure was brought about by large

permanent deformations of the aperture of the corner fitting and/or shearing at the

perimeter of the aperture (Figs. 5.4 and 5.5). A relatively small bearing area of the cone

on the corner fitting caused a concentration of force near the edge of the aperture, and as

a result, the edge of the cone sheared through the top plate of the corner fitting.

Figure 5.6 shows the fracture of the shaft after the cone has partially dislodged from the

corner fitting. A large deformation of one side of the aperture of the corner fitting

created a high bending moment in addition to the axial load on the shaft. This caused the

shaft to fracture, and at the same time the cone to shear the corner fitting.
18

The latch locks were tested with varying degrees of extension of the latch. At 100 %

extension, the latch of TYS-L-N latch locks extends 10.7 mm (0.42 in) from the housing.

Two additional tests were made with the latch extended 76 % and 57 %.

Because there is only one latch, a latch lock applies an eccentric load on the corner

fitting. This results in prying action in addition to tension load on the corner fitting. The

degree of deformation and shearing of a part of the corner fitting depends on the amount

of contact between the latch and the inner surface of the corner fitting. Figure 5.7 shows

how the latch sheared through the edge of the aperture of a corner fitting.

The maximum test loads ranged from 408 kN (69 500 lbf) to 710 kN (159 000 lbf) for the

twistlock-corner fitting assemblies, whereas the maximum test load for a latch lockcorner fitting assembly with 100 % latch extension was 309 kN (69 500 lbf). When the

latch was extended to only 76 % and 57 % of full extension, the maximum loads dropped

substantially to 203 kN (45 700 lbf) and 90 kN (20 300 lbf), respectively. Thus, the

maximum test loads of the latch lock-corner fitting assemblies are considerably less than

those of the twistlock-corner fitting assemblies.

The ISO Standard 1496-1 [2] requires that the entire container structure be able to

support twice the maximum gross weight of the container without consideration of any

dynamic or impact effects (at rest state). If dynamic effects are considered together with

this implied factor of safety of 2, the corner fitting must have an ultimate capacity of at

least 300 kN (67 400 lbf). This requirement applies to lifting the full weight of a

container with cargo. The test loads indicate that twistlock-corner fitting assemblies

would most likely meet this requirement. On the other hand, even with the latch fully

extended, latch lock-corner fitting assemblies would less likely meet this requirement.

5.3 Compression capacity of twistlock and latch lock assemblies

Twistlocks and latch locks must remain functional after being subjected to compression

force. As described in Section 4.3, the test specimens were unloaded beginning at

222 kN (50 000 lbf) to examine whether the connector functioned properly and to check

for visible damage. Subsequently, the specimens were examined at each 56 kN

(10 000 lbf) increment. The maximum capacity was determined as one load increment

level below the load at which disfunctioning of the twistlock or latch lock was first noted.

The maximum loads so determined for six tests are listed in Table 5.3. Figures 5.8

through 5.13 show damage to the twistlocks and latch locks assemblies. Comparing the

tensile and compressive load capacities in Tables 5.2 and 5.3, it is observed that the

capacities of the assemblies in compression are considerably higher than their capacities

in tension.
19

5.4 Tensile strength of twistlock shafts

The tensile stress-strain relationships of the shaft materials of ASM-T-N, CON-T-N, and

P&H-T-N (two specimens each) are shown in Figs. 5.14 through 5.20. The stress is

computed by dividing the applied load by the cross sectional area of the shaft. The

strains were measured using strain gages. Strain values in the non-linear range beyond

0.02 may not be true strains because strain values have exceeded the capacity of the

gages. Estimated yield strengths from the stress-strain curves range from 660 MPa

(95 ksi) to 950 MPa (135 ksi). The ultimate tensile strength of the six tests ranged from

792 MPa (116 ksi) to 1065 MPa (155 ksi), and are listed in Table 5.4. These results

indicate that the materials used for the tested twistlocks can be classified as high-strength

steel. The test specimens of each group after fracture are shown in Figures 5.21, 5.22 and

5.23.

