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8/13/2019 A Simplified Approach for Seismic Calculation of a Tall Building Braced by Shear Walls and Thin-walled Open Section Structures
1/10
Engineering Structures 29 (2007) 25762585
www.elsevier.com/locate/engstruct
A simplified approach for seismic calculation of a tall building braced byshear walls and thin-walled open section structures
Sid Ahmed Meftah, Abdelouahed Tounsi, Adda Bedia El Abbas
Laboratoire des Materiaux et Hydrologie, Universite de Sidi Bel Abbes, BP 89 Cite Ben Mhidi 22000 Sidi Bel Abbes, Algerie
Received 18 January 2006; received in revised form 2 November 2006; accepted 19 December 2006
Available online 22 February 2007
Abstract
In this paper an approximate hand-method for seismic analysis of an asymmetric building structure having constant properties along its height
is presented. The building is stiffened by a combination of shear walls and thin-walled open section structures. Based on the continuum technique
and dAlemberts principle, the governing equations of free vibration and the corresponding eigenvalue problem are derived. By applying the
Galerkin technique, a generalized method is proposed for the free vibration analysis of coupled vibration of a building braced by shear walls and
thin-walled open section structures. Simplified formulae are given to calculate the circular frequencies and internal forces of a building structure
subjected to earthquakes. The utility and accuracy of the method is demonstrated by an numerical example, in which the proposed method is
compared with finite element calculations.c2007 Elsevier Ltd. All rights reserved.
Keywords:Tall building; Eigenfrequency; Continuous approach; Thin-walled structures; Coupling vibration; Internal forces
1. Introduction
During an earthquake, damage to buildings is largely caused
by dynamic loads. Therefore, in order to design buildings
resistant to earthquakes, the dynamic characteristics of the
building must be known. The important characteristics, such
as circular frequencies and mode shapes, can be calculated
by numerical means such as the Finite Element Method
(FEM). While such methods are necessary for the final design,
approximate analyses are most helpful in preliminary designs.
A continuous approach to analysis for tall building structures
is available and has been used in the preliminary stages of the
design of high-rise structures subjected to lateral loading [14].
Over the years the method has been extended to an eigenvalue
problem, including free vibration and buckling analysis [59].
A generally asymmetric tall buildings may consist of any
combination of structural forms, such as frames, shear walls,
structural cores, coupled shear walls. These types of structure
have been widely analysed by many authors [511]. However,
Corresponding author.E-mail addresses:[email protected](S.A. Meftah),
[email protected](A. Tounsi), [email protected](A.B. El Abbas).
very few publications are available on the coupled vibrationcharacteristics of tall buildings braced by shear walls and
thin-walled open section structures [12]. Recently, Kuang and
Ng [13] presented an analytical method for the triply coupled
vibration of asymmetric shear wall structures. In their theory,
however, the flexural displacement and the offsets of the
flexural centre are determined with respect to the axes which
are perpendicular and parallel to the floor axis. The equations
are formulated in such a way that the principal axes of the walls
coincide in directions with those of the floor plan.
In this paper, a dynamic analysis of tall buildings braced by
shear walls and thin-walled open section structures as shown in
Fig. 1is presented. In such a structural configuration, the lateraldisplacements in two perpendicular (not necessary principal)
directions and the torsional rotation can no longer be treated
separately due to their coupling in the governing differential
equations of free vibration. Hence, if the flexural vibrations
in one direction are coupled with the torsional vibrations, the
resulting phenomenon is called double coupling; whereas if
the flexural vibrations in two mutually perpendicular directions
are all coupled with the torsional vibrations, it is referred to
as a triple coupling. Emphasis in the analysis is placed on
the lateraltorsional coupled vibration characteristics of the
0141-0296/$ - see front matter c2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.engstruct.2006.12.014
http://www.elsevier.com/locate/engstructmailto:[email protected]:[email protected]:[email protected]://dx.doi.org/10.1016/j.engstruct.2006.12.014http://dx.doi.org/10.1016/j.engstruct.2006.12.014mailto:[email protected]:[email protected]:[email protected]://www.elsevier.com/locate/engstruct8/13/2019 A Simplified Approach for Seismic Calculation of a Tall Building Braced by Shear Walls and Thin-walled Open Section Structures
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S.A. Meftah et al. / Engineering Structures 29 (2007) 25762585 2577
Fig. 1. Floor plan of an asymmetric building braced by shear walls and thin-walled open cross section columns.
