School chairs for developing countries: designing for strength and durability, simplicity, and...

Abstract

Persistent shortages of school furniture pose serious educational problems in underdeveloped and developing countries. Research has shown that strong durable school furniture can be produced with simple tools and production processes. Thus, the opportunity exists to solve school furniture

problems by utilizing participatory cottage industry at the local level as well as larger manufacturers at the regional or country level. To do so, however, it is necessary to have designs that inherently ensure long maintenance-free service life, using parts that are easy to manufacture and constructions that are easy to assemble. This paper describes a design for school chairs, based on proven round mortise and tenon construction, which is not only strong and durable, but also easy to assemble and manufacture.

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In developing countries of the world, persistent shortages of school furniture pose serious educational problems (Quesada, 2001, Haviarova, 2000). Shortages exist because furniture is one of the most expensive components of education (Haviarova 2000); it has a low priority in limited school budgets, and frequent replacements are needed owing to the short service life of the furniture, often as short as 1 to 3 years.

Initial research has shown that strong durable school furniture can be produced with simple tools and production processes (Eckelman et al. 2001; Haviarova et al. 2001a, 2001b). Thus, the opportunity exists to solve school furniture problems by utilizing participatory cottage industry at the local level as well as larger manufacturers at the regional or country level. To do so, however, it is necessary to have designs that inherently ensure long maintenance-free service life, using parts that are easy to manufacture and constructions that are easy to assemble.

The objective of this project was to develop a design for school chairs, based on proven round mortise and tenon construction, that not only would be strong and durable, but also easy to assemble and manufacture.

Design criteria: Overall design considerations

In general, the chair was to be designed for children in the 9- to 10-year-old age group, but the design was to be of such a type that it could be scaled to size for essentially all school grades from kindergarten through high school. Construction was to be simple, and equipment requirements limited to a table saw and drill press. As much as possible, the chair was to be constructed of small parts, which in many cases could be machined from sawmill waste. The desired simplicity of design and manufacture dictated that the chair have either a straight or straight slanted back. Furthermore, assuming a bandsaw is not available to cut curved surfaces, the back slats would necessarily be straight. Since a slanted back would allow for a slight back slope, it became the preferred design. It was recognized, however, that in many areas, practical considerations would dictate the use of straight rather than slanted back posts. Thus, a design was needed that would allow the use of both. Comfort considerations also dicta te that the seat should have a slight front to back slope and that the seat should cover the tops of the front posts. Finally, strength, durability, ease of parts manufacture and chair assembly mandated that round mortise and tenon joints be used in construction of the chair.

The front and side view of a chair that satisfies these criteria in most essentials is shown in Figure 1. Height of the seat is 14 inches, which makes it suitable for children in the 9- to 10-year-old age group (Garcia et al.). The chair has three side stretchers, which ensures high front to back strength and durability. It also has a front stretcher and front rail, which ensures satisfactory sidesway strength of the front frame. Finally, the chair has a back stretcher and top and bottom back seat rails, a top rail, and back slat, all of which provide sidesway strength to the back frame and also help to reinforce the front frame.

The front edges of the back posts are slanted in order to provide a small amount of slope to the backrest. These posts measure 1-1/2 by 2-1/4 inches in cross section, whereas the front posts measure 1-1/2 inches square. All of the other members are constructed of 7/8-inch stock, except the front seat rail, which is constructed of 1-inch-thick material to accommodate the recess holes for the seat bolts. The seat is supported by the front and back seat rails and has a half-inch slope to the rear.

Round mortise and tenon joints

Round mortise and tenon joints were chosen for construction of the chair because round tenons are simple to cut with deep hole saws (plug cutters will also work) and mortises are simple to machine with conventional wood bits. Also, it is relatively easy to obtain a close fit between the round tenon and the mortise into which it fits by matching the drill bit used to machine the mortises to the hole saw used to cut the tenons. This ability to match the diameter of the tenon to the diameter of the hole without the need for strict quality control procedures and thereby obtain a tight force fit provides the key to the construction of uniformly durable furniture. Additionally, "shrink and swell fit" construction (Boggs 2001) may be used to augment "force fit" construction.

After the tenons are cut, excess material surrounding the tenons must be removed. This is done in such a way that a "shoulder" is formed at the point where the tenon emerges from the part. Subsequently, when the chair is assembled, the tenon is pressed into the mortise until the "shoulder" of the tenon presses against the side of the part into which the tenon is inserted. In constructions with non-slanting legs, this practice assures the dimensional accuracy of the construction without resorting to gages and other measuring devices, assuming that the desired shoulder to shoulder distance is maintained.

Strength requirements

Universal strength requirements for school furniture of the type described here have not been established, but useful estimates can be obtained from work published on the performance of library chairs, particularly, the work on front to back and side thrust load tests (Eckelman 1995, 1977). Both tests are described below in the section dealing with performance tests. In the case of front to back load tests, acceptance levels of 250, 350, and 450 pounds are defined for categories of light, medium, and heavy duty. Experience indicates that if chairs in university libraries do not have a front to back strength of at least 250 pounds, a significant number will fail during the first 2 years of service. Few chairs with a front to back strength of 350 pounds ever fail, however. Comparable values for side load tests are 200, 250, and 300 pounds. The higher values tend to reflect the strength of chairs produced (and tested), however, rather than strength needed to survive in service. Thus, as a first estimate, it app ears reasonable that if chairs designed for lower grades meet the lower requirements and chairs for upper grades meet the higher requirements, both designs should have sufficient strength and durability to survive in service.

Structural considerations

As a close approximation, for chair side frames in which the front and back legs are large in cross section compared to the stretchers, and the stretchers are of identical cross section, the external bending force acting on a side frame is equally distributed to each stretcher (and to its accompanying tenons). Thus, for a front to back force applied to the top edge of the front post of a side frame (Fig. 2) the internal bending force acting on each tenon is given by the expression:

[f.sub.4] = [F.sub.2] x h/n in.-lb.

where:

[F.sub.4] = bending force acting on the end of the stretcher

[F.sub.2] = force applied to the side frame in a front to back direction

h = vertical distance to the load point

n = number of tenons

This expression indicates that for a given external front to back force acting on a side frame, the greater the number of stretchers, the smaller the internal bending force acting on each tenon. Hence, strength and durability are obtained by using sufficient stretchers to ensure that the bending force acting on each joint does not exceed the strength of the corresponding tenon.

The stress generated in a round tenon is given by the expression:

[S.sub.4] = 32[f.sub.4]/[pi][D.sup.3]

where:

[f.sub.4] = bending force

[S.sub.4] = corresponding bending stress

[pi] = 3.14

Previous tests indicate that under cyclic loading conditions, round tenons fail at stress levels at least equal to the modulus of rupture (MOR) of the material. The resistance of a side frame to front to back loads, therefore, can be estimated by setting [S.sub.4] equal to MOR in the above expression, rearranging terms, and then solving for [F.sub.2] in terms of [S.sub.4]:

[F.sub.2] = [pi][D.sup.3][S.sub.4]/32 x n/n = [pi][D.sup.3].MOR/32 x n/n

In the case of yellow-poplar, for example, the MOR at 12 percent moisture content is about 10,100 psi. A side frame with three stretchers constructed with 3/4-inch-diameter tenons and a front post height of 13-1/4 inches, therefore, would be expected to have an ultimate resistance to front to back forces applied to the top of the front post of:

[F.sub.2] = 10,000 x 3.14 x [(0.75).sup.3]/32 x 6/13.25

= 1891b.

The complete chair (with two side frames), therefore, would be expected to have an ultimate resistance to front to back cyclic loads of 2 x 189, or 378 pounds.

Had the chair been constructed with 5/8-inch-diameter tenons, the expected front to back strength would be [(0.625).sup.3]/[(0.75).sup.3], or, 58 percent as great, i.e., it would be expected to have a front to back strength of 0.58 x 378, or 219 pounds.

