Design of steel structure (CE5G)-compression member.ppt

Steel

1. 1
V.N.Kundlikar
Design of Compression Members (Limit
State Method)
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2. Course Content
•Design philosophies
•Introduction to Steel Structures
•Design of Welded connections
•Design of Bolted connections
•Design of Tension Members
•Design of Compression Members
2

3. Course Content
•Design of Column Bases
•Design of Beams
•Design of Composite Beams
•Design of Plate Girders
3

4. 4
Lecture 01 Dsign Philosophies

5. Topics to be covered
•Design philosophies
•Limit States
•Design Considerations
•Allowable Stress Design (ASD)
•Load and Resistance Factor Design (LRFD)
•Design process
5

6. Design Philosophies
•A general statement assuming safety in
engineering design is:
•Resistance ≥ Effect of applied loads ---(1)
•In eq(1) it is essential that both sides are
evaluated for same conditions and units e.g.
compressive stress on soil should be compared
with bearing capacity of soil
6

7. Design Philosophies
•Resistance of structures is composed of its
members which comes from materials & X-
section
•Resistance, Capacity, and Strength are
somewhat synonym terms.
•Terms like Demand, Stresses, and Loads are
used to express Effect of applied loads.
7

8. Limit States
•When particular loading reaches its limit,
failure is the assumed result, i.e. the
loading condition become failure modes,
such a condition is referred to as limit state
and it can be defined as
•“A limit state is a condition beyond which a
structural system or a structural
component ceases to fulfill the function for
which it is designed.”
8

9. Limit States
•There are three broad classification of limit
states:
1.Strength limit states
2.Serviceability limit states
3.Special limit states
9

10. Limit States
10
Strength Limit States:
• Flexure
• Torsion
• Shear
• Fatigue
• Settlement
• Bearing

11. Limit States
11
Serviceability Limit States:
• Cracking
• Excessive Deflection
• Buckling
• Stability

12. Limit States
Special Limit States:
•Damage or collapse in extreme
earthquakes.
•Structural effects of fire, explosions, or
vehicular collisions.
12

13. Limit States
•Design Approach used must ensure that the
probability of a Limit State being reached in
the Design/Service Life of a structure is
within acceptable limits;
•However, complete elimination of
probability of a Limit State being achieved
in the service life of a structure is
impractical as it would result in
uneconomical designs.
13

14. Design Considerations
•Structure and Structural Members should
have adequate strength, stiffness and
toughness to ensure proper functioning
during service life
•Reserve Strength should be available to
cater for:
– Occasional overloads and underestimation of loads
–Variability of strength of materials from those specified
–Variation in strength arising from quality of
workmanship and construction practices
14

15. 15
•Structural Design must provide adequate
margin of safety irrespective of Design
Method
•Design Approach should take into account the
probability of occurrence of failure in the
design process
Design Considerations

16. 16
•An important goal in design is to prevent limit
state from being reached.
•It is not economical to design a structure so
that none of its members or components
could ever fail. Thus, it is necessary to
establish an acceptable level of risk or
probability of failure.
Design Considerations

17. Design Considerations
•Brittle behavior is to be avoided as it will imply
a sudden loss of load carrying capacity when
elastic limit is exceeded.
•Reinforced concrete can be made ductile by
limiting the steel reinforcement.
17

18. 18
•To determine the acceptable margin of safety,
opinion should be sought from experience and
qualified group of engineers.
•In steel design AISC manuals for ASD & LRFD
guidelines can be accepted as reflection of
such opinions.
Design Considerations

19. 19
•Any design procedure require the confidence
of Engineer on the analysis of load effects and
strength of the materials.
•The two distinct procedures employed by
designers are Allowable Stress Design (ASD)
& Load & Resistance Factor Design (LRFD).
Design Considerations

20. 20
•Safety in the design is obtained by specifying,
that the effect of the loads should produce
stresses that is a fraction of the yield stress f
y,
say one half.
Allowable Stress Design (ASD)

21. 21
•This is equivalent to:
FOS = Resistance, R/ Effect of load, Q
= fy/0.5fy
= 2
Allowable Stress Design (ASD)

22. •Since the specifications set limit on the
stresses, it became allowable stress design
(ASD).
•It is mostly reasonable where stresses are
uniformly distributed over X-section (such
on determinate trusses, arches, cables etc.)
22
Allowable Stress Design (ASD)

23. 
i
n
Q
R





CE-411:Lecture No. 1 23
Mathematical Description of A S D
R
n = Resistance or Strength of the component being designed
Φ = Resistance Factor or Strength Reduction Factor
= Overload or Load Factors
= Factor of Safety FS
Q
i

= Effect of applied loads
Allowable Stress Design (ASD)
Prof. Dr. Akhtar Naeem Khan

24. 






