I want to do some time history nonlinear analysis on a couple of 1-storey, 1-bay steel moment frames and I want to design them in a way to ensure that nonlinear deformations will occur in these frames (my earthquake records are far-field records and their PGA is around 0.35g utmost). The challenge is, when I design these frames according to gravity loads, soft story happens and the beams won't bear nonlinear deflections. On the other hand, when I want to design the frame according to codes and standards and follow "strong column-weak beam" method to avoid soft story, the earthquake must be scaled with a large factor to make the frame nonlinear. Can you suggest a design procedure which is more appropriate?
Did you apply the R factor (Seismic Response Modification Factor) in your analysis? It seems to me, your excitation is not high enough to make your structural elements behave non-linearly.
let me define my question in another way. I want to design my frame as weak as possible (only resistant against gravitational loads) but I do not want the occurence of soft story.
When we are dealing with gravity loads it's a different case.But once you start applying horizontal loads on a building,the behaviour is totally different.Soft storey occurs only when you apply horizontal loads and there are no infill walls.So if you add enough stiffness to your frame in place of infill walls then soft storey should not occur.
It depend to your assumption, when you design a moment frame in life safety performance level for a specific hazard level, it means that generally your frame have a nonlinear response in the earthquake not for all earthquake only for strong motions that are equal or more stronger than expected hazard level
First, I have some questions to understand your problem:
- Soft story mechanism is a problem of multi-story frames (plastic mechanism in one story instead of the whole frame). How do you define soft story mechanism in an 1-storey frame?
-Do you have any boundary conditions for your frame (span, height, mass) or should it be a generic frame?
In any case, I propose to design the frame with a linear static method (R-factor in US or q-factor in Europe). To design a generic frame you can use following procedure:
1.) Select the first Eigenperiode of your frame (e.g. T1 = 1 s)
2.) Choose the mass m of the frame and determine the stiffness of your frame (cross section of beam and columns) that the first Eigenperiode is equal to T1
3.) Generate the elastic response spectra of your record
4.) Determine the Ordinate of response spectra at the first Eigenperiode Sa(T1)
5.) Choose the force reduction factor q (e.g. q = 4)
6.) Determine the equivalent seismic (design) force Fb,d = Sa(T1)*m/q
7.) Determine the strength of the frame (yield strength as section is defined in 4.): determine by elastic static analysis for Fb,d and vertical loads (m*g) the maximum bending moments and choose the yield strength that the plastic moment capacity of the columns (or beams) are equal to the maximum bending moment (for columns maybe M-N-interaction has to be considered)
By changing Eigenperiode and q-factor you change the ratio between horizontal (seismic) and vertical (gravity) loads.
Dear Max Gundel, Thank you, I know this procedure. By the way I do not want my columns be nonlinear prior to beams. and i want my frame to be weak as possible it can be.
Dear Arman,
I am not exactly clear, what kind of analysis you want to carry out. Still let me suggest you to go for a detailed push over analysis using any standard software. This will give you a better understanding about the response of the frame to lateral loads. There by you can decide which member should yield first and the remaining hierarchy of yielding by changing suitable sections for the frame members.
Oh, sorry for not mentioning that ( i thought when i am talking about earthquake acceleration records it will be clear). I want to do nonlinear time history analysis.
Dear friend,
I am presuming the following two things for giving my comment on your topic.
1) The structure is a moment resisting frame with no additional energy dissipation mechanism.
2) For non-linear time-history analysis, you need to avoid soft story response by allowing some intended level of rotation/yielding both in beams and columns.
To obtain a structure with well controlled response as briefed above, we can use Performance Based Plastic Design (PBPD) method.
For details you can go through the text book " Performance Based Plastic Design Earthquake Resistant Steel Structures" by Subhash C. Goel and Shin-Ho Chao.
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