Dasse_Cost_Study_of_BRBF.pdf

7/29/2019 Dasse_Cost_Study_of_BRBF.pdf

1/9

May 27, 2009

COST ADVANTAGES OF

BUCKLING RESTRAINED BRACED FRAMEBUILDINGS

INTRODUCTION


2/9

May 27, 2009 1

Through the development of hypothetical model buildings, this study investigates the potential material savingsand cost advantages of BRBF systems relative to Special Concentrically Braced Frame (SCBF) systems. Modelbuildings of two heights are developed for each of the lateral force resisting systems. Elements of the lateral forceresisting system are designed for each scenario.

While Buckling Restrained Braced Frames (BRBF) have been tested and used in practice for some time, the lack ofprovisions for their design in most building codes could make gaining jurisdictional approval somewhat

cumbersome. Owing largely to the efforts of AISC and the Structural Engineers Association of California(SEAOC), design provisions can now be found in recent editions of ASCE 7 and AISC 341.

Design seismic forces are lower for each of the BRBF buildings than their SCBF counterparts. This results fromcode provisions recognizing the favorable ductility of BRBFs as well as their higher fundamental periods.

Estimates of material quantities and costs of the model buildings show that structures utilizing the BRBF systemcan be more economical than those utilizing SCBF systems, despite higher costs. The study also indicates thatamount of these savings is related to building height.

ASSSUMPTIONS AND DESIGN CRITERIA

The model buildings developed for the purposes of this study are regular, steel frame and composite deck structureswith lateral force resisting systems located at the perimeter walls. The model buildings are assumed to be located in

downtown Los Angeles, CA and of office occupancy.

Code: 2006 IBC, ASCE 7-05, AISC 341-05

Building Location: Los Angeles, CA

Seismic Design Category: D

Occupancy Category: II (Office)

Importance Factor: 1.0

Short Period Spectral Acceleration, Ss: 2.2

1 s Period Spectral Acceleration, S1: 0.74

Fa: 1.0

Fv: 1.5

Analysis Procedure: Equivalent Lateral Force

Allowable Soil Bearing Pressure: 5000 psf

Allowable Pile Capacity: 200 k

Allowable Pile Uplift Capacity: 70 k

Design Coefficients and Factors:

System R 0BRBF 8.0 2.5SCBF 6.0 2.0


3/9

May 27, 2009 2

BUCKLING RESTAINED BRACED FRAME SYSTEMS

Buckling restrained braced frames (BRBFs) are a type of braced frame in which the braces consist of a steel coreand an outer casing. Brace axial forces are resisted only by the steel core, which is restrained from buckling by theouter shell.

As the core is restrained from buckling and the development of plastic hinges, it can be expected to developcompression-yielding similar to its tension-yielding. This characteristic results in a highly ductile lateral forceresisting system, which is recognized by the higher response modification coefficient, R, in ASCE 7-05.

Traditional SCBF systems have bracing members with relatively long unbraced lengths and therefore large areas. Asthe load-bearing core in a BRBF brace is continuously braced it typically has a smaller cross sectional area and canbe more flexible than a SCBF brace. The resulting BRBF system has a higher flexibility and will often be drift-controlled. This flexibility can also yield a higher fundamental building period.

Model Buildings

The model buildings developed to investigate the BRBF and SCBF systems are rectangular structures with fourperimeter braced frames. The three-story buildings are 66,000 gsf, the six-story buildings are 132,000 gsf. Typical

building floor plans and frame elevations are shown in Figure 1.

Figure 1: Model Building Floor Plan and Braced Frame Elevations

Lateral Analysis

As anticipated, the BRBF buildings are found to have lower design base shear coefficients than the SCBF buildings.This results both from a higher response modification coefficient (R) as well as a higher fundamental buildingperiod. The building code recognizes the higher flexibility of the BRBF system through a higher Ct factor, yieldinga higher calculated period.

The effect of each model buildings period on spectral acceleration is demonstrated in Figure 2; both heights ofBRBF buildings have lower accelerations than their SCBF counterparts.