Similarly, as a first approximation, the front frame of the chair, with seat rail and one stretcher, might be expected to have about two-thirds of the strength of one side frame, i.e., about 2/3 x 189, or, 126 pounds for 3/4-inch-diameter tenons, and 84 pounds for 5/8-inch tenons. Including the strength of the back frame, the resistance of the chair as a whole to sidesway forces would be at least double these values.

It should be noted that these values apply to one of the "weaker" woods. Had sugar maple (Acer sacharum) been used, for example, with an MOR of 15,800 psi at 12 percent moisture content (FPL 1999), these values would be increased by 15,800/10,100, or, 56 percent. For northern red oak (Quercus rubra), another of the "stronger" woods, the corresponding increase would be 14,300/10,100, or 42 percent.

These preliminary calculations tend to indicate that a chair constructed from even a relatively low-strength wood species with nominal 3/4-inch-diameter tenons should have sufficient strength to satisfy medium strength requirements as defined by the American Library Association (ALA) (Eckelman 1995, 1977). Chairs constructed from "stronger" species would be expected to satisfy heavy-duty requirements.

Cross mortises in legs

Pilot studies (Eckelman et al. 2002) have quantified the loss of strength that occurs when holes or mortises intersect at right angles in chair and table legs and similar parts. In tests carried out with legs 1-3/8 inches square with intersecting 3/4-inch-diameter mortises, the strength of the leg was reduced to 31 percent of solid wood strength. With 5/8-inch-diameter holes, strength was reduced to 40 percent.

As the separation between the crossed mortises increased, however, leg strength increased substantially. In the case of 3/4-inch tenons, maximum strength values were obtained with crossed axes separations of as little as 1-1/2 inches, namely 50 percent of solid strength. At this point, the residual external bending strength, [F.sub.4], could be calculated by means of the conventional bending stress expression for beams on the basis of net remaining section:

[F.sub.4] = (t-d)([w.sup.2])/6x[S.sub.4]

where:

t = thickness of the member (in.)

w = width of the member (in.)

d = diameter of the tenon (in.)

Given the above results, the maximum possible practical distance was maintained between mortises for side rails and the mortises for front and back rails in the chair shown in Figure 1, in most instances, a spacing of 1-3/8 inches.

In the case of 1-1/2-thick by 2-inch-wide yellow-poplar back legs with this mortise spacing, the above expression indicates that the ultimate carrying capacity of a back leg with a 3/4-inch-diameter mortise should be about 5,050 in.-lb. Since even a 500-pound front to back load applied to the front of the seat would produce a bending force acting on each back leg at the lower side stretcher joint of only (500/2) x 5.5 = 1375 in.-lb., these legs would be expected to carry far larger loads than what might reasonably be expected in service. From a practical viewpoint, however, this over-design provides some measure of protection against the use of wood with knots or other defects in this area.

Front to back loads acting against the backrest must also be considered. For an extreme case in which the load is applied to the top edge of a 16-inch-high backrest, each backrest would be able to resist a front to back load of 5,050 in.-lb./16 inches, or, 316 pounds for a total back load of 632 pounds, which would satisfy ALA requirements for medium back strength.

Seat construction and attachment

Several seat constructions are possible with this chair. The simplest is the one-piece seat constructed of 3/4-inch medium density fiberboard shown in Figure 1. This seat has enough bending strength that it need be supported only by the front and back seat rails. Considered as a simple beam in bending, for a material bending strength of 2,400 psi, this seat should be able to support a concentrated load at midspan of 1,100 pounds. The comparable value for a 3/8-inch-thick seat would be 286 pounds. Since impact must be considered, the latter thickness would not be expected to be satisfactory for all applications.

To use thinner seat material, it is necessary to add seat support rails, as shown in Figure 3. Addition of the rails makes construction more labor intensive, but assuming the rails are cut from waste material, this practice may result in sufficient composite board savings to justify the practice.

