BRACED STEEL FRAMES IN EARTHQUAKE

BRACED STEEL FRAMES
IN EARTHQUAKE
Sajid Iqbal_

Why Steel Structures ?
• Reduced construction time & no seasonal
effect.
• Light weight and reduced foundation cost.
• Durable , Long Lasting and Recyclable.
• Easier to modify and reinforce if required.
• Fabrication off-site possible.
• On site erection is a time saving process.
3

Alcoa Building
San Francisco California
The Bow
Alberta, Canada
4

Why Bracing ?
• Steel structures are widespread.
• They exhibit ductile behaviour when
subjected to transient lateral loading,
caused by wind or earthquake action.
• Steel bracings are lateral load resisting
system.
5

Why Braced Frames ?
• Braced frame is the economical method of
resisting lateral load in multi storey
structural frame.
• Bracing creates triangular configuration in
the structures.
• Bracing increases structural response in
steel moment resisting frames.
6

Types Of Braced Frames
Concentrically braced
frames (CBF)
Eccentrically braced
frames (EBF)
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Arrangement Of Bracings
Diagonal
Cross /
X - type
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Methods Used For Analysis
• Equivalent Static Analysis
• Response Spectrum Analysis
• Time History Analysis
10

Equivalent Static Analysis
• It is the simplest method of analysis. Here, force depend
upon the fundamental period of structures defined by IS
Code 1893:2002 with some changes.
• First, the design base shear is computed for the whole
building.
• It is then distributed along the height of the building.
• The lateral forces at each floor levels thus obtained are
distributed to individual lateral load resisting elements.
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Response Spectrum Analysis
• It is a linear dynamic analysis method.
• In this approach multiple mode shapes of the
building are taken into account.
• For each mode, a response is read from the
design spectrum, based on the modal frequency
and the modal mass.
• They are then combined to provide an estimate
of the total response of the structure using
modal combination methods.
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Time History Analysis
• To perform such an analysis, a representative
earthquake time history is required for a
structure being evaluated.
• In this method, the mathematical model of the
building is subjected to accelerations from
earthquake records that represent the expected
earthquake at the base of the structure.
• The method consists of a step- by- step direct
integration over a time interval.
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(Front elevation for cross bracing model) (Side elevation for the model)
15

(3D view of the typical steel frame with diagonal bracing)
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The three earthquake used for analysis are as follows:
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Lateral Load Profile In Equivalent Static Analysis :
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 Cross bracing has the highest lateral
stiffness as compared to diagonal bracing,
and obviously to frame without bracing.
Graphical Observation :
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Story Drift Of The Model In Equivalent Static Analysis :
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 On an average, 87% decrement in story
drift is observed by installing cross or
diagonal bracing on the model as
compared to that of the model without
bracing. Now, cross bracing and diagonal
bracing undergo almost same drift. This is
because, one of the diagonal of cross
bracing remains inactive during the
analysis.
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Story Drift Of The Model In Response Spectrum Analysis :
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 On an average, 28% decrement is
observed by installing cross bracing
instead of diagonal bracing. Cross bracing
is obviously more laterally stiffer than
diagonal bracing, and hence the
decrement is observed.
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Story Drift Of The Model In Time History Analysis :
X D W
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 Time history is a linear analysis and hence the
effect of decreased roof loading doesn’t affect
the final drift profile. IS code ground loading has
the highest peak ground acceleration as
compared to the other two earthquake loadings.
Therefore, highest story drift is observed in IS
Code as compared to the other two earthquake
loading.
 Cross bracing has the most lateral stiffness and
hence in both the earthquake loading it shows
least story drift.
25

W X
Shear Force At The Corner Columns :
X
D
W
In Time History AnalysisIn Response Spectrum Analysis
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 Both the graphs, represent a lower value
of shear force for cross bracing as
compared to diagonal bracing and frame
without bracing. For both the analyses, it
can be concluded that by increasing the
bracing, or by increasing the lateral
stiffness shear force in columns tend to
decrease.
27

W
Bending Moment At The Corner Columns :
X
In Response Spectrum Analysis In Time History Analysis
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 Both the graphs, represent a lower value
of bending moment for cross bracing as
compared to diagonal bracing and frame
without bracing. So, by increasing the
lateral stiffness of the moment resisting
frame, increasing the bracing bending
moment force applied at the columns tend
to decrease.
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Axial Force At The Corner Columns When Subjected To IS Code Loading :
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 The graph represents a higher value for
axial force at column ends for cross
bracings followed by diagonal bracing and
frame without bracing. So, with increase in
bracing axial forces in the columns tend to
increase.
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CONCLUSION :
• Braced steel frames have more base
shear than unbraced frames.
• Cross bracings undergo more base shear
than diagonal bracings.
• Bracings reduce the lateral displacement
of floors.
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• Cross bracings undergo lesser lateral
displacement than diagonal bracings.
• Axial forces in columns increases from
unbraced to braced system.
• Shear forces in columns decrease from
unbraced to braced system.
• Bending moment in column decreases
from unbraced to braced system.
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REFRENCES
• Tremblay, R.; et al., Performance of steel structures during the 1994
Northridge earthquake, Canadian Journal of Civil Engineering, 22, 2,
Apr. 1995, pages 338-360.
• Khatib, I. and Mahin, S., Dynamic inelastic behaviour of chevron
braced steel frames, Fifth Canadian Conference on Earthquake
Engineering, Balkema, Rotterdam, 1987, pages 211-220.
• Meher Prasad: “Response Spectrum”, Department of Civil
Engineering, IIT Madras.
• David T. Finley, Ricky A. Cribbs: “Equivalent Static vs Response
Spectrum – A comparison of two methods”.
• IS 1893 (Part 1):2002, “Criteria for Earthquake Resistant Design of
Structures”.
• Hassan, O.F., Goel, S.C.(1991).”Modelling of bracing members and
seismic behaviour of concentrically braced steel frames”.
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Thank You
PREPARED BY :
SAJID IQBAL
Class – B.C.E-4
Roll - 001410401094
UNDER THE GUIDANCE OF :
DR. KALYAN KR. MANDAL
Assistant Professor
Department of Civil Engineering
Jadavpur University
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BRACED STEEL FRAMES IN EARTHQUAKE

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