Comparative SeismicPerformance of Four Structural Systems

8/11/2019 Comparative SeismicPerformance of Four Structural Systems

1/13

Comparative Seismic Performance of Four Structural Systemsand Assessment of Recent AISC BRB Test Requirements

Ronald L. Mayes, Staff ConsultantSimpson Gumpertz & Heger Inc.

San Francisco, CA

Craig B. Goings, Project ManagerWassim I. Naguib, Senior Engineer

Stephen K. Harris, Senior Project ManagerSimpson Gumpertz & Heger Inc.

San Francisco, CA

Abstract

This paper provides a comparison of the maximum inter-story drifts and floor response spectra of both athree and nine story building each with four different steel structural systems moment frame, bucklingrestrained braced frame, viscously damped frame and a base isolated braced frame. Each of the buildingmodels were analyzed as fully non-linear structures and subjected to a total of 10 time histories each. Oneset of five time histories was representative of a 50% in 50 year earthquake, while the other set wasrepresentative of a 10% in 50 year earthquake. Both sets were developed for the Los Angeles area. Theresults of each set of five were averaged and reported separately.

The results of the non-linear time history analyses on the buckling restrained braced frame and those performed by others are used to assess the most recently recommended AISC test requirements for buckling restrained braced frames. The AISC provisions requires that the braces have adequate performance, as demonstrated by tests, for deformations corresponding to 2.0 times the design drift. Thisrequirement needs to be increased to 3.0 times the design drift or a drift of 3% (0.03 times story height)

because the drifts that result from the non-linear response of these frames for the design earthquake(especially for buildings less than approximately 6 stories high) may have significant non-lineardeformations concentrated in one story height. This concentration of deformation can lead to much higherductility demands in selected braces.

Introduction

The introduction of the buckling restrained brace (BRB) in the US was as a bracing system that was a goodenergy dissipater, and early BRB projects were based on the draft NEHRP energy dissipation design

procedures. This meant the BRB was in the same category as viscous dampers, friction and hystereticdamping elements. The use of the NEHRP provisions requires more sophisticated analytical effort plus

prototype testing and production testing, as well as peer review of the design and testing of the devices. The

proponents of the BRB technology in the US quickly realized that this positioning of the product was amistake. They repositioned them as a brace that is superior to a concentric brace in that it does not buckle;therefore the design could be covered by the steel design provisions, which were less stringent with regardto peer review and testing requirements. The BRBs are now established in the code design process as analternate to the eccentric braced frame (EBF) with the advantage that they will not locally damage the floorframing during an earthquake and, if necessary, can be replaced more easily than the floor beam link beamin an EBF. As such, a designer can now use an R-factor of 7 for a BRB system to get the required yieldforce in the brace and, provided a brace of a similar size has been tested, no prototype or production tests arerequired.


2/13

For other energy dissipation elements (viscous and hysteretic dampers) the trend is to design a momentframe using an effectively increased R that results in a reduced base shear of approximately 75% of therequired base shear for a full moment frame, without regard to inter-story drift limits. The drift limits of thecode are met by designing the energy dissipation element to reduce the inter-story drifts to at or below thecode minimums. In the early application of energy dissipation elements, the moment frame had to satisfythe code minimum base shear.

Base isolation design has had its own set of design provisions since 1991. The design requirements for thestructure above the isolators is much more stringent than BRBs or Viscous Dampers and ensures that thestructure remains essentially elastic as the maximum R-Factor is 2.2. This results in much lower ductilitydemand on the structure than a BRB and viscously damped system but it also results in a higher structuralcost relative to other lateral force resisting systems. .

Unfortunately there has not been a comprehensive study that looks at the relative performance of thesenewer structural systems (BRBs, viscous dampers, base isolation etc.) comparing both drift andacceleration performance. As part of a recent research and development project, SGH had the opportunity tocompare the seismic performance of four different structural systems in the three and nine story SAC

buildings. The four lateral load systems were: moment frames, buckling restrained braced frames, viscouslydamped frames and a base isolated braced frame. The moment frame, buckling restrained braced frame(R=7) and base isolated braced frame were designed following the requirements of the 1997 UniformBuilding Code (UBC) whereas the viscously damped frame used the 2003 NEHRP requirements.

3 and 9 Story Building Configurations

The three- and nine-story building configurations were the same as those used in the SAC studies and hadsimilar floor plans with 30-foot bay spacing. The three-story building had 6x4 bays with equal story heightsof 13 feet. The nine-story building had 5x5 bays with a first story height of 18 feet, with the remaining floorheights at 13 feet. Figures 1 and 2 show the moment and braced frame configurations.

