NORTH AMERICAN STRUCTURAL PERFORMANCE TESTS OF LOW-RISE WOOD-FRAME BUILDING SYSTEMS

Transcription

1 NORTH AMERICAN STRUCTURAL PERFORMANCE TESTS OF LOW-RISE WOOD-FRAME BUILDING SYSTEMS Ronald W. Wolfe and Russell C. Moody ABSTRACT Low-rise wood-frame buildings have been viewed by the research and design communities as a collection of individual structural subassemblies. In many cases, these subassemblies are viewed as collections of individual framing elements forced to work together by load-distributing elements. During the past 40 years, many independent research studies have evaluated subassembly load capacities. These studies consider different load and assembly configurations. Recently, more powerful computers and analytical models have permitted scientists to develop analytical tools to evaluate stress-strain distributions within subassemblies and entire building systems. Structural performance data needed to verify these models are available but are not compiled in one source. This paper summarizes the available subassembly and assembly test data published by North American researchers during the past 40 years. INTRODUCTION Design and construction of low-rise, wood-frame buildings in North America have evolved as a combination of art and science. Although the art influence may be classified as cyclical, the science influence has focused the evolution on more efficient use of labor and materials to provide safe and affordable buildings. Projections for the future suggest continued trends toward use of more cost-effective methods and materials. This implies a growing need for engineering design support for structural applications of wood in low-rise buildings. In North America, wood has held a dominate position as a structural material for low-rise buildings for over 200 years. Throughout this time, many changes in construction details and structural wood components have reduced labor requirements and improved structural performance. Dimension lumber fastened with nails replaced heavy timbers connected with mortice and tenon joints. Panelized sheathing replaced board sheathing that required more than twice as many nails to achieve comparable diaphragm performance. Light-frame plate-connected roof trusses have been widely accepted in place of dimension lumber rafters as providing superior structural performance and reducing on-site construction time. Walls sheathed with gypsum wallboard and reinforced with steel strap diagonal braces or panelized sheathing also reduce on-site labor costs compared with walls constructed prior to 1940, which used lath and plaster interior and horizontal or diagonal board sheathing exterior to resist racking due to wind or seismic loading. Engineered structural components developed over the past 20 years are being used to stretch both roof and floor spans, providing greater versatility in the use of interior space. Performance standards for low-rise buildings are also in a constant state of evolution. Current standards developed to provide a basis for the acceptance of subassembly designs are based on experience with conventional building methods. However, as conventional methods change, Research Engineer and Supervisory Research Engineer, respectively, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain and not subject to copyright. XXII-1

2 some of these standards have been found lacking in aspects not previously considered. For example, with the development of longer floor spans resulting from improved stress grading, and engineered components such as wood I-beams and parallel chord trusses, the static midspan deflection limit of 1/360 of the floor span has been found to be an inadequate measure of acceptable floor performance. Natural frequency of vibration and damping have received recognition as possibly being more important than static deflection. Tests of rafter roof assemblies suggest that the conventional rafter-joist heel connection details are insufficient to resist design load with less than in. (0.38 mm) of slip. Should these connections be improved, or is the joint specification too stringent for this application? Serviceability limits for shear wall displacement under design seismic load is times the story height, or 0.5 in. (12.70 mm) in the case of conventional low-rise buildings. Is this adequate to prevent binding of tighter doors, which could block exits? Changing performance requirements for low-rise buildings are creating a demand for more highly engineered components and assemblies. Although segments of the wood construction industry have placed greater emphasis on meeting this demand, it is also attracting attention from competing materials industries (plastics and steel). These industries have proposed alternatives for conventional wood-frame construction. To continue to provide safe and economical structures, a better understanding of wood structural assembly interactions and system design is needed. OBJECTIVES AND SCOPE The purpose of this paper is to summarize test results for low-rise building subassemblies and systems. This summary is intended to serve as a basis for evaluating the strengths and weaknesses of these buildings and for evaluating the adequacy of innovative design alternatives. Computer models developed to predict assembly behavior may be verified or refined on the basis of these results. This summary will also provide a basis for judging where research effort is needed in order to permit characterization of assembly response to all anticipated loads. SUBASSEMBLIES Low-rise building subassemblies discussed include floors, walls, and roofs. In conventional light-frame structures, each of these subassemblies share common structural characteristics, but each must satisfy a different set of serviceability criteria. They are each composed of a set of parallel framing members (joists, studs, rafters) that define the primary plane of the system. These framing members are attached to a sheathing element that serves as a combination plate and diaphragm to distribute loads. In each case, these subassemblies are built with a degree of redundancy in that rarely does a single critical member exist whose failure at design load would constitute failure of the subassembly. High variability in strength of framing members thus leads to ductile type assembly failures rather than brittle type assembly collapse. As framing member strength variability is reduced, these assemblies may exhibit a threshhold failure mechanism or domino effect. If the first failure occurs at a load close to the average strength of all framing members, the redistributed load will most likely exceed the capacity of adjacent members. Floors While a floor system does serve as a horizontal diaphragm for purposes of seismic design, the response that gets the greatest attention in design is its reaction as a plate in resisting vertical loads applied perpendicular to its principal plane. Design needs in this area focus on defining ac- XXII-2