Edge-glued solid wood seats could also be used when they are available and cost effective. Seats may also be constructed of narrow, often scrap, boards, as shown in Figure 4, without edge-gluing the slats together. Fabrication of these seats is labor intensive, but is justified when low-cost labor is plentiful. The seat itself consists of a front edge piece, or lip, and several seat slats, usually at least four. Half-inch diameter tenons are cut on the front edge of the seat slats. Corresponding round mortises are cut into the back edge of the lip. A notch is cut into the rear edge of each slat. This notch prevents the seat slat from sliding rearward beneath the back upper seat rail. The seat slats should be sufficiently long to allow about 1/4 inch of the front edges to rest on top of the front rail. This allows the seat loads to be transferred directly to the front rail rather than to the tenons. A rabbet is cut on the top of the seat along the back edge. The shoulder of the rabbet butts up against the upp er back seat rail and prevents the seat slats from sliding to the rear. The seat rail must be repositioned downward 1/8 inch from its normal position to engage the shoulders of the slats. The lip of the seat is attached to the front rail with two carriage bolts, whereas the rear ends of the slats are allowed to "float" between the upper and lower back seat rails. Notches cut into the rear end of the outer two seat rails prevent the rear of the seat from sliding sideways.

Top rails and back slats

Ideally, the top rails and back slats used in the backrests should be curved to provide maximum comfort to the user. It is common practice, however, to use straight rails. A curve can be cut on the inside face of these members with a bandsaw in order to obtain some relief, as shown in Figure 1.

Separate top and back rails are normally used. With wide top rails, there is the chance of the end joints loosening with time so that the top rail rotates. A solution to this problem was devised, as shown in Figure 5. In this construction, a wide top rail is used that has two 3/4-inch-diameter tenons on each end. Use of two tenons provides significant strength to the top rail, prevents it from rotating in service, and allows the slant of the top rail to be altered by changing the location of the holes for the tenons in a front to back direction.

In constructing this top rail, the material between the two tenons is removed by first cutting a 3-inch-diameter hole in the flat plane of the rail at each end with centers located 1-1/2 inches from the end and edge of the rail. Tenons are then cut on the protruding stubs.

An alternative slatted backrest construction was devised, as shown in Figure 6. This backrest makes use of small pieces of material, but the fabrication process is labor intensive. The complete backrest consists of the top rail, the backrest rail, and, in this case, three backrest slats. Somewhat wide slats are shown in the figure; narrower, or even square, slats could be used instead. Half-inch diameter tenons are cut on the ends of the backrest slats. These fit into corresponding round mortises cut into the bottom edge of the top rail and the top edge of the bottom rail. Three-quarter-inch-diameter tenons cut on the ends of the top rail and backrest rail fit into corresponding mortises cut in the sides of the backrest posts. Use of this construction eliminates the need for careful spacing of the top rail and backrest tenons to match the corresponding mortises in the back posts as is necessary with the construction shown in Figure 5.

Parts manufacture

Parts are cut first to rectangular shape. Mortises are then cut in them with the aid of the jigs shown in Figure 7. These jigs are placed on the bed of a drill press and clamped in such a position that with the backstop properly adjusted, holes are located in the desired position with respect to the width of the leg. Position of the mortises along the length of the leg is governed by the dowel pin stops that may be extended or retracted as desired. Toggle clamps may be used to hold parts in place while the mortises are being drilled. Use of the jigs significantly speeds the mortising process and ensures the proper location of the mortises.

The tapers on the back post are then cut with the jigs also shown in Figure 7. Use of these jigs significantly simplifies the cutting of the tapers on the back posts and also ensures the accuracy of the cuts.