structures. Based on the continuum technique and dAlemberts
principle, the governing equations of free vibration are derived,
which consist of a set of differential equations of two lateral
flexure vibrations coupled by Vlasov torsion vibration [14]
(i.e. St Venant + warping torsion vibrations). By employing
the Galerkin approach, a generalized method for solution of the
eigenvalue equations of the problem is proposed for analysingthe lateraltorsional coupled vibration of buildings braced
by shear walls and thin-walled open section structures. The
analysis is extended to calculate the base internal forces arising
from earthquakes. The proposed method is simple and accurate
enough to be used at the concept design stage in particular. It
can be useful to verify the results of the FEM where the time-
consuming produce of handling all the data can always be a
source of error.
2. Statement of the problem
We consider a building structure which contains a
combination of lateral load-resisting systems formed by shear
walls and thin-walled open section structures as shown in Fig. 1.
The arrangement of the stiffening system is either symmetrical
or arbitrary.
The arrangement of the lateral load-resisting systems is
identical at each floor. The stiffnesses at every storey are also
identical, the masses of the individual floors and their horizontal
distribution are the same.The walls and thin-walled open section structures will
deform predominantly in flexural mode.
The P -Delta effect induced by the second-order overturning
moment is neglected in this study. However, this effect must
be taken into account in the final analysis stage of tall building
structures.
The proposed method is applicable only to buildings whose
vertical bracing elements develop no or only negligible axial
deformation.
Our aim is to develop a simple approximate expression for
the calculation of the eigenfrequencies and seismic loads of the
considered building structures.
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3. Method of analysis
3.1. Basic assumptions, approach
We assume that the material behaves in a linearly elastic
manner.
The floor slabs of the building are very rigid in-plane but
very flexible out-of plane, such that the whole structure is
assumed to deflect as a rigid section displacement.
ConsiderFig. 1which shows typical asymmetric shear walls
and thin-walled open section structures of total height H. In the
analysis, the structure is considered as an equivalent flexural
cantilever, which is located at the centre of flexural rigidity
O. Under the action of lateral loading, the flexural cantilever
beam may undergo deformations of both lateral flexure and
torsion. The vertical z -axis is chosen over the structural height
and through the centre of flexural rigidity, O. The co-ordinate
(xC,yC)represents the position of the geometric centre of the
floor plan,C, in thex O yco-ordinate system.
It is also assumed that the structure has a uniformlydistributed mass m, flexural stiffnesses E Ix and E Iy in the x
andy directions, respectively, coupling stiffnessE Ix y , warping
torsion stiffness E I and St Venant torsion constant G J along
the structural height. Stiffness E Ix y is the flexural coupling
stiffness between the flexural motions in the x andy directions.
3.2. Governing differential equations
Based on dAlemberts principle, the governing equations of
free vibration of the structure can be derived by substituting
inertial forces into the equations of static equilibrium given by
E Ix 4
u(z, t)z4
+E Ix y 4
v(z, t)z4
+ m2
t2[u(z, t) yC(z, t)]= 0 (1)
E Iy4v(z, t)
z4 +E Ix y
4u(z, t)
z4
+ m2
t2[v(z, t)+xC(z, t)]= 0 (2)
and
E I 4(z, t)
z4 G J
2(z, t)
z2
m2
t2
yCu(z, t)xcv(x , t) R
2(z, t)
=0 (3)
where u and v are deflections of the centre O in x and y
directions, respectively, and is angle of rotation of the floor
plan about the point O at the height z (0 z H). R is the
inertial radius of gyration given in theAppendix.Details of the
calculation of flexural and torsional stiffnesses and geometric
properties are also given in theAppendix.
