US 3531904 A
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Oct. 6, 1970 A. c. SANFORD 3,531,904
REINFORCED CONSTRUCTION FOR WOOD STRESS MEMBERS Filed June 17, 1968 3 Sheets-Sheet 1 INVENTOR. ARTHUR CAROL SANFORD BY WM, W W
ATTORNEYS 1970 Q A. c. SANFORD 3,531,904
REINFORCED CONSTRUCTION FOR WOOD STRESS MEMBERS Filed June 17, 1968 5 Sheets-Sheet INVENTOR. ARTHUR CAROL SANFORD BY @13 iii,
ATTORNEYS REINFORCED CONSTRUCTION FOR WOOD STRESS MEMBERS i-lled June 17, 1968 3 Sheets-Sheet 5 INVENTOR. ARTHUR CAROL SANFORD RM ia/W ATTORNEYS United States Patent Ofice 3,531,904 Patented Oct. 6, 1970 3,531,904 REINFORCED CONSTRUCTION FOR WOOD STRESS MEMBERS Arthur Carol Sanford, P.O. Box 1177, Pompano Beach, Fla. 33061 Filed June 17, 1968, Ser. No. 737,585 Int. Cl. E04c 3/17, 3/292 US. Cl. 52730 1 Claim ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION In the construction of low cost housing, various stressed members such as trusses are constructed of wood components in standard size dimensional lumber such as 2 X 4s, 2 X 6s and the like. In wooden trusses in which the components are all in one plane, the wood components are usually connected at their joints by toothed metal plates having their teeth embedded into the wood of the abutting components. Such truss plates are shown, for example, in my prior Pat. No. 3,104,429 and in McAlpine Pat. No. 3,377,905.
In my copending application Ser. No. 588,167 now US. Pat. No. 3,498,170 there is disclosed a connector plate combination for connecting wood members in which a toothed plate overlies at its central portion a perforated grommet plate of smaller area with the teeth of the over lying plate penetrating through the perforations into the wood members. The purpose of the grommet plate is to increase the cross-sectional tensile and diagonal shear strength of the toothed plate at the critical joint area between the abutting wood members.
In these days of vast construction requirements the better quality woods such as good grades of fir and yellow pine are not only difficult to obtain but their cost is substantially prohibitive for use in low cost housing. The result is that softer woods such as hemlock and spruce are being used. The fiber strength of average grade yellow pine and Douglas fir is on the order of 1500 psi. in tension and 1200 p.s.i. in compression, while that of average grade hemlock is somewhat less in tension and compression.
Accordingly, in order to obtain the requisite strength in the components of wood trusses using these softer woods, it has been necessary to increase the transverse dimension of the wood, as for example by using 2 X 6s instead of 2 x 4s. Obviously, this increases the cost of the truss to the point where it may approach the cost of a truss using the harder, more expensive woods with the original design size components. Moreover, there are many cases where the extra weight or space required by the larger components is not desirable.
SUMMARY OF THE INVENTION I have discovered that by applying one or more flat metal toothed reinforcing strips longitudinally to the wood components, at least those subjected to the greatest stress, the strength of the component in tension and compression is greatly increased with no material increase in transverse dimension, and with much less added expense than would be involved in using the harder wood components or softer wood components of increased transverse dimen- When the teeth of the reinforcing strip are fully embedded into the wood so that the body of the strip abuts the wood, tension and compression stresses applied to the wood are transmitted by the teeth through the strip, and the engagement of the strip and the teeth with the wood prevents buckling of the strip under compression, so that the full compressive strength of the steel strip is utilized.
The result is that the cheaper softer wood components of original transverse dimension can be used because the strength of the wood in both tension and compression is in effect upgraded to be more than equal to harder wood components of the same transverse dimension. Moreover, the wood components can be made proportionately stronger in areas requiring greater strength by using additional strips in those areas, either side by side with the first strips or in underlying relation thereto. Thus, in a long span truss, for example, the bottom chord may have a long toothed metal reinforcing strip applied over the greater part of its length just short of the ends, and at least one shorter strip underlying or alongside of the central portion of the chord. The additional or underlying strips may not be required to be toothed but are perforated to allow the teeth of the overlying strip to penetrate therethrough into the wood.
