Peer-to-Peer Standards Exchange

  • 1.  ASCE 7-16, Chapter 13, Item 3.3.1.1

    Posted 03-03-2023 12:28 PM

    ASCE 7 -16 Chapter 13 discusses requirements for support of non-structural components such as cable trays.

    ASCE 7-16, Chapter 13, Item 3.3.1.1 gives some equations for horizontal forces for seismic design for components that include an importance factor. The importance factor is given by that stated in Section 13.1.3 and is either 1.0 or 1.5.

    For a facility currently in the design phase, I would use 1.0 for the importance factor for non-structural components.

    Ii is possible that if these loads were secured to a larger structure, that there would be no question about the importance factor.

    If an independent structure will be used to support these non-structural components (cable Trays), does the independent structure (which could be a T-post) use the same coefficients as given in Section 13.1.3?

    should the seismic design loads be adjusted by the importance factors given in Table 1.5-2 based on Risk Category. For Risk Category III, the importance factor would be 1.25.

    Which importance factor should be used?

    Please provide some insight?



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    David Brosig P.E., M.ASCE
    Engineer
    Abilene TX
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  • 2.  RE: ASCE 7-16, Chapter 13, Item 3.3.1.1

    Posted 03-05-2023 11:29 AM

    If cable tray supported from a major structure then the factor used for that structure is governed. I believe your question is more if an independent post or support to be used for cable trays. Then the question becomes what conduits are carried by these supports for a critical unit. If this unit shot down is critical to operation the higher coefficient shall be used such as post disaster structures or important structures. Always engineers to be proactive on using higher importance factor but if with discussion the owner can prove or assure that these structures failure in case of disaster will not vital then the normal Importance factor or risk factor to be used.



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    Yahya Hematy P.E., M.ASCE
    PRINCIPAL ENGR
    Sarnia ON
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  • 3.  RE: ASCE 7-16, Chapter 13, Item 3.3.1.1

    Posted 03-06-2023 06:39 PM

    A big issue I have is that the independent structures to be designed will be secured to an existing 8 inches thick concrete slab and the anchorage may be quite difficult to achieve with the overstrength factor.

    I do not want to overdesign, but still want to comply with the minimum ASCE 7-16 Code requirements.

    For independent structures supporting mechanical or electrical appurtenances, it seems that Risk Categories are not involved per Chapter 13 of the ASCE 7-16 code. (Just an importance factor of 1.0 or 1.5 based on what is supported.)

    Does the Risk Category become involved for mechanical or electrical items? If so, for Risk Category III, the importance factor is 1.25 which is less that the 1.5 per ASCE 7-16, Chapter 13.

    It also seems that if an individual structure supports different components, each component should be assigned an appropriate importance factor.

    It seems that one can use the importance factors given by ASCE 7-16 for an independent support structure regardless of the Risk Category.

    Can you advise? Thanks.



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    David Brosig P.E., M.ASCE
    Engineer
    Abilene TX
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    Original Message


  • 4.  RE: ASCE 7-16, Chapter 13, Item 3.3.1.1

    Posted 03-05-2023 09:52 PM

    A few points to consider:

    If the project is governed by the IBC, use the Risk Category Table 1604.5.  It supersedes the table in ASCE 7.  The table in the IBC is much more comprehensive than the table in ASCE 7.  The abbreviated table in ASCE 7 can be used in regions of the world where the IBC isn't adopted.  I believe that the commentary in ASCE 7 explains this.

    The Risk category in Chapter 13 is the risk category for the occupancy being served.  If the cable tray is attached to equipment in an Emergency Operating Center, for example, Ip is 1.5, regardless as to how the cable tray is supported or if the tray is inside or outside the building.  An example of why this policy is in place is the collapse of the canopy over the ambulances at Olive View Hospital.  This canopy collapsed in the 1971 San Fernando Earthquake, crushing many of the ambulances that were parked at the hospital.  The canopy was across the drive from the hospital, and was not attached to it.  Today the canopy would be a RC IV , since the ambulances serve the RC IV occupancy. See Photo 4 at URL: https://www.nbclosangeles.com/news/earthquakes/1971-sylmar-san-fernando-earthquake-california/3104/



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    Edwin Huston P.E., S.E., MIStructE, F.SEI, M.ASCE
    Vice Pres
    Huston Structural Engineering, PLLC
    Shoreline WA
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  • 5.  RE: ASCE 7-16, Chapter 13, Item 3.3.1.1

    Posted 03-10-2023 11:43 AM

    I agree with Edwin.  My interpretation is that the importance factor in Chapter 13 supersedes the Chapter 1 table, which is intended for the design of buildings rather than nonstructural components.  Chapter 13 is independent of risk category--except for a few provisions which are triggered only for RCIV, such as note 13.1.3.3.

    One wrinkle here is whether the failure of that anchor would cause unacceptable damage to the building.  In most cases, it would be a localized spall and not a big deal, but if there is a scenario where it compromises part of the building structure that would need to be considered in selecting the 1.5 or 1.0 importance factor.  As Edwin suggests, it comes down to what the consequence of the failure of your component would be.

    My firm recently designed some exhaust vent supports for the emergency power generators at a large hospital.  After discussing internally, we concluded that we needed a high importance factor because, in an extreme event requiring emergency power, the failure of the flue supports could potentially make part of the hospital unusable which is counter to the intent of the code for essential facilities.  Others may have taken a different approach, but I suspect most engineers would lean conservative on this if there's any scenario where the use of the facility would be impacted.  It would be quite a shame if the emergency room was out of commission in an emergency, especially if it was only to save a couple pounds of miscelaneous metals.



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    Christian Parker P.E., M.ASCE
    Structural Project Engineer
    Washington DC
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