Patent Application
20080028865
Certain embodiments of the invention are shown in the drawings as nonlimiting examples; however, reference should be made to the claims to define the scope of the invention. In the drawings,
As shown in
A deck 25 of the type of structure shown typically comprises a plurality of parallel planks 24 forming a surface, mounted closely adjacent to one another and affixed to parallel underlying spaced joists 26 by fasteners 28 such as screws or nails. In many deck designs, particularly outdoor constructions that advantageously drain readily, admit air flow and are tolerant of thermal expansion, each of the planks 24 is spaced from adjacent planks by a narrow gap. Examples are decks that sometimes are used as porch or patio areas, swimming pool surrounds and the like. The joists 26 are in turn supported by beams on posts or on headers affixed to building panels, which are not shown.
In decks comprising planks spaced by a gap, each plank forms an independent load bearing member over the span between the joists 26 to which the planks are affixed. The joists 26 are typically parallel spaced members disposed at an angle to the planks 24, often perpendicularly, but possibly at an oblique angle. There are numerous varied arrangements of planks and joists possible, wherein the planks might be rectilinearly cut and/or equal in width, or otherwise shaped. There are any number of specific deck assemblies and related structures possible, wherein joists and/or planks may be equal or may vary in elevation or may be parallel or may be divergent at angles.
The invention is generally applicable to most or all arrangements wherein planks or plank-like members extend between spaced points of support such as joists, and over the span between such supports, the planks or other members need to support a static or live weight load. Various structures qualify but differ in one way or another from the conventional one-level parallel plank and perpendicular joist construction. A deck may have different grade levels. In stair steps, the stair treads function as load supporting planks at different elevations, carried on stringers that function as joists supporting the treads at spaced points. The deck planks may vary in orientation, possibly reflecting angles of joists that diverge (for example, in hexagonal or octagonal plank layouts), or possibly for other reasons such as to form a herringbone pattern.
When planks are oriented perpendicular to spaced parallel joists, the span of the planks between the joists is the minimum possible. If instead the planks are oriented at an angle, the span is longer according to the triangular geometry involved. For example, the span of a plank at 45 degrees to spaced parallel joists is equal to 1.414 times the perpendicular space between the joists (i.e., by the square root of two). The span of a plank at 30 degrees is twice the distance between the joists.
The extent to which a plank can support a weight is inversely related to the unsupported length that the plank spans. The plank functions as a resilient or elastic structure with a spring constant. At a distance from a point of support (or at a point between two spaced points of support), the plank can be deflected vertically from a rest plane by a distance that is related to the plank material stiffness (the dimensions and material modulus of elasticity), and the applied weight. This vertical deflection aspect can form the basis of a measurement for rating the load bearing capability of a plank or plank faced structure. Standards have been established with respect to the extent of vertical deflection permitted for deck planks when stressed by a load.
Other things being equal, a deck may adequately support a relatively greater load (more weight) without undue vertical deflection, either by substituting stronger and stiffer planks, or by spacing the joists more closely. Planks can be made stronger by using a stronger material or by making the planks thicker. It is generally undesirable to use planks that are thicker than necessary or joists that are more closely spaced than necessary, because these entail overuse of materials.
The present invention can be used with any flexing plank material, whether natural material such as wood or man made materials such as molded or extruded polymer material. The invention is particularly advantageous when used to assess the deflection characteristics of plank material made from composite material, such as a polymer matrix containing a particulate filler material, optionally provided with a wear resistant or wood simulative outer surface. Various products that simulate wood but are more durable and decay resistant are marketed. An example comprises wood flour filler in a polyvinyl chloride (PVC) matrix, optionally with a distinct outer surface (cap stock) comprising a different polymer material or a different mix of components.