Frame assembly

Side frames are constructed first. These frames are assembled by inserting the stretcher tenons into their corresponding mortises and then pulling the assembly together with bar clamps. The clamps are tightened until the shoulders of the tenons press against the sides of the front and back legs. Thus, shoulders must be cut at points on the ends of the rails that result in the desired leg to leg spacing and thereby ensure that the assembly, when clamped together, is dimensionally accurate. The tenons on the ends of the cross stretchers, rails, and slats are then inserted into their corresponding mortises in the side frames. Again, the assembly is pulled together with bar clamps until the shoulders on the tenons press against the inside surfaces of the legs and backrest. This assembly procedure is inherently simple and produces frames that are both dimensionally accurate and square without the use of jigs or other types of clamps.

The walls of the mortises are coated liberally with a polyvinyl adhesive before the tenons are inserted. Excess glue pushed out the end of through-mortises is wiped off immediately. Before the adhesive dries, an adjustable wrench may be used to rotate any misaligned members into alignment.

Attachment of seats

The rear edge of the seat is inserted in the slot between the top and bottom back seat rails (Fig. 1). With the chair standing in an upright position, #10 round head carriage bolts are then inserted through the holes near the front edge of the seat and into the corresponding pilot holes in the front rail. A hex serrated flange-head nut is then inserted in the open end of a deep socket wrench with the socket held in the vertical position. The socket with nut is then inserted in the recess hole for the nut (drilled vertically in the underside of the front rail) and the nut is screwed onto the end of the carriage bolt (Fig. 1). Use of hex serrated flange-head nuts greatly simplifies the threading of the nuts on the bolts and is greatly preferred over the use of a two-part nut and washer combination.

Chair construction

Essential details of the basic chair developed in the study with seat height of 14 inches is shown in Figure 1. This chair can be scaled up or down in 2-inch increments to produce chairs with 12- or 16-inch seat heights. Aside from differences in the lengths of the members, the only major difference between chairs is that the small chair is constructed with two side stretchers rather than three. These are positioned at points 6-1/4 and 9 inches above the floor, whereas in the case of chairs with a 16-inch seat, the side rails are positioned at heights of 5-1/2, 9-1/4, and 13 inches. These chairs are suitable for children in kindergarten and beginning grades, middle grades, and upper grades and some adults, respectively. The cross-sectional areas of comparable members are the same in all three chairs.

Two chairs of each seat height were constructed of yellow-poplar (Liriodendron tulipifera) and two of red oak (Quercus rubra) for a total of 12 chairs. The wood used in the study was cut from green logs by means of a portable band mill. Chair parts were subsequently cut from "scrap" lumber procured from the logs. Parts in the rough were visually inspected to eliminate severe defects, but in general, an attempt was made to avoid sorting for quality Subsequently, the ends of the rough mill parts to be converted into rails and stretchers were visually inspected for soundness and freedom from defects to ensure that sound tenons could be cut on their ends. An additional four chairs were constructed with a seat height of 14 inches from material salvaged from scrap loblolly pine (Pinus taeda) 2 by 4's. In general, parts cut from this material were clear and largely free of defects. These chairs were included in the tests because school furniture may be constructed from scraps of structural timber left over from con struction of school buildings, and it was thought worthwhile to determine the performance of chairs fabricated of this material. Tenons were cut with a 3/4-inch-diameter hole saw that produced nominal 0.725-inch-diameter tenons. Mortises were drilled with a 46/64-inch-diameter twist drill. Before assembly, walls of the mortises were coated liberally with an aliphatic resin adhesive.

Performance tests: Front to back load test and side load test on seats

The front to back load test on seats as defined in the performance test method for library chairs developed by the ALA (Eckelman 1995) was used to evaluate the strength and durability of the chairs in the critical front to back direction. This performance test method was selected initially because of the similarity of the service conditions in schools and libraries. In addition, the tests provide a straightforward procedure for relating field performance to laboratory test results based on historical data. Finally, this method of test maximizes the amount of useful engineering data obtained.