Eqs.(1) and (2)describe that the sum of all forces applied
on the floor plan in x and y directions should be equal to zero;
Eq.(3) describes that the sum of all moments about the centre
O applied on the floor plan should be equal to zero.
3.3. Eigenvalue equation
Since the motion of the structure in free vibration at
any point of the structural height z is harmonic and the
corresponding deflection shape is independent of time t, the
displacement and the torsional rotation may be expressed in the
form
u(z, t)
v(z, t)
(z, t)
=
u( )
v()
( )
sin t (4)
in which = z/H, is the circular frequency, and the mode
shape vector is
D( )=
u( )
v()
( )
. (5)
By substituting Eq. (4) into Eqs. (1)(3) and carrying out
the necessary differentiation, the eigenvalue equations of the
problem can be obtained:
E Ix
m H4
4u( )
4 +
E Ix y
m H4
4v()
4 2 [u( ) yC( )]= 0 (6)
E Iy
m H4
4v()
4 +
E Ix y
m H4
4u( )
4 2 [v()+xC( )]= 0 (7)
E I
H4
4( )
z4
G J
H2
2( )
2
+ m2yCu( )xcv() R
2( )
=0. (8)
The boundary conditions of the above equations are:
D( )= D( )= 0 at =0 and
D
( )= 0 and
u( )= v ( )= E I( )G J( )= 0 at =1.
(9)
3.4. The Galerkin method
Based on the Galerkin technique [15] a method of solution
is proposed for solving the eigenvalue problem given in
Eqs.(6)(8). According to the principle of the Galerkin method,
the solution of the eigenvalue equations for a continuous
structural system can be expressed in a form of a linear
combination of arbitrarily selected shape functions u ( ), v()and ( ), i.e.,
u( )
v()
( )
=
i
ai f( )
bi g( )
ci k( )
(10)
in whichai , bi andci are the constants corresponding to thei th
vibration mode.
When using the Galerkin method with the chosen shape
functions there is no need to satisfy differential equation which
define the eigenvalue problem. But it is necessary and sufficient
to satisfy only the geometric boundary condition. Such shape
functions are classified as admissible functions.
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3.5. Uncoupled vibration
The uncoupled natural frequencies of lateral flexural
vibrations in the two directions (not necessarily the principal
directions) and St Venant warping torsion about the vertical
axis of the structures, represented by x , y and , can be
determined from the solutions of the frequency equations:
1+cosh jcos j =0 (j = x ,y) (11)
1+
1+
4
2()2
cosh 1cos 2
+2
2()sinh 1sin 2 = 0 (12)
where
1 =
()2 +
4
4 +
2
2 and
2 =
()
2 +4
4
2
2 .
(13)
The solutions of Eq. (11) are given by Timoshenko [16] as
follows:
(1)x =(1)y =1.875
(2)x =(2)y =4.694
(3)x =(3)y =7.855
(4)x =(4)y =10.996
(i)x =(i)y =
i
1
2
(i =5, 6, 7, . . . n. . .).
