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Answer: Automatic hinge properties for steel members are based on Table 5-6 of FEMA-356, and for concrete members, Tables 6-7 and 6-8.
Overwrite options are available for the manual definition of hinge behavior or certain hinge properties.
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Answer: The yield rotation of a hinge is calculated as yielding curvature (My/EI) multiplied by hinge length.
For the automatic FEMA frame hinges, the yield rotation is specified in the code (ASCE 41-06, Table 5-6) as the chord rotation of the full member length due to plastic moment at one end. Specifically, the yield rotation is Mp/(6*E*I/L), where Mp = Z*fy, and L is the frame object (not element) length and this is how the yield rotation is determined in the program.
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Answer: Several reasons as to why hinge results may deviate from a given backbone curve include:
Strength loss (degradation), indicated by the negative slope of a backbone curve, is automatically limited to 10% of the frame-element elastic stiffness. Rationale is explained in the CSI Analysis Reference Manual (Strength Loss, page 135). A hinge-overwrite option is available through the Assign > Frame > Hinge Overwrites menu such that users may specify steeper strength degradation by using a small relative length on the order of 0.02.
Answer: Frame hinges must be specified to model nonlinear frame behavior. Nonlinear material parameters are then associated with hinge response, including the interaction surface and the moment-rotation curves which describe post-yield behavior. When implementing fiber hinges, material definition controls the stress-strain relationships of individual fibers.
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Answer: The energy dissipation which occurs during time-history analysis may be modeled using hysteretic links. Links are useful for capturing dynamic loading and unloading because of their multi-axial response.
Isotropic, kinematic, Takeda, and pivot hysteresis models are available for single DOF hinges. For isotropic hysteresis, hinges unload elastically, parallel to the initial stiffness tangent (A-B slope), while for other hysteresis types, unloading follows a more complex nonlinear relationship.
For links, several additional hysteretic models are available. Hysteretic behavior may be specified for multiple degrees-of-freedom using a single link.
Additional information can be found in the Hinge and link comparison article.
Answer: Yes, energy is dissipated for PM and PMM hinges. In fact, these always use isotropic dissipation, which dissipates more energy than the kinematic, Takeda, and Pivot rules available for the single DOF hinges.
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Answer: For response, please see the CSI Analysis Reference Manual (Scaling the Curve, page 135).
Answer: A comparison between hinges and links, along with the advantages of each, is given in the Hinge and link comparison article.
Answer: The graphical display of two hinges is merely a convention which shows the same hinge on either side of a meshed joint into which other members frame.
Answer: To assign hinges to a steel pipe section, define a User Hinge, then assign this hinge type to the section. As an alternative, fiber hinges may be assigned. The software will then automatically determine the fiber layout according to the section shape.
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Answer: For response, please see the P-M2-M3 hinge moment-rotation curve article.
Answer: Hinge states A, B, C, D, and E are used to define the moment-rotation curve of a coupled P-M2-M3 hinge. These parameters are not applicable to fiber P-M2-M3 hinges, therefore fiber-hinge state is always given as A ≤ B because computation does not involve their values.
Fiber-hinge response is derived from the nonlinear constitutive model defined for each material within the frame-element cross section. Plastic force-displacement and moment-rotation curves are obtained by integrating the axial behavior of the individual fibers which populate the cross section.
Users may display the cross-section moment-rotation curve, or individual fiber stress-strain curves, through the Display > Show Hinge Results menu.
Answer: For steel members, ductile hinges are based on effective strengths, which are the expected material properties, and according to FEMA-356, are recommended for deformation-controlled actions.
For steel members, brittle hinges are based on minimum strengths, which are the lower bounds of material properties, and are recommended for force-controlled actions.
For reinforcement in concrete members, minimum strengths are currently being used for both deformation-controlled and force-controlled hinges.
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Answer: The hinge interaction surface is considered to be a property of the cross section, and not the entire member. Therefore unbraced length is not considered during interaction-surface calculation. The interaction surface envelopes all yield points which characterize the onset of plasticity in extreme fibers under combined loading conditions. Hinge capacity is associated with the frame or tendon cross section local to hinge location. Flexural and buckling capacity are two parameters which are associated with unbraced length.
Users who wish to consider unbraced length during interaction-surface calculation have the option to manually define P-M hinge properties and yield criteria. Moment-rotation curves may be developed through nonlinear analysis of members modeled using shell elements which would capture localized buckling of slender members.
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Answer: Please note that hinges are not active during Fast Nonlinear Analysis (FNA), only during nonlinear static and direct-integration time-history analyses.
FNA is based upon the mode-superposition method, intended for primarily linear-elastic systems which may have a limited number of predefined nonlinear elements. An elastic building with isolation and damping devices, for instance, would be suitable for dynamic-linear analysis.