US2601910A - Composite structural member - Google Patents

Description

T. F. NICHOLS COMPOSITE STRUCTURAL MEMBER July 1', 1952 Filed May 16, 1944 5 Sheets-Sheet 2 INVENTOR W1 7210/1/15 f. N/cwoas RNEY ly 1952 T. F. NICHOLS ,910

COMPOSITE STRUCTURAL MEMBER Filed May 16, 1944 5 Sheets-Sheet 4 ATTORNEY July 1, 1952 c o s 2,601,910

COMPOSITE STRUCTURAL MEMBER Filed May 16, 1944 5 Sheets-Sheet 5 J 2 mu m III m INVENTOR 7770M A /V/c//04S Patented July 1, 1952 COMPOSITE STRUCTURAL MEMBER Thomas F. Nichols, Utica, N. Y.

I Application May 16, 1944, Serial No. 535,777

This invention relates to the reinforcement of structural members which are subject to bending, and particularly to a novel method of effecting the desired reinforcement of such members readily and with assurance of effective results. More particularly the invention relates to reinforced structural members which have applied thereto an improved system of metal reinforcements which have been suitably proportioned and prestressed, as hereinafter more fullyset forth.

The invention is applicable to beams, slabs, arch-ribs, etc. of timber and of other materials which have elastic properties similar to, but not necessarily identical with, those of timber. For purposes of illustration, the invention will be de-' scribed herein particularly in its application to the reinforcement of timber structures.

The principles of design which are here given are alike for all of the materials to which the invention is applicable, the only changes necessary in the control formulas for the different materials being in the numerical values of the symbols employed. The metal reinforcements which characterize the present invention are usually applied near that face which would have been subject to tensile stress under design loading if the beam were without prestressed reinforcements. They may be located directly upon that face (hereinafter sometimes referred to as the tensile face) or along the sides of the beam or either within or without the body of the beam. That face of the beam which would have been subject to compressive stress under design-loadin if the beam were without prestressed reinforcements will be hereinafter sometimes referred to as the compressive face.

For simplicity of description of the invention the term beam will be used to include allstructural members subject to transverse bending, a slab being consideredmerely' as the union of a series of such beams'placed side by side. 'Also the reinforcements, which are a characteristic feature of the invention, will be consideredas being applied at a distance above the'tensile face of the beam, this distance being indicatedhereinafter by r d, where T represents a fractional number, usually small, which is positive when the reinforcementsare upward, zero when the reinforcements are on the face referred to, and negative when they are somewhat external and below theface. y

As above stated; the invention relates particularly to the application to beams and'other struc. tural members of metal reinforcements which have been proportioned, as; hereinafter imore 1 Claim. (01. 189-40) fully set forth, such application being made while the said reinforcements are held under a tensile stress of an intensity approximating the intensities shown in the control formulas given later.

It will be understood that the invention is independent of the mechanical means which are used to produce, within the metal reinforcements, the prescribed tensile stress specified to be therein prior to loading and that it is also independent of the mechanical character of the fastenings which may be installed to connect the reinforcements with the nucleus of the beam referred to. It will be apparent to anyone skilled in the art that the desired prestress in the metal reinforcements may be produced in a great variety of ways and that this prestress may be maintained in the reinforcements, While they are being secured to the body of the beam with which they are associated, by a great variety of expedients. It will also be apparent that there are many different types of mechanical fastenings that may be used to connect the reinforcements with the main body of the beam. Moreover, in most reinforced structures to which the invention is applicable it is usually possible to use the main body of the beam or other structure as strut to act as the necessary reaction to the tensile stress in the reinforcements while that stress is being built up, thereby producing a compressive resultant within the main body of the beam at the same time that the tension is being built up in the metal reinforcements associated with the beam. For convenience and simplicity in illustrating and describing the invention, it will first be explained in its application to the Workings of a beam composed of timber, either in a single piece or built up by lamination, when this beam is simply supported and has reinforcements installed under tension of an intensity which, shown more fully hereinafter, may be represented as at the depth r-d above the lower face. In this explanation it will be assumed that, at or near the ends of such a beam, substantial anchorages either temporary or permanent in character have been installed from which the reinforcements referred to may be stretched to the amount desired.

The-range of possible determinations as to the amount of reinforcement used in relation to-the size of the beam, its span length and the loading to be provided for is relatively wide. "Further.- more, there is a wide range of possibilities as to the intensity of the. prestress to be employed. Nevertheless, there is'either danger or waste in making an installation of a beam having prestressed reinforcements without proper consideration of the relationships hereinafter shown.

The theoretical formulas here given have been formulated with a view to economy of materials and to efliciency in resisting the effects of loading. It will be understood, therefore, that these formulas may be susceptible to considerable modification as, for example, when there is a desire for greater stiffness'or' when it is necessary to secure adaptation to the dimensions of the material available such, for example, as the size of the reinforcing bars available or the thickness of flat metal that may be obtainable. Furthermore, the physical characteristics of timber are not uniform throughout any given member and its modulus of elasticity is never exactly known and is believed to vary throughout the depth of the beam with changes in imposed stress and possibly somewhat differently in compression as compared to tension. In spite of this uncertainty, however, it has been found practicable, while stretching the steel and using the timber beam as a strut to furnish the necessary reaction, thereby to determine an ideal value for the modulus of the particular timber which will serve satisfactorily in the following formulas in place of the actual value. If. the modulus of elasticity for the timber be represented by ET and that of the reinforcements by Es the value of the ratio ES/ET is represented by m, i. e., m=Es/Er.

The operation of stretching the reinforcements to give them the desired prestress tends to produce in the timber associated with the reinforcements a compressive stress in the face nearest to the reinforcements and tensile stresses in the opposite face, thereby causing the timber to bow upwards somewhat away from the reinforcements. The resultant deformation is a uniform bending except as this may be modified by dead load, this bending being similar to the bending caused by negative moments applied at the two ends of the timber.

Although timber has been in general use structurally since prehistoric times and has proved to have great strength in relation to its weight and to be exceptionally long-lived when suitably protected or when treated with proper preservatives, nevertheless, as it occurs naturally and also as it is commercially produced, there is a great variation in the physical characteristics of different pieces. This has resulted in the adoption of standard working stresses far below those that might have been established had the physical characteristics of similar pieces of timber been more uniform.

In recent times an improved process of pressure gluing has been developed whereby laminated timber units are being built up of selected pieces and in some cases with the pieces which are of exceptional quality distributed throughout the body of the laminated whole where the strength requirements are severest.

