COST ADVANTAGES OF BUCKLING RESTRAINED BRACED FRAME BUILDINGS

Transcription

1 COST ADVANTAGES OF BUCKLING RESTRAINED BRACED FRAME BUILDINGS May 27, 2009 INTRODUCTION

2 Through the development of hypothetical model buildings, this study investigates the potential material savings and cost advantages of BRBF systems relative to Special Concentrically Braced Frame (SCBF) systems. Model buildings of two heights are developed for each of the lateral force resisting systems. Elements of the lateral force resisting system are designed for each scenario. While Buckling Restrained Braced Frames (BRBF) have been tested and used in practice for some time, the lack of provisions 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 from code provisions recognizing the favorable ductility of BRBF s as well as their higher fundamental periods. Estimates of material quantities and costs of the model buildings show that structures utilizing the BRBF system can be more economical than those utilizing SCBF systems, despite higher costs. The study also indicates that amount 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 structures with 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 Building Location: Los Angeles, CA Seismic Design Category: D Occupancy Category: II (Office) Importance Factor: 1.0 Short Period Spectral Acceleration, S s : s Period Spectral Acceleration, S 1 : 0.74 F a : 1.0 F v : 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 Ω 0 BRBF SCBF May 27,

3 BUCKLING RESTAINED BRACED FRAME SYSTEMS Buckling restrained braced frames (BRBF s) are a type of braced frame in which the braces consist of a steel core and an outer casing. Brace axial forces are resisted only by the steel core, which is restrained from buckling by the outer shell. As the core is restrained from buckling and the development of plastic hinges, it can be expected to develop compression-yielding similar to its tension-yielding. This characteristic results in a highly ductile lateral force resisting system, which is recognized by the higher response modification coefficient, R, in ASCE Traditional SCBF systems have bracing members with relatively long unbraced lengths and therefore large areas. As the load-bearing core in a BRBF brace is continuously braced it typically has a smaller cross sectional area and can be more flexible than a SCBF brace. The resulting BRBF system has a higher flexibility and will often be driftcontrolled. 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 four perimeter 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 building period. The building code recognizes the higher flexibility of the BRBF system through a higher C t factor, yielding a higher calculated period. The effect of each model building s period on spectral acceleration is demonstrated in Figure 2; both heights of BRBF buildings have lower accelerations than their SCBF counterparts. May 27,

4 Figure 2: Response Spectrum The lower spectral response acceleration, along with the higher R value, yields lower BRBF base shear coefficients for both building heights: Six-Story BRBF: C s = 0.08 W (T = 1.23 s) Six-Story SCBF: C s = 0.16 W (T = 0.76 s) Three-Story BRBF: C s = 0.13 W (T = 0.73 s) Three-Story SCBF: C s = 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 model building. Regular in plan and with braced frames located at the perimeter of the buildings, the redundancy factor ( to 1.0 for the four model buildings as determined by analysis. ) is equal 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 lighter than those required for the SCBF buildings. Additional savings are realized by the BRBF system as the beams intersected by braces in chevron configurations are not required to be designed for unbalanced brace loading. Framing sizes are summarized in Table 1. May 27,

5 Table 1: Member Sizes Shallow and deep foundation systems were designed for each of the model buildings. As a result of the higher overturning forces, the SCBF foundation demands are significantly larger. For taller SCBF buildings on weaker soils this could require that additional bays of bracing be added to reduce the foundation sizes to a more reasonable and economical size. Table 2: Foundation Design As bracing connections are designed to exceed expected brace strength, the smaller net area characteristic of buckling restrained braces can yield lower connection design forces. Material savings are realized through reduced gusset plate sizes and weld lengths. Typical BRBF and SCBF connection details are shown in Figure 3. May 27,

6 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. While BRBF 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 are based upon average values seen in projects of this size. The costs and material quantities listed represent the amounts required in addition to that required for the gravity load carrying system. Table 3: LFRS Material Quantities and Costs May 27,

7 The cost savings generated by the BRBF systems is more significant at taller buildings, as the greater quantities of material utilized offsets the premium paid for the BRBF members. In addition, the period and base shear advantage of BRBF buildings to SCBF buildings increases with building height. Figure 4 demonstrates LFRS cost relative to building height for each of the model buildings. Figure 4: LFRS Cost Relative to Building Height May 27,

8 The total structure cost for the 6 story SCBF building on piles is estimated to be $5 M ($38/sf), with the LFRS being $930 k ($7 /sf) of that amount. The $611 k ($4.6/sf) cost of the BRBF system corresponds to a unit savings of $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 deep foundations. 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 over conventional 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 quantities of materials offset the more expensive braces. The recent inclusion of BRBF systems in building codes has made their design and approval an easier process. The system is considered to have favorable seismic performance over traditional braced frmaes, making it an attractive option to structural engineers. This, in concert with the potential cost savings, is also making BRBF s an attractive option to building owners and developers. As designers, building departments, contractors, and material suppliers become more familiar with this systems the benefits should only increase. May 27,

9 REFERENCES ASCE 7-05, Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers, Reston, VA AISC , 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. May 27,