Figure 1. Three and Nine Story MomentFrame Configuration

Figure 2. Three and Nine Story Braced FrameConfigurations


3/13

Table 1. Three Story Building Design Summary

The required number of moment frame bays was significantly greater than the required number of bracedframe bays due to drift limitations and the new redundancy ( ) factor provisions for special moment-resisting frames in the 1997 UBC. For the three-story building, only two exterior bays of braces or damperson each exterior perimeter were necessary. For the nine story building, three bays of bracing and two baysof dampers were required. Tables 1 and 2 provide a brief summary of the design data for each of thedesigns. For the three story building the buckling-restrained bracing system was designed as both aconventional building with an R-Factor of 7 and as an essential facility using an R-Factor of 3.5. Theviscous damped frame was initially designed to meet the minimum code requirements (133 K Dampers)and then the system was redesigned with an almost doubling in the damping coefficient (220 K Dampers) ofthe viscous dampers. These two additional designs were performed in order to study the relative

performance of structural systems that could meet lower drift limits required for essential facilities. Thefollowing summarizes the designs performed for both the three and nine story buildings:

Moment frame both the three and nine story building were controlled by the 2% inter-story driftrequirement and as noted above by the redundancy factor of the 1997 UBC. The redundancy factorresulted in a lateral system that required more moment frame bays than braced frames. Momentframes extend the full building height and Figure 1 shows the moment frame configurations used.

Buckling-restrained braced frames with R-factors of 7 and 3.5 both with 45 ksi yield strength steel.BRBs are trending towards higher yield stress materials after originally being introduced by Nipponsteel with 22 ksi yield strength braces. Figure 2 shows the bracing configurations used.

Viscous damped moment frame the moment frame was initially designed to resist 75% of the codecalculated base shear (V). The viscous dampers were initially sized to satisfy the 2% driftrequirements of the 1997 UBC and had a velocity coefficient of 0.4. For the three story building andthe conventional code design the force capacity of the dampers was 133 kips at velocity of 15 in/sec.For the higher performing essential building performance the damper force was increased to 220kips at velocity of 15 in/sec. For the nine story building, the top four stories used dampers with acapacity of 148 kips at a velocity of 15 in/sec. while the lowest five stories used dampers with acapacity of 295 kip at a velocity of 15 in/sec. There were two bays of dampers on each of the foursides of the building for both the three and nine story buildings.

Base isolated conventional braced frame this system was designed following the requirements ofthe 1997 UBC using a 2.5 sec. isolated system with a yield level of 0.4W.

.

Moment Frame - Method BT1 = 0.94 sV = 602 k

Story Design DriftBrace Axial

Capacity Design Drift

Brace Axial

Capacity Design Drift(2.0% limit) (kips) (2.5% limit) (kips) (2.5% limit)

3 1.93% 135 1.21% 242 0.72%2 1.97% 236 1.17% 439 0.71%1 1.16% 236 1.33% 439 0.80%

Story Dam per Force at 15 in/s D esign DriftDrift w/oDampers

DamperForce at15 in/s Design Drif t

Drift w/oDampers

(kips) (2.0% limit) (kips) (2.0% limit)3 220 1.41% 3.44% 133 1.98% 3.44%2 220 1.46% 3.27% 133 1.85% 3.27%1 220 1.06% 2.14% 133 1.26% 2.14%

T1 = 1.25 T 1 = 1.25V = 634 k - Method A V = 634 k Method A

V = 1026 k V = 2053 k

Viscou s Damped Frame - 220K Dampers Viscou s Damped Frame - 133K

Unbonded Braced Unbonded BraceT1 = 0.72 T 1 = 0.56


4/13

Table 2. Nine Stor Buildin Desi n Summar

The buildings were modeled using the 3D RAM PERFORM computer program recently developed byProfessor Graham Powell of Berkeley and sold by RAM International. Design of the braced frame andmoment frame components of the building were done with RAM STEEL. Nonlinear time history analyseswere performed on each building with the non-linear element being the actual BRB, viscous damper,isolator, brace and moment frame connections and their immediate surrounding frame as appropriate.