At the present time it is considered practicable to produce structural units of laminated timber having safe working stresses materially in excess of the 1000 pounds per square inch now commonly specified in connectionwith the use of commercially obtainable lumber. Such laminated timber structural shapes are now being constructed successfully of lengths and cross sections much greater than obtainable previously in single sticks. It is believed, however, that prior to the present invention structural units of timber, either in its natural state or in laminated form, have not been strengthened efiiciently, systematically or economically by metal reinforcements as has been done with concrete beams.

In the development of such a system of reinforcement it is obviously necessary to adapt it to use in connection with timber of various grades and limiting strength. Formulas have been developed accordingly which use symbols to which special values are to be assigned in their individual application according to the physical characteristics of the timber to be employed. Moreover, the physical characteristics of the metals available for use as reinforcement vary widely. Any numerical values assigned herein, therefore, either for the strength of the lumber concerned or for the strength of the steel or other metal associated therewith as reinforcement, are to be considered as illustrative only and as in no way limiting the applicability of the control formulas hereinafter set forth.

In the followingv description the word beam" is used to indicate any structural member whose principal loading takes place transversely to its length in such a manner that we may analyze its effect without undue error by considering it as acting in a plane of loading so-called. Thus, for convenience there may be included under this one descriptive term both those mem-- bers which are more commonly referred to as beams and which have considerable depth in. relation to their width and also those other members, sometimes called planks, flooring, roofing, slabs, etc., which have greater width in relation to their depth, these latter, however, being considered in the following description with regard to their capacity for carrying transverse load, i. e., their capacity for beam action. It will be understood, however, that with suitable modification the invention is applicable also to struts, posts, columns, etc. that are or may be subject to eccentric loadingv and also to arches and to so-called rigid frames.

In order that the advantages of this invention may be more clearly shown, reference is here made to the strength characteristics of similar structural members of prior practice. In this connection reference is made to Applied Mechanics by Charles E. Fuller, S. B., and William A. Johnston, S. B., vol. II, Strength of Materials, published by John Wiley 8; Sons, Inc. of New York, pages 103 to 309;

The Pocket Companion24th edition-copyrighted by Carnegie Steel Co. in 1934, pages 127, 153;

The Douglas Fir Use Book, published by The West Coast Lumbermens Association, 364 Stuartv Bldg, Seattle, Wash.

The Glued Laminated Wooden Arch, by T. R. C. Wilson, Senior Engr., Forest Products Laboratory;

U. S. Department of Agriculture, Technical Bulletin No. 691.

In the prior practice, one of the factors limiting the use of timber for beams and other structural members has been the relatively low modulus of elasticity of the timber concerned. In many cases this has resulted in excessive deflection thereby limiting the load or the span length prior to reaching the limiting fiber strength in the faces of the beam concerned. Moreover, the

strength in. bending of such structural members made of timber has been small relatively to the size of the section as compared with steel.

One of the main purposes of the present invention is to eliminate the limiting factors which have heretofore prevented the more extensive use of timber beams and other structural members by substituting for the plain timber beam a composite structural unit of timber and metal so connected and fastened together as to form a substantially unitary structure that will have a much wider field of utility and which in addition will have new, useful and extraordinary characteristics.

A further important purpose of this invention is a novel process of producing a structural unit of the type to which the invention is directed and in the practice of which certain newly discovered relations between the elements of the structure are taken advantage of to facilitate the building up of a structural unit having the desired characteristics.

With the foregoing and other purposes in view, the invention aims to produce, and has succeeded in producing, a beam having numerous improved characteristics over timber beams heretofore employed, and among them, a much greater permissible length of span for a given section, loading, and unit stress in the extreme fibres, in respect to both deflection and resisting moment. An additional advantage is found in the fact that higher working stresses are permissible in the prestressed reinforcement than in like material without prestress, and this without sacrificing safety as to overload. c

In general, the invention resides in the conception that by introducing into a timber structural unit, such as a beam, a slab, etc., suitable metal elements under a suitable intensity of artificially induced tensile prestress and so positioning, these in the structural unit that the metal elements suply a major part or all of the tensile components of the resisting couple induced at the critical section by loading, anew and useful structural member of reinforced timber is provided, having advantages much like those of a beam with negative end restraints, but superior to such in that it is independent of its supports, except as to bearing, and in that the negative moment effect may he graduated throughout thelength of the beam rather than be of an unchanging intensity throughout said length as is the case when a negative moment is applied by external means at the ends of a similar beam. Although emphasis has been put in this statement upon providing reinforcement to supply a major part or all of the tensile components of the resisting couple, it will be understood that such a structural member may also be provided with metal reinforcements on that face which is subject to compression under loading in order to reduce the intensity of the compressive stress in the midsection under extreme loading, and/or of the tensile stress in the top fibres near the ends of the beam under zero or small loading. For simplicity, however, consideration of such reinforcement will be omitted from the initial discussion of the control formulas and only those will be set forth which have been derived for a beam having reinforcements on its tensile side only.

In reinforcing a timber beam in accordance with the present invention, steel or other metal reinforcements, in the suitable amount hereinafter more fully set forth, are installed at critical sections on or near that face of the beam at which the timber would otherwise be at tensile stress when loaded. These reinforcements are so installed andfastened to that face of the beam,

in the aforementioned suitable amount, while they are held under a tensile prestress of suitable intensity, such as is also more fully set forth hereinafter, this arrangement being made with a view to what may be called balanced design.

It is obvious that the maintenance of such a tensile stress in the reinforcements, while they are being secured to the timber element associated with them, indicates the presence of a suitable strut or anchorages to supply the necessary reaction to the tensile forces existing in the reinforcements referred to. Naturally, too, the timber elements associated with them are almost universally found to provide the most convenient, suitable and economical reaction to the tensile forces referred to. In practice, therefore, the intensity of prestress in the reinforcements is commonly induced by pulling or stretching them from anchorages either fixed to the timber elements associated with them, or temporarily externally applied.

It is convenient here, however, first to proceed by assuming a prestress in the reinforcements which has been created by stretching them from anchorages outside of and independent of the timber associated with the reinforcements while the timber is without stress and straight throughout. It is believed, however, that the directions here given are sufiicient to enable any competent designing engineer to take care of any modifications made necessary by possible changes in the manner of producing desired prestress in the reinforcements.