5.5 Measurement of load bearing areas

The ISO Standard 1161 [1] requires a minimum bearing area on the horizontal part of the

inner top surface of the top corner fitting of 800 mm2

 (1.24 in2). Recently, an amendment

to the ISO Standard 3874 [5] was proposed and approved (January 1999) at the

committee level relative to the bearing area requirements. This amendment requires that

the bearing area be at least 600 mm2 (0.93 in2) for connectors bearing on only one side of

the corner fitting such as latch locks. Using a special test apparatus, the footprints of

bearing of the top and bottom cones were measured. Since the ISO Standard 1161 [1]

allows a tolerance of +1.5 mm (+1/16 in) on the 63.5 mm (2 1/2 in) width of the cornerfitting aperture, the bearing areas were measured for 63.5 mm and 65 mm wide openings.

The measured bearing areas are listed in Table 5.5. It is seen in the table that except for

the latch lock, all twistlocks in the 63.5 mm (2 1/2 in) opening have a sufficient bearing

area to meet the ISO 1161 requirement. In the 65 mm opening, three twistlocks failed

barely to meet the requirement. On the other hand, even with the latch fully extended,

the bearing area of the latch lock is less than 50 % of the current requirement of the ISO

Standard 1161, as well as the amended requirement of the ISO Standard 3874. The

bearing area would be substantially smaller when the latch is not fully extended.
20

Table 5.1 Tension Capacity of Twistlock Shafts

Specimen Maximum Test Load

Kilonewton (kN) Pound-force (lbf)

Mode of Failure

ASM-T-N 562 126 400 Brittle *

ASM-T-N 563 126 500 Brittle

CON-T-N 602 135 500 Ductile shear *

CON-T-N 591 132 900 Ductile shear

P&H-T-N 651 146 400 Ductile *

P&H-T-N 574 129 000 Ductile

TYS-T-N 687 154 400 Ductile shear

TYS-T-N 713 160 200 Ductile shear

ASM-T-U 778 175 000 Ductile

ASM-T-U 802 180 300 Ductile

CON-T-U 746 167 800 Brittle

CON-T-U 712 160 000 Brittle

* Brittle, ductile, and ductile shear ruptures occurred in the shaft.
21

Table 5.2 Tension Capacity of Twistlock and Latch Lock Assemblies

Specimen Maximum Test Load

Kilonewton (kN) Pound-force (lbf)

Failure Mode

ASM-T-N 436 98 200 Pointed cone cut through

corner fitting

CON-T-N 408 91 800 Top cone pried off from

shaft in tension and bending

P&H-T-N 474 106 500 Pointed cone cut through

corner fitting

TYS-T-N 636 143 000 Corner fitting deformed, top

cone dislodged

ASM-T-U 579 130 200 Corner fitting deformed, and

pointed cone cut through

corner fitting

CON-T-U 710 159 000 Shaft ruptured after pointed

cone cut partially through

corner fitting

TYS-L-N

(100% extension)

309 69 500 Latch cut through corner

fitting

TYS-L-N

(76% extension)

203 45 700 Latch cut through corner

fitting

TYS-L-N

(57% extension)

90 20 300 Latch cut through corner

fitting

Table 5.3 Compression Capacity of Twistlocks and Latch Locks

Specimen Maximum Test Load

Kilonewton (kN) Pound-force (lbf)

Figure No.

ASM-T-N 667 150 000 5.8

CON-T-N 534 120 000 5.9

P&H-T-N 756 170 000 5.10

P&H-T-N 667 150 000 5.11

TYS-T-N 667 150 000 5.12

TYS-L-N 445 100 000 5.13
22

Table 5.4 Tensile Strength of Twistlock Shafts

Specimen Tensile Strength in MPa (ksi)

Test 1 Test 2 Average

Mode of Failure

ASM-T-N 896 (130) 881 (128) 889 (129) Brittle

P&H-T-N 792 (116) 818 (119) 805 (118) Ductile

CON-T-N 1000 (145) 1065 (155) 1033 (150) Ductile shear

Table 5.5 Bearing Areas on Corner Fitting

Bearing Area in mm2 (in2) Specimen Cone

Position

63.5 mm Slot 65 mm Slot

ASM-T-N Top

Bottom

800 (1.24)

1741 (2.70)

787 (1.22)

1703 (2.64)

CON-T-N Top

Bottom

903 (1.40)

916 (1.42)

812 (1.26)

851 (1.32)

P&H-T-N Top

Bottom

1690 (2.62)

838 (1.30)

