Page 181 - Marine Structural Design

Chapter 9 Buckling and Local Buckling of Tubular Members 157

Taking E and cy as 21,000 kgf /mm2 and 32 kgf /mm2, respectively, the critical D/t is 73. If
D/t is beyond the critical ratio, local buckling may occur before the plasticity of the cross-
section. it is easily known that D/t of 124 for the existing bracing member is far above the
critical value.
From this exercise, it may be concluded that local buckling takes place before the hlly plastic
condition is satisfied at the cross-section where internal forces are most severe. A welded tube
on the market is selected as test specimens, whose collapse behavior is expected to be close to
the above-mentioned bracing member. The dimensions of the test specimens are shown in
Table 9.1. Their diameter is 508 mm, and their length is taken to be 8,000 mm so that their
slenderness ratio will close to that of the existing bracing member. The scale factor is 113.5,
and this specimen is referred to as a large scale test specimen. The D/t is 78, which is small
compared to that of the existing one. However, it is still larger enough for local buckling to be
a concern.

G
,IO
46.4 -t----
f
- T 508+ 8000 750 I
”*_ _-_
b
C 7504

JL3.5
9 1

(b) Detafl at G (c) End fixture

Figure 9.1 Large Scale Test Specimen and Its End Fixture
The large-scale test specimen is illustrated in Figure 9.1. The tube’s wall thickness is 6.4 mm.
However, within 750 mm from both ends, the thickness is increased to 10 mm to avoid the
occurrence of local collapse near the ends.