FS
F
For
FS
F
Ff
cr
b
y
bb
M
FS
M
n

cI
M
cI
cI
FS
F
y
//
/

CE-411:Lecture No. 1 24
Mathematical Description of Allowable Stress Design
In ASD we check the adequacy of a design in terms of stresses
therefore design checks are cast in terms of stresses for
example if:
M
n
=

Nominal Flexural Strength of a Beam
M = Moment resulting from applied unfactored loads
FS = Factor of Safety
Allowable Stress Design (ASD)
Prof. Dr. Akhtar Naeem Khan

25. 25

Section Modulus:
S ≥ effect of load/Allowable stress
= M/f
b ------(ii)
Section Modulus

26. 26
• Implied in the ASD method is the
assumption that the stress in the member is
zero before any loads are applied, i.e., no
residual stresses exist from forming the
members.
ASD Drawbacks

27. 27
Material A has more Residual Stresses due to:
1. Non uniform cooling
2. Cutting a plate into smaller
pieces reveals the stresses
Variation of Residual Stress with
Geometry

28. 28
•ASD does not give reasonable measure of
strength, which is more fundamental
measure of resistance than is allowable
stress.
•Another drawback in ASD is that safety is
applied only to stress level. Loads are
considered to be deterministic (without
variation).
ASD Drawbacks

29. •To overcome the deficiencies of ASD, the
LRFD method is based on:
Strength of Materials
•It consider the variability not only in
resistance but also in the effects of load.
•It provides measure of safety related to
probability of failure.
29
Load and Resistance Factor
Design (LRFD)

30. 
30

Safety in the design is obtained by specifying that the reduced
Nominal Strength of a designed structure is less than the effect of
factored loads acting on the structure

in
QnR 
R
n = Resistance or Strength of the component being designed
Q
i = Effect of Applied Loads
n = Takes into account ductility, redundancy and operational imp.
Φ = Resistance Factor or Strength Reduction Factor
= Overload or Load Factors


= Factor of Safety
Load and Resistance Factor
Design (LRFD)

31. 31
Ductility: It implies a large capacity for inelastic
deformation without rupture

Ductility will ensure
redistribution of load through
inelastic deformation.
The role of ‘n’

32. 32
Redundancy:
1.A simply supported beam is a determinate
structure so it has no redundant actions.
2.A fixed beam is indeterminate by 2 degrees
so it has two redundant actions.

The role of ‘n’

33. 33
Yielding will initiate at mid span due to maximum moment at mid span
with no Redistribution of load
Redundancy

34. 34
Yielding will initiate at supports due to maximum moment at supports
Redundancy

35. 35
Redistribution of load to mid span after yielding of section at supports
Redundancy

36. 36
Operational Importance:

A hospital and a school require more
conservative design than an ordinary
residential building.
The role of ‘n’

37. 37
→ hospital
→ park
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38. •LRFD accounts for both variability in
resistance and load.
• It achieves fairly uniform levels of safety for
different limit states.
38
LRFD Advantages

39. •It’s disadvantage is change in design
philosophy from previous method.
39
LRFD Disadvantages

40. 40
•ASD combines Dead and Live Loads and
treats them in the same way
•In LRFD different load factors are assigned
to Dead Loads and Live Loads which is
appealing
•Changes in load factors and resistance
factors are much easier to make in LRFD
compared to changing the allowable stress
in ASD
Comparison of ASD and LRFD Design
Approaches

41. 41
•LRFD is intrinsically appealing as it requires
better understanding of behavior of the
structure in its limit states
•Design approach similar to LRFD is being
followed in Design of concrete structures in
form of Ultimate Strength Design -- why not
use similar approach design of steel
structures?
Comparison of ASD and LRFD Design
Approaches

42. •ASD indirectly incorporates the Factors of
Safety by limiting the stress whereas LRFD
aims to specify Factors of Safety directly by
specifying Resistance Factors and Load
Factors
•LRFD is more rational as different Factors
of Safety can be assigned to different
loadings such as Dead Loads, Live Loads,
Earthquake Loads and Impact Loads
42
Comparison of ASD and LRFD Design
Approaches

43. 43
•LRFD considers variability not only in
resistance but also in the effects of load which
provides measure of safety related to
probability of failure
•It achieves fairly uniform levels of safety for
different limit states.
•ASD still remains as a valid Design Method
Comparison of ASD and LRFD Design
Approaches

44. )/(1
)/(07.18.0
67.167.1
78.133.1
DL
DL
LD
LD
ASD
LRFD






)/(1
93.0
67.167.1
56.1
DLLD
D
ASD
LRFD




44
In LRFD For Tension Members:
1.2D + 1.6 L = 0.90 R
n  1.33D + 1.78 L = Rn (LRFD)
In ASD Factor of Safety FS = 1.67, Therefore:
1.0D + 1.0 L = R
n / 1.67  1.67D + 1.67D L = Rn (ASD)
In LRFD For Dead Load Case:
1.4D = 0.90 R
n  1.56D = Rn (LRFD)
…. (A)
…. (B)
Comparison of ASD and LRFD Design
Approaches

