Download Case study of a 40-storey buckling-restrained braced frame building

THE STRUCTURAL DESIGN OF TALL AND SPECIAL BUILDINGS
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
Published online 30 October 2009 in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/tal.543
CASE STUDY OF A 40-STOREY BUCKLING-RESTRAINED
BRACED FRAME BUILDING LOCATED IN LOS ANGELES
ANINDYA DUTTA* AND RONALD O. HAMBURGER
Simpson Gumpertz & Heger, San Francisco, California, USA
SUMMARY
Simpson Gumpertz & Heger has prepared two prototypical designs for a 40-storey buckling-restrained braced
steel-framed office building located at a generic site in Los Angeles, CA. One of these designs conforms in all
respects, except height limits with the design criteria contained in the 2007 California Building Code and ASCE
7.05 Standard for Minimum Design Loads for Buildings and Other Structures. The second design has been conducted using a performance-based approach generally based on the criteria contained in guidelines published by
the Los Angeles Tall Buildings Council. The performance-based design incorporates fewer bays of bracing and
lighter members than the code-based design, but is intended to provide performance at least equivalent to that
anticipated for code-designed buildings. The purpose of this work was to permit study of the performance capability of buildings designed to alternative criteria. This work was performed in support and under funding provided
by the Pacific Earthquake Engineering Research Center’s Tall Buildings Initiative. Copyright © 2009 John Wiley
& Sons, Ltd.
1.
OBJECTIVE AND SCOPE OF WORK
Simpson Gumpertz & Heger, Inc. developed two alternative designs of a 40-storey steel building on
behalf of the Pacific Earthquake Engineering Research Center (PEER) under its Tall Buildings Initiative (TBI). The objective of the TBI is to develop recommended performance-based design criteria
for tall buildings as an alternative to the criteria contained in present building codes and standards.
These alternative performance-based criteria are intended to provide performance that is at least
equivalent to that intended for buildings designed in conformance with the code. The buildings presented in this paper, together with designs using other structural systems developed by other designers,
will be used by PEER to assess the cost and performance capability of buildings designed using
alternative design approaches and criteria. This information will guide the development of the PEER
recommendations.
We developed the designs to a schematic level. We sized the gravity load system, including floors,
systems and columns considering blanket superimposed and live loads, but neglecting miscellaneous
openings, cladding supports and similar information that typically becomes available later in the design
process. Seismic analysis, including linear response spectrum analysis for the code-based design and
nonlinear response history analysis for the performance-based design, was performed, and members
of the seismic force-resisting system were proportioned as required to meet the respective criteria. We
designed the foundations to a sufficient level to determine mat thickness, but did not determine reinforcing. Detailing of the structure was not performed.
* Correspondence to: Anindya Dutta, Simpson Gumpertz & Herger, The Landmark at One Market, Suite 600, San Francisco,
CA94105. E-mail: [email protected]
Copyright © 2009 John Wiley & Sons, Ltd.
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A. DUTTA AND R. O. HAMBURGER
In parallel with our work, two other firms performed similar designs of buildings using reinforced
concrete structural systems. All buildings were assumed to be located on the same generic site in
downtown Los Angeles. PEER provided site criteria for us in our design including site class, permissible bearing pressures, spectral response ordinates and ground motion acceleration histories.
Our scope of work included the following:
(1) Meet with representatives of PEER and other design consultants engaged in the TBI project to
develop the general criteria for the designs.
(2) Develop a design, to a schematic level, for an essentially code-conforming steel-framed
building.
(3) Develop a design, to a schematic level, for a performance-based steel-framed building, of similar
height and footprint, using the procedures contained in design recommendations prepared by the
Los Angeles Tall Buildings Council.
(4) Prepare schematic-level drawings documenting the design.
2.
BUILDING DESCRIPTION
Both designs have typical above-grade floors comprised of 6 ¼ in. lightweight concrete fill on metal
deck supported by composite steel framing with a foot print of 170 by 107 ft. Both designs have four
basement levels with a foot print of 227 by 220 ft. Both structures have lateral force-resisting systems
comprised of buckling-restrained braced frames without backup moment frames.
The site class was assumed as class C. Spectral response ordinates were: Ss = 2·15 and S1 = 0·72.
For the performance-based design, we used a suite of seven acceleration histories provided by
PEER.
3.
3.1
CODE-BASED DESIGN
Purpose
This section provides a brief overview of the code-based design. As discussed earlier, we used the
2007 California Building Code to perform this design.
3.2
Design description
This design was conducted in conformance with all the prescriptive provisions of the 2007
California Building Code and its referenced standards, except for the limitation on the building
height. The California Building Code requires that buildings in excess of 160 ft and located on the
site selected for this building incorporate a special moment-resisting frame capable of resisting at least
25% of the specified design seismic forces. This design did not incorporate such a frame. Figure 1
shows building plan views at various levels, and salient features of this design are described further
below.
3.3
Gravity analysis and design
We designed the building’s vertical load-resisting system using gravity loads that include a combination of structure self-weight and additional superimposed loads. The design criteria document provided
to us by PEER specified the superimposed loads at various floors shown in Table 1. We combined
the element self-weights with the superimposed loading from this table, and performed a gravity-load
design using the RAM Structural System software, version 12.1.
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
ARCHETYPICAL DESIGNS FOR A 40-STOREY BRB
79
Figure 1. Plan view of the building at various floors
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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3.4
A. DUTTA AND R. O. HAMBURGER
Lateral analysis
Following design of the vertical load-carrying elements, we performed a lateral analysis of the building. We considered both wind and seismic forces, and designed each element for the most severe
requirements. A brief description of the wind and seismic design is given below.
3.4.1 Wind analysis
We used ASCE 7-05, method 2 to calculate the wind pressures. Table 2 lists the various parameters
used. We considered the four cases depicted in figures 6–9 of ASCE 7-05. Each of these cases includes
the direct wind force in one of four orthogonal directions either alone or in combination with storey
torsional moment caused by the eccentric application of this load. We calculated the gust effect factor
(Gf) following section 6.5.8.2 of ASCE 7-05 for flexible or dynamically sensitive structures with 1%
assumed damping.
We calculated wind base shears of 1436 and 2629 kip, respectively, along the two orthogonal
directions.
3.4.2 Seismic analysis
We used linear response spectrum analysis to calculate seismic forces and displacements. Per ASCE
7-05, Cl 12.9.4, we scaled the forces to 85% of the base shear obtained from the equivalent lateral
force procedure of Cl 12.8. Table 3 lists the various parameters that we used for the seismic design.
Table 1. Gravity loading criteria
Description/Location
Roof
Mechanical, electrical at roof
Residential including balconies
Corridors, lobbies and stairs
Retail
Parking garage, ramp
Construction loading
Cladding
Superimposed dead
Live load
Reducability
28 psf
Total of 100 kip
28 psf
28 psf
110 psf
3 psf
3 psf
15 psf
25 psf
–
40 psf
100 psf
100 psf
40 psf1
30 psf
–
Yes
–
Yes
No
No
Yes
No
–
PEER document showed 50 psf. SGH considered 40 psf in keeping with ASCE 7-05.
Table 2. Wind design criteria
Parameter
Value
Basic wind speed, 3 s gust (V)
Basic wind speed, 3 s gust (V), for serviceability wind demands based on a 10-year mean
recurrence interval
Exposure
Occupancy category
Importance factor (Iw)
Topographic factor (Kzt)
Exposure classification
Internal pressure coefficient (GCpi)
Mean roof height (h)
Wind base shear along two orthogonal directions
Copyright © 2009 John Wiley & Sons, Ltd.
