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This page is devoted to *frequently asked questions* (FAQ) related to [hinges|kb:Hinge].
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h2. To modelShould nonlinear frame elements, shouldbe Imodeled definewith nonlinear material properties or assign frame hinges?

To model [Framenonlinear|kb:Nonlinear] frame behavior, [frame|kb:Frame] [hinges|kb:Hinge] must be specified to model [nonlinear|kb:Nonlinear] frame behavior. 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 axial stress-strain relationshiprelationships of individual fibers. {hidden-content} {verify} {hidden-content}


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

TheEnergy 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. When I plot theWhy do hinge results theydeviate dofrom notthe follow thedefined hinge backbone?
as
defined.Several Theyreasons generallyas followto thewhy overallhinge backbone,results butmay deviate noticeably.from Whatthe causesbackbone this?curve 
There could be several reasons for thisdefined include:

* If you are running nonlinear static analysis, make sure that you specified A sufficient number of multiple states toshould be savedspecified on the "Results Saved for Nonlinear Load Cases" form menu when running [nonlinear static|kb:Nonlinear] analysis.

* Strength The program automatically limitsloss (degradation), indicated by the negative slope of a hingebackbone tocurve, beis noautomatically stifferlimited thanto 10% of the elastic stiffness of the Frame element containing the hinge and this could cause some deviation from the backbone curve in the negative slope portion of the hinge curve. If you need a sharper drop-off, you can assign [hinge overwrites] (via "[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 command)such that users may specify steeper strength degradation by using a small relative length, say on the order of 0.02.

* ForThe backbone coupledcurve hinges,for thea backbonecoupled curvehinge is only valid only if the yield point on the interaction surface does not change. For example This may occur with [P-M2-M3 hinges|kb:P-M2-M3 hinge moment-rotation curve], for example, PMMwhen hinge,P theor hingeM3 responsechange, forcausing M2 mayto deviate from the backbone curve,. ifPlease Pkeep orin M3mind change,that orthe evenbackbone ifcurve theyrepresents doa nottriaxial changerelationship butbetween theeach surfaceof grows due to hardeningthese parameters.


h2. How does the SF for moment curvature defined hinges affect the analysis?

Please refer to [CSI 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], chapter(Scaling "Frame Hinge Properties"the Curve, section "Scaling the Hinge"page 135).


h2. Should I use a hinge or a link?

_[See "Link/FAQ" page.|Link FAQ]_

h2. Why does the program graphically show result for two hinges at either side of the actual hinge, when only one hinge was defined at the given location?

When hinges are defined such that they would be located at a meshed joint where other member(s) are framing into the frame in question, the program graphically shows the state of theA 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 the joint. Please note that this is merely a convention used for graphical display a [meshed|kb:Meshing] [joint|kb:Joint] into which other members frame.


h2. CouldIs youthere explainan theexplanation meaningfor of Rp, Rp2and andthe Rp3 and thecoupled P-M-M hinge equation Rp = Rp2 * cos (theta)Φ + Rp3 sin (theta) for coupled PMM hingesΦ?

_[See "Moment rotation curve for coupled For response, please see the [P-M2-M3 hinge" page.| moment-rotation curve|kb:P-M2-M3 hinge moment-rotation curve]_ article.


h2. Why is the fiber-hinge state foralways fibergiven hinges alwaysas "A <= B"?

*Extended Question:* When the stress and strain of some fibers beyond the corresponding yield point, both my fiber state and hinge state are "A <=B". I do expect my beam reach at plastic region. Why is the program indicating "A <= B" hinge state≤ B, even when plastic behavior is achieved?

*Answer:* For fiber P-M2-M3 hinges, the hingeHinge states A, B, C, D, and E are notused reallyto applicable,define because the hinge responsemoment-rotation curve isof calculateda directly from the response of individual section fibers (rather than specifying the values for each hinge state). Therefore, the [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 notalways graphicallygiven displayedas onA the≤ deflectedB shapebecause ofcomputation thedoes structurenot andinvolve thetheir hingevalues.
state
onFiber-hinge theresponse "Hingeis Results"derived formfrom is not applicable for fiber hinges.

Using "Display > Show Hinge Results" menu command, you can display moment-rotation curve for the entire section or stress strain-curves forthe [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 aswhich shown inpopulate the figurecross belowsection. For

coupled P-M2-M3 hinges,Users may display the shape of thecross-section moment-curvaturerotation curve is defined by specifying the coordinates of critical points A, B,or C,individual D, and E on the curve. This is not required for fiber hinges, because the moment curvature curve is obtained by integrating the response from individual fibersfiber stress-strain curves, through the Display > Show Hinge Results menu.


h2. What is the difference between Deformation Controlleddeformation-controlled (Ductileductile) and Forceforce-controlled Controlled (Brittlebrittle) hinge types?

Ductile hinges are based on effective strengths, and brittle hingeswhich are based minimum strengths. Effective strengths represent the expected material properties, and according to FEMA -356, recommends that these strengths be used are recommended for deformation-controlled actions. 
On
Brittle thehinges otherare hand,based theon minimum strengths, which representare the lower boundbounds of material properties, and are recommended to be used for force-controlled actions. {hidden-content} {verify} {hidden-content}

{hidden-content}
*Related IncidentsIncident:*
* Incident 25372, 6/14/2010 email: hinge response when there are dimples in hinge interaction surface.
{hidden-content}


h2. Does the program consider column unbraced length forinfluence the calculationFEMA ofP-M anhinge interaction surface?
for a FEMA P-M
The hinge? interaction Unlikesurface foris theconsidered PMMto interactionbe equationa usedproperty forof the design check ofcross section, and not the entire column member,. theTherefore unbraced length is not considered induring theinteraction-surface calculation. ofThe hinge interaction diagram,surface becauseenvelopes theall hingeyield interactionpoints diagramwhich ischaracterize considered to be a propertythe onset of theplasticity cross-sectionin ratherextreme thanfibers aunder propertycombined ofloading theconditions. entireHinge membercapacity (thatis wouldassociated bewith affected by the unbraced length). The points on the hinge interaction diagram capture the combination of moments and axial force when the extreme fiber reaches yield stress.

Please note that you can define your own interacting PM hinge for which the unbraced length could be considered by defining the appropriate interaction surface and moment-curvature curves. These curves could be developed by performing a nonlinear analysis of the member modeled by shell elements thatthe [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 IncidentsIncident:*

* {incident:no=32980|comment=Effect of unbraced length on the hinge interaction surface.}
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h2. Why I cannot see the plastic hinges colors , whileare plastic-hinge colors not displayed when viewing the deformed of a FNAFNA deformation?

PlesePlease note that [hinges|kb:Hinge] are not active during activatedFast inNonlinear aAnalysis ([FNA analysis, they are only activated in a nonlinear static, or direct integration time history analyses. The Fast Nonlinear Analysis (FNA) method, is based on mode superposition, and it is intended for use with structural systems that are primarily linear elastic, but which |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, suchfor asinstance, buildingswould withbe basesuitable isolators and/or dampers.for [dynamic-linear|kb:Nonlinear] analysis.