Corrugated strandboard structural panels.

Abstract

The use of a corrugated shape as a way of improving the strength and stiffness efficiencies of a structural panel has long been used in metal and plastics. This paper presents the design, development and evaluation of a shallow corrugated panel made from strandboard material. The results show that an efficient structural panel can be successfully produced using conventional mat forming and hot pressing techniques. The panels in the study had a thickness of 0.375 in, an overall depth of 1.125 in and a wave length of 8 in. They gave primary direction stiffness and strength exceeding typical 23/32 in single floor panels.

For several decades, considerable effort has been devoted to research on improving the mechanical properties of flat composite wood panels, particularly OSB (OSB), by optimizing the resin content, flake geometry, flake alignment, additives, etc. It appears that the performance of flat composite panels is nearing the limits of current material technology. One option to further improve the structural properties of a panel is to alter the shape to a more efficient geometry. Higher stiffness and flexural strength can be obtained by moulding the strand mat into a corrugated shape. The idea of using a corrugated panel is not new. It is very common in the plastic and sheet metal industries. Nevertheless, there is no commercial production of corrugated wood panels as decking materials for floor or roof systems.

In the mid 1970s, Price and Kesler (1974) molded relatively shallow small corrugated panels (16 inches by 18 inches trimmed size, 30-degree pitch with 5.63-in period, 45-degree pitch with 4.00-in period and 45-degree pitch with 5.63 in period) by placing a flat wood flake mat on a set of fixed corrugated platens. The thickness of all these panels was 0.25 in and the total depth was 1.25 in. The corrugated panels tested by Price and Kesler did not exhibit good bending properties. Lower maximum stress and lower modulus of elasticity were reported for these panels compared to flat panels with similar configuration. The lower strength properties reported may have been due to bad flow properties of the mat in the cross corrugation direction because the initial flat mat had to elongate to assume the shape of the corrugated platens. This suggests that the moulding process needs to be refined in order to produce corrugated boards with better strength properties.

Later in the 1980s, Michigan Technological University (MTU) performed extended studies on the moulding behavior and structural performance of deep corrugated panels molded by fixed corrugated platens (Sandberg et al. 1989, DeBruine et al. 1990, Haataja et al. 1991). These studies produced deep 4-ft by 8-fl corrugated panels. These panels ranged from 3/8" to 7/16" thick and 3 inches to 4 inches deep. Excellent strength and stiffness properties were observed. However, the manufacturing process was not readily adaptable by existing OSB mills due to the complexity in the mat forming techniques.

Lau and Knudson (1990) developed a ribbed panel with one flat face and solid ribs on the opposite face. This concept has the potential advantage of avoiding the need for an underlayment layer to cover the openings in a uniform thickness corrugated panel. Stacking of a solid ribbed panel for shipping and storage could pose some problems.

Bach (1989) produced deep drawn corrugated panels on a set of articulated platen assemblies that went from an initial flat configuration to the final wave configuration during the pressing stroke. This process eliminated transverse density variation that can result from fixed corrugated platens. In spite of that, the panels did not go into commercial production, possibly because of the complications involved in fabricating, operating and maintaining the mechanical platens.

The objective of this paper is to present the design, development and evaluation of a shallow corrugated panel made from strandboard material, without the need for special mat forming methods or articulated platens.

Preliminary investigation

A corrugated structural panel must meet two major criteria to be practical. First, it should be suited to production with only minor modifications to current OSB technology. Most important here, the formation of the mat should be feasible with conventional techniques and equipment. The dies necessary to press the panels should not occupy excessive press daylight, so that the number of panels per press cycle is similar to that for OSB production. This criterion suggests a fairly shallow panel with moderate draw depth and draw angle. Secondly, the panel must be efficient in application, such that the construction industry finds it attractive as an alternative to OSB. This implies a panel shape compatible with the basic 4 ft module in domestic construction, with sufficient stiffness and strength to compare favorably to the thicker single floor OSB systems. It also suggests that the channel openings on the upper surface be narrow enough to allow use of a reasonable underlayment thickness and of a depth to make any blocking for load transfer possible with material of common thickness. These considerations led to the panel section of Figure 1.

