US3471881A

US3471881A - Method of constructing a reinforced suspension bridge by applying pre-stress

Info

Publication number: US3471881A
Application number: US598668A
Authority: US
Inventors: Tadao Kawada; Higashi Tonami-Gun
Original assignee: Kawada Industries Inc
Current assignee: Kawada Industries Inc
Priority date: 1966-12-02
Filing date: 1966-12-02
Publication date: 1969-10-14
Anticipated expiration: 1986-10-14

Description

Oct. 14, 1969 TADAO KAWADA 3. 3

METHOD OF CONSTRUCTING A REINFORCED SUSPENSION BRIDGE BY APPLYING FEE-STRESS Filed Dec. 2, 1966 .2 Sheets-Sheet 1 Fig.2. q 6666'6'6' 321123 9 .L J if Fl 9 .3 3 4 5 l I 5 3 a 2 1 1 2 5 I n U Q I 2 I t 117 'a '2 '1 1 2 [7 c Oct. 14, 1969 TADAQ KAWADA 3,471,881

METHOD OF cons'rauc'rme A REINFORCED SUSPENSION BRIDGE BY APPLYING PBS-STRESS Filed D80. 2, 1966 .2 Sheets-Sheet 3,471,881 METHOD OF CONSTRUCTING A REIN- FORCED SUSPENSION BRIDGE BY APPLYING PRE-STRESS Tadao Kawada, Fukunomachi, Higashi Tonami-gun, Japan, assignor to Kawada Kogyo Kabushiki Kaisha, Tonami-gun, Japan, a corporation of Japan Filed Dec. 2, 1966, Ser. No. 598,668 Int. Cl. E0111 11/00; E04b 5/18, 1/20 US. Cl. 14-18 2 Claims ABSTRACT OF THE DISCLOSURE In the building of a suspension bridge, the span of girder members is so constructed that the opposite ends thereof are first positioned a short distance above the shoes on the opposite banks to pre-stress the girders. The concrete floors is then placed on the span so that the opposite ends thereof then rest on the shoes so as to cancel the pre-stress.

This invention relates to a method of constructing a suspension bridge by applying pre-stress thereto. When constructing a suspension bridge minute care should be paid to fabricate concrete slabs, because it is usual to fabricate or assemble concrete slabs only after completion of constructing reinforcing girders and after fabrication of concrete girders a portion of the dead load of the girders is borne by the reinforcing girders themselves instead of cables of the suspension bridges. Such loading upon reinforcing girders results in the fiexure thereof and by this reason, bending moment, shearing force or forces of structure members which can be calculated by dividing these moment or force with the girder height will remain in the reinforcing girders as an asymmetrical stress after securing floor plates or beds. Such residual stress act as a compression stress for the upper member or the girder whereas as a tension stress for the lower member as more fully described later. Such stresses are of course undesirable when one considers that actual live load is superposed thereon. Residual of such remaining stress contravenes one of the assumptions of the elastic theory of a suspension bridge that all dead load is carried by cables so that no dead load stress is created in the reinforcing girder.

However, in prior calculation sheets the effect of such a residual stress was not generally considered.

It is, therefore, the principal object of this invention to eliminate such difliculties encountered in the prior method of constructing a suspension bridge thereby enabling the construction of most theoretical suspension bridges.

A further object of this invention is to readily construct a suspension bridge with a reinforcing girder free from any residual stress.

These and further objects can be attained according to this invention by providing a method of constructing a reinforced suspension bridge by applying a pre-stress which comprises the steps of pre-stressing a reinforcing girder by adjusting the length of the hanger means ac cording to the result of calculation by means of a turnbuckles and the equivalent means, and then fabricating a concrete slab so as to cancel said pre-stress thus assuring substantially no residual stress to remain in said reinforcing girder.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as this invention, it is believed that the invention will be better understood from the following description taken in connection with the accompanying drawings in which:

nited States Patent 0 "ice FIG. 1 is a side view of a reinforcing girder to explain stress acting upon the reinforcing girder after constructing a concrete slab;

FIGS. 2 to 4 inclusive are side views of a suspension bridge to explain various steps of constructing a reinforced suspension bridge in accordance with the invention; and

FIGS. 5 and 6 are side views to explain a prior method of constructing a suspension bridge.

Referring now to the accompanying drawings, FIG. 1 shows residual stress remaining in a reinforcing girder of a suspension bridge after it has been constructed or fabricated in the field. As shown by arrows, such a residual stress acts as a compression stress c for the upper member of the girder while as a tension stress t for the lower member.

As has been pointed out before, in the prior method of constructing and designing the bridge, due consideration has not been made for the effect of the residual stress created in the auxiliary girder at the time of constructing the concrete slab, but where the undesirable residual stress reaches an unexpectedly large value by some reason, for example by the elongation of the cable, or collapse of the supporting towers and the like, it has been proposed to provide a hinge 2 at the center of the bridge or between sections of reinforcing girders 1 and 1' as shown in FIG. 6. However, such means is effective only to locally eliminate difficulties mentioned above. This is because each of the divided half of the reinforcing

girders

1 and 1 accompanies said problem and because the span was reduced to one half of the original value. However, increase in the number of hinges decreases the rigidity of the bridge itself, which, of course, is undesirable from the standpoint of vibration and the like.

