Regulatory and design resistance of concrete and reinforcement.

Regulatory and design resistance of concrete.

The strength characteristics of concrete have variability. Even samples from one batch of concrete will show different strength during testing, which is explained by the heterogeneity of its structure and unequal test conditions. Of all the possible values ​​of strength, it is necessary to enter into the calculation one that with the necessary reliability would ensure the safe operation of the structure.

According to SniP "Concrete and reinforced concrete structures" the main characteristic of concrete is its normative resistance, representing the strength of a concrete cube with an edge of 15 cm, which determines the class of concrete in compressive strength - "B".

The standard resistance of a concrete prism to axial compression Rbn (prism strength) is determined by the standard value of cubic strength, taking into account the relationship connecting prism and cubic strength. The values ​​of Rbn are given in the table.

Standard concrete resistance to axial tension of Rbtn

The design resistances of concrete for limit states of the 1st group Rb and Rbt are determined by dividing the standard resistance by the corresponding coefficients of concrete reliability under compression bc or tensile bt.

Rb = Rbn / bc Rbt = Rbtn / bt

For heavy concrete bc = 1.3; bt = 1.5

The design resistances of concrete for the limiting states of the second group are taken to be equal to the standard resistances. This is explained by the fact that the onset of limiting states of the 2nd group is less dangerous than the 1st one, since it does not lead to the collapse of structures and their elements.

When calculating concrete and reinforced concrete structures, the calculated resistances of concrete are multiplied by the coefficients of working conditions bi = 0.85 - 1.1, taking into account the duration of the load, production conditions, the nature of the work.

Regulatory and design resistance of reinforcement.

Regulatory and design resistances of reinforcement Rsn are taken equal: for rod reinforcement, high-strength wire and reinforcing ropes - yield strength; for an ordinary reinforcing wire, the stresses are 0.75 times the temporary tensile strength.

The design resistance of reinforcement to tension Rs is determined by dividing the standard resistance by the corresponding safety factors for reinforcement s.

When calculating according to the limiting states of the 2nd group, the value of the safety factors for all types of reinforcement is assumed to be 1, that is, the calculated resistances are numerically equal to the normative.

Questions for self-test.

1. What are we building structures for?

2. For what reasons can a construction lose its performance?

3. By what method are building structures calculated?

4. What does this method guarantee for a building structure?

5. Describe the 1st group of limit states.

6. Describe the 2nd group of limit states.

7. What is the regulatory burden?

8. What is the regulatory load?

9. What is the design load?

10. What is the reliability factor for the load introduced?

11. What are the loads?

12. What are the combinations of loads?

13. What concrete classes do you know?

14. What reinforcement classes do you know?