Each of the models was analyzed using a total of 20 time histories selected from those developed for LosAngeles as part of the SAC program. One set of five time histories is representative of a 50% probabilityof exceedance in 50 year design event for Los Angeles (72 year return period), five time histories arerepresentative of a 10% probability of exceedance in 50 year design event for Los Angeles (475 yearreturn period), five time histories are representative of a 2% probability of exceedance in 50 year designevent for Los Angeles (2500 year return period) and five are representative of severe near fault ground

motions. The results of each set of five are averaged. Due to the space limitations of this paper only themore frequent 72 year and the 475 year design event results are presented.

Discussion of Results

The results obtained from the three and nine story building are different and will be discussed separately.

There are two equally important variables that should be assessed when evaluating the seismic performance of a structural framing system. The first and almost universal variable is the inter-story drift.This is a code design parameter and is something most engineers focus upon during the design process.The other key parameter, from a performance perspective, is the floor acceleration as characterized by thefloor response spectra. This is rarely assessed as part of the design process because it requires a time

history analysis to obtain it.

The primary results of our analysis are the average inter-story drifts and floor response spectra from eachof the five non-linear time history analyses. There are a number of different ways to present them. For thethree story building the average inter-story drift results of the four code designed buildings are given inFigure 3 and the average floor response spectrum at the 3 rd floor level is given in Figure 4 for the 10% in50 year earthquake time histories. The two essential facility building designs were not included in these

plots in order to maximize clarity.

Moment FrameT1 = 2.04 sV = 1076 k

Story Design Drift

Brace

AxialCapacity Design Drift

DamperForce at15 in/s Design Drift

Drift w/oDamper s

(2.0% limit) (kips) (2.0% limit) (kips) (2.0% limit)9 1.17% 149 1.12% 148 0.83% 2.65%8 1.68% 162 1.33% 148 1.19% 3.41%7 1.72% 248 1.28% 148 1.37% 3.76%6 1.67% 297 1.22% 148 1.38% 3.92%5 1.77% 365 1.15% 295 1.17% 3.77%4 1.75% 392 1.12% 295 1.16% 3.95%3 1.70% 446 1.01% 295 1.15% 3.97%2 1.78% 459 1.01% 295 1.18% 3.94%1 1.77% 527 0.91% 295 1.14% 3.63%

V = 2286 k V = 807

Unbonded Braced Viscously Damped FrameT1 = 1.54 T 1 = 3.02


5/13

Figure 4. Average Inter-story Drift for ThreeStory Building 10% in 50yr Event

Figure 5. Average Floor Spectra for Three Story Building (3rd Floor) 10% in 50yr Event

0

50

100

150

200

250

300

350

400

450

500

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0Interstory drift %

S t o r y

H e

i g h t ( i n

)

Moment Frame

Unbonded Braced Frame

Viscous Damped Frame

Base Isolated

The inter-story drifts given in Figure 3 are plotted such that the average drift between the ground level

and the 2 nd story is plotted at the 2 nd floor level (156 inches height) while that between the 3 rd and 2 nd floor is plotted at the 3 rd floor level (312 inches height) etc. We can then take the maximum value of eachof these three floors and these are 0.3, 2.95, 2.33 and 2.12 for the base isolated braced frame, buckling-restrained braced frame, moment frame and viscously damped braced frame respectively. Thesemaximum results over three floors can then be normalized using one of the framing schemes and Figure 5

provides a plot of the maximum inter-story drifts of all six framing schemes normalized to the inter-storydrift of the base isolated braced frame. Note that the buckling-restrained braced frame has the maximumdrift and it is concentrated in the 2 nd floor. The buildings designed for the essential facility performance(R=3.5 for the buckling-restrained brace design and 220K for the viscous damped) reduce the maximumdrift by approximately 45%. If we had averaged the results of each of the three floors, the results would

be 0.3, 1.73, 1.77 and 1.80 for the base isolated braced frame, buckling-restrained braced frame, momentframe and viscously damped braced frame respectively and the concentration of drift at the 2 nd floor level

of the buckling-restrained braced frame would not have been so obvious in the comparisons. The meanvalues of the 3 floors are also consistent with the design values given in Table 1. The concentration ofdrift at the 2nd floor level in the 3 story building has been observed in other nonlinear time historyanalyses is not intuitively obvious when the design values of Table 1 are assessed.

