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

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

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.

Background

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)

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)

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

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

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

Constructability Challenges for Typical Weak-Story or Open Front Structures

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

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

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

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 1.27 - 1.80 Undeformed Deformed

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

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

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.

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

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.

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

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

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

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

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

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

DamperFrame Testing at UC San Diego Test Configuration

DamperFrame Testing at UC San Diego Cape Mendocino Rio MCE

DamperFrame Testing at UC San Diego Telescoping Frame

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

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

DamperFrame Testing at UC San Diego Large Pivot Frame

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.

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