A thin steel toothed strip of 18 gauge hot rolled low carbon steel has approximately the same strength in tension as the average wooden member having sixteen times as much cross-sectional area, so that two steel 18 gauge strips 1 /2 wide applied to a 2 X 4 would be equivalent to adding a wood strip 1%" wide (actual 2 X 4 width) and slightly more than 1 inch thick in respect to the increase in tensile strength.
It is therefore an object of the present invention to provide an economical means for reinforcing an elongated wooden stress member whereby the strength of said member in tension and compression is substantially increased without materially increasing its transverse dimensions.
Another object is to provide an improved reinforced construction for an elongated wooden stress member comprising a toothed metal strip applied longitudinally to said member to increase its tension and compression strength.
A further object is to provide an improved reinforced construction for progressively increasing the strength in tension and compression of a wooden stress member by applying at least one additional metal strip to said member in the area of its greatest stress.
These and other objects are accomplished by the improvements comprising the present invention which are shown in the accompanying drawings and hereinafter described in detail. Various modifications and changes in details of construction are intended to be within the scope of the appended claim.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation of a flat wood and metal truss having the novel metal reinforcing strips applied to its top and bottom wood chords.
FIG. 2 is a bottom plan view, partly broken away to show additional perforated strips underlying the toothed strips.
FIG. 3 is a side elevation of a triangular wood truss having the novel metal reinforcing strips applied to its bottom chord.
FIG. 4 is a side elevation of a triangular wood truss in which the top and bottom chords are composed of several relatively short wood pieces, and having the novel metal reinforcing strips applied to both the top and bottom chords.
FIG. 5 is an enlarged partial sectional view as on line 55 of FIG. 3 showing a perforated strip and an overlying toothed strip applied to the bottom of a wood chord member.
3 FIG. 6 is an enlarged partial isometric view showing the toothed strip and perforated strip of FIG. in detached position relative to the wood chord member.
DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIGS. 1 and 2, the flat truss indicated generally at 10 has a wooden top chord 11 and a wooden bottom chord 12 parallel thereto. The chord members 11 and 12 may be dimensional lumber such as 2 x 4s with their longest transverse dimension extending horizontally. The top and bottom chords are connected by metallic web members 13 which resist the tensile and compressive forces between the chords and also impart unity to the truss.
The diagonal web members 13a, 13b, 13c, and 13d each have a strut portion 14 extending between the chords and a flat load transfer flange 15 at each end. The strut portions 14 may be longitudinally corrugated for increased strength, and the strut portions 14a, 14b, 14c and 14d are progressively narrower since the stress on the diagonal web members at the bearing wall is greatest and becomes less and less toward the center of the truss span. Also, as indicated in FIG. 1, the diagonals on one half of the truss may be inclined in the opposite direction to those on the other half. As indicated in FIG. 2, the diagonal members may be duplicated on the other side of the truss.
The load transfer flanges 15 are provided with a plurality of perforations arranged to register with the teeth in toothed connector plates 16 which penetrate the perforations and are embedded into the sides of the wood chord member-s to connect the load transfer flanges thereto. The detailed construction of the perforations and the teeth penetrating them is not part of the present invention, and an example of such construction is shown in my copending application Ser. No. 591,788, filed Nov. 3, 1966 now Pat. No. 3,416,283.
As shown in FIG. 1, vertical wood compression posts 17 may be positioned between the top and bottom chords 11 and 12 adjacent the connector plates 16, and these spacer posts may be standard dimensional lumber, such as 2 x 4s.
The reinforcing strips of the present invention may be applied to the top surface of the top chord 11 to reinforce it against compressive stress, and to the bottom of the bottom chord to reinforce it against tension stress. As seen in FIGS. 2 and 6, the strips 20 are narrow elongated strips extending over a substantial part, and preferably a major part, of the chords. The strips are preferably 18 gauge, hot rolled steel.
