Page 214 - Ship Construction.DJ Eyres 6Ed

Ch18-H8070.fm Page 203 Wednesday, October 18, 2006 6:58 AM
Bulkheads and Pillars 203
CONSTRUCTION OF THE SHAFT TUNNEL The thickness of the tunnel
plating is determined in the same manner as that for the watertight bulk-
heads. Where the top of the tunnel is well rounded the thickness of the top
plating may be reduced, but where the top is flat it is increased. Under
hatchways the top plating must be increased in thickness unless it is covered
by wood of a specified thickness. Vertical stiffeners supporting the tunnel
plating have similar scantlings to the watertight bulkhead stiffeners, and
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their lower end is welded to the tank top (see Figure 18.6). On completion
the shaft tunnel structure is subject to a hose test.
At intervals along the length of the shaft, stools are built which support
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the shaft bearings. A walkway is installed on one side of the shaft to permit
inspection, and as a result, in a single screw ship the shaft tunnel will be
offset from the ship’s centre line. This walkway is formed by gratings laid on
angle bearers supported by struts, etc., any piping is then led along under-
neath the walkway.

Pillars

The prime function of the pillars is to carry the load of the decks and
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weights upon the decks vertically down to the ship’s bottom structure
where these loads are supported by the upward buoyant forces. A sec-
ondary function of pillars is to tie together the structure in a vertical
direction. Within the main hull of a cargo ship two different forms of pil-
lar may be found, those in the holds invariably fulfilling the first function,
and those in the machinery spaces fulfilling the latter function. Hold pillars
primarily in compression are often without bracket connections at their
ends, whilst machinery space pillars are heavily bracketed at their ends
to permit tensile loadings. This latter type of pillar may also be found in
tank spaces where the crown of tank under pressure can put the pillar in
tension.

SPACING OF HOLD PILLARS Since pillars located in holds will interfere
with the stowage arrangements, widely spaced pillars of large fabricated
section are used rather than small, solid, closely spaced pillar systems. The
arrangement most often found in cargo ships, is a two-row pillar system,
with pillars at the hatch corners or mid-length of hatch supporting deck
girders adjacent to the hatch sides. As the deck girder size is to some extent
dependent on the supported span, where only a mid-hatch length pillar is
fitted the girder scantlings will be greater than that where two hatch corner
pillars are fitted. In fact pillars may be eliminated altogether where it is
important that a clear space should be provided, but the deck girder will
then be considerably larger, and may be supported at its ends by webs at the