US2642598A - Rigid tension-truss bridge - Google Patents

Description

June 23, 1953 J. W. BERETTA RIGID TENSION-TRUSS BRIDGE Filed Dec. 9, 1946 4 Sheets-Sheet l @Hou/unda June 23, 1953 v J, w BERETTA 2,642,598

RIGID TENSION-TRUSS BRIDGE v Filed Deo. 9, 1946 4 Sheetsshaet :e

attenua@ J. W. BERETTA RIGID TENSION-TRUSS BRIDGE June 23, 1953 4 Sheets-Sheet 3 Filed Deo. 9, 1946 June 23, 1953 J. w. BERETTA 2,642,598

RIGID TENSION-muss BRIDGE Filed Dec. 9, 1946 4 Sheets-Sheet 4 Patented June 23, 1953 JohnW. Beretta, SanAntonio, Tex. Application December'9, 1946, Serial No. 715,119

3 Claims. (Cl.V 14- 18).

(Granted under Title 35, U'QS; Code (1952),

The invention described herein may be manufactured andV used by or for the Government for governmental purposes, Without payment to me of any royalty thereon.

This invention relates to trusses an more particularly to bridge trusses.

An object of this invention is to provide a truss bridge having the necessary qualities o :fA stability, rigidity and ycarrying capacity, both vertically and laterally, in which the main structural members,

other than the end members, are capable of taking tension only. The utilization of members capable of taking tension only, such as cables and rods, makes it possible to break down the component parts of the bridge into relatively small and light unitswhich can be transported to comparatively inaccessible spots over poor roads such as are found, for example, in the mountains, and there assembled into a long-span, high-capacity bridge. K This type of construction is also highly advantageous in more approachable construction areas Where access is had to the bridge site over relatively goodroads, in that it makes possible 'a much lighter'bridge structure than the comsion to an amount such that the tensional stresses are never reversed by compressive stresses set up by live loads on any portion of the bridge or even by the vcombined dead loads and livev loadsdue to Wind, impact and temperature stresses. The top and bottom chords are connected by members which are also capable of taking tensional vstresses only, these members being arranged to form, in conjunction Withthe top and bottom chords, a series of triangles.

The procedure, therefore, in design, Will be assumptions of dead and live load conditions and determination of preliminary stresses and sections. On the basis of these determinations, static lanalysis under dead loadand elastic analysis under live load can be made. Stresses and sections then can be corrected, and on the basis of this correction, the stressing of the top and bottom chord members can be determined', taking into account the elastic deformation of the members under thevarious conditions of loading to produce maximum stresses.

Upon determining k these maximum stresses and elastic deformations,

the I'amounts of prestressing necessaryto prevent reversal of stress can be computed;

Another object of the invention is to provide Va truss bridge including a floor consisting of transverse structural members, longitudinal strucvture members and means for xedly connecting certain endsrof the longitudinal structural members to the transverse structural membersand hingedly connecting certain of the other endswith the transverse members, whereby the longitudinal structural members may be articulated on the transverse members, thus providing a series of nodes in each panel which dampen lout any tendency for the .bridge to have vibrations of Wide am"- pltude in a vertical plane, over a largev portion nof its length. Another object of the 'invention is toprovide a multi-span truss bridge composed of vtrusses having top chords, bottom chords and web members 4all being capable of taking tension only."

In a conventional suspension bridge there are three essential main `elements Which are the cables, towers and anchorages and various auxiliary members comprising Suspenders, noor. beams, stringers, and lateral system. There are, also, the stiiening trusses or girders for distributing the concentrated live loads in such manner that these live loads are applied to the main cables by all vsuspendersin substantially equal amounts, so that the normal parabolic curve of the main 'cables v'will not be changed materially under live load. Thelstiii'ening truss carries no deadload as it' is supported from the main cables. Furthermore, it carries only substantially twenty per `-cent -of thelive loadgthe balance, approximately "eightyper' cent of the live load, is actually carried by themaincables. The stiiiening truss on fthe conventional suspension bridge is therefore a -very uneconomic member considering thejworkl `1t actually performs, as, for example, for twolane 'suspension bridges of military design, ranging in lengths from one hundred 'andnfty feet to six hundred feetof fty ton capacity, the Stifffening truss Weighs approximately fty ,perA cent :of the total vstructural steel work inthe bridges exclusive of main towers. lThe top chords',4 ,bottom chords and the diagonal members of the stiffening truss must be designed to take either tension or compression, and the permissible tension stress must be. reduced to provide for `the proper slenderness ratio of the compression unit stress ".formula.` In the stiieningtruss whileone chord memberis taking compression stressesthe other chord Will take tension stresses and the bottom chords act also as the Wind chord of the hori- Vextending between the towers.

creased to provide for these additional stresses. 'Ihe lateral members are generally made of ordinary structural steel. In the main cables of all suspension bridges, and in inverted or storm cables where they are utilized, the horizontal components of the stresses in such members are equal throughout their lengths. Therefore, it can be seen that if conventional stiffening trusses on suspension bridges could be dispensed with by suitable means that would simultaneously guarantee a level roadway without objectionable wave action under loads and without distortion of-'the f parabolic curve of the cable, asaving;.particu1 arly in weight, could be had. y

It is, therefore, another aim and purpose'of this invention to provide a rigid tension-truss bridge in which the conventional stiffening truss is eliminated thereby making possible a large saving in the material required for the construction of the bridge.

