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# Modeling

## Which link property is most suitable for modeling reinforced-concrete shear behavior?

Answer: The multi-linear plastic-link property, with pivot hysteresis type, would be most suitable for modeling the shear behavior of reinforced-concrete members.

## How is friction modeled between two surfaces?

Answer: We recommend using nonlinear link/support elements to model friction behavior between two surfaces. The friction isolator is best for SAP2000 models, and the Isolator 2 is best for ETABS models.

Use engineering judgement when deciding upon linear and nonlinear stiffness properties.

# Formulation

## What are the Link Directional Properties?

Answer: Three options are available when defining Link Directional Properties. Their descriptions are given as follows:

1. Leave the Direction option unchecked, as shown in Figure 1 with U1. This will keep the link from contributing stiffness to the direction specified.

2. Select the Direction and Fixed option, as shown with U2. In this case, the link will not experience any deformation in the direction specified because relatively large stiffness will be applied to that component.

3. Select the Direction and Nonlinear option, as shown with U3. Here, the link will contribute the stiffness specified when Modify/Show is selected from below the Properties heading.

Figure 1 - Link directional properties

## How is link stiffness calculated?

Answer: For the various link objects available, force-deformation relationships may be defined along each degree-of-freedom (DOF), including axial, torsional, major and minor shear, and major and minor bending. These properties may be specified to simulate the behavior desired of bearings, isolators, soil springs, and other structural components modeled using link objects.

The CSI Analysis Reference Manual (The Link/Support Element - Basic > Internal Deformations and Link/Support Properties, pages 238 and 240) provides additional details on force-deformation relationships and internal link deformation and forces. The Derivation of link equations article may also be useful.

# Reporting

## What are the units for link effective damping?

Answer: For all link properties, damping input is actual damping, given in units of Force-Second/Length, or Force/Velocity.