1658 (2.57)

774 (1.20)

TYS-T-N Top

Bottom

1070 (1.66)

941 (1.46)

1032 (1.60)

870 (1.35)

ASM-T-U Top

Bottom

1716 (2.66)

800 (1.24)

1645 (2.55)

716 (1.11)

CON-T-U Top

Bottom

987 (1.53)

935 (1.45)

896 (1.39)

845 (1.31)

TYS-L-N Latch

Bottom

380 (0.59)

1109 (1.72)

329 (0.51)

980 (1.52)
23

Figure 5.1 Brittle Mode of Failure of Shaft (ASM-T-N)

Figure 5.2 Ductile Mode of Failure of Shafts (CON-T-N)
24

Figure 5.3 Ductile-Shear Mode of Failure of Shafts (TYS-T-N)

Figure 5.4 Damaged Corner Fitting in Tension Test of

Assembly with Twistlock
25

Figure 5.5 Shearing Failure of Corner Fitting

Figure 5.6 Fractured Shaft and Shearing Failure of Corner Fitting
26

Figure 5.7 Shearing Failure of Corner Fitting by

Latch Lock

Figure 5.8 Specimen ASM-T-N after Compression Test
27

Figure 5.9 Specimen CON-T-N after Compression Test

(Note: twistlock depressed into corner fitting.)
28

Figure 5.10 Specimen P&H-T-N after Compression Test

(Note: cracks in intermediate plate)

Figure 5.11 Specimen P&H-T-N after Compression Test

(Second Test)
29

Figure 5.12 Specimen TYS-T-N after Compression Test

(Note: spreading of intermediate plate at center)

Figure 5.13 Specimen TYS-L-N after Compression Test

(Note: cracking at top center of cone)
30

0

200

400

600

800

1000

0

29

58

87

116

145

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Stress (MPa)

Strain

Stress (ksi)

E = 2.07*105 MPa

 = 3.00*104

 ksi

Figure 5.14 Stress-Strain Curve for ASM-T-N

(First Specimen)

0

200

400

600

800

1000

0

29

58

87

116

145

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Stress (MPa)

Strain

Stress (ksi)

E = 2.06*105 MPa

 = 2.99*104

 ksi

Figure 5.15 Stress-Strain Curve for ASM-T-N

(Second Specimen)
31

0

200

400

600

800

1000

0

29

58

87

116

145

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Stress (MPa)

Strain

E = 2.08*105 MPa

 = 3.02*104

 ksi

Stress (ksi)

Figure 5.16 Stress-Strain Curve for CON-T-N

(First Specimen)

0

200

400

600

800

1000

0

29

58

87

116

145

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Stress (MPa)

Strain

E = 2.07*105 MPa

 = 3.00*104

 ksi

Stress (ksi)

Figure 5.17 Stress-Strain Curve for CON-T-N

(Second Specimen)
32

0

200

400

600

800

1000

0

29

58

87

116

145

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Stress (MPa)

Strain

Stress (ksi)

E = 2.03*105 MPa

 = 2.94*104

 ksi

Figure 5.18 Stress-Strain Curve for P&H-T-N

(First Specimen)

0

200

400

600

800

1000

0

29

58

87

116

145

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035

Stress (MPa)

Strain

E = 2.09*105 MPa

 = 3.03*104

 ksi

Stress (ksi)

Figure 5.19 Stress-Strain Curve for P&H-T-N

(Second Specimen)
33

Figure 5.20 Brittle Failure of Tensile Test Specimen (ASM-T-N)

Figure 5.21 Ductile-Shear Failure of Tensile

Test specimen (CON-T-N)
34

Figure 5.22 Ductile Failure of Tensile Test specimen (P&H-T-N)
35

6.0 CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

At the request of the Occupational Safety and Health Administration (OSHA), the

National Institute of Standards and Technology (NIST) carried out an experimental

program to evaluate the strength characteristics of interbox connectors for intermodal

containers. New semi-automatic twistlocks from four different manufacturers and new

latch locks from one manufacturer were included in the program. To gain information on

wearing and aging effects on the strength, used twistlocks from two different

manufacturers were also tested.

