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DESIGN AND PERFORMANCE EVALAUATION OF CONCENTRIC BRACED FRAME USED

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DESIGN AND PERFORMANCE EVALAUATION OF CONCENTRIC BRACED FRAME USED Powered By Docstoc
					  DESIGN AND PERFORMANCE EVALAUATION OF
CONCENTRIC BRACED FRAME USED AS A SEISMIC LOAD
            RESISTING SYSTEM (SLRS)


                 Presentation by:
                  Devin Huber




              CE 697R Class Project
                December 5, 2006
            PRESENTATION OVERVIEW

•   Introduction of Project
•   Overview of SLRS Design
    – Procedure used, comments on design
    – Design members and connection detail
    – Problems encountered in design
•   Summary of Performance Evaluation and Results
    – Approach used for evaluation
    – Unique problems encountered for SLRS used
•   Summary and Conclusions
          INTRODUCTION OF PROJECT

•   Design and evaluation of concentrically braced frame
    (CBF) used as SLRS
•   SLRS part of a structure located in Northridge, CA
    – Location is in highly seismic area
•   Intent of project was to design structure according to
    relevant AISC and ASCE specs and evaluate the
    performance of the design using accepted procedure
    – Procedure used for performance evaluation was non-linear
       static pushover analysis
          INTRODUCTION OF PROJECT

•   Structure Description
    – 10 story office building
    – Regular and symmetric structure
             OVERVIEW OF SLRS DESIGN

•   Determination of forces
    – Gravity forces and Seismic forces determined per ASCE 7-
      05
    – Seismic forces determined using Equivalent Lateral Force
      Procedure

      1.4 D                       (1)    Story Level   Distribution   Total Lateral Force
                                                       of Force at      at Story (kips)
      1.2 D + 0.6 L + 0.5 Lr      (2)                  Story (kips)
                                             10            397               397
      1.2 D + 1.6 Lr + L          (3)        9             384               781
                                             8            305.5             1086.5
      1.2 D + 1.6W + L + 0.5 Lr   (4)        7             236              1322.5
                                             6             175              1497.5
      1.2 D + 1.0 E + L           (5)        5             124              1621.5
                                             4              81              1702.5
      0.9 D + 1.6W                 (6)       3              47              1749.5
                                             2              23              1772.5
      0.9 D + 1.0 E                (7)       1               7              1779.5
          OVERVIEW OF SLRS DESIGN
•   Design forces on members were determined with FEM software
    SAP 2000
    – Previously mentioned load combinations used
    – Two dimensional wire frame model used
    – Only exterior frames modeled
    – P-delta effects were accounted for
          OVERVIEW OF SLRS DESIGN
•   Design forces on members were determined with FEM software
    SAP 2000
    – Previously mentioned load combinations used
    – Two dimensional wire frame model used
    – Only exterior frames modeled
    – P-delta effects were accounted for
          OVERVIEW OF SLRS DESIGN
•   Design forces on members were determined with FEM software
    SAP 2000
    – Previously mentioned load combinations used
    – Two dimensional wire frame model used
    – Only exterior frames modeled
    – P-delta effects were accounted for
     DESIGN OF STRUCTURAL MEMBERS

•   Columns, braces, and girders were designed within SLRS
•   Relevant AISC Specifications were used for design
•   Member design considered both strength and drift limits
•   Important to note that design was done using a LINEAR
    static analysis
    – How does this capture non-linear dynamic effects that would
       actually occur in seismic event?
    – Design philosophy is simplification of what actually is
       happening
     DESIGN OF STRUCTURAL MEMBERS
•   Columns in braced frames
    - Columns chosen were seismically compact
    - Columns are designed with strong column – weak beam
      philosophy
       - Pu/φPn<0.4 than use ρ=1.3
       - Pu/φPn>0.4 than use Ω=2
     DESIGN OF STRUCTURAL MEMBERS
•   Columns in braced frames
    - Columns chosen were seismically compact
    - Columns are designed with strong column – weak beam
      philosophy
       - Pu/φPn<0.4 than use ρ=1.3
       - Pu/φPn>0.4 than use Ω=2
     DESIGN OF STRUCTURAL MEMBERS

•   Girders in SLRS
    - Members were seismically compact
    - Designed with load combinations using overstrength
      factor
    - Assumed lateral bracing at 10’ spacing
    - Very little unbalanced load on girders due to two story
      X-bracing that was used
     DESIGN OF STRUCTURAL MEMBERS

•   Girders in SLRS
    - Members were seismically compact
    - Designed with load combinations using overstrength
      factor
    - Assumed lateral bracing at 10’ spacing
    - Very little unbalanced load on girders due to two story
      X-bracing that was used
    DESIGN OF STRUCTURAL MEMBERS
•   Braces in SLRS
    - Designed to be ‘fuses’ in the structure
    - Designed with load combinations using ρ factor
    - Designed to be seismically compact and meet AISC
      Slenderness requirements
    - For connection design the strength was multiplied by Ry
      factor (1.1)
    DESIGN OF STRUCTURAL MEMBERS
•   Braces in SLRS
    - Designed to be ‘fuses’ in the structure
    - Designed with load combinations using ρ factor
    - Designed to be seismically compact and meet AISC
      Slenderness requirements
    - For connection design the strength was multiplied by Ry
      factor (1.1)
              DRIFT CONSIDERATIONS

