Page 392 - Marine Structural Design

368 Part III Fatigue and Fracture

cyclic wave pressure. In double-hull tankers, however, the double bottom seems to be the
main problem area due to a very high differential pressure between laden and ballast
conditions. A large number of cracks have been observed in inner bottom structures of several
double-hull tankers (Cheung & Slaughter, 1998). Many cracks occur along the flat-bar weld,
between frame vertical stiffeners and inner bottom longitudinals.
Similarly, many survey reports show that cracks also occur in way of connection of
longitudinals with transverse floors inside double bottom and hopper of bulk carriers (IACS,
1994).
The flat bar appears to be the weakest link in the connections. Some survey reports list up to
hundreds of flat bar failures in a single vessel (Ma, 1998, Bea, et al., 1995). This subject was
investigated by Glasfeld et a1 (1977) which concluded that approximately 75% of the total
number of cracks found around slots are at flat bars.
Cracking around end connection typically follows a sequence. The first crack normally
appears along the footprint of flat bar on the flange of longitudinal (type B crack in Figure
19.2). Extensive corrosion, commonly observed at these cracks, indicates that crack growth
rate is slow. As the flat bar cracks grow slowly with time, stresses redistribute to the web
frame through collar plates. Once the flat bar has cracked through, it loses its load-carrying
capability completely and the additional load transfers to the remaining one or two collar
connections. If this defect is not found and rectified, a second crack will start at the radius of
cutout (type D in Figure 19.2) and a third crack eventually occurs at the fillet weld on shell
plate (type C or C1 in Figure 19.2). This crack sequence has been confirmed by many survey
reports and field observations. These show that a cutout radius crack is only found when flat
bar has completely cracked through.

19.6.2 Stress Criteria for Collar Plate Design
In Ma et a1 (2000), simple criteria have been developed for ship designers to perform a quick
check of their designs of end connections. The criteria require two checks be performed for
each design of end connection. First, the calculated mean normal stress in flat bar, up,
should be less than an allowable value (see Eq. (19.17)). Second, the calculated mean shear
stress at collar plate, zdc, should also be less than its allowable value (see Eq. (19.18)).

(1 9.1 7)

(19.18)

Here p, s and 1 represent static design pressure, panel width and panel length (see Figure
19.3), respectively. A,, A, and A, are flat bar footprint area, direct connection area, and
collar connection area (see Figure 19.4), respectively. Units are in millimeters, Newtons or
their combinations.