The strips 20 have longitudinal rows of teeth extending throughout their lengths. Two rows of teeth 21 are shown, but one or more additional rows may be provided The teeth 21 may vary in configuration, and a preferred construction is shown in which the teeth are struck from the flat body portion of the strip in opposed pairs, forming openings 22. Each tooth has a relatively wide base portion 23 and a narrow tip portion 24. One edge of the tooth has inclined portion 25 connecting the base and tip portions. These teeth are preferably of the same configuration as the teeth on the toothed connector plates, and further details of their construction are set forth in my copending application Ser. No. 591,788.
As shown in FIG. 2, the chord 12 has two strips 20 applied to its bottom surface in side-by-side relation. These strips may be about 1 /2" in width, and the chord 12 may be a standard 2 X 4 with its widest transverse dimension (3%") extending horizontally or crosswise of the truss. Thus, the outer edges of the two strips 20 may be flush with the sides of the chord, leaving a space of about /3" between the strips. As shown in FIGS. 5 and 6, a single strip 20 applied to the narrower side of a 2 x 4 member 26 will extend over substantially the entire width of the surface.
In order to make the chords or wood stress members progressively stronger with the greatest strength in areas of greatest stress one or more strips 27, preferably of the same width as strips 20 but shorter in length, may be placed in underlying relation with strips 20. The strips 27 have two rows of holes 28 spaced longitudinally in pairs so as to register with the pairs of teeth 21. The holes or perforations 28 are shown as circular, but the shape of the holes may vary as long as their maximum dimension permits the base 23 of a tooth 21 to fully penetrate the hole and bring the body of strip 20 into flat abutment with the body of strip 27, as shown in FIG. 5. The fit or engagement between the holes 28 and the teeth 21 should be snug so as to lock the perforate plates against lateral movement. Such snug fit reinforces the teeth against bending when the wood member is stressed, thereby increasing the holding power of the teeth.
As shown in FIGS. 1 and 2, the central portions of the chords 11 and 12, which are at the central part of the span of the truss and hence subjected to the greatest stress, have two perforated strips 27a and 27b underlying the toothed strips 20. Strips 27a are the shorter ones in abutment with the central portion of the chords, and strips 27b overlie and extend beyond the ends of strips 27a, with the extending portions in abutment with the chords. The toothed strips 20 overlie and extend beyond the ends of perforated strips 27b with the body part of their extending portions in abutment with the chords.
Using teeth 21 which are long and both the toothed strip 20 and perforated strips 27 of 18 gauge steel, no more than two perforated strips 27 should be used, so as to obtain adequate penetration of the teeth into the wood to give effective holding power. Of course, by using longer teeth additional perforated strips 27 may be used. On the other hand, additional holding power may be obtained by providing at least one of the perforated strips with teeth.
Accordingly, when the wood chords are subjected to stress, they are reinforced by the steel reinforcing strips 20 and 27 to increase their strength, with no material increase in cross-sectional dimension, and the amount of reinforcement is applied progressively over the length of the chord in accordance with the progressive application of stress to the chord; in other words, the greatest amount of reinforcement is applied at the area of greatest stress. It will be understood that the number, length and location of the reinforcing strips used will depend upon the length and strength of the wood stress member and the amount and kind of loads to which it is subjected. The improved reinforcing strips thus afford wide flexibility in adapting to a wide variety of such conditions.
An 18 gauge steel strip 1 /2" wide has been determined to be equivalent in tension to a member of standard grade fir or pine having approximately sixteen times the cross-sectional area of the steel. Thus, two rows of three steel strips as shown in FIGS. 1 and 2 are equivalent in tension to 6.6 square inches of wood, which is more than adding another 2 x 4 having a finished cross-sectional area of 5 .89 square inches.
