Performance-Based Engineering for Light-Frame ...

Performance-Based Engineering for Light-Frame Wood Construction in the United States: Status and Challenges B.R. Ellingwood (College of Engineering Distinguished Professor) School of Civil and Environmental Engineering, Georgia Institute of Technology Atlanta, Georgia, USA J.W. van de Lindt (Associate Professor) Department of Civil Engineering, Colorado State University Fort Collins, CO, USA D.S. Gromala (Senior Engineer) Weyerhaeuser Company Federal Way, Washington, USA. D.V. Rosowsky (A.P. and Florence Wiley Chair Professor) Department of Civil Engineering, Texas A&M University College Station, Texas, USA. R. Gupta (Associate Professor) Department of Wood Science and Engineering, Oregon State University Corvallis, Oregon, USA. S. E. Pryor (Building Systems Research and Development Manager) Simpson Strong-Tie Co. Dublin, California, USA. Summary

Performance-based engineering of light-frame wood construction has yet to be formally developed within a well-articulated philosophy. U.S. investment in woodframe structures makes it critical that they perform to the level(s) expected by society during their service lives. This paper summarizes the current status of performance-based engineering of woodframe construction in the United States for natural phenomena hazards such as earthquake and wind, and identifies future challenges that include: (1) defining performance levels for woodframe structures; (2) development of appropriate design tools and supporting databases for PBE; and (3) dealing with broader impacts of PBE on residential construction. 1. Introduction

Light frame wood construction makes up more than 80% of the building stock in the United States, and its widespread use in residential construction makes it a major individual, community, and national investment. Damage to residential construction often represents a substantial portion of the financial losses incurred as a result of natural disasters such as the Northridge Earthquake or Hurricane Andrew. The occurrences of large financial losses are troubling because the vast majority of these buildings performed very well from a structural standpoint – there were relatively few structural failures that resulted in loss of lives. In fact, most of the financial losses were due to failures of the so-called non-structural systems (interior finish cracking in earthquakes, cladding failures followed by water intrusion in hurricanes). Even modest increases in the performance of 1

woodframe structures exposed to natural hazards might lead to a significant reduction in losses. The newly evolving paradigm of performance-based engineering (PBE) seeks to provide building designers and owners with flexibility to reduce losses through better defined performance objectives, limit states that are connected to those performance objectives within the design framework, and targeted investments in risk mitigation. An extensive body of literature on PBE has been developed in the past decade which can be consulted for general details and background. PBE is an engineering approach that is based on (1) specific performance objectives and safety goals of building occupants, owners, and the public, (2) probabilistic or deterministic evaluation of hazards, and (3) quantitative evaluation of design alternatives against performance objectives; but does not prescribe specific technical solutions [1]. While one might argue that the design of civil engineering structures has been based on performance for many years, any performance objectives beyond life safety have not been well articulated in building codes. It has been assumed that performance would be acceptable if the structural system or its components or subassemblies were designed with sufficient strength that the safety of the building occupants and the public is not endangered. The performance of buildings during earthquakes and hurricanes in the 1980’s and 1990’s demonstrated that design based on safety is necessary but not sufficient for owner and occupant satisfaction. This is particularly true for woodframe structures which were severely damaged during the 1994 Northridge, California earthquake. Woodframe1 structures, by number, comprise the vast majority of the building stock in the United States. The majority of woodframe structures in the United States are designed using prescriptive construction techniques. Such practices encompass gravity loads, fire resistance, serviceability issues, and even building retrofit for natural hazards, but do not address performance requirements directly. Thus, the development of a generalized performancebased engineering philosophy for woodframe structures, particularly single family residential structures, presents a significant challenge for engineering research and professional practice, and the nation’s home builders. An Invitational Workshop on Performance-Based Design of Woodframe Structures was held in August, 20052 to discuss these issues and challenges [2]. This paper is the result of discussions during and following that workshop. As will become evident, most of these challenges are not country-specific. Since that workshop, it has become apparent that the majority of active woodframe researchers in the United States and a substantial fraction of the building industry believe that PBE is a viable and desirable alterative to the traditional approach to design and construction.

2. PBE in the United States: Current Status The concept of PBE is familiar to engineers in automotive and aerospace engineering, where large numbers (fleets) of nominally identical systems are manufactured and where prototype testing is a normal part of the design process. In the building engineering community, in which design codes play a major role in safety assurance and product 1 2

“Woodframe” is used here to describe a system of repetitive horizontal and vertical wood light-framing members. This workshop was supported, in part, by the Structural Engineering Institute of the American Society of Civil Engineers.

