Journal of Structural Engineering

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.

Woodframe buildings are unique in that the nonstructural finishes such as gypsum wall board and stucco provide significant stiffness and strength relative to the lateral force resisting system, e.g., wood shear walls. Wall finishes, or components within a woodframe wall subassembly, can consist of multiple layered modern and/or archaic elements such as wood planks, drywall, plaster on lathe, stucco, or plywood. There exist significant differences in ductility among these materials, raising questions about how best to superimpose single-degree-of-freedom hysteretic models or backbone curves during nonlinear time history analysis or when combining backbone curves for design and retrofit. This paper presents the method and results of an experimental study of 18 walls installed with one, two, or three of the previously described finishes. Testing was performed to determine the best approach to add the sheathing layers numerically when combining the backbone curves for analysis and design. Nonlinear dynamic analyses were conducted to quantify the difference between the behavior of the combined sheathing test and the superimposed single layer sheathings.

To study the lateral resistance of bolted glued-laminated (glulam) timber post and beam structures, two monotonic tests and eight cyclic tests were performed on full-scale, one-story, one-bay timber post and beam construction specimens. Four laterally strengthened structural systems were considered: a frame with an X-brace, a frame with a K-brace, a frame with a knee-brace, and a frame filled with light wood shear walls. Seismic performances were evaluated according to the experimental phenomena, hysteretic characteristics, stiffness degradation, strength degradation, and energy dissipation capacity. The results showed that the frame with an X-brace system and the frame with a K-brace system provided markedly better elastic stiffness but lower ductility. The frame with a knee-brace system showed increased elastic stiffness compared with the simple frame system and provided higher ductility compared with both the frame with the X-brace system and the frame with the K-brace system. Additionally, the frame filled with the light wood shear walls system performed well in terms of elastic stiffness and ductility. Finally, some suggestions were presented to improve the lateral resistance and ductility of each structural system.

Soft-story wood-frame buildings have been recognized as a disaster preparedness problem for decades. There are tens of thousands of these multifamily three- and four-story structures throughout California and other parts of the United States. The majority were constructed between 1920 and 1970 and are prevalent in regions such as the San Francisco Bay Area in California. The NEES-Soft project was a five-university multiindustry effort that culminated in a series of full-scale soft-story wood-frame building tests to validate retrofit philosophies proposed by (1) Federal Emergency Management Agency’s recent soft-story seismic retrofit guideline for wood buildings and (2) a performance-based seismic retrofit (PBSR) approach developed as part of the NEES-Soft project. This paper is the first in a set of companion papers that presents the building design, retrofit objectives and designs, and full-scale shake table test results of a four-story $\text{370-}{\mathrm{m}}^2$ ($\mathrm{4,000}\text{-}{\mathrm{ft}}^2$) soft-story test building. Four different retrofit designs were developed and tested at full scale, each with specified performance objectives, which were typically not the same. Three of these retrofits were stiffness or strength–based strategies and one applied supplemental damping devices in a pinned preassembled frame. This paper focuses on the building and retrofit design methodologies and specifics and the companion paper presents the experimental results of full-scale shake table tests ranging from 0.2- to 1.8-g spectral acceleration for all four retrofits.

Soft-story wood-frame buildings have been recognized as a disaster preparedness problem for decades. The majority of these buildings were constructed from the 1920s to the 1960s and are prone to collapse during moderate to large earthquakes due to a characteristic deficiency in strength and stiffness in their first story. In order to propose and validate retrofit methods for these at-risk buildings, a full-scale four-story soft-story wood-frame building was constructed, retrofitted, and subjected to ground motions of various intensities. The tests were conducted to validate retrofit guidelines proposed in a “Federal Emergency Management Agency’s recent soft-story seismic retrofit guideline for wood buildings” and a performance-based seismic retrofit (PBSR) methodology developed as part of the NEES-Soft project. This paper is the second in a set of companion papers and presents the full-scale shake table test results using the two new approaches. The companion paper to this paper presents the design philosophies, design details, and numerical analysis of the retrofitted building for each of the four retrofits.