As will be seen from the following example, the use of this novel reinforcement can substantially increase the effective span of wooden structural members. Thus, the reinforced truss 10, of the depth depicted in FIG. 1, could be used over a larger span than possible for a truss of that depth without the reinforcing strips, or, if the depth of the truss is itself a crucial design consideration, these reinforcing strips permit a shallower truss than could heretofore be utilized.
Strips of the typical dimensions heretofore described whether toothed or grommet, have an individual design strength of 1250 pounds per square inch in tension. Ac cordingly, with the two rows of three strips-each row having two grommet and one toothed stripaflixed to that portion of the lower chord member 12 spanning the center bay A, the strength of that portion is increased by 7,500 pounds of tensile design strength. Thus, the height of the compression post members 17 can be reduced as compared to like members in a truss without reinforcing strips. In fact, the height of posts 17 can be reduced at least to that point at which the stress in the portion of chord member 12 spanning center bay A does not ex ceed the available strength of the chord member in combination with the reinforcing strips.
By fundamental methods of analysis, the stress in the lower chord member spanning successive bays toward the support S (which is of progressively lesser magnitude than the stress in the portion of chord member 12 spanning the center bay A) is computed until it is determined that the grommet strips 27a can be eliminated. As depicted in FIG. 1, calculations might indicate that the grommet strips 27a are no longer required in bay C, and strips 27a would, therefore, be terminated within bay C, just to the left of plate 150, as shown.
Similar calculations will determine at what point grommet strip 271) can be discontinued, which might be within bay D, as indicated.
Additional considerations must be evaluated in determining the termination of toothed strips 20. The necessity for the toothed strips 20 to bolster the tensile strength of the lower chord beyond the termination of grommet strip 27b must be first computed. Thereafter, one must determine the number of teeth required to effect the load transfer between the wooden chord member 12 and the reinforcing strips.
Extensive tests have determined that a proper design value for effecting this load transfer with teeth of the type heretofore described is approximately pounds per tooth. Accordingly, one first calculates the total number of teeth required to effect load transfer for the maximum stress to be carried by the two grommets and one toothed strip in each row and then provides at least a sufficient length of toothed strips to assure the number of teeth required to transfer that load. Toothed strips of the type heretofore described preferably have four teeth per lineal inch. Additionally, the number of teeth is computed which is required to transfer the load to be carried by the toothed strips across the last bay where such strips would be required for tensile strength to assure that the proper number of teeth to transfer this load would be embedded beyond the last bay.
To exemplify this situation, should the load carried by the strips 27a, 27b and 20 secured to that portion of the lower chord member 12 spanning the center bay A require a number of teeth such that the strips 20 would extend to point 30 in bay D, and should that portlon of the lower chord member 12 spanning bay D itself require reinforcement, the toothed strips 20 must extend beyond bay D a sufficient distance to transfer the load from bay D into the chord member 12. As shown, the toothed strlps 20 would extend to point 31 in Bay E.
Conversely, should the length of the toothed strips 20 required to transfer the load carried by the strips 27a, 27b and 20 across bay A extend beyond point 31, then the toothed strips 20 must extend beyond point 31 even if that portion 'of chord member 12 spanning bay D required no reinforcement.
A similar arrangement of one toothed strip 20' overlying a shorter grommet strip 27a is shown applied to the bottom chord 26 of a W type peak truss in FIG. 3 to reinforce the chord in tension. The top chords 34 are connected to the bottom chord at the heel joints by toothed plates 35 similar to the plates 16 in FIG. 1, and the strut members 36 and 37 are also connected to the chords by similar toothed plates 38, 39 and 40.
It will be understood that under conditions of less stress requirements one or more toothed reinforcing strips 20 may be sufiicient to reinforce a wood stress member without using any underlying perforated strips. Moreover, one or more reinforcing strips may be applied to a prestressed wood member to maintain it in pre-stressed condition until loaded, and thus further increase its strength.
