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  • 1.  Tabulated "Rain Loads" in Structural Drawings

    Posted 04-22-2019 01:47 PM
    Edited by Tirza Austin 04-22-2019 01:53 PM
    What Rain Loads are being used to design roofs as per IBC and ASCE 7?  Is there any specific USA Map with Rain loads that can be referenced similar to Snow, Wind, or Seismic Maps?

    If there is a requirement to include Rain Loads in the set of Structural Drawings as per IBC 2018, Where are these rain loads taken from? Who takes the responsibility to specify them, Mechanical Engineer, Architect, Structural Engineer, Building Official, Owner? This might be a very GRAY area. Would it then be the IBC or ASCE 7 to take the lead here and provide a USA MAP with contour lines of tabulated RAIN LOADS in psf per each State?

    A relevant article has just been published in the STRUCTURE Magazine issue of April 2019, What do you think?
    Do Structural Engineers Design for Rain Loads?
    Structuremag remove preview
    Do Structural Engineers Design for Rain Loads?
    During its lifetime, a building roof is subjected to a number of different structural loads - roof dead loads and roof live loads (principally snow, wind, and rain). Depending upon the location, one of these will be the controlling roof live load.
    View this on Structuremag >

    Pedro R. Munoz Ph.D., P.E., Archineer
    Principal Engineer
    PRM Engineering, LLC
    Methuen MA
    (978) 738-8001

  • 2.  RE: Tabulated "Rain Loads" in Structural Drawings

    Posted 04-23-2019 09:58 AM
    Edited by Tirza Austin 04-23-2019 09:57 AM
    I agree completely that this is a critical and fully overlooked area of the Code.  The requirements and responsibility have changed hands over the years, and currently the IBC makes reference to the IPC.  I don't have a copy of the IPC and don't wish to buy another code book just for rain drainage.  But as the SEOR I believe it is on me to know what forces I need to design to. 

    In a large building with parapets, I doubt that the plumbing designer is looking at collection area, roof slope, and height of parapet; all which play a prominent role in the final applied loads. Depending on roof shape, the rain loads can easily outgrow other design live loads.  Sizing and location of drains can dissipate the loads OR the structure can be designed to withstand the loads.  Someone needs to be overseeing the big picture, and the building codes need to define who that is.

    Jerry Coombs M.ASCE
    Coombs Engineering
    Plano TX

  • 3.  RE: Tabulated "Rain Loads" in Structural Drawings

    Posted 04-24-2019 07:36 AM
    Edited by Tirza Austin 04-24-2019 07:35 AM
    The International Plumbing Code is available online for free, https://codes.iccsafe.org/content/IPC2018?site_type=public.  You cannot cut and paste, but at least you can view it.  Rainfall intensities can be found at https://hdsc.nws.noaa.gov/hdsc/pfds/.

    Richard Bennett M.ASCE
    Univ Of Tennessee
    Knoxville TN

  • 4.  RE: Tabulated "Rain Loads" in Structural Drawings

    Posted 04-24-2019 10:06 AM
    Edited by Tirza Austin 04-24-2019 10:06 AM
    ​The ICC site was down yesterday morning but is back up again today.

    Sure enough - looking in the 2018 Plumbing Code (Section 1108 - Secondary (Emergency) Roof Drains), it still says "Secondary (emergency) roof drain systems shall be sized in accordance with Section 1106 based on the rainfall rate for which the primary system is sized."  And section 1106 still refers to the 100-year hourly rate, not the 100 year 15 minute rate.

    It sounds as though ASCE and the ICC need to get together and resolve this - or more specifically - the ICC needs to resolve this.  The 2018 IBC refers to both the 2016 ASCE 7 and the 2018 IPC....which are in conflict with each other for the rainfall rates for secondary roof drainage.

    Thanks to the article authors and Pedro for elevating this issue.

    Greg Thein, PE
    Cleveland, OH

  • 5.  RE: Tabulated "Rain Loads" in Structural Drawings

    Posted 04-23-2019 05:14 PM
    Edited by Tirza Austin 04-23-2019 05:13 PM
    Generally the rain load is included in load combination 1 through 6 (ASC 7).  The rain load unlike the snow load, doesn't withstand load on the roof. Thus far, most of the occasions, the rain load doesn't come to play in the load combinations. However, in special cases, the rain load might add on loads; for instance, on a roof without drainage or on a flat roof and not having drainage. ASC 7 provides a factor of 0.5 times the rain load. This shall be also noticed that fluid load (F load)  can be applied as according ASC for very special cases in which with the same load factor as Dead load assigned to be.

    Sayed Maqsood
    Oakland CA

  • 6.  RE: Tabulated "Rain Loads" in Structural Drawings

    Posted 04-24-2019 10:03 AM
    Edited by Tirza Austin 04-24-2019 10:03 AM
    The Steel Joist Institute published their Technical Digest​ 3 which is a tremendous resource for Rain Loading calcs overall and great guidance for proper documentation within construction drawings.

    Judian Duran EIT,A.M.ASCE
    Seminole FL

  • 7.  RE: Tabulated "Rain Loads" in Structural Drawings

    Posted 04-25-2019 07:42 AM
    Edited by Tirza Austin 04-25-2019 07:42 AM
    The calculation of rain loads is fairly straightforward using the equations in ASCE 7. The latest change in ASCE 7 -16 is the use of the 100-yr 15 minute duration rain load instead of the 100 yr 1 hr duration wind load used in previous editions of of ASCE 7. An example showing the calculation of rain loads is covered in "Structural Steel Design", 2nd edition (Pearson) by Aghayere and Vigil. As indicated above, use the 100yr 15 minute duration rain loads as the rainfall intensity in the equations. These rainfall intensities can be found in the NOAA website.

    Abi Aghayere
    Avondale PA