3 ceptability limits for static deflection and dynamic response. Studies cited in this paper are those addressing the characterization of floor response to vertical loads. Static Response -Before the widespread use of panel sheathings, diagonal board subflooring was the traditional method of covering wood floor joists. The composite structural action between the nailed board subfloor and the joists was relatively small (Russell 1954). The development and widespread use of both panel products and elastomeric adhesives since the 1950s have resulted in potentially much stiffer floors because of the T-beams or composite action between sheathing and joists. However, design procedures have generally not recognized this potential. Only floors with glued plywood have been permitted to include composite action in the design and then only for stiffness, not strength (Rose 1970). Several research programs since the early 1960s demonstrated the amount of composite action obtained by either nailing or both nailing and gluing plywood sheathing to the floor joists. A test series by the National Association of Home Builders (NAHB 1961) showed a 13-percent increase in stiffness with nailed plywood and a 38-percent increase with both nailing and gluing. Tests by Corder and Jordan (1975), Hurst (1970), Johnson and Angleton (1970), Polensek and Atherton (1970), Polensek (1969), Polensek and others (1972), Rose (1970), Williston and Abner (1962), and Penner (1972) showed considerable stiffness increases due to composite action. Other tests by Vanderbilt and others (1974), McCutcheon (1977), Foschi (1982), and Wheat and others (1986) also characterize stiffening effects due to composite action in floors. A summary of test results is provided by Sherwood and Moody (1989). Dynamic Response -Although code limitations reference only the static performance requirements, the majority of service problems with light-frame floors have to do with human response to floor vibration. Polensek (1969) and Polensek and Atherton (1970) were among the first to try to evaluate vibration characteristics of wood floors and to identify limits of acceptability. A number of other studies have been conducted to identify variables affecting wood floor vibration characteristics (Hanson 1972, Johnson and Angleton 1970) and to identify limits of acceptibility (Atherton and others 1976, Onysko 1988, Foschi 1982). Studies show a wide range of acceptability for transient vibration in floors, making it difficult to reach a consensus regarding design limits. The Canadian standards governing design of wood structures have proposed more stringent static deflection requirements, which increase the stiffness requirements for longer spans. This should increase natural frequency and decrease amplitude of vibration. Although this topic has generated much discussion over the past 10 years, research needs summarized by Vokey and Jones (1981) on the basis of their 1981 review of the state of the art in building vibrations are still valid in They cited a need for standardized vibration tests, design criteria based on results of those tests, and methods to improve the vibrational performance of existing floors. Walls Light-frame wall systems arc multifunctional. From a structural standpoint they must (a) resist normal wind loading and transfer this load to the foundation, floor, or roof diaphragms or to another perpendicular wall, (b) transfer gravity loads from upper floors or roof to the foundation, and (c) act as shear diaphragms in transmitting lateral (wind or earthquake) loads to the foundation. These three functions require that the wall behave as a plate, a column, and a diaphragm. Normal wind loading results in a plate-bending action. In this case, the composite wall section, consisting of stud and sheathing layers, may respond as an I-beam or simply as independent plate elements separated by the stud frame elements, depending on the rigidity of the sheathing-to-frame connection. Gravity load transferred to the wall results in axial compressive stresses carried primarily by the framing. In this case, the sheathing and its connections to the XXII-3

4 studs serve to prevent stud buckling in the plane of the wall. Shear loads are transferred through the walls to the foundation by way of shear stresses in the sheathing transmitted through the sheathing-to-frame connections. These shear stresses are typically not assumed to interact with the stresses produced by normal wind and gravity loads. In all three types of loading, the sheathing frame connections are stressed in lateral shear and their rigidity has a strong influence on how stresses are distributed among sheathing and framing members. Despite the occurrence of normal wind and bearing loads, the majority of research on light-frame walls has concentrated on their racking behavior. Racking -Racking or shear loads can be applied as a combination of a uniform load applied perpendicular to the wall length, in the plane of the wall, and a concentrated load or distributed live load from the roof diaphragm. The stiffness of walls under these types of shear loading is the most important factor influencing the distribution of lateral loads within the structures. Several techniques for analytically describing the nonlinear behavior of nailed racking walls have been proposed. Until the 1940s, conventional wood-frame structures used diagonal bracing or lumber sheathing for shear resistance. In 1949, guidelines were issued by the Federal Housing Administration for acceptance of panel sheathing to be used for shear resistance. These guidelines form the basis of standard panel tests. A standard procedure for the evaluation of racking strength of different sheathing materials is provided by the American Society of Testing Materials (ASTM 1977). This procedure (ASTM E 72-77, Standard Method of Conducting Strength Tests of Panels for Building Construction) is designed for static tests only and utilizes an 8-ft (2.4-m) square wood frame. Although this is a standard test, much of the research incorporating this standard deviates in speed of testing and panel configuration. Sherwood and Moody (1989) summarized most of the North American studies on wall racking that have influenced design decision-making during the 40-year period from 1948 to The National Research Foundation (NAHB 1967, 1971) reported results of a series of wall racking tests conducted in the late 1960s and early 1970s to evaluate effects of materials and wall configurations that were relatively new at the time. Two studies conducted under contract with the U.S. Department of Housing and Urban Development (HUD) included 15 tests of long walls with nominal 2- by 3-in. (standard 38- by 64-m) framing and various sheathing combinations and a series including tests of 11 exterior and 11 interior wall configurations using the ASTM E 72 test procedure. The first study consisted of three wall tests in each of five configurations. Each sheathed wall included sheathing on both sides of the wall and all walls were 12 ft (3.6 m) long by 8 ft (2.4 m) high constructed using single end studs, spaced 16 in. (406 mm) on center with double top plates. The second study involved variations in framing member width (1.5 in (38 mm), 2.5 in. (64 mm), and 3.5 in. (89 mm)), stud spacing (16 in. (406 mm) and 24 in. (610 mm)), diagonal bracing, sheathing (gypsum wallboard, low density fiberboard, plaster/gypsum lath, plywood), and the sheathing-to-frame connection (nails, screws, adhesive). Reported results included loaded deflections and residual deflection for loads of 1,200 lb (5 kn) and 2,400 lb (10 kn) as well as maximum load. Tuomi and Gromala (1977) investigated the relative effects of deviations in the ASTM E standard test. These effects included the speed of testing and panel configuration. The effectiveness of plywood sheathing in resisting lateral loads as compared to traditionally used diagonal corner bracing was investigated by Tissell (1983). Eighteen 8-ft (2.4-m) square shear walls were statically tested. Results showed that a wall only partially sheathed with plywood will develop racking resistance comparable to that of a wall with corner bracing. XXII-4

5 Lyon and Barnes (1979) evaluated the racking resistance of wall components with particleboard sheathing. Results of full-scale tests showed that panels oriented parallel to studs were stiffer than those oriented perpendicular. Price and Gromala (1980) investigated the racking strength of walls sheathed with structural flakeboards. Full-size panels (8 ft (2.4 m) square) and small panels (2 ft (0.61 m) square) were used in racking tests. They concluded that flakeboard walls were slightly stronger than plywood walls. Dolan and Foschi (1991) tested seven 8-ft (2.4-m) walls; four with waferboard and three with plywood sheathing. While their computer model showed the waferboard walls to be slightly stronger, the test results suggested that the waferboard walls were initially stiffer but any difference in strength was insignificant. Wolfe (1982) reported the results of a series of tests conducted to assess the contributions of the wall frame, diagonal bracing, corner attachments, and gypsum sheathing to the racking resistance of conventional light-frame walls. Evaluation of the contribution of gypsum wallboard also considered the effects of panel orientation, wall length, and the existence of door and window openings. Results suggested that (1) the frame, bracing, and wallboard racking strength contributions are additive, (2) horizontal panel orientation gives greater racking strength than vertical orientation, (3) wall strength is linearly proportional to uninterrupted wall length, and (4) corner connections can have a significant effect on wall racking strength and stiffness. Patton-Mallory and others (1984) conducted small-scale shear wall tests to investigate the effect of wall length, the additive nature of sheathing on two sides, and relative strength of interior and exterior wall construction. They conducted 200 tests of walls constructed using nominal 2- by 4- (standard 38- by 89-mm) framing similar to conventional walls except that the studs were 19 in. (0.48 m) long and spaced 12 in. (0.30 m) on center. A pinned steel frame was used to force the frame to deform as a parallelogram. This frame restrained separation of the top and bottom plates due to rotation of the studs and sheathing panels. These tests included four wall lengths in each of five wall configurations. Results indicated that shear capacity is proportional to wall length, double-sided shear behavior may be predicted as the sum of individual single sided wall racking behavior, and the racking resistance of interior walls with gypsum wallboard on two sides is more than 57 percent that of exterior walls consisting of plywood and gypsum sheathings on opposite sides of a stud frame. Tissell (1990) summarized results of 181 shear wall tests conducted by the American Plywood Association (APA) to update the data base used to support code values for shear wall capacity. Tests evaluated the effects of blocking, stud spacing, fastener type, size; and spacing, sheathing placement variables, and metal studs. Test walls were built and loaded in accordance with provisions of ASTM E72. Timber frame shear wall performance under cyclic and dynamic loading has received limited attention in North America. Young and Medearis (1962) tested shear walls sheathed with plywood to determine energy absorption characteristics. They found damping ratios averaging 0.10 for shear walls with plywood on both sides and 0.07 for shear walls with plywood on one side were recommended. Freeman (1977) tested several building partitions constructed of wood or metal studs and sheathed with gypsum board or plywood. Esimated damping ratios ranged from 0.07 to Polensek and Laursen (1984) tested six 8-ft- (2.4-m-) square walls under cyclic load at varying displacement amplitudes and free vibration to quantify the effect of construction variations and cyclic loading to various displacements on wall stiffness and natural frequency. The natural frequency of the test walls decreased from 30 to 10 Hz, with initial displacements changing from 0.1 to 0.55 in. (2.5 to 14.0 mm). Gray and Zacher (1988) summarized a study of 13 wall tests subject to cyclic loading in which the major focus was the effect of overdriven nail and staple fasteners on the energy dissi- XXII-5