Composite and synthetic planks and similar building materials are manufactured and engineered to meet specifications providing for a predetermined degree of flexibility, and often the resulting synthetic products are relatively flexible compared to similarly dimensioned wood. The characteristics of all planks of a given product description and size are more equal than is the case with natural wood. For these reasons, the deflection characteristics of synthetic and composite materials are important.
Natural and synthetic products both might be tested, classified for stiffness and sorted for flexibility and other characteristics. The repeatability of synthetic and partly synthetic (composite) materials is such that it is possible to remain close to the structural recommendations for decking construction, such as the recommended joist spacing under load, and thereby avoid unnecessary use of materials. However, some variation occurs among products. Also dimensional variations can occur during installation, including the spacing of joists, such that it is advantageous to test the deflection characteristics of planks after they are installed.
For this purpose, the apparatus shown in
The apparatus is arranged to act only on the plank 24 being tested, in that the apparatus is supported at points 34 other than the plank 24 being tested, and comprises a movable weight application mechanism whereby a weight can be applied to the plank 24. The vertical displacement of the mechanism is noted and determined to be the deflection of plank 24 under a load equal to the applied weight and over the span equal to the distance between support points, in this case joists 26.
The apparatus enables assessment of the deflection of decking planks 24, installed independently on deck 25, and carried on supports spaced by a measurable span. The apparatus as shown in
The plank under test could be a single plank or a subset of two or more adjacent planks that are coupled together, or even two or more non-adjacent planks provided they are coupled together or are arranged such that the load weight can be applied to them as a structural unit. Thus, plural planks that are coupled by a batten (not shown) could be regarded as a structural unit. Plural planks that are not coupled together by structure of deck 25 could be regarded as a structural unit by providing a mechanism that applies the weight to the plural planks at the same time, thus being coupled by the mechanism that applies the weight. As another alternative, a panel that encompasses an area such as a square panel as opposed to an elongated plank, could be tested for deflection at a midpoint between supports at its corners, provided that the apparatus is supported independently of the panel during the testing.
The legs 44 and feet 45 hold at least part of the bridge member at a space over the plank to be tested (which as described can be any subset of planks that are coupled or can be loaded as a structural unit. A tup or contact member 47 is movably mounted on the bridge member 42, extending downward to the decking 25. The tup 47 is dimensioned exclusively to contact the subset of planks (one or more planks) that is to be tested for deflection under load.
For applying the test deflection load (not shown in
A distance displacement measurement scale or mechanism 52 is provided and is mounted between fixed and movable points on the bridge member 42, which remains stationary, and the tup or the mechanism associated with the tup, which moves vertically when a weight on the platform 48 or other receptacle loads the plank 24 and produces vertical downward deflection from a rest position. The operator either zeroes the measurement scale or mechanism 52 before applying the weight, or notes the scale reading before and after applying the weight. The measurement scale can be mounted on either the bridge 42 or the platform 48, provided that the relative movement of the other of the bridge 42 and platform 48 is measured.
The tup 47, platform 48 and sliding connecting shaft 49 comprises structures that have a nonzero weight, constituting a tare value. The tare weight should be small compared to the typical live load specification for the decking, typically 100 or 200 lbs. per square foot. The tare weight can be minimized by appropriate choices of materials and structures for the platform 48, shaft 49 and tup 47. For example, the platform 48 can be relatively thin steel plate or sheet metal. The shaft 49 can be a pipe section, etc. The tare weight can be partly offset by including a spring (not shown) such as a compression spring between the platform and the upper side of the bridge member 42.
If minimal, the tare weight can be ignored. Generally, even a significant tare weight can be taken into account by judging deflection as the difference in deflection between loading equal to the tare and loading equal to tare-plus the applied test weight, instead of between zero and the test weight.
A particular measurement arrangement that has proved to be workable with the invention is the Mitutoyo Electronic Indicator, Model 543-453B, coupled to a Digimatic Gauge Counter, Model EC-10D. This arrangement has a zero to one inch measurement span. It reads out to a resolution of 0.0001 inch, with a nominal accuracy of ±0.00012 inch. Other specific measurement devices and techniques likewise can be used in this application.