-..In order to prevent waste, and uneconomical,

- inefficient or dangerous design in the exploitation of the invention, there is given hereinafter a guide. setting forth the proper approach when preparing astructuralmember embodying or employing this invention. It will be understood. however, that no theory of design which is here set outas to the working of such a member is to be taken as in any way limiting the invention or obscuring the substantial concrete results that have been obtained in the practice of the invention. g

In order to arrive latbenefitssubstantially as great as practicable or as great as may be predetermined, it is necessary, in the practice of this invention, properly to proportion, at the critical section, both the amount of the reinforcements used and the intensity of a tensile prestress tobe induced in them. It will further be found that the amount of reinforcement at the other sections of the beam and/or the amount of prestress in them preferably tends to decrease progressively from the critical section referred to toward both ends of the beam.

The control formulas here given have been devised with the purpose of arriving approximately at the maximum possible resisting moment of a timber beam of given dimensions, having physical characteristics approximately as shown, and of doing this in such manner that both theex'treme fibers of the timber element and the metal reinforcements associated therewith shall simultaneously approach satisfactorily close to their permissible maximum working unit stress, that is, the timber in compression and the metal in tension,and that this shall happen while the metal reinforcements furnish all or nearly all the tensile components of the resisting couple referred to. Such an arrangement has been referred to above as constituting balanced design. It will be understood, however, that the invention includes also less efficient designs which have advantages more or less proportional to the degree of their approach to the ideal referred to above as constituting balanced. design.

Itwill' be apparent that for the best results mechanical means must be provided for producing the desired tensioning' of the reinforcements referred to and for bringing about their installation. in. the structural unit while they are held satisfactorily close to the desired intensity indicated in the following control formulas. Mechanical means which perform. these operations satisfactorily may include, for example, pulling bolts for stretching the reinforcements toward end anchorages, astrain gauge, Ames type dial or other adequate means for measuring the stretch of the reinforcements and permitting computation of the stress with respect to the modulus of the elasticity of the reinforcements concerned. Less satisfactory but usable mechanical. means for performing the aforementionedfunctions have M comprised leverages actuated by known weights. Furthermore, jacks are available for providing the stretch and some of these come with gauges which measure more or less satisfactorily the number of pounds of pressure applied;

The proportioning of the reinforcements, their tensioning and their installation do not, however, require accuracy impracticable of attainment in order to secure great advantages over the timber beams of prior practice. Within reasonable limits, readily obtainable, these advantages approximate those theoretically indicated.

In the designing of the novel timber beam of the present invention, certain assumptions. are

made which are somewhat similar to those made in connection with the beam theory, so-called, of prior practice. One of these assumptions is the soecalled straight line relationship between unit stress and unit deformation. Furthermore, the beam is considered as being straight, of uniform cross section, as lying in a horizontal position and loaded on its top surface while simply supported at each end. Obviously the formulas derived with these assumptions will require modification readily derived corresponding to conditions other than thosehere assumed.

In the novel beam of the present invention, however, there is found no fixed neutral axis passing longitudinally through the center of gravity of all transverse. sections as in the old type beam and, therefore, we are without this aid to the derivation. of stress diagrams and the strength coeficient of, the beam concerned at a given section.

There beingsno such neutral axis, a new mode of procedure must be provided with a new structural analysis, new stress diagrams and new formulas so properly associated with this new type of structural member and so properly controlling its production that the result is a structural member having new, useful, desirable. and extraordinary structural characteristics.

I In the following control formulas which have been derived for use in the design of the novel structural unit of the present invention, tension is considered positive and is indicated by positive numerical values. Compressive stress. is considered negative and. is indicated: by negative numerical values. 'Ili'e equations are so written as either to be true algebraically, with due regard to the signs above indicated, or, if not, a notation is made that the relation is nu merical only. All formulas and equations given are to be interpreted in conformity with the above.

Consistently with the foregoing, in those of the accompanying drawings, which show stress diagrams at the various beam sections, tensile stresses are indicated as extending to the right of a neutral axis O--Y and compressive stresses are shown as extending to the left of such axis.

In the: drawings:

Figure 1 shows an elevation of a composite structural member of the new type having metal reinforcements above the face subject to extension under contemplated loading, as assumed primarily in the accompanying specifications;

Figure 2 shows the stress diagram in the wooden portion of the beam only, at a section of the beam of Figure 1, .in the unloaded condition of'the beam and with extreme compression in the lower face, tension in. the upper face and a point G at which there is substantially unchanging compressive stress as loading increases up to the maximum contemplated;

Figures 3, 4 and 5-show optional stress diagrams that can be determined upon respectively to occur at the critical section of the beam of Figure 1 under contemplated loading.

Figure 6 shows an unloaded beam embodying the present invention with prestressed reinforcements which have been put under a design tensile stress by means of pulling bolts acting through. anchorage plates shown in the figure;

Figure '7 is a section on the line 7-: of Fi ure6;

Figure 8 shows another method of anchorage by means of bolts passing through. the body of the beam at such an angle as to assist in resisting shear stress near the end of the beam;

Figure 9 shows still another embodiment of the invention in which the connections between the anchorage in the top face of the beam and the reinforcements on: the bottom face comprise plates applied to the sides of the beam instead of bolts through thebeam;

Figure 10 illustrates the method of applying reinforcements, of the type shown in Figure 8', by reversing the beam top for bottom, deforming. the beam by loads contrariwise to the manner of its deflection-under loads in its normal position, and in excess thereof, then securing the reinforcements to the ends of the beam and putting them under tension, before the intermediate fastenings are permanently installed by the aid of the resilience of the timber when the reverse loading is removed;

Figure 11. illustrates how the reinforcements of a timber beam embodying the present invention pass from the top-to the bottom face in the case of acontinuous beam on supports or beam fixed at the ends;

Figure 12 is amodification of the embodiment of the invention shown in Figure 11- and illustrates how the haunch at the support may, if desired, be replaced by additional reinforce merits; i

Figure 13 shows another embodiment of the invention in which: there are reinforcements bothon the tensile face and on the compressive face of the beam;

Figure 14 illustrates a beam of the type shown in Figure 12 having bothtensile and compressive reinforcements as well as supplementary reinforcements at the point of support;

Figure 15 shows a section of a laminated timber beam which inaddition to the other reinforcements of the present invention may be provided with supplementary reinforcement consisting ofa wire winding transverse to the laminations and located either in the midsection of the beam where the bending is most severe or near each end where the shear stresses are higher or at all such points;

Figure '16 shows another form of supplementary shear reinforcement near the end of a beam embodying the present invention, these reinforcements being similar in their mode of action to the so-called vertical stirrups in reinforced concrete beams, but having their rods prestressed;

Figure 17 shows another method of prestress ing the reinforcements and securing them to the beam in its upside-down position;

Figure 18 shows the use of a removable an chorage plate for effecting the prestressing, and

Figure 19 shows a section of a two-hinged arch of laminated timber having prestressed metal reinforcements embodied therein in accordance with the teachings of the present invention.