4/9

May 27, 2009 3

Figure 2: Response Spectrum

The lower spectral response acceleration, along with the higher Rvalue, yields lower BRBF base shear coefficientsfor both building heights:

Six-Story BRBF: Cs= 0.08 W (T= 1.23 s)Six-Story SCBF: Cs= 0.16 W (T= 0.76 s)

Three-Story BRBF: Cs= 0.13 W (T= 0.73 s)Three-Story SCBF: Cs= 0.24 W (T= 0.35 s)

Lateral forces are distributed vertically and horizontally according to code provisions, including accidental torsion.As each building is symmetrical with frames of similar size and configuration, one frame is designed for each modelbuilding.

Regular in plan and with braced frames located at the perimeter of the buildings, the redundancy factor ( ) is equalto 1.0 for the four model buildings as determined by analysis.

Design diaphragm and collector forces for each of the buildings are governed by the minimum, location dependant

requirements. As a result, deck and collector designs are identical for each of the similar buildings.

Model Building Designs

Having lower design base shears, buckling restrained braced frame member sizes were typically found to be lighterthan those required for the SCBF buildings. Additional savings are realized by the BRBF system as the beamsintersected by braces in chevron configurations are not required to be designed for unbalanced brace loading.Framing sizes are summarized in Table 1.


5/9

May 27, 2009 4

Table 1: Member Sizes

Shallow and deep foundation systems were designed for each of the model buildings. As a result of the higheroverturning forces, the SCBF foundation demands are significantly larger. For taller SCBF buildings on weakersoils this could require that additional bays of bracing be added to reduce the foundation sizes to a more reasonableand economical size.

Table 2: Foundation Design

As bracing connections are designed to exceed expected brace strength, the smaller net area characteristic ofbuckling restrained braces can yield lower connection design forces. Material savings are realized through reducedgusset plate sizes and weld lengths. Typical BRBF and SCBF connection details are shown in Figure 3.


6/9

May 27, 2009 5

Figure 3: Connection Details

Material Quantities and Costs

A summation of the material quantities finds that significant savings can be realized by the BRBF systems. WhileBRBF bracing members can be more expensive than HSS sections, the cost is offset by material savings in the

columns, frame beams, connections and foundation.As unit material and construction costs tend to vary significantly from project to project, the costs in table 3 arebased upon average values seen in projects of this size. The costs and material quantities listed represent theamounts required in addition to that required for the gravity load carrying system.

Table 3: LFRS Material Quantities and Costs


7/9

May 27, 2009 6

The cost savings generated by the BRBF systems is more significant at taller buildings, as the greater quantities ofmaterial utilized offsets the premium paid for the BRBF members. In addition, the period and base shearadvantage of BRBF buildings to SCBF buildings increases with building height. Figure 4 demonstrates LFRS costrelative to building height for each of the model buildings.

Figure 4: LFRS Cost Relative to Building Height


8/9

May 27, 2009 7

The total structure cost for the 6 story SCBF building on piles is estimated to be $5 M ($38/sf), with the LFRSbeing $930 k ($7 /sf) of that amount. The $611 k ($4.6/sf) cost of the BRBF system corresponds to a unit savingsof $2.40/sf.

Table 4: Unit Costs and Savings

As shown in Figure 5, the BRBF system can yield a 34% saving in LFRS cost for the 6 story buildings on deepfoundations.

Figure 5: Relative Cost of LFRS Elements

6 Story Building with Pile Foundation

CONCLUSION

Lateral-force-resisting systems with buckling restrained braces can yield significant structural cost savings overconventional SCBF systems. These savings are the result of decreased material quantities and foundation demands

due to the reduced base shear and brace areas. The savings increase with building height, as the greater quantitiesof materials offset the more expensive braces.

The recent inclusion of BRBF systems in building codes has made their design and approval an easier process. Thesystem is considered to have favorable seismic performance over traditional braced frmaes, making it an attractiveoption to structural engineers. This, in concert with the potential cost savings, is also making BRBFs an attractiveoption to building owners and developers. As designers, building departments, contractors, and material suppliersbecome more familiar with this systems the benefits should only increase.


9/9

May 27, 2009 8

REFERENCES

ASCE 7-05, Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers,Reston, VA

AISC 341-05, Seismic Provisions for Structural Steel Buildings, American Institute of Steel Construction, Chicago, IL

IBC, International Building Code, 2000 Edition, International Code Conference, Country Club Hills, IL.

Documents

Dasse_Cost_Study_of_BRBF.pdf