The test itself consists of pushing from front to back on the seat of a chair (or on the front rail). This action produces internal resisting forces in the side frame of the chair similar to those caused by the action of someone tilting backward. The chair is mounted for testing as shown in Figure 8. Reaction brackets are placed behind each of the back legs to prevent the chair from sliding backwards. A strap is then passed over the seat from front to back and attached to a small clevis connected to the rod end of an air cylinder that applies loads to the chair. The other end of the belt is dropped over the front edge of the seat, allowed to hang vertically, and attached to a crossbar located directly below the front edge of the seat. As the seat is pulled to the rear, the chair tends to tip over backward. As it begins to tilt slightly, however, its motion is resisted by that portion of the strap that hangs vertically from the front edge of the seat and is anchored below; in effect, the vertical portion of t he strap always provides the exact force needed to keep the chair from overturning.

Horizontal loads are applied to the chair seat in a front to back direction at a rate of 20 cycles per minute. Tests were conducted until a chair suffered disabling damage, which, in all cases, was fracture of one or more tenons on the ends of the stretchers.

The horizontal side load test on seats is identical to the front to back load test except that the load is applied to the seat in a sideways direction, and reaction brackets are placed on one side of the chair rather than behind the chair.

Results and discussion

Results of the tests are presented in Table 1. Estimated and failing strengths for the front to back load tests are given in Table 2.

The loblolly pine chairs were tested first. Owing to uncertainty concerning their expected performance, tests were started at the 50-pound load level and increased in increments of 25 pounds. As a result of the use of these conservative starting loads and load increments, the loblolly pine chairs were subjected to a much larger number of load cycles than the remaining chairs. In the front to back tests, these chairs failed at 475 and 450 pounds, respectively. In the side load tests, the chairs failed at 375 and 350 pounds, respectively.

All remaining tests were started at the 150-pound load level and increased in increments of 50 pounds. In the case of the chairs with a 12-inch seat height, the red oak chairs failed at 450 and 400 pounds, respectively, in the front to back load and the side load tests. Comparable values for the yellow-poplar chairs were 350 pounds and 350 pounds, respectively. As expected, the red oak chairs performed better than the yellow-poplar chairs.

In the case of the chairs with a 14-inch seat height, the red oak chairs failed at 700 and 450 pounds, respectively, in the front to back load and the side load tests. Comparable values for the yellow-poplar chairs were 550 pounds and 300 pounds, respectively. In the case of the chairs with a 16-inch seat height, the red oak chairs failed at 450 pounds in both the front to back load and the side load tests. Comparable values for the yellow-poplar chairs were 350 pounds in both tests.

Conclusions

Results of the study indicate that strong durable school chairs can be produced from only a few configurations of simple parts that are easy to manufacture and assemble into finished furniture. Round mortise and tenon joint construction produces tight-fitting, strong, durable joints with minimum quality control requirements. Furthermore, dimensionally accurate tenons may be machined with simple equipment and mortises cut with conventional drill bits. Simple jigs ensure the accurate placement of the mortises and speed up part production. Shoulders machined at the root of the tenons allow rails and legs to be assembled into dimensionally accurate frames without the use of special jigs. Use of an upper and lower back seat rail provides a simple fool-proof means of anchoring the rear of the seat. Attachment of the front of the seat to the front seat rail with two #10 carriage bolts provides an unobtrusive yet positive means of anchoring the front of the seat to the chair frame. Use of hex serrated flange-head nut s allows the nuts to be rapidly threaded onto the bolts, even in the deeply recessed access holes.

Results of the tests indicate that all of the chairs met either the medium or high acceptance levels for front to back load tests specified by the ALA. Furthermore, all of the chairs met the high acceptance levels for side load tests. To put these results into perspective, it must be realized that adult library reading rooms (especially those located at universities) represent a severe use environment and acceptance levels, accordingly, are set high relative to other use environments. Chairs that meet only the "low" ALA acceptance level, for example, have given good service in fast food restaurants. Furthermore, chairs intended for home use often fall considerably short of satisfying the "low" ALA acceptance level. Thus, a very high level of strength was achieved in the test chairs, which, presumably, can be attributed both to their design and method of construction.