6 pation characteristics of plywood-sheathed shear walls. These tests indicated a wide disparity between the ductile behavior of walls with flush driven nails and the brittle behavior of walls with nails over-driven one third the depth of the plywood panels. Gray also concluded from the four gypsum wall tests conducted in this study that the 1987 code values for seismic shear resistance of gypsum-sheathed panels are much too high. Oliva (1990) showed that walls tested under cyclic loading exhibited a brittle failure mechanism not apparent under static monotonic loading. This behavior was accompanied by a damage mechanism that resulted in lower ultimate strengths for the cyclic loaded walls than was obtained for static tests. The shear (racking) loads introduced by wind or seismic loads are normally a small percentage of the shear buckling capacity for light-frame walls. Thus, shear buckling is not normally a concern with conventional light-frame walls. The latest concern in the area of racking performance is the shear capacity of internal partitions. Problems with the standard test procedures used to compare shear wall sheathing performance (ASTM E 72) and shear wall racking performance (ASTM E 564) were first brought to light in tests of mobile home shear walls conducted by the National Association of Home Builders (NAHB 1990). They concluded that tests of walls supported on a flexible foundation had 70 percent of the shear capacity of the same wall tested on a rigid foundation. Phillips (1990) also showed a wall stiffness anomaly that could possibly be explained by differences in the foundation support. The committee responsible for building shear wall test standards obviously must address the question of racking strength sensitivity to boundary conditions. Bending -Conventional light-frame walls are known to adequately resist most bending loads caused by wind; in fact, bending failures do not occur in service. One full-scale test (Tuomi and McCutcheon 1974) showed that the initial failure location was at the wall-to-floor connection, not in the wall itself. Only after reinforcing this connection did a wall stud fail. Actual tests of walls (Gromala 1983, Polensek 1976) constructed using common wood-building materials show that conventionally built walls can resist high bending loads. Although wall strength is not a serious concern, wall stiffness greatly influences the distribution of wind loads to the roof diaphragm, end walls, and interior walls. An analysis technique to account for both composite and two-way action of walls under both axial and bending loads was developed by White (1975) and Polensek (1976). This technique was used in a finite element computer program called FINWAL, which was developed for the analysis of laterally loaded bearing walls. This model predicts both strength and stiffness of walls. The wall strength analysis is based on a wall stud bending failure mode. Strength can be predicted even after one stud fails, provided that stud is assumed to carry no load after rupture. Several related studies by Polensek (1976) and Polensek and White (1979) examined the characteristics of different input parameters. Gromala (1983) tested the sensitivity of wall response to some of these parameters. Compression -In most light-frame applications, the axial compression load is only a small percentage of the buckling load for the wall. However, for buildings containing more than two stories, bearing capacity of the first floor walls can present a design problem. The low incidence of wall failure due solely to compressive loads has caused this wall failure mechanism to receive little attention from researchers specializing in low-rise light-frame structures. Espenas (1970) compared the performance of gypsum-sheathed Douglas fir and Engelmann spruce stud walls in compression. For walls with 10-ft- (3-m-) long studs, the studs failed due to stress concentrations at knots or brace notches. For walls with 8-ft (2.4-m) studs, initial failure was due to compression perpendicular to the grain in the plates. A diagonal brace and the gyp- XXII-6

7 sum sheathing provided sufficient restraint to prevent buckling in the plane of the walls, and the 2 by 4 studs had sufficient section to resist out-of-plane buckling. Although buckling of nailed walls is not believed to be a practical consideration, the possibility of buckling should be considered with thinner walls, such as those commonly used in the interior partitions of many mobile homes. Buckling of isotropic and orthotropic plates is treated extensively in engineering texts on buckling. Roofs Roof assemblies for low-rise buildings cannot be treated as generically as floors and walls. A wide variety of roof configurations might be classified as conventional. Both roof shape and construction details contribute significantly to the structural performance of the building system. In general, roofs are classified as flat (pitch less than 0.25 in 12) or pitched, rafter or trussed. Flat roofs may be treated like floors in that they behave like a plate in resisting vertical loads and like a diaphragm in distributing horizontal loads to supporting walls. However, pitched roofs are constructed with a wide range of roof slopes and framing details that affect how loads are distributed. There is also some variation in the load-distributing elements used for pitched roofs. The majority of these roofs are constructed with sheathing attached to the framing members, but some roofs designed to support a tile wear surface use battens spaced 12 in. (305 mm) on center. The slope of a pitched roof affects load distribution. Both horizontal and vertical loads will have force components in the plane of the sheathing as well as perpendicular to it. The magnitude of each is related to the angle between the principal axis of the force and the plane of the roof sheathing. Resistance to horizontal and vertical displacement affects how a roof assembly distributes load to the rest of the building system. Unless a load is oriented normal to the surface of the roof, both horizontal and vertical force components will be transferred to the roof surface reactions. In the case of a triangular-shaped roof truss, a large portion of the horizontal force component is resisted by the horizontal bottom chord member. For dimension lumber rafter assemblies, a nailed lap joint commonly used to fasten the rafter to a ceiling joist is slightly less effective in resisting these forces, and framing members used to support a sloped ceiling provide little or no resistance to horizontal forces. In the latter cases, walls take on the added function of resisting horizontal thrust developed in response to gravity loads on the roof. The stiffness and strength variability of the framing members affect the importance of the load-distributing element. Pitched trusses, normally designed for strength with little concern for design load deflection, exhibit relatively low stiffness variability compared to dimension lumber rafters. Thus, for a uniformly distributed design load, the load distribution element of a rafter roof plays a much larger role than that of a trussed roof. The importance of the load-distributing element increases as load approaches the capacity of the weakest framing member. By distributing load away from soft spots, the load-distributing element shifts the roof load capacity behavior from that best represented by a weakest-link model toward that predicted by a parallel system model. If the strength of the weakest member is close to that of the strongest, the loaddistributing element will have little effect on load capacity of the roof. However, if there is a wide disparity between weakest and strongest framing members, the load-distributing element could have a major impact on roof load capacity. Another major influence on load capacity of pitched roofs is the stiffness and strength of the sheathing. To distribute load away from a limber or failed framing member, the sheathing strength and stiffness must be large enough to permit it to carry a significant portion of the load over a span of two framing spaces without deflecting enough to exceed the strain capacity of the framing member. This capability is influenced by the sheathing connections and by its mechanical and physical properties. XXII-7

8 Research conducted to characterize the behavior of pitched roof systems commonly used in North America is sparse compared to that for floors and walls. This is partly because of so few complaints about roof system behavior. Problems that have prompted special research attention include truss uplift and roof tiedowns. However, most roof assembly research has focused primarily on understanding the load distribution mechanisms in an attempt to develop more efficient system design procedures (Wolfe 1990). Uplift -Uplift has two common meanings related to roof assemblies. One refers to seasonal arching of trusses (truss uplift), and the other refers to the upward force on a roof surface due to the partial vacuum resulting when wind blows across the ridge of a pitched roof. Truss uplift is a seasonal movement of trusses that causes gaps to appear at the intersection of the ceiling and interior partitions oriented perpendicular to the span of the roof trusses. A number of researchers (Percival and Comus 1979, Onysko and others 1979, HUDAC 1980, Gorman 1984, and Lischkoff 1985) studied this problem and agreed that it was caused by a combination of the increased dimensional change with moisture characteristic of juvenile wood and heavy insulation over the bottom chord, which accentuated the difference in change of moisture content of top and bottom chords during the heating season. Roof tiedowns are often discussed after a major wind event. This is the primary cause of failures of roofs subject to wind uplift forces. Conner and others (1987) used wind speeds measured in tornados to assess the expected uplift load at the rafter to bearing plate connection. Comparing these loads to results of uplift tests conducted on conventionally nailed roof-to-wall connections, he concluded that while these connections do not have the strength to resist most tornado-force winds, connections could readily be engineered to withstand these uplift forces. Vertical Load Distribution -Research conducted to characterize roof structural behavior has concentrated on interactions between sheathing and framing elements as the mechanisms that distribute loads and cause the roof to behave as an assembly rather than a series of independent framing members. These load distribution mechanisms have been placed in two general categories: composite action and load sharing. Composite action covers the interaction of sheathing and framing members to increase the effective stiffness of the framing member. Load sharing covers a combination of effects that result in loads being distributed away from weak members. The most important considerations in this area are the diaphragm and plate action of the roof sheathing. Work by Rose (1970) and Wolfe and LaBissionere (1991) suggests that composite action for nailed connections reaches its maximum due to excessive nail withdrawal before the framing members begin to fail; therefore, for nailed-only connections, the composite action influence on strength is minimal and highly variable. A number of studies have been conducted in an attempt to shed light on the evasive concept of load sharing. A load sharing factor for repetitive member assemblies is justified on the premise that the strength of an assembly is greater than that predicted on the basis of weakest link assumptions. Thornburn and Schriever (1962) tested a variety of rafter roofs with plywood sheathing and board sheathing. Their comparison of rafter deflections and strengths inside an assembly to those tested outside the assembly did little to support the 15 percent load sharing increase factor proposed by ASTM (1962). They concluded that the plywood had varying effects on strength, ranging from 8 percent to 50 percent, but no effect on stiffness. Board sheathing had no significant effect on rafter stiffness or strength. More recent studies conducted on trussed roof assemblies (NAHB 1975, Nicol-Smith 1977, Tuomi and McCutcheon 1974, Wolfe and McCarthy 1989, Wolfe and LaBissonere 1991, LaFave 1990) all indicated that uniformly applied loads were not uniformly distributed among trusses in XXII-8

