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2 Stage Analysis

2 Stage Analysis

2 Stage Analysis

Hello all,

For 2 stage analysis, if we have a wood framed upper structure above a concrete podium I know that I have to treat the upper and lower parts as separate buildings. But do I add (R upper)/(R lower)x V upper to its base shear? In equation form I mean this: V lower = (R upper / R lower)*V upper + Cs*W (lower) ? or is it just ( R upper/ R lower )* V upper for the lower building?
Sorry, it may seem like a silly question and I don't have much experience with 2 stage analysis but your expert advice would be greatly appreciated. Thank you in advance

RE: 2 Stage Analysis

The podium level is design for its own seismic weight (Cs W (lower)) + the scaled reactions from the upper building. One way to think about it is that the podium is designed as a single story building where the seismic mass is (W lower + W upper). This may not be a detailed enough method to capture the overturning effects of the upper structure but the shear at the podium would be the same V lower = (R upper / R lower)*V upper + Cs*W (lower) = Cs (W lower + W upper).

The main benefit of this is that the upper structure doesn't need to be included with the lower structure when performing the vertical distribution of seismic base shear.

Your second proposed equation is wrong because you need to account for the podium seismic weight on the lowest level lateral system.

RE: 2 Stage Analysis

The copy and paste of the relevant clauses;
c. The upper portion shall be designed as a separate structure
using the appropriate values of R and ρ.
d. The lower portion shall be designed as a separate structure
using the appropriate values of R and ρ. The reactions from
the upper portion shall be those determined from the
analysis of the upper portion amplified by the ratio of the
R∕ρ of the upper portion over R∕ρ of the lower portion.
This ratio shall not be less than 1.0.
That's how I read the standard;
Vtotal = (R upper / R lower)*V upper + Cs*W (lower)

Probably it would be better if you post a sketch showing the story shears , reactions of upper portion to get better responds..

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

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