Download II. Overview Of structure

International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Study On Torsional Response of High-Rise Steel
Frame Building Under Eccentricity Changes
Nan Yu Thwe, Kyaw Moe Aung

Abstract— In this paper, study on torsional response of
irregular 12- storyed steel building due to eccentricity
changes is presented. The structure is situated in seismic
zone 4 and composed of special moment resisting frame.
Dead loads, live loads, wind and seismic loadings data are
considered based on UBC 97 (Uniform Building Code).
Required data for design specification of structural
elements are considered according to AISC-LRFD 99.
Structural steel used in the building is A572 Grade 50
steel. Torsional behavior of asymmetric building is one of
the most frequent cause of structural damage and failure
during eccentricity changes. In this work, a study on the
influence of the torsion effects on the behavior of
structure is carried out. To study torsional response of
steel building, three different eccentricity values such as
10%, 15% and 20% are considered and analysis results
are also compared. In this paper, torsion in a proposed
building is studied by eccentricity changes. And then,
analysis results of proposed building are compared .The
comparison of storey forces are compared due to
eccentricity changes.
Index Terms— Dynamic analysis, Response spectrum analysis,
Torsional response, Eccentricity changes, Torsion
effect of steel structure.
I. INTRODUCTION
Myanmar is a developing country in South-East Asia.
Nowadays, the multi-storeyed buildings are widely used in
Myanmar and the potential of its popularity will be greater in
future. The proposed building is superstructure of 12- storey
steel building. Steel have been used for many years in
building construction. Steel structures are stiff; lesser time is
required for construction; they are easier for fabrication than
reinforced concrete structures; lesser weight will be resulted
out for using it. Myanmar is situated in a secondary seismic
belt which is in the junction of two major belts called Alps
Himalaya and Circum Pacific belts. It is likely to meet
destructive damage of earthquake to the buildings in some
areas. Therefore, in constructing of high-rise buildings, it
should be designed to resist the earthquake effects.
The proposed building is 12-storeyed steel building. From
a structural engineering standpoint, one of the major
distinguishing characteristics of high-rise building is the need
to resist large lateral forces due to wind or earthquake. In most
Manuscript received Oct 15, 2011.
Nan Yu Thwe, Department of Civil Engineering, Mandalay
Technological University,(e-mail:[email protected]). Mandalay,
Myanmar +95 9 91054737
Kyaw Moe Aung, Associate Professor, Head Department of Civil
Engineering, Mandalay Technological University, Mandalay, Myanmar,
(e-mail: @gmail.com).
of buildings, torsion in buildings during earthquake shaking
may be caused from a variety of reasons, the most common of
which are non-symmetric distributions of mass and stiffness.
Modern codes deal with torsion by placing restrictions on the
design of buildings with irregular layouts and also through the
introduction of an accidental eccentricity that must be
considered in design. Most of the current structural design
provisions require to consider the torsional behaviour using
the design eccentricities, which take into account both natural
and accidental sources of torsion. The natural eccentricity is
generally defined as the distance between the center of mass
(CM) and the center of rigidity (CR) for a considered floor,
while accidental eccentricity generally accounts for factors
such as the rotational component of ground motion about the
vertical axis, the difference between computed and actual
values of the mass and rigidity and an unfavourable
distribution of live load mass. Torsional forces also affect the
way an L-shaped building reacts during an earthquake.
Torsional forces are forces that make an object rotate. If
buildings are not designed to resist torsional forces,
considerable damage or collapse could occur.
II. OVERVIEW OF STRUCTURE
Earthquakes generate torsional movements of
structures for three reasons(1) an eccentricity can exist at
every storey between the storey’s resultant force, which
coincides with the mass centre CM of the storey, and the
centre of rigidity CR of that storey, (2) ground movement has
rotation aspects which affect very long structures, (3) even in
a symmetrical building, there is an uncertainty on the exact
location of the CM and design codes impose consideration in
the analysis of an ‘accidental’ eccentricity equal to 5% of the
building length perpendicular to the earthquake direction
being considered, in addition to the computed CM-CR
distance.
The centre of rigidity CR is the point where the
application of a force generates only a translation of the
building parallel to that force. The effects of torsion have to
be determined based on the CM-CR distance and on the
accidental eccentricity in either a + or - sense. In irregular
structures, the computation of torsional effects resulting
from the non – coincidence of CM and CR can only be done in
a 3-D model. The effects of accidental eccentricity can be
found applying at every level a torque computed as the
product of the storey force by the CM-CR distance. The
effects of those two terms of torsion are then “combined”,
which means that effects of accidental eccentricity have to be
considered with + and – signs.