9 the assembly. A number of these studies indicated that loads were redistributed among the various trusses to the point that all trusses were carrying close to their full load capacity at the same time. When one truss failed, the assembly failed. In the study by Tuomi and McCutcheon (1974), loads were redistributed after the premature failure of a truss web. A 35-percent increase in load beyond that of the first failure resulted in a sudden failure of the assembly. In many cases, when the assembly threshhold load was reached, all trusses in the assemblies exhibited the same failure mode, suggesting that the first truss to fail set the trend by focusing a sudden stress increase at that location in each adjacent truss and causing a dominoing failure. These results do not support the premise that assembly load capacity is equal to the sum of individual truss capacities, but it does indicate that the assembly capacity is not limited to the strength of the weakest truss. Studies by Wolfe (1989, 1991) and LaFave (1990) indicated that the distribution of loads is affected by stiffness variability, boundary conditions, and truss location in the assembly. Whenever there is a wide variation in truss stiffness, the stiffer members will attract more than their share of the load. When there is little variation in truss stiffness, truss load may be determined by its location with respect to an end wall or it may be influenced by slight variations in camber of adjacent trusses. Percival and Comus (1980) studied the distribution of loads from the area of a roof hip detail to the truss that supported the upper ends of the hip rafters. They found that the truss carried more than 30 percent of the load applied in the area of the roof hip. This is one of the few studies conducted on load distribution in hip roofs. Researchers dealing with structural testing and modeling of roof assemblies would benefit from the development of assembly database guidelines. Many roof assembly tests have answered specific questions about. stiffness and strength of a particular roof configuration. Lack of sufficient test data make it difficult to use this information to check computer model predictions of assembly load distribution characteristics to the point of failure. Recent developments in computer monitoring systems, such as those reported by Wolfe and McCarthy (1989) and Lafave (1990) facilitate the measurement of all assembly reaction as well as essential displacements. A guideline covering properties to be measured and database storage format recommendations would add value to assembly test results by allowing them to be easily incorporated in a universal data bank of roof assembly test results. Folded Plate Roofs -Brown (1958) presented test results and structural analysis methods to demonstrate the feasibility of using a folded plate roof design. This design method takes advantage of the diaphragm stiffness of plywood sheathing panels to provide a relatively inexpensive yet reliable roof assembly. Diaphragms In considering full-structure behavior of wood frame buildings subject to wind or seismic loading, the performance of horizontal as well as vertical diaphragms is a key issue in characterizing load distribution and energy dissipation. Two extensive bibliographies have been published on this subject (Carney 1977, Peterson 1983). Allowable shear tables for horizontal diaphragm design currently used by code agencies originated from static floor tests performed by the Douglas Fir Plywood Association (Countryman 1952, 1955). Various types of plywood sheathing, fasteners, and framing were considered. These tables were later expanded as a result of additional tests performed by Tissell (1967, 1977). XXII-9

10 Similar tables for shear walls sheathed with plywood and particleboard have been published, again the result of experimental testing. In the 1980s, a number of studies were conducted to form a basis for judging the contribution of structural diaphragms to system response to static, cyclic, and dynamic loads. Atherton (1981) conducted cyclic static tests on several 16- by 48-ft (4.9- by 14.6-m) diaphragms with waferboard and particleboard sheathing. He concluded that no significant increase in stiffness or ultimate load resulted from increasing nail size from 8d to 10d. However, increasing panel thickness (7/16 in. to 5/8 in. (11.1 mm to 15.9 mm)) or increasing the number of nails was shown to increase strength and stiffness. Staggered panel patterns were found to be slightly stiffer than nonstaggered panel patterns at ultimate loads. It was concluded that static load cycling had no effect on ultimate strength. Falk and Itani (1987) tested 10 diaphragms to evaluate natural frequencies, damping, and stiffness properties. These diaphragms ranged in size from 8 by 24 ft (2.4 by 7.3 m) (walls) to 16 by 28 ft (4.9 by 8.5 m) (ceilings and floors). The study concluded that natural frequencies range from 8 to 29 Hz, damping ratios range from 0.09 to 0.34, and the stiffness of wall diaphragms with openings is porportional to the uninterrupted wall length. Polensek and Laursen (1984) tested six walls to study the effects of material properties on seismic response of wood-frame building systems. In addition to all the attention given wall and floor diaphragms, Johnson (1972) and Zahn (1972) also investigated the shear stiffness of roof systems. Hoagland (1981) discussed the diaphragm design of post-frame buildings. Tests were performed to determine the in-plane strength and stiffness of 8- by 12-ft (2.4- by 3.6-m) roof panels with metal sheathing. Polensek (1975) tested 34 wood joist floors for damping capacity using horizontal vibrations. These specimens ranged in size from 6 by 20 ft (1.8 by 6.1 m) to 16 by 24 ft (4.9 by 7.3 m) and were sheathed with various materials. Damping ratios for the floors ranged from 0.07 to GangaRao and others (1980) studied the behavior of timber diaphragms subjected to harmonic vibrations. Five full-scale (16- by 24-ft (4.9- by 7.3-m)) diaphragms were dynamically tested. The dynamic behavior and failure patterns of diaphragms under dynamic loads were very similar to those under static loads, except that the ultimate failure load capacity was reduced to about half that of the static load capacity. Ewing and others (1980) reported their ongoing analytical and experimental investigation of wood diaphragms and unreinforced masonry walls subjected to seismic loads. Nineteen wood diaphragms (20 by 60 ft (6.1 by 18.3 m)) and 22 masonry walls of different configurations were designed and tested under static and dynamic loads. A lumped parameter analytical model including a nonlinear hysteretic element was developed. Results were not given, although dynamic tests were performed. Diaphragm performance could play a vital role in future design of wood structures. The diaphragm contribution of wood-based sheathing materials currently receives little recognition in conventional wood-frame structures due to their conservative repetitive member nature. As the state of the art in design of wood structures progresses and wood-building systems are developed to rely less on redundancy and more on engineered structural components, greater attention will be given to the design of wood-based diaphragms. This suggests a growing need for information on the design and construction of wood diaphragms. XXII-10