85 mph
67 mph
B
II
1·0
1·0
Enclosed
±0·18
544 ft, 6 in.
1436 and 2629 kip
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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ARCHETYPICAL DESIGNS FOR A 40-STOREY BRB
Table 3. Seismic design parameters
Parameter
Value
Building latitude/longitude
Occupancy category
Importance factor (Ie)
Spectral response coefficients
Seismic design category
Lateral system
Undefined
II
1·0
SDS = 1·145; SD1 = 0·52
D
Buckling-restrained braced frames, non-moment-resisting
beam column connections
7
5·5
2·0
3·16 s
0·051 W (governed by Cs-min from equation 12.8-5)
3504 kip (85% of static base shear)
Modal response spectral analysis
Response modification factor (R)
Deflection amplification factor (Cd)
System overstrength factor (Ω0)
Building period (T) using Cl. 12.8.2
Seismic response coefficient Cs (Eq. 12.8-1)
Scaled spectral base shear
Analysis procedure
Actual period from dynamic model: TY = 5·05 s; TX = 3·62 s.
Spectral Acceleration, Sa (g)
1.6
1.2
0.8
0.4
0
0
2
4
6
8
Period (sec)
Figure 2. Response spectrum for code analysis
We performed the lateral analysis using ETABS, version 9.5.0. We used the 5% damped, acceleration response spectrum shown in Figure 2. We included 120 modes to obtain participation of at
least 90% of the structure’s mass. We scaled the results of the response spectrum analysis such that
the base shear immediately above the ground floor matched 85% of the static base shear from the
equivalent static force analysis. Note that for levels below the ground floor, the mass of the perimeter
walls is automatically calculated by the program.
The 2007 California Building Code requires calculation of the redundancy factor (ρ) for seismic
design. In accordance with ASCE 7-05 section 12.3.4.2, buildings qualify for a value of ρ = 1 provided
that the removal of an individual brace or connection will not result in more than a 33% reduction in
storey strength, nor will the resulting system have an extreme torsional irregularity. Extreme torsional
irregularity is defined to exist when the maximum storey drift computed including accidental torsion,
at one end of the structure transverse to an axis, is more than 1·4 times the average storey drifts at the
two ends of the structure.
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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A. DUTTA AND R. O. HAMBURGER
In order to calculate the redundancy factor for seismic forces acting in the north–south direction,
we removed a brace from line 2 along the full height of the building. The code would actually permit
this to be done one storey at a time; however, recognizing that we were being conservative, we
removed the brace from all stories simultaneously, to reduce analysis effort. We then subjected the
building to the static lateral forces along with the 5% accidental torsion. We term these forces as
EQYPL (with the accidental eccentricity towards the positive right) and EQYMN (with the accidental
eccentricity towards the negative left). Since the building has at least six bays of braced frame in each
principal direction, loss of a brace or connection will never reduce the capacity by 33%. We found
that for the north–south direction, the largest ratio of maximum storey drift over the average drift over
the height of the building was 1·39 at storey 13. Since this is less than 1·4, we concluded that the
redundancy factor in the north–south direction is 1. Figure 3(a) shows the elevation at line 2 after the
removal of the brace.
We performed a similar analysis in the X (east–west direction). In this analysis, we removed a single
brace from line D and subjected the building to static forces EQXPL (with the accidental eccentricity
towards the positive top) and EQXMN (with the accidental eccentricity towards the negative bottom).
Figure 3(b) shows the elevation of the frame along line D with the brace removed. We then calculated
the ratio of maximum to average drifts, and obtained ratios ranging from 1·468 to 1·408 from the roof
to storey 36. However, section 12.3.4.2 stipulates that this ratio should strictly be studied for a floor
resisting more than 33% of the base shear which is numerically equal to 1156 kip. This only happens
below storey 35. In addition, as noted previously, our simultaneous removal of a brace at all levels is
conservative. Since the ratio of maximum to average drift is only marginally greater than 1·4 at two
stories with a maximum value of 1·408 at storey 34 (D/C of 1·008). We declared that this design met
the requirements of ASCE 7-05 for a redundancy coefficient, ρ = 1.