1
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Torsional effects in earthquakes can occur even where the
centers of mass and resistance coincide. For example, ground
motion waves acting on a skew with respect to the building
axis can cause torsion. Cracking or yielding in an usymmetric
fashion also can cause torsion. These effects also can magnify
the torsion due to eccentricity between the centers of mass and
resistance. Torsional irregularities are defined to address this
concern.
Figure.1 Torsion
III. OBJECTIVE OF STUDY
The objectives of this study are as follows:
[1] To study the torsional response of high rise steel
structure due to eccentricity changes.
[2] To investigate the effects of torsion by increasing
eccentricity 10%, 15% and 20%.
[3] To compare analysis results of proposed building
due to eccentricity changes.
IV.
PREPARATION FOR DESIGN CALCULATION
A. Structural Framing System
This structure is designed based on UBC - 97. It is located
in Mandalay area which is UBC seismic zone 4. The structure
has not only 169 ft of long axis of periphery line but also 83 ft
of short axis. The grandstand has a total building height of
129 ft and total building area of 14027 ft2. STADD-PRO
software is used to analyze the superstructure.
B. Material Properties of Structure
Material properties of structure are as follows:
1) Material Property of Steel
i.
Analysis property data
- Unit weight of steel
Modulus of elasticity
Poisson ratio
Coefficient of thermal expansion
Minimum yield strength, Fy
Ultimate tensile strength, Fu
= 490 pcf
= 29 x 106 psi
= 0.3
=6.5 x10-6in/in
per degree F
= 50 ksi
= 65 ksi
C. Loading Consideration
Loads are forces tending to effect and produce
deformations, stresses or displacement in structure. They are
gravity loads and lateral loads. Gravity loads are caused by
the gravitational pull of the earth and act in vertical direction.
Gravity loads are further classified as dead loads and live
loads. The two primary lateral loads on structure are wind and
earthquakes. Design load combinations are also used.
1) Dead Loads: Dead loads consist of the weight of all
material and fixed equipments incorporated into the building.
-unit weight of concrete
= 150 pcf
- 9 " thick brick wall weight
=100 pcf
- 4.5 " thick brick wall weight
= 55 pcf
- superimposed dead load for
finishing
= 25 psf
- Weight of finishing & ceiling
= 25 psf
-weight of lift
= 3 tons
2) Live loads: Live loads shall be the maximum loads
expected by the intended use or occupancy. They may be
fully or partially in place or not present at all.
- Live load on typical floor
= 40 psf
- Live load on roof
= 20 psf
- Live load on lift
= 100 psf
- Live load on stair case
= 100 psf
- Live load on lobbies
= 100 psf
- Live load on platforms
= 100 psf
- Unit weight of water
= 62.4 pcf
3) Wind Load: The determination of wind design force on
a structure is basically a dynamic problem because a building
will be continually affected by gusts and other aerodynamic
force. Required Data in designing for wind load:
- Exposure type
= Type B
- Basic wind velocity
= 80 mph
- Method used
= Normal
Force
Method
- Windward coefficient
= 0.8
- Leeward coefficient
= 0.5
- Importance Factor
= 1.0
4) Earthquake Load: Nowadays, the structures are
designed to resist in an earthquake according to lateral force
design. Effects on earthquakes on structures are as follows:
(i) Seismic importance factor, I
(ii)Seismic zone factor, Z
(iii)Soil profile types, S
(iv)Seismic source type
(v)Near - source factors, Na and Nv
(vi)Seismic response coefficients, Ca and Cv
(vii) Response Modification Factor, R
- Seismic zone
= zone (4)
- Seismic source type
=A
- Soil profile type
= SD
- Structural system
= SMRF
- Seismic zone factor
= 0.4
- Seismic importance factor, I
= 1.0
- Response modification factor, R = 8.5
-Seismic response coefficient, Ca = 0.44 Na
-Seismic response coefficient, Cv =0.64 Nv
- CT value
= 0.035
5) Load Combinations
In dynamic case, the following load combinations are
considered according to AISC-LRFD-99 code.
