Transcript Introduction to Lateral Force Resisting Systems

CE 636 - Design of Multi-Story Structures
T. B. Quimby
UAA School of Engineering
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Many different types of Tube structures:
framed-tube, tube-in-tube, bundled tube,
braced-tube, and composite tube
All forms evolved from the traditional rigid
jointed structural frame.
The basic design philosophy is to consider the
entire floor plan as one section and place as
much as possible the load carrying material
around the external periphery of the building to
maximize the flexural rigidity of the cross
section.
Consists of four orthogonally rigidly jointed frame
panels place around the perimeter of the structure
forming a tube in plan.
 The columns are closely spaced and connected by
deep, stiff beams. The columns are aligned such that
the strong bending plane is in the plane of the panel.
 Placing the frames on the exterior of the building
maximizes the inertia of the tube section.
 The stiff beams mobilize the “flanges” of the tube.
The stiffer the beams, the less shear lag, and more
participation by the columns in the frames normal to
the direction of force.
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The uniformity of the members makes it possible to
use mass production techniques in the construction.
Interior layout is very flexible, making this system
good for office buildings. There are no interior shear
walls or braced frames in the way.
The closely spaced columns create problems where
large openings are required. Can resort to transfer
trusses/girders or inclined columns to solve the
problems.
The closely spaced columns and uniformity are not
always considered to be aesthetically pleasing.
Repetitive floor systems are possible.
Complex behavior: Acts like a perforated tube,
which is somewhat more flexible than a solid
tube.
 Frames parallel to the neutral axis take much of
the moment in tension/compression. (Column
stress ~ Mc/I) Shear lag resulting from
deformation in the spandrel beams tends to
keep all columns from participating equally.
 Frames perpendicular to the neutral axis take
the shear.
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The function of the floor system is to transmit
the horizontal forces to the different vertical
structural elements.
 Floor diaphragms are rigid in plane. This prevents
distortion of the “tube” section.
 Floor diaphragms are flexible out of plane. Out of
plane actions are generally negligible.
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Most tubes are doubly symmetric, a feature
that can make analysis much simpler.
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Additional web frames are added to reduce
shear lag in the flange frames.
Resulting effects:
 stresses in flange columns is more uniform
 reduced shear forces in web frames (interior
frames constrained to act with exterior frames by
rigid floor diaphragms.)
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Diagonal bracing is added to the frames.
The addition of bracing stiffens the web frames
and reduces shear lag in the flange frames.
Behavior is much closer to that of a true
cantilevered tube.
May reduce the number of columns in the
frames.
Large scale bracing tends to be more practical
than single bay/story bracing due to reduced
number of connections. All columns connected
to the bracing.
Bracing constrains all the columns to deflect
together axially, creating a redistribution (or
equalization) of gravity loads among the
columns.
 Many of the columns may actually be in tension
when subjected to gravity loads.
 Spandrels must be able to take the tension
created by the compressive forces in the
diagonals under gravity loading and the
compression created the tensile forces in the
diagonals under lateral loading.
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Generally core structures make use of an
elevator or utility core in the building by
enclosing the core with shearwalls or braced
frames.
 The core behaves as a single large structural
component (a vertical cantilever thin walled
beam) with large moments of inertia.
 Due to the thin wall nature, the section is prone
to warping resulting from torsion and/or thin
plate buckling/distortions resulting from
bending compressive stress.
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Most cores are open thin walled elements.
Under the influence of torsional forces the
internal forces are a result of both pure shear
and warping shear.
Warping may result in large normal stresses.
This is the same as seen in thin walled
structural steel sections subjected to torsion.
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Warping of the core has a significant effect
on the response of the core.
See Mechanics of Material texts and Steel
texts for detailed discussion of torsion in thin
walled elements. These principles can be
applied to core structures.
Hand methods require the solution of the differential
equations associated with the warping behavior.
 Computer finite element membrane analysis
eliminates the need for in-depth understanding of
warping behavior and the computation of warping
section properties. Auxiliary beams may be required
to make rigid connections to the membrane
elements.
 An analogous frame can be created if membrane
elements are not available.
 The problem can be simplified according to the two
column analogy or a single warping column model.
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Consist of a core which is stiffened by
attachment to axial force columns by flexurally
stiff “outriggers”. Stiff spandrel girders located
at the end of the outriggers are used to mobilize
all the columns on a building face.
The core takes all the shear.
Most of the moments is taken by the columns at
the ends of the outriggers.
Outriggers must be very stiff.
The structure may be unsymmetrical.
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Approximate Analysis
 Four simplifying assumptions (see pg. 356 of text)
 May be used in optimizing the location of the
outriggers.
 The approach uses compatibility of rotations.
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Final Analysis
 Stiffness computer methods.