Assumptions The assumptions which are made in connection with the derivation of the formulas hereinafter given which control the design and construction of the beams of this invention are, for the most part, those which are familiar to engineers in connection with well-established theories governing the design and construction of homogeneous beams.

Thus, to recapitulate in part, they include the following: The beam is straight and deflections are negligible insofar as they affect the derivations concerned.

It is acted upon by a balanced system of forces, external thereto, and applied at right angles to its central axis.

It will be considered as lying in a horizontal position, for convenience in reference, with the assumption that all derivations may be considered as applying in any other position, when properly modified.

The material of the timber component of the composite member will be considered as homogeneous with a constant modulus of elasticity throughout its body, unchanging asthe stressincreases.

It will be considered as completely elastic with full recovery from the effects of all loadings upon the removal of such loadings.

The beam is of uniform symmetrical cross section with respect to a vertical plane.

The loading is applied in equilibrium to the reactions at the end supports in such a manner as to cause no torsion, the loading being considered as though it were all acting in the vertical plane of symmetry, often called the plane of loading.

Whenever the term cross section is used it signifies a section perpendicular to the central axis of the beam.

The length of the beam is large in proportion to its sectional dimension.

The reinforcements are so firmly connected to the timber element associated with them that they may be considered as deforming together with the filament of the timber most closely associated with them, although of different unit stress, without slip or crawl beyond that negligible practically. V

The beam sections under loading are subject to uniformly varying stress, but without any fixed longitudinal axis of zero stress such as is found in homogeneous beams and therein called the neutral axis, from which the uniform variation of stress may be reckoned.

sociated with them the following notation is used:

b=width of the beam.

dzdepth of the timber element of the beam ir respective of the metal reinforcements.

r-d depth of the resultant stress of the reinforcements from the bottom face of the beam, r being a numerical multiplier, positive when the resultant referred to is above the bottom face and negative when it is. below the bottom face, with all formulas interpreted accordingly.

p a numerical multiplier such that p-b-d=the sectional area of the reinforcements referred m=Es/ET where Es represents the modulusof elasticity of the reinforcements and ET represents the modulus of elasticity of the timber concerned. l

fizthe unit horizontal stress component in the top fiber of the timber element of the beam, this component varying with the condition of loading.

.fo the unit horizontal stress component in the bottom filament or the timber element of the beam, this component varying with f1.

fe -the unithorizontal stress component in that fiber of .the timber which lies at a distance :r-d above the bottom face of thebeam.

:an ideal constant tensile stress in the reinforcements at each beam section, separately, (this stress being not necessarily identical throughout the length of the beam) which stress, if imposed on the reinforcements prior to their attachment to the timber element associated with them, would, upon such attachment and subsequent release from outside restraint, compresssuch timber while the reinforcement is losing tensile stress and is shortening in length, along with the shortening "of the timber filament adjacent to it produced by this compression, so that, upon the completion of these changes, the reinforcements and the timber elements associated therewith shall be in the same condition of stress as that which is brought about when the timber element is used to supply the reaction necessary for the stretching of the reinforcements, the beam thus acquiring compressive stress progressively and being meanwhile in an unloaded con dition.

This represents at the section considered, the loss in unit stress in the reinforcements from the'amount F referred to above, due to the production of stresses indicated by f1 and ft in the top and bottom filaments of the timber element,

respectively; whether these arise from the re'- silien'ce of the reinforcements upon their release from outside stress or from external loading combined with such resilience;

F2:the residual unit stress-at the section considered which-remains in l the reinforcements 'silience, to compress the timber element associated with them and because of their eccentric position they cause the timber element to "tend to bend upward.

Figure 3 shows a stress diagram of :a beam in which, at the critical section and under maximum contemplated loading a portion of the tensile component of the resisting couple is furnished by a part of those fibres of the nucleus that would furnish the entire tensile resultant of the unreinforced nucleus, with reinforcements providing the necessary additional tensile component required to provide balance to the increased compressive component above thatfound in the unreinforced nucleus. There is also a point G at which there .is unchanging compressive stress when loading varies up to the maximum contemplated.

Figure 4 shows a stress diagram that may be substantially realized, at a critical section of the beam of Figure 1, under maximum contemplated loading by suitable choice of the amount, position and intensity of stress of the reinforcements under zero loading, with a point G at which there is unchanging compressive stress as the loading varies and with the unit stress of the lower face substantially equal to zero.

Figure 5 shows a type of stress diagram that may be found in the beam of Figure .1, at .a critical section and under maximum contemplated loading, with compressive stresses extending completely through the body of the beam and with compressive stress in that face which is sub- J'ect to extension under loaded conditions of the unreinforced nucleus. Thereis also a point G at which there is unchanging compressive stress under all variations of loading.

These Figures .2, 3, 4 and '5 are not intended to indicate to any scale actual numerical values of stress intensity but only the character of stress that may be realized, each fig-ure being independent of each of the others.

The following formulas express the relations following immediately from the definitions given above and include a repetition of the expression for F1 in order that this may be numbered serially with the others.

F -p-b'd= b-d (the condition for equilibrium) f1+fo] IV 2p-F=- [1+2p.m.rl f1- [1+2p.m(1-r) lfo(combining 1,11 and III) Theoretically in beams of this character there is a longitudinal axis, sometimes called a gravity axis or a G-axis, extending longitudinally'through the center of gravity of all sections, if, in them, we consider the reinforcements as replaced by timber having a sectional area equal to m-p-b-d although of negligible thickness. On this axis the horizontal component of the compressive stress in the timber elements is theoretically constant and is here indicated by j v V g %(1+2p-r-m p A numerical mu'ltiphersuch that g-d equals the ordinate of the gravity axis, referred to above, above the bottom face of the beam.

' l-i-p-m Attention is called to the fact that, although at each section the formulas given above apply, yet in some respects it has been found advantageous to decrease theprod-uot p-F progressively from the critical section toward the points of zero moment. The numerical value of the symbols used are often, therefore, quite different at different beam sections. I

The matter of design is a relatively simple one and involves the choice of one of the stress diagrams 3', 4 and 5 as the desideratum to end up with when the beam is fully loaded. Usually. then, f1 equals the maximum permissible compressive stress. For good grades of timber such a value will be found to be between -1,000 p. s. i. and -2,000 p. s. i. The final value of the unit stress in the bottom fiber may be chosen as a small tensile stress, as shown in Figure 3, or zero, as shown in Figure 4, or small compressive stress, as shown in Figure 5.

