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DNV-OS-C201 Structural Design of Offshore Units (WSD method) |
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| Sec.9: Weld Connections |
| — | Important cross connections
in structures exposed to high stress, especially dynamic, e.g. for
special areas and fatigue utilised primary structure. | — | All welds with abutting plate panels forming boundaries
to open sea. | — | All external welds in way of opening to open sea e.g.
pipes, seachests or tee-joints. | |
| — | stresses in the weld are mainly shear |
| — | direct stresses are moderate and mainly static |
| — | dynamic stresses in the abutting plate are small. |
| — | oiltight and watertight connections |
| — | connections at supports and ends of girders, stiffeners and pillars |
| — | connections in foundations and supporting structures for machinery |
| — | connections in rudders, except where access difficulties necessitate slot welds. |
| — | chain weld |
| — | staggered weld |
| — | scallop weld (closed). |
Fig. 1 Tee or cross joints
Fig. 2 Intermittent welds
Fig. 3 Slot welds
Fig. 4 Lap joint
If the yield stress of the weld deposit is higher than that of the base metal, the size of ordinary fillet weld connections may be reduced as indicated in 103.
The yield stress of the weld deposit shall in no case be less
than given in DNV-OS-C401.
Sec.9 C
102 Welding consumables used for welding of normal steel and some
high strength steels are assumed to give weld deposits with yield
stress sfw as
indicated in Table C1. If welding consumables with deposits of lower yield
stress than specified in Table C1 are used, the applied yield strength
shall be clearly informed on drawings and in design reports.
Sec.9 C
103 The size of some weld connections may be reduced:
| — | Corresponding to the strength
of the weld metal, fw:
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| — | Corresponding to the strength ratio value fr, base metal to weld metal:
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| fy | = | characteristic yield stress of base material, abutting plate (N/mm2) |
| sfw | = | characteristic yield stress of weld deposit (N/mm2) |
Ordinary values for fw and
fr for normal strength
and high strength steels are given in Table C1.
Sec.9 C
104 When deep penetrating welding processes are applied, the required
throat thicknesses may be reduced by 15% provided sufficient
weld penetration is demonstrated.
Sec.9 C
| Table C1 Strength ratios, fw and fr. | |||||||||||||||||||
| Base metal | Weld deposit | Strength ratios | Strength group | Designation | Yield stress | sfw (N/mm2) Weld metal
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Base metal/weld metal
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Normal strength steels | NV NS | 355 | 1.36 | 0.75 | High strength steels | NV 27 | NV 32 NV 36 NV 40 375 | 375 375 390 1.42 | 1.42 1.42 1.46 0.75 | 0.88 0.96 1.00 | ||
| fr | = | strength ratio as defined in 103 |
| t0 | = | net thickness (mm) of abutting plate. For stiffeners and girders within 60% of the middle of span, t0 need normally not be taken greater than 11 mm, however, t0 shall in no case be less than 0.5 times the net thickness of the web. |
| — | 50% of total length for connections in tanks |
| — | 35% of total length for connections elsewhere. |
Double continuous welds shall be adopted at stiffener ends
when necessary due to bracketed end connections.
Sec.9 C
204 For intermittent welds, the throat thickness tW is not to exceed:
| — | chain welds and scallop welds
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| — | staggered welds
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If the calculated throat thickness exceeds that given above, the considered weld length shall be increased correspondingly.
| fw | = | strength ratio as defined in 103 |
| s | = | calculated maximum tensile stress in abutting plate (N/mm2) |
| r | = | root face (mm), see Fig.1 b |
| t0 | = | net thickness (mm) of abutting plate. |
| — | welded directly to the web plate on one or both sides of the stiffener |
| — | connected by single- or double-sided lugs |
| — | with stiffener or bracket welded on top of frame |
| — | a combination of the ways listed above. |
In locations where large shear forces are transferred from
the stiffener to the girder web plate, a double-sided connection
or stiffening should be required. A double-sided connection may
be taken into account when calculating the effective web area.
Sec.9 C
402 Various standard types of connections between girders and
stiffeners are shown in Fig.5.
Fig. 5 Connections of stiffeners
Sec.9 C
403 Connection lugs should normally have a thickness not less
than 75% of the web plate thickness.
Sec.9 C
404 The total connection area a0 (parent material) at supports
of stiffeners should not be less than:
| c | = | detail shape factor as given in Table C2 |
| sp | = | permissible stress (N/mm2) |
| = | h0 fy | |
| h0 | = | allowable usage factor, see Sec.2 |
| fy | = | minimum yield strength, see Sec.4 |
| l | = | span of stiffener (m) |
| s | = | spacing between stiffeners (m) |
| p | = | lateral pressure (kN/m2). |
Sec.9 C
| Table C2 Detail shape factor c | |||||||||
| Type of connection (see Fig.5) | I Web to web connection only | II Stiffener or bracket on top of stiffener Single-sided | Double-sided | a | b c 1.00 | 0.90 0.80 1.25 | 1.15 1.00 1.00 | 0.90 0.80 | |
| fr | = | strength ratio as defied in 103 |
| a0 | = | connection area (mm2) as given in 404. |
The throat thickness is not to be exceed the maximum for scallop
welds given in 204.
Sec.9 C
406 The weld connection between stiffener end and bracket is principally
to be designed such that the shear stresses of the connection correspond
to the permissible stress.
Sec.9 C
407 The weld area of brackets to stiffeners which are carrying
longitudinal stresses or which are taking part in the strength of
heavy girders etc., shall not be less than the sectional area of
the longitudinal.
Sec.9 C
408 Brackets shall be connected to bulkhead by a double continuous
weld, for heavily stressed connections by a partly or full penetration
weld.
Where the shear stresses in web plate exceed 75 N/mm2, double continuous boundary fillet welds should have throat thickness not less than:
| t | = | calculated shear stress (N/mm2) |
| fw | = | strength ratio as defined in 103 |
| fr | = | strength ratio as defied in 103 |
| t0 | = | net thickness (mm) of web plate |
Guidance note: ---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
In general this will be satisfied if the capacity of the weld
is not less than 80% of the capacity of the weakest of
the connected parts.
| s^ | = | normal stress perpendicular to the throat |
| t^ | = | shear stress (in plane of the throat) perpendicular to the axis of the weld |
| t || | = | shear stress (in plane of the throat) parallel to the axis of the weld, see Table C3 |
| fu | = | nominal lowest ultimate tensile strength of the weaker part joined |
| bw | = | appropriate correlation factor, see Table C3 |
| h0 | = | basic usage factor, see Sec. 2 E |
Fig. 6 Stress components at a fillet weld
Sec.9 C
| Table C3 The correlation factor bw | |||||||||||||||||||||||
| Steel grade | Lowest ultimate tensile strength fu | Correlation factor bw NV NS | 400 | 0.83 | NV 27 | 400 | 0.83 | NV 32 | 440 | 0.86 | NV 36 | 490 | 0.89 | NV 40 | 510 | 0.9 | NV 420 | 530 | 1.0 | NV 460 | 570 | 1.0 | |
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