As shown in FIG. 4, by the use of steel reinforcing strips a truss can be constructed with the chords made up of short pieces of wood connected together with their ends in abutment. The peak truss in FIG. 4 may have an upper chord comprising components 42 and 43, and a bottom chord comprising components 44 and 45. The wood strut member 46 is connected to upper chord component 43 and lower chord component 44 by toothed connector plates 47 and 48, and a longitudinally corrugated metal strut 49 (similar in construction to the metal struts 13 in FIG. 1) connects chord component 44 to component 43 at the peak of the truss by overlying toothed plates 50 and 51 respectively. A metal strut 49' from the other side of the truss is connected to the reverse side of the peak joint.
In this case a toothed metal reinforcing strip 52 is embedded in the bottom chord components 44 and 45 and extends the full length of the bottom chord to connect the components and reinforce the chord in tension. Similarly, a toothed metal reinforcing strip 53 is embedded in the top chord components 42 and 43 and extends the full length of the top chord to connect the components and reinforce the chord in compression. To facilitate assembly of the truss in a structure, the strip 52 may be applied to the top surface of the bottom chord and the strip 53 may be applied to the bottom surface of the top chord. Short toothed plates 54 and 55 are preferably applied at the joints between components 42 and 43 and between components 44 and 45, respectively, to hold the abutted components in alignment.
If necessary, a perforated strip or strips may be applied underlying one or both of the reinforcing strips 5t2 and 53, at the chord portions subjected to the greatest s ress.
it will be seen that the improved metal reinforcing strips provide quickly and easily applied means for greatly increasing the tension and compression strength of wood stress members without materially increasing the transverse dimensions of the members, and for proportionately increasing the strength of the wood stress members in the areas of greatest stress. Accordingly, the metal reinforcing strips add a safety factor to the wood stress members to which they are applied, and assist in obtaining less deflection of the wood members under load.
Stated another way, the metal reinforcing strips provide a novel mechanical means of gripping the wood member in its unstressed position so as to cause the wood to stretch with the metal when the wood member is stressed, for example as in a beam.
1. In a structure, at least one wooden component having at least one surface with an elongate dimension, said structure resting on supports and subjected to a load imparting fiber stresses of varying magnitude along the elongate dimension of said wooden component, the stress along a portion of said elongate dimension exceeding the allowable stress for said wooden component, at least one reinforcing steel strip of the order of 18 gauge in thickness extending along and beyond the elongate dimension of the wooden component subjected to stress in excess of that allowable for said wooden component, said strip having a fiat body portion with integral teeth projecting therefrom substantially at right angles thereto throughout the length of said strip, the effective cross section of said strip being such as to bear certain excess stress to which said Wooden component is subjected, said teeth being fully embedded into the wooden component with said flat body portion in substantial abutment with one surface thereof, sufficient teeth being provided within the length of the strip overlying the portion subjected to excess stress to transfer the excess stress into said body portion, the length and the number of teeth in the portion of said strip extending beyond said portion of the wooden component subjected to excess stress being sufficient to transfer the excess stress back into said wooden component along an elongate portion thereof to which the loading of said strip can be applied Without exceeding the allowable stress in said Wooden components, and at least one perforated strip shorter in length than said toothed strip underlying the longitudinally medial portion of the toothed strip with the teeth thereof penetrating the perforations of the perforated strip to bear additional excess stress to which said elongate portion of said wooden component is subjected, the teeth on said toothed strip snugly engaging said perforations to preclude relative lateral movement between said plates, the length of said perforated plate being suflicient that the excess stress required to be borne thereby can be transferred thereto by the number of teeth penetrating the same.
References Cited UNITED STATES PATENTS 1,335,609 3/1920 Schneller 52730 2,039,398 5/1936 Dye 52730 5 2,738,832 3/1956 Torkelson 52225 X 3,067,544 12/1962 Willatts 52-642 3,344,225 9/1967 Jureit et a1 287--20.92 X 3,416,283 12/1968 Sanford 8513 X 10 FOREIGN PATENTS 165,619 10/1955 Australia. 391,25 7 4/ 1933 Great Britain.
15 ALFRED C. PERHAM, Primary Examiner US. Cl. X.R. 52-642, 693