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API-RP-2A, ISO 19902 and AISC 360-10/16

Here some notable issues are discussed that were encountered when comparing the three standards.

The AISC code is one of the most flexible standards compared to the others. API and ISO consider the beam connection as rigid (fully restrained). In AISC more options are available to also partially restrain the moment connection between brace and chord and also allow rotation between the members. This is mentioned in AISC as “simple connections”.

However, the consideration how the connection is treated should be consistent throughout the whole structural calculation (see AISC 360-10 B3.6b). All three standards treat torsion and axial stresses separately, which make these standards less suitable for supported monopile structures like e.g. windmills.

For joints all standards use the same equation to combine axial load and moments:

equation to combine axial load and bending moment

But only in AISC it is stated that this equation is only valid for TY and X joints.  For K joints AISC states: "K-connections with moment loading on the branches are not covered by this specification", see Commentary K3; HSS to HSS moment connections.

API and ISO are for (offshore) tubular structures only. Use API-RP-2A when WSD method is required and ISO-19902 for LRFD method. AISC seems somewhat simpeler with less complicated formulas, but this leads also to more conservative answers.

Furthermore, the standards according to API and ISO allow the first order analysis as acceptable, while using AISC, the engineer needs to (at least) consider a second order analysis including P-Δ and P-δ effects and other effects of initial imperfections and stiffness reductions due to inelasticity (see AISC 360-10 C2).

Note that the one-third stress increase is removed from AISC 360-10/16.

ASCE/SEI standards no longer permit the familiar one-third stress increase in allowable stress design. In practical terms, what does this mean for the structural engineer?

AISC is originally a code for buildings that where exposed to a load combination of dead weight, live load and wind. The new approach is to specify the loadcases for buildings in standard ASCE 7. It is funny to read the history of the one-third increase, see also "The Origin of the One-Third Stress Increase." For offshore and maritime industry that use the AISC for certification, unity factors should be chosen in accordance with the requirements of the applicable certification authority. For clearness the one-third increase corresponds to a unity factor of 0.8. Therefore the FEMDS software still contains the opportunity to select the correct unity factor.

For more background information about the decision to remove the 1/3 stress increase from AISC also check: "The One-Third Stress Increase. Where is it now?".

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