Laterl Analysis Question

[kngpenn]

I have a project that involves and existing building with a two-way slab floor system with concrete shear walls. I want to add steel braced frames as some of the shear walls are failing due to the additional mass of an additional floor we are adding. When using a mixed system for lateral such as this, do you use the lesser of the two R values?

 

[MiketheEngineer]

You can or beak it our as two problems. I generally take the safer approach and quicker and use the lower R.

 

[DST148]

ASCE 7-05, section 12.2.3.2 - Where a combination of different structural systems is utilized to resist lateral forces in the same direction, the value of R used for design in that direction shall not be greater than the least value of R for any of the systems utilized in that direction. However, there are exceptions which are given in this section.

 

[steellion]

Keep in mind that the system will still want to transmit the lateral loads based on relative stiffness of the members. The concrete shear walls are likely to be much more stiff than the steel braced frames.

 

[kngpenn]

Ordinary concrete shear walls R=5
Ordinary concentrically braced frame R=3.25
Therefore the R used should be 3.25, correct DST148?

This is true steellion.
I'm having to keep adding braces as the concrete shear walls want to pick up more of the load just as you said steellion. Another problem is that the building was originally designed in 1978 therefore the design needs to be brought to current code which is more then likely generating more seismic loading on the shear walls then it was originally designed....not to mention the additional loading coming for the new floor level we are adding.

 

[DST148]

kngpenn - Yes, R to be used should be 3.25. Also depending on the location of the project, check the structural system limitations.

As an aside - I live on the east coast with low seismic risk level.
To avoid using AISC 341-05 : Seismic Provisions for Structural Steel Buildings, we use R = 3 for structural steel systems not specifically detailed for seismic resistance. See ASCE 7-05, section 14.1.2 for guidance.

 

[kngpenn]

Thanks DST148!!
I'm experiencing its better to have a highr R value as it produces a lower Cs used in the Base shear formula. The R=5 presents a Cs=.069 (7% of the building weight) where as the R=3.25 presents a Cs=1.07 (107% of the building weight).

 

[DST148]

kngpenn - Cs = 1.07? (107% of the building weight?).
We better close this thread.

 

[kngpenn]

Typo.....I meant .107 (10.7%)