11 lntercomponent Connections An understanding of component interactions is essential for the evaluation of individual components as well as structural assemblies. The interaction of structural components define boundary conditions that affect the way loads are distributed in an assembly. While the importance of component interactions is obvious, their effect is often underestimated or ignored by standard tests of individual components or subassemblies. Standard tests of walls do a poor job of defining the true boundary conditions imposed by connected wall, floor and ceiling components. Truss tests rarely consider the effects of interior partitions, which can be fastened indirectly by way of ceiling sheathing, blocking nailed between bottom chords of adjacent trusses, or oriented perpendicular to the span of the truss and not fastened but still providing an interior bearing. Floor system tests also often ignore the effects of attached interior partitions that link the floor to the outer walls and ceiling of a building. Studies by Doyle (1969), Tuomi and McCutcheon (1974), Wolfe (1982), Polensek and Laursen (1984), Conner (1987), Phillips (1990), Gebremedhin (1991) and NAHB (1990) provide good examples of how component interactions affect the apparent stiffness and strength of the components as well as the assembly. These studies emphasize sensitivity of the structural performance of building subassemblies to possible boundary conditions imposed by component interactions. Characterizing the contribution of intercomponent connections is not easy. It requires a combination of three-dimensional analytical models and assembly tests designed to provide information about loads and compliance of adjacent components or subassemblies. Full-Scale Building Tests The performance of light-frame structures in North America under simulated wind loading has been reported in the following, which are described in this section: Dorey and Schriever (1957), Hurst (1965), Yokel and others (1973), Yancy and Somes (1973), Tuomi and McCutcheon (1974), Stewart and others (1988), and Phillips (1990). Dorey and Schriever (1957) evaluated a one-story 24- by 36-ft (7.3- by 11.0-m) house under simulated wind and snow loads. The ceiling joist-rafter roof system withstood a 73-lb/ft 2 (3.5 kn/m 2 ) snow load before failure, which was 43 percent above the 50-lb/ft 2 (2.4-kN/m 2 ) design load. The walls were constructed with 1 by 4 let-in diagonal wind braces and covered on the inside with 3/8-in. (9.5-mm) gypsum wallboard, but the exterior sheathing was purposely omitted. These walls were loaded with the equivalent of a 120-mile/h (193 km/h) (36.8-lb/ft 2 (1.8 kn/m 2 )) wind load, which was considerably above the design speed. Lateral distortions were relatively small at this load (<0.12 in (<3.0 mm)), but there was some evidence of cracking in the gypsum wallboard. Their test demonstrated that light-frame structures built without exterior Sheathing can be capable of good field performance. The experimental house evaluated by Hurst (1965) was specifically built for his study and was evaluated during various stages of construction and dismantling. Wall movement under simulated wind loading was resisted primarily by the 3/8-in. (9.5-mm) plywood exterior wall sheathing, and it was difficult to quantify additional contributions of interior walls and gypsum wallboard sheathing. Specific information on the load versus deformation behavior of the shear walls is not included. Foundation failure was noted at 20-lb/ft 2 (0.96-kN/m 2 ) lateral loading. The minor racking distortions of the end walls and ballooning of the loaded walls were similar to those found in the work by Dorey and Schriever (1957). XXII-11

12 Yokel and others (1973) evaluated the performance of a conventional two-story house under simulated wind loads up to about 25 lb/ft 2 (1.2 kn/m 2 ). Walls were constructed with gypsum wallboard on both faces and diagonal 1 by 4 wood braces in the corners. Lateral translation at the story levels, called drift, was considerably less than the level that forms the basis for the design of medium- and high-rise buildings at a lateral load of 25 lb/ft 2 (1.2 kn/m 2 ). The floorceiling diaphragm between the first and second story acted rigidly, while the upper ceiling diaphragm experienced significant in-plane distortions. Yancy and Somes (1973) evaluated both the stiffness and strength of a housing unit typical of a factory-built module. This unit was considerably more flexible than the conventional house tested by Yokel and others (1973). Tuomi and McCutcheon (1974) evaluated the lateral resistance of a single-story light-frame structure during various stages of construction to determine the effects of window and door openings, interior gypsum board, and other structural components. They found that the addition of gypsum wallboard and siding nearly doubled the lateral stiffness of the structure as sheathed with 3/8-in. (9.5-mm) plywood. The simulated wind loads were progressively increased, and initial failure occurred at the sole plate at a lateral pressure of 63 lb/ft 2 (3.0 kn/m 2 ). Upon reinforcing this weakness, a pressure of 123 lb/ft 2 (5.9 kn/m 2 ) was reached before the structure failed in the foundation. Nelson and others (1985) studied the structural behavior of wood shear wall assemblies through experimental testing. Seven wall assemblies used in manufactured buildings were tested. The assemblies consisted of