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PRIMARY STRESS ALLOCATION, LOADS AND DESIGN APPROACH
Loads
Primary – hull girder stresses
y
M
I
σ
=
Secondary/Tertiary
Other
Primary – Hull Girder
Hogging/Sagging
σ
DSM
σ
DHM
1
0.5
0
0.5
1
1
0
1
hogging moment
sagging moment
(<= compressive)
stress
(tension =>)
distance from neutral axis
above neutral axis
+ stress hogging
below neutral axis
+ stress sagging
Treat “corners” of the plot i.e.
σ
at deck (D), hogging(H), maximum(M);
σ
DHM
σ
at deck (D), sagging(S), maximum(M);
σ
DSM
and
at keel (K);
σ
KHM
,
σ
KSM
For first approximation treat internal and external and external structure differently:
1
notes_21_primary_stress_loads.doc
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External – Design Philosophy governs at least to start
1
0.5
0
0.5
1
1
0
1
hogging moment
sagging moment
hogging external
sagging external
sagging external
hogging external
(<= compressive)
stress
(tension =>)
distance from neutral axis
first set tension at neutral axis at half the maximum in tension and compression ;
1
max
2
DHM
TNA
KSM
σ
≡
1
max
2
DSM
CNA
KHM
≡
Then linear
Design Philosophy further allocates a fraction of allowable stress to primary
MS
8.5
TSI
19.04 KSI
HTS
9.5
TSI
21.28 KSI
HY80
10.5
TSI
23.52 KSI
HY100
11.5
TSI
25.76 KSI
2
notes_21_primary_stress_loads.doc
And applies margin of
1 TSI combatant
0.5 TSI auxiliary and patrol craft
0
0.5
1
1
0
1
stress allocated to primary
1 => total allowable stress
distance from neutraal axis
If required, bending moments could be estimated as follows:
With sufficient knowledge of the design, a bending moment can be calculated (static or stochastic etc.) Frequently to
get started on the design spiral an initial estimate of the structural weight and scantlings is desired. Estimates can be
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This note was uploaded on 02/24/2012 for the course MECHANICAL 2.082 taught by Professor Davidburke during the Spring '03 term at MIT.
 Spring '03
 DavidBurke
 Stress

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