In assessing the floor response spectra results of Figure 4 there are two variables that can be used forcomparison purposes. The first is the peak floor acceleration which is the floor acceleration at a period ofzero seconds and is the value that impacts all rigid elements at the upper floor levels of a building. The

peak floor acceleration results of the moment frame show a little more amplification than those of the buckling-restrained braced frame and the viscously damped frame indicating a lesser amount of inelasticresponse in the moment frames. A structure that remains essentially elastic would be expected to amplifythe ground PGA by a factor of 2.5 to 4 whereas structures responding inelastically have much less

amplification of the peak floor acceleration. The other variable of interest is the peak value of the averagefloor response spectra of Figure 4 as this provides some measure of the maximum forces that the flexiblecontents or equipment of a building will be subjected to. Note that the peaks in Figure 4 for the differentframing schemes all occur at different periods that are essentially the 1 st fundamental mode of the

particular framing scheme. These peak spectral acceleration values can then be normalized to the valuesof one of the framing schemes and Figure 6 provides a comparison of all framing schemes normalized tothe base isolated braced frame.

0

1

2

3

4

5

6

0 1 2 3 4 5 6T (sec)

S p e c

t r a l

A c c e l e r a

t i o n

A ( g )

Moment Frame

Ground Input

Braced Frame

Viscous Damped

Base isolated


6/13

Figure 5. Normalized Maximum Value of Inter-story Drifts for Three Story Building -10% in 50 yr Event

Figure 6. Normalized Peak Spectral Accelerations for Three Story Building (3rd floor)- 10% in 50yr Event

The peak spectral acceleration of the moment frame from Figure 4 is 5.5g and is much higher than the2.8g of the buckling-restrained brace (R=7) and the 2.1g of the viscously damped frame. The baseisolated braced frame produces significantly lower floor accelerations (0.3g) as well as inter-story drifts.What is interesting to note as we go from a code designed buckling-restrained braced frame (R=7) to anessential facility design (R=3.5) the maximum drifts (Figure 5) are reduced by 45% as expected but the

peak spectral floor accelerations (Figure 6) are increased substantially and are almost as high as those ofthe code designed moment frame. For the viscously damped frame as the damper force is increased froma code design (133k dampers) to an essential facility design (220k dampers) the drifts are also reducedapproximately 45% and the peak floor accelerations are also reduced; this is very desirable from a seismic

performance perspective.

In order to simultaneously assess both the drift and floor acceleration results, Figure 7 provides a plot ofthe peak value of the floor inter-story drift versus the peak spectral floor acceleration at the 3 rd floor for allsix of the framing schemes for the 10% in 50 year results. Clearly the closer to the origin of both the driftand acceleration axes the better the overall seismic performance of the structural system. Rather than

providing Figures 4 through 8 for the 50% in 50 year time histories, Figure 8 provides a summary plot ofthe peak floor inter-story drift versus the peak spectral floor acceleration at the 3 rd floor level for all six ofthe framing schemes for the 50% in 50 year results.

Figures 7 and 8 provide a very good summary of the relative performance of the four framing schemes forthe three story building. It is clear that the base isolated braced frame has significantly better performancethan all of the other framing scheme with inter-story drifts reduced by factors of 3 to 9 (Figures 6, 9 and10) and peak floor accelerations reduced by similar factors (Figure 8, 9 and 10) relative to the other threeframing schemes. From an inter-story drift perspective the buckling-restrained braced frame has theworst performance for the 10% in 50 year event due to the concentration of inter-story drift at the 2 nd floorlevel whereas for the 50% in 50 year event the moment frame has the worst inter-story drift performance.This is because for the more frequent lower level event the drift is not concentrated at the 1 st floor level ofthe buckling-restrained braced frame. The viscously damped frame performs better from a drift

perspective than both the buckling-restrained braced frame and the moment frame although onlymarginally better than the buckling-restrained brace for the 50% in 50 year event. When the inter-storydrift performance of the viscously damped and buckling-restrained braced frames designed as essentialfacilities are compared the viscously damped frame performs better for both design events.

0

3

6

9

12

1

Structure Type

D r i

f t / B a s e

I s o

l a t e d D r i

f t

Base Isolated

MomentFrame

UnbondedBrace

R=7

UnbondedBraceR=3.5

ViscousDamped

133K

ViscousDamped

220K

0.00

3.00

6.00

9.00

12.00

1

Structure Type

P e a

k S p e c

t r a

l A c c n .

/ B

a s e

I s o

l a t e d A c c n .

