Modal Analysis or Equivalent Lateral Force?
Modal Analysis or Equivalent Lateral Force?
(OP)
Hey, all.
I have a 3+3 podium-like structure. RC moment frames at the lower 3 levels and wood bearing/shear walls at the upper 3 levels. I have determined that 2 stage ELF analysis is not likely going to work because of the stiffness requirements between the upper and lower, so I have to figure out what I want to do next.
I see two options (unless someone can point me in a different direction)
1. Calculate the period of the structure using FEM software (I use RISA 3D) and then use that value to determine the base and story shear with ASCE 7-10 12.8 ELF procedure. I am not aware of any prescriptive or "simple" analytical procedures to conservatively approximate the period for a structure like this.
2. If I do indeed have to model the structure in FEM in order to calculate the period, I feel like by the time I put all of that effort into imputing the structure into RISA I might as well have RISA complete the modal analysis and provide the base and story shears for me. I have never performed a modal analysis, and I am feeling very cautious to do so. But, if it isn't terrible complicated than I would rather "kill two birds..."
Hopefully one of our RISA experts (I think Josh) notices this post.
I have a 3+3 podium-like structure. RC moment frames at the lower 3 levels and wood bearing/shear walls at the upper 3 levels. I have determined that 2 stage ELF analysis is not likely going to work because of the stiffness requirements between the upper and lower, so I have to figure out what I want to do next.
I see two options (unless someone can point me in a different direction)
1. Calculate the period of the structure using FEM software (I use RISA 3D) and then use that value to determine the base and story shear with ASCE 7-10 12.8 ELF procedure. I am not aware of any prescriptive or "simple" analytical procedures to conservatively approximate the period for a structure like this.
2. If I do indeed have to model the structure in FEM in order to calculate the period, I feel like by the time I put all of that effort into imputing the structure into RISA I might as well have RISA complete the modal analysis and provide the base and story shears for me. I have never performed a modal analysis, and I am feeling very cautious to do so. But, if it isn't terrible complicated than I would rather "kill two birds..."
Hopefully one of our RISA experts (I think Josh) notices this post.






RE: Modal Analysis or Equivalent Lateral Force?
RE: Modal Analysis or Equivalent Lateral Force?
My gut instinct matches asixth's comment. Similar to how we'll model a steel braced penthouse on top of a main building as a rigid mass at the top of the main roof. That being said, this is a little different and I don't know of a code provision that sanctions this approach. Maybe the provisions for rigid equipment? A three story "penthouse" on top of a three story base building seems like a bit of a stretch. I like the RISA modelling approach though.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Modal Analysis or Equivalent Lateral Force?
I knew you would be one of the first to hit this post up, KootK. In Chapter 15 (nonbuilding structures), there is a section on nonbuilding structures supported by another structure. If the nonbuilding structure mass is greater than 25% of the supporting structure AND the fundamental period is less an 0.06, you can do as asixth has mentioned. If the nonbuilding structure is less than 25% of the effective seismic mass of the supporting structure, it is deemed a nonstructural component (chapter 13)
The problem is that last time I checked, a condominium or apartment building is not a nonbuildinng or nonstructural component (nice double negative). Besides, my stick-framed portion will have a period on the order of 7 times higher than 0.06. So, I guess it would make sense that this method is not even included in chapter 12.
Looks like I may be leaning towards RISA to determine the fundamental period and plug into the ELF procedure.
RE: Modal Analysis or Equivalent Lateral Force?
You could also try to use Rayleigh's equation for the period Link
If you have a good estimate of the stiffness of each level you can get a reasonable estimate on the period (I found it to be within a few percent generally) without modeling the structure in RISA, and follow an ELF approach.
RE: Modal Analysis or Equivalent Lateral Force?
I have a feeling that period will be similar to the RC frame alone - somewhere around T = 0.4 sec. or less. I just can't see the upper stories contributing much to raising the period considering how lacking of mass they are. I really one envision the mode shape for the whole building being different from the existing lower 3 stories.
RE: Modal Analysis or Equivalent Lateral Force?
If you end up going with a response spectra / modal analysis procedure, be ready for something of a learning curve. The analysis results will be fine in RISA, but will often cause a lot of confusion to engineers not familiar with the procedure. It all relates to the loss of relative signs when doing an RSA analysis. Some RISANews blog topics on the subject to get you started:
http://risa.com/news/why-does-deflection-plot-resp...
http://risa.com/news/why-dont-response-spectra-rea...
RE: Modal Analysis or Equivalent Lateral Force?
As a side thought, due to the information in my recent post, "Analyzing a 1950's Concrete Frame", I am more than happy to have a super conservative period.
RE: Modal Analysis or Equivalent Lateral Force?
Now, what to do with the upper 3-stories?. I would model it seperately with spring supports derived from the above analysis and apply the seismic load obtained at the mass from the first analysis. Not sure if this is feasible until I would have tried it and evaluated the results.
RE: Modal Analysis or Equivalent Lateral Force?
See this link for very preliminary arch plans and elevations: Link
Of course there is a darn courtyard from level 4 and up, so the mass and stiffness are going to be off-center from the structure below. Because it will be light-framed bearing walls above, I am going to be generous with assigning shear walls throughout. If I can get it to work out, I may be able to balance the mass and stiffness of the upper floors where it will not induce much torsion on the lower RC structure. I want a concentric lateral response in both directions throughout.
RE: Modal Analysis or Equivalent Lateral Force?
This will require some judgement to avoid excessive conservatism but I think that it's a valid approach.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Modal Analysis or Equivalent Lateral Force?
Here is his "backdoor escape" verbiage from our proposal:
At least the Windsor Probe is in there??? - we own one. I guess I should just be happy for what I can get.
RE: Modal Analysis or Equivalent Lateral Force?
RE: Modal Analysis or Equivalent Lateral Force?
RE: Modal Analysis or Equivalent Lateral Force?
FYI - GPR is not accurately able to determine rebar diameter to the precision of 1/8" increments of rebar sizing.
RE: Modal Analysis or Equivalent Lateral Force?