The amount of reinforcement required will be determined from Formula III by substituting therein valuesof f1, f0 and F2 chosen to occur under maximum loading, indicated herein by f'i, fo and F's.

The value of the resisting moment is thengiven by the formula:

2 VII Zlrf= -g-[(2-3r)f +(l-SrJfQJ and the resisting moment considered. negative when the loading causes a positive bending moment.

The value of F, showing the-amount of prestress necessary in order to arrive at the chosen stress diagram under full loading using an outside strut to furnish the reaction so as not to cause any accompanying compression in the beam itself may be found by the aid of Formula IV, substituting therein the chosen values of f1 and fo, together with m, p and r as assumed or given or previously found. After such prestress F is so induced in the reinforcements they are attached to. the beam, freed from the outside strut referred to, and allowed to compress the same as herein described.

If on the contrary, as is usually the case, we wish to create the prestress in the reinforcements by using the timber to furnish the necessary reaction, then .it may be convenient, theoretically to consider the beam so supported that there is zero moment at that section where, under loaded conditions, a maximum bending moment will occur. v

The intensity of prestress then to be applied, compressing the timber simultaneously, is the amount F2 when the bending moment is zero rather than the value F previously ascertained from Formula .IV. The condition for zero moment is seen to he (24m f1+(.l3r) fo=u. By combining this condition with Formulas I, II, Ill and IV, we find that where A=1+4mp(1-3r+3r This then represents the intensity of stress which would remain in the reinforcements if F were imposed by outside means and then allowed to compress the timber until equilibrium were obtained at a section where M :0. Equally it is the amount of prestress in the reinforcements at the critical section that may be built up gradually, using the timber as the reaction, equilibrium being retained throughout the prestressing, while at that section M=0. This value of F2 may be indicated by Fp signifying the intensity of prestress required when the timber is used as a reaction and the beam is so supported that the external moment is zero at the section considered. Obviously when the beam is otherwise supported a corresponding value of prestress required can be determined using the timber as reaction.

Attention is again called to the fact that for the symbol 1' in the above formulas positive numerical values must be substituted when the reinforcements lie above the bottom face of the timher and negative when they are below it with 1:0 when the resultant tensile stress of the reinforcement is coincident with that bottom face.

As a numerical example We may assume that under design load Substituting these values in Formula III, the condition for equilibrium, we find Assuming further that A m=l3 and. r=-0.1

and then inserting these values in Formula IV, together with p, f'i and f'o as above, we find F=23,130 (approximate) M=405(b-d (i. e. K=405) For timber of this quality, without prestressed reinforcements, we ordinarily have K =16'7. This indicates that by such reinforcement the strength of the given timber in bending is multiplied by 2.42.

A computation has also been made showing the maximum intensity of shearing stress on a right section to be located at the gravity axis referred to above. Its value is found to be 2 b-d (l+p-m)A where A has the same value as given before and S equals the total external shearing force at that section considered.

.While this intensity is somewhat less than for the timber alone without reinforcements, the im provement in this regard is somewhat less favorable than that found for K the strength coeiiicient. Thus in beams of this invention it is 8716 p.s.i.

H iliary anchorages.

necessary to examine them with regard to shear intensity as well as for strength in bending.

It should be understood that the assumptions made in connection with the derivation of these. control formulas are approximately true only. By actual test, however, they have been found to result in the construction of beams which safely support the loads indicated, with deflections and unit stresses satisfactorily close to those theoretically to be expected, as indicated by the formulas above given. It shouldibe noted in this connection that by making simultaneous meas-. urements of deformation in the reinforcements and in the timber elements associated therewith while effecting the prestress inthe reinforcements (the timber being used for reaction), a very satisfactory estimate can be made of a suitable assumed value for m. I

The following detailed memoranda have been made for assistance in the actual production of satisfactory beams of this invention:

(a) For assistance in securing a satisfactory close estimate of the numerical value to be used for m, it has been found adequate to hold the reinforcements in place by clamps or otherwise, 31-.- lowing the reinforcements to slip longitudinally but preventing wide separation between the reinforcements and the timber as the timber tends to bow up. I V

(b) The maximum variation in temperature to be expected should be considered in connection with the maximum allowable working stress in.

the reinforcements. This allowance should be made with provision of smaller prestress intensities under higher temperatures and higher prestress intensities during temperatures lower than those to be expected after construction.

(0) The beam of this invention may require protection from extreme changes in moisture content. This may be accomplished by the use of water repellent impregnation under adequate pressure, a treatment usually ,possible with commercially obtainable preservatives. Other means that may be used include various types of superficial paint coats and protective coatings ofmetals, plastics, glues, thin sheets of resin-im pregnated veneers, or auxiliary metal. I

(d) It is sometimes desirable to apply preloading which will so act that the beam shall not bow up to an extent causing excessive tensile stress in the top fibres, and that, under extreme load, such preloading shall no longer act so as to add to the total bending moment imposed.

In the beams of this invention there are mechanical fastenings securing the reinforcements to the timber associated with them. While these may be integral with the reinforcements, either altogether or in part, it is convenient to consider them separately.

They may act in either one of two ways or in a combination of both. They may act as clamps forcing the two constituent parts into close contact or they may be fastened to the reinforcements on one side and to the timber associated therewith on the other side, much as partial aux-.

Usually, except when the reinforcements are entirely outside the body of the beam the fastenings are designed to'serve a dual purpose, acting as partial anchorages. at thepoint of fastening and at the same time forcing the two materials closely together so as to build up frictional resistance to slippage throughout the interval between fastenings. The load bearing capacity of fastenings is estimated longitudinallyof the grain with regard to longitudinal shear,

so called. The Fir Use Book previously referred to contains information in this regard, both as to the mechanical connection contemplated and as to the frictional resistance to longitudinal shear Where the timber and reinforcements are in contact. This latter may sometimes be increased by the application of suitable glues, cements or plastics that harden with tire on the surfaces in contact.

Obviously the more thoroughly the two materials are joined together the more nearly will the composite whole act as an integral structural member. Practically, however, it has been found adequate to install fastenings on each side of the critical section such that their combined load bearing capacity is approximately equal to the maximum tensile force to be found in the reinforcements. These are spaced at decreasing intervalsfrom the critical section toward thebearings, possibly so that their respective distances from the critical section vary approximately as the square roots of their serial numbers counted therefrom; or, more or less in accord with the requirement for resistance to increasing longitudinal shear.