(1) 1.4DL
(2) 1.2DL+1.6LL
2
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
(3) 1.2DL+1.6LL+1.3WX
(4) 1.2DL+1.6LL-1.3 WX
(5) 1.2DL+1.6LL+1.3WZ
(6) 1.2DL+1.6LL-1.3 WZ
(7) 1.2DL+0.8 WX
(8) 1.2DL-0.8 WX
(9) 1.2DL+0.8 WZ
(10) 1.2DL-0.8 WZ
(11) 0.9DL+1.6WX
(12) 0.9DL-1.6WX
(13) 0.9DL+1.6WZ
(14) 0.9DL-1.6WZ
(15) 1.4DL+LL+1SPECX
(16) 1.4DL+LL-1SPECX
(17) 0.7DL+SPECZ
(18) 0.7DL-SPECZ
Figure 5, 3D view of proposed building
IV. ANALYSIS RESULTS OF THE PROPOSED BUILDING
B. Design Results for Frame Members
Design results for frame members are mentioned below.
Beam and column sections of the proposed building are listed
in Table I.
TABLE I
COLUMN AND BEAM SECTIONS OF PROPOSED BUILDING
A. Site Location and Profile of structure
Type of building
Type of occupancy
Location
Shape of building
Typical story height
Bottom story height
Size of building
12-storeyed steel building
Residential
Seismic zone 4
Irregular shape (L-shape)
= 10 ft
= 11 ft
Length
= 169 ft
Width
= 83 ft
Plan, Elevation and 3D view of proposed building are
shown in Figure 1, 2, 3 and 4.
Figure 2.Plan view of proposed building
Column
Section
Storey 1
to 3
Storey 4 to
6
C1
W12x136
W12x120
C2
W14x132
C3
W12x152
C4
Storey Level
Storey 7 to
9
Storey
10 to 12
W12x106
W12x65
W12x106
W12x96
W12x65
W12x120
W12x106
W12x65
W12x170
W12x136
W12x106
W12x65
B1
W12x35
W12x35
W12x35
W12x35
B2
W12x30
W12x30
W12x30
W12x30
B3
W12x26
W12x26
W12x26
W12x26
Stair
Roof
W12x65
W12x30
C. Analysis Results for steel structure
When the proposed building is dynamically analyzed with
eccentricity changes, it is found that some frame members are
not satisfied to design criteria. In 10% eccentricity, it has been
observed that 8 numbers of beams are failed. In 15%
eccentricity, it has been observed that 10 numbers of beams
are failed. In 20% eccentricity, it has been observed that 13
numbers of beams are failed. Columns are not failed for the
proposed building in all cases of eccentricity. The fail beams
are summarized in Table II.
TABLE II
FAIL MEMBER SECTIONS OF PROPOSED BUILDING
Eccentricity 10%
Figure 3, Line Plan of proposed building
Beam
No
231
Eccentricity 15%
Section
Beam
No
W12x30
231
234
4483
W12x30
W12x30
4487
4488
4493
4496
4498
Eccentricity 20%
Section
Beam
No
Section
W12x30
231
W12x30
232
234
W12x30
W12x30
232
234
W12x30
W12x30
W12x35
W12x35
W12x35
4650
4483
4488
W12x30
W12x30
W12x35
4650
4483
4487
W12x30
W12x30
W12x35
W12x35
W12x35
4496
4497
W12x35
W12x35
4493
4496
W12x35
W12x35
4510
4522
W12x35
W12x35
4498
4499
W12x35
W12x35
4510
W12x35
4522
4523
W12x35
W12x35
Figure 4.Elevation view of proposed building
3
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
V. STABILITY CHECKING OF SUPERSTRUCTURE
According to UBC 97, checking for story drift, checking
for P- Δ effect, checking for overturning, checking for sliding,
checking for torsion are needed to be safe for the structure.
There are five portions to check stability of the structure.
A. Checking for story drift
Story drift is lateral displacement of one level of structure
relative to the level below. In UBC-97, for structure having a
fundamental period of greater than 0.7 second, the calculated
story drift shall not exceed 0.020 times the story height. The
story drift shall not exceed 0.025 times the story height for
structures having the fundamental period of 0.7 second or
lesser. These limitations are to ensure a minimum level of
stiffness so as to control inelastic deformation and possible
instability.
ΔM= 0.7 R Δs
Where,
ΔM = Maximum displacement
R = Response modification factor= 8.5
Δs = Deformation
According to analysis results by STADD-Pro software, the
value of story drifts for proposed portions are within story
drifts limitation. Thus, the superstructure is stable.
respectively. In eccentricity 20%, factor of safety in Xdirection is 6.2 and that of Z- direction is 4.92. Thus, the
structure will be safe for sliding.
E. Checking for torsion
Accidental torsion that occurs due to uncertainties in the
building’s mass and stiffness distribution must be added to the
calculated eccentricity. The torsional effect is checked
between the most two distant points in a structure. In this
structure, the safety factors for X and Z-directions are 1.07
and 1.239 for eccentricity 10%, and 1.17 and 1.247 for
eccentricity 15% and 1.17 and 1.273 for eccentricity 20%.
The torsional irregularity cannot be neglected because the
safety factors of this structure are not within the limit. The
amount of torsion by eccentricity 10%, 15% and 20% are as
follow.
Figure 6, Checking of Torsion for proposed building
B. Checking for P- Δ Effect
According to UBC-97, the resulting member forces and
moments and story drifts included by P- Δ effects shall be
considered in the evaluation of overall structural frame
stability. In seismic zone 3 and 4, P- Δ effects need not be
considered when the story drift (Δ) is less than or equal to
0.02 hx/R. So, the proposed building is located in seismic
zone 4. The maximum drift ratio of stadium is satisfied with
the limitation, we can neglect P- Δ effect.
C. Checking for Overturning Moment
Every structure shall be designed to resist the overturning
effects caused by earthquake forces. The distributed of
earthquake forces over the height of a structure causes to
experience overturning moment. The UBC-97 requires that
every designed structure be able to resist overturning effect
included by earthquake forces. Based on analysis result of
structure, the safety factor is not only 49.47 for X- direction
but also 25.38 for Z- direction for eccentricity 10%. In
eccentricity 15%, the safety factor of X- direction is 47.86
and that of Z- direction is 24.47. For eccentricity 20%, factor
of safety for both X and Z-directions are 47.78and 24.35
respectively. So, the proposed building is able to resist
overturning effect as the safety factors for X and Z directions
are greater than 1.5.
D. Checking for Sliding
Factor of safety for sliding is the ratio of the resistance due
to friction to sliding force. From UBC-97, the safety factor of
sliding must be greater than 1.5. For the proposed building,
the factors of safety for sliding in both X and Z- directions are
5.69 and 5.89in eccentricity 10%. For eccentricity 15%,
safety factor for both X and Z directions are 5.7 and 5.37
VI. COMPARISON OF STABILITY RESULTS
In this study, the design results for members are carried
out load combinations based on AISC-LRFD (99) code. The
comparison of structural performance results of response
spectrum analysis are graphically described.
A. Comparison of Storey Drift in X direction
This section discusses the relative story drift value for
response spectrum analysis and all these values are
graphically represented in Figure 7.
Figure 7.Comparison of Story Drift in X- direction
It can be found that, storey drift value in X-direction
between 10% and 15% is increased about 1.2 times and
between 15% and 20% is increased about 1.1 times due to
SPECTRUM X.
B. Comparison of Storey Drift in Z-direction
This section discusses the relative story drift value for
response spectrum analysis and all these values are
graphically represented in Figure 8.
4
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
Figure 11.Comparison of Story Moment in X- direction
Figure 8.Comparison of Story Drift in Z- direction
It can be found that, storey drift value in X-direction
between 10% and 15% is increased about 1.4 times and
between 15% and 20% is increased about 1.1 times due to
SPECTRUM Z.
C. Comparison of Storey Shear in X-direction
This section shows story shear for eccentricity changes in
each story level and shown in figure 9.
This figure shows storey moment values in X-direction are
slightly decreased from 1 to 12.Maximum value of storey
moment is 39700 kips-inches in storey 1 at 20% eccentricity
due to SPECTRUM X.
F. Comparison of Storey Moment in Z- direction
This section discusses the storey moment value for
eccentricity changes and it is represented on figure 12.
Figure 12.Comparison of Story Moment in X- direction
Figure 9.Comparison of Story Shear in X- direction
In comparison of storey shear in X- direction, between
10% and 15% is increased about 1.06 times and between 15%
and 20% is increased about 1.2 times due to SPECTRUM X.
D. Comparison of Storey Shear in Z-direction
This section shows story shear for eccentricity changes in
each story level and shown in figure 10.
This figure shows storey moment values in X-direction are
slightly decreased from 1 to 12.Maximum value of storey
moment is 79600 kips-inches in storey 1 at 20% eccentricity
due to SPECTRUM Z.
G. Comparison of Point Displacement in X- direction
This section discusses the point displacement value for
eccentricity changes and it is represented on figure 13.
Figure 10.Comparison of Story Shear in Z- direction
In comparison of storey shear in Z- direction, maximum
value of 7356 kips is occurred at storey 2 in SPEC Z.
E. Comparison of Storey Moment in X- direction
This section discusses the storey moment value for
eccentricity changes and it is represented on figure 11.
