TESTING OF A FULL-SCALE HOUSEUNDER SIMULATEDSNOWLOADS AND WINDLOADS

USDA FOREST SERVICERESEARCH PAPER

FPL 2341974

FOREST PRODUCTS LABORATORY • FOREST SERVICE U.S. DEPARTMENT OF AGRICULTURE • MADISON, WIS.

ABSTRACT

The structural performance of a full-scale conventional house is not well under- stood. Past work has generally been limited to the assessment of parts and pieces, but little testing has been accomplished to evaluate the final structure as a three-dimensional unit.

To determine the influence of each com-ponent as it contributes to overall performance, one house was constructed and tested during progressive stages of construction. Resistance to lateral forces was measured during five stages of construction using concentrated forces. The sixth stage utilized uniform loads and induced progressive modes of structural failures.

The racking resistance of the walls parallel to the simulated windload was adequate. Structural failures occurred in connections between the loaded wall and the floor, and later at the sill plate. A relationship between concentrated forces and uniform loads is presented.

Finally, the trussed roof system, commer-cially fabricated for a 40-pound-per-square-foot design load, was tested to failure. One web member failed under a uniform live load, of 100 pounds per square foot, and ultimate load reached 135 pounds per square foot.

TESTING

UNDER SIMULATEDSNOWLOADS ANDWINDLOADS

OF A FULL-SCALE HOUSE

ByROGER L. TUOMI, Engineer

andWILLIAM J. McCUTCHEON, Engineer

Forest Products Laboratory, 1Forest Service

U.S. Department of Agriculture

INTRODUCTIONWhy bother to test conventional full-scale

houses? Millions have been built during the past couple hundred years, and they are per-forming capably. The current construction rate is near 2 million starts per year, which accounts for over 50 percent of the annual softwood harvest of the Nation’s forests.

If they perform so well, why not leave a good thing alone? The answer is simple. There is reason to believe that conventional houses are over built in many respects. House construction thus offers the greatest single area of potential savings of our timber resource, and these savings can probably be achieved without sacrificing any of the structural in-tegrity in the house.

Little structural engineering goes into the design of a conventional house. The present framework has evolved from our predecessors. The walls, floors, and roof systems are essential-ly the same in all houses.

Some engineering breakthroughs have been made. The trussed rafter system is a good example of improved efficiency in labor and material through engineering. The glued floor system is a more recent example where savings can be realized by taking advantage of the composite action between joist and sub-floor. Still, all in all, each member is assumed to act as an individual element without regard

or understadding of how they all interact in the final structure.

Present Design Criteria

Experience has shown that conventional construction provides a safe and serviceable structure. Present design criteria are that all structures, whether completely innovative or partially modified, must perform as well as, or better than, a conventional house. But what this base really is has never been defined. Neither are there accepted or standard pro- cedures for determining a base level of accep- tance. There are, however, performance require-ments for individual components or partial assemblies of a house.

The racking, or shear, resistance of a conventional wood-frame house is assumed to be provided solely by the end walls (parallel to wind direction). Therefore, present perfor-mance standards are based on the racking strengths and racking resistances of 8- by8-foot wall panel assemblies, typically con-structed of 2- by 4-inch framing members and covered only with a sheathing material. Gen- erally the sheathing consists of two 4- by8-foot sheets of panel material and must pro- vide resistance to racking at least equal to that

1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin.

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afforded by horizontal wood board sheathing and let-in corner bracing. The exact load-deflection levels and ultimate strength re-quirements are specified in Federal Housing Administration Technical Circular No. 12 (6)2

and the test procedure is defined by ASTM Standard E 72 (1).

However, the conventional wood-framehouse is made up of more than framing members and 8 feet of sheathing. It is a three-dimensionalstructure with sidewalls, interior partitions, floors, and a roof. In addition to sheathing, walls have interior coverings, such as plaster or drywall, and generally have exterior siding applied over the sheathing. Also walls are likely to be 24 feet or more in length, providing far greater racking resistance than 8-foot test panels. All of these factors contribute to the overall racking resistance of end walls. Neglecting the composite action afforded by the complete house usually leads to over-building and material waste.

Operation Breakthrough Guide Criteria (8) specify that racking distortion, if tested, be conducted on completed dwellings, whereas other criteria are related to 8-foot-long panels. The allowable racking distortion, or drift, is the limit which normally applies to medium-and high-rise buildings.

Roofs are designed to carry deadweight plus a live load dictated by the geographical location of the house. A 15 percent increase in allowable stresses is normally permitted for repetitive member loading. Deflection require-ments, rather than stress, often control. For example, deflection due to live load is often limited to the span length divided by 360 (L/360) for plaster ceilings and L/240 for flexible-type ceilings such as drywall (11). Forsimplicity, forces in truss members are deter-mined by assuming that the members are pin-connected. Actually the joints lie somewhere between pin connections and rigid joints. Recently, computer programs have been developed to model these joints accurately (9), but this technique is not yet widely used.

Both the racking requirements of walls and deflection limitations of roof systems are presently based more on physical performance tests than on engineering. The interaction between components is neglected. This ap-proach is used because there are neither analog-ous structural models nor direct equations avail-able to analyze these composite systems.

Past and Current Work

Only recently has any significant effort been expended toward testing full-scale con-

ventional houses, and information on this topic is limited. Ironically, there are numerous reports on full-scale tests of stressed-skinhouses, prefabricated buildings, arctic struc-tures, precast multistory apartments, and other nonconventional structures.