(14)
The uncoupled natural frequencies can then be obtained using
x =2x
x, y =
2y
y(15)
in which x and y are the characteristic parameters, defined
by
2x =m H4
E Ix, 2y =
m H4
E Iy. (16)
The solution of the frequency equation (12)give the values of
(i)
(i = 1, 2 . . . n. . .), the characteristic parameter and
given as:
2
=m H4
E IwR2, 2 =
G J H2
E I. (17)
The associated mode shapes, u( ), v() and ( ) for each
vibration mode are determined by
u(i )( )
v(i )( )
(i)( )
=
a(i )
b(i)
c(i )
N(i)x ( ) N(i )y ( ) N(i ) ( ) (18)
where a, b and c are indeterminate constants and N(i )( ) are
the non-normalized shape functions(N = f, g, k),
N(i)j = cosh
(i )j cos
(i )j
cosh (i)j +cos
(i)j
sinh (i)j +sin
(i )j
(sinh
(i )
j sin
(i)
j ) j = x y (19)
N(i)
= cosh
(i )
1cos
(i)
2
(i)
1
2cosh
(i)
1+(i)
2
2cos
(i )
2
(i)
1
2sinh
(i)
1+() sin
(i)
2
sinh
(i )
1
(i)
1
(i)
2
sin (i)
2
. (20)
3.6. Internal forces about the floor axis
In the response modal analysis of buildings subjected to
earthquakes an equivalent load is determined in each mode of
vibration. For spatial vibration[17]
f(i )xf(i )y
f(i )
=
10 {D( )}
T [M] d{l}10 {D( )}
T [M] {D( )} dm[M] {D( )}SAi (21)
where f(i)x and f
(i )y are the horizontal distributed forces in
the x and y directions, respectively, f(i) is the resultant
moment about thez-axis,SAi is the spectral acceleration (which
depends on the period of vibration, damping and ground peak
acceleration), and {l} is the influence vector, which representsthe direction of excitation. Matrix[M]is given by
[M] =
1 0 yc0 1 xc
yc xc R2 x 2c y
2c
. (22)
The total horizontal load, which is identical to the base shear
forces, is obtained by integrating Eq.(26)over the height of the
building as
V(i)xV(i)y
V(i)
= H
1
0
f(i)xf(i)y
f(i)
d . (23)
The base overturning moment can be obtained with
M(i )yM(i )xB(i )
= H2
10
f(i)xf(i)y
f(i)
d. (24)
3.7. Internal forces according to the principal direction axis
The design value of the shear forces and the base overturning
moment can be calculated according to the principal axis
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xOy (Fig. 1) defined by the angle from the x0y axis
given by
=Arctg
2IxyIx Iy
2
. (25)
The shapes modes for free vibration of the building structure
according to the principal axis are given by:
D( )
=
u(i)p ( )
v(i)p ( )
(i )p ( )
=
cos sin 0sin cos 0
0 0 1
u(i )p ( )
v(i )p ( )
(i)p ( )
(26)
and the location of the geometric centre Cin the x Oy co-
ordinate system is:
x C =x Ccos + yCsin (27a)
yC = yCcos xCsin . (27b)
The design shear base loads are determined when the
earthquake ground motion is in the principal axis direction. For
each mode this forces are given by
V(i )x
V(i )y
V(i )
= H
10
10
D( )
T[M] d{l}1
0 {D( )}T [M] {D( )} d
m[M]
D
( )
SAi d (28)and the base overturning moment
M(i )y
M(i )x
B(i )
= H2
10
10
D( )
T[M] d{l}1
0 {D( )}T [M] {D( )} d
m[M]D( )
SAi d (29)
where M is the mass matrix about the principal direction axis
written as
[M]
= 1 0 y
c
0 1 xc
yc xc R
2 x 2c y2c
. (30)
4. Computation procedure
4.1. Natural frequencies and associated mode shapes
The procedure of computation for determining natural
frequencies and associated mode shapes of tall buildings braced
by shear walls and thin-walled open section structures in
coupled vibration is presented as follows.
Step 1: Calculate lateral and torsional stiffnesses E Ix , E Iy ,
E Ix y ,E Iand G Jusing Eqs.(A.2)and(A.3)in theAppendix,
and geometric properties xc,yc and R using Eqs.(A.1),(A.2)
and(A.4).
Step 2: Calculate the uncoupled frequencies (i)x ,
(i)y and
(i)
using Eqs.(15)and(12).