In view of such defects of prior method, the girders are pre-stressed to construct floor plate or bed so as to cancel the stress acting in the reverse direction and created after constructing the floor bed with said pre-stress so as to obtain an excellent suspension bridge having substantially no stress after constructing the floor bed. which has been long desired in the art The method of this invention comprises following three principal steps:

1) The reinforcing girder members are hung by means of hanger means prior to constructing the floor bed so as to impart a predetermined camber to the girder members and then these members are riveted together to bring the entire girder to a non-stressed condition.

(2) After completion of riveting to impart sufiicient rigidity to the girder, hanger members are adjusted to prestress the reinforcing girder by its own weight.

(3) When the floor bed concrete is constructed, said pre-stress is cancelled and the reinforcing girder becomes non-stressed condition.

Turning now to FIGS. 2 to 4, towers 3 are first erected on the opposite banks of a river, inlet and the like and cables 4 are passed over the top ends of the towers 3. A plurality of hanger members 5 are hung from cables 4; said hanger members, excepting the one at the center, being provided with turnbuckles 6 6 6 and 6' 6' and 6' so that their length may be adjustable. Then reinforcing girders 1 are suspended by the hanger members 5 and the respective members are uniformly riveted so that an ultimate camber will be given after constructing the floor bed concrete. Thus, the entire reinforcing girders will be suspended by the hanger members 5 in a non-stressed condition. Under this condition the opposite ends 7 of the reinforcing girders 1 should be displaced by predetermined gaps 9 which were determined by calulation from the shoes 8 on the abutments 8, as shown in FIG. 2.

After completing the installation of the reinforcing girders in this manner, turnbuckles 6 6 6 and 6' 6' and 6' included in respective hanger members are successively loosened starting from the center to the opposite ends of the bridge. Then each of the nodal points 1 1 1 and 1' 1' 1' will be lowered correspondingly according to the reinforcing girder itself until finally the opposite ends 7 of the girder will come to rest upon the shoes 8' on the abutment 8 and secured thereto, as shown in FIG. 3.

Consequently, as shown in FIG. 3 the upper member of the reinforcing girder will be subjected to a tension stress 1, whereas the lower member thereof to a compression stress. This means that the reinforcing girders are subjected to a pre-stress by the foregoing steps of this invention, and according to this invention the magnitude of the pre-stress is selected such that it can be cancelled by the sag due to the concrete slab which is to be constructed later after fabrication of the reinforcing girder.

After pre-stressing the reinforcing girders in the manner mentioned above, the concrete slab will then be constructed as shown in FIG. 4. Then, by the weight of the concrete slab, girders 1 will be stressed in the direction opposite to said pre-stress thus cancelling it. Accordingly, the camber of the reinforcing girders after constructing the concrete slab will be the same as the camber shown in FIG. 2, thus completing the construction of the bridge without remaining any internal stress.

The novel method of constructing a suspension bridge can provide a suspension bridge without any internal stress in reinforcing girders after completion of the construction. Thus, the novel method fully satisfies the fundamental assumption in the elastic theory of suspension bridges that all dead loads are born by the cable and that the reinforcing girders will not be subjected to any dead load stress. Moreover, filed works can be performed relatively readily without necessitating any complicated steps. More particularly, the required quantity of prestress can be imparted to the reinforcing girders by availing the weight thereof and only by manipulating turnbuckles included in hangers. Thus, with design calculations and tests of wire ropes which are to be made in the factory, the novel method enables to construct ideal suspension bridges without necessitating any troublesome field works.

Although in the foregoing embodiment pre-stress was given to the reinforcing girders by first hanging these girders with their opposite ends positioned at positions shortly above abutments and then by elongating hangers to cause girders to lower due to their own weight. Alternatively, reinforcing girders may be lowered without any gap between their opposite ends and the abutments and then the central portion of the girder may be elevated by the adjustment of hangers to pre-stress the girders.

As a matter of practice, however, it is more convenient to lower the girders by using their weight rather than by decreasing the length of hangers.

In accordance with the provisions of the patent statutes, I have explained the principle and operation of my invention and have illustrated and described What I consider to represent the best embodiment thereof. However, I desire to have it understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

What is claimed is:

1. A method of constructing a stiffened suspension bridge comprising the steps of suspending a stiffening girder span above end supports in a non-stressed condition from cables passing over supporting towers through a plurality of hanger members, successively increasing the length of said hanger members starting from the center of said girder to the opposite ends thereof so as to impart a predetermined camber to said stiffening girder span by its own weight with the ends of said girder resting on said end supports, then constructing a concrete slab on said girder span whereby to cancel said camber thus preventing any residual stress remaining in said girder span.

2. A method of constructing a stiffened suspension bridge comprising the steps of suspending a stiffening girder span in a non-stressed condition from cables passing over supporting towers by means of a number of longitudinally spaced apart hanger means such that the opposite ends of said girder span rest upon shoes of abutments, elevating the central portion of said girder span by adjusting the length of said hanger means, so as to ive a predetermined pre-stress to said girder, and constructing a concrete slab on said girder span to cancel said pre stress in said stiffening girder.

References Cited UNITED STATES PATENTS 965,358 7/1910 Aylett. 2,096,921 10/1937 Sahlberg 52-334 2,380,183 7/1945 Maney 14-24 XR 2,642,598 6/1953 Beretta 14-18 JACOB L. NACKENOFF, Primary Examiner US. Cl. X.R. 52-334