Dual System Structures

1. CE 636 - Design of Multi-Story Structures T. B. Quimby UAA School of Engineering Dual System Structures

2. Definition & Uses • A combination of SHEAR WALLS or BRACED FRAMES used in conjunction with MOMENT FRAMES. • The walls and frames are constrained to displace laterally together by the rigid floor diaphragm connecting them. • Since the deflection characteristics are different for each system, the coupled system results in an increase in stiffness for structures in the 50 story height range. • The potential advantages of a dual system structure depends on the amount of horizontal interaction of the systems.

3. Behavior • Horizontal interaction is governed by the relative stiffness of the walls and frames. • Stiffening the frames increases the interaction • Increasing height tends to reduce the wall stiffness, increasing the interaction. • Old assumption was that the shear walls took 100% of lateral forces and frames designed for gravity loads. • Little error in shorter buildings (20 stories) • Likely to be overly conservative for taller buildings. • Height is the major factor in determining the influence of the frame on the lateral stiffness of the dual system.

4. Advantages • The estimated drift may be significantly less than if the walls alone were considered to resist the horizontal loading. • The estimated bending moments in the walls or cores will be less than if they were considered to act alone. • The columns of the frames may be designed as fully braced. • The estimated shear in the frames may be approximately uniform throughout the height, allowing floor framing to be designed on a repetitive basis.

5. Displacement Characteristics • Shear wall deflection is characteristically flexural (i.e. concave down wind) • Moment frame deflection is characteristically shear (i.e. concave up wind) • Combining the two results in reversed curvature in tall buildings. • The shear walls (flexural deflection) will dominate the lower levels. • The frames (shear deflection) will dominate the upper levels.

6. Deflected Shapes

7. Analysis • The author presents approximate methods for hand analysis, however computer modeling is much quicker and easier with a program like ETABS. • Be aware that hand methods are available and that you can find them in the text if you need them. • The approximate method given in the text does not account for changing member properties or axial deformation of columns and should only be used for preliminary design. • A computer model may be simplified using the principles learned in Chapter 5.

8. “Tuning” the System • By working with the relative stiffness of the frame and walls, it is possible to make the shear load in the frame nearly constant over the height of the structure. • When the shear is constant, the member forces are constant and the member design becomes repetitive.

9. Effect of Eliminating Shear Wall in Upper Levels • Note in a system with full height shear walls that the upper level of the walls end up with a reversed story shear, increasing the shear in the frame. • Increase shear in the frame results in large moments and story shears in the frame, increasing the natural deflection of the frame • Removing the shear wall above the inflection point stiffens the structure and reduces story forces.