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This page contains frequently asked questions related to hinges.

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h1. FAQ

h2. When modeling nonlinear frames, should I utilize nonlinear material properties or define frame hinges? What are the advantages and disadvantages of either approach?

To model nonlinear frames, frame hinges must be used. The nonlinear material behavior is used to develop interaction surface that is utilized in calculation of moment-rotation or moment-curvature curves or for other response curves for the hinges. For [fiber hinges|fiber hinge], the nonlinear material behavior controls the behavior of individual fibers .{verify}

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

Hysteretic behavior will enable you to model [energy dissipation] during time history analysis (this is because loading and unloading of the link can follow different paths).

As of V14.1.0, only [isotropic|isotropic hysteresis], [kinematic|kinematic hysteresis], [Takeda|Takeda hysteresis] and [pivot hysteresis|pivot hysteresis] models are available for single degree of freedom hinges. For isotropic hysteresis, the hinges unload elastically along the A-B slope, while the unloading curve is more complex for the other types of hysteresis.

Several additional hysteretic models are available for links and a single links enables to specify hysteretic behavior for multiple degrees of freedom.

Please see the [Comparison of hinges and links|Comparison of hinges and links] for additional information.

h2. When I plot the hinge results they do not follow the hinge backbone as defined. They generally follow the overall backbone, but deviate noticeably. What causes this?

There could be several reasons for this:

* If you are running nonlinear static analysis, make sure that you specified sufficient number of multiple states to be saved on the "Results Saved for Nonlinear Load Cases" form.

* The program automatically limits the negative slope of a hinge to be no stiffer than 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 "Assign > Frame > Hinge Overwrites" menu command) using a small relative length, say 0.02.

* For coupled hinges, the backbone curve is valid only if the point on the interaction surface does not change. For example, for PMM hinge, the hinge response for M2 may deviate from the backbone curve, if P or M3 change, or even if they do not change but the surface grows due to hardening.

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

Please refer to [CSI Analysis Reference Manual|doc:Analysis Reference Manual], chapter "Frame Hinge Properties", section "Scaling the Hinge".

h2. Should I use hinge or 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 the hinge on either side of the joint. Please note that this is merely a convention used for graphical display.

h2. Could you explain the meaning of Rp, Rp2 and Rp3 and the equation Rp = Rp2 * cos (theta) + Rp3 sin (theta) for coupled PMM hinges?

_[See "Moment rotation curve for coupled P-M2-M3 hinge" page.|Moment rotation curve for coupled P-M2-M3 hinge]_

h2. Why is the hinge state for fiber hinges always "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?

 *Answer:* For fiber P-M2-M3 hinges, the hinge states A, B, C, D and E are not really applicable, because the hinge response curve is calculated directly from the response of individual section fibers (rather than specifying the values for each hinge state). Therefore, the hinge state is not graphically displayed on the deflected shape of the structure and the hinge state on the "Hinge Results" form 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 for individual fibers as shown in the figure below. For coupled P-M2-M3 hinges, the shape of the moment-curvature curve is defined by specifying the coordinates of critical points A, B, C, 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 fibers.

h2. What is the difference between ForceDeformation Controlled (BrittleDuctile) and DeformationForce Controlled (DuctileBrittle) hinge type?
types?

Ductile hinges are based on effective strengths and brittle hinges are based minimum strengths. Effective strengths represent the expected material properties and FEMA 356 recommends that these strengths be used for deformation-controlled actions. On the other hand, the minimum strengths represent the lower bound material properties and are recommended to be used for force-controlled actions.

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