Page 320 - Marine Structural Design

296 Part 11 Ultimate Strength

EXAMPLE 14.3: Tubular Space Frame Under Impact Load
The tubular space frame shown in Figure 14.4 (a) is subjected to a step load as illustrated in
Figure 14.4 (b).
In a MARC FEM analysis, element type 14 is used and each structural member is discretized
by 10 elements as shown in Figure 14.4 (c). The element is a three-dimensional thin-walled
tubular beam- column. There are three Gaussian points, which are further divided into 16
Simpson integration points along the circumferential direction. Plasticity is taken into account
at these integration points by applying the Von Mises's yield condition and taking the stresses
due to axial forces, two bending moments, and torsional moments into account. Therefore, the
plastic yield condition used in the present analysis is taken to be:

(14.23)

where M, is the sectional torsional moment and My and M, are the bending moments.

The time history of the impact displacement is presented in Figure 14.4(d).

L = 0.71m
E = 2.106r10"Nm*
0 : 0.7923 I lO"Nni*
0.- 2.7 a IO' Nm"
I&'= E/49
D i 0.06073 m
t =0.00579m
P = 7800 kg
u = impact dhplocmmmi

Figure 14.4 Dynamic Elastic-plastic Behavior of Space Frame under
Impact Load
a. Calculation model
b. Applied load-time relationship
c. FE model in MARC analysis
d. Time history of displacement at impact point