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development, the concept is more novel, with roots in the HUD Operation Breakthrough Program of the early 1970s [3]. Advances in PBE of woodframe structures have been most notable in the areas of fire protection [4] and earthquake engineering [5,6]. Additional consumer-driven issues directly related to serviceability of woodframe floor systems have been addressed within the industry (e.g. [7]). Most recent proposals for PBE have been built around a matrix in which one axis describes severity of hazard (minor, severe) and the second identifies expected performance (continued occupancy, life safety). Measurement of performance (or the confidence of achieving the stipulated performance level for a given hazard) is accomplished probabilistically, wherever possible, to account for uncertainties in a rational and consistent way. Such methods have become widely accepted in the past two decades, and are embedded in ASCE Standard 16-95 on LRFD for Engineered Wood Construction [8]. 2.1 Serviceability Long before PBE was a term known to the vast majority of the structural engineering research community, Weyerhaeuser’s Trus JoistTM embarked on an internal effort to develop a floor system that met certain (often qualitative/subjective) performance objectives desired by their customers [7]. Trus JoistTM received numerous complaints of “bouncy” wood floor systems throughout the 1980’s when the floors were designed and built to comply with the bare minimum requirements of the building code. To address these complaints, an internal task group was formed in 1991 to investigate the static, dynamic, and subjective performance of floor systems. The efforts of this team led to a floor rating system based on human perception of acceptable floor vibrations. This is a direct example of PBE for serviceability, presented to the builder in a prescriptive fashion, i.e. the attributes of alternative floor systems are selected from performance rating tables based on the geometric configuration, load demands, and most importantly, the nature of the structure, including owner demands. This is quite different from historical problem resolution, wherein serviceability problems related to woodframe structures in the U.S., once identified and deemed unacceptable, are addressed by industry through product modification and new products. 2.2 Fire Protection Engineering Performance-based engineering for fire protection has the potential to reduce property and life losses while promoting fire safety through innovation. Many countries around the world have implemented PBE codes. The motivation for PBE in the area of fire protection is strongly economic. Fire protection engineering traditionally has relied on component qualification testing (according to ASTM Standard E119 or ISO Standard 834). Acceptance criteria are based on the component or substructure surviving a “standard” fire exposure for a prescribed rating period (in hours) without collapse or fire spread to compartments that are adjacent to the one in which the fire initiates. The rating period depends on the element, and is achieved mainly through insulation or other passive protection measures. Such acceptance criteria have been used for nearly a

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century. However, this approach does not encourage innovation in design, and the level of fire protection afforded is undefined. The standard ASTM E119 fire exposure is unrealistic, as it represents a fire with an inexhaustible supply of fuel, and does not take into account differences in fire loads, compartment ventilation and other factors that are known to impact the severity of the fire [9]. This approach also neglects issues related to the time required to safety exit the building versus the expected “human viability” time (i.e., the amount of time that humans can be expected to survive within the building). The Society of Fire Protection Engineers (SFPE) and National Fire Protection Association (NFPA) have developed an engineering guide to performance-based fire protection analysis and design of buildings, which outlines a process for using a performance-based approach in the design and assessment of building fire safety [5]. The intent of the guide is to provide a process by which engineers can develop fire protection measures that provide acceptable levels of safety without imposing unnecessary constraints on other aspects of building design and operation. 2.3 Earthquake-resistant design and rehabilitation Developments in performance-based seismic engineering (PBSE) were stimulated by the Northridge Earthquake of 1994, where the less-than-satisfactory performance of many code-compliant buildings led to efforts to improve design practices so as to minimize economic losses as well as to protect public safety. The SAC Project for Steel Moment Frames and the CUREE-Caltech Woodframe Project are two products of these recent efforts. PBE is not limited to the design of new construction. The U.S. Federal Emergency Management Agency (FEMA) Prestandard and Commentary for Rehabilitation of Buildings [5] provides a framework for performance-based seismic retrofit of existing structures including woodframe buildings. Table 1 shows a short excerpt from the FEMA Prestandard demonstrating the proposed connection between performance objectives and seismic hazard level. 2.4 Wind Engineering Guidelines for performance-based engineering for extreme winds and hurricanes in the U.S do not currently exist. Significant advances in wind load determination through a combination of computational and wind tunnel modeling have been made in the past three decades. Extreme winds are not viewed as being as significant a threat to human life as earthquakes primarily because of the opportunity for prior warning. Post-disaster surveys indicate that the major economic losses stem from damage to the building cladding and contents rather than the main load-bearing structural system. At present, most wind-load/structural-response models can only capture the structural behavior to the point of first failure, e.g. loss of roof sheathing; once local damage occurs, the distribution of external and internal pressures changes and the model is no longer valid. A database to describe wind pressures on low-rise buildings, developed from extensive wind tunnel testing on scale rigid models, is available (see [10] for an example) but still only provides pressures for use in first-failure models or for comparison to prescriptive techniques (e.g. combining ASCE 7-05 with AF&PA/ASCE 16) [11,8]. Because of the nature of the economic losses that woodframe buildings are likely to incur in extreme

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winds, advances in PBE in this area are likely to require integrated models of buildings in which the response of load and non-load bearing systems can be analyzed together. Table 1. Target Building Performance Levels from FEMA 356 (2000) Target Building Performance Levels (FEMA 356)1

Overall Damage General

Nonstructural components

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Collapse Prevention Severe Little residual stiffness and strength, but load bearing columns and walls function. Large permanent drifts… Building is near collapse. Extensive damage.

Life Safety Moderate Some residual strength and stiffness left in all stories. Gravityload-bearing elements function... Building may be beyond economic repair. Falling hazards mitigated but many architectural, mechanical, and electrical systems are damaged.

Immediate Occupancy Light No permanent drift. Structure substantially retains original strength and stiffness. … Fire protection operational Equipment and contents are generally secure, but may not operate due to mechanical failure or lack of utilities.

Operational Very light No permanent drift. Structure substantially retains original strength/stiff… All systems important to normal operation are functional. Negligible damage occurs. Power and other utilities are available, possibly from standby sources.

Excerpted (in part) from [6] Table C1-2.

2.5 Status of PBD for Flood and Snow The writers are unaware of any performance-based engineering standards or guidelines that specifically address either flood or snow induced loading. Snow loads are a significant source of economic damage to building roofs in the northern tier of the United States. Snow loads historically have been treated within ASCE Standard 7 [11]; criteria for flood loads on structures located in river and coastal areas was added to the Standard in 1995. In both cases, the focus of their application is on the collapse prevention performance level of assemblies or systems. The consideration of economic consequences above and beyond life safety in PBE might place these loads in a different light. For example, durability considerations might require that flooding, snow or rain be considered when damage due to mold is a consideration. 2.6 Status of PBD for Durability Durability of woodframe structures, components, and assemblies is currently prescriptive in the United States. Durability issues such as termites, decay, and molds pose a problem that is somewhat unique to the woodframe industry in that these are seen as a weakness of the material and not a design or engineering defect. According to Leicester [12], durability issues such as these can be analyzed in the same manner as structural attributes. Leicester proposes the establishment of hazard maps similar to wind speed maps and the development of reliability-style calculations to estimate the probability of reaching

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various limit states depending on the durability-related design solution selected. There is a similar effort underway within the ISO Technical Committee TC98 (Design of Structures), Subcommittee 2 (Reliability).

3. PBE of Woodframe Structures: Challenges During the 1st Invitational Workshop on Performance-Based Design of Woodframe Structures [2] five performance objectives for woodframe structures were outlined by participants from academia and industry for further development and investigation. These five performance objectives included (1) occupant comfort, (2) continued occupancy, (3) manageable loss/acceptable damage/limited damage, (4) injury/life safety, and (5) general structural integrity/collapse. Each performance objective (level) must be combined with one or more hazards for which that performance objective might be pertinent. Table 2 presents the five performance objectives outlined during the workshop with six possible sources of environmental hazard. An “X” indicates which performance objectives might be tied to which of the seven hazards. Table 2. Tentative Performance Objectives and Hazards for Woodframe Structures Performance Objective Occupant comfort Continued occupancy Manageable loss/damage Injury/life safety General structural integrity / collapse

Hazard Flood Snow

Seismic

Wind X

X

X

X

X

X

X

X

X

X

X X

X X

X X

X X

X X

X X

Durability X

Fire

Associated with each performance objective must be a system or component response/indicator variable (deformation, force, damage, etc) that measure whether the objective is met and a probability that measures the likelihood that the objective is not met, taking into account the uncertainties in demand placed on the building system and the capacity of the system to withstand those demands. While absolute reliability in the presence of uncertainty is ephemeral, building performance can be enhanced through investments in risk reduction through good building practices. A probability-based approach facilitates sound tradeoffs between investment and performance. 3.1 Defining performance This is one of the most challenging aspects of PBE for woodframe structures. One must answer the questions: “What limit state(s) should be connected to each performance objective, and how should those limit states be defined quantitatively?” For example, the vast majority of researchers and practitioners in the field of earthquake engineering recognize that inter-story drift has become the default indicator of performance of woodframe buildings under seismic effects. As an indicator of incipient collapse, 3% inter-story drift (ISD) has been utilized. For life safety issues, 2% ISD has been used; 6

