2 Stage Analysis & Fundamental Period

[eisparky]

I have a major problem. I have a 2 story Ordinary Steel Braced frame with the second floor having a 5" thick concrete deck & a braced roof. Someone, however, altered the building by cutting some braces in the transverse direction of the building at both levels. I've been ordered to find a solution to this but the simplest of re-attaching the braces is out. Next solution would be to add light frame shear walls (top level) & CMU Shear wall(lower level) at the exterior along the grid lines where the braces have been cut. This would be the new lateral system. ASCE 7-05 & 2010 California Building Code require that I design my shear walls using R, omega, and Cd of an OSCBF since there are steel braces left intact. Using values for an ordinary system, however, results in Higher seismic forces. I also noticed that ASCE section 12.2.3.1 allows me to use a two stage analysis as long as the lower portion of my structure is 10x stiffer than the upper portion, period of upper portion shall be 1.1x greater than the total period, & the upper portion shall be flexible. Doing this I believe would require that the we get rid of all the braces in order to replace them with the new CMU & wood shear wall system which also allows me to use seismic forces generated by an R=5 & R=6.5.

Now in order to have a flexible upper level compared to the (E)lower concrete deck & (N)CMU SW, and be able to design for lower seismic forces. Would it be right to suggest that the remaining steel braces in the upper level be cut so that I can design for a more flexible upper section having wood shear walls(R=6.5 compared to R=3.25 of an OSBF)? Or can I keep my steel braces and just design wood shear walls for R=6.5 at the grid lines w/o braces per ASCE sec.12.2.3.2 "horizontal combination" and is the dual combination of wood SW along with Steel Braced frames allowed?

First floor is 9ft high & total height is 29ft this gives me a Ta=0.2499sec(total) & Ta=0.1891sec(upper level) doing 1.1Ta(upper)=0.208sec < 0.2499(total) using ASCE approximate method so I don't think I can use ASCE sec.12.2.3.2 two stage analysis unless I do a dynamic analysis to show that the periods are ok. Or by me using a stiffer lower (CMU Shear Wall) portion mean that I can automatically use wood shear walls in the upper level with out having to comply ASCE Vertical & horizontal combination requirements as it is done when designing a two story residence with a CMU or Concrete lower level and light frame upper? The building is also symmetrical w/o any irregularities.

Hopefully someone here can help.

 

[jittles]

I don't have an answer for you, but I noticed one thing

Or by me using a stiffer lower (CMU Shear Wall) portion mean that I can automatically use wood shear walls in the upper level with out having to comply ASCE Vertical & horizontal combination requirements as it is done when designing a two story residence with a CMU or Concrete lower level and light frame upper? The building is also symmetrical w/o any irregularities.

The only reason that is allowed is under the exceptions to 12.2.3.1 for one and two family dwellings of light frame construction. I am guessing that this is not what you have.

 

[eisparky]

No. I do have a 2 story, OC.II though. Mine is more of an apartment structure.

 

[CCox]

I am not familiar with the CBC, but here are my thoughts on the ASCE requirements. If you consider the structure to be broken into a 2-stage analysis, the requirements of section 12.2.3.2 would apply to the second floor, independent of the ground floor. The forces from the upper floor are superimposed onto the ground floor analysis, amplified accordingly. If you keep the braced frames in, it sounds like you would have to perform a dynamic analysis to determine if you qualify for a 2-stage analysis because you do not meet the approximate fundamental period requirements from section 12.2.3.1. To determine the period from eqn. 12.8-10 for concrete or masonry shearwalls, I would think all the shear elements at the ground floor would need to consist of these type of elements.

If you do qualify for a 2-stage analysis, it sounds like the upper floor meets the three conditions allowing you to utilize the separate seismic factors (R, Cd,overstrength)for the new wood shearwalls and the existing braced frames. If it were me, I would have no problem allowing multiple R-values because of separate shear elements, as long as they are in independent lines of each other. One thought: the seismic force could also be reduced by considering the existing braced frames as SCBF. However, this increases detailing requirements and you may not have easy access to these.

However you slice this, it is a major headache. Hopefully, you have the budget to look at different options. It never ceases to amaze me that someone could do something like cut through a braced frame, especially in CA.

Good Luck,

Cody