The frictional resistance to slippage in those areas between fastenings may be included in the estimate of load bearing capacity of the mechanical fastenings referred to. The end anchorages may be omitted from the computation referred to and their load bearing capacity serve as providing a factor of safety so called. The number and locations of such fastenings having been determined upon, preferably equal reductions in the tensile force in the prestressed reinforcements are assigned to be made at each of them. Thus, beginning at the critical section the first fastenings on each side are installed while the full amount of design prestress and the design value of 10 are maintained. Thereafter, as each fastening is installed, the tensile force immediately preceding is serially decreased. either by releasing a portion of the pres-tress previously existing in the reinforcements or by decreasing the amount of reinforcement that continues beyond the fastening referred to, or by some convenient combination of these reductions.

Obviously, if control of the intensity of prestress exists only at one end of the beam the procedure indicated above should be modified so as to provide for building up the tensile force of the prestress from the opposite end, till the critical section is reached, and then reducing it as indicated from that point on.

If or when reinforcements are added near the face of the beam which is subject to compression under loading, a modification of the formulas already given must be made but is easily derived. In such case .glflzhe intensity of pres-tress in an y top reinforcements if or when it is induced while the fiber of the timber adjacent to it is under zero stress.

,q=the residual prestress remaining in the top reinforcements referred to after the loss indicated by and which is necessary to-complete the requirement for the equilibrium equation H :0.

p =a number such that p -b-d=the sectional of the top reinforcements referred to.

r numerical multiplier such that r -d=the distance below the top face of the beam of the stress resultant of the top reinforcements referred to.

In this connection it will be understood that positive numerical values should be substituted for 1-, when their reinforcements lie below the top face of the beam and negative values when they lie outside that face.

Assuming now reinforcements near the bottom face, as before, which are combined with top reinforcements, as indicated, the following relations are easily derived:

Throughout this specification understanding of theinventicn has been facilitated by referring to one specific type of reinforced structural member, namely the simple beam and slab. It will be apparent, however, that the advantages of this invention are adaptable to struts subject to eccentric loading by using prestressed reinforcements so placed and proportioned as to neutralize the bending effect to which such struts are otherwise subject.

In arch ribs advantages accrue also by so modi fying top and bottom reinforcements applied to the arch ribs that either positive bending or negative bending may occur in those sections where such bending is to be expected.

Referring now to the illustrative embodiments of the invention shown in Figures 6 to 19, inclusive, of the drawings, Figure 6 is a view of an unloaded beam embodying the present invention in which the reinforcements, such as metal strips or plates 2, are put under a design tensile stress before they are attached to the beam 4 by the fastening means 6, such, for example, as lag screws. The means shown for prestressing the reinforcement 2 before it is attached to the beam 4 comprises an anchorage plate 8 secured to one end of the beam 4 and a second anchorage plate l9 secured to the other end of the beam. The anchorage plate Ill, if desired, may be integral with the reinforcement 2 and the anchorage plate 8 may be angular in shape and have an overhanging portion l2 adapted to be secured to the top face of the beam 4. Pulling bolts M passing through plate l6 and the anchorage plate 8 serve to exert a pull on the reinforcement 2 to prestress the reinforcement. The plate It may, if desired, be integral with the reinforcement 2 or otherwise connected thereto. Nuts 1 8 bearing against washers 20 may be turned on the threads on the bolts M to exert pull on the reinforcement 2. It will be understood that all this prestressing is done before the fastenings, such as the lag screws 6, are inserted through holes drilled in the reinforcing metal strips 2 and screwed into the beam 4.

In Figure 8 is shown another method of providing anchorage for the pulling bolts. In this case the reinforcing strips 2 having integral with or connected thereto the plates [6 through which the-pulling bolts 14 pass, are shown as located on the under face of the beam 4 the same as in Figure 6. In this modified embodiment of the invention, however, an angle plate 22 is arranged to serve as the anchor plateagainst which the head of the bolt It bears when prestressing the strip 2. The angle plate 22 is anchored to the beam by means of bolts 24 passing obliquely through the beam and having heads bearing against that part of the angle plate Which rests against the beam and its threaded ends extending through another plate 26 which has an angle arm secured by a screw 28 to the end of the beam and an inclined abutment portion'30 against which the nuts 32 may be screwed to secure the anchor plate 22 firmly in position on the timber.

In the structural member shown in Figure 9 the anchorages are used to hold the prestress but do not cooperate with pulling bolts. In the embodiment shown in Figure 9 the prestressed metal reinforcements 2 are supplemented by a prestressed cover plate 34 and the anchorage plate 36 to which the prestressed strip 2 is directly connected is itself connected to an angle plate 38 on the top and end corner of the beam by a strap 40 extending over the side'face of the beam. Similarly the anchor plate 42 to which the cover plate 34 isdirectly connected isconnected to a top plate 44 on the top face of the beam by side straps 46.

In Figure a method of securing the prestressing of reinforcements such as shown in Figure 9 is illustrated. It will be seen that the structural member, the reinforcements of which are to be prestressed, is reversed top for bottom and the timber part 4 of the beam is deformed by loads W1 approximately contrariwise to the manner of its deflection under loads in its normal position. The reinforcements 2 and 34 are then secured to the beam at the-ends respectively, the reinforcements 2 being secured to anchor plates 48 directly at the ends and the reinforcement 34 being-secured by the bolts, 24 passing diagonally through the timber as in the form of the invention shown in Figure 8 and secured to anchor plates 50 on what will eventually be the top face of the beam. 1 The beam is then further depressed by loads whereby any tension in the reinforcements is decreased and'the reinforcements are then secured to, the timber by the fastenings 6 progressively fromthe ends toward the middle while the loads fare also progressively decreased; This methodresults in a graduatedprestress greatestat themidsection which is'where the greatest prestress is needed. Ob viously experience is required to graduate the prestress so as to bring it intoapproximation to the theoretically indicated unit stress at the midsection.

" In'Figure ll is sho'wn a beam thatextends over a series of supports-this figure illustrating the necessity for transferring the reinforcements from the bottom to the top face in the case ofa continuous beam or a beam fixed at the end with 1 a haunch at the support. Figure 12 shows how the haunch appearing at Figure 13 shows a structural unit having, prestressed reinforcements 2 and 34 on its tensile face and; also havingqa reinforcement 50v onits compressive face. These reinforcements are prestressed by any suitable means with anchorage for the reinforcements 2 at the beam ends for the reinforcements 34 at points between the beam ends and also foranchorage for the prestressed compressive reinforcements 50 between the beam ends.

In Figure 14 is shown a beam having prestressed reinforcements 2 on its tensile face, prestressed reinforcements 50 on its compressive 'face and also having supplementary reinforcements 52 and 54 at the points of support, these supplementary reinforcements serving substantially the same purpose as those shown in Figure 12.

