Response Modification Coef.

[MOOK]

If you have 3 story building, Response medication factor for the first floor is R = 5, the second floor R= 4, and the third floor R = 3.

What is the design value of the response modification factor for the first floor?

Some Reference used 4 and the other used 3.

What do you think?

Thanks

 

[outatime2002]

If you have 3 story building, Response medication factor for the first floor is R = 5, the second floor R= 4, and the third floor R = 3. What is the design value of the response modification factor for the first floor?

Some Reference used 4 and the other used 3.

What do you think?

Thanks

Refer to ASCE 7-05 Section 12.2.3: Combinations of Framing Systems in the Same Direction. My interpretation is that the entire system shall be designed for the smallest seismic response coefficient in that particular orthogonal direction as the provision stipulates. So, I would say R = 3. With that being said, in cases where combinations of structural systems are used, the structural engineer should use his/her best judgment in selecting an appropriate R value. It isn't uncommon to use a composite R value for the overall structural system as long as it's justified.

 

[jamie]

my review instructor advised us "R used for design at any story shall not exceed the lowest value of R that is used in the same direction at any story above that story." what that basically means is that starting at the top story, R cannot increase as you go down story by story. So for this problem, since the top floor has R=3, the entire building has to be design with R=3. There are some exceptions to this rule for single family homes and some other two story buildings. However, although R cannot increase as you go down, it can decrease. Had the R values been reversed, with R=5 for the third floor, then each floor could be designed with it's own R value, with the top floor R=5, the second R=4, and the first floor R=3.

 

[outatime2002]

my review instructor advised us "R used for design at any story shall not exceed the lowest value of R that is used in the same direction at any story above that story." what that basically means is that starting at the top story, R cannot increase as you go down story by story. So for this problem, since the top floor has R=3, the entire building has to be design with R=3. There are some exceptions to this rule for single family homes and some other two story buildings. However, although R cannot increase as you go down, it can decrease. Had the R values been reversed, with R=5 for the third floor, then each floor could be designed with it's own R value, with the top floor R=5, the second R=4, and the first floor R=3.

I think that this is a reasonable interpretation of what the code is saying. For talking purposes, if 3 different seismic force resisting systems (R =3,4,5) are employed in the same orthogonal direction at a particular story, the most stringent R value (3) should be used for the design of that story (given that there isnt a smaller R value in any of the stories above in that orthogonal direction).

 

[sac_engineer]

Just remember that "R" represents the structure's ductility. The lower the R-value, the less ductile the system, and less ability to sustain cyclic elastic deformations. That's why "R" is in the denominator of the seismic response coefficient equation and would result in higher value as ductility decreases.

Thus, for a vertical combination structure, you would use the lowest R-value above the story you're analyzing.

Good luck!