In the ETABS program, when you define concrete materials, in the Specified concrete compressive strength, fc section, you must enter the characteristic strength included in the calculation maps. This resistance corresponds to a cylindrical sample with a height of 30 cm and a diameter of 15 cm at the age of 28 days. When entering it, you should pay attention to several points:
1- In general, the value of the characteristic resistance should be between 200 and 500 kg/cm2.
2- Pay attention to the provisions of table 9-6-1 of the ninth topic. In this table, minimum values for the characteristic strength are given to maintain the reliability of concrete. For example, buildings on the ground in the areas near the coast are placed in category B and the W/C ratio should be at most 0.45 and also the characteristic strength of concrete should be at least 300 kg/cm2.
3- In the seismic design of concrete structures with medium ductility, the minimum characteristic strength is 200 kg/cm2 and for frames with special ductility, the minimum characteristic strength is 250 kg/cm2.
4- For concretes with a strength of more than 500 kg/cm2, the maximum strain of concrete is no longer 0.003 and the defaults of the program are not established for them.
5- In most of the big cities of the country, concrete production companies are easily able to produce concrete up to the characteristic strength of 400 kg/cm2. Of course, the cost of concrete with a strength of more than 250 kg/cm2 will increase. Concretes with higher strengths require a special mixing plan, and only the volume ratio of cement is not important.
6- High strength concrete can be used in tall structures and to reduce the displacement of floors. In this case, the elasticity coefficient of the material increases and the displacement of floors decreases. You can enter the elasticity coefficient of concrete in the modulus of elasticity section. This number is determined based on the relation 1-9-13 of the ninth topic.
7- Unfortunately, some designers use unreasonable characteristic resistances such as 210 kg/cm2 or 280 kg/cm2 in the maps, which is irrelevant. The characteristic resistance included in the drawings must be a factor of 50. The standard categories of typical structural concrete strength are: 200, 250, 300, 350, 400, 450 and 500 kg/cm2.
8- The characteristic resistance of 300 kg/cm2 (or C30) is suitable for most common structures and is recommended.
This post is written by AminNajafgholizadeh