The railway bridge over the Karacum river

13.07.2023

Author of the design model: Design Bureau of Gorodetsky

Location:  Turkmenistan

Full name of the project

RAILROAD BRIDGE AT THE CROSSING OF THE CONNECTING CHANNEL ''-3 OF THE RAILWAY LINE "ATAMURAT-IMAMNAZAR", 256 METERS LONG, AND RAILROAD BRIDGE OVER THE RIVER KARAKUM, 363 METERS LONG

The tools involved

LIRA-FEM - VISOR-SAPR; CS-SAPR (for creating stiffnesses of non-standard sections); dynamics module (39) SNT 2.01.08-99; requirements of section 3, SNT 2.01.08 - 2001 are realized via response-spectrum (41); DCL analysis; analysis of loads on a fragment; adaptation of steel structures design mode (SNiP II-23-81*) to requirements of SNiP 2.05.03-84.

Design solution

The span of the railroad bridge is 110 m, with a ride below with welded elements of closed section and mounting connections on high-strength bolts. The main trusses of the span are 15 m high. The distance between the axes of the main trusses across the bridge is 5.8 m. The main trusses include upper and lower girders, as well as braces of box section with perforation of one wall, struts and hangers with I-beam section. The nominal length of the panel is 11 m. The upper longitudinal connections are cross system with a panel 5.5 m, the elements of the lower connections are T-section to ensure the inclusion of the roadway in joint operation with the main trusses. Portal and transverse connections are located in the plane of portal braces and struts. The roadway includes longitudinal and transverse beams of I-section.

Description of the design scheme

The analysis is based on the finite element method with the use of displacements and rotations of the nodes of the design scheme as the main unknowns. Therefore, the idealization of the structure is made in a form adapted to the use of this method, namely, the system is represented as a set of bar elements corresponding to the structural solution of the bridge span. The selected type of finite element is determined by its geometric shape, the rules defining the relationship between the displacements of the finite element nodes and the nodes of the system, the physical law defining the relationship between internal forces and internal displacements, and the set of parameters (stiffnesses) included in the description of this law, etc.

The design scheme is defined as a system with a general feature whose deformations and its basic unknowns are represented by linear displacements of nodes along the X, Y, Z axes and rotations around these axes.

The analysis of the spatial truss was performed, taking into account the joint operation with the supporting parts. The design model contains 2,622 nodes and 2,717 elements. The selection of the finite element step is conditioned by the peculiarity of the structural solution of the main trusses, namely the perforation step.

The description of the stress state of bar elements is related to the local coordinate system, in which the X1 axis is oriented along the bar, and the Y1 and Z1 axes are oriented along the principal axes of inertia of the cross section. Some bars are connected to the nodes through offsets in bars, by means of which the eccentricities of the node connections are taken into account. All nodes of the main trusses are represented by a spatial rigid body.

The choice of hinged joint of bar elements made of single profiles is conditioned by the design of joints.

The boundary conditions of the spanning structure are determined by the design of the supports that are used. The frictional resistance in the moving bearing parts is modeled by introducing the stiffness of the joint along the corresponding direction.

In the design model of the span structure longitudinal and transverse beams of the roadway do not have breaks, therefore, on the basis of paragraph 4.71 of SNiP 2.05.03-84 the analysis was performed in the elastic stage taking into account additional forces from their joint operation with the main truss belts. The forces in the roadway elements from joint operation with the main trusses are determined assuming that in the horizontal plane the longitudinal beams are hinged to the transverse beams.

Perforations in elements of main trusses and transverse links are modeled by introducing corresponding sections along the length of the bar with a weakened section. Centering of the perforated sections is performed by means of offsets in bars in the corresponding direction of the local axes.

The required set of loads used in the design model is given in accordance with the requirements of SNiP 2.05.03-84. The number of loads in the design model is 103. The number of considered load combinations is 136.

The temporary load C-14 is modeled by a set of loads on the span from 1 to 110 meters. Each of the C-14 loads is accompanied by a corresponding braking load on the section under consideration.

The loads, which according to the standard must be applied at the level of the roadway (rail head), are applied taking into account the eccentricity of its transfer to the idealized model.