Watertight work, around bulkheads seams and boundary bars for example, should not exceed three and a half diameter spacing and, if the bulkhead is to be oil tight, that should be reduced to two and a half diameters only. As noted above and under class rules, ships over 100 m (300 feet) length would have the shell seams at the quarter lengths fitted with an extra row of rivets to take account of the high sheer forces at those points in the structure. Note: Seam lands hem, butt lands lap.
The points of rivets will often be found pitted and that is usually, but not necessarily, due to galvanic action between the rivet and the mild steel plate.
If the vessel is not built to classification society rules, the first point to consider when riveting the shell plates together is the diameter of the rivets which is fixed by two considerations:
1. the minimum diameter of a hole that can be punched into a plate of given thickness.
2. the equality of the shear strength of the rivet and that of the plate from edge of the rivet hole to the edge of the plate.
The first of those two points is not of great seriousness and the minimum diameter of a punched hole is usually taken as numerically equal to no more than twice the thickness of the plate. With regard to the second condition, consideration must always be given to the fact that the riveted seam must be caulked watertight. That cannot be done of the rivets are too far away from the plate edge and the usual practice is to make the outer line of rivets one and a half diameters from the edge of the plate which is the distance accepted by the classification societies.
Sometimes it may be necessary for the marine surveyor in preparing a structural report on a riveted boat to measure the diameter of a rivet that is part of an existing structure such as a shell seam or a frame to shell attachment. It is useless to measure the diameter of the point because, although it is approximately round in shape, that shape is very irregular due to the method of closing the rivet and the marine surveyor has no means of assessing the size of the countersink even if there is such.
Figure 9 Detail of a Single Riveted Shell Seam Lap Joint In the analyses the following symbols will be used: A
Ac
Aes As At Fl Fs Fp P
Ps Pt Rc Rs Rt d
dh e n p
tbs tp
wL ηJ σc
σt τs
σs σc
= = = = = = = = = = = = = -
= = = = = = = = = =
= =
= =
area of the joint faying surface area of rivet resisting crushing the area resisting such shear area of the rivet in shear area of plate in tension friction load
shear force on a rivet shear force in plate tensile load on a joint
Figure 9 Detail of a Single Riveted Shell Seam Lap Joint
mm2 mm2 mm2 mm2 mm2
N/mm2 N N N
shear load carrying capacity of the joint N tensile load carrying capacity of the joint l/mm2 the plate/rivet’s resistance to crushing the plate/rivet’s resistance to shearing the plate’s resistance to tearing the diameter of the rivets diameter of the rivet hole the set back or edge set number of rivets in one line the rivet longitudinal pitch thickness of the butt strap thickness of the plate land width
the joint’s efficiency
the compressive stress in or the bearing stress between the rivets and plates the tensile stress in the plate the shear stress in the rivets
0.6σt 2τs
= 1.2σt with any real degree of accuracy.
mm mm mm mm %
N/mm2 N/mm2 N/mm2
- -
(1) (2)
The marine surveyor should note that it is extremely difficult to assess the value of σc
The Report • June 2021 • Issue 96 | 47
N/mm2 N/mm2 N/mm2 mm mm mm -
The only place where a measurement is possible is the head. In shipbuilding, by far the majority of the rivets are of the cone or pan head type. The head, mathematically, is the frustum of a cone. The top of the head is usually badly deformed due to the riveting process but the bottom of the head where it fays against the inside of the shell plate retains its original diameter. That is partly due to the fact that it suffers no hammering and partly due to the friction between the underside of the head and the inside of the shell plate.
The dimension dH in the Figure 9 should be measured with callipers or Vernier
gauge and then multiplied by 5/8. The result will be the diameter of the rivet. When that diameter is multiplied by 1½ it will give the distance from the centre of the rivet to the plate edge known as the set back and when multiplied by 3½ will give the distance known as the pitch between the rivets centre to centre. It can be shown that for the rivet and plate to fail at the same shear stress, the diameter of the rivet should be twice the value of t the thickness of the plate. That fact can be used to check the value of the diameter obtained by the outlined method. For rivets of iron or mild steel and plates of mild steel, it is usually assumed that the following is applicable.
The frame spacing should be measured over the square body frames. The rivet through the plate land is called a seam rivet and the one through the frame only a frame rivet.
dH
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