What Is an Arch Truss?

Truss arch bridge refers to an arch bridge with solid webs in the middle and arched truss plates on both sides. The truss arches are connected as a whole by a bridge deck system and a horizontal connection system (transverse brace, scissors brace). The characteristic is that the solid web section and the arched truss plates on both sides play the role of arch force, and the arch feet have horizontal thrust to reduce the mid-span bending moment; this bridge has a reasonable force than the ribbed arch bridge with general arch construction. Save material, reduce self-weight, suitable for occasions with poor foundation. ^[1]

Truss arch bridge is a combination of two structural systems, arch and truss. Each member of the truss part mainly bears axial force, which has the characteristics of the force of ordinary truss. The solid web section has the characteristics of arch force: the horizontal thrust of the arch reduces the mid-span bending moment, so that the mid-span real web section under axial load mainly bears axial pressure; under live load, it will bear the bending moment and become eccentric. Pressure member. The truss arch integrates the favorable factors of the truss and the arch. It mainly supports the axial force. It can be built with masonry materials and can give full play to the characteristics of masonry materials. At the same time, the structure on the arch and the ribs have formed a truss, which can give full play to the role of the full-section material. Compared with the same-span beam bridge, it saves more steel and the amount of masonry is close to the beam bridge, but less than the same-span arch bridge. In addition, since the two-hinged structure is usually used on the outside of the truss arch, the additional internal force generated by the foundation displacement and temperature changes is small. Therefore, the truss arch has the characteristics of reasonable structural force, strong integrity, material saving, and low self-weight. It has good adaptability to soft ground.

Most of the components of the arch bridge are prefabricated and installed. At the same time, the construction process is small, and the adaptability to lifting capacity is strong. The prefabrication of truss arch members can be operated in parallel with the construction of the substructure, and the construction period can be shortened accordingly. In addition, because the truss nodes are rigidly connected, the vertical and oblique rods meeting at the nodes are prone to crack, which affects the overall rigidity and durability and is difficult to maintain and maintain. Therefore, the application range of truss arch bridge is suitable for a medium span of 20-50m. However, if a prestressed truss arch is used, the problem of cracking of the tension member can be overcome, and the span can be increased. ^[2]

According to its structure, truss arches can be divided into four types: oblique (web) bar type, vertical (web) bar type, truss rib arch type and combined type.

(A) oblique (belly) pole type

Inclined-bar truss arch bridges can be divided into four types: triangular, triangular with vertical bars, oblique compression bars and cable stayed bars. The total length of the triangular web is the shortest, so the web uses less material and the overall stiffness is greater. However, when the arch span is large and the internodes are too long, the upper chord rods need more steel bars to support the local bending moment. Therefore, vertical bars should be set to reduce the internode length and become a triangular truss arch with vertical bars. Depending on the tilt direction of the inclined rod, there are two types of inclined pressing rod and inclined rod. The oblique rod type oblique rod is compressed, the vertical rod is pulled, and the length of the inclined rod varies with

The construction of truss arch bridges is mainly the prefabrication and installation of components.

(1) Prefabrication of components

The prefabricated members of truss sections of truss arches are generally prefabricated horizontally, and the components of solid webs are prefabricated vertically. Therefore, after the truss section members are lifted from the prefabricated base and pits are installed, the prefabricated components need to be "turned over" at a certain stage , That is, from the horizontal state to the vertical state, the basic steps are: lift the truss section member horizontally off the ground brake the lower chord sling tighten the upper chord sling, or brake the upper chord sling slowly relax the lower chord The pole sling can realize the turning of the component in the air.

(Two) installation of truss arch

The truss arch bridge lifting construction process includes: lifting the prefabricated section members of the truss arches to the bridge holes, closing them in place, and handling joints; installing transverse connectors to integrate the truss arches into one piece; laying prefabricated micro-curved plates or bridge decks; installation Sidewalk cantilever beams and sidewalk boards.

The installation work of truss arches is divided into: with bracket installation and without bracket installation. The former is suitable for bridges with small spans and relatively flat riverbeds and shallow bridges when installed; the latter is suitable for bridges that span deep water and valleys or multi-span and long-span bridges.

1. With bracket installation

With bracket installation method, a temporary rack must be set under the bridge hole. The position of the shelf is determined according to the joint position of the truss arch. Each rack is generally a double rack, in order to support the two adjacent ends of two connected members respectively, and the joint concrete is poured or the joint steel plates are welded on it. During installation and construction, after transporting the first side section truss prefabricated component to the bridge hole, it is hoisted and installed by a floating crane or a gantry crane, etc., and the truss section is temporarily fixed with diagonal braces; the remaining side section truss members are hoisted and installed, and cross braces The front piece is temporarily connected; the installation span is in the middle of the solid web; the truss arches are longitudinally connected with the joints of the horizontally-connected ties; when the joint concrete meets the strength requirements, the temporary horizontal braces are removed for unloading.

2. Without bracket installation

Installation without brackets refers to the installation process of joints and closures in the prefabricated section of truss arches when suspended by a crane. Commonly used are: tower inclined cable installation, multi-machine installation, cable crane installation, cantilever assembly and arch rib installation.

The installation method of the inclined cable of the tower is to set a tower on the top of the pier, and after the side section of the truss arch is lifted, it is stabilized by the diagonal cable (also known as the buckle) and the wind cable and then installed in the middle section. Generally, the oblique cable is loosened after closing the dragon, and then the tower is moved to install the next piece.

For multi-machine installation, each prefabricated section of a truss arch is hoisted by one crane, and it is put together in place. After the joint is completed, the crane loosens and leaves.

Arch rib installation method, that is, the truss arch is divided into the lower chord member and some triangular members for prefabrication; during the installation, the lower chord member is first made into an arch according to the installation method of the rib arch bridge, and then the triangular member is installed on it. ^[3]