In current force-based seismic design procedures, structures are allowed to behave inelastically during significant seismic events. For this reason, the applied design base shear of a structure is calculated by reducing the elastic strength demand by a ductility factor that represents the ability of a structure or structural system to deform inelastically beyond yielding. Whereas North American building codes provide ductility factors for traditional structural systems, including timber-based or steel-based systems, there are currently no design provisions available for novel hybrid structural systems. Thus research is required to define the force reduction factors, and specifically the ductility factor, $R_d$, to safely and efficiently design novel systems within the existing seismic design provisions. One such novel system is the so-called finding the forest through the trees (FFTT), proposed in 2012, where mass-timber panels act as shear walls and are connected to each other and to perimeter frames through steel beams. The research reported in this paper presents nonlinear dynamic analyses to evaluate the suitability of $R_d$ factors ranging from 1.5–6.0 to design Option 1 of the FFTT system, which is proposed for buildings up to 12 stories tall. Potential $R_d$ factors are evaluated by limiting interstory drift as the main performance criterion in nonlinear time history analyses to an acceptable limit of 2.5% drift with 90% probability of nonexceedance. Based on the analyses presented in this paper, a ductility factor of 5.0 is suggested for the considered layout of the novel timber–steel hybrid FFTT system.

Increasing the lateral strength and stiffness of light-frame structures can lead to reduced damage in the event of an earthquake. Because low-rise wood and cold-formed steel-framed buildings often contain many partition walls, their performance can be improved economically by more effective engagement of gypsum-sheathed walls. This paper describes a series of tests of enhanced screw and adhesive connections between gypsum wallboard and wood and cold-formed steel-framing members. Small wall panels constructed with the improved connections were also tested and compared with wallettes built with conventional drywall screws. Test results indicate that while enhanced screws can increase connection and wall strength modestly relative to conventional screws, a much more significant increase in strength and stiffness can be obtained by using construction adhesives. Test results show that supplementing conventional screws with adhesives increased connection strength and stiffness by a factor of four to five. These adhesive-supplemented connections in light-frame wallettes increased strength by approximately two times and stiffness by approximately three times when compared with wallettes with conventional screw connections alone. This paper describes the response and damage progression of gypsum wallboard connections and framed wallettes using the enhanced screw and adhesive connections.

This paper discusses the structural characterization of a novel hybrid shear-wall system formed by coupling standard platform-frame panels with an external reinforced concrete shelter formed of precast slabs screwed to the wooden frames. The external RC skin is intended as a supplementary bracing system, increasing strength and dissipative capacity of the bare timber frame. The structural performance of such hybrid shear wall under monotonic and cyclic loading was first theorized analytically on the basis of code provisions and then confirmed via experimental tests. The novel shear walls demonstrated to fulfill the requirements prescribed by Eurocode 8. In particular, the analyzed system belongs to high ductility class (HDC). Finally the seismic response of a reference building realized with the innovative hybrid shear walls was simulated by means of a numerical model validated on experimental tests; the suitable behavior factor for the building was estimated.

The paper presents a comprehensive study on oriented strand boards (OSB) and gypsum fiber boards (GFB) as sheathing materials for light-frame wall elements. The study is based on the experimental results of the research project, optimization of multistory timber buildings against earthquake impact (OptimberQuake). In the framework of this project, 20 light-frame wall elements and 58 sheathing-to-framing connections with different layouts have been tested under cyclic and monotonic loading. The experimental results have been analyzed and compared in terms of ductility, energy dissipation as well as deformation, and load-bearing capacity. Furthermore, the performance of the walls was evaluated with the probabilistic approach in accordance with current methodology. For this purpose, a simplified numerical model was used in which the walls were represented by an equivalent nonlinear shear springs. The characteristics of the springs were determined based on the experimental results wherein the response was described by a hysteretic model. As a result of probabilistic analysis, behavior factor (i.e., force reduction factor) was evaluated for walls with different types of sheathing. The analysis of the experimental results and probabilistic approach led to the same conclusions. The results show a comparable cyclic response of timber framed walls covered with OSB or GFB boards in terms of energy dissipation and deformation capacity. The probabilistic assessment also suggests that the same behavior factor can be used for both types of wall if the basic requirements regarding the minimum thickness and detailing of the connections are met. A higher value for the minimum thickness of GFB compared with OSB will be recommended.

