Reinforcing device for wood beams with end splits

Details Reinforcing device for wood beams with end splits Reinforcing device for wood beams with end splits

1999-05-10

2001-01-23

Kent, Christopher T. (Department: 3635)

Static structures (e.g., buildings)

With means for split-prevention or damaged part repair

C052S696000, C052SDIG006, C052SDIG007, C052S749100

Reexamination Certificate

active

06176058

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed to dimensional lumber and, more particularly, to end-split wood beams which are reinforced to inhibit the end splits from propagating toward the middle of the wood beam.
BACKGROUND OF THE INVENTION
End splitting is a phenomenon associated with the drying and shrinking processes of wood and is a common defect in dimensional lumber. End splitting occurs when the ends of a board dry at a faster rate than the interior region. This higher moisture content gradient at the ends of the beam results in differential shrinkage. When the tensile strength perpendicular to the grain is exceeded, an end split forms.
The incidence of end splits typically increases as the depth of the wood increases. For example, dimensional 2×10 and 2×12 lumber have a higher occurrence of end splits when compared to 2×4 and 2×6 lumber. End splits also are more common on the higher-density species of wood such as Douglas Fir, and less common on lower-density species such as Spruce and Pine. On average, about 20% of a quantity of dimensional lumber will have end splits.
The moisture content of lumber varies and is dependent on its exposure to weather. End splits can occur in a piece of lumber at any time, i.e., during the period from initial milling up until the time the piece reaches moisture content equilibrium in its final structure. An end split can start small and become longer, propagating toward the middle of the piece of lumber.
As end splits are a common defect in lumber, specified lengths of end splits are permitted by lumber grading rules. Typically, the higher the grade of lumber, the smaller the allowable end split. Indeed, the grade of a piece of lumber can be governed by an end split. For example, a piece of lumber which is Select Structural grade in all respects except for the permissible length of an end split, could be downgraded to a No. 3 grade.
Plates have been applied to composite wood beams to increase shear capacity. For example, Knowles U.S. Pat. No. 4,637,194 describes applying shear stress plates to opposite faces of a web of a wooden I-beam near the ends of the beam. The plates may be formed of a plywood and secured to the web by means of nails, staples, adhesive, or a combination thereof. The shear stress plates are said to increase the thickness of the web only in the shear-field or area needed. Alternately, a stiff metal plate may be used, similar to that used in the truss industry. The metal plates are said to reinforce the ends of the web in a manner similar to thickening the ends of the web in the area or field of maximum shear stress.
The shear plates disclosed by Knowles function by the classical formula for calculating maximum longitudinal shear capacity of wood beam:
V=⅔Af
v
V=maximum shear force
A=cross-sectional area of wood beam
f
v
=allowable unit shear stress of wood
From this classical formula, the maximum shear force in a wood beam is directly proportional to the cross-sectional area of the beam. Based on the uniform shear stress distribution shown in
FIG. 14
of Knowles, it is apparent that no consideration was given to the end split condition. The classical formula described by Knowles is not valid for end-split lumber, which does not have a uniform shear stress distribution.
The use of metal plates also is disclosed by Birckhead U.S. Pat. No. 4,442,649, wherein a fabricated wooden beam has at least two longitudinal wooden members spaced parallel and apart by a plurality of wooden blocks spaced between and along the length of the longitudinal members. A plurality of truss plates, each installed across one side of a block and spanning between each longitudinal member, are said to provide structural integrity. The truss plates may be installed in pairs with one across either face of each block at an acute angle, i.e., a 45° angle, to the longitudinal axis of the beam. Narrow truss plates may be provided at each end, tieing the ends of the longitudinal members and the end block more securely together.
Although Knowles and Birckhead describe applying shear plates to composite wood beams to increase shear capacity and/or assemble the composite wood beams, neither reference addresses the use of lumber with end splits or the problem of inhibiting end splits from propagating in dimensional lumber to upgrade or avoid downgrading of the lumber.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, lowering of the grade of a piece of end-split lumber to a lesser grade is prevented or substantially avoided by inhibiting the propagation of the end split. The propagation of the end split is inhibited by attaching at least one reinforcing plate to the piece of lumber covering at least a portion of the end split.
According to another aspect of the invention, the grade of a piece of end-split lumber can be increased by attaching at least one reinforcing plate to the piece of lumber covering at least a portion of the end split. The reinforcing plate nullifies the effects of the end split which otherwise would result in the piece of lumber being assigned a lesser grade.
The reinforcing plate preferably is steel and has teeth which are angled relative to the grain of the wood. In a preferred embodiment, first and second portions of the teeth are oriented oppositely from one another, i.e., 90° relative to one another.


REFERENCES:
patent: D. 202925 (1965-11-01), Mort
patent: 2470514 (1949-05-01), Muller
patent: 3094748 (1963-06-01), Sanford
patent: 3234841 (1966-02-01), Adams
patent: 4442649 (1984-04-01), Birchkead et al.
patent: 4637194 (1987-01-01), Knowles
patent: 4710083 (1987-12-01), Wolf
patent: 5116179 (1992-05-01), Matlock
patent: 5234279 (1993-08-01), Poutanen
patent: 5551819 (1996-09-01), Stern
patent: 5553961 (1996-09-01), Olden
patent: 5896716 (1999-04-01), Jalla
patent: 613247 (1979-09-01), None

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