Transcript Introduction to Lateral Force Resisting Systems

CE 636 - Design of Multi-Story Structures
T. B. Quimby
UAA School of Engineering
The LFRS is used to resist forces
resulting from wind or seismic activity.
 Buildings are basically big cantilever
beams. They are supported on one end
only and the loads are perpendicular to
the beam.
 As in a beam, buildings are designed for
strength (shear and flexure) and
serviceability (deflection).
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Strength
 Shear
 Flexure
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Serviceability
 Deflection
 Spatial Requirements
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Braced Frame (Vertical Truss)
Moment Frame
Infilled-Frame
Shear Wall (~ solid beam)
Tube systems
Combinations of the above
Braced Frames are basically vertical truss
systems.
 Almost exclusively steel or timber.
 Highly efficient use of material since forces are
primarily axial. Creates a laterally stiff building
with relatively little additional material.
 Has little or no effect on the design of the
horizontal floor system.
 Good for buildings of any height.
 Bracing may intrude on the spatial constraints.
 May be internal or external.
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Different types of bracing
 Single Diagonal
 Double Diagonal
 Chevron Bracing
 Story height knee bracing (eccentricity braced
frames
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May be single story and/or bay or may span
over multiple stories and/or bays
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Multiple Floors
Multiple Bays
Columns and Girders joined by moment resisting
connections
 Lateral stiffness of the frame depends on the the
flexural stiffness of the beams, columns, and
connections.
 Economical for buildings up to about 25 stories.
 Well suited for reinforced concrete construction
due to the inherent continuity in the joints.
 Design of floor system cannot be repetitive since
the beams forces are a function of the shear at
the level in addition to the normal gravity loads.
 Gravity loads also resisted by frame action.
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Note the bending in the typical beam, column
and joint.
Common in many countries.
Used for buildings up to 30 stories.
Steel or concrete frame infilled with concrete or
masonry.
 Infill behaves as a strut in compression.
 Tension contribution is ignored.
 Due to random nature of masonry infill, it is
difficult to predict the stiffness and strength of
this system.
 No method of analyzing infilled frames has
gained general acceptance.
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Generally constructed with concrete, masonry,
or plywood. Sometimes steel.
 Shear walls have high in-plane stiffness and
strength.
 Well suited for tall buildings up to about 35
stories.
 Shear walls may intrude on the spatial
constraints. Best suited to residential and hotel
construction.
 Can be used around elevator and/or stair cores.
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Special case of shear walls.
Two or more shear walls in-plane, coupled
with a stiff beam or slab at each level.
Tends to behave like a moment frame system
with very stiff columns.
The coupling reduces lateral deflections.
Forces in the coupling elements can be quite
large.
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Free body of left shear wall has additional
reactions from the coupling members.
Combination of shear walls and rigid frames or
combination of braced and rigid frames.
 Shear walls and braced frames tend to deflect in
a flexural mode while the rigid frames tend to
deflect in a shear mode.
 In a wall-frame structure, both the shear walls
and rigid frames are constrained to act
together, resulting in a stiffer and stronger
structure.
 Good for structures in the 40-60 story range.
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The basic idea is to make a rectangular tube out the
the perimeter of the building.
The tube is made up of closely spaced columns
connected by stiff spandrel beams creating very stiff
moment frames.
Frames parallel to direction of force act like webs to
carry the shear.
Frames perpendicular to the direction of force act as
flanges. Flange forces are not uniform.
Best applied to rectangular or circular plans.
Suitable for both steel and concrete.
Use for buildings of 40 stories or more.
Frames are repetitive and easily constructed.
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Gravity Loads taken by
frames and interior
columns.
Aesthetically, the
system gets mixed
reviews because of the
small windows and the
repetition.
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Tube-in-Tube or Hull-Core
 Inner tube is usually around an elevator or service core and
can be made very stiff with shear walls or braced frames.
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Bundled Tubes
 Introduces additional “web frames” which reduces shear
lag which makes flanges more efficient.
 Allows for more architectural variation.
 Sears Tower, Chicago
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Braced-Tube
 Utilizes a large scaled braced frame in place of rigid frames
 Allows for wider columns spacing and smaller spandrels.
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Structural “depth” is increased (i.e. the
moment of inertia of the structure is
increased)
Shear strength is unchanged.
Utilizes a braced core with stiff outriggers to
mobilized outer columns in tension and
compression.
4 to 5 outriggers appear to be the economical
limit.
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Under Lateral Loads:
 Columns on one side are
in tension
 Columns on other side
are in compression.
Used primarily to achieve some architectural purpose.
Floor are hung from a truss on an upper level
Tension members can be smaller than columns would
be in same place.
 Accumulated lengthening of tension members may
cause extreme deflection problems at lowest hung
floor. This can be controlled by hanging 10 or less
floors from a single truss.
 Limited to “shorter” structures since structural depth
is small at base, making lateral deflections large.
 There are several variations on the theme.
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Suspension does little to help the LFRS.
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Core carries all gravity and lateral forces.
Core may be braced frame or shear wall.
Floors are cantilevered off of the core.
Creates a column free interior.
Building width is limited by capabilities of the
cantilever.
Building height limited by stiffness of core.
Structurally inefficient.
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Three dimensional triangulated frame.
Highly efficient and relatively light weight.
Bank of China building in Hong Kong is a
classic example.
Ingenuity required to get the gravity and
lateral loads from the floors into the space
frame.
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Combinations of the various types of
systems.
There are almost limitless combinations.
May be necessary to achieve architectural
goals. (“Postmodern” architecture
intentionally tries to get away from simple
prismatic building shapes.)
The development of large scale computer
based analysis has made design of odd
shapes possible.