for
Earthquake Resistance
Philosophy of Structures
Date April 5, 2002.
Sanaullah Khan Kakar
Contents
► Shear Walls
► Moment Resisting Frames
► Braced Structures
► Framed Tube Structures
► Hybrid Structures
► Conclusion
Shear Walls
First used in 1940, may be described as
vertical, cantilevered beams, which resist
lateral wind and seismic loads acting on a
building transmitted to them by the floor
diaphragms.
Effects in Earthquake
Building frame system
with Shear Walls
Shear Walls
Typical arrangement of shear walls
Shear Walls
A simple building with shear walls at its
ends. Ground motion enters the building
and creates inertial forces which move
the floor diaphragms. This movement is
resisted by the shear walls, and the
forces are transmitted back down to the
foundation.
Shear wall: vertical analogy
as cantilever beams
Moment-Resisting Frames
Moment-resisting frames are structures having
the traditional beam-column framing. They carry
the gravity loads that are imposed on the floor
system. The floors also function as horizontal
diaphragm elements that transfer lateral forces
to the girders and columns. In addition, the
girders resist high moments and shears at the
ends of their lengths, which are, in turn,
transferred to the column system. As a result,
columns and beams can become quite large.
Effects of Earthquake
Moment Resisting
Frame System
Moment-Resisting Frames
Moment-Resisting Frames
■ Moment-resisting frames can be constructed of
steel, concrete, or masonry.
■ Consist of beams and columns in which bending of
these members provides the resistance to lateral
forces.
■ There are two primary types of moment frames,
ordinary and special.
■ Special moment-resisting frames are detailed to
ensure ductile behavior of the beam-to-column joints
and are normally used in zones of higher seismicity.
■ Steel moment-resisting frames have been under
intensive study and testing.
Braced Structures
Braced frames develop their resistance to
lateral forces by the bracing action of
diagonal members. The braces induce
forces in the associated beams and
columns so that all work together like a
truss with all members subjected to
stresses that are primarily axial.
Braced Structure
Braced Structures
■ Braced frames act in the same manner as
shear walls, though they may be of lower
resistance depending on their detailed design.
■ Bracing generally takes the form of steel
rolled sections, circular bar sections, or tubes;
vibrating forces may cause it to elongate or
compress, in this case it loses its effectiveness
and permits large deformations or collapse of
the vertical structure.
Braced Structure
Braced Structures
Comparison of Moment-Resisting Frames and Braced Structures
Braced Structures
Bracing can be used to stop buildings swaying
over. It helps buildings stand up to
the
sideways forces that can occur during
earthquakes or high winds. Bracing members
can work in tension or in compression.
Effects of Earthquake
Framed Tube Structures
A framed tube structure is a new addition to the
structural systems. Khan is generally credited
with its invention in the 1960s.
"a three dimensional space structure
composed of three, four, or possibly more
frames, braced frames, or shear walls, joined
at or near their edges to form a vertical tubelike structural system capable of resisting
lateral forces in any direction by cantilevering
from the foundation."
Dr.Fazlur Rehman Khan
Framed Tube Structures
The framed-tube structure has its columns
closely spaced around the perimeter of the
building, rather than scattered throughout the
footprint, while stiff spandrel beams connect
these columns at every floor level. This
structural system was first implemented in
1964 in the construction of the DeWittChestnut Apartments in Chicago, a 43-story
reinforced concrete tower designed by
Dr.Fazlur Rehman Khan and his colleagues at
Skidmore, Owings & Merrill (SOM). Because of
its great relative strength and stiffness, the
tubular form immediately became a standard in
high-rise design.
The revolutionary framed-tube
structural system was first seen
in the construction of the
DeWitt-Chestnut Apartments in
Chicago.
Framed Tube Structures
Looking up at the Sears Tower, the worlds tallest
building for over twenty years. The design for this
1454-foot tall tower introduced the bundled tube
structural system, as well as a new vocabulary in
architectural form.
Framed Tube Structures
In 1998 the city of Chicago named the intersection
of Jackson and Franklin Streets (located at the
foot of the Sears Tower) “Fazlur R. Khan Way.”
Hybrid Structures
Structures are often built in which the lateral
resistance is provided by a mixture of
structures. The most common are moment
resisting frames combined either with structural
walls or diagonal bracing.
Both externally and internally
the new Toyota building is
visually exciting
Hybrid Structures
■ £25 million Toyota headquarters on the North
Downs near Epsom racecourse in Surrey.
■ The
hybrid
structural
frame
which
successfully combines precast and insitu
concrete with structural steelwork.
■ The resulting hybrid structure was erected at
a speed to rival that of steel construction and
has the bonus of a high quality exposed finish
that absorbs temperature fluctuations. To
paraphrase the Toyota advertisement: “the There is a refined juxtaposition
structure in front is a hybrid”.
of the structural elements
Conclusion
Structural systems that are suitable for
different ranges of number of stories