Code Update: An Advanced Approach to Seismic Code-Check of Steel Connections
About half the buildings designed and constructed in North America have some level of seismic design requirements. Seismic design has evolved—engineers now face terms such as capacity design, dissipative item, plastic hinge and deformation capacity—so this column hopes to shed some light on recent developments.
The objective of capacity design is to confirm that a building is capable of a controlled ductile behavior to avoid collapse in a design-level earthquake. This involves designing the structure to allow yielding at key predictable locations within the structure and prevent brittle failure types occurring near these locations or elsewhere in the structure.
In other words, in a structure that contains both brittle and ductile elements, the capacity design is a method to provide the structure with an overall ductile characteristic.
Some members are considered dissipative and others non-dissipative. Dissipative elements are expected to undergo significant plastic deformations during seismic load cases. Seismic energy may be depleted at these deformations, and the seismic load is therefore significantly lower. On the other hand, dissipative elements must be able to withstand the cyclic strains without any cracks, and all non-dissipative elements must be able to transfer the load induced by formation of plastic hinges in dissipative elements.
All this can be modeled and analyzed by an innovative component-based finite-element method (CBFEM), which has been implemented into applications such as IDEA StatiCa Connection and Hilti PROFIS Engineering.
CBFEM is a synergy of the component method and finite-element analysis, and it’s based on the idea that most of the verified and useful parts of the component method should be kept. The weak point of the component method—generally when analyzing stresses of individual components—was replaced by modeling and analysis using the finite-element method. (Find more information at www.CBFEM.com.)
Structural engineers and fabricators can leverage models from various software to export and synchronize necessary data with CBFEM software. All connections then can be code-checked according to seismic clauses from several national codes: American Institute of Steel Construction (AISC), Eurocode (EC) and other standards (CSA/AUS/SP).
A dissipative item is a weak link in the joint where plastic hinge is expected. It’s modeled with an increased strength and modified material diagram. Overstrength factor Ry and strain-hardening factor Cpr are defined for dissipative item. A dissipative item is excluded from the strain check of plates.
IDEA StatiCa Connection checks the connection on applied design load, which should create a plastic hinge in the selected dissipative item, usually the beam. The plastic strain in the dissipative item should be around 5%. This can serve as a confirmation that the magnitude and position of loads were determined properly.
To better understand the principles, assumptions and limitations of the CBFEM method, it’s helpful to investigate the book Component-based finite element design of steel connections published by professor Frantisek Wald and his team.
AISC Seismic Connections
Structural engineers in seismically active areas face connections of three levels of difficulty:
1. Simple: Connections defined in AISC 358 Design Guide.
2. Moderate: Structural engineers have to take into account a mix of code requirements from AISC 341 and AISC 360.
3. Complex: These usually require either a special experiment to validate the results or a complex finite-element analysis performed by a specialist.
IDEA StatiCa, for example, can accurately code-check the simple connections, but using an Excel spreadsheet with prequalified seismic joints may still be faster (and will not miss any detailing checks). IDEA StatiCa and similar products shine with the second and third group, where engineers step outside the templates and want to have a flexible tool to validate the constructability of various seismic connections.
Eurocode Seismic Connections
Eurocode seismic provisions aren’t so heavily composed of detailing checks, so engineers using IDEA StatiCa can leverage the huge library of connections. Designing moderate, complex and simple connections becomes dramatically faster than with any other tool.
Verification studies were performed for European prequalified joints in the scope of EQUALJOINTS project, which covers moment-resisting joints. The CBFEM analysis is general, and any plate can be selected as dissipative (a place where the plastic hinge is expected), and the forces acting on the connection should be set accordingly. However, the connection should conform to one of the experimentally prequalified connections, and its prescribed detailing should be kept. The detailing requirements can be found in the EQUALJOINTS project, AISC 358-20 or AISC 341. Only then is the sufficient deformation capacity of the connection ensured.
Designing a structural steel joint intended as a part of a seismic-resisting system is one of the most difficult and complex tasks in structural engineering. Currently available advanced software based on finite-element modeling can be very helpful in supporting engineering decisions and proper design according to the requirements of local standards and codes. Despite their validated accuracy, engineers should always be able to justify their designs and calculations according to the code requirements. Software, however, can support their confidence and provide more security to the examined joint.