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By Dr. Graham H. Powell, Professor Emeritus, UC Berkeley

Introduction

In PERFORM, coupling beams for shear walls can be modeled using Frame elements or Wall elements. This note considers some of the modeling issues and suggests methods for modeling coupling beams.

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Deep coupling panels are most likely to be controlled by shear, rather than bending. Because of this they can be the simplest to model. Figure 1 shows models some possible models.

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Figure 1: Deep Coupling Panels

The following are some points to consider:

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Slender coupling beams may be controlled by bending or shear. These are also relatively simple to model, using Frame elements. Figure 2 shows some possible models.

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Figure 2: Slender Coupling Beam

1. The coupling beam element can be modeled with moment and/or shear hinges in the usual way, depending on whether bending or shear governs. If the beam acts compositely with the floor slab, this should be taken into account when calculating the beam stiffness and strength.
2. The beam element must be connected to the piers by "imbedded" beam elements. If this is not done, the coupling beam will be effectively pin-connected to the wall.
3. If a pier is modeled using a single element across its width, as in Figure 2(b), the imbedded beam element will extend across the width of the pier. If the pier is modeled using several elements across the width, the imbedded beam can extend over one or several elements. In Figure 2(c) the Imbedded beam extends over the full width of the pier.
4. Figure 2(d) shows one model for the imbedded beams. The beams are stiff in bending, to provide a stiff connection between the pier and the coupling beam. The imbedded beam should have only a small axial stiffness, to avoid adding stiffness to the wall elements.
5. Figure 2(e) shows an alternative model for an imbedded beams. In an actual wall, there may be substantial local deformation where the beam connects to the pier. In Figure 2(e), the rotational stiffness of the elastic connection component can be chosen to provide an appropriate amount of fixity (if it is known). It may be noted that the degree of fixity may have an effect on the elastic stiffness of the structure, but after the coupling beam yields, in bending or shear, the amount of end fixity is likely to have little effect.

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Moderately deep coupling beams are the most complex to model. Figure 3 shows possible models for the case where the floor level is at the top of the beam. 

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Figure 3. Moderately Deep Coupling Beam, Floor Level at Top

The following are some points to consider. 

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Figure 4 shows possible models for the case where the floor level is within the depth of the coupling beam. The points to consider are similar to those in the preceding section.

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Figure 4. Moderately Deep Coupling Beam, Floor Level Within Beam Depth

Conclusion

A coupling beam of any depth can be modeled using either Frame or Wall elements. Although it can require more initial effort to use Frame elements, they have the advantage that they give you better control over the properties of the analysis model. Wall elements can be simpler to use, but their behavior is less certain, and possibly inaccurate.

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