The main regulatory document in the design is the technical specifications. With the development of the theory of structures, the emergence of new building materials and structural forms, as well as the accumulation of experience in operating existing bridges, the technical conditions are revised and supplemented from time to time. The current technical specifications in the field of steel structures of bridges are currently the "Technical specifications for the design of railway, road and city bridges and pipes" CH 200-62. The technical conditions given below, when covering the issues of calculation and design, are designated for brevity "TU" with the number of the corresponding paragraph.
The technical conditions provide a uniform approach of numerous design organizations to checking the dimensions of elements of bridge structures and establish a number of design requirements, the fulfillment of which contributes to the improvement of the manufacturability of the structures and to their durability.
Technical requirements for bridge structures can be united by one concept of "reliability", which is increasingly acquiring the rights of a generalized quality criterion in all areas of technology.
Scientific substantiation of the reliability of bridge structures, while maintaining their efficiency, comes down first of all to determining reasonable safety factors in the form of design factors: overload n, uniformity k and working conditions m. The need to introduce these factors into the calculations is caused by the known uncertainty of the true values of the initial calculated data: design loads, mechanical strength characteristics of the building materials used, as well as some discrepancy between design schemes and the actual operation of structures, possible deviations of the actual dimensions of the elements from those taken in the calculation.
Possible combinations of simultaneously acting loads are divided into three groups: basic combinations, which include permanent loads and temporary vertical loads; additional combinations, in which, in addition to the loads included in the main combinations, other loads (wind, ice, braking), except for construction loads and seismic ones, can be included, and, finally, special combinations, which include, in addition to other loads, also seismic and construction load.
The greater the intensity of possible loads simultaneously taken into account by the calculation, the less the probability of coincidence in time of their maximum values.
This circumstance is taken into account by a decrease in the overload coefficients n when calculating strength and stability for temporary loads included in additional and special combinations.
Overload factors for constant loads are given in two oscillations: a larger and a smaller unit (SN 200-62, p. 115), since there is a possibility of deviation of constant loads in both directions from the values of the standard loads, and the greatest absolute value of the design force is achieved, in some cases, with the lowest values of the calculated constant load. An example of such a case is the calculation of the calculated effort from the combined action of constant and temporary vertical loads with a two-digit line of influence for the sought effort, when there is an inequality: m> n - 1, where m is the ratio of the intensity of the temporary load to the intensity of the constant one, and n is the ratio of the greater , in magnitude, the area of the section of the line of influence of one sign to the area of another sign.
The technical specifications give an indication of the incompatibility of individual loads included in additional and special combinations. The simultaneous action of such loads is unrealistic. So, for example, the maximum wind pressure cannot act on the superstructure simultaneously with horizontal transverse impacts of a railway moving load, since at the design wind pressure, the wheel flanges are tightly pressed against the leeward rail and, therefore, cannot produce impacts.
Calculations of metal bridge structures are performed according to the first and second limiting states. The calculations for the first limit state include checks for strength, stability and endurance.
Strength is characterized by the ability of the structure to resist possible loads and influences without breaking the integrity and without the development of externally noticeable plastic deformations.
Violation of shape stability is characterized by the loss of the design shape (rectilinear, flat) by structural elements under the action of compressive stresses that have reached a critical value.
When the stability of the position is lost, a shift or overturning of the structure occurs.
For the loss of strength and stability, it is sufficient that the possible loads and effects at least once during the service life of the structure exceed the permissible limits.
Therefore, when determining the forces in the strength and stability calculations, the possible maximum values of the forces in the tested structural elements must be taken into account, by means of the overload coefficients n and the dynamic coefficient 1 + JJ.
Endurance may not be guaranteed if the design effort can be repeated multiple times. Such a repeatability of efforts is possible only under normal operating loads that do not exceed their standard values.
Therefore, when checking endurance, the overload factors are not taken into account, that is, they are assumed to be equal to 1. The dynamic factor is taken into account in the endurance calculations, since the dynamic effect of a moving load is regular.
The calculations for the second limiting state include checking the rigidity of the structure, which is understood as its ability to undergo elastic deformations under a temporary vertical load that do not exceed the normalized limits.
In these calculations, the overload factor and the dynamic factor are not considered.
It should be noted that the stiffness standards have not yet had sufficient scientific basis and need to be clarified through additional research.
The need for a scientific substantiation of the stiffness standards is becoming especially acute in connection with the expanding use of high strength steels in bridges with an increased ratio of the yield strength to the elastic modulus.
The coefficient of inhomogeneity k is taken into account by the Technical Conditions during the transition from standard resistances of building materials to design resistances, which are usually used in calculations.
The values of the coefficients of the working conditions m in those special cases when they are not equal to 1 are given in further instructions for the calculation.
Recently, calculations on electronic computers have become widespread, performing not only arithmetic, but also logical operations, which makes it possible, in particular, to automatically obtain an optimal solution when inputting initial data into the machine in various versions.
It is difficult to overestimate the role of using such machines in engineering calculations.
However, this circumstance not only does not eliminate, but even increases the need for the engineer to understand the “play of forces” in the structure and the effect on the magnitude of the efforts of certain changes in the initial data. With the widest use of electronic computers, the engineer must also have the ability to quickly determine the approximate values of the forces using a slide rule for the so-called "sketch" calculations.
The material presented below is intended to highlight the procedure for determining efforts and making approximate calculations.