One full-scale test was performed by the National Research Council of Canada (4). Theroof sustained a load of 73 pounds per square foot (lb/ft2) (143 pct of design load). Racking distortion was insignificant on end walls under simulated windloads of 37 lb/ft2. More critical were the ballooning deformations of the wind-ward and leeward walls. Foundation anchorage was adequate. Yet this house was deficient in many respects according to FHA Minimum Property Standards. The house was constructed without exterior sheathing although 1- by4-inch let-in braces were used at the corners. Batten strips, nominal 1 by 6's, spaced 12 inches on centers, were used instead of roof sheathing.

Another study was conducted at Virginia Polytechnic Institute under the sponsorship of the National Forest Products Association (7). A foundation failure occurred under a simulated windload of 20 lb/ft2. This report confirmed the results of the Canadian study with respect to minor racking distortion of end walls and significant ballooning of the leeward wall,

The National Bureau of Standards has tested an Operation Breakthrough module in the laboratory (13), and field tested a two-storydetached house (14). The latter house construc-tion was considered typical for the eastern United States. Lateral loads were applied as concentrated forces in line with floor or ceiling diaphragms. Both houses satisfied drift requirements of height/500 as applied to med-ium- and high-rise buildings under a static wind pressure of 20 lb/ft.2

OBJECTIVES AND SCOPE

The primary objective of this study was to determine the structural responses of a con-ventional house to selected imposed forces. These were limited to horizontal loads on walls and vertical loads on the roof. An impor-tant component, the floor, was not included since contemporary studies are underway eleswhere. Because there are many facets in the unknown character of a house, the study was broken down as follows:

2 Numbers in parentheses refer to Literature Cited at end of report

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1. Measure the racking resistance of walls under concentrated loads during pro-gressive stages of construction as the house advances from a simple wall panel to a com-plete three-dimensional structure.

2. Define serviceability limits, such as a distortion level that would render doors and windows inoperable or produce objectionable cracks in wall coverings.

3. Correlate the relationship between uniform loads (windload) and concentrated forces as they affect racking distortion.

4. Determine the ultimate state limits or weak links in the house that lead to structural failure.

One full-scale house was tested under simu-lated snowloads and windloads. A series of six tests were conducted under horizontal forces to assess the racking properties of the house, and one test was performed to deter-mine the strength of the trussed rafter system. The first five of the racking tests utilized con-centrated loads during progressive stages of construction to measure stiffness, and the final test employed uniform loads to determine both stiffness and strength.

RESEARCH MATERIAL AND CONSTRUCTION METHODS

The test house was 24 feet wide in the truss direction, and 16 feet in length. This size was limited by the dimensions of the structural test facility. The 24-foot width is fairly typical, but the 16-foot length is no more than one-half a house in length. This is, how-ever, a reasonable distance from an end wall to a partition. The two end walls (parallel to load) contained windows and doors, but the sidewalls did not. The window and door locations, floor plan, and elevation are shown in figure 1.

No attempt was made to procure high-quality or specialty material for this study. All material was essentially the minimum size, thickness, and quality allowed under the Federal Housing Administration Minimum Property Standards, FHA No. 300 (5). Theconstruction details and nail schedules were also in accordance with the above standard except for the floor-to-sill connection. No special conditioning of material was involved. The material was used “as delivered” from the suppliers as would be the normal situation in house construction.

Figure 1.-Plan and elevation of test house. M142077

Foundation and Floor System

The test house was supported on four wide-flange beams of the structural test facility. Nominal 2- by 6-inch lumber was bolted to the steel to serve as sill plates. Bolts, 1/2 inch in diameter, were located at each corner of the house and at 8-foot intervals to tie the sill plate to this “pseudo” foundation.

The floor system was not part of this study and was constructed primarily as a work platform. Floor joists were nominal 2 by 6’s spaced 16 inches apart and spanned the 16-foot direction (fig. 1). Two intermediate sup-ports were used under the joists. The joists were end nailed to the band or header joist with two sixteenpenny nails and every other joist was toenailed to the sill plate with one sixteenpenny nail. There was no connection between the band joist and sill plate. This

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resulted in a sill connection that was inferior to good construction practices. On the other hand, houses are often constructed without any mechanical connections between sill and foundation.

The subfloor consisted of 1/2-inch-thickplywood applied with face grain perpendicular to the joists and nailed with eightpenny nails spaced 6 inches on center at the edges and 8 inches on center over intermediate joists.

WallsConventional platform framing was used

throughout. Stud grade Douglas-fir 2 by 4’s (1.5 by 3.5 in. standard dressed size) were spaced on 16-inch centers, Double end studs with intermediate blocking, double top plates, and single sole plates were used. Tiltup construction was utilized, permitting horizontal assembly of the walls. Two sixteenpenny nails were end nailed at the stud to plate connections. Top plates, double end studs, and sole plate-to-joist connections were face nailed with sixteenpenny nails spaced 16 inches on center.

Sheathing was 4- by 8-foot sheets of Douglas-fir plywood, Standard C-D Interior grade 3/8 inch thick. Sheathing was applied vertically with sixpenny nails spaced 6 inches on center along the perimeter and 12 inches on center over intermediate supports. The edge distance was approximately 3/8 inch.

The interior wall covering was gypsum board drywall, 1/2 inch thick, complying with ASTM C 79 (2). The gypsum board was applied vertically and secured with 1-1/4-inch ring-shank nails spaced 8 inches apart.

Western redcedar siding was used for exterior cladding. The siding was nominal 1/2 by 6 inches in size applied horizontally with a 1-inch lap. Siding was nailed with one seven-penny aluminum nail (11-1/2 gage by 2-1/4 in.) per course at each stud crossing at a height sufficient to clear the lower course. Horizontal siding was carried up to the apex on the gabled ends.

