Topic Thread

  • 1.  Importance factor using 2 stage analysis

    Posted 08-19-2022 04:06 PM
    We are designing a hotel with the wood framed rooms (risk category II, ignoring floors below) built over a multi level concrete podium.  One floor down from the wood framed hotel rooms, on the concrete podium, there is a ballroom/conference center (risk category III, based on occupancy).
    Seismically we are approaching this as a 2 stage analysis, by designing the wood portion, independently of the concrete pier, but then adding the mass at the corresponding center of gravity to the concrete podium below.
    The question is, given the approach above, is it permissible to use an importance factor of 1.00 for the upper wood portion and 1.25 for the lower higher occupancy/use concrete podium design?  1.5.2 seems to indicate this may be possible.  We know this is specifically applicable to horizontal separation of portions of the structure.  We don't know if this is applicable vertically as well.
    #ASCE7-16
    #Seismic

    ------------------------------
    Mario Giampa S.E., M.ASCE
    SR PROJECT ENGINEER
    Ficcadenti Waggoner and Castle
    Irvine CA
    ------------------------------


  • 2.  RE: Importance factor using 2 stage analysis

    Posted 08-19-2022 06:19 PM
    Hi Mario,

    Section 1.5.2 is the appropriate provision.  It states that where the portions of a building have independent structural systems, different Risk Categories may be assigned.  However, the word independent is key.  Since the upper wood frame portion is supported by the lower concrete frame portion, some building officials will interpret that the higher Risk Category must apply to the entire structure, since if the upper structure collapsed, it would impact the lower structure.  It would be best to check with the local building official.

    ------------------------------
    Huajie Liu P.E., M.ASCE
    Engineering Manager
    LCRA
    Austin TX
    ------------------------------



  • 3.  RE: Importance factor using 2 stage analysis

    Posted 08-22-2022 10:57 AM
    Hi Mario,

    I agree with Huajie and would also add that the Two-Stage Procedure (Section 12.2.3.2) does not explicitly address a change in Importance therefore I would assume this is not anticipated – or maybe that it was discussed and the authors could not envision such a scenario.  In adjusting the forces per the ratio R/ρ for the upper and lower systems (Section 12.2.3.2, item d), note that Ie is not included.  Would you augment this to also include the ratio of Ie for these systems? The ratio of these two (R/Ie) is found pretty much everywhere else so being that it is not in this section allows for an interpretation that the authors thought that a change in Ie couldn't exist when using this procedure. 

    I mention this to add to Huajie's point that collapse of the upper portion could easily be argued as not 'independent' to the lower portion (i.e., 'independent' being a key word used in the Provision).  Unless this sort of thing is precluded, my opinion would be to use the larger Ie for all.  Even more directly to your question, yes this is allowed horizontally but no I don't see this working vertically unless provisions are made to assure the systems are independent (i.e., with respect to failure). 

    The building official may allow it but before inquiring you might consider if you're capturing the intent expressed in the Commentary 1.5.1 (as you've probably seen there is no commentary for 1.5.2):

    A rational basis should be used to determine the Risk Category for structural design, which is primarily based on the number of persons whose lives would be endangered or whose welfare would be affected in the event of failure.

    I'd expect that this will help you decide if you even want to pursue it.  If so, I'd think you'll need to have a good answer to this for discussion with the building official anyway. 

    Hope this helps,
    Keith



    ------------------------------
    Keith MacBain Ph.D., P.E., A.M.ASCE
    Associate & Chief Analyst
    Geiger Engineers, Campus 2
    Suffern NY
    ------------------------------



  • 4.  RE: Importance factor using 2 stage analysis

    Posted 08-22-2022 12:12 PM
    Thank you for the responses.  It does indeed that the section allows for a different importance factor only with independent systems, and this is not the case I described.  There was some discussion in our office and indeed it appears that your answers are accurate with the intent (and really the plain reading) of the code.  Thank you again.

    ------------------------------
    Mario Giampa S.E., M.ASCE
    SR PROJECT ENGINEER
    Ficcadenti Waggoner and Castle
    Irvine, CA 92692
    ------------------------------