Figure 13.Comparison of Point Displacement in X- direction
This figure shows point displacement values in
X-direction are slightly increased from 1 to 12.Maximum
value of point displacement is 3.97inches in storey 12 at 20%
eccentricity due to SPECTRUM X.
5
All Rights Reserved © 2012 IJSETR
International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
H .Comparison of Point Displacement in Z- direction
This section discusses the point displacement value for
eccentricity changes and it is represented on figure 14.
Figure 14.Comparison of Point Displacement in Z- direction
This figure shows point displacement values in
Z-direction are slightly increased from 1 to 12.Maximum
value of point displacement is 7.96inches in storey 12 at 20%
eccentricity due to SPECTRUM Z.
VII. CONCLUSIONS
In this study, 12-storeyed steel building with L shaped in
plan is selected as a proposed building. The main aim is to
study torsional response of proposed building. The building is
analyzed for eccentricity changes 10%, 15% and 20%
respectively and then story responses are also compared as a
analytical result. The structure is analyzed and designed
according to STADD-Pro software, UBC-97 and
AISC-LRFD 1999 specifications. Structural steels used in the
building are A572 Grade-50.Wide flanges W-sections are
used for frame members. Torsional irregularity may exist in
structures, which are symmetric regarding to both plan
geometry and stiffness distribution of structural elements.
Firstly, the proposed model is analyzed with static analysis.
If the static condition is satisfied, the model is also analyzed
with response spectrum analysis for dynamic approach. And
then, three eccentricity changes (10%, 15% and 20%) are
considered and analyzed to compare the analysis results.
After the analysis with three eccentricity changes10%, 15%
and 20%, 8 numbers, 10 numbers and 13 numbers of beams
are not satisfied for torsion criteria. Most of beams are failed
in Z direction. Columns are not failed in this structure
although three different eccentricity changes are considered.
Checking for storey drift, P- Δ effect, overturning and sliding
are satisfied and checking for torsion are not satisfied in
stability checking. Checking for torsion are satisfied in
X-direction and not satisfied in Z-direction for eccentric 10%,
15% and 20%.And then, the amount of torsion are slightly
increased 1.239, 1.247 and 1.273 by changing 10%, 15% and
20%. In comparison of analysis results, storey drift, storey
shear, storey moment and point displacement during
eccentricity changes are compared due to Spectrum X an Z.
The storey drift and shear are maximum values at 2nd floor and
minimum values at 12th floor in X direction and Z direction.
The storey moments are slightly decreased from 1 to 12 storey
in X-direction and Z-direction due to Spectrum X and
Z-direction. From this study, all analysis results are slightly
increased due to eccentricity changes.
VIII. ACKNOWLEDGMENT
First of all, the author wishes to express her deep gratitude
to Excellency Minister Dr. Ko Ko Oo, Ministry of Science
and Technology for opening the Master of Engineering course
at Mandalay Technological University.The author's deep
thanks are due to Dr. Myint Thein, Rector of Mandalay
Technological University, for his invaluable directions,
managements and helps. The author deeply thanks to her
supervisor, Dr. Kyaw Moe Aung, Associate Professor and
Head, Department of Civil Engineering, Mandalay
Technological University, for his supervision, for his valuable
advices, true – line guidance and encouragement during the
preparation of this study.
The author is indebted to all of her teachers from Civil
Engineering department for their patient guidance, suggestion
and encouragement for completion of this study. The author
wishes to convey her special thanks to all her friends and all
persons who helped directly or indirectly towards the
successful completion during the preparation of this study.
Finally, the author is deeply grateful to her beloved parents
for their supports, advice and encouragement to attain her
destination.
REFERENCES
[1] De La Llera, J.C., and Chopra, A.K. 1994. Accidental and natural
torsion in earthquake response and design of buildings. Report 94/07,
Earthquake Engineering Research Centre,University of California,
Berkeley, Calif.
[2] Akbar RTamboli: Handbook of Structural Steel Connection Design
and Details, McGraw-Hill Co.Inc.(1997).
[3] Bungale S.Taranath: Steel, Concrete and Composite Design of Tall
Building,2nd Edition, Mc Graw Hill Co.Inc., (1997).
[4] Uniform Building Code, Volume 2. Structural Engineering Design
Provisions1997,8th International conference of Building Officials.
(1997).
[5] Charles G. Salmon, John E.Johnson: Steel structure (Design and
Behavior), 3rd Edition, Harper Collins Publishers, (1986).
6
All Rights Reserved © 2012 IJSETR