[FIGURE 1 OMITTED]

Specimen preparation

Initial evaluations of the panel design were on specimens made with a set of 18- by 18-inch aluminum dies in a laboratory press. The furnish was typical aspen OSB strands (averaging 2in by 0.50in by 0.025in) at 5 percent MC, blended with 5 percent polymeric methylene diphenyl diisocyanate (pMDI) adhesive. No wax or water repellent was added to the mat. Target oven-dry density was 40 pcf. The mats were hand formed on a caul sheet, which was slipped out from under the mat after positioning on the lower die. The press was closed to stops as quickly as possible, to minimize density profile effects. The panel was pressed for 3 minutes at 375[degrees]F. Samples were made in: (A) a 3/8 in thickness and random orientation, (B) 3/8 in thickness and three layer OSB alignment with the surfaces aligned in the direction parallel to the channels, and (C) 1/2 in thickness and random orientation. The pressing stroke was recorded on video. This indicated minimal lateral movement of the mat edges, an important factor for the production of full-width panels. Visual inspection of the specimens indicated good uniformity in density across the channels with no evidence of mat tearing.

All panels were equilibrated to standard conditions for 3 weeks in a controlled conditioning room at 70[degrees]F and 65 percent relative humidity. The panels were sealed in plastic bags, before removal from the conditioning room and unsealed immediately prior to the testing. The Type A had an average thickness of 0.376 in, an oven-dry density of 40 pcf, and an equilibrium MC (EMC) of 5.5 percent at time of testing. The Type B specimens averaged 0.369-in-thick, were somewhat denser at 43 pcf, and had an EMC of 5.2 percent. The Type C specimens averaged 0.493-in-thick and had essentially the same density and EMC as the Type A panels.

Small panel testing

The evaluation of the small panel specimens involved six types of tests; strong direction bending shear, and bearing (Fig. 2), weak direction bending (Fig. 3), edge loading (Fig. 4), and evaluation of the lateral density variation across the channels (Fig. 5). The strong axis bending tests and the shear and bearing tests were done on different specimens. Shear tests were done on one end of a specimen and the undamaged far end was used to test bearing capacity.

[FIGURES 2-5 OMITTED]

Strong direction bending tests were conducted on the full 16-in by 16-in panels, using an adaptation of ASTM D1037 (ASTM 1999). The span, in the direction of the channels, was 14.5 inches. End support and center load were through 1.5-in-diameter steel tubes. Deflection was measured at the load. The edges of the panel in the span direction, parallel to the channels, were lightly clamped to prevent spreading.

Weak direction tests were also adapted from ASTM D 1037. Panels were cut across the channels into 3-in strips. The strips were loaded on the central upper deck and supported at the outer edges of the two lower decks with a 10-in span. This test was intended to give data on the resistance to secondary moments that might occur in a structural panel during handling and construction. Also, the test gives some indication of whether flaws developed in the panel because of mat tearing in the moulding process.

The shear test evaluated the capacity of the corrugated section for conditions where shear loads are more critical than bending moments. This is seldom a problem in conventional OSB applications, but required evaluation here to rule out any unexpected behavior. The test set up was similar to the strong-direction bending test, except the load point was shifted from midspan to a point 3 inches from one of the supports.

The bearing test was conducted on the opposite end of specimens previously used for the shear test. Its purpose was to give an indication of the ability of the section to transmit loadings from the bottom plate of an upper wall to the top plate of a wall directly beneath. The loading tube was placed directly over the support tube, with the far end support inside of the shear test failure location. Load and displacement were recorded up to a crushing deformation in the vertical direction of 0.2 inch.