Twistlock shafts were tested in tension without corner fittings. In addition, assemblies of

connectors with corner fittings were tested in tension and compression. The stress-strain

relationships of twistlock shaft materials were determined from tensile strength tests on

machined twistlock shafts. Load bearing areas of twistlocks and latch locks on the top

and bottom corner fittings were measured. These values were compared with the ISO

minimum bearing area requirements.

Based on the results of the experimental study presented in this report, the following

conclusions are drawn:

1) The maximum load carrying capacity of twistlocks (not considering corner

fittings) in direct tension is governed by the fracture strength of the shaft. The tensile

load capacity of the shafts ranged from 562 kN (126 400 lbf) to 802 kN (180 300 lbf).

2) When the top and bottom corner fittings, which are interconnected by a twistlock

or a latch lock, are pulled apart, the corner fitting will most likely undergo large

deformation. This condition will cause the twistlock cone to disengage from the corner

fitting. Furthermore, the cone or the latch will shear the corner fitting at its opening.

3) The maximum tensile loads for the assemblies with twistlocks ranged from

408 kN (91 800 lbf) to 710 kN (159 000lbf). For the assembly with a fully extended

latch lock, the maximum test load was only 309 kN (69 500 lbf). It is important to note

that the failure load was reduced significantly when the latch did not extend fully. When

the latch was extended to only 57 % of full extension, the maximum tensile load was

reduced to 90 kN (20 000 lbf).

This study clearly showed that the tensile capacities of the latch lock-corner fitting

assemblies are substantially less than those of the twistlock-corner fitting assemblies.

This is attributed to the fact that only one latch engages with the corner fitting.

4) When compression load was applied to the top of the connector seated in a corner

fitting, no large deformation and/or disfunctioning of twistlocks or latch locks were
36

observed up to 445 kN (100 000 lbf). The compression load at which twistlocks or latch

locks began to disfunction varied from 445 kN (100 000 lbf) to 756 kN (170 000 lbf).

5) The measurements of bearing area indicates that the twistlocks would most likely

meet the requirement of the ISO Standard 1161, that is, 800 mm2

 (1.24 in2). With the

latch fully extended, however, the bearing area of the latch lock is less than 50 % of this

requirement, and is less than required by the ISO 3874 proposed amendment of 600 mm2

(0.93 in2) for one-sided bearing.

The conclusions listed above are based on limited NIST tests of twistlocks and latch

locks produced by a limited number of manufacturers. In particular, the corner fittings

used in the tests were from one manufacturer. Since corner fittings are manufactured in

accordance with the ISO Standard 1161, dimensional variations would be minimal from

one manufacturer to another. Thus, the elastic stiffness characteristics of steel corner

fittings are about the same regardless of manufacturers. However, the load carrying

capacity of the assemblies of connectors and corner fittings would vary depending upon

the material strength of corner fittings. The modes of failure of the assemblies may also

differ.

6.2 Recommendations

1) At present (year 2000), only one requirement is specified in the current ISO

standards related to interbox connectors, i.e., the load bearing area of the connector on the

corner fitting. This study showed that the tensile capacity of the connection between two

vertically joined containers is governed by the strength of the connector-corner fitting

assembly. The assembly strength is strongly related to the bearing area of the cone of the

connector on the corner fitting. Based on these observations, it is strongly recommended

that ISO standards be developed to cover design, manufacturing, testing and standards

practice on use of interbox connectors in lifting.

2) A limited number of used connectors were tested. In order to attain reliable data

on the bearing area of connectors in service, additional measurements of the bearing area

between connector and corner fitting should be made for various types of connectors.
37

7.0 REFERENCES

1. International Organization for Standardization, “ISO Standard 1161-Series 1

Freight Containers-Corner Fittings-Specification,” Fourth Edition, 1984-12-15.

2. International Organization for Standardization, “ISO Standard 1496-1-Series 1

Freight Containers-Specification and Testing,” Fifth Edition, 1990-08-15.

3. Annual Book of ASTM Standards, Vol. 01.04, “ASTM A 370-99a Standard Test

Methods and Definitions for Mechanical Testing of Steel Products,” 1999.

4. Anderson, L.D., “Container Lashing,” Swedish National Testing and Research

Institute, SP Report 1997:22, 1997.

5. International Organization for Standardization, “ISO Standard 3874-Series 1

Freight Containers-Handling and Securing,” Fourth Edition, 1988-08-01.