With the above and other objects and advantages in view the invention consists of certain features of construction and operation of parts which ,hereinafter will be described and shown in the accompanying drawings, in which:

. Fig. 1 is a diagrammatic side view of a truss bridge embodying the novel Websystem of .the instant invention;

Fig. 2 is la diagrammatic top plan View thereof; Fig. 3 is a diagrammatic side view of the truss ,bridge illustrated in Figs. 1 and 2 showing a different arrangement of certain of the diagonal fmembers of the trusses;

parts broken away and in section;

Fig. 8 is a fragmentary sectional view thereof taken on line VIII-VIII of Fig..7;

Fig. 9 is a diagrammatic view of another modified form of truss bridge embodying three spans;

Fig. 10 is a .diagrammatic top'view thereof;

and

Fig. 11 is a diagrammatic view showing a modivfled arrangement of the diagonal-membersI of the trusses in Fig. 10. Y

Referring more specifically to the.v drawings, there is diagrammatically illustrated in Figs. 1 and 2 one form of the improved rigid tensiontruss bridge constructed in accordance with my invention which includes spaced pairs of end ,tower columns I0 mounted on piers I I', with the trusses indicated generally by A connected to and The truss A is composed entirely of tension members of steel wire ropes, cables or the like, including upper V'and lower chords prestressed in tension. The

other structural or supporting units, such as the flooring of the bridge, may be composed of structural units capable of taking tension, compression, flexure, or a combination Vof, such stresses.

The tension members in the truss are arranged in a series of triangles and include top tension chords or cables Il, bottom tension Ychords or cables I2, diagonal intersecting flexible members yI3 and vertical flexible members I4. yThe top 4 chords II are passed over'the towers I0 and extensions thereof are anchored in abutments VI6 in a conventional manner. Tension is applied to the top chords II by turnbuckles I8', which ,are similar to turnbuckles I8, or other functionallysimilar tensioning means which are preferably connected to the extensions I5. The amount of tension applied to the top chords is such that it is greater than any compressive stresseswhich maybe transmitted by the members I3, I4.

The bottom tension chords I2 extend along the Y outer side of the outside lines of stringers or vlongitudinal members I'I of the floor system,

more particularly indicated in Fig. 7, and are connected at their ends to the bottom of the `towers I0 through turnbuckles I8 and shackles I9 engaging pins 20 on brackets 2| mounted on V the tower cross-beams I I'I, so that the tension stress to which the bottom chords I2 are lsubjected isV transmitted as an axial compression Yload to the lines of stringers I'I and I1. Instead of connecting the ends of the bottom chords I2 to thetower cross-beams lrrthey may be connected to the end stringersof the outside line of stringers Il and the bottom chords maybe connected to the transverse iloor beams 22`by clamps 23 or the like.

The diagonal and vertical flexible members I3 and I4 respectively, are. connected at their ends4 at spaced points 24 on the upper tension chords I I and are connected yat their lower ends to plates 25 connected to brackets 26 mounted on the ends of the floor beams 22 as more particularly illustrated in Figs. 4, and 6 to 8, inclusive. The diagonal members I3 serve to distribute any local loads to several points 24 along the top chords II instead of having these local loads taken to the top chords by the vertical members I4.

At the panel points 21, one end of each of the stringers I'I and II of the floor system is attached at 28 to the transverse floor beams 22 by a rigid connection, while one end of each of the other ends of the stringers are hingedly connected to the floor beams by pins 29, so that the Y local deflections at any panel point are taken care of by the hinge connection. The hinge pins 29 are located below the neutral axis of the stringers I1 and I1' so that compression in the stringers produces an upward bending of the entire floor system. This construction takes care of the panel point deflections under the passage of loads and provides articulation in each panel, with the articulation providing a series of nodes i in each panel and damping out any tendency l.for

the bridge to have vibrations of wide amplitude in a vertical plane over a large portion of its length.

