1. A New Approach to Increasing Seismic Resistance of Wood-Frame Structures
C. E. Meyer, United States Geological Survey
Thesis defense:
SEAOC Convention
Seattle, Washington
September 11, 2015

2. Acknowledgements Co-Authors:
Steve Yang, graduate student, Dept. of Civil and Env. Engineering, Rensselaer Polytechnic Institute (RPI)
Michael Symans, Assoc. Prof., Dept. of Civil and Env. Engineering, RPI
David Lee, Taylor Devices
DamperFrame Testing:
Gilberto Mosqueda, Assoc. Prof., Structural Engineering, UC San Diego
Alireza Sarebanha, graduate student, Structural Engineering, UC San Diego

3. Background
In recent years, there has been growing interest in increasing the seismic resistance of existing wood-framed buildings, especially those with identified structural deficiencies such as weak story or open front structures.
Two major California cities; San Francisco and Los Angeles, have a significant stock of older light-framed wood multi-family residential structures that exhibit the weak story deficiency.
These structures are not only a potential threat to life safety, but also threaten the resiliency of the respective communities and their ability to recover from a significant earthquake.
San Francisco is two years into their “Mandatory Soft Story Retrofit Ordinance”.
In addition, the city of Los Angeles recently proposed legislation to mandate the retrofit of soft story apartment structures within five years.

4. Background

5. Background
Looking to improve the expected seismic performance of wood-framed construction, there have been a number of recent large-scale research projects within the United States including the CUREE-Caltech Woodframe Project, the NEES-Wood Project (van de Lindt et al. 2010) and the NEES-Soft Project (van de Lindt et al. 2014a).
While looking at a number of different methods to strengthen wood structures, all three of these projects included research on the application of energy dissipation devices (such as viscous dampers) for seismic strengthening (Symans et al. 2002, 2010, & 2012)

6. Seismic Retrofit of Wood Structures
“Tried and True” methods of retrofit usually involve adding strength and stiffness to increase force capacity of the structures
Effective strengthening using viscous dampers, instead, increases effective damping of the structures thereby lowering the accelerations and drifts (damage)

7. Retrofit Methods and Goals
FEMA P-807 (2012)
Guidelines for retrofit of weak ground story wood-frame buildings
Goal: Retrofit ground story without transferring excessive force to upper stories
FEMA 365 (2000)
Drift Guidelines:
1%: Immediate Occupancy
2%: Life Safety
3%: Collapse Prevention
Source: FEMA P-807, 2012

8. Viscous Damper use in Wood Construction
Source: FTF ENGINEERING, INC.
Use of viscous dampers to address structural challenges and to provide better seismic performance

9. Constructability Challenges for Typical Weak-Story or Open Front Structures
Source: Tian 2014
Lack of space: Typically it is desirable that any retrofit solution be able to fit in small space. Effective use of dampers in these cases requires amplification.
High aspect ratio shear walls or exposed solutions may not be the best option

10. Constructability Challenges for Typical Weak-Story or Open Front Structures

11. Benefit of Damper Frames with Displacement Amplification (F = CVα)
Frame with Traditional Diagonal Damper
ΔFloor = 1”
(1% Drift)
Floor Force = 10 Kips
Δdamper = 0.3”
Damper Force = 33.5 Kips
Width-to-Height Ratio = 2.5:8
Amplified Frame.
Overall Amplification ( Δdamper ΔFrame ) = 1.5-to-1
ΔFloor = 1”
(1% Drift)
Floor Force = 10 Kips
Δdiagonal = 0.3”
Diagonal Force = 33.5 Kips
5-to-1 Lever Arm
Δdamper = 1.5”
Damper Force = 6.7 Kips
Displacement & Velocity
Force

12. Amplification Frame Study
Three amplification mechanisms:
Toggle (shown)
Four-bar Linkage*
Collinear (Telescoping) Tube*
Three aspect ratios:
2:8, 3:8, and 4:8
*New designs motivated by NEES-Soft Project, developed by MiTek/Hardy Frame and Taylor Devices

13. Frame Aspect Ratio (B:H) Amplification Factor over +/-5% Drift (η)
Analytical Study of Four-Bar Linkage Frame: Displacement Amplification Factor
3:8 Aspect Ratio
Frame Aspect Ratio (B:H)
Amplification Factor over +/-5% Drift (η)
2:8
1.06 – 1.14
3:8
1.62 – 1.74
4:8
2.03 – 2.24
Undeformed
Deformed

14. Amplification Factor over +/-5% Drift (η)
Analytical Study of Collinear Tube Frame: Displacement Amplification Factor
3:8 Aspect Ratio
Aspect Ratio (B:H)
Amplification Factor over +/-5% Drift (η)
2:8
0.88 – 0.94
3:8
1.18 – 1.38
4:8
Undeformed
Deformed

15. Analytical Study of Toggle Frame: Displacement Amplification Factor
3:8 Aspect Ratio
Aspect Ratio (B:H)
Amplification Factor over +/-1% Drift (η)
2:8
0.7 – 0.9
3:8
1.1 – 2.0
4:8
1.4 – 2.5
3.39:8 (UCSD)
1.22 – 2.27

16. Pairs of damper frames installed in opposing directions
Installation of Damper Frames: Orientation to Reduce Displacement Dependence
Ground story
Pairs of damper frames installed in opposing directions

17. Summary of Displacement Amplification Factors
Deamplification
Amplification
Narrow frames have smaller amplification factor (but still more efficient than diagonally-braced damper frames)
Wide frames have about 50% or more amplification
(2.5:8)
(2.5:8)
(3.4:1)
Diagonally-braced damper means a damper installed along the diagonal of a rectangular frame. For such a case, the amplification factor, for small drifts, is equal to the cosine of the angle of inclination.

