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  • 1.  ASCE 7 Horizontal irregularity #5 - Nonparallel system

    Posted 03-08-2023 02:17 PM

    Hello fellow engineers!

    I would like to get an interpretation on how to define Horizontal irregularity #5 - Nonparallel system per ASCE 7-16, Table 12.3-1.

    Per the table, "Nonparallel system irregularity is defined to exist where vertical lateral force-resisting elements are not parallel to the major orthogonal axes of the seismic force-resisting system."

    My question is:  Which part of the vertical lateral force-resisting elements?  Does it have to be the entire element on one side, or any part of the system will trigger this nonparallel system irregularity? 
    Please take a look at the floor plan below for a concrete tilt-up warehouse with flexible roof diaphragm.  On the south side, about half of the concrete shearwall is parallel to the major orthogonal axis, the other half is skewed and not parallel.
    Would you define this as nonparallel?


    If nonparallel system exists here, then Section 12.5.3 and 12.7.3 are applicable.
    12.5.3 tells you how the structure shall be loaded.
    12.7.3 tells you how the structure shall be analyzed and modeled.  For irregularity #5, a 3D model is required; the only exception is provided for a flexible diaphragm structure if it has horizontal irregularity #4.

    As computer modeling becomes widely available, I do not see an issue for analyzing this kind of building using a 3D model to get a more realistic results, and I think it's a great way to learn how the building would behave in different kinds of irregularities.

    Thanks!


    #ASCE7-16
    #Seismic

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    Manhin Lee Aff.M.ASCE
    Plan Check Professional
    City of Fremont
    Fremont CA
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  • 2.  RE: ASCE 7 Horizontal irregularity #5 - Nonparallel system

    Posted 03-08-2023 02:19 PM

    The structure you show in plan would be considered by most engineers to have horiozntal irregularity type 5.  It should be analyzed and deisnged for 100% of the forces in Direction X combined with 30% of the forces in Direction Y, and 100% of the forces in Direction Y combined with 30% of the forces in Direction X.



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    Ronald Hamburger, SE
    Consulting Principal
    Simpson Gumpertz & Heger
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  • 3.  RE: ASCE 7 Horizontal irregularity #5 - Nonparallel system

    Posted 03-09-2023 11:50 AM

    Great!  Agree on the applications of loads.

    The follow-up question would be, is it required to be analyzed using 3D model per ASCE 7-16, 12.7.3?
    12.7.3 states that for horizontal irregularity 1a, 1b, 4, and 5, a 3D model is required; the only exception is provided for a flexible diaphragm structure if it has horizontal irregularity 4 (out-of-plane offset).



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    Manhin Lee Aff.M.ASCE
    Plan Check Professional
    City of Fremont
    Fremont CA
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  • 4.  RE: ASCE 7 Horizontal irregularity #5 - Nonparallel system

    Posted 03-09-2023 12:26 PM

    In my opinion, a 3D model is not required because this building has a flexible diaphragm.  However, even if hand calculations and statics are used ot determine the forces on the walls, one has to consider the 3D behavior of the system.



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    Ronald Hamburger, SE
    Consulting Principal
    Simpson Gumpertz & Heger
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  • 5.  RE: ASCE 7 Horizontal irregularity #5 - Nonparallel system

    Posted 04-27-2023 04:20 PM

    If hand calculations are done such that the stiffnesses of the shear walls are considered as well as the location and direction of those stiffnesses AND the center of force along with the eccentricities of the load (both actual and minimum) are taken into account, what else must one do to "consider the 3D behavior of the system"? IMO, if the roof diaphragm were considered as semi-rigid, i.e., the stiffness of the diaphragm considered, then I would totally agree with a 3D computer model to solve the problem. Otherwise, I don't see how one can do anything more to "consider the 3D behavior of the system". Not being critical for you have much more knowledge and experience in this area than I do. I'm sincerely curious.

    Thank you,



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    Bill Allen P.E., S.E., M.ASCE
    Principal
    Allen Designs
    Mission Viejo CA
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  • 6.  RE: ASCE 7 Horizontal irregularity #5 - Nonparallel system

    Posted 05-01-2023 08:03 AM

    It's seem that horizontal irregularities geometric stability is there but a cutouts necessitate to reduce moment at the centre and avoiding tilted. This is my humble opinion. I wish to hear from you
    Alex Thomas MIE REng CEng M ASCE 



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    Alex Thomas R.Eng, C.Eng, M.ASCE
    Senior Site Engineer
    Geo Structurals Pvt Ltd
    CochinAlexThomasR.Eng, C.Eng, M.ASCEIndia
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  • 7.  RE: ASCE 7 Horizontal irregularity #5 - Nonparallel system

    Posted 04-25-2023 06:13 PM

    ASCE 7 does require a 3D model  



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    Ronald Hamburger, SE
    Consulting Principal
    Simpson Gumpertz & Heger
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  • 8.  RE: ASCE 7 Horizontal irregularity #5 - Nonparallel system

    Posted 04-27-2023 08:02 AM

    Do expansion joint provide for vertical irregularities. ?
    If a capstan is necessary arises carryover moment. 
    Alex Thomas MIE REng CEng M ASCE



    ------------------------------
    Alex Thomas R.Eng, C.Eng, M.ASCE
    Senior Site Engineer
    Geo Structurals Pvt Ltd
    CochinAlexThomasR.Eng, C.Eng, M.ASCEIndia
    ------------------------------