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Solid Mechanics
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Mechanics of Solids (VDB1063)
Column Stability
Lecturer: Dr. Montasir O. Ahmed
LEARNING OUTCOMES
•To determine the critical load on a column with various types of supports
LECTURE OUTLINES
• Critical Load
• Ideal Column with Pin Supports
• Columns Having Various End-Conditions
• Long slender members subjected to an axial compressive force are called columns,
and the lateral deflection that occurs is called buckling.
• The maximum axial load that a column can support when it is on the verge of buckling
is called the critical load.
• Additional loading will cause the column to buckle .
CRITICAL LOAD
1. Stable equilibrium: Restoring force is greater than the applied load (F > 2×P tanθ) OR (k Δ > 2P tanθ)
OR (k θ L/2 > 2P tanθ) OR (P < k L/4).
2. Unstable equilibrium: Restoring force is less than the applied load (P > k L/4).
3. Neutral equilibrium: Restoring force is equal to the applied load (P = k L/4)
• Three different states for the column members:
CRITICAL LOAD
IDEAL COLUMN WITH PIN SUPPORTS
• Ideal column characteristics:
It is perfectly straight before loading
Both ends are pin-supported
Loads are applied throughout the centroid of the cross section
Euler load
𝑃𝑐𝑟 =𝜋2𝐸𝐼
𝐿2where
Pcr = Critical or max. axial load on the column just before it begins to buckle. This load must not cause the
stress in the column to exceed the proportional limit.
E = Modulus of elasticity
I = least moment of inertia for column’s cross sectional area
L = Unsupported length of the column, whose ends are pinned.
Note: The column will buckle about the principal axis of the cross section having the least I
• For purpose of design, Euler formula can be written in a more useful form by
expressing I = Ar2, where A is the cross sectional area and r is the radius of gyration
of the cross sectional area. Thus
𝑃𝑐𝑟 =𝜋2𝐸𝐼
𝐿2𝑃𝑐𝑟 =
𝜋2𝐸(𝐴𝑟2)
𝐿2𝑃
𝐴 𝑐𝑟=
𝜋2𝐸
𝐿/𝑟 2 𝜎𝑐𝑟 =𝜋2𝐸
𝐿/𝑟 2
• where
• 𝜎𝑐𝑟 = critical stress, which is the average normal stress in the column just before the
column buckles. This stress is an elastic stress and therefore 𝜎𝑐𝑟 ≤ 𝜎𝑌• E = Modulus of elasticity
• L = Unsupported length of the column, whose ends are pinned.
• r = smallest radius of gyration of the column, determined from r = 𝐼/𝐴, where I is
the least moment of inertia of the column’s cross sectional area A.
• (L/r) = slenderness ratio.
IDEAL COLUMN WITH PIN SUPPORTS
The critical-stress curves are hyperbolic, valid only for σcr is below yield stress
IDEAL COLUMN WITH PIN SUPPORTS
EXAMPLE 1
Copyright © 2011 Pearson Education South Asia Pte Ltd
The A-36 steel W200 46 member shown in Fig. 13–8 is to be used as
a pin-connected column. Determine the largest axial load it can
support before it either begins to buckle or the steel yields.
EXAMPLE 1 (cont)
Copyright © 2011 Pearson Education South Asia Pte Ltd
• From Appendix B,
• By inspection, buckling will occur about the y–y axis.
• When fully loaded, the average compressive stress in the column is
• Since this stress exceeds the yield stress,
Solutions
2N/mm 5.3205890
10006.1887
A
Pcrcr
46462 mm 103.15,mm 105.45,mm 5890 yx IIA
kN 6.1887
4
1000/1103.15102002
4462
2
2
L
EIPcr
(Ans) MN 47.1kN 5.14725890
250 PP
COLUMNS HAVING VARIOUS END-CONDITIONS
Le = KL. where Le is the effective length, K is the effective length factor, L is the column length
• The Euler formula for column's with various end conditions is
• (KL/r) is called the column’s effective slenderness ratio 2
2
2
2
/,
rKL
E
KL
EIP crcr
EXAMPLE 2
Copyright © 2011 Pearson Education South Asia Pte Ltd
A W150 24 steel column is 8 m long and is fixed at its ends as shown
in Fig. 13–11a. Its load-carrying capacity is increased by bracing it
about the y–y (weak) axis using struts that are assumed to be pin
connected to its mid-height. Determine the load it can support so that
the column does not buckle nor the material exceed the yield stress.
Take Est = 200 GPa and σY = 410 MPa.
EXAMPLE 2 (cont)
Copyright © 2011 Pearson Education South Asia Pte Ltd
• Effective length for buckling about the x–x and y–y axis is
• From the table in Appendix B,
• Applying Eq. 13–11,
Solutions
mm 2800m 8.22/87.0
mm 4000m 485.0
y
x
KL
KL
46
46
mm 1083.1
mm 104.13
y
x
I
I
(2) kN 8.460
2800
1083.1200
(1) kN 2.16534000
104.13200
2
62
2
2
2
62
2
2
y
xcr
x
xcr
KL
EIP
KL
EIP
EXAMPLE 2 (cont)
Copyright © 2011 Pearson Education South Asia Pte Ltd
• By comparison, buckling will occur about the y–y axis.
• The average compressive stress in the column is
• Since this stress is less than the yield stress,
buckling will occur before the material yields.
• Thus,
• From Eq. 13–12 it can be seen that buckling will always
occur about the column axis having the largest
slenderness ratio, since it will give a small critical stress
Solutions
MPa 6.150N/mm 6.150
3060
108.460 23
A
Percr
(Ans) kN 461crP
Important Points in this Lecture
• The three states for the column member are:
1. Stable equilibrium: P < k L/4
2. Unstable equilibrium: P ˃ k L/4
3. Neutral equilibrium: P = k L/4
• The Euler formula for column's with various end conditions is
• σcr ≤ σY
22
2
2
/,
rKL
E
KL
EIP crcr
Next Class
FINAL EXAM