this ISD must be related to other life safety issues such as component and assembly failure. For performance-based wind engineering, component failures such as the loss of roof sheathing or roof truss connection failures are justifiable limit states. Both flood and fire will most likely be based on some level of damage sustained as a result of their occurrence. Continued occupancy (serviceability) must be tied to vibrations (such as the Trus JoistTM study discussed earlier), noise level, lighting, etc all of which are subjective and related to numerous “human factors”. 3.2 Design tools and supporting databases There is currently no software, commercial or otherwise, available for wind loading beyond first failure. Models must begin to address issues such as roof system response during pressure/suction, progressive failure, window effect, roof-to-foundation load path discontinuities, and their effect on the system as a whole. For modeling the response of woodframe structures to earthquake ground motion, the available models more suitable for research investigations than for engineering practice. Issues such as axial effects, diaphragm flexibility, corner effect, uplift modeling, and the ability to apply modular input of sub-assemblies (if desired), will need to be addressed for performance-based seismic engineering to become a reality for woodframe structures. A concentrated effort will be required to implement these advanced behavioral models into computer platforms suitable for engineering design office use, since PBE requires quantitative evaluations of system behavior for comparison with performance objectives. 3.3 Broader Impact of PBE on Woodframe Residential Construction Implementation of PBE for woodframe construction offers huge potential advantages for owners, builders, and manufacturers. As stated earlier, building codes typically focus on one type of limit state (strength) with the goal of eliminating structural collapse under extreme loading conditions. Historically, from an owner’s perspective, there are only two choices – either the building is code-compliant or it is not. Further, the owner assumes that “code-compliant” building implies full satisfaction of all of his/her needs. With PBE, clear statements can be provided of a much broader range of performance objectives. For example, what if an owner would like a building in which there may be loss of roofing but no water intrusion during a design level hurricane? What if a builder could offer potential buyers a building that can be affordably restored after a flood? PBE can provide the tools for manufacturers and installers of building systems to customize the structure to the needs and desires of the buyer. Insurance companies also have a financially vested interest in the reduction of losses for woodframe structures and this may offer another opportunity to further PBD.

4. Conclusions With the move toward performance-based engineering, there is the potential to clarify the objectives of design and to facilitate understanding of building code provisions by engineers and building owners. This move will enable the building developer or owner (consumer) to manage risk effectively and economically (without sacrificing life safety) by choosing to increase initial investments in design, quality of construction, and

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reduction in uncertainty against reduced future losses from natural and human-induced hazards. PBE will encourage structural designers to develop innovative solutions that might not otherwise be supported by prescriptive code provisions but that will enable the performance expectations of the owner to be met equally well. Such innovation will encourage competition in the building industry, as was envisioned during Operation Breakthrough in the 1970’s [3], and will inject a healthy measure of professionalism to the practice of engineering and construction of woodframe buildings.

5. References [1]Ellingwood, B. (1998). "Reliability-based performance concept for building construction." in Structural Engineering World Wide 1998, Paper T178-4, Elsevier (CDROM). [2]van de Lindt, J.W. (2005). “E-proceedings of the 1st International Workshop on Performance-based Design of Woodframe structures.” Fort Collins, CO, USA July 28-29, 2005; Available on-line at http://www.engr.colostate.edu/~jwv/ . [3] Performance criteria resource document for innovative construction, Report NBSIR 77-1316 National Institute of Standards and Technology, Washington, DC (available from NTIS). [4] SFPE 2005 SFPE Engineering Guide to Performance-based Fire Protection Analysis and Design of Buildings. Draft 2nd Edition (May 2005), Society of Fire Protection Engineers and National Fire Protection Association. [5]FEMA (2000). “Prestandard and Commentary for the Seismic Rehabilitation of Buildings.” Rep. No. 356, Federal Emergency Management Agency, Washington, D.C. [6]ATC (2003). “Preliminary Evaluation Methods for Defining Performance.” ATC-58-2, Applied Technology Council, Redwood City, California, USA.

Rep.

[7]Trus Joist (2005). “Floor Performance – TJ-ProTM Rating System.” A background document available at http://www.trusjoist.com/EngSite/. [8]AF&PA/ASCE 16 (1996) Standard for Load and Resistance Factor Design (LRFD) for Engineered Wood Construction [9] Buchanan, A.H. 1999 Implementation of performance-based fire codes. Fire Safety Journal, 32(4): 377-383. [10] Rigato, A., Chang, P., and Simiu, E. (2001). “Database-Assisted Design, Standardization, and Wind Direction Effects.” Journal of Structural Engineering, 127(8), 855-860. [11]ASCE-7 (2005). “Minimum Design Loads for Buildings and Other Structures.” American Society of Civil Engineers, Reston, Virginia, USA, 424pp. [12] Leicester, R.H., G. C. Foliente, W. Ganther, X. Wang, C-H Wang, and M. Nguyen. (2004). “Australian Houses: Monitoring and predicting Microclimates and the Durability of the Housing Envelope.”, Forest Products Society, Woodframe Housing Durability and Disaster Issues Conference, Las Vegas, NV.

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