Roof SystemThe roof system consisted of seven W-type

commercially produced trussed rafters spaced 2 feet on center and two gable end sections. The span was 24 feet and the pitch was 4 in 12. Sheet metal saddle plates were used to anchor the trusses to the wall plates. Joint connections were made with 20-gage metal toothed plates. The design load was 40 lb/ft2.A 1.15 duration of load factor (2-mo snowload) adjustment to the allowable unit stresses

was included. Roof sheathing was 3/8-inch-thick Stand-

ard Interior grade plywood with a 24/0 identification index number. The sheets were applied normal to the trusses and fastened with sixpenny nails spaced 6 inches apart at the ends and 12 inches apart over intermediate supports. The ceiling was covered with 1/2-inch-thick gypsum board installed perpendicular to the bottom chords and secured with 1-1/4-inch ring-shank nails spaced 6 inches on center.

Windows and DoorsTwo windows and one door were installed

in each end wall (racking wall). The windows were installed side by side with the mullions at the centerline of the wall. The doors were installed with the outside jamb 26 inches from the end of the walls and were located at the opposite ends of the house. The windows were 32- by 24-inch, two-light, double-hung wood-framed windows. Doors were 3- by 6-foot8-inch hollow core, 1-3/4 inches thick, exterior flush type, without lights. Clearance at the threshold was 1/32 inch and 1/8 inch at the top.

TEST FACILITIES The test facilities consisted of a structural

loading frame, loading apparatus, measuring instrumentation, and a data acquisition system. A more detailed description of the facility is presented in another FPL report (3).

Structural Loading Frame The structural loading frame consisted of

12 base members over a 26- by 36-foot floor area, 12 vertical columns 16 feet long, perimeter collar beams, and four overhead box beams. The collar beams and overhead box beams were adjustable and could be raised or lowered to accommodate variable load positions or speci-men heights. The members were 12- by 12-inchby 120-pound wide flange beams except the overhead box beams which were builtup sec-tions. The complete frame weighed approx-imately 60 tons and is shown in figure 2.

Loading Apparatus Loads were applied by means of hydraulic

jack actuators and an air bag system. The jacks were single-acting spring return units with a 10-ton capacity. Each jack was equipped with 2-inch-diameter ball-and-socket universal joint and a base plate for mounting to the structural frame. The jacks had connections

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Figure 2.—Structural loading frame and test house with roof partially complete.

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for attaching electronic load cells with a capacity of 20,000 pounds. Each load cell was equipped with a sphere and socket. A single manual hydraulic pump was used for applying oil pressure to one, two, or four jacks. Multiple jacks were installed on a single manifold system and each jack had a separate load cell.

An air bag system was used to apply a uniform load to one wall of the house. A bulk-head was erected against the structural frame leaving a 4-inch cavity between the bulkhead and house wall. A polyethylene bag, 4 mils thick, was fabricated to cover the entire vertical wall surface and inserted into the cavity (fig. 3). Perimeter seams were made with an impulse heat sealer. Pressure was supplied from an 80-pound-per-square-inch (lb/in2) plant air system and manually controlled through an air regulator and bleed-off valve. A simple water manometer was used to monitor supply pressure

while the precise system pressure was measured with an electronic pressure transducer.

The polyethylene bag was not wholly satisfactory as leaks began to develop at pres-sures above 75 lb/ft2. This probably did not influence the test results, but it was often necessary to stop testing and make repairs. A superior envelope material would be highly desirable.

lnstrumentationInstrumentation consisted of load cells,

linear variable differential transformers (LVDT’s), and pressure transducers. Man-ometers and X-Y recorders were also utilized, but primarily to monitor or control load applica-tion.

The load cells (strain-gage type electronic transducers) had a capacity of 20,000 pounds each with an accuracy of ±1/4 percent at full load. The pressure transducer used with the air

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Figure 3.–Air bag suspended between bulk-head and house for applying uniform horizontal load: (1) bulkhead; (2) airbag; (3) test house.

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system had an upper limit of 100 inches of water with the same degree of accuracy. Deformation measurements up to 2 inches were possible with some of the LVDT’s. All of the electronic transducers were excited by a single 6-voltd.c. power supply. The displacement trans-ducers were mounted on stands constructed of perforated angle sections, and were struct-urally independent of the loading frame.

Data Acquisition System As many as 100 transducers could be

manually or automatically scanned for informa-tion. However, only 22 channels were utilized in the wall and roof tests. The output from each transducer was observed on a digital voltmeter having an accuracy of ±3 microvolts on the 100-millivolt range. This apparatus was equipped with a coupler controller that switched the voltage values in the digital voltmeter to a calculator where the voltage values were con-verted directly into engineering units by appropriate calibration constants. The coupler controller also directed the calculator output to teletype and punch paper tape recording systems. The teletype printout was for immediate observation of data whereas the punch paper tape was a data storage device for future computer analysis. A schematic of this system appears in figure 4, and the apparatus is shown in figure 5.

Data transfer from transducers to calculator memory required only milliseconds so that change in deformation was minimal. Printout time was 1-1/2 seconds per transducer; how-ever, load levels could be changed as desired during the printout period.

Data could not be observed prior to print-out. The operator would activate the system at predetermined levels of load or deformation observed on the monitor control system. The operator’s control readings were used as a check against the data acquisition values.

RESEARCH METHODS The walls were tested first under lateral

loads. Little structural distress was evident at the conclusion of wall tests, and vertical loading of the roof followed. Since there were no standard procedures to serve as guidelines, these testa were somewhat exploratory in nature. Also there was not sufficient back-ground information on structural responses to predict what the loads should be. Therefore, the loadings were kept simple and direct to limit the number of variables. Close control of load application was required to avoid premature structural failure.

