The ultimate goal of the calculation of any design is to check the suitability of the structure for operation, combined with cost-effectiveness.
Design calculations by the method of limit states.
Under the limit, they imply such states, upon reaching which the structures do not meet the requirements for them during operation. Limit states are divided into two groups.
I For loss of bearing capacity (in view of loss of stability, durability)
II Unsuitable for normal operation due to the appearance of unacceptable deformations.
The design calculation consists in determining the forces arising in them from the actual loads and assignments, the required dimensions of the cross sections of the elements.
Calculation of structures for the limiting states of group 1 is made in the general case for all stages of the operation of structures and its elements, manufacturing, transportation, construction and operation.
The main type of calculation for 1 group of limit states - calculation by bearing capacity is made according to the formula:
γ n * Q (q n, γ f, γ n) <Ф (S, Rn, γ d, 1 / γ m)
In the left part of the formula Q - force (moment, longitudinal or transverse force), depends on the regulatory loads qn multiplied by the reliability coefficient for the intended purpose γn, which takes into account the class of responsibility of the building, the coefficient γn is taken by SNiP from 0.9 to 1.
In the right-hand side of the formula Ф, the function is the geometry of the characteristics of the section S, the standard resistances of materials Rn, the coefficient of working conditions γd and the reliability of the material γm. The physical meaning of the formula is that the maximum possible force (moment, force) in the cross section of the element q. δ. less, or at most equal to the minimum possible bearing capacity of the section.
The calculation for group 1 of limit states is obligatory and is made based on design loads and calculations of the resistance of materials.
Calculation of structures for limiting states of group 2 - for unsuitability for normal operation.
This calculation takes into account regulatory loads.
As a result of the calculation, a certain deflection of the structure from the action of normal loads is compared with the maximum permissible deflection established in the norms for this type of structure. f <f prev. where f / l = 1 / h0. f values before given in the SNiP.
Under the action of constant, long-term and short-term loads, the deflection in all cases should not exceed 1 / n0 = 1/150 of the span length and the directional deflection of 1/75 deflection of one cantilever arm departure t = MH * l2 / 10E *.
Regulatory Resistance
rolling steel under tension
compression and bending.
As a standard resistance in the calculation of compression, bending for steels of ordinary and high strength, the SNiP adopted the yield strength σt, the corresponding standard resistance, denoted RHT. In special cases, (when the development of plastic deformations for these steels is acceptable, the resistance time (tensile strength) σв is used as a standard resistance. In this case, the normal resistance is designated Rвн values σт, σв, set at GOST, are defective and are monitored during inspection and acceptance rolled.
ESTIMATED RESISTANCE.
At steel mills, the yield strength of steel is controlled selectively, so a material with worse properties than that set by a guest can get into the construction. The calculated resistances for rolling steel R and Rв are equal to the standard RH, Rвн divided by the reliability coefficient for the material γm; numerically, the value of γm is taken 1.025 ... .1,15.
Regulatory and design rolling resistance for steel structures are given in SNiP. R = Rтн / γm; Rv = Rvn / γm
Что бы оставить комментарий войдите
Комментарии (0)