Seismic Rehabilitation of Extreme SoftStory School Building with Friction Dampers Using the ASCE 41

1
Seismic Rehabilitation of Extreme Soft-Story
School Building with Friction Dampers Using the
ASCE 41 Standard

• Presented by Paul E. Oyen

2
Overview

• Description of Building
• Project Background
• Two Solution Procedures
• Code-Based
• Performance-Based (ASCE 41)
• Friction Dampers
• Comparison of Solutions
• Conclusions

3
Description of Building

• Glendale HS - Glendale, CA
• Built in 1968
• Administrative Offices Library
• Structural System
• Roof
• Concrete Slab
• Steel Girders
• 2nd Story
• Perimeter Bearing Walls
• Reinforced Masonry Concrete
• 2nd Floor
• Reinforced Concrete Pan-Joist
• 1st Story
• Reinforced Concrete Columns
• Foundation
• Drilled-piers
• 18f x 30 deep
• Groups of 2-4 _at_ each column

4
Project Background

• Following the 1994 Northridge Earthquake, the
  soft-story condition is identified
• In 2006, Glendale USD funds rehabilitation as
  part of overall campus modernization.
• Architectural requirements dictate perimeter
  braces for any strengthening stiffening.
• Project falls under DSA jurisdiction as a
  voluntary seismic upgrade.

5
Deficiencies

• Global
• Soft-Story
• Weak-Story
• Local (Non-Ductile Detailing)
• Little confinement in columns
• Shear critical beams and columns
• No top bars in pile caps
• Partial height reinforcement in drilled-piers

6
Code-Based Solution

• Design Steps
• Size steel braces to eliminate code-defined
  soft-story
• Check braces for elastic design forces
• Check existing foundations for elastic design
  forces
• Design foundation strengthening as necessary
• Detail braces per code
• Design brace frame connections to building with W
  forces
• Analysis
• Linear Dynamic
• ETABS

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Code-Based Solution

• Results
• Added (10) concentric steel braces frames
• W14x233 W14x311 braces
• Add (20) drilled piers to existing pier groups
• Add new pier groups at each of (4) corners
• Required frame to existing foundation connection
  strength could not be attained.

8
Performance-Based Solution (ASCE 41)

• Design criterion
• Basic Safety Objective for structural components
• Life Safety in 10/50-yr event
• Collapse Prevention in 2/50-yr event
• Design Steps
• Define desired yield mechanism
• Add ductility to yielding components
• Add stiffness to building
• Analyze check deformation limits
• Iterate to solution
• Analysis
• Nonlinear Static (pushover)
• PERFORM-3D

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Friction Dampers

• Two steel plates sandwiched together by
  pretensioned bolts in long slotted holes
• Calibrated to slip at specified axial load
• Provide elastic-perfectly- plastic behavior
• Two functions in this design
• Allow large displacements without buckling brace
• Limit force on foundation and connection to
  structure
• ASCE 41 does not allow added damping in pushover
  analysis for friction dampers.

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Performance-Based Solution (ASCE 41)

• Results
• Add ¼ steel jackets to (20) columns
• Add braces at each corner with friction dampers
• Friction dampers have 200-kip slip load /- 4½
  displacement capacity
• Selectively cut longitudinal column bars to
  protect against shear failure

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Comparison of Solutions

• Code-Based
• Conventional technology
• Elastic analysis
• 64,000 lbs of steel
• (32) new drilled piers
• Frames could not be adequately connected

• Performance-Based
• Additional testing approval required
• Nonlinear analysis required
• 30,000 lbs of steel
• (12) new drilled piers
• Predictable behavior ensured

12
Conclusions

• Prescriptive code requirements led to a costly
  and infeasible solution.
• Performance-based design concepts allowed for
  reduced construction costs and improved seismic
  performance.

13
Acknowledgements

• Skip Freet
• Craig Goings
• Ron Hamburger
• James Parker
• Jaime Rosenbach

14

• Questions?