Moment Frame

UnbondedBrace R=7

UnbondedBraceR=3.5

ViscousDamped

133K

Base Isolated

ViscousDamped

220K


7/13

0

1

2

3

4

5

0 0.5 1 1.5 2 2.5

Peak Value of Drift - %H

P e a

k S p e c

t r a

l A c c e

l e r a

t i o n -

3 r d

F l o o r -

% g Moment Frame

Unbonded Brace - R7

Unbonded Brace - R3.5

Viscous Damped - 133K


Base Isolated

Figure 7. Peak Inter-story Drift vs. PeakSpectral Acceleration for Three Story Building -10% in 50 yr Event

Figure 8. Peak Inter-story Drift vs. Peak Spectral Acceleration for Three Story Building 50% in 50Yr Event

In assessing the acceleration performance, the moment frame has the worst performance because it hasless inelastic response for both of the design events. The viscously damped structures have better

performance than both of their buckling-restrained braced frame counterparts and this difference issignificantly better for the building designed as an essential facility (Figures 8, 9 and 10) for both designevents. It is very clear from Figures 9 and 10 that as a designer attempts to improve the drift performanceof a buckling-restrained braced frame by going from an R-Factor of 7 to 3.5, the acceleration performancedeteriorates significantly for both design events. For a viscously damped frame the opposite occurs - asthe inter-story drift performance is improved by increasing the force capacity of the dampers from 133kips to 220 kips the acceleration performance also improves for both design events. The viscouslydamped frame is significantly better than the buckling-restrained braced frame which in turn is better thanthe moment frame for both the 10% (475 Year) and 50% (72 Year) in 50 year design events.

In summary, for the three story building the base isolated braced frame has the best overall relative performance of the four framing schemes and by a significant margin. The viscously damped frame issignificantly better than the buckling-restrained braced frame which in turn is better than the momentframe for both the 10% and 50% in 50 year design events.

Figures 9 and 10 provide the summary plots of the peak floor inter-story drift versus the peak spectralfloor acceleration for the four framing schemes for the 10% in 50 year and 50% in 50 year results for thenine story building. There were no essential facility designed nine story buildings. For the nine story

building the base isolated braced frame again has the best overall relative performance of the four framingschemes. The viscously damped frame is significantly better than the moment frame which in turn is

better than the buckling-restrained braced frame for both the 10% and 50% in 50 year design events. Note that the relative performance of the buckling-restrained braced frame and moment frame arereversed for the nine story building when compared to the three story building performance It is alsonoteworthy that the acceleration performance of the viscously damped nine story building is much closerto that of the base isolated structure than the three story building. However the inter-story drift

performance of the viscously damped frame is a factor of 4 to 5 higher than the base isolated structure.

0

1

2

3

4

5

6

0 0.5 1 1.5 2 2.5 3 3.5Peak Value of Drift - % H

P e a

k S p e c

t r a

l A c c e

l e r a t

i o n -

3 r d

F l o o r -

% g

Moment Frame

Unbonded Brace - R7

Unbonded Brace - R3.5



Base Isolated


8/13

0

0.5

1

1.5

2

2.5

0 0.25 0.5 0.75 1

Peak Value of Drift - % H

P e a

k S p e c

t r a

l A c c e

l e r a

t i o n -

% g

Moment Frame

Unbonded Brace

Viscous Damped

Base Isolated

Figure 9. Peak Inter-story Drift vs. PeakStructural Acceleration for Nine Story Buildings 10% in 50yr Event

Figure 10. Peak Inter-story Drift vs. PeakStructural Acceleration for Nine Story Buildings 50% in 50yr Event

Required Tests of Buckling Restrained Braces

One of the important requirements for the buckling restrained braces (BRBs) is that the braces are able toaccommodate the displacements that result from the earthquake ground motion. The newly adopted AISCseismic provisions require that the braces have adequate performance, as demonstrated by tests, fordeformations corresponding to 2.0 times the design drift where the minimum design drift for these testrequirements is 0.01.

There have been a number of other analytical studies that have focused on the non-linear response ofBRBs and the results are very consistent. Table 3 summarizes the results of three recent studies (Sabelli2001, Fahnstock et. Al. 2003, Mayes et. al. 2005) of the non-linear time history behavior of 3, 6, 9 and 20story BRBs. The time histories used in the SGH (Mayes et. Al.) and Fahnstock et al. studies were thosedeveloped for the SAC project at the Los Angeles site. The mean and mean-plus-one-standard-deviationresults are remarkably similar for the range of design events included in each of the studies. The demandsfrom the 3-story buildings are generally the highest except for Sabellis 2% in 50 year results for his 6story building. An interesting issue is what inter-story drift demands should be used to determine the testrequirements for the braces from this relatively limited number of studies. Options and their implicationsinclude:

The maximum value from the 50% in 50 year event this would require a test requirement of2.3%

The mean, mean plus one standard deviation or maximum value from the 10% in 50 year event this would require a test requirement of 1.8%, 2.8% and 4.5% respectively.