Figure 4.–Schematic of data acquisition system showing apparatus to sense, trans-mit, reduce, and record data.

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Wall Tests A series of six tests were conducted in the

wall study. The first five tests were designed to evaluate stiffness or racking resistance during progressive stages of construction, and the final test was aimed at defining stiffness, strength, and load distribution for the com-pleted structure. The changes in construction stages and objectives were:

Stage 1.–Measure the stiffness of a basic 24-foot end wall covered only with 3/8-inch plywood sheathing.

Stage 2.–Install two windows and a door and define reduction in racking resistance due to standard openings.

Stage 3.–Apply interior gypsum drywall and exterior wood siding to evaluate the gain in Stiffness due to normal wall coverings.

Stage 4.–Add the two sidewalls to assess the influence of structural components placed perpendicular to the loading walls.

Stage 5.–Add the roof system and measure the racking resistance of a complete three-dimensional structure.

Stage 6.–Apply a uniform load to one sidewall, and determine stiffness, ultimate strength, and net force distributed to end walls.

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Figure 5.–Load, monitor, and data acquisi-tion apparatus: (1) calculator; (2) tele-type; (3) coupler-controller; (4) digital voltmeter; (5) scanner; (6) monitor; and (7) hydraulic pump.

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During the first five test stages, concen-trated loads were applied to the top corners of the walls parallel to the wall direction (fig. 6). Since these first five stages were designed to evaluate racking stiffness within the elastic range, and since the same structure would be reused in successive test stages, it was necessary to limit deflections. Earlier laboratory wall tests indicated that the elastic limit is reached at approximately 0.15 inch, and deflections were not allowed to exceed this value. Load-ing to failure was reserved for the final stage.

Test stages 1 through 3 were conducted on the two-dimensional walls. Eight displace-ment readings plus the load levels were record-ed. Load application position and LVDT locations are shown in figure 6. Both walls were symmetric except that the door openings were reversed relative to the loaded end. The LVDT’s were numbered 31 through 38 on the west wall and 41 through 48 on the east wall. Transducers with the same second digit were in the same relative locations. The west load cell was identified as No. 1 and the east load cell No. 11.

Transducers X1 and X2 (X being either transducer 31 or 41) measured the total move-ment near the top of the wall at opposite ends (fig. 6). Likewise X3 and X7 recorded displace-ments near the sole plate, and X4 and X8 were

positioned near the sill plate. The differences between translation readings at the sill plate (foundation) and the sole plate indicate slip-page between the wall and floor. Numbers X5 and X6 were mounted vertically to measure uplift. LVDT X5 was mounted to the wall and X6 was mounted to the band joist. Therefore, X5 measured total uplift and the difference between X5 and X6 was the separation be-tween wall and floor line.

For stage 6 loading, five additional LVDT’s (numbers 51 through 55) were added to measure deformation of the north wall, which was subjected directly to the uniform load (fig. 7). Displacement instruments numbered 51 and 55 were mounted to measure midspan deflec-tion of studs located at the quarter points of wall length. Instruments numbered 52, 53, and 54 recorded midspan and quarter-point deflec-tion of the stud at the centerline of the wall.

Pairs of roller guides were mounted near the third points of the walls to keep the walls alined vertically during the first three stages. The guides were installed relatively loose to minimize friction and a small amount of bowing was possible. During stage 4, with the sidewalls in place, guides were used only on the outside surfaces of the walls. No guides were used when the roof system was completed for stages 5 and 6.

Load-deflection data were desired to a deformation level near 0.15 inch. It was vis-ually observed that the uplift displacements were limited to local bending and distortion at the loaded end and did not reflect rigid body rotation. Also, since the width of the wall was three times the height, rotational consid-erations were not significant with respect to horizontal deformation, and deflection was taken as the difference between the displace-ments near the top and bottom plates of the wall.

Deflection near the top was generally greater near the loaded end than at the oppo-site end, while the bottom translations near the sole plate were reasonably close. This was probably due to a slight amount of bowing (as mentioned earlier, the lateral guides were left relatively loose to minimize binding and frictional restraints) and possibly to a degree of compressive strain within the double top plates. Therefore, deflection was taken as the difference between the top plate and sole plate measurements at the loaded end of the wall.

The load-deflection relationships were not linear and it was necessary to control displacement rather than load in order to stay within the elastic range of wall deforma-

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Figure 6.–Locations and numbers of dis-placement instrumentation.

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tion. Displacement of the LVDT at the top loaded corner of the wall was monitored through an X-Y recorder wired in parallel with the data acquisition system. The operator con-trolled displacement and activated the data acquisition system at predetermined levels of deformation.

Loads were applied in 0.025-inch displace-ment increments. The load rate was controlled so that each increment was reached in not less than 2 minutes. Following each new level of deformation, the load was released and set readings were recorded after a 2-minute re-covery period.

Loading continued in progressive incre-ments until the net displacement was between 0.125 and 0.150 inch. A cycle was complete when the net deflection was within these limits. The wall was cycled four additional times in this same manner to evaluate residual set.

During the first three stages, each wall was structurally independent and therefore loaded

separately. The walls were physically connected for subsequent stages and were loaded simul-taneously. Two jacks were loaded from a single hydraulic pump through a manifold system. A switchbox was installed into the monitor system whereby deflection of either wall could be observed. The wall having the greater deflection was then selected for load control.