Posttensioning low-damage technologies were first developed in the late 1990s as the main outcome of the U.S. Precast Seismic Structural System (PRESSS) program coordinated by the University of California, San Diego, and culminated with the pseudo-dynamic test of a large-scale five-story test building. The extension of posttensioned techniques to timber elements led to the development of new structural systems, referred to as Pres-Lam (prestressed laminated timber). Pres-Lam systems consist of timber structural frames or walls made of laminated veneer lumber, glue laminated timber (Glulam), or cross-laminated timber (CLT). Pres-Lam walls consist of a rocking timber element with unbonded posttensioned tendons running through the length and attached to the foundation, which provides a centering force to the wall, while energy dissipation is supplied by either internal or external mild steel dissipaters. Previous tests carried out on posttensioned timber walls focused on small-scale (one-third) specimens with the main objective of evaluating the general response of the system. The main objective of the experimental program herein presented is the testing and estimating of the response of a series two-thirds-scale posttensioned walls, with alternative arrangements and combination of dissipaters and posttensioning, focusing on the construction details adopted in real practice. The paper first presents a brief discussion on the seismic demand evaluation based on the displacement-based design approach. The construction detailing of the steel dissipater connections, posttensioning anchorage, and shear keys are then presented. The main objectives of this experimental program were the investigation of the experimental behavior of large-scale posttensioned timber walls, with particular focus on the system connection detailing and optimization of posttensioning anchorage, fastening of the dissipation devices, and shear keys. The program consisted of several quasi-static cyclic tests considering different steel dissipater configurations, different levels of posttensioning initial stress, and different dissipater options were considered: both internal and external mild steel tension-compression yield devices were used. The experimental results showed the performance of posttensioned timber wall systems, which provide a high level of dissipation while showing negligible residual displacements and negligible damage to the wall element. The final part of the paper presents the experimental evaluation of the area-based hysteretic damping for the tested specimens, and the results highlight the great influence of the connection detailing of the dissipaters.

A 2-story full-scale model of a cross-laminated timber (CLT) house was tested under quasi-static monotonic and cyclic loading. The primary objectives were to investigate 3D system performance of a CLT structure subjected to lateral loads in terms of lateral strength and deformability capacity, global behavior of the structure, frequency response of the structure before and after each test, and performance of anchoring connectors (hold-downs, brackets) and connections between CLT panels. The house was $6.0\times 4.8\mathrm{m}$ in plan with a height of 4.8 m. A total of five (one push-over and four cyclic) quasi-static tests were performed, one direction at a time. Parameters, such as the direction of loading, number of hold-downs, and number of screws in perpendicular wall-to-wall connections, were varied in the tests. The CLT structure performed according to the design objectives, with the ultimate resistance being almost identical in both directions. Failure mechanisms, i.e., shear failure of nails in the brackets in the first story as a result of sliding and rocking of the CLT wall panels, were similar in all tests. Even after the maximum force was reached, no global instabilities of the house were detected. Torsion effects did not compromise the integrity, stability, or the lateral resistance of the building. The outcomes of the full-scale CLT house tests will be used for further analytical and numerical analyses to help the implementation of CLT as a structural system in the North American building codes and material standards.

