What if the SIF is Not Accurate?
The figure below shows an X at 100% of a typical allowable
stress and a Z at the mean of the failure data. The ratio
between Z and X is the “safety” when the design is at 100%
of the allowable stress. For fatigue, the ratio between Z
and X is about 2.0.
If the SIF is off by 2.0 and the ratio between design and
mean failure is 2.0, then at 100% of the Code allowable
stress there is no safety factor.
The design stress is a product of the calculated load and
the SIF.
If the calculated load is off by more than 2.0 and the
ratio between design and mean failure is 2.0, then at
100% of the allowable stress, there is more than a 50%
probability the component will develop a crack during its
lifetime.
Some Calculated SIFs are compared to below to B31 Code
SIFs from Appendix D for large diameter 90 and 60 degree
large diameter intersections with d/D = 1 and 0.4.
Differences can be seen to be both conservative and
nonconservative by a factor of about 2.0
Unreinforced Fabricated Tee
|
Description |
Angle |
ii (B31.3) |
io (B31.3) |
ii (FEA) |
ii (FEA) |
|
30x30x0.375
|
90 |
8.079 |
10.439 |
10.63 |
13.75 |
|
30x12x0.375 |
90 |
8.079 |
10.439 |
4.56 |
13.22 |
|
30x30x0.375
|
60 |
8.079 |
10.439 |
9.23 |
10.9 |
|
30x12x0.375 |
60 |
8.079 |
10.439 |
4.64 |
11.07 |
Fabricated Tee With 0.25 inch Reinforcing Pad
|
Description |
Angle |
ii (B31.3) |
io (B31.3) |
ii (FEA) |
ii (FEA) |
|
30x30x0.375
|
90 |
5.1 |
6.46 |
6.82 |
12.32 |
|
30x12x0.375 |
90 |
5.1 |
6.46 |
3.98 |
11.09 |
|
30x30x0.375
|
60 |
5.1 |
6.46 |
6.22 |
10.87 |
|
30x12x0.375 |
60 |
5.1 |
6.46 |
4.07 |
9.54 |
Note that for laterals and hillside junctions torsional moments
may interact with inplane and outplane moments to produce
situations where the calculated and B31 SIF factors are
different by much greater than 2.0. In these situations
users are urged to use additional caution.