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This page is devoted to *frequently asked questions* (FAQ) related to [hinges|kb:Hinge].
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h2. Should frame plasticity be modeled withusing nonlinear material properties or frame hinges?

To model [nonlinear|kb:Nonlinear] frame behavior, [frame|kb:Frame] [hinges|kb:Hinge] must be specified. Nonlinear material parameters are associated with hinge response, including the interaction (yield) 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. {hidden-content} {verify} {hidden-content}


h2. What is the difference between using hinges and hysteretic links?

Energy dissipation which occurs during [time-history|kb:Time-history analysis] analysis may be modeled using [hysteretic|kb:Material nonlinearity#Hysteretic cycle] links. Link objects may capture shearing behavior in the 2D plane orthogonal to their orientation, capturing the arbitrary pattern of loading and unloading characteristic of two-component ground-motion records.

Isotropic, kinematic, Takeda, and pivot hysteresis models are available for single degree-of-freedom 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|kb:Hinge and link comparison] article.


h2. Why do hinge results deviate from the defined hinge backbone?

Several reasons as to why hinge results may deviate from the backbone curve defined include:

* A sufficient number of multiple states should be specified on the Results Saved for Nonlinear Load Cases menu when running [nonlinear static|kb:Nonlinear] analysis.

* Strength loss (degradation), indicated by the negative slope of a backbone curve, is automatically limited to 10% of the [frame|kb:Frame]\-element elastic stiffness. Rationale is explained in the {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} [_Analysis Reference Manual_|doc: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.

* The backbone curve for a coupled hinge is only valid if the yield point on the interaction surface does not change. This may occur with [P-M2-M3 hinges|kb:P-M2-M3 hinge moment-rotation curve], for example, when P or M3 change, causing M2 to deviate from the backbone curve. Please keep in mind that the backbone curve represents a triaxial relationship between each of these parameters.


h2. How does the plastic-hinge deformation-curve scale factor affect analysis?

For response, please see the {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} [_Analysis Reference Manual_|doc:Analysis Reference Manual] (Scaling the Curve, page 135).


h2. Should I use a hinge or a link?

A comparison between [hinges|kb:Hinge] and [links|kb:Link], along with the advantages of each, is given in the [Hinge and link comparison|kb:Hinge and link comparison] article.


h2. Why are two hinges shown, one on each side, when only one hinge is defined?

The graphical display of two hinges is merely a convention which shows the same hinge on either side of a [meshed|kb:Meshing] [joint|kb:Joint] into which other members frame.


h2. Is there an explanation for Rp and the coupled P-M-M hinge equation Rp = Rp2 cos Φ + Rp3 sin Φ?

For response, please see the [P-M2-M3 hinge moment-rotation curve|kb:P-M2-M3 hinge moment-rotation curve] article.


h2. Why is fiber-hinge state always given as A ≤ B, even when plastic behavior is achieved?

*Answer:* Hinge states A, B, C, D, and E are used to define the moment-rotation curve of a [coupled P-M2-M3 hinge|kb:P-M2-M3 hinge moment-rotation curve]. 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|kb:Material nonlinearity] constitutive model defined for each material within the [frame|kb: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.


h2. What is the difference between deformation-controlled (ductile) and force-controlled (brittle) hinge types?

Ductile hinges are based on effective strengths, which are the expected material properties, and according to FEMA-356, are recommended for deformation-controlled actions.

Brittle hinges are based on minimum strengths, which are the lower bounds of material properties, and are recommended for force-controlled actions. {hidden-content} {verify} {hidden-content}

{hidden-content}
*Related Incident:*
* Incident 25372, 6/14/2010 email: hinge response when there are dimples in hinge interaction surface.
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h2. Does unbraced length influence the FEMA P-M hinge interaction surface?

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|kb:Frame] or [tendon|kb:Tendon] cross section local to hinge location. Flexural demand 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|kb:Material nonlinearity] analysis of members modeled using [shell|kb:Shell] elements which would capture localized buckling of slender members.

{hidden-content}
*Related Incident:*
* {incident:no=32980|comment=Effect of unbraced length on the hinge interaction surface.}
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h2. Why are plastic-hinge colors not displayed when viewing FNA deformation?

Please note that [hinges|kb:Hinge] are not active during Fast Nonlinear Analysis ([FNA|kb:Comparison of FNA and direct-integration time-history analysis]), only during [nonlinear static|kb:Nonlinear] and [direct-integration|kb:Comparison of FNA and direct-integration time-history analysis] [time-history|kb:Time-history analysis] analyses.

FNA is based upon the [mode-superposition|kb:Modal analysis] 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|kb:Nonlinear] analysis.