The mean value from the 2% in 50 year event this would require a test requirement of 4.5%. The mean value of the near fault time histories this would require a test requirement of 3.0%

0

0.5

1

1.5

2

2.5

3

3.5

0 0.4 0.8 1.2 1.6

Peak Value of Drift - % H

P e a

k S p e c

t r a

l A c c e

l e r a

t i o n -

% g

Moment Frame

Unbonded Brace

Viscous Damped

Base Isolated


9/13

Table 3 Summary of Analysis Results

Design SGH Fahns tock e t al. Sabelli Sabe lli SGH SGHEvent 3 Story 3 Story 3 Story 6 Story 9 Story 20 Story

50% in 50 Mean 0.80% 0.70% 0.50%

Years Mean+Std. Dev. 1.40% 1.20% 0.70%

Maximum 2.30% 2.10% 1.20%

10% in 50 Mean 1.70% 1.80% 1.40% 1.60% 1.00% 0.80%

Years Mean+Std. Dev. 2.80% 2.20% 2.10% 2.20% 1.70% 1.10%

Maximum 4.50% 2.60% 3.10% 1.40%

2% in 50 Mean 2.90% 3.00% 4.50% 1.50% 1.40%Years Mean+Std. Dev. 4.60% 4.00% 6.60% 2.30% 1.80%

Maximum 6.90% 4.40% 3.40% 2.30%

Near Fault Mean3.00% 1.60% 1.10%

Mean+Std. Dev. 5.10% 2.70% 1.50%

Maximum 8.70% 5.40% 1.90%

The results of the 3-story building reported herein will be discussed in some detail, as they permit a focuson the detailed results of each story rather than looking at the mean results presented in Table 3. Thesestory-by-story results are important as they demonstrate that inelastic deformations may be moresignificant in the lowest story of a building than in the others levels. The 3-story building floor plan was6 bays by 4 bays with 30 ft. bay spacing and equal story heights of 13 ft. Figure 2. The seismic design

parameters of the 2000 IBC are shown in Tables 4, 5 and 6. Two buildings were designed with an S s =1.50, S 1= 0.60 and Soil Type D. One building used a 45 ksi yield stress for the BRB (Table 4) and the

other used a 30 ksi yield stress for the BRB (Table 5). The 3rd

building (Table 6) was designed for nearfault condition (S s=2.0 and S 1=0.75) and an I e factor of 1.5. This represents the strongest braces thatwould be required by the IBC for essential facilities.

Table 4 - 3 Story Building with R=7 and 45 Ksi Yield Strength Steel(IBC Design Parameters Ss=1.50, S1=0.60, Ie=1.0, Soil Type D)

BraceArea

BraceStrength

Design Displ.From Equiv.Lat. ForceProcedure

(in.)

InterstoryDispl. fromEquiv. Lat.

ForceProcedure

(in.)

xe (% )Drift fromEquiv. Lat.

ForceProcedure

Design Drift (%)

=(Cd/Ie)* xewhere Cd=5

2 * DesignDrift forTesingBRB's

AnalysisResults

10% in 50Year Dri ftResults -Avg of 5

Ratio of 10%in 50 YrDrift to

2*DesignDrift

3 3.00 135.0 1.18 0.38 0.24 1.22 2.44 1.18 0.48

2 5.25 236.3 0.80 0.38 0.24 1.22 2.44 1.05 0.431 5.25 236.3 0.42 0.42 0.27 1.35 2.69 2.95 1.10

Mean 1.26 2.52 1.73 0.67

Table 5 - 3 Story Building with R=7 and 30 Ksi Yield Strength Steel(IBC Design Parameters Ss=1.50, S1=0.60, Ie=1.0, Soil Type D)


10/13

BraceArea

BraceStrength

Design Dis pl.From Equiv.Lat. ForceProcedure

(in.)


ForceProcedure

(in.)

xe (% )Drift fromEquiv. Lat.