3.5
Lateral force-resisting member design
3.5.1 Buckling-restrained brace (BRB) design
We designed the BRBs for the worst of the wind and seismic loads. In almost all cases, seismic loads
governed the design. Brace capacity in tension and compression was taken as φAsFy, with φ = 0·9 and
Fy = 38 ksi.
3.5.2 Braced frame column design
AISC’s Seismic Provisions for Structural Steel Buildings (ANSI/AISC 341-05) require columns in
buckling-restrained braced frames to be checked for:
(1) Axial load and moment interaction for code level forces
(2) Axial load only corresponding to the sum of the vertical component of the strength of all BRBs
that frame into the column along with tributary gravity loading
We checked the columns for both of the above criteria, and found that the latter generally produced
larger D/C ratios. Since the current configuration uses columns that form part of lateral framing in
two orthogonal directions, we used the 100%–30% combination to calculate the maximum compression from the braces. We established the maximum compression forces from the brace as RyωβAsFy,
where Ry = 1·1, ω = 1·25 and β = 1·1.
We used built-up square box columns infilled with high strength ( f ′c = 10 ksi) concrete to resist the
large compression force demands. The plate thicknesses of the box columns ranged from 1·5 in. at
upper stories to 3 in. at lower ones. The plan dimension of the box columns ranged from 18 to 57 in.
We calculated the strength, axial area and stiffness of infill columns using the provisions of chapter I
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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ARCHETYPICAL DESIGNS FOR A 40-STOREY BRB
(a)
(b)
Figure 3. Elevation of frame along lines 2 and D following removal of a brace. (a) Line 2 and (b) line D
(‘Design of Composite Members’) of the 13th edition LRFD Steel Code (ANSI/AISC 360-05). We
included this enhancement of axial area and stiffness over the bare steel section properties in the
ETABS analysis model as well. We did this by defining a steel box section in ETABS and then using
the property modifier to account for the infill concrete.
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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A. DUTTA AND R. O. HAMBURGER
3.5.3 Braced-frame beam design
We designed the braced-frame beams for the horizontal component of the brace compression along
with the unbalanced upward component of the BRB given by RyωAsFy(β-1)sinα, where α is the brace
angle. Since BRBs are always stronger in compression than in tension, this moment is opposite in
direction to the gravity moment.
3.5.4 Storey drifts for code design
We calculated inter-storey drifts using response spectrum analysis. ASCE 7-05 stipulates the use of a
response spectrum that is reduced by the response reduction factor R. It also states that the drifts
obtained from this reduced spectral analysis be amplified by Cd to yield design storey drifts. Table 4
shows the design inter-storey drifts (amplified by Cd) obtained at various levels both for the X and Y
directions. As can be seen, the maximum storey drifts are less than the permissible value of 0·02 at
all levels.
4.
PERFORMANCE-BASED DESIGN
We based the seismic design for this alternate on the seismic design criteria (dated 29 April 2008)
published by the Los Angeles Tall Buildings Structural Design Council (LATBSDC 2008). Since this
design was not required to meet all of the prescriptive criteria contained in the building code, we were
able to reduce the size and number of bays of the buckling-restrained frames. Specifically, along lines
2 and 7, we omitted two bays of bracing below level 10. Table 5 lists the performance objectives we
followed for this design.
