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FAQ
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 only used to develop the moment rotation or other response curves for the hinges. Verify
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, kinematic, Takeda and 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 for additional information.
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.
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This page is devoted to frequently asked questions (FAQ) related to hinges. |
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Default hinge properties are derived from which source?
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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How is yield rotation calculated?
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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Why do hinge results deviate from the defined hinge backbone?
Answer: Several reasons as to why hinge results may deviate from a given backbone curve include:
- A sufficient number of multiple states should be specified on the Results Saved for Nonlinear Load Cases menu when running nonlinear static analysis.
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.
- The backbone curve for a coupled hinge is only valid if the yield 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.
How does the SF for moment curvature defined hinges affect the analysis?
Please refer to CSI Analysis Reference Manual, chapter "Frame Hinge Properties", section "Scaling the Hinge".
Should I use hinge or link?
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.
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.
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?
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- This may occur with P-M2-M3 hinges, 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. See test problem Hinge response when yield point changes for an example.
Should frame plasticity be modeled using nonlinear material properties or frame hinges?
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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What is the difference between hinge and hysteretic-link application?
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.
Can PM and PMM hinges be used to model energy dissipation?
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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How does the plastic-hinge deformation-curve scale factor affect analysis?
Answer: For response, please see the CSI Analysis Reference Manual (Scaling the Curve, page 135).
Should I use a hinge or a link?
Answer: A comparison between hinges and links, along with the advantages of each, is given in the Hinge and link comparison article.
Why are two hinges shown, one on each side, when only one hinge is defined?
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.
How are hinges assigned to steel pipe sections?
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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Is there an explanation for Rp and the coupled P-M-M hinge equation Rp = Rp2 cos Φ + Rp3 sin Φ?
Answer: For response, please see the P-M2-M3 hinge moment-rotation curve article.
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. These parameters 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.
Hinge
</html>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.
What is the difference between deformation-controlled (ductile) and force-controlled (brittle) hinge types?
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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Does unbraced length influence the FEMA P-M hinge interaction surface?
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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Why are plastic-hinge colors not displayed when viewing FNA deformation?
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.