7-53.External forces and their classification (Volume, surface, accumulated, distributed, static, dynamic, constant, time, cycle). To the details of the structure and machineIt is divided into volume and surface forces depending on the application of external forces. Forces acting on all internal points of the body are called volume forces. These forces are distributed over the entire volume of the body. Volumetric forces include the body's own gravity, magnetic attraction, inertial forces, etc. Volumetric forces with the amount of force per unit volume, i.e. tk/ m 3 , kgk/ m 3 , It is measured in n/m 3 . Forces acting on a certain part or all of the structure's surface are called surface forces. External forces are concentrated or distributed (spread).
The forces acting on the surface of the structural elements, which are very small compared to their size, are conventionally called concentrated forces. The concentrated force is the equal effector of the forces acting on a small area. The amount of accumulated forces is measured in newtons (N, kN) in the international system of units.
The forces acting on a certain surface or length of the construction part are called distributed (spread) forces. Distributed force intensity with the unit of force per unit area, i.e. tk/m 2 ,kgk/m 2 etc., and in the international system of units, it is measured in newtons ( n/m 2 ) corresponding to the surface unit. Distributed forces include the pressure of the snow on the roof of the building, the pressure of the gas on the tank walls, and the pressure of the water on the dam. If the intensity of the distributed forces does not change over the entire surface or length, such forces are called evenly distributed forces. When solving some problems of material resistance, the forces distributed over the surface are replaced by the force per unit length of the beam. The intensity of the force per unit length is found by multiplying the intensity of the force distributed over the surface by the width of the beam and it is measured by kgk/m, tk/m, N/m, kN/m .
External forces are divided into static and dynamic forces depending on their change over time.
Static force is the force that is gradually applied to the construction parts and brought to its final value. Due to the fact that the speed and accelerations that occur during the movement of structural points due to static force are very small, the inertial forces that appear during movement are not taken into account.
Dynamic forces are forces that change their magnitude and point of contact in a very short period of time. Due to the influence of dynamic forces, the movement of construction points occurs with great speed and acceleration, as a result, inertial forces arise in the construction masses.
Forces are divided into permanent and temporary forces according to the duration of impact on the structure. A permanent force is a force that acts continuously during the entire service life of the structure. An example of this is the construction gravity.
Temporary forces act on the structure for a limited period of time. An example of it is the pressure of the train on the bridge, the weight of snow, etc.
8.-14Theories (criteria) of strength. (strength condition, elongation - compression, stress: permissible, dangerous, head; classical: largest normal, linear deformation, effort, relative potential energy of deformation)