William F. Cofer

Base Isolation and Supplemental Damping Systems for Seismic Protection of Wood Structures: Literature Review

This paper provides a literature review on the application of base isolation and supplemental damping systems for seismic protection of wood structures. The review reveals that both elastomeric bearings and sliding bearings have been considered for implementation within base isolation systems of wood-framed buildings. In addition, friction dampers, viscoelastic dampers, hysteretic dampers, and fluid viscous dampers have been considered for implementation within the framing of wood buildings. Although there are a number of impediments to the widespread implementation of such advanced seismic protection systems, the reviewed literature clearly demonstrates that advanced seismic protection systems offer promise for enabling light-framed wood structures to resist major earthquakes with minimal damage.

Seismic Behavior of Wood-framed Structures with Viscous Fluid Dampers

The suitability of viscous fluid dampers for seismic protection of light-framed wood buildings is investigated in this paper. Nonlinear finite-element models of wood building components (shear wall) and systems (three-dimensional buildings) are developed and numerical analyses are performed to evaluate their response to seismic loading. For both the single wall and the building system, seismic protection is provided by installing viscous fluid dampers within the wall cavities. The results of the numerical analyses demonstrate the ability of fluid dampers to dissipate a significant portion of seismic input energy, reducing the inelastic strain energy demand on the wood framing system. In addition, the study revealed some important practical issues associated with implementation of fluid dampers within light wood-framed buildings.

A comparison of current computer analysis methods for seismic performance of reinforced concrete members

In this paper, an evaluation of current capabilities in the area of cyclic analysis of reinforced concrete components is presented through the application of several analysis methods to a benchmark problem. Data from experimental tests involving the flexural and shear performance of a reinforced concrete column was used for comparison. Analyses for cyclic and monotonic load cases were performed using a degrading plastic hinge model, a fiber beam model, and a three-dimensional finite element model. Details of the data preparation and analysis results are presented and evaluated. Recommendations are made regarding the relative effectiveness of the methods for practical dynamic analysis.

Effects of blast loading on prestressed girder bridges

Since the events of September 11th, increased attention has been given to the effects of blast loading on structures. Bridges are especially important due to their potentially critical role in the economy and for emergency response. Prestressed concrete highway bridges are very common, representing 11 percent of state bridges nationwide. Yet, very little is known about how prestressed concrete bridges respond to blast loading.

Performance of lightly confined reinforced concrete columns in long-duration subduction zone earthquakes

The main goals of this research were to evaluate typical 1950s and 1960s as-built bridge columns in western Washington State in large subduction zone earthquakes and to investigate the dependency of failure mechanisms on loading history. Eight displacement histories were applied to eight nominally identical, half-scale, circular reinforced concrete columns expected to respond primarily in flexure (flexure-dominated). The main design deficiencies were a short longitudinal reinforcement lap splice at the base of the column (35db) and inadequate transverse reinforcement. Test results showed that the failure mode of reinforced concrete columns was controlled by the column loading history. Three distinct failure mechanisms were observed for columns with an aspect ratio of approximately 4.2, assuming symmetric, double-curvature behavior. Large initial displacements greater than six times the effective yield displacement (Δ y ) were likely to result in shear failures. Columns experiencing many displacements less than 4Δ y were likely to fail because of longitudinal reinforcement buckling. Columns subjected to several displacement excursions less than 4Δ y followed by an excursion greater than 6Δ y were likely to fail by longitudinal reinforcement slipping within the splice region. Despite the deficiencies present in circular reinforced concrete bridge columns built before 1975 in western Washington State, this study showed that flexure-dominated columns with a 35db lap splice in multiple-column bent, three-or four-span bridges were not likely to experience significant damage in the predicted Cascadia Subduction Zone earthquake. However, other components of the bridge need to be assessed to determine whether the global bridge response is acceptable under the predicted Cascadia Subduction Zone earthquake.

Part 5: Seismic Design of Bridges: Performance of Lightly Confined Reinforced Concrete Columns in Long-Duration Subduction Zone Earthquakes

The main goals of this research were to evaluate typical 1950s and 1960s as-built bridge columns in western Washington State in large subduction zone earthquakes and to investigate the dependency of failure mechanisms on loading history. Eight displacement histories were applied to eight nominally identical, half-scale, circular reinforced concrete columns expected to respond primarily in flexure (flexure-dominated). The main design deficiencies were a short longitudinal reinforcement lap splice at the base of the column (35db) and inadequate transverse reinforcement. Test results showed that the failure mode of reinforced concrete columns was controlled by the column loading history. Three distinct failure mechanisms were observed for columns with an aspect ratio of approximately 4.2, assuming symmetric, double-curvature behavior. Large initial displacements greater than six times the effective yield displacement (Δyy) were likely to result in shear failures. Columns experiencing many displacements less ...