•   Last aspect checked for design was drift
•   Calculated earthquake forces were used to check drifts
•   Deflections were initially obtained in SAP model
•   Deflections were then increased by amplification factor
    specified in ASCE 7-05 (5 for SCBF)
•   Calculated inter-story drifts checked against allowable drift
    of 2% of story height
              DRIFT CONSIDERATIONS

•   Last aspect checked for design was drift
•   Calculated earthquake forces were used to check drifts
                        Total Height Amplified Interstory Allowable
                Floor
•                           (ft)         Drift (in.)
    Deflections were initially obtained in SAP model
                 1           15              1.2
                                                          Drift (in.)
                                                              3.6
                  2          27              1.5              2.9
•   Deflections were then increased by amplification factor
                  3
                  4
                             39
                             51
                                             1.6
                                             2.2
                                                              2.9
                                                              2.9
                  5          63              2.2              2.9
    specified in ASCE 7-05 (5 for SCBF)
                  6          75              2.4              2.9
                  7          87              2.5              2.9
•   Calculated inter-story drifts checked against allowable drift
                  8
                  9
                             99
                            111
                                             2.7
                                             2.6
                                                              2.9
                                                              2.9
                 10         123              2.3              2.9
    of 2% of story height
FINAL MEMBER DESIGNS


      Girders have little
      unbalanced force to resist


       W shapes used for braces
       due to large seismic loads
       generated

        Strong Columns – take
        large seismic loads
                  CONNECTION DETAIL

•   Show detailed connection at first floor
CONNECTION DETAIL
CONNECTION DETAILS
CONNECTION DETAILS
CONNECTION DETAILS
                OVERALL COMMENTS

•   Members in braced frames were resisting very high forces
    and were thus ‘bulky’ members
    – Designs could have been further optimized by putting braced
       frames in more bays
•   Using ‘bulky’ members lead to complex connections
    requiring much capacity
    – Connections would be very costly to fabricate and erect
•   SCBF is likely not the most efficient SLRS that could be
    used in this application
    PERFORMANCE EVALUATION OF SLRS
•   Performance evaluation was conducted on SLRS following
    design of system
•   Static non-linear (pushover) analysis conducted on system
    – Guidelines from FEMA 356 were used in analysis
    – SAP 2000 software was used to conduct analysis
NON-LINEAR STATIC PUSHOVER ANALYSIS

•   Force-deformation relationships for members used were
    those prescribed in Table 5-6 and 5-7 in FEMA 356
MEMBER FORCE-DEFORMATION RELATIONSHIPS
MEMBER FORCE-DEFORMATION RELATIONSHIPS
NON-LINEAR STATIC PUSHOVER ANALYSIS

•   Overall Frame behavior is idealized into bi-linear
    relationship
•   Target displacement is calculated
    – Two load cases are considered: (1) uniform loading and (2)
       modal type loading
•   Performance criteria are checked against behavior at
    target displacement
NON-LINEAR STATIC PUSHOVER ANALYSIS
MEMBER FORCE-DEFORMATION RELATIONSHIPS

•   Problems with force-deformation relationship of braces
    – Negative post buckling stiffness defined in braces caused
      numerous convergence problems within SAP 2000
    – To try and deal with this, model was run with two different
      assumptions
        • One model defined elastic-plastic behavior for compression
          where strength was defined as residual strength
        • Other model defined ‘softer’ post buckling curve
BRACE FORCE-DEFORMATION MODELS

           Ft




                       Δ
NON-LINEAR STATIC PUSHOVER RESULTS
•   Elastic-Plastic Compressive Brace Model – Modal Load
NON-LINEAR STATIC PUSHOVER RESULTS

Development of hinges – Modal Load Distribution (EP Model)
NON-LINEAR STATIC PUSHOVER RESULTS
What happens if we include initial buckling strength of brace?
                     COMMENTS ON ANALYSIS

•   This EP model is conservative for brace capacity since it does not
    consider initial buckling strength of braces
     – This assumptions leads to underestimating forces on beam and columns
•   EP Model predicts overall deformation capacity of SLRS adequately,
    however target displacement is overestimated
     – Under predicts effective stiffness of structure
•   Including initial buckling strength increases effective stiffness but
    model cannot converge to final displacement
     – SAP 2000 has problem in dealing with negative stiffness in this model
                             NLSP Conclusions

•   Modeling SCBF structure for NLSP analysis is difficult
     – Difficulty arises from negative stiffness occurring in braces after buckling
     – Simplifying assumptions can be made for better convergence
•   The structure at hand appears to be ‘ok’ from a performance
    perspective
•   The moral of the story is that SAP 2000 cannot effectively handle the
    negative stiffness occurring in the structure
     – We must ‘massage’ the model to get it to work for us
     – What are other possible ways to handle the problem at hand?
QUESTIONS? COMMENTS? CONCERNS?

				
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