Table 1. - Results of front to back and side thrust load tests on chair
frames.


Chair   Seat                                   Initial
no.    height    Wood      MC   Type of test    load    Load step
        (in.)              %                         (lb.)

1        12     Red oak   9.07  Front to back    150       50
2        12     Red oak   9.13   Side load       150       50
3        12    Y-poplar   9.82  Front to back    150       50
4        12    Y-poplar   9.51   Side load       150       50
5        14     Red oak   7.11  Front to back    150       50
6        14     Red oak   7.53   Side load       150       50
7        14    Y-poplar   9.83  Front to back    150       50
8        14    Y-poplar   8.90   Side load       150       50
9        14    Lob, pine  7.96  Front to back     50       25
10       14    Lob, pine  7.96   Side load        50       25
11       14    Lob, pine  7.96  Front to back     50       25
12       14    Lob, pine  7.96   Side load        50       25
13       16     Red oak   9.36  Front to back    150       50
14       16     Red oak   9.51   Side load       150       50
15       16    Y-poplar   9.45  Front to back    150       50
16       16    Y-poplar   9.13   Side load       150       50

                                    Cycles at    Total
Chair   Seat              Ultimate  ultimate    cycles
no.    height    Wood       load      load     completed
        (in.)               (lb.)

1        12     Red oak     450       3040      153,040
2        12     Red oak     400      23,200     148,200
3        12    Y-poplar     350      17,400     117,400
4        12    Y-poplar     350      21,300     121,300
5        14     Red oak     700       4,543     254,543
6        14     Red oak     450       5,063     155,063
7        14    Y-poplar     550       5,102     205,102
8        14    Y-poplar     300      10,341      85,341
9        14    Lob, pine    475      16,532     241,532
10       14    Lob, pine    375       3,130     328,130
11       14    Lob, pine    450      23,100     423,100
12       14    Lob, pine    350       2,350     302,350
13       16     Red oak     450      11,300     161,300
14       16     Red oak     450       2,360     152,360
15       16    Y-poplar     350        20       100,020
16       16    Y-poplar     350        75       100,075

Table 2. - Comparison of estimated and failing load values in front to
back load tests.

Chair    Wood    Moisture   Seat   Front post   Modulus    Est. failing
no.    species   content   height    height    of rupture      load
                   (%)           (in.)           (psi)         (lb.)

1      Red oak     9.07      12      11.25       15,976        416
3      Y-poplar    9.82      12      11.25       10,981        286

5      Red oak     7.11      14      13.25       17,098        567
7      Y-poplar    8.9       14      13.25       11,352        377

9      Lob pine    7.96      14      13.25       14,868        493
11     Lob pine    7.96      14      13.25       14,868        493

13     Red oak     9.36      16      15.25       15,810        456
15     Y-poplar    9.45      16      15.25       15,444        321

Chair    Wood    Actual failing  Actual/
no.    species        load        est.
                      (lb.)        (%)

1      Red oak        450         107.4
3      Y-poplar       350         122.4

5      Red oak        700         123.5
7      Y-poplar       550         145.9

9      Lob pine       475          96.35
11     Lob pine       450          91.3

13     Red oak        450          98.7
15     Y-poplar       350         109.9

Literature cited

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(11.) USDA Forest Products Laboratory (FPL). 1999. Wood Handbook: Wood as an Engineering Material, Gen. Tech. Rept. FPL-GTR-l13. USDA Forest Serv., Forest Prod. Lab., Madison, WI.

C.A. Eckelman *

Y.Z. Erdil *

E. Haviarova *

* Forest Products Society Member.

The authors are, respectively, Professor, Dept. of Forestry and Natural Resources, Purdue Univ., West Lafayette, IN 47907-1200; Assistant Professor of Wood Science, Mugla Univ., Faculty of Technical Education, Kotekli/Mugla/48000, Turkey; and Wood Manager, Wood Research Lab., Purdue Univ. This paper was received for publication in October 2001. Article No. 9375.