STRUCTURAL EVALUATION OF ENGINEERED WOOD COMPOSITES FOR NAVAL WATERFRONT FACILITIES

The paper describes ongoing comprehensive research effort to develop wood-plastic composite lumber for use as an alternative to preservative-treated wood in fender systems. The objectives of the research were to evaluate material and structural demands on members composed of the new material, to establish appropriate design criteria, and to verify component performance. The paper also presents procedures for structural modeling for component testing, and the resulting formulation of design equations.

Fluid Dampers for Seismic Protection of Woodframe Structures

In the recent past, a large number of steel-framed buildings have been constructed or retrofitted with supplemental energy dissipation systems for the purpose of seismic protection. However, the application of such systems to woodframe structures has been essentially non-existent except for a limited number of experimental laboratory studies. This paper presents a numerical study of the application of fluid dampers for seismic protection of wood-framed structures. Such dampers dissipate energy via orificing of a fluid. The seismic response of a wood-framed shear wall with and without dampers is evaluated via nonlinear finite element analyses. The results of the analyses demonstrate that the dampers are capable of dissipating a large portion of the seismic input energy while simultaneously relieving the inelastic energy dissipation demand on the shear wall. Introduction Light-framed wood construction has generally been regarded as performing well during moderate to strong earthquakes. Such performance is primarily due to the low mass of lightframed construction combined with its ability to deform inelastically without inducing collapse of the structure. Although light-framed wood structures typically do not collapse during moderate to strong earthquakes, the inelastic response is generally associated with significant structural and non-structural damage that may be very costly to repair. As an example of the magnitude of the damage to light-framed wood buildings during a moderate earthquake, consider the 1994 Northridge Earthquake (Moment Magnitude = 6.8) in which there was in excess of 20 billion dollars worth of damage to such structures (Kircher et al., 1997). Obviously, the 1994 Northridge Earthquake provides clear evidence that conventional light-framed wood buildings are prone to significant damage when subjected to strong earthquake ground motions. One approach to mitigating the effects of strong earthquakes on light-framed wood buildings is to incorporate an advanced seismic protection system within the building. For example, introducing a supplemental damping system within the framing of a building can reduce its seismic response. The supplemental damping system dissipates a portion of the seismic input energy, thereby reducing the amount of energy dissipated via inelastic behavior within the structural framing. The number of applications of advanced seismic protection systems within Reference: Symans, M.D., Cofer, W.F., Du, Y. and Fridley, K.J. (2001). “Fluid Dampers for Seismic Protection of Woodframe Structures,” Proc. of the 2001 Structures Congress and Exposition, ASCE, Edited by P.C. Chang, Washington, D.C., May, available on CD-ROM only. 2 buildings has been steadily growing within approximately the past ten years. Nearly all of these applications have been within either steel or concrete structures (e.g., see Soong and Constantinou, 1995). There are a wide variety of supplemental damping systems available for implementation in buildings (Constantinou et al., 1998 and Constantinou and Symans, 1993a). However, the most rapid growth in the application of supplemental damping systems to buildings has occurred for fluid dampers. Since the first experimental studies on a scale-model steel building frame in 1993 (Symans and Constantinou, 1993b), the number of implementations of fluid dampers within major bridge and building structures has grown to 49 with installation pending in 17 additional structures. Although there are many factors that have contributed to this rapid growth, one of the primary reasons is the high energy dissipation density of fluid dampers (i.e., fluid dampers are capable of dissipating a large amount of energy relative to their size). Relatively few studies have been conducted on the application of supplemental damping systems for seismic protection of wood frame structures. Filiatrault (1990) performed a numerical study to evaluate the seismic response of a woodframed shear wall with friction dampers at the corners of the wall and Dinehart and Shenton (1998) and Dinehart et al. (1999) experimentally evaluated the seismic response of a woodframed shear wall with viscoelastic dampers located at various positions within the wall. The results of these studies clearly demonstrate that supplemental damping systems have the potential for significantly improving the seismic response of woodframed buildings. Note that Symans et al. (2001) provides a comprehensive