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Truss structure analysis

What structural analyses are generally required besides the elastic beam theory?

Industrial FEA software like FEMAP or ANSYS don't contain practical solutions for a check if your truss structure is according to the requirements of standards like ISO19902, API-RP-2A, Eurocode 1993-1 or AISC. The only straight forward check done by FEA is based on the elastic beam theory, but this isn't enough to cover checks for e.g. buckling, crippling, built in tolerances and joint fatigue. Such an evaluation if the truss structure complies with the required standards can be very time consuming. Analysing each joint with FEA only is impracticle.

The FEMDS software checks truss members and joints and provides an overview of all checks in 3D. It includes a strength check, like buckling and a punching shear. All provided in no time.

Differences between standards API, ISO and AISC

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 also allow rotation between the members, which is mentioned 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). API-RP-2A and ISO 19902 are specifically for fixed offshore platforms. Since they treat torsion and axial stresses separately, these standards are not suitable for monopole structures like e.g. windmills.

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).

The one-third stress increase is removed in AISC code

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

Since AISC is a originally a code for buildings, for AISC 360-10 the loadcases are specified in ASCE 7.  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.  Therefore the FEMDS software still contains the opportunity to select the correct unity factor.

For more background information also check:

  • The one-third stress increase. Where is it now?
  • The origin of the one-third stress increase.