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The "What is the difference between hinge and hysteretic-link application?" section states that "Links are useful for capturing dynamic loading and unloading because their behavior couples all six degrees-of-freedom (DOF)." I do not think that this is quite correct, because not all the nonlinear link properties have coupled DOFs. In fact, my understanding is that coupled DOFs are available only linear elastic links and some DOFs of the friction isolator links. -ok
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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 using nonlinear material properties or frame hinges?

Frame [hinges|kb:Hinge] must be specified to model [nonlinear|kb:Nonlinear] [frame|kb: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. {hidden-content} {verify} {hidden-content}


h2. What is the difference between hinge and hysteretic-link application?

The energy dissipation which occurs during [time-history|kb:Time-history analysis] analysis may be modeled using [hysteretic|kb:Material nonlinearity#Hysteretic cycle] links. Links are useful for capturing dynamic loading and unloading because their behavior couples all six degrees-of-freedom (DOF).

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|kb:Hinge and link comparison] article.

h2. Can PM and PMM hinges be used to model energy dissipation?

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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Related Incidents: {incident:no=44641|comment=Energy dissipation at hinges}
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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: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.

* 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:CSI 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}

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*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.

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*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.