literature review on the application of advanced seismic protection systems (both base isolation and supplemental damping systems) to wood-framed structures. To the knowledge of the authors, the research presented herein represents the first study on the application of fluid dampers within wood-framed structures for seismic energy dissipation. Description of Finite Element Model A nonlinear finite element model of a wood-framed shear wall was developed for the numerical analyses using the commercial program ABAQUS (ABAQUS, 1998) (see Figure 1). The dimensions of the shear wall were 2.44 m x 2.44 m (8 ft x 8 ft). The framing of the wall consisted of 38.1 mm x 88.9 mm (nominal 2 in. x 4 in.) lumber. The vertical studs were spaced at 60.96 cm (24 in.) on center. The wall was sheathed with 1.22 m x 2.44 m (4 ft x 8 ft) waferboard sheathing panels having a thickness of 9.53 mm (3/8 in.). The connections between the sheathing and framing consisted of 6.35 cm (2.5 in.) 8d galvanized common nails. The field and perimeter nail spacing was 15.24 cm (6 in.). The weight at the top of the wall was 44.5 kN (10 kips), which is intended to represent the tributary weight if the wall were located at the first story of a three-story building. The weight was distributed at the nodes along the top plate. The bottom plate is assumed to be fixed to the foundation. The framing members and sheathing panels were modeled as 2-D isoparametric beam elements and 2-D isoparametric quadratic plane stress elements, respectively. The connection model for each nail was based on the hybrid Stewart-Dolan connector model, as depicted by the hysteretic loop shown in Figure 2. The parameters of the connection model were obtained from experimental test data provided in Dolan (1989). Note that, as a simplification, the stiffness degradation indicated in Figure 2 was not included in the analyses presented in this paper. Reference: Symans, M.D., Cofer, W.F., Du, Y. and Fridley, K.J. (2001). “Fluid Dampers for Seismic Protection of Woodframe Structures,” Proc. of the 2001 Structures Congress and Exposition, ASCE, Edited by P.C. Chang, Washington, D.C., May, available on CD-ROM only. 3 System identification of the wall model was performed via eigenvalue analysis wherein the damping matrix of the wall (without dampers) was assembled using a Rayleigh damping formulation. As obtained from the eigenvalue analysis, the natural frequency and damping ratio in the fundamental mode were 4.18 Hz and 2.1%, respectively. The fundamental mode shape is shown in Figure 3. Figure 1 Finite Element Model of Wood-Framed Shear Wall. Hybrid Stewart Dolan Nail Connector Model -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -20 -15 -10 -5 0 5 10 15 20 Displacement (mm) L o ad ( kN ) Figure 2 Hysteretic Behavior of Nonlinear Connection Element. Reference: Symans, M.D., Cofer, W.F., Du, Y. and Fridley, K.J. (2001). “Fluid Dampers for Seismic Protection of Woodframe Structures,” Proc. of the 2001 Structures Congress and Exposition, ASCE, Edited by P.C. Chang, Washington, D.C., May, available on CD-ROM only. 4 Figure 3 Fundamental Mode Shape for Shear Wall. Description and Configuration of Fluid Dampers Fluid viscous dampers offer considerable promise for application within wood-framed buildings due to their high energy dissipation density. The high energy dissipation density allows the dampers to be conveniently located within the walls of a wood-framed structure. For example, in this study, the damper was positioned along the diagonal of the wall (see Figure 4). In this configuration, dual let-in rods are used to connect the lower corner of the wall to the end of the damper. One rod is located on each side of the wall and small plates are used to prevent the rod from buckling outward. One advantage to this configuration is that the damper force lies within the plane of the wall and thus there are no bending moments applied to the wall at the corner connections. In contrast, a disadvantage to this configuration is that the effectiveness of the damper is reduced by 50% (for a square wall) due to the diagonal orientation. In addition to their high energy dissipation density, the behavior of fluid dampers is quite unique in that they are incapable of developing appreciable restoring forces for the frequencies of motion expected during an earthquake (Symans and Constantinou, 1998). Thus, the dampers behave essentially as pure energy dissipation devices. The design of structures that incorporate such dampers becomes simplified since the dampers may be regarded as simply adding additional energy dissipation capacity to the structure. Of course, one must recognize that the installation of supplemental dampers will alter the load path for the transfer of forces within the structure. Reference: Symans, M.D., Cofer, W.F., Du, Y. and Fridley, K.J. (2001). “Fluid Dampers for Seismic Protection of Woodframe Structures,” Proc. of the 2001 Structures Congress and Exposition, ASCE, Edited by P.C. Chang, Washington, D.C., May, available on CD-ROM only. 5 Piston Head Piston Rod Pinned Connection Pinned Connection