This study examines the mechanical performance of large and small cross-laminated timber (CLT) wall panels with different applications. Two CLT structures were subjected to reversed cyclic lateral loads. One structure consisted of 90-mm-thick, large CLT wall panels ($6\times 2.7\mathrm{m}$), and the other consisted of 90-mm-thick, small CLT wall panels ($1\times 2.7\mathrm{m}$). A weight was installed on the roof of two-story structures to simulate the weight of three-story structures, designed by elastic calculations with a base shear coefficient of 1.0. The number of screws for each joint was determined by a linear finite-element method (FEM) analysis. The experimental results showed that the ultimate capacity of these structures is 60–80% higher than the design load, indicating high structural performance. A three-dimensional, nonlinear analysis was conducted via the FEM, which accurately predicted the mechanical performance of the CLT structures. This design procedure, based on linear analysis, resulted in a conservative design, and simulation using the nonlinear FEM provided an effective tool for design optimization.

This paper presents a numerical study conducted on a seven-story timber building made of cross-laminated (X-lam) panels, equipped with a linear translational tuned mass damper (TMD). The TMD is placed on the top of the building as a technique for reducing the notoriously high drifts and seismic accelerations of these types of structures. TMD parameters (mass, stiffness, and damping) were designed using a genetic algorithm (GA) technique by optimizing the structural response under seven recorded earthquake ground motions compatible, on average, with a predefined elastic spectrum. Time-history dynamic analyses were carried out on a simplified two-degree-of-freedom system equivalent to the multistory building, while a detailed model of the entire building using two-dimensional elastic shell elements and elastic springs for modeling connections was used as a verification of the evaluated solution. Several comparisons between the response of the structure with and without TMD subjected to medium- and high-intensity recorded earthquake ground motions are presented, and the effectiveness and limits of these devices for improving the seismic performance of X-lam buildings are critically evaluated.

Recent earthquakes have demonstrated the vulnerability of soft-story wood buildings. A performance-based retrofit option focusing on stiffening/strengthening only the soft ground story, as regulated by FEMA guidelines, was proposed in an attempt to achieve collapse prevention of the ground story while at the same time ensuring damage control of the upper stories. This paper focuses on an alternative retrofit approach that focuses on energy dissipation, in order to achieve a performance that is essentially comparable to the inherent performance target defined in FEMA guidelines. A viscous damper retrofit is designed based on conclusions drawn from parametric studies. Then, the proposed retrofit is validated via slow pseudodynamic hybrid testing of a full-scale three-story building. The test data are presented, interpreted, and compared against both pretest numerical predictions and posttest simulation results after model calibration. It is demonstrated that a damping retrofit, as compared to a stiffening/strengthening retrofit, better complies with the FEMA retrofit standards due to its effectiveness in reducing ground-story peak deformations while being less likely to propagate damage to the upper stories.

Recent experiments on a $2.44\times 2.44\mathrm{m}$ rigid timber wall panel with slip-friction connectors have demonstrated the feasibility of enabling elastoplastic behavior in structures that would otherwise be essentially rigid. The slip-friction connectors are adopted as the hold-downs that anchor the ends of the wall to the foundation. These replace the traditionally used steel bracket hold-downs, which relied on inelastic damage to the screw or nail connections for energy dissipation. Overturning resistance of the wall directly relates to the slip-force in the slip-friction connectors. On the slip-force being reached, the intention is that the wall rocks in a controlled manner. A numerical study demonstrates the energy dissipation advantages of this approach. A direct-displacement-based design procedure is proposed for a multistory wall with slip-friction connectors. The wall is numerically modeled, and its response to earthquake time-history loadings compared with that of an idealized structure with a single plastic deformable hinge at the base. Results show that when gravity is not considered, the wall structure with slip-friction connectors behaves almost identically to that of its idealized equivalent. Taking into consideration higher mode effects of multi-degree-of-freedom (MDOF) rocking structures, base shears, and response accelerations are capped to the level expected, but residual displacements are significant. However, with self-weight considered, residual displacements of the wall are trivially small, and maximum displacements are also, in general, reduced. For the wall configurations investigated, the results suggest that a rocking timber wall unit at the base of a multistory shear wall will not only provide the load limiting benefits of a plastically deformable hinge, but also minimize maximum drifts, and allow for the structure to restore to its original position.