ForceProcedure

Design Drift (%)

=(Cd/Ie)* xewhere Cd=5


AnalysisResults

10% in 50Year DriftResults -Avg of 5

Ratio of 10 %in 50 YrDrift to

2*DesignDrift

3 4.50 135.0 0.80 0.26 0.17 0.83 1.67 0.88 0.53

2 7.88 236.4 0.54 0.26 0.17 0.83 1.67 1.18 0.71

1 7.88 236.4 0.28 0.28 0.18 0.90 1.79 2.88 1.60

Mean 0.85 1.71 1.65 0.95

Table 6 - 3 Story Building in Near Fault Region with Ie=1.50 and R=7 Design and 45 Ksi Yield Strength Steel(IBC Design Parameters Ss=2.0, S1=0.75, Ie=1.5, Soil Type D)

BraceArea

BraceStrength

Design Displ.From Equiv.Lat. ForceProcedure

(in.)


ForceProcedure

(in.)

xe (%)Drift fromEquiv. Lat.

ForceProcedure

Design Drift (%)

=(Cd/Ie)* xewhere Cd=5 and

Ie=1.5


AnalysisResults

Near FaultDrift

Results -Avg. of 5

Ratio of NearFault Dr ift to

2*DesignDrift

3 5.38 242.1 1.42 0.46 0.29 0.99 1.98 2.57 1.30

2 9.75 438.8 0.96 0.45 0.29 0.97 1.93 1.74 0.90

1 9.75 438.8 0.51 0.51 0.33 1.10 2.19 2.53 1.16

Mean 1.02 2.03 2.28 1.12

The far right hand column of Tables 4, 5 and 6 compares the actual drift obtained from the average of 5non-linear time history analyses for the 10% in 50 year design event with the test requirements assuming2 times the design drift. In the first two designs the non-linear response is concentrated in the 1 st story.This is probably due to the use of similar core plate sizes in both the 1 st and 2 nd stories not an optimaldesign but not an uncommon one. In the response of the near fault design there is a more a more uniformdistribution of inelastic deformation in the braces on the various floors. It should be noted in Table 5 thatthe lower bound value of the design drift for the purpose of establishing the test requirements is 1%. If weconsider the mean values of all the story drift values the for the conventional designs then the testrequirement of 2 times the design drift test requirement (minimum design value of 1%) is adequate but itis not adequate for the near fault design of Table 6. If it was desired that the test requirement enveloped

the average values of drift at each story level then the test requirement should be increased to 3 times thedesign drift a 50% increase over the current requirement. Another approach that maybe favored by thefabricators is to require that the test deformations correspond to a specific 3% drift requirement. Note thatseveral of the non-linear time history results are close but just below a drift of 3%. This would be similarin concept to the testing requirement for SMRF connections.

The ductility demands on the BRB are a function of the yield stress of the material, the bay width andstory height and the length of the connections of the BRB. The ductility demands for a variety ofvariables (bay width of brace B, connection length of brace, story height of 13 ft.) are plotted in Figure 13for a drift demand of 3%. In order for a fabricator to qualify a brace for a variety of buildingconfigurations they would need to demonstrate that they could achieve a maximum ductility capabilityshown in Figure 13 (i.e. 30) if they wanted to cover all possible combinations of bay widths and

connection lengths for a 13 ft. story height.

Summary and Conclusions

This paper has presented a summary of the comparative seismic performance (inter-story drifts and floorresponse spectra) of three- and nine-story buildings, each designed with four different structural systemsdesigned following the 1997 UBC provisions. The four framing schemes were a buckling-restrained

braced frame, viscously damped frame, moment frame and a base isolated braced frame. For the three-story building, the buckling-restrained braced frame and the viscously damped frame were also designed


11/13

as essential facilities where the R-Factor for the buckling-restrained braced frame was reduced from 7 to3.5 and the force capacity of the viscous dampers at 15 in/sec. were increased from 133 kips to 220 kips.Each of the building models was analyzed as a complete non-linear structure using a total of ten timehistories. One set of five time histories was representative of a 50% in 50 year (72 year return period)earthquake, while the other set was representative of a 10% in 50 year (475 year return period)earthquake. Both sets were developed for the Los Angeles area. The results of each set of five wereaveraged and reported separately.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

4.5 4 3.5 3 2.5 2

Connection Length, each end (ft.)

B R B D u c

t i l i t y D e m a n

d

B=10B=12B=15B=20B=25B=30

Figure 13 Ductility Demand Ratios for Drift ratio of 3%

Two equally important variables were assessed in order to evaluate the seismic performance of thestructural framing systems. The first and almost universal variable for comparing seismic performance isthe inter-story drift. This is a code design parameter and is something most engineers focus upon duringthe design process. From a damageability perspective it is a measure that impacts damage to the framingsystem, building faade and windows, partitions, piping and ductwork. One interesting result that wasobtained from the non-linear time history analyses was the concentration of inter-story drift that occurredat the 2 nd story of the three-story buckling-restrained brace building. Although not presented herein thedrifts in the 2 nd story became quite significant (>5%) for the near-fault and 2500 year time histories.