In the embodiments of
In
One advantageous aspect of the invention is that the apparatus is portable. The weight bearing platform and the mechanism for selectively applying the loading force to the planks is easily manipulated to deploy and locate the bridge member 42 in the correct position for temporary placement of a weight on the platform 48 and thereby to selectively apply the loading force via to plank 24 via the connection between the platform 48 and the tup 47, which is movable vertically relative to the bridge member 42. For this purpose, handles 62 are attached to the bridge member 42 and extend upwardly where they can be grasped to lift and move the apparatus.
The apparatus might have a permanently attached weight on platform 48 or may receive a movable free weight as in
In the embodiment shown in
In order to facilitate moving the device around, the apparatus can be made from sheet metal and hollow parts. The feet 45 on one side or end of the device can be replaced with wheels (not shown) for rolling the apparatus into position like a shipping dolly.
The operator's weight is previously known. The length of the span of the tested plank 24 between joists 26 might be accepted as nominal. For example bracing (not shown) is sometimes used between joists (not shown) and could be noted as an indication of the joist spacing. More preferably, the distance between the joists 26 is measured because the joist spacing may not be nominal. A measurement of the span length is also useful if the planks 24 are oriented oblique to the joists 26 instead of perpendicularly, resulting in a span length that is determined by the oblique angles as well as the joist spacing.
In order to verify operation of the apparatus for measuring deflection, and optionally to calibrate the measurements that are taken for an unknown load weight (for example if the operator's weight is not accurately known), a calibration fixture 80 can be provided as shown in
In
In the embodiments of
The apparatus is useful to measure the bending deflection of a deck plank, especially after installation on joists. This can be a quality/safety assurance step. For example, the invention can be used to determine compliance with a structural standard (or an arbitrary requirement) such as the L/360 rule, generally providing that the vertical deflection of a deck plank under load must be less than one 360th of the span between the support joists.
The invention as discussed above, in particular together with a knowledge of the span between adjacent joists to which the planks are fastened and optionally knowledge of the plank width, enables a determination as a function of the weight, the span and the deflection of the plank, whether the deck meets certain predetermined loading criteria. Such loading criteria can be, for example, an L/360 standard at a given live load weight for a plank of a given width such as 200 lbs. per square foot. If a given deflection is measured under the weight of the operator (or some other known weight that is applied as a test load), a deflection at a nominal load can be inferred, at least over a deflection range in which the deflection of the plank has a known relation to load weight (e.g., a linear or spring-constant relationship). The deflection at nominal load such as a weight equal to the rated live load for the applicable area of plank, in that case produces a deflection in a ratio to the observed deflection under the operator's weight, that is in the same ratio as the ratio of the respective weights of the operator and the rated live load.
In order to employ the invention for assessing compliance with such standards, and assuming that a range of different spans between joists and other particular deck characteristics are encountered, it is advantageous of if the invention is provided as a kit for assessing deflection of installed decking under load. The kit includes a span measuring device for determining a span distance between the spaced supports along a line parallel to elongation of a subset of planks, and for indicating said distance, together with the deflection measuring device for determining displacement of the subset of planks between at least two measurements taken in a direction perpendicular to said line parallel to elongation of the subset of planks at a point between the spaced supports, namely with and without the applied loading weight. The deflection measuring device comprises the elements discussed above.
The said span measuring device as discussed and shown in
Another element of the kit can be the calibration reference, with a test member that deflects upon application of a load, wherein the calibration reference is configured to engage with the deflection measuring device in lieu of the subset of planks. The calibration as discussed and shown in
The invention has been disclosed in connection with a number of examples and preferred arrangements. However the specific examples should not be regarded as limiting and reference should be made to the appended claims rather than the foregoing specification to determine the scope of the invention and the scope of exclusive rights claimed.