Table 4. Storey drifts for code design
Storey
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
Y drifts
X drifts
Storey
Y drifts
X drifts
0·013
0·014
0·014
0·014
0·014
0·014
0·014
0·014
0·014
0·014
0·014
0·014
0·014
0·013
0·013
0·013
0·013
0·012
0·012
0·012
0·012
0·011
0·005
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
0·006
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0·011
0·011
0·010
0·010
0·010
0·009
0·008
0·007
0·004
0·004
0·004
0·004
0·004
0·004
0·003
0·003
0·003
0·002
0·006
0·006
0·006
0·006
0·005
0·005
0·005
0·005
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·004
0·003
0·002
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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ARCHETYPICAL DESIGNS FOR A 40-STOREY BRB
Table 5. Seismic performance objectives
Level of earthquake
Earthquake performance objectives
Frequent/Service: 25-year return period,
2·5% damping
Maximum considered earthquake (MCE):
As defined by ASCE 7-05, section
21.2, 2·5% damping
Serviceability: Essentially elastic performance with minor yielding
of brbf-s. Drift limited to 0·5%
Collapse prevention: Extensive structural damage, repairs are
required and may not be economically feasible. Drift limited
to 3%
Spectral Acceleration, Sa (g)
0.4
0.3
0.2
0.1
0
0
2
4
6
8
Period (sec)
Figure 4. Response spectrum for serviceability analysis
4.1
Service level earthquake evaluation
We used response spectrum analysis for this evaluation. We used the response spectrum shown in
Figure 4, which was provided by PEER. The damping level was set at 2·5%. We neglected accidental
torsion. Inter-storey drifts obtained for the service level earthquake are shown in Table 6. As can be
seen, the drifts are less than 0·5% at all levels. We also evaluated the building for the same wind forces
that we used for the code-based design described in the previous section. We found that wind always
governed the required brace strength as compared with service level earthquake demands.
4.2
Maximum considered earthquake evaluation
As required by the LATBSDC document, we performed a nonlinear response history analysis of the
building for maximum considered earthquake shaking as defined by the building code. We used seven
pairs of acceleration histories provided to us by PEER for this purpose. We used CSI Perform, version
4.0.1 with a constant modal damping level of 2·5% and 0·1% Rayleigh damping. We explicitly modelled nonlinearities in the BRBs. We modelled columns and beams as elastic elements, and later
verified that demands on these elements remained within their elastic capacities. Details of the element
modelling are discussed below.
4.2.1 Modelling of BRBs
We modelled the BRBs using the perform BRB inelastic element in series with an elastic spring
that models the end attachment of the braces. The BRB backbone curves were modelled as shown in
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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A. DUTTA AND R. O. HAMBURGER
Table 6. Storey drifts for service level earthquake
Storey
Y drifts
X drifts
Storey
Y drifts
X drifts
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
0·0031
0·0032
0·0033
0·0033
0·0034
0·0033
0·0033
0·0033
0·0032
0·0031
0·0031
0·0029
0·0028
0·0027
0·0026
0·0026
0·0025
0·0024
0·0023
0·0022
0·0021
0·0021
0·0014
0·0015
0·0016
0·0017
0·0017
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0018
0·0017
0·0017
0·0017
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0·002
0·002
0·0019
0·0019
0·0018
0·0018
0·0017
0·0017
0·0016
0·0016
0·0016
0·0015
0·0015
0·0015
0·0014
0·0014
0·0012
0·0008
0·0017
0·0017
0·0017
0·0016
0·0016
0·0016
0·0016
0·0016
0·0017
0·0017
0·0017
0·0017
0·0017
0·0017
0·0017
0·0016
0·0014
0·001
1.25%Ko
ωRyFyAs
RyFyAs
Tension
20Δy
10Δy
Ko
Δy
Δy
10Δy
20Δy
Compression
RyFyAs
βωRyFyAs
Figure 5. Presumed backbone curve for buckling-restrained braces
Figure 5 with Ry = 1·1, ω = 1·25 and β = 1·1. Figure 6 shows a screen shot of the Perform input form
for the BRB definition. Note that the initial stiffness (K0) of the BRB is based on AsE/L with L equal
to 70% of the actual centre-to-centre length of the brace. The remaining 30% of the length is modelled
as an essentially rigid element.