45. 45
0.7
0.8
0.9
1.0
1 2 3 4 5 6
0.93
0.12
0.83
1.4D
1.2D + 1.6L
L
R
F
D
A
S
D
Live Load
Dead Load
3%
Comparison of ASD and LRFD Design
Approaches

46. •AREA Stands for American Railway Engineers
Association (AREA)
•Railway Bridges and Structures are usually
designed using provisions of the AREA Code
•AREA Code uses only the Allowable Stress
Design Method. However, the allowable
stresses and design requirements may differ
from AISC/ASD method
46
AREA Code for Design of Railway
Structures

47. •AASHTO Stands for Association of American
State and Highway Transportation Officials
(AASHTO)
•Highway Bridges are usually designed using
provisions of the AASHTO Code
•AASHTO Code uses both ASD and LRFD Design
Methods
47
AASHTO Code for Design of Highway
Bridges

48. •It is very difficult to devise a design code that is
applicable to all uses and all types of structures
such as buildings, highway bridges, railway bridges
and transmission towers
•The responsibility of infrastructure on roads,
bridges and electrical transmission towers rests
with the organization responsible for approving,
operating and maintaining these facilities
48
The role of various Codes

49. 49
•Uses and critical loads may be different in
different types of structures and no one code
can cater to all the different important
considerations
•For above reasons different codes prevail and
will continue to do so
•AISC ASD Code and LRFD Code primarily is
pertinent to Building Structures.
The role of various Codes

50. Overview of LRFD Manual
•Part 1: Dimensions and properties
•Part 2: General Design considerations
•Part 3: Design of flexural members
•Part 4: Design of compression members
•Part 5: Design of Tension members
•Part 6: Design of members subject to
combined loading
50

51. Overview of LRFD Manual
•Part 7: Design considerations for bolts
•Part 8: Design considerations for welds
•Part 9: Design of connecting elements
•Part 10: Design of simple shear connections
•Part 11: Design of flexible moment
connections
51

52. Overview of LRFD Manual
•Part 12: Design of fully restrained (FR)
moment connections
•Part 13: Design of Bracing connections and
truss connections
•Part 14: Design of Beam bearing plates,
Column base plates, anchor rods,
and column splices.
52

53. Overview of LRFD Manual
•Part 15: Design of Hanger connections,
Bracket plates, and Crane-rail
connections
•ANSI/LRFD Specifications for structural steel
Buildings.
53

54. Design Process
54
1. Functional planning
• Development of a plan that will enable the structure to
fulfill effectively the purpose for which it is to be built

55. 55
The involvement of Structural engineer in the functional planning is very imp
because an Architect can suggest a plane which is practically not possible.
Design Process

56. 56
2.Structural scheme
Design Process

57. 57
2.Structural scheme (Contd.)
Design Process

58. •Deflection Considerations
•ASD Commentary L3.1 suggests following Limits:
CE-411:Lecture No. 1
58
3. Preliminary Member Sizing of Beams
For fully stressed Beams & Girders
)(
800
KsiFD
L
y

20
D
L
)(
800
KsiFD
L
y

For Beams & Girders subject to
vibrations
For Roof Purlins
Design Process
Prof. Dr. Akhtar Naeem Khan

59. •Strength/Capacity Considerations
CE-411:Lecture No. 1
59
3. Preliminary Member Sizing of Beams
T
r
ib
u
ta
ry
A
re
a
Beam
Unbraced Length
D
e
s
i
g
n

M
o
m
e
n
t
Design Process
Prof. Dr. Akhtar Naeem Khan

60. •Strength/Capacity Considerations
CE-411:Lecture No. 1
60
3. Preliminary Member Sizing of Columns
Tributary Area
•Use of Tributary Areas and
Column Tables
Design Process
Prof. Dr. Akhtar Naeem Khan

61. 61
Tributary Area

62. Design Process
62
4.Structural Analysis - Modeling

63. Design Process
63
4.Structural Analysis - Analysis

64. Design Process
64
•Must be chosen so that they will be able to resist,
within appropriate margin of safety, the forces
which the structural analysis has disclosed.
5.Design Review/ Member Modification

65. Design Process
65
•Make a tentative cost estimates for several
preliminary structural layouts.
•Selection of constructional material based on:
•Availability of specific material
•Corresponding skilled labor
•Relative costs
•Wage scales
6.Cost Estimation

66. Design Process
66
7. Preparation of Structural Drawings & Specifications

67. 67
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