Step 3: Determine the coupled frequencies using
2
x
2
yc
2
2Y 2 xc
2
yc2 xc
2 R2(2
2)
(i)
=0 (31)
where
=4
x;y
and =
m H4
E Ix y(32)
and the associate mode shapes using
u(i)p ( )
v(i)p ( )
(i )
p ( )
=
a(i )p
b(i )p
c
(i )
p
N(i)x ( ) N(i)
y ( ) N(i)
( )
(33)
where the vibration mode number i = 1, 2, 3, . . . n. . .and the
shape number p = 1, 2, 3; the constants a(i )p , b
(i)p , c
(i )p satisfy
the following relations:
a(i )p
c(i )p
=
(i )2
p ((i)2
y yc + (i )xc
(i )2
p yc)
(i)2
(i )2
y (i )2
x +(i)2
y (i)2
p +(i )2
x (i )2
p (i)4
p
(34)
and
b(i)p
c(i )
p
=
(i )2
p ((i)2
x xc+ (i )yc
(i )2
p xc)
(i)2
(i )2
y (i )2
x +(i)2
y (i)2
p +(i )2
x (i )2
p (i)4
p
.
(35)
It is seen from Eq.(21)that this equation is a cubic equation for
coupled frequency 2. In the i -th vibration mode, the solution
of Eq.(21)will give three values of coupled frequency: (i )1
(i )2
(i)3 . Each of the three frequencies for a given mode
(i)p
corresponds to a particular pattern of vibration.
Step 4: Determine the base shear forces and overturning
moments.
The base shear forces in the y and x directions are calculated
from Eqs.(23)and(28)for the principal axis directions. In the
same way, the base overturning moments are calculated fromEqs. (24) and (29). Finally, the design value of the internal
forces can be calculated by combining the modal response,
using the method of Square Root of Squares (SRSS).
5. Numerical example
In order to verify the accuracy of the mechanical concept of
the proposed method, a building braced by either shear wall and
angle type thin-walled open section structures is analysed. The
floor plan given inFig. 2 is that of a 25 storey building. The
modulus of elasticity is E = 25,000 MN/m2, shear modulus
G = 10,420 MN/m2, the storey height is h = 3 m, the total
height of the building is H = 75 m and the thickness of the
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Fig. 2. Floor plan of example structure.
Fig. 3. Modelling procedure using the FEM (SAP 2000[18]).
Fig. 4a. Vibration frequencies and associated mode shape for the first mode.
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Fig. 4b. Vibration frequencies and associated mode shape for the second mode.
shear wall and the thin-walled angle cross sections are equal to
0.3 m. The thickness of the floor slab is 0.15 m. Assuming that
the weight per unit volume of the building is 25 kN/m2, the
mass density per unit length is m = 114,365 t/m.
It is required to determine the natural frequencies of the
first three modes for coupled lateral flexural-warping torsion
vibration and the associated mode shapes.
Step 1: The geometrical properties of the structure are given in
Table A.1. Ix ,Iy ,Ix y,I and Jare determined using Eqs.(A.2)
and(A.3)in theAppendix.
Ix =65.30 m4, Iy =70.69 m
4, Ix y = 32.55 m4,
J =0.508 m4 and I = 7588.09 m6.
The location of the geometric centre C of the floor is
determined from formulae(A.2)and(A.4)
xc = 6.222 m and, yc = 7.577 m.
Step 2: The characteristic structural parameters are calculated
by employing Eqs.(16),(17)and(22):
= 4.446 s2, x =1.448 s1, y =1.431 s
1,
= 0.396 s1
and =1.914 s1
.
By employing Eq. (15) and solving Eq. (12) the uncoupled
natural frequencies can then be determined.
Step 3: The coupled natural frequencies can be determined by
solving the cubic equation(21)for each vibration mode. The
results are shown in Table A.2,these are later compared with
those obtained by employing a FEM analysis package SAP
2000 [18], using a fine mesh model, subdivided into a large
number of shell type finite elements as shown inFig. 3.Using
Eqs. (23)(25),the three first natural mode shapes of coupled
vibration for the example structure are determined and plotted
inFig. 4.
Step 4: The seismic forces are determined using the response
Modal Analysis. The calculation was carried according to
spectral acceleration shown inFig. 5.The base shear forces and
overturning moments of the three firsts modes in the case of
different earthquakes excitations are given in Tables A.3and
A.4.