4.2.2 Modelling of the columns and beams
We modelled the columns using the non prismatic steel sections in Perform. The moment of inertia
input in the form was adjusted to account for the additional stiffening due to the presence of the infill
concrete. The axial load and moment strength for the various columns were defined in Perform using
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
ARCHETYPICAL DESIGNS FOR A 40-STOREY BRB
87
R yF yA s
70% Actual
1.25% Ko
Figure 6. BRBF property definition in Perform
Figure 7. Column strength definition in Perform
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Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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A. DUTTA AND R. O. HAMBURGER
the same process as for the elastic analysis. We assumed nominal strengths for the columns without
any strength reduction or φ factors. Figure 7 shows the column axial load a representative flexural
strength definition form in Perform.
The compound column element associated with the geometric location of the column utilizes the
cross-sectional information and assembles the column element as an elastic element. We have ensured
that the column element always stays elastic during the analysis by monitoring the axial flexure interaction ratios so that they are always less than 1.
Beams are modelled in an identical fashion as the columns, except that standard W sections are
used. Similar to the columns, the beams are also modelled as elastic elements.
4.2.3 Modelling of the diaphragms at upper, ground and basement levels
We modelled rigid diaphragms at all elevated floors. However, we modelled diaphragms at the ground
and all basement floors with elastic shell elements using 30% of the gross cross-section properties.
We considered an effective thickness comprising the total thickness of the topping plus half the rib
thickness. Thus, the ground floor was modelled as 10·5-in. thick slab as it represents a 9-in. topping
and 1·5-in. half rib height. We followed the same approach for the basement slabs except that the
modulus of elasticity also included the λ modifier corresponding to lightweight concrete.
Peak Interstory Drift in Y Direction
40
35
30
Story
25
20
15
10
δy
5
0
-5
0.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
Peak Interstory Drift (%)
Average
Denali Ps
Landers Yermo
LP Gilroy
LP Saratoga
Nridge Sylmar CS
Nridge Sylmar Hosp
Parachute
Figure 8. Peak inter-storey drifts in the Y direction
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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ARCHETYPICAL DESIGNS FOR A 40-STOREY BRB
4.2.4 Modelling of perimeter walls
We modelled the perimeter shear walls as elastic wall elements with cracked stiffness equalling 50%
of the gross stiffness. The elastic shear modulus was considered to be 40% of the gross elastic
(Young’s) modulus per ASCE 41, supplement 1.
4.2.5 Selected results from nonlinear analysis
4.2.5.1 Storey drifts
Figures 8 and 9 present the storey drift profiled obtained from the analyses. Figure 8 shows the drift
in the Y direction and Figure 9 the X direction. The maximum drift in the Y direction is around 1·97%,
while the maximum in the X direction is at 2·25%. As can be observed from the figures, the drifts are
less than the permissible value of 3% in both directions, and the mean peak drifts are generally on the
order of 1·25% or less.
4.2.5.2 Column axial flexure interaction ratio
Since the beam columns were modelled as elastic elements, we monitored the interaction ratios closely
and ensured that they remain less than unity signifying elastic behaviour. We found that the maximum
mean interaction ratio was 0·63. The absolute maximum from the seven analyses is 0·83, justifying
our assumption regarding the elastic behaviour of the columns.
