Members under compression – concept of columns and struts, concept of buckling

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Members under compression – concept
of columns and struts, concept of
buckling
Aditya Mondal 1802216
Akash Singh 1802217
Kshitij Anand 1802239
Kumar Kartikey 1802240
Nilabhra Ghosh 1802248
Shuvranil Biswas 1802268
Zeeshan Hussain Khan 1802280

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TopicsCovered
» Columns
» Equilibrium, instability, and loads
» Types of Columns
» Struts
» Application of Struts
» Buckling
» Types of Buckling

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Strut is a structural member
which is subjected to an axial
compressive load. Strut
generally work by resisting
longitudinal compression they
may also serve in tension
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What is a Strut?

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Applications of Struts in Industries
• Architecture
Strut channel made from steel ,aluminum ,or fiber-reinforced plastic is used
in the building industry and is often used in the support of cable trays and
other forms of cable management ,and pipes support systems.
• Aircraft
Strut have also been widely used for purely structural reasons to attach
engines ,landing and other loads. The oil-sprung legs of retractable landing
gear are still called oleo struts.
• Automobiles
Struts are used to reinforce the chassis /or body ,or active components of
suspension. Struts provide structural support for the vehicles suspension.
Struts affect riding comfort and handling, As well as vehicle control, braking

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Strut
 A structural member subjected to axial compressive force is called strut.
 Strut may be vertical horizontal or inclined.
 The cross- sectional dimensions of strut are small.
 Normally struts carry smaller compressive loads.
 Struts are used in roof truss and bridge trusses.

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Uses of struts
» In the middle 1970's, domestic automakers began the transition from
producing large rear-wheel drive vehicles to producing lighter, more fuel-
efficient front-wheel drive vehicles. Along with this transition came many
changes to the typical suspension system. For decades, the majority of
passenger cars came equipped with short-arm/long-arm suspension systems,
which are frequently called SLA's.
» But with the advent of smaller, front-wheel drive vehicles, under-hood space
became a premium and most front-wheel drive vehicles simply don't have
enough room for a short-arm/long-arm suspension system. As a result, the
MacPherson strut suspension is now the standard suspension for all front-
wheel vehicles and most rear-wheel drive vehicles.
» When comparing the typical SLA suspension with the strut suspension we
see that the strut suspension is taller than the SLA but does not require an
upper control arm, pivot shaft or bushings. This reduction in parts helps allow
the strut suspension to provide a lightweight, space efficient suspension
system that is ideal for a variety of applications.
 .

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Strut Operations
 Struts perform two main
jobs. First, struts perform a
shock damping function
like shock absorbers.
Internally, a strut is similar
to a shock absorber. A
piston is attached to the
end of the piston rod and
works against hydraulic
fluid to control spring and
suspension movement.
Just like shock absorbers,
the valving generates
resistance to pumping
forces created by the up
and down motions of the
suspension.

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Typesof Strutsused
SpringSteelStruts
» One of the most common landing strut systems on
general aviation aircraft is the spring steel strut.
These aircraft use strong, flexible materials like
steel, aluminum or composites to help absorb the
impact of a landing.
» As the plane touches down, the springs flex upward,
dissipating and transferring the impact load to
airframe at rate that doesn't bend the plane. Spring
steel is popular because it's mechanically simple,
typically lightweight, and needs little to no
maintenance.

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RigidStruts
» Rigid struts were the original type of landing gear.The
idea was simple: weld the wheels to the airframe.The
problem was the imperfect landing; a hard touchdown
meant the strong shock load transfer went directly into
the airframe.
» Soon after, aircraft engineers started putting inflatable
tires on aircraft, and the air softened the impact load.
» While we don't see them as often these days, you can still
find rigid struts on the ramp. Almost all helicopters use
them, in the form of metal skids attached to the frame of
the helicopter.

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BungeeCords
» Bungee cords are often found on tailwheel and
backcountry airplanes. One of the most popular
examples PiperCub.
» Bungee cords are just that - a series of elastic cords
wrapped between the airframe and the flexible gear
system, allowing the gear to transfer impact load to
the aircraft at rate that doesn't hurt the plane.While
some aircraft use a donut-type rubber cushion, most
of them use lots of individual strands of elastic
material to dissipate the shock, like the one pictured
below.