The unbonded posttensioned rocking and dissipative technology was first developed as the main outcome of the PREcast seismic structural systems (PRESSS) Program in the United States. After the first developments and significant refinement, the technology was extended to steel and, more recently, timber structures. The timber version, referred to as prestressed laminated (Pres-Lam) system can be either implemented for timber walls (single or coupled) or frames or a combination of the above, with unbonded posttensioning and supplemental dissipation devices. In unbonded posttensioned dissipative wall systems a combination of recentering capacity and energy dissipation leads to a controlled rocking mechanism, which develops a gap opening at the wall base. This generates an uplift displacement that is transferred to the floor diaphragm. This vertical displacement incompatibility can represent a potential issue if the connection detailing between floor and lateral resisting system is not designed properly. The same issue can be mitigated by adopting an alternative configuration of the rocking/dissipative wall system, based on the use of a column–wall–column posttensioned connection. This concept, originally proposed for precast concrete walls and referred to as prestressed wall with end column (PreWEC), has been extended and adapted to posttensioned timber structures and validated through experimental testing. The paper presents the design, detailing, and experimental testing of a two-thirds scale wall specimen of this alternative configuration. Different wall configurations are considered in terms of posttensioning initial force as well as dissipation devices layout. The experimental results confirm the excellent seismic performance of the system with the possibility to adopt multiple alternative configurations.

This paper investigates the seismic analysis of multistory cross-laminated timber (XLAM) buildings. The influence of different parameters such as wall geometry, vertical load level, friction, and, most importantly, connection stiffness, strength, and ductility is assessed. Linear and nonlinear finite-element (FE) analyses are carried out on a hypothetical 4-story case study building. The actual load-carrying capacity of the case study building is calculated with the nonlinear static pushover method and assessed using displacement-based design. The XLAM building behavior factors are then derived for different cases using a simplified method. Values in the range 2–3 have been obtained depending on whether monolithic or segmental walls (namely made of approximately 2-m-wide panels screwed to the adjacent ones) are used. Further nonlinear dynamic analyses carried out on a part of the case study building show that friction may have a beneficial effect on the seismic resistance of XLAM buildings. However, it is advised that its influence is conservatively neglected until further investigations are performed. Lastly, the importance of considering the overstrength concept in design of XLAM buildings is presented via a wall case study. Obtained results provide an important insight for both academics and practicing engineers in the FE modeling and design of XLAM buildings using different code-based approaches. This data is also crucial for the preparation of new seismic design codes on XLAM timber buildings.

This paper presents a new alternative energy dissipation solution to be used with cross-laminated timber (CLT) self-centering walls. CLT is a relatively new building product in North America and could potentially be used for high-rise construction. The development of high-performance seismic design solutions is necessary to encourage innovative structures and the design of these structures to new heights. The objective of this paper is to propose a wall-to-floor connection system that is easy to install and replace (structural fuse) after the occurrence of a large damaging event. The proposed energy dissipators are fabricated following concepts used in developing steel buckling restrained steel braces (BRB), having a milled portion, which is designed to yield and is enclosed within a grouted steel pipe. The connection system is investigated experimentally through a test sequence of displacement-controlled cycles based on a modified version of the test method developed by the American Concrete Institute (ACI) to facilitate development of special precast systems (ACI T1.1-01 Acceptance Criteria for Moment Frames Based on Structural Testing). Digital Image Correlation (DIC) was used to analyze strain behavior of the milled portion, as well as track movement of the panels during quasi-static uniaxial and cyclic testing. The results show the yield behavior and energy dissipation properties of the connection system. Damage was focused primarily in the energy dissipators, with negligible deformation and damage to the CLT panels and connections.