Peak Interstory Drift in X Direction
40
35
30
Story
25
20
15
10
5
δx
0
-5
0.00
0.50
1.00
1.50
2.00
2.50
Peak Interstory Drift (%)
Average
Denali Ps
Landers Yermo
LP Gilroy
LP Saratoga
Nridge Sylmar CS
Nridge Sylmar Hosp
Parachute
Figure 9. Peak inter-storey drifts in the X direction
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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A. DUTTA AND R. O. HAMBURGER
Line 2
Line 3
Line 4
Line D
Figure 10. BRBF core strain DCRs along lines 2, 3, 4 and D
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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ARCHETYPICAL DESIGNS FOR A 40-STOREY BRB
Accumulated Peak Story Shear in Steel Superstructure in Y Direction
40
Vy
35
30
Story
25
20
15
10
5
0
-5
0
2500
5000
7500
10000
12500
Accumulated Peak Story Shear (kips)
Average
Denali Ps
Landers Yermo
LP Gilroy
LP Saratoga
Nridge Sylmar CS
Nridge Sylmar Hosp
Parachute
Figure 11. Accumulated peak storey shear in steel superstructure in Y direction
4.2.5.3 BRB core strain
We also monitored strain in the BRB cores, and ensured that the mean strain is less than of 0·013
(10εy) obtained from the test results conducted at the University of Utah by Romero and Reavely.
Figure 10 shows the BRB core strain for lines 2, 3, 4 and D, respectively. The results are presented
as D/C ratio with the capacity being 0·013. The ratios are all less than unity, and most are substantially
less than unity, implying acceptable performance. DCRs indicated in yellow in the figure indicate
values in excess of 75% of the 0·013 strain limit.
4.2.6 Storey shears
We also monitored storey shears over the building height. Figures 11 and 12 plot the storey shears in
the Y direction and X direction, respectively. The effect of the stiff ground floor diaphragms is easily
observable from the plots. We have proportioned the ground floor slab and the collectors so that it is
capable of transferring this large reaction.
5. SUMMARY AND CONCLUSIONS
Simpson Gumpertz & Heger performed two alternative designs of a 40-storey building located at a
hypothetical site in Los Angeles. Both buildings utilize buckling-retrained braced frames as the lateral
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
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A. DUTTA AND R. O. HAMBURGER
Accumulated Peak Story Shear in Steel Superstructure in X Direction
40
35
Vx
30
Story
25
20
15
10
5
0
-5
0
2000
4000
6000
8000
10000
Accumulated Peak Story Shear (kips)
Average
Denali Ps
Landers Yermo
LP Gilroy
LP Saratoga
Nridge Sylmar CS
Nridge Sylmar Hosp
Parachute
Figure 12. Accumulated peak storey shear in steel superstructure in X direction
system. The first design strictly follows the provisions of the 2007 California Building Code except
that it exceeds code limitations on the height for structures not having backup special moment frames.
The columns and beams are designed using the relevant requirements of AISC 341.05 seismic
provisions.
The second design uses the concepts of performance-based engineering and is based on the
LATBSDC. A nonlinear time history analysis is performed on this design using maximum credible
earthquake input to ensure a safe design.
Generally, the second design produced fewer bays of bracing, smaller sizes of the BRBs and
columns. The main savings, however, is likely to be from the design of the foundation. The code
design requires that the foundation be designed for the accumulated vertical component of the brace
forces over the height of the building. This essentially implies exclusively first-mode behaviour which
does not occur in tall buildings. Using the actual demands from the time history analysis, we obtained
forces that are smaller and hence more manageable than the code-required forces.
REFERENCES
(American Institute of Steel Construction). ANSI/AISC 341-05, 2005 Seismic Provisions for Structural Steel
Buildings. AISC: American Institute of Steel Construction, Chicago, IL.
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal
ARCHETYPICAL DESIGNS FOR A 40-STOREY BRB
93
ANSI/AISC 360-05, 2005, Specification for Structural Steel Buildings, American Institute of Steel Construction,
Chicago, IL.
ASCE 2005. Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers,
Reston, VA.
LATBSDC (Los Angeles Tall Buildings Structural Design Council). 2008. An Alternative Procedure for Seismic
Analysis and Design of Tall Buildings Located in the Los Angeles Region. LATBSDC: Los Angeles, CA.
Copyright © 2009 John Wiley & Sons, Ltd.
Struct. Design Tall Spec. Build. 19, 77–93 (2010)
DOI: 10.1002/tal