two side walls and one interior shear wall. For the loading configuration used, interior shear walls located on the windward side of the assembly assumed a larger portion of the applied load than interior shear walls located on the leeward side of the assembly. Stewart and others (1988) tested two full-scale manufactured houses to evaluate their structural response to horizontal wind loading. The study attempted to identify the contribution of transverse shear walls to the structural capacity of the building system subjected to concentrated as well as uniformly distributed transverse loading. Due to torsion of the building shell, the interior walls did not exhibit as much racking deformation as the end walls. Due to flexibility and energy dissipation characteristics, the building systems were able to sustain loads three times that expected from a design wind with relatively minor damage. This study will serve as a guide to future tests and provides a basis for analytical modeling of these structural systems. Phillips (1990) tested a full-scale building to evaluate the distribution of building shear loads to interior and exterior shear walls of varying stiffness. Wall sheathings included gypsum, plywood, and T1-11 siding. His study evaluated four different shear wall configurations. Each was tested under a load of A800 lb (±3.5 kn) cycled three times. Test boundary conditions included individual walls with one side sheathed, then two sides sheathed, then attached to transverse walls. A roof to ceiling diaphragm was then attached, and the entire building was loaded in horizontal shear under single cycles of load at 800-lb (3.5-kN) increments from ±2,200 lb (±9.8 kn) to ±7000 lb (±31.1 kn). Wall test variables included openings, sheathing materials, and boundary conditions. Two of the walls were supported on a relatively rigid foundation, and two were supported on simple span floor joists. Stiffness did not appear to be the determining factor in regard to wall shear load. Possibly due to the flexible support under the interior walls, their contribution to system performance was not proportional to their relative shear stiffness. Due to the wide variation in structural configurations of low-rise buildings, developing a standard test procedure for full-scale structural assemblies is not feasible. However, it would be beneficial to provide a guideline to highlight those aspects of building performance necessary for the development and verification of three-dimensional structural models. Such a guideline could XXII-12

13 tell what to test, how to test it and how to document the test results so that they can be easily retrieved and used. CONCLUDING REMARKS A review of available literature dealing with tests of low-rise building structural components, subassemblies, and full-scale buildings indicates that the areas of greatest concern have been floors and walls. For floors, static response to gravity loads has received the greatest attention; for walls, the focus of past research has been racking resistance. Sufficient work has been done in these areas to give us a thorough understanding of wall and floor subassembly response to these loads. While floors have been tested for dynamic response to vertical loads and diaphragm stiffness, no standard test procedures have been established, and there are no internationally recognized acceptance criteria. At the present time, racking is still the primary concern for wall design. However, standard test procedures may need to be revised to account for boundary conditions actually imposed on shear walls in service as well as combined bearing and lateral wind forces. Low-rise building roof assemblies have not received much research attention. It is difficult to get support for tests of subassemblies with the performance record of trussed roof assemblies. The greatest problem facing light-frame roof assemblies is damage due to wind uplift, and that is primarily a problem of poor connection details and lack of attention to detail by building inspectors. As long as current construction methods remain competitive, there will be little demand for further roof assembly testing. If conventional building spans start to increase, or pressure from competing materials forces truss designers to extend truss spacing, it would be beneficial to know the extent to which conventional roof reliability depends on load redistribution and how that is affected by sheathing stiffness and truss spacing. Finally, the area that has received the least research attention is the subassembly interaction mechanisms. To model full structure performance on the basis of a collection of subassembly models, it is necessary to know how each subassembly affects the boundary conditions of adjoining subassemblies. These are difficult mechanisms to study and often require the instrumentation and testing of full-scale buildings. A few such tests have been conducted, but with the development of microcomputer technology, it is now more feasible to monitor a large number of channels simultaneously to get a more complete picture of how these subassemblies interact. Advancements in the areas of data collection systems and three-dimensional analytical models have opened the door to characterizing the structural performance of full-scale structural assemblies. The expense and complexity of these tests warrant extra care in collecting any information of importance to analytical model development and verification. A guideline developed to highlight and prioritize important assembly performance parameters could facilitate the collection and use of assembly test results. Such a guideline would discuss assembly parameters, data collection devices, data filtering and reduction, and database formats, all oriented toward future use for analytical model development and verification. Low-rise wood-frame building technology has come a long way during the past 50 years, yet we still seem to be on a threshold of new and innovative developments. The fact that we can identify research needs indicates that there is still room for improvement. Combining advancements in computer technology, developments in the areas of engineered structural components, and a major advance into the commercial-industrial building markets during the past 20 years, the future appears bright for low-rise wood structural systems in North America. XXII-13