Loads for the first two stages were applied only from the south. At the conclusion of stage 2 it appeared that the east wall possessed greater stiffness than the west wall. The door opening in the east wall was located near the loaded end. Therefore, the loads were applied from both south and north during stages 3 and 5 to assess directional stiffness charac-teristics.

Stage 6-employed the air bag loading system, which was installed at the north end of the house. The X-Y recorder monitor system was replaced with a water manometer for load

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Figure 7.–Locations and numbers of dis- placement instruments on north wall under uniform load.

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control. The operator regulated air pressure and activated the data acquisition system in approximate 5 lb/ft2 load increments as dictated by the visual manometer readings. The visually controlled pressures were gener- ally within ± 1.0 lb/ft2 of the pressure trans- ducer measurements.

The pressure was released following each load increment. However, due to a slow bleed- off rate at pressures below 5 lb/ft2, it was not practical to wait for zero pressure. There- fore, loading was resumed once the residual pressure dropped below 5 lb/ft2 and true set readings were not obtained. A vacuum pump would have been helpful in this phase of testing.

The house was loaded in 5 lb/ft2 increments to a pressure of 40 lb/ft2 and cycled three times to that level. The third cycle loading was continued above 40 lb/ft2 until failure occurred. As before, the load was increased in 5 lb/ft2

increments and the pressure was released

between loads. The first failure occurred at the sole plate

of the loaded wall at a pressure of 63 lb/ft2

and the center stud deflection had reached approximately 1 inch. But racking distortion was not yet significant. Therefore, the wind- ward or loaded wall was stripped of drywall and reinforced with double 2- by 4-inch studs, and sole plate blocking (fig. 8) in order to resume loading to determine the next mode of failure.

Doors and windows were operated manually at various times when the house was under varying degrees of racking distortion. This was done to determine a functional limit or distortion level that rendered doors and win- dows inoperable. Also, the gypsum board drywall covering was checked after each new level of deflection to determine when cracks might develop.

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Figure 8.–Reinforcement of windward wall with double studs and sole plate blocking following initial failure under uniform load.

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Roof Test The house had not been significantly

damaged during racking tests, and it was decided to test the roof system. Wood blocks were inserted between the structural steel supports and the bottom of the floor at the third points along the north and south bear- ing walls to compensate for floor joist dis- turbance under racking forces. These modi- fications were below the stud walls and were strictly precautionary to guard against sudden lateral displacement.

Roof loading was accomplished through a “whiffletree” arrangement whereby the loads from four jacks were distributed through stacks of simple beams positioned so as to

produce equal forces at 32 points on the roof. The loading system is shown in figure 9 and the locations of the 32 load points are pre- sented in figure 10. The load points were midway between trusses and a t the quarter points of the top chords. Four distribution beams, 5-1/2 inches wide by 2-1/2 inches deep, were placed normal to the trusses to prevent puncture of the sheathing at the load points.

The deadweight of this loading system was 1,940 pounds, which produced a distribut- ed load of 4.8 lb/ft2 over the entire roof. The four jacks were equipped with individual load cells and were loaded by a single hydraulic pump through a manifold system. The forces were applied between the node points so as to introduce bending stresses in the top chords.

The four jack loads and 18 deflection points were recorded during the roof test. Six deflection readings were taken along the top chord of the center truss, nine readings along the bottom chords, and three readings along the center stud of the south wall. The locations and numbers of the displacement instruments are shown in figure 11.

The top chord displacements were taken relative to the center joint and heel joints, and the stud movement relative to the top and bottom plates. Bottom chord deflections were related to the floor of the house.

The roof was loaded in 4,000-pound incre- ments, up to 16,000 pounds, above the dead- weight of the loading system and then cycled four times to that level (approximately 45 lb/ft2). Above 16,000 pounds, the increments were reduced to 2,000 pounds and loading continued to failure. During all the testing, the load was released between increments in order to observe recovery.

ANALYSIS OF RESULTS

Load cycling was considered essential to simulate inservice conditions. Also, the charac- teristic stiffness curve (load-deflection re- lationship) could not be defined on the first or second load cycle. Initial loadings produced residual set in the walls (fig. 12). Set is probably related to seating of the nail fasteners into the wood foundation, a phenomenon known as slip. This inelastic movement between nails and wood fibers decreased under cyclic loading. The racking resistance of the walls is afforded by the lateral resistance of the nails through the wall coverings. The shape of the load- deflection curves for the walls is similar to the load-slip pattern for individual. nail joints (12).

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Figure 9.–Loading system to distribute jack loads equally over the roof surface.

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In general, there was little, if any, increase in set after the third load cycle. However, the walls were cycled five times during each test phase (three times for airbag loading), and the last cycle was used for determination of stiffness properties. The residual set, whichaveraged around 0.03 inch, was subtracted from the measured deflection levels to determine net displacement under load.

Stiffness Properties The load-displacement relationships

for the walls at progressive stages of con-struction were curvilinear in nature. To easily compare the responses of the walls without interpolating, it was desirable to derive simple equations which accurately describe these relationships. It was found that simple power curves could be fit to the data by using the method of least squares.

Figure 10–Location of 32 load points for

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roof test. M142110

Figure 11.–Location and numbers of dis-placement instruments during roof test.

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The curves are of the general form: containing the door opening than when loaded in the other direction. This may explain the apparent transposal in stiffnesses during

where P is racking load, A is net deflection stage 4 which was loaded from the north end (gross minus residual set) at load P, and a,b only. The west wall was stiffer in that direction are coefficients determined by least squares while the east wall was less stiff when loaded analysis. from the north. The directional differences

Figures 13 and 14 show the curves which were not significant during stage 5 with the roof were computed. The correlation coefficients, system in place. r, indicate an excellent fit. In the following Had this directional anomaly been discussion, the stiffnesses of the walls were anticipated at the outset, load positions would compared at a deflection of 0.10 inch. have been alternated during all test phases.

