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Component Manufacturing dverti$er
Don’t Forget! You Saw it in the
Adverti$er
January 2019 #11234 Page #50
Referring to the 2015 IBC, Section 1608 Snow Loads: “1608.3 Ponding instability. Susceptible bays of roofs shall be evaluated for ponding instability in accordance with Section 7.11 of ASCE 7.” (Underlined for emphasis.) The emphasis of the IBC language is structural (ponding instability) and points to the need for additional structural work when “susceptible bays” exist on the completed roof construction. Referring to the 2015 IBC, Section 1611 Rain Loads: “1611.2 Ponding instability. Susceptible bays of roofs shall be evaluated for ponding instability in accordance with Section 8.4 of ASCE 7.” (Underlined for emphasis.) Again, the emphasis of the IBC language is structural (ponding instability) and points to the need for additional structural work by the design team. To summarize—based on the IBC code provisions cited above, the typical truss design note that appears on a truss design drawing with a top-chord design slope of ¼:12 or less is clearly a structural issue, requiring additional consideration and design work by the design team.
3. To whom is the instruction “Provide adequate drainage” directed? The intended recipient of the note to “provide adequate drainage” is not clear. In addition, the note in total does not give guidance as to what actions or professional work is required to prevent water ponding. Knowing the structural complexity to evaluate a roof having “susceptible bay(s)” for ponding instability, uncertainty of the actual dead loads (uniform and concentrated) on the completed roof construction, and final roof-surface profile, we believe clarity in stating the truss design assumptions is critical information for the design team members in their review and approval of the proposed truss design drawings. Other typical design notes, such as wind load design data, special live loads, concentrated loads, snow loading, IBC referenced load standard and edition used, and so on, are a statement of “design assumptions” that can be reviewed and approved by the structural engineer and/or architect and general contractor and returned to the Component Manufacturer (CM). Even if the truss designs are not reviewed and approved by the structural engineer and/or architect and returned to the CM, the stated design assumptions stand alone as the basis of the proposed truss design.
Suggestions and Guidance We offer the following ideas for educating and communicating the substance and importance of this low-slope roof ponding issue.
1. Consider revising the “water ponding” truss note to be a truss design assumption consistent with other notes that spell out the parameters of the design. For example, this type of language could be used:
“This design does not include loads that could occur from the ponding of roof water in susceptible bays (2015 IBC, 1608.3 and 1611.2).” This could be a truss design assumption made consistