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How do you calculate weak story?? 1

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NewbieInSE

NewbieInSE

Structural
Dec 19, 2019
234
Hello Dear Engineers,
How do you calculate weak story??

Definition: A weak storey is one in which the storey strength is less than 80
percent of that in the storey above. The storey strength is the
total strength of all seismic-resisting elements sharing the storey
shear for the direction under consideration.

What does the shear strength mean here? Is it sectional nominal shear capacity (2sqrt(f'c)bd) or shear produced to balance the probable moment at column ends (i.e, (Mpr1+Mpr2)/story height)?

Thanks.

 
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True but add (...is less than 70% of that in the story above..). Pls look also ( 1b. Stiffness–Extreme Soft Story Irregularity ).


If SFRS is moment frames,the definition; ∑ (Mpr1+Mpr2)/story height) of SFR columns in case of story mechanism should be OK.. . But , in case of sway mechanism ; ( weak beam - strong column ) this definition is not OK. Moreover, if the SFRS with bracing , shear walls, this assumption will not be valid.

 
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Thanks HTURKAK.
In a pure Special Moment Frame system, Sum of (MPR1,MPR2)/STORY HT. should be ok, right?

What is the procedure in case of sway mechanism/shear wall system?
 
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In sway mechanism, maybe beam end seismic capacity plays role in the calculation, i think.
But i don't know how to calculate weak story in case of shear wall or braced frames? For shear wall system, do we use 2*sqrt(f'c)bd for the shear wall and other remaining moment frames?
 
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I am sorry if i disappoint you ..but NO!!!!. For SMF systems ,strong column-weak beam design is mandatory requirement to make sure sway mechanism WILL develop..not the story mechanism..So, the story strength is controlled by the beams plastic moment strength .

That is, the shear story strength V = ∑ ( Mpi+Mpj)/ H . Mpi and Mpj are plastic moment str. of beam k ,at joints i and j.

sway_vs_story_mechanism_uexbfp.png



In case of shear walls with moment frame etc. not so easy..you should perform inelastic analysis.
 
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Thanks HTURKAK for clarifying my confusion.
I understood the reason of using beam plastic moment. Since we already ensured strong column-weak beam (by a moment capacity ratio of 1.2), so beams will reach seismic plastic moment. Therefore, in calculating column shear strength, that resulting moment from beam has to be used.
 
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