`In order to provide forr the requisite rigidity tol withstand horizontal wind and other lateral forces, the floor system is provided with lateral bracing 30 and horizontal wind cables 3| Ymade of steel wire ropes orY strands, Figs. 2 and A slightly different arrangement of the.. diagonal members is vshown in Fig. 3 from .that shown in Fig. 1. IIn this arrangement certain diagonal members 32 extend over more than one panel near the towers. I0 in order to secure the requisite rigidity to transport vehicles andthe like satisfactorily, with the diagonal members I3 in the middleY of the bridge remainingthe same as in Figs. 1 and 2. Other arrangements -of the diagonal members would also provide. the

same results. l

Bridges embodying this inventionV may take other forms than that disclosed in Figs.4 1, tof3.

inclusive, which show trusses in parallel over a single span. In Figs. 9 and 10 there is shown a three span bridge consisting of a center span indicated generally by M having top and bottom chords 33 and 34, respectively, connected to and between main tower columns 35, a web system including diagonal exible members 36 and vertical flexible members 31 and end spans indicated generally by S. Each of the end spans S includes top and bottom chords 38 and 39, respectively, connected to and extending between the tower columns 35 and end columns 40, a web system including diagonal flexible members 4l and vertical flexible members 42, with anchored backstays 43 for stressing the main span top chords 33 and span top chords 38 in Ytension to such an amount that this tension is never entirely neutralized by the compression stresses transmitted to it by members 36, 4i as a result of live load, impact, wind and temperature stresses. The top and bottom chords may be continuous in all the spans, Whereas the web system may be separate for each span. In order to provide for the requisite rigidity to withstand horizontal wind and other lateral forces, the floor system is provided with a center span, diagonal bracing 44, end span diagonal bracing 45, center and end span wind cables 46 and 41, respectively, center span transverse iloor beams 48, and end span transverse floor beams 49.

In Fig. 11 a slightly different arrangement of the diagonal members is disclosed from that disclosed in Fig. 9. In this arrangement certain diagonal members 50 of the center span M', extend over more than one panel near the tower columns 35 both in the center span M and end span S in order to secure the requisite rigidity to transport vehicles and the like satisfactorily, with certain of the diagonal members 36 and other structural units remaining the same as in Fig. 9.

It will thus be seen that there has herein been provided a new, novel and efficient form of truss bridge having a similarity in appearance to suspension bridges, but which acts as a true truss throughout its length and under normal loads. The main chords act as the top chords and the inverted curve bottom chords become the bottom chords and under abnormal loads such as aerodynamic uplift the action reverses with the vertical and diagonal members of the bridge being designed to take tension stresses only; merely going in and out of action under various combinations of loading with one diagonal acting in any one panel at a time.

Even though the basic principle of the instant invention is to provide a more economic structure for truss bridges over rivers and the like and having certain features of construction and operation of parts, it is nevertheless to be understoodv that the structure may be used for other purposes such as long span trusses in buildings and that various changes may be made therein, if the changes do not depart from the spirit or scope of the claims.

Having thus described my invention, what I claim as new and wish to secure by Letters Patent is:

l. A truss bridge comprising tensioned upper cables anchored at the ends and supported intermediate their anchored ends on spaced towers to form an upper chord, a cross-beam mounted on each tower below the upper cables,` an articulated floor system extending between said tower crossbeams, said floor system being attached tosaid tower cross-beams and including a plurality of pivotally connected sections, ilexible vertical suspenders secured to the floor system at the points of pivotal connection therein and to the upper chord at spaced points, tensioned lower cables secured at their ends to said tower cross-beams and intermediate theirend-s to the floor system at the points of pivotal connection therein to prestress said floor system in compression, and flexible diagonal members connected between said spaced'points and said points of pivotal connection to form a truss structure.

2. A truss bridge comprising parallel tensioned upper cables anchored at their ends and supported intermediate their anchored ends on spaced ltowers to form an upper chord, flexible suspenders secured to each upper cable at' corresponding opposite spaced points and depending vertically therefrom, said towers each including a transverse tower cross-beam, a floor system attached to and extending between the towercrossbeams and including a plurality of interconnected longitudinal stringers and spaced transverse beams, said spaced beams being connected to said suspenders to determine panel points, tensioned lower cables connected at their ends to the tower cross-beams and supported intermediate their ends by said spaced beams to prestress said stringers in compression, and intersecting flexible diagonal members connected between said panel points and the spaced points on `said upper cables to complete a truss structure.

3. A truss bridge comprising parallel tensioned upper cables anchored at their ends and supported intermediate their ends on spaced towers to form an upper chord, flexible suspenderssecured to each upper cable at corresponding opposite spaced points and depending vertically therefrom, said towers each including a pair of spaced vertical tower columns and a transverse tower v cross-beam interconnecting the columns of each tower, a floor system attached to and extending between the tower cross-beams and including a plurality of floor panel sections, said hoor panel sections each including parallel'stringers and a transverse floor beam rigidly connected at one end of' said parallel stringers, the opposite ends of said stringers being pivotally connected to the transverse floor beam of the adjacent panel, said transverse floor beams being connected to said Suspenders, tensioned lower cables connected at their ends to the tower cross-beams and supported intermediate their ends by said transverse floor beams to prestress said stringers in compression, and intersecting iiexible diagonal meinbers connected between said transverse floor beams and the said opposite spaced points on the upper cables to complete a truss structure.

JOHN W. BERETTA.

References Citedl in the file 0f this patent UNITED STATES PATENTS OTHER REFERENCES Engineering News-Record, April 24, 1941, pages 64 and e5.

Engineering News-Record, March 7, 1946, pages 91-94.

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