Page 187 - Marine Structural Design

Chapter 9 Buckling and Local Buckling of Tubular Members 163

9.2.3 Buckling Test Procedures
For large-scale test specimens, axial compressive loads are applied with eccentricity using
large-scale model testing machines of 3,000 tons installed at Hiroshima University. Simply
supported end conditions are simulated at both ends with pinned joints. Both ends of each test
specimen are attached to loading heads through cylindrical plugs as illustrated in Figure 9.1
(c). The eccentricity of the axial load is taken to be 1/8, 1/4, and 3/8 times of the outer
diameter. These eccentricities are obtained by changing the position of the plug relative to the
loading heads. This testing machine is a horizontal type, and the test specimens are placed
horizontally. Therefore, an initial deflection of 0.63 mm is produced due to the specimen's
own weight.
For small-scale test specimens, two types of loads are applied, axial compressive loads with
eccentricity and pure bending loads. Eccentric axial loads are applied through a plug and a
spherical support as illustrated in Figure 9.4. The pure bending is applied using four point
bending as illustrated in Figure 9.5. Rigid tubes are inserted into both ends of the specimen so
that the specimen does not deform locally at the loading points. A test specimen is connected
to rigid tubes with friction bolts.
Unloading and reloading are performed several times during the experiment especially after
the occurrence of local buckling. The strain in axial and circumferential directions, lateral
deflections, and load-line displacements, are measured during the experiment.

9.2.4 Test Results

Eccentric Axial Compression Tests Using Large Scale Specimens
Axial loads vs. lateral deflection relationships are plotted using solid lines as shown in Figure
9.7. In all cases, no significant deformation of cross-sections is observed until the ultimate
strength is attained. After reaching the ultimate strength, the load decreases as lateral
deflection increases, local buckling takes place near a mid-span point, and the load carrying
capacity suddenly decreases. The local buckling mode in terms of cross-sectional deformation
may be approximated by a cosine mode as illustrated in Figure 9.8 (a). The wavelength of this
local buckling mode is almost a half circle in the circumferential direction and is very short in
the axial direction. With a further increase of lateral deflection, local denting deformation
takes place at the foot of the initial cosine-buckling wave as illustrated in Figure 9.8 (b).
The horizontally flattened part grows and folds toward the inside of the cross-section c-c'. At
the same time, a similar phenomenon is observed at the cross-section a-a', but with two dents,
A-B and A-C. The horizontally flattened part of the cross-section c-c' grows until it becomes
nearly equal to a quarter circle, see B'-C' in Figure 9.8 (c). Then, two other dents, A'-B' and
C'-D', begin to grow as illustrated in Figure 9.8 (c). At this stage, significant deformation is
observed at the cross-section b-b'. A local cosine-buckling wave occurring in the area of
maximum compressive strain is followed by the formation of dents at both sides of the wave.
Such collapse mode is observed in all large-scale test specimens regardless of the magnitude
of eccentricity. It should be noticed that the length of a fully developed buckling wave (B'-C'
in Figure 9.8 (c)) is close to that of shell buckling under pure compression.