The other key parameter from a performance perspective is the floor acceleration as characterized by thefloor response spectra. This is almost never assessed as part of the design process, because it requires atime history analysis to obtain it. From a damageability perspective it is the measure that impacts damageto the ceiling and lights, electrical and mechanical equipment, elevators and the building contents.

The seismic performance of both the three and nine-story base isolated structures were shown to besignificantly better than the other three framing schemes. The viscously damped three- and nine-storyframes were in turn much better than both the buckling-restrained braced frames and the moment frames.The three story buckling-restrained braced frame had better performance than the three story momentframe building whereas the reverse was true for the nine story buildings.

The results clearly show the importance of assessing both the inter-story drift and the floor accelerationswhen comparing the relative seismic performance of different structural systems. Had only inter-story


12/13

drift been used to compare the seismic performance there would not have been such a clear cut difference between the viscously damped frame, the buckling-restrained braced frame and the moment framealthough the base isolated braced frame would still have been the best performing system by a significantmargin.

As the profession progresses with performance based design we will eventually be able to convert thevalues of inter-story drift and floor accelerations presented herein to the dollar cost of damage andresulting downtime; these measures of performance will be more meaningful to the owners of buildings.However, until that time occurs, it is important that the structural engineering design profession assess

both inter-story drift and acceleration performance when selecting a structural system for buildings andespecially for essential facilities such as hospitals, high tech manufacturing facilities, essential service

buildings etc. that should be operable after a significant event. The base isolated building had the best performance by a significant margin and the viscously damped system was significantly better than boththe buckling-restrained braced fame and moment frame.

It is also clear that if a buckling-restrained brace system is being considered for a project that requireshigher performance than life safety then a viscously damped frame should be considered as a designalternate as its seismic performance has been shown to be significantly better and no other layout or

architectural changes are required.

Some members of our design profession are proposing the use of buckling-restrained brace systems forstructures that require higher performance capabilities, going so far to state that base isolation has givenway to BRBs as the new technology for higher predictable performance. This statement demonstrates areal lack of awareness of what causes the large dollar costs of earthquake damage as well as theunfortunate fact the our design codes do not require a designer to assess floor accelerations as part of thedesign process. Structural engineers should consider acceleration performance rather than just inter-storydrift in the selection of the structural system, especially when higher performance is desired.

There also appears to be a need to increase recently agreed upon AISC inter-story drift demands for thetesting of BRBs from 2 times the design drift (minimum of 2% drift) to 3 times the design drift or 3% of

the story height. This is a critical performance issue especially for buildings less than 6 stories in heightwhere it appears that the inter-story drift may concentrate in the lower level of the building. In summary,the results presented herein indicate that the performance of a building and its contents is dependent uponthe structural system chosen for the building.


13/13

References

1. SAC, 2000a, Recommended Seismic Design Criteria for New Steel Moment-Frame Buildings, prepared by the SAC Joint Venture, a partnership of the Structural Engineers Association of California, the appliedTechnology Council, and California Universities for Research in Earthquake Engineering; published bythe Federal Emergency Management Agency (FEMA- 350 Report), Washington, DC.

2. BSSC, 2001, NEHRP Recommended Provisions for Seismic Regulations for New Buildings and OtherStructures (2000 Edition) , prepared by the Building Seismic Safety Council; published by the FederalEmergency Management Agency (FEMA 368 Report), Washington, DC. Universities for Research inEarthquake Engineering; published by the Federal Emergency Management Agency (FEMA- 351Report), Washington, DC.

3. Sabelli, R., 2001 Research on Improving the Design and Analysis of Earthquake Resistant SteelBraced Frames, EERI/FEMA NEHRP Fellowship report, Oakland, California.

4. Fahnstock, L.A., Sause, R., Ricles, J.M. and Le-Wu Lu 2003, Ductility Demands on Bucklingrestrained Braced frames Under Earthquake Loading, Earthquake Engineering and engineeringVibration, Vol.2, No. 2 December 2003.

5. Mayes, R.L., Goings, C.B., Naguib, W.I. and Harris, S.K. 2005, Comparative seismic Performance ofFour Structural Systems, Proceedings of 2005 SEAOC Convention, San Diego Ca., September 2005.

Documents

Comparative SeismicPerformance of Four Structural Systems