18. Building Model for Numerical Simulations: NEES-Soft Test Specimen
Four-story Wood-frame structure
Soft ground story
Uniaxial seismic shake table testing at NEES-UCSD
Seismic
Motion
Source: Tian 2014

19. Numerical Model
Numerical model developed in Seismic Analysis of Woodframe Structures (SAWS)
Simplified component behavior
Rigid diaphragm
Zero-length shear walls
Inherent rate-dependent energy dissipation modeled with Rayleigh damping: 1% in 1st and 3rd modes
Hysteretic behavior of shear walls represented by 10-parameter model
Each damper frame represented by linear viscous damping model with damping coefficient set equal to effective damping coefficient.

20. Earthquake Records Earthquake Records Seismic Intensity
1989 Loma Prieta EQ; Gilroy record; 0 degree component
1992 Cape Mendocino EQ; Rio record; 360 degree component
Seismic Intensity
10% POE in 50 years; ~500 year return period; Design Basis Earthquake (DBE)
2% POE in 50 years; ~2500 year return period; Maximum Considered Earthquake (MCE)
Gilroy DBE
Rio MCE

21. Simulation Results for Retrofitted Structure: Peak Drift for Rio Records
Peak drifts are <2% for most cases
Aspect ratio has strong influence on performance
Upper story drift response is opposite of ground story drift response
Rio DBE
Rio MCE

22. 4:8 Telescoping Tube Frame
Simulation Results for Retrofitted Structure: Hysteretic Response for Very High Damping
For wide frame (high displacement amplification) and multiple dampers, the frame provides very high effective damping.
Drift demand in ground story is small and hysteresis loop is elliptical
Ground story is protected while damage propagates to upper stories (increased drift demand in upper stories)
Capacity of 3rd story is almost reached (negative tangent stiffness)
4:8 Telescoping Tube Frame
Rio MCE

23. Simulation Results for Retrofitted Structure: Hysteretic Response for Very Low Damping
For narrow frame (low displacement amplification) and single damper, the frame provides very low effective damping.
Soft story behavior occurs with permanent ground story drift
For a given aspect ratio, an appropriate frame must be selected (for this 2:8 case, a collinear tube frame would provide improved performance relative to a toggle frame)
2:8 Toggle Frame
Rio MCE

24. 2:8 Telescoping Tube Frame
Simulation Results for Retrofitted Structure: Hysteretic Response for Moderate Damping
For narrow frame (low displacement amplification) and multiple dampers, the frame provides a moderate level of effective damping.
Drift demand in ground story is larger than upper stories with significant energy dissipation from dampers (elliptical hysteresis loop)
Minor damage is propagated to upper stories
2:8 Telescoping Tube Frame
Rio MCE

25. DamperFrame Testing at UC San Diego
Large Capacity
Frame
Toggle Frame
Telescoping Frame

26. DamperFrame Testing at UC San Diego
Test Configuration

27. DamperFrame Testing at UC San Diego
Cape Mendocino Rio MCE

28. DamperFrame Testing at UC San Diego
Telescoping Frame

29. DamperFrame Testing at UC San Diego
Telescoping Frame
Cape Mendocino Rio DBE

30. DamperFrame Testing at UC San Diego
Toggle Frame
Cape Mendocino Rio DBE

31. DamperFrame Testing at UC San Diego
Large Pivot Frame

32. Observations and Conclusions
Limited space typically available in open front or tuck-under buildings dictates the need for retrofit solutions with high aspect ratios. In such limited space steel frames with viscous damping and amplification mechanism can provide an economical and efficient retrofit.
For a given frame height, wider frames provide higher levels of damping than narrow frames. However, care should be taken to avoid frames that provide damping that is very large (damage may propagate to upper stories).
For a given aspect ratio, the telescoping tube damper framing system provided much higher effective damping than the toggle frame and slightly more than the four-bar linkage frame.
For practical application of damping amplification frames in soft-story wood-framed structures, narrow (2:8) collinear tube frames provide a moderate level of damping that protects the ground story while preventing excessive transfer of forces to the upper stories.
Controlling slip and excessive displacement of attaching elements (drags, pivots, base attachments) is necessary to take full advantage of added damping, but sufficient damping can still be achieve with an increase in drifts.

33. Thank You
From left: Jingjing Tian, Prof. Symans, Steve Yang