Transcript Introduction to Lateral Force Resisting Systems

CE 636 - Design of Multi-Story Structures
T. B. Quimby
UAA School of Engineering
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.
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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.
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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.
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Height is the major factor in determining the
influence of the frame on the lateral stiffness of the
dual system.
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.
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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.
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.
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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.
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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.