2007 Div 2 Limit Load vs elatic plastic

[tschul1]

Section 5.2.3 Limit load Analysis FE method uses elastic-perfectly plastic material model with small displacement theory. the acceptance criteria calls for Table 5.4

Section 5.2.4 Elastic plastic Analysis FE method calls for elastic-plastic material model and non-linear geometry. The acceptance criteria calls for Table 5.5

The elastic plastic anlysis of section 5.2.4 is more accurate so why is the load factors of table 5.5 higher than the load factors of table 5.4?

 

[jte]

tschul-

Well, an elastic-plastic material model would be expected to show a higher load factor for a given design vs an elastic perfectly plastic (zero tangent modulus) model. This will be particularly true for many common vessel steels which have a relatively low yield to ultimate ratio. If your analysis technique is going to result in a higher indicated capacity (since you are taking advantage of a non-zero modulus) then it makes sense that to maintain equivalent criteria the required load factor combinations would be higher. The factor against failure should be about equal. Run your model both ways; technically if either way passes, the design is satisfactory. Ideally, both methods of analysis would give you the same margin against their respective load factor criteria.

Note that the next edition of Div. 2 should see some changes to the factors in Tables 5.4 and 5.5, but not huge ones. For example, the 2.6 factors in Table 5.5 2), 3), and 4) will become 2.7.

jt