In Figure 15 is shown a section of a laminated timber beam in which, in addition to the prestressed reinforcement not shown there is prefera'bly a' supplementary reinforcement consisting of a wire winding 56 transverse to the laminations. This supplementary reinforcement may be located either in the midsection of the beam where bending is most severe, or near the end where the shear stresses are higher, or wherever it appears desirable.

InFigure 16 the supplementary reinforcement is shown as comprising top and bottom straps 58 connected by rods 611 to form clamps, these reinforcements acting in a manner similar to that of the so-called vertical stirrups in a-reinforced concrete beam but more efliciently because of the prestress in the rods 60.

Figure 18 shows a slight modification of the structural member shown in Figure 6 and illustrates a method of securing prestress in the reinforcement by pulling from a removable anchor plate; In this modification of the form shown in Figure 6 the anchor plate 8 is temporarily secured to the end of the timber 4 by means of a lag screw 6 or other suitable means and the beam is placed up-side-down to aid convenient operation thereon, an angle plate I2 serving the same purpose as the part 12 of the anchor plate shown in Figure 6 helps hold the anchor 8 against the beam end. The anchor plate 8 in Figure 18 extends beyond the bottom face of the beam sufficiently' to receive the pulling bolts [4 Which pass through a stud or plate [6 that is integral with the reinforcement 2 and extends away from the bottom face of the timber 4. When the nut I8 is turned to exert pull on the reinforcement strip or plate 2 which is anchored at the other end of the timber 4 in the same manner as shown in Figure 6 there will first be applied a small tension to the reinforcement 2. Before further tension is applied the fastening or fastenings will be inserted'near the end remote from the anchor plate Band then fastenings will be inserted progressively as the tension caused by turning up the nut 18 is integrally increased. This progressive incre ment of tensioning is so graduated that it reaches its required maximum at the mid-point of the beam simultaneously with the insertion of the fastenings at the mid-point. The tension is then preferably gradually decreased as the fastenings are installedfr'om the mid-point of the beam toward the point-of pull. .After the fastenings have been installed the plates 8 and I2 and the pulling bolt 14 may all be removed. I

Figure 1'7 shows an arrangement'for'securing prestress in the reinforcements in a beam of the character of that shown in Figure 10 and operat ing on the principle illustrated diagrammatically in Figure 10. In this view the beam is shown as turned .top for bottom so that reinforcements which are, to be on the bottom face underloaded conditions are here shown as above the beam and in the position which they occupy prior to prestressing and preparatory thereto. In this drawing the ends of the beam are shown as resting on supports 62 and anchorages Ed are provided in the base 86 to which rods i8 threaded at their upper ends to receive nuts 79 are connected, the nuts bearing on cross straps '12 by which the timber part 4 of the beam may be deflected below the reinforcements 2 and 3d. Blocks 74 may be placed temporarily between the: timber 4 and the reinforcements 2 and 34 to support the reinforcements in substantially their straight condition until they are secured at the ends to the end plates 76, in turn secured to the ends of the timber 4.

After the reinforcements have been anchored the blocks 74 and the straps 72 are removed and the reinforcements are then drawn down and attached to the timber, starting first at the points 18 and working progressively toward the center and then starting at the ends and working also toward the center. It is probable that some graduation of the prestress will be obtained by the procedure just described. In other words, that it will be graduated so that it is greatest at the mid section, but there will not be with this procedure as close an approximation to the theoretically indicated unit stress at the mid section as can be obtained by the procedure illustrated in Fig. of the drawings.

Figure 19 shows a section of a two-hinged arch laminated timber having prestressed metal reinforcements 80 on the outer surface of the ring and also having prestressed reinforcements 82 on the intrados near the crown through the necessary area extending from point 84 to point 86. For convenience the terminal points 84 and 86 are shown as connected with the terminal points 88 and 9B of the outer reinforcements by bolts 92 passing through the arch ring, the bolts 92 thereby serving as anchorages for both exterior and interior reinforcements. It is obvious that in installing the interior reinforcements in a structural member of this type when the ends of the reinforcements are secured the intervening parts of the reinforcements tend to be not in immediate contact with the arch ring. In .pulling them down to secure this contact the tension is then obviously increased. Therefore, in this portion ofthe reinforcement the fastenings are first made at the ends 84 and 86 and progressively from there to the'crown of the arch where the maximum stresses are desired. The reinforcements on the extrados may be pulled over the curving surface by means similar to those shown in Figure 18 or the equivalent at the ends nearest the springing, and subsequently fastened and anchored.

It will be understood, of course, that in view of the, great variety of beams, arches and other structural units of the general types referred to to which the invention is applicable and in view of the widepossibilities of arranging and anchoring the reinforcements to meet the conditions of the present invention, that the embodiments shown in-Figures 6 to 18 inclusive are merely illustrative and that the invention has a wide application to other structural forms than those here specifically shown.

It is felt that the guide already given for prestressed reinforcements to a simply supported timber beam is sufi'icient to enable any competent engineer to provide satisfactory prestressed reinforcements for beam bridges already inplace, whether the beams "concerned are timber, concrete or steel. This, of course, involves the devis-- 20 ing of fastenings which are satisfactory in respect to the material concerned.

So, too, it is thought that herein maybe found a satisfactory guide to the. design of beams of concrete, aluminum bronze, magnesium and steel, etc, using prestressed reinforcements of a type that will not form an electrolytic couple with the material concerned. In particular in the reinforcement of steel I beams or T beams or similar shapes the use of high tensile steel as reinforcements is preferably indicated, prestressed and secured to the lower flange or stem while held undersuitable tensile stress.

In connection with the use of prestressed reinforcements for concrete beams, reference is hereby made to U. S. Patent No. 2,035,977. The physical characteristics obtained under the practice there outlined have been found very satisfactory under repeated tests.

The present invention, when applied to concrete beams, serves to provide a cheaper method of production than that outlined in the patent above referred to and to obviate the necessity for the provision, therein mentioned, of additional prestress to neutralize the effects of shrinkage and flow. Thus, when the procedure herein described is employed in contrast to that described in Patent No. 2,035,977, the concrete ispoured in place with provision for thereafter attaching, on the tensile face, the required reinforcements; also with provision for allowing the concrete matrix to set and harden and so to be used as a strut from which the reinforcements are to be stretched in place and thereafter attached to the body of the beam with intermediate fastenings spaced and handled in a manner closely resembling the practice in connection with timber beams already stated in more detail. I 7

Obviously, however, because of the difficulties of making insertions into the body of hardened concrete, it is preferable to include metal insets within the body of the concrete when poured, these insets being adapted to receive clamping devices whichserve to fasten the reinforcements to the tensile face of the concrete at points established inamanner-similar to that employed in connection with the timber beams above referred 'to, provision being likewise made for decreasing the tensile force in the reinforcements serially from the critical section toward the ends in a manner similar to that already outlined. for timberlbeams.

As in thecaseof timberbeams concrete beams with such .prestressed reinforcements may be bolted downslightly in front of the end bearings soas -to. prevent excessive upspring when loading is small, and this maybe done also .-in such .a way that thebolts referredto cease to act. whenconsiderable loading. .comes on and so do not limitinany manner the amount of load permitted. By limiting the upspring, however, they also limit the amount of stress variation in both top and bottom fibres due .to variable; loading and all changes in form of the beam.

So beams of all materials having such prestressed reinforcements maybe used on spans much longer relatively to'their .depth than other beams of similar material not. so reinforced with negligible change in profile due to great variations in loading. So thenstress intensities are such atall times as notto be favorable-to flow and under loading any such 'ilow tends to be opposite inv character to anyexpectable under dead-load only. -Withsuchreinforcements flow ceases to be any obstacle causing trouble in connection with design or causing any progressive deformation with time. It vmay be neglected with less resulting discrepancy than is the case in'connection with reinforced concrete of prior practice and in connection with prior practice its consideration has been found necessary only in isolated cases. forcement and accompanying preload it is considered entirely negligible. r

In order to clarify the advantage found in prestress relative to the permissible working stress, the following explanation is given. In the beam of this invention the rate of increase in the unit stress of the reinforcements ,per unit of load is given by the expression where F2 represents the unit stress in the reinforcements under design load; Fp represents the intensity of prestress in said reinforcements under zero loading, and W represents design load. Any additional overload -W'will accordingly cause additional unit stress r Fg-F,

above thedesign unit stress F2. In order that this may not exceed the yield point of the material, it is necessary that: I Y Ea= -P whereY-P represents the yield point referred to. Contrariwise, metal without prestress increases in unit stress from zero intensity up to design stress as loading comes on. Thus in this case the rate of increase per unit of load is given by where S represents the working stress and the W represents design loading. Without prestress then an overload C-W produces an increase in unit stress represented by 0-8 and a total stress equal to S(l+C).

Thus for the same fractional overload represented by 0-W the working stress of a metal without prestress must be such that it does not exceed in order that the stipulated overload may not cause a unit stress exceeding the yield point of the metal concerned.

With prestress, however, the condition is seen to be somewhat different. In this case we may have YP 0-F,

obviously exceeding the stress permitted without prestress by the amount The beam of this invention having considerable compressive intensity throughout its depth is more favorable to a rectangular cross section than beams without prestress. Other forms of section are premissible however and formulas for such are derivable from those hereinbefore given for rectangular sections and those well known formulas applying to homogenous structural members.

'With such prestressed rein-- Consider,.forexample, a beam of this invention not rectangular in section, referred to as beam B, of which the dimensions are known, the area A, the amount of reinforcement p-A, the intensity of prestress F. (prior toany'compression of .the compressive element), and the ordinate of the center of .gravity G of the transformed section,.so called, such transformed section being identical with the section of B referred to exceptthat the reinforcements are considered as replaced by .m times their sectional area of the same material as that making up the rest of the compressive element, so that this transformed section is: homogeneous and susceptible of analysis :according to well known laws. The depth of' the. section being denoted by d, the ordinate (above the lower face) of the resultant stress in the reinforcements isv denoted by r-d .and of G .by g-d. Without the reinforcements, .the center of gravity is indicated as at hcl and the moment of inertia as I.

. A'stress diagram at any section of such a beam B maybe considered as the result of addingthe abscissas at corresponding points .having identical ordinates of corresponding stress diagrams of two ideal'beams B and B"," of which B is identical with B when the unit stress through its entire depth is ',F F1 the intensity of stress in the reinforcements being 1 B being identical with B as to dimensions but with the transformed section above referred to forming a homogeneous beam of which the moment of inertia may be obtained, here indicated by I, and here being considered when the unit stress in the extreme top fibre has the value f1 being the unit stress in the corresponding fibre of B. The resisting moment of the beam B at its critical section M can then be written as the sum of the resisting moments M and M forcements when under zero or small loading, the longitudinal profileof the face most closely associated with the reinforcements is preferably convex toward the reinforcements to a degree such that, under all loadings, the reinforcements, following a like contour, tend to press against such face, due to the prestress in them rather than to pull away from it. 7

By the use of improved glues the number of mechanical fastenings required to secure the reiniorcements in the desired .tensioned relation to the structural shapecan be reduced .to limits .not yet fully determined, possibly to the complete elimination of such mechanical fastenings. The use of end anchorages is, however, considered desirable and is recommended, regardlessof the efiiciency of the best glues obtainable, even if their use .is only to provide an additional factor of safety.

Then, too, many of the glues best suited for substitution forifastenings in securing the reinforcements in their desired prestres'sed relation .to the structural shapes are thermosetting and are therefore applied hot and usually under pres; sure. When reinforcements are installed and held by hot glues, obviously the extension of the reinforcements will be in part or wholly accomplished by the heat of the glue and the mechanical stretching hereinabove referred to must,

therefore, be adjusted or omitted according to u ject to bending under design load and which comprises a structural shape of suitable structural material, constituting the nucleus of said memher and of substantially the dimensions thereof. in combination with metal reinforcements thereill for so intimately incorporated under stress in said composite structural member that said structural shape is under a resultant compressive stress in the unloaded condition of the composite member and that. the reinforcements are under a. resultant tensile stress and in which the reinforcements are fastened to the structural shape at intervals so progressively decreasing, substantially throughout the extent of the reinforcements, from a critical section toward the end anchorages, as to provide systematically for increasing shear as the anchorages are approached.

THOMAS F. NICHOLS.

REFERENCES CITED The following references are of record in the tile of this patent:

UNITED STATES PATENTS Number Name Date 133,306 Cutler Nov. 26, 1872 424,798 Lazell Apr. 1, 1890 1,042,808 MacManus Oct. 29, 1912 1,368,594 Aatila Feb. 15, 1921 1,770,932 Leake July 22, 1930 2,035,977 Nichols Mar. 31, 1936 2,039,398 Dye May 5, 1936 2,172,703 Freyssinet Sept. 12, 1939 2,380,953 Dubassofi Aug. 7, 194.5

FOREIGN PATENTS Number Country Date 5,344 Great Britain 1---- Mar. 11, 1899 393,576 Great Britain May 26, 1933

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