Journal of Wind Engineering and Industrial Aerodynamics, 36 (1990) 699-707 699 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Improving Wind Resistance of Wood-Frame Houses

Henry Liu I, V. S. Gopalaratnam 2 and F. Nateghi 3

ABSTRACT

Wood-frame houses (light-frame timber construction) are one of the types of buildings most extensively damaged in high winds. Post- disaster investigations have identified certain weak links in this type of building, including the use of toenails to anchor rafters and ceiling joists to the bearing plates, the lack of wall resistance to both uplift and racking, the lack of wall anchorage to foundation and so on. Techniques to improve these weak links are discussed. A new research project aimed at developing an analytical method to calculate the response of wood-frame houses to high winds is described. Successful development of this method will enable the determination of the forces and moments on the joints of conventional wood-frame houses caused by high winds under various conditions, and a systematic assessment of the adequacy of the joints to resist high winds.

INTRODUCTION

Each year, high winds generated by hurricanes, tornadoes, downbursts, and mountain downslope winds cause several billion dollars of damage to buildings in the United States. The damage is most wide-spread for non-engineered and marginally engineered wood-frame houses (light-frame timber construction)-- the way most homes, motels, stores, shopping centers, office buildings, schools and churches are built. An improvement in the wind-resistance of such buildings will go a long way toward reducing wind damage. The purpose of this paper is to identify the problems with conventional light-frame timber houses with respect to their vulnerability to winds, and to offer possible practical solutions. Research to better understand the response of wood-frame houses to high winds, including a new NSF (National Science Foundation) research project at the University of Missouri-Columbia aimed at developing an improved analytical method to predict the response of

i. Professor of Civil Engineering, University of Missouri- Columbia, Columbia, MO 65211, USA.

2. Assistant Professor of Civil Engineering, University of Missouri-Columbia, Columbia, MO 65211, USA.

3. Instructor, Department of Technology, Lincoln University, Jefferson City, MO 65101, USA.

0167-6105/90/$03.50 © 1990--Elsevier Science Publishers B.V.

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wood-frame houses to high winds, will be discussed. This paper is based on the reports of numerous investigations conducted immediately after wind disasters (namely, post-disaster investigation reports), on recent studies to determine the effectiveness of wood-frame joints, on an ASCE Task Committee Report (Liu, Saffir and Sparks, 1988), and on preliminary findings of the NSF project at UMC. Opinions expressed herein are strictly those of the authors, and not of NSF or the ASCE Task Committee.

FINDINGS FROM POST-DISASTER INVESTIGATION

The poor performance of conventional wood-frame houses in hurricanes and tornadoes has been reported in numerous post-disaster investigation reports conducted in the United States such as (Dikkers, Marshalls and Thom,1970), (Mehta et ai,1975 & 1981), (Chiu et ai,1983), (Savage et ai,1984), (Kareem,1985), and (Sparks,1985). These investigations repeatedly found that the most common damage to wood-frame houses by wind is roof failure, and that once the roof of a house has failed in a wind storm, the house loses its structural integrity and total destruction of the house often follows immediately. The cause for roof failure is generally attributed to poor tiedown between the roofs and the walls. Similar conclusions have been reached from post-disaster investigations conducted in Australia such as (Walker,1975) and (Leicester and Reardon,1975). Other frequent observations are described below:

-Roof damages by wind are generally caused by uplift forces. Light-weight roofs are far more vulnerable to wind than heavy roofs.

-Roof collapses are often preceeded by window or door failures on the windward side. Once a windward window or door is broken or forced open by high winds or wind-generated missiles, the internal pressure of the house suddenly rises. The combined high internal pressure and the high external suction lifts the roof off the house.

-Occasionally, the entire house (roof-and-wall assembly) is lifted off its foundation. This happens when the tiedown of the roof is good, but the wall is not properly anchored to the foundation.

-Occasionally, the large thrust of wind causes wood-frame houses to sway excessively until they fail in the racking mode. Lack of shear walls or lack of in-plane wall bracing is the cause of this type of failure.

-Gable roofs and flat roofs are more susceptible to wind damage than hip roof. The reason hip roofs are less vulnerable is that the strong suction normally generated at roof corners by wind is significantly reduced when the roofs are hipped.

-Roofs with large spans, carports, large doors and windows, and large roof-overhangs are all vulnerable to high winds.

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-Wind damage to roof covering, such as shingles or corrugated metal sheets, is initiated by stress concentration and fatigue failure at locations of contact with the fasteners (nails or screws). Failure to distribute the loads from fasteners to a large area of the covering, caused by the use of inadequate washers or load spreaders, is the principal cause of wind damage to covering.

RESULTS FROM EXPERIMENTAL INVESTIGATIONS

Three types of tests have been conducted in the past to determine the behavior of wood-frame houses under the action of strong winds. They are full-scale house tests, dissected joint tests, and fatigue tests of fasteners. They are briefly reviewed as follows:

Full-Scale-House Tests

Full-scale tests of wood-frame houses under simulated wind loads have been conducted both in the United States and in Australia. In the U.S., the tests were carried out at the U.S. Department of Agriculture's Forest Products Laboratory in Maddison, Wisconsin (Tuomi and McCutcheon,1974). The house tested was 24 ft wide (in the direction of joists or span), 16 ft long, and 8 ft tall (height of walls). The house had a gable roof of 1:3 slope, with rafters made of 2"x6" lumber at 2 ft spacing. Roof covering consisted of 3/8" thick plywood panels fastened to the rafters with six-penny nails. The wall studs were made of 2"x2" lumber spaced 2 feet apart, and were covered by 3/8" plywood panels. Construction details (nailing) were similar to that of conventional wood-frame houses.

The house was tested for its response to simulated wind loads at five different stages of construction. During each stage, a horizontal load was applied to the building in the spanwise direction. The displacements of the building at key locations were measured. The results were used to determine the wind resistance of the building in various stages of construction. The study found that while the racking resistance of the house was more than adequate, the connection between the sole-plate and the floor needed strenthening. This is the only known testing of a full-scale house for wind effects conducted in the U.S.

In Australia, full-scale testing of wind effects on wood-frame houses is being conducted at the Cyclone Structural Testing Station (CSTS), James Cook University, Townsville, Queensland, since 1983. Four full-scale houses have been tested so far; they are described in (Boughton,1983), (Boughton and Reardon,1983), (Reardon and Boughton,1984), and (Boughton and Reardon,1984). The first house tested was a 40-year-old house that had been condemned for sanitary reasons; the other three were new houses.

One of the new houses was a wood-frame house set on concrete stumps; the house had a corrugated steel roof. Simulated static and

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dynamic wind loads, both horizontal and vertical (uplift), were applied to the building by hydraulic rams. The tests consisted of three stages: (i) non-destructive tests to determine the response of the house to lateral loads, (2) destructive tests of the entire house to see how it fails under wind load, and (3) destructive tests of remaining structural components after the building has failed. Work is still continuing in these areas.

Dissected-Joint Tests

Conner et al (1987), at the University of Oklahoma, tested dissected rafter-to-top-plate joints under uplift forces, using both conventional joints connected by toenails and special joints connected by strapping, clip angles, extra nailing, 5-inch lag bolts and 8-inch lag bolts. They found that all the special joints, especially the 8-inch lag bolts, make stronger joints than toenailed joints for resisting uplift. They also calculated the anticipated wind speeds that would have caused the joints to pull out, assuming pure uplift created by wind.

Nateghi, Liu and Gopalaratnam (1987) tested dissected roof joints by applying both a force and a moment to each test joint. The angular deformation of the joint (i.e., the rotation of one joint member relative to another in the same joint) was measured and related to the applied force and moment to determine the stiffness of the joint. The joint stiffnesses were then used in a flexible joint model to predict the response of wood-frame houses under different loading conditions, including wind load. More work along this line was done by Nateghi (1988).

Fatigue Tests of Fasteners

Wilkinson (1976) conducted a set of experiments to assess the strength of wood joints to resist vibrating loads. He used both nailed and bolted joints and a sinusoidal load. It appears to be the only fatigue test of wood joints conducted in the U.S.

In Australia, Beck (1978) conducted a set of tests of the fatigue failure of corrugated roof--a common roofing material used for homes in Australia. He found that the use of a special washer- -"cyclone washer"-- on each screw can prolong the fatigue life of the corrugated roof by a hundred times! Tightening each screw also substantially increases the fatigue life. This shows the great importance of paying attention to details such as the type of washers used and the tightening of screws.

ANALYTICAL MODELS

Wood-frame houses are extremely difficult to analyze due to the diaphragm action of the walls and the ceilings, flexibility of wood joints, and the complex ways the joints are constructed. Due to these difficulties, no satisfactory methods exist today for calculating the forces and moments on individual members of wood~

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frame houses caused by wind load. Several researchers have attempted to develop methods to analyze such structures. Their works are

described next.

Maraghechi (1982) and Maraghechi and Itani (1984) developed a method to analyze the response of wood-frame trusses that use toothed-plate connectors. Each truss was modeled by using two types of elements: beams and joints. These elements were used in a matrix method to formulate the stiffness matrix of the structure. The joints were considered to be semi-rigid. Each joint was represented by three springs: rotational, tensile and shear. The spring constants were determined from tests. The method provides a way to analyze the response of wood trusses, but it cannot yet be used to predict the behavior of wood-frame houses because it does not have provisions for analyzing the diaphragm action of walls and ceilings.

Gupta and Kuo (1987) developed a finite element method to predict the behavior of wood-frame houses under lateral forces of wind or earthquake. This method takes into account of the effect of shear walls (diaphragm action). The response of the house was analyzed by using nine degrees of freedom: five representing horizontal displacements at the roof-ceiling level, two representing vertical displacements of windward walls, and two for uplift in studs at windward walls. They checked their model by comparing with test results reported in (Tuomi and McCutcheon,1974) based on full- scale testing. Even though the comparison seems to be satisfactory, many parameters had to be adjusted arbitrarily in order to match the test results. For this reason, the method cannot be used for predicting the response of houses that have not been tested before.

Nateghi, Liu and Gopalaratnam (1987), and Nateghi (1988) developed a flexible-joint model whereby the response of a wood- frame house can be analyzed. The model is two-dimensional, and the diaphagm action of walls parallel to wind is modeled by a pair of cross braces. Each flexible joint was modeled by a spring of rotational stiffness K. The angular deformation of any two-member joint (i.e., the rotation of one member relative the other) was assumed to be linearly proportional to the moment at the joint, with the proportionality constant being K--the rotational stiffness. The uniqueness of this model is that the value of K can be determined from tests of dissected joints. Two shortcomings of this model are: (i) The model is two-dimensional and hence it cannot simulate the diaphragm action of the ceiling which is in the horizontal plane-- the third dimension. (2)The model does not provide a way to determine the properties of diaphragms. The size of the cross braces to represent diaphragm action is determined from the overall response of the house rather than from the properties of each wall. This limits the usefulness of the model to houses that have been tested--the same problem that some of the other models have.

ONGOING NSF PROJECT

In 1988, the National Science Foundation (NSF) awarded a

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research project to the University of Missouri-Columbia to study the response of wood-frame houses to high winds. The main objective of the study is to develop a realistic and practical analytical model that can be used to predict the response of wood-frame houses under the action of high winds. Once such a model is developed, it can be programed on a computer to determine the response of various types and sizes of wood-frame houses under various conditions--a study difficult (time-consuming and costly) to do experimentally using full-scale houses. Such a study will yield reliable information on how to improve the design and construction of wood- frame houses in order to make them more wind-resistant.

To accurately determine the response of wood-frame houses to high winds requires two things: First, the wind loads must be predicted or determined accurately; and secondly, the response of the houses must be modeled accurately. For design purpose, wind loads are normally determined from building codes or standards such as ANSI A58.1-1982. However, for accurate assessment of wind effects on structures, one must consider many conditions that codes and standards do not consider, such as what happens to the internal pressure when a windward window is broken by wind, or what external pressure coefficient should be used for the corner of a hip roof as compared to that of a gable roof. Consequently, the wind loads used for this NSF study will not be simply those specified by codes; rather, they will be determined from current information availabe in wind engineering literature such as from wind tunnel data or field measurements.

The analytical model used for predicting structural response of wood-frame houses will be based on an improvement of the earlier model developed by the authors (Nateghi, Liu and Gopalaratam,1987), and (Nateghi,1988). The model improvement will consist of: (1)Making the model three-dimensional. This can be done, for instance, by representing each wall and the ceiling of each room by a pair of cross braces as was done for the walls in the two- dimensional model. (2) The properties of the cross braces that represent each shear wall should be determined from the physical properties of the wall that the braces represent, rather than from full-scale testing of the entire building. (3) The current model is over-simplified. For instance, the model has rafters and joists but has no collar beams and no vertical members to connect rafters to joists at midspans. These members must be added to the model to make it more realistic in representing common construction practices.

The model developed under NSF sponsorship will be verified using results from full-scale tests published in the literature, such as those conducted at the Forest Product Laboratory in Wisconsin, and at the Cyclone Structural Testing Station in Australia. More recently, the Washington State University (WSU) has received a grant from the Forest Product Laboratory to build and test a full-scale house. Results of this WSU study will also be utilized by the authors to test the validity of the model.

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CONCLUSION

Wind damage to wood-frame houses is a serious national problem that demands public attention. While much can be done to reduce wind damage to such structures by using existing knowlege through technology transfer, research is needed to refine and improve existing knowlege so that the solutions offered to mitigate wind damage to houses can be more effective and economical.

While post-disaster investigations have been the principal source of information on wind damage to wood-frame houses, more experimental and analytical studies are needed to further advance the state of the knowledge in this field. The main advantage of a good analytical model is that it can be used to predict the effect of wind on wood-frame houses under a great variety of conditions-- conditions difficult to vary and simulate in full-scale tests. Full-scale tests are needed to develop analytical models and to gain new information on building behavior before good analytical models can be developed.

A good analytical model must incorporate both the diaphragm action of walls, ceilings and floors, and the behavior of flexible joints. The model must be able to get the properties of the diaphragms and the properties of the joints either from theory or from testing of components. It must not require full-scale tests of the entire building for each case. The model also must be three- dimensional, and it must represent the modeled house realistically. Work is progressing at UMC to develop such a model.

ACKNOWLEDGMENT

This study is sponsored by the National Science Foundation's Natural and Man-Made Hazards Mitigation Program, under Grant No. CES-8808425 (Program Director: Eleanora Sabadell.)

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