6. Conclusions
A generalized hand method for seismic analysis of
asymmetric structures braced by shear walls and thin-walled
open section columns is presented. Based on the continuum
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Fig. 4c. Vibration frequencies and associated mode shape for the third mode.
Fig. 5. Spectral accelerations.
approach and dAlemberts principle, the governing differential
equation on free vibration and the corresponding eigenvalue
equation coupled lateral flexuralSt Venant and warping torsion
vibration have been derived. The Galerkin method is applied for
solving the eigenvalue equation. A computational procedure is
presented to determine the natural frequencies and associated
mode shapes in coupled vibration. This study was next
extend to determine the design internal forces of asymmetric
structures braced by shear walls and thin-walled open section
columns subjected to earthquake excitation. The results from
the proposed method are in good agreement with those from
a comprehensive package program for analysis of the building
structure stage and for final analysis.
Appendix
For buildings of rectangular plan shape and subjected to a
uniformly distributed mass at floor level, the radius of gyration
is obtained from
R =
L2 + B 2
12 +x 2C+y
2C
. (A.1)
The location of the centre of flexural rigidity for a general
asymmetric shear wall/open thin-walled cross section structure
as shown inFig. 1is given by:
xO =
q
xqE Iy,q
q
E Iy,q, yO =
q
yqE Ix ,q
q
E Ix ,q(A.2)
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Table A.1
Flexural and torsional moment of inertia of shear wall and thin-walled angle cross section structures
Bent x y Ix (m4) Iy (m
4) Ixy (m4) J(m4) I (m
6)
1 23 1 4.0157 4.0157 2.3933 0.072 1304.13
2 24 12 0.0135 5.4 0 0.054 109.906
3 23 23 4.0157 4.0157 2.3933 0.072 2085.63
4 1 23 4.0157 4.0157 2.3933 0.072 1064.715 2.25 2.25 45.598 45.598 27.320 0.162 2013.32
6 15 15 7.645 7.645 7.626 0.0763 1010.38 65.304 70.690 32.553 0.5083 7588.094
Table A.2
Comparison of circular frequencies (rad/s) of the proposed method and FEM analysis
Frequencies 1(1) 2
(1) 3(1) 1
(2) 2(2) 2
(3) 1(3) 2
(3) 3(3)
Proposed method 1.648 1.787 4.620 10.197 11.147 28.806 28.490 31.193 80.600
FEM analysis 1.628 1.976 5.091 9.912 11.919 28.128 26.563 31.851 75.973
Table A.3
Comparaison of the internal forces under the x O yaxis
Mode Vx (kN) Vy (kN) My (kN m) Mx (kN m)
Present FEM Present FEM Present FEM Present FEM
1 951.73 1090.09 777.68 745.14 51,863.16 60,431.83 42,378.38 41,338.41
2 1611.70 1829.57 1234.85 1221.19 25,268.72 28,498.63 19,411.17 19,158.35
3 887.75 1083 654.44 661.28 8,314.62 10,064.02 6,497.30 5,977.06
SRSS 2071.58 2389.25 1599.35 1576.02 58,287.46 67,568.20 47,063.10 45,952.49
Table A.4
Comparison of the internal forces under the principal direction axis
Mode Vx (kN) My (kN m)Present FEM Present FEM
1 1581.75 1566.39 86,183.97 86,887.16
2 2536.96 2581.42 39,812.39 40,319.50
3 1359.50 1430.89 13,010.68 13,159.43
SRSS 3284.25 3341.37 95,822.65 96,686.15
and the flexural and warping properties of the equivalent
cantilever are
Ix =q
Ix,q , Iy =q
Iy,q , Ix y =q
Ix y,q ,
I =
q
[( xq xO )2Iy,q + ( yq yO )
2Ix,q ]
J =
q
Jq (A.3)
where Ix ;qIy,qIx y,q , I,q and Jq are the flexural and warping
properties of theq-th shear wall/thin-walled open cross section.
As shown inFig. 1the location of the geometric centreCof
the uniform floor slabs in the co-ordinate system x O y is given
by
xC = xC xO , yC = yC yO . (A.4)
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