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ShockStruts
» The last type of strut is the only one that is a
true shock absorber. Shock struts, often called
oleo or air/oil struts, use a combination of
nitrogen (or sometimes compressed air) and
hydraulic fluid to absorb and dissipate shock
loads on landing.
» Shock struts use two telescoping cylinders,
both of which are closed at the external ends.
The top cylinder is attached to the aircraft, and
the bottom cylinder is attached to the landing
gear.The bottom cylinder, typically called the
piston, can also freely slide in and out of the
upper cylinder.

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Concept of Column
Column or pillar in architecture and structural engineering is a structural element that transmits,
through compression the weight of the structure above to other structural elements below, in other
words a column is a compression member.
The term column applies especially to a large round support with a capital and base and made of
stone, or appearing to be so , for the purpose of wind or earthquake engineering, columns may be
designed to resist lateral forces.
Other compression members are often termed "columns" because of the similar stress conditions.
columns are frequently used to support beams or arches on which the upper parts of walls or ceilings
rest.
In architecture "column" refers to such a structural element that also has certain proportional and
decorative features. a column might also be a decorative or triumphant feature but need not be
supporting any structure.
Column-a vertical strut which is liable for failure due to buckling or bending is called column.

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Causesof failureof Columns
The failure of a column takes place due to the anyone of
the following stresses set up in the columns:
 Direct compressive stresses.
 Buckling stresses.
 Combined of direct compressive and buckling
stresses.
The mode of failure of columns depends
upon their lengths and depending on the
mode of failure columns are classified as:
a. Short columns
b. Long columns
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FailureinColumns
FAILURE DUE TO DIRECT COMPRESSIVE
STRESS OR CRUSHING LOADS
Short Column
When a column is subjected to loads and is
increased, the column will reach a stage when it will
be subjected to ultimate crushing stress. Beyond this
stage, the column will fail by crushing. The load
corresponding to the crushing stress is called
crushing load. The load carrying capacity of a short
column depends only on its cross sectional area (A)
and the crushing stress of the material(ςcu). The
crushing load Pcu for axially loaded short column is
given by Pcu = ςcu × A . The safe load on the column
is obtained by dividing the crushing load by suitable
factor of safety. i.e Psafe =Pcu/ FS
FAILURE OF A LONG COLUMN BY BUCKLING
Long Columns
Long columns, which are also called slender columns,
when subjected to compression, deflects or bends in a
lateral direction as shown in the figure. The lateral
deflection of the long column is called buckling. The long
column fails when there is excessive buckling .ie when the
load on the column exceeds critical load. Critical load
denotes the maximum load carrying capacity of the long
column. The load carrying capacity of long column
depends upon several factors like the length of the
column, M.I of its cross–section, Modulus of elasticity of
the material, nature of its support.
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Buckling
In science, buckling is an instability that leads to structural
failure.
“OR”
When a slender member is subjected to an axial compressive
load, it may fail by a condition called buckling.
When a structure is subjected to compressive axial stress,
buckling may occur. Buckling is characterized by a sudden
sideways deflection of a structural member.This may occur
even though the stresses that develop in the structure are well
below those needed to cause failure of the material of which
the structure is composed.

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BucklingMechanism
Stable Equilibrium
» If the load P is sufficiently small, when the force F is
removed, the column will go back to its original straight
condition.
Unstable Equilbrium
» When the column carries a load which is more than critical
load, 𝑃cr (Increased value of the load P) and a lateral force F
is applied and removed, the column will bend considerably
and it grows into excessively large deflection.

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UnstableEquilbrium
» When the column carries critical load 𝑃cr (Increased value of the load P) and a lateral force F is applied and
removed, the column will remain in the slightly deflected position.
» Deflection amount depends on magnitude of force Elastic restoring force is sufficient to prevent
» excessive deflection.

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References
» Honda, M. et al. (2004). Analytical approach on seismic resistance capacity of
structures with hysteretic seismic contro devices. JSCE J. Struct. Eng. 50A, 539-550.
»
» JSSC (1998). Seismic Responses and Seismic Design Methods of Frames with
Hysteretic Dampers,Tokyo.
»
» Kanaji, H. et al. (2004). Fundamental study for damage control structure of an
integrated column by multi steel pipes.
»
» https://en.wikipedia.org/wiki/Strut
»
» https://en.wikipedia.org/wiki/Buckling#Cause
»
» https://1civilengineering.blogspot.com/2016/11/columns-struts.html

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