The average stiffness of the east wall The apparent difference in stiffness of the appeared to be slightly greater than that of two walls was not too great when compared the west wall (fig. 15), with the exception of relative to the location of openings and load stage 4. At the conclusion of stage 2, it appeared (figs. 16 and 17). that the walls possessed directional properties. The changes in stiffness during pro-Therefore, the walls were loaded from both the gressive stages of construction are presented south and the north for stages 3 and 5 to in table 1. Average values for both walls were determine whether the location of openings used here to factor out directional properties relative to load was a factor. Both walls were as well as normal variability in materials and stiffer during stage 3 when loaded at the end workmanship.

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Figure 12.–Typical load-deflection pattern for cyclic loading of walk.

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Figure 13.–Stiffness characteristics of west wall under concentrated and uniform loads when loaded from north.

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Figure 14.–Stiffness characteristics of east wall under concentrated and uniform loads when loaded from north.

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Figure 15. –Average stiffness properties of racking walls during progressive stages of construction. M142105

Figure 16.–Average stiffness of racking wallswhen loaded at end containing dooropening. M142105

Figure 17.–Average stiffness of racking walls when loaded at end opposite door opening.

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Table 1.--Average racking stiffness of two walls during progressive stages of construction

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Distribution of Uniform Load Uniform loadings, which employed the

air bag system, were applied only from the north end of the house. Siding was left off this wall to provide a smooth surface to load against.

Stiffness curves were derived using the same mathematical model as were used for concentrated loads. Comparisons of stiff-ness curves are presented in figures 13 and 14. It should be noted that the shapes of the two curves for north loadings are nearly identical. The only significant differences are in the scale factor coefficients, a, which define the magnitude of load.

Table 2 illustrates the method of relating uniform loads to concentrated forces. The pressure measured on the loaded wall multipli-ed by the surface area equals the total applied load. Deflection measurements on each wall at each applied pressure were converted to equi-valent concentrated forces from the regression equations presented in figures 13 and 14. About 75 percent of total uniform force was resisted by the two racking walls over a pressure range of 9.6 to 71.1 lb/ft2. Loading was carried to a level of 123 lb/ft2. However, displacement measurements were not reliable above 71.1 lb/ft2 due to excessive translation at the sill plate.

At a deflection level of 0.1 inch, the relationship between pressure, p, and concen-trated load, P, took the form:

p = 0.0212 P for the east wall

p = 0.0203 P for the west wall and

An attempt was made to develop a structural model for the two load conditions to complement the above empirical relation-ship.

However, the racking deflection of a stud wall is not a phenomenon which can be re-presented with a simple model. The com-posite material and geometric properties cannot be defined by direct means and the data from one series of tests are not sufficient for developing an accurate model.

A deformation diagram of the house under simulated windload is presented in figure 18. The stiffness properties of the wall studs were not measured, but the deflection of the loaded wall is reasonably symmetrical. The stiffness of the wall coverings was sufficient to introduce a degree of plate action. That is, part of the force was transmitted directly to the racking walls and the rest was carried by the roof diaphragm and foundations. This

would be analogous to an orthotropic plate supported on four sides.

An oversimplified approach would be to assume that 25 percent of total force is trans-mitted directly to the roof, 25 percent to the foundation, and 26 percent to each of the two end walls. The load from the roof diaphragm would divide equally into two concentrated forces at the top of the end walls. This procedure would direct three-eighths of the total windload to each racking wall, which agrees with the empirical results of these tests.

Serviceability Limitations

Windows and doors were checked while the house was under varying degrees of racking distortion to determine the level that renders them inoperable. The windows were equipped with pressure side plates for the purpose of installing or removing them from the frames. This provided a degree of flexi-bility along the contact edges of the window stiles and considerable deformation could be tolerated.

The door near the loaded end of the house started to bind at the threshold at wall deflec-tions in excess of 0.10 inch. The door, however, could be opened and closed with slight force. This was accompanied by an annoying vibration at the moment of release.

It should be noted that the threshold clearance was minimal, only about 1/32 inch. The door at the opposite end gained threshold clearance under load and functioned properly. The top clearance, however, was about 1/8 inch and could therefore accommodate greater displacements before any binding could develop.

No visible damage in the gypsum drywall was noted even under midheight stud deflec-tions of 1 inch. This is 2-1/2 times the normal acceptance level (L/96 vs. L/240). The joints were not taped, however, and some objectional visual distress might have appeared in taped joints. The ceiling drywall broke loose con-currently with the structural failure in the trusses.

Ultimate Strength

Loading to failure disclosed structural weak links in the construction of the house. These failures occurred at levels too low to assess the full racking strength of the house. Testing was terminated before the house experienced any serious racking distress.

The initial failure occurred in bending of the sole plate on the loaded wall under a uniform

18

Table 2.--Equivalent concentrated forces as estimated from measured deflections under uniform loads

pressure of 63 lb/ft2. The sole plate split in line with the sixteenpenny nails which secured the north wall to the floor system (fig. 19). The base of the wall was pushed in about 5-1/2 inches at time of failure but recovered somewhat following release of air pressure (fig. 20). The nailing of the sole plate to the header joist was necessarily near the edge of the sole plate, and this undoubtedly contributed to the split failure. This condition prevails along the walls parallel to the joists and also when the studs are placed directly over the floor joists.

Centerline deflection of the middle stud measured 0.986 inch just prior to sole plate failure. Corresponding deflections of the two studs located at the quarter points of the loaded wall averaged just under 0.7 inch. The modulus of elasticity for the center stud, under a uniform load of 63 lb/ft2 and a deflec-tion of 0.986 inch, was calculated to be 1.46 million lb/in.2; this value agrees well with the listed MOE of stud-grade Douglas-fir2 by 4's.

As mentioned earlier, the loaded wall

19

Figure 18.–Deformed shape of house under a uniform pressure of 60 lb/ft2. M142082

Figure 19.–Failure at the sole plate of loaded wall under a pressure of 63 lb/ft2. M14099-17

20

Figure 20.–Extent of displacement of sole plate immediately following failure with a recovery of about 3 inches after release of pressure.

M140999-13

had to he reinforced to resume testing. There-fore an actual stud failure was not experienced during testing. However, it was estimated that failure was likely to occur at a pressure of 70 lb/ft2, which would induce a bending stress of 3,000 lb/in.2 approximately three times the allowable design stress for Douglas-firstuds.

Loading beyond 70 lb/ft2 was encumbered by leaks in the air hag and excessive translation of the house along the sill line. It was frequently necessary to stop testing to repair the air hag or reset displacement instruments hack within their travel limits. Finally, at a pressure of 123 lb/ft2, the joint at the sill plate failed and testing was terminated (fig. 21). The total lateral force at that time was nearly 16,000 pounds. The sill plate connection was inferior to that used in good construction. With proper fastening, the house would cer-tainly have sustained a load considerably higher than that reached.

The sequence of failures and corresponding load levels are presented in table 3, along with the calculated equivalent wind veloci-ties in miles per hour. The standard relation-ship between pressure, p, and velocity, V, (mi/h), p = 0.002558V2, was used.

Roof System

The load-deflection relationship produced a reasonably smooth curve up to an equiva-lent uniform load level of 94.8 lb/ft2 (fig. 22), the load level prior to the first structural fail-ure. The deflection limitation, L/360, was reached at a load level of about 80 lb/ft2,and structural failure preceded the L/240 limitation.

Figure 23 presents a deflection diagram at the house centerline under a load of 79.5 lb/ft2. Deflections of the top chords were charac-terized by double inflection due to the reactions of the compressive web members. The instrumented south wall exhibited a slight outward bowing.

Load-deflection data are presented in table 4 for the bottom chords of the center truss and the two trusses positioned at the east and west quarter sections of the house. These data exhibit reasonably close symmetry. The centerline deflections of the two quarter-point trusses averaged 75 percent of the center deflection of the middle truss. This undoubtedly is due to the influence of gable end supports.

In the span direction the deflection profiles were quite flat over the middle half of the

21

Figure 21.–Failure along sill plate connection M141561 under a uniform load of 123 lb/ft2.

Table 3.--Sequence and modes o f f a i l u r e i n house subjected t o hor izonta l uniform loads

22

Figure 22.–Load-deflection relationship at midspan of center roof truss prior to initial failure.

M142078

trusses. The quarter-span deflections averag- ed nearly 90 percent of the centerline deflec- tion. Most of the truss distortion occurred near the bearing walls where the truss depths were relatively shallow.

Table 5 contains similar data for the top chords and south wall at the middle of the house. The lowest deflection occurred near the mid-points of the top chords (gage Nos. 33 and 36) over the compression web members. Maximum deflections occurred at the lower quarter points above the heel joints. These members were subjected to combined bending and axial forces which greatly amplify both

deflection and stress. Outward bowing of the south wall was not significant.

The first structural failure occurred in the tension web member in the south half of the center truss (fig. 24) at a load near 100 lb/ft2. This failure was not visibly discernible due to the ceiling covering. However an abrupt change in the deflection reading of gage 36 (table 5) at the next load increment above 94.8 lb/ft2 indicates when the failure occurred. The tension member was of very low-quality lumber with a large spike knot extending completely through the narrow face near the joint.

23

Figure 23.–Deflection diagram at house centerline under uniform roof load.

M142081, M142080

24

Figure 24.–Single wood failure in a tension web member at a large spike knot under a uniform roof load near 100 lb/ft2.

M141099-21

25

Table 4.--Load-deflection relationships of three bottom chord truss members

After this failure, the roof continued to support further loadings to a maximum of 135 lb/ft2. The travel of some of the LVDT’s was exceeded and some of the deflection data were not reliable following the initial failure. All of the plate connectors along the north wall sheared cleanly along the juncture of the top and bottom chords at the heel joint (fig. 25). There was visible distress in the heel joints along the south wall, but an actual rupture did not occur. Possibly the web fail-ure in the south half of the roof caused a

redistribution of forces to the north. The axial load at failure was estimated to be 8,800 pounds. This would induce shear stresses in the heel plates approaching 40,000 lb/in.2,assuming the net area between perforations as 60 percent of the gross area along the shear line.

The tension plates and the plates at the apex were only slightly deformed and appeared to be performing adequately. The gypsum ceiling covering broke loose at the time of failure (fig. 26).

26

Table 5. --Load-deflection relationships of center top chord truss members and south wall

27

Figure 25.–Shear failures in the metal plate connectors ut the heel joints.

M141100-7

Figure 26.–Gypsum drywall on ceiling broke

M141099-13loose following truss failure.

28

SUMMARYFirm conclusions cannot be derived from The deflections under uniform loads

a single test. It would be ill-advised to draw indicate that about three-eighths of the total generalizations through some inductive process force was resisted by each of the two racking based on the performance of one structure. walls. In a longer house with interior walls, Therefore, only observations as they relate to these so-called nonstructural interior parti-this particular house are presented. tions would be forced to assume a portion of

The racking resistance of this house under this load, and probably of a greater magnitude these test conditions proved to be more than than the two outside end walls. All partitions adequate. From this one might conclude that are a part of the house and whatever structural the racking properties are high enough that role they play should he considered in the over-they need not be considered in the design all design. of the house. However, other sheathing mat- The boundary support afforded to the rials are often used and their racking properties loaded wall by the racking walls, and by the are different from those utilized in this gabled ends to the trusses, were sufficient to particular study. introduce a degree of two-way action. The

The weak links observed in this study were maximum deflections and corresponding the connection systems between the sole plate maximum stresses occurred near the center and the floor and later the sill plate connec- portions of the walls or the roof systems. tions. As mentioned earlier, the sill plate Generally it is assumed that walls or floors connection in this study was probably some- or roof systems are infinitely long perpendicular what deficient. However, it should be noted to the span direction so that the end supports that in many areas there are no requirements have no effect on performance. It appears for mechanical anchorage between the house possible that within limits a degree of plate and the foundation. Both of these connection action is developed. Interior walls and parti-defects could easily be rectified with only tions provide this type of support within a minor modifications to the connection details. typical house.

More serious were the ballooning deforma- Trusses, the most highly engineered tions and bending forces along the loaded components of a typical house, did not windward wall. The same observations were provide any great surprises. The roof system noted in two of the other references studied sustained an equivalent live load of nearly that involved loading near the midheight 135 lb/ft2. The deflection limitations were of the wall system. Design changes in this not a serious factor. Most of the deflection area may be somewhat more difficult and, in occurred near the bearing walls and was fact, this may be the limiting factor in the relatively flat over the middle half of the structural integrity of a conventional wall. truss system. The mode of failure in shear A balanced design may therefore be controlled at the heel plate was not anticipated. Shear by the bending capabilities of the windward tests are not normally required for truss plates. walls.

29

LITERATURE CITED

30

Comments on Full-Scale House Testing

There was little published information to serve as a guide on full-scale house testing. Only through experience can optimum test-ing be developed. It should be noted that the results of three of the referenced studies on full-scale houses were not available before the conclusion of this study. In comparing the results of this study to the limited number of other studies, it is striking to notice the great differences in construction, testing techniques, and loading conditions. It would be difficult at best to define consistent trends in the performance of these test houses. The experience gained from these past tests suggests the need for established guidelines for any future testing on full-scale houses. These include defining the construction of the conventional full-scale house, developing test procedures that best assess ita performance, and developing load conditions that best simulate the in situ forces imposed on the house in service.

Standard House

Conventional construction to one re-searcher may be far different from conventional construction to another. Should the house be one-story or two? Two bedrooms or three? An infinite number of combinations in con-struction details and choices of materials further emphasize the need to define a basic structure before any meaningful comparisons can be made.

This basic structure must be at sea level and not at the summit of Mount Everest. The basic structure must be safe and struc-turally adequate; durable but not superfluous. Without such a base it would be difficult, if not impossible, to realistically assess the infinite number of alternatives in design and material options.

Standard Test Procedure

Test procedures may be relatively simple or highly complex. The load condition might simulate a single force, such as a snowload, or represent combinations of simultaneous forces, such as snowload and windload together. Also the test procedure might be designed to assess a single performance attribute or to uncover a structural weak link within the house system.

Testing directed to measure one particular property, such as racking or bowing for example,

is obviously important. This provides a key to where material can be saved or better utilized. However, of equal importance are tests that uncover weak links in the structure. Knowing what the weak links are, or where they are, might disclose the areas that limit the ultimate performance of the structure. On the other hand, these weak links might be easily corrected with only minor modifications in design or detail.

The ultimate goal is balanced design. Excessive strength in one area will not compen-sate for a weakness elsewhere. The optimum performance of materials cannot be achieved without ensuring equal performance of the joints.

Standard Loads

There is a need for realistic load conditions that best simulate site conditions. In design numerous approaches are being employed, particularly in respect to windloadings. One approach considers a uniform load over the projected area of the windward surface. Another considers the structure to be hermeti-cally sealed. Still another considers the num-ber of openings contained in the walls of the structure. This "openings" approach assumes that the windows or doors are either open or broken and attempts to resolve both internal and external forces.

It would be desirable to provide as many load cases as possible within the limitations of the apparatus. However, from a testing standpoint the assumed loading conditions can greatly influence the sophistication of the test setup. The influences of pitch of roof, overhang, ridge turbulence, and openings could require variable loading on several surfaces of the house simultaneously. Such a setup would be extremely complex and difficult to control.

The National Bureau of Standards has an experimental house located in Great Falls, Mont. This house is highly instrumented to measure differential pressures across the walls and roof under actual wind conditions. Other means available would include wind tunnel test or the use of fans. One study incorporated the use of the prop blast of an airplane anchored near an experimental structure. Information of this type, which clearly defines the forces imposed by nature, is needed to develop realistic load conditions.

The testing of full-scale structures is

31

a new approach to the field of light-frame construction. The experience gained from the tests conducted to date should serve to develop guides to all future testing, The know- ledge gained in this manner can change the field of housing from a concept born on tradition to one based on utility.

A paper recently released appraises research needs in light-fxame construction in great depth (10). Our study, of far lesser

scope, affirms at least two important require- ments contained therein. First, testing of full- scale structures is needed to measure the struct- ural attributes of the so-called conventional house. Secondly, with this as a base, the application of more precise engineering techniques can develop more efficient structures and extend our timber resource through improved utilization.

U.S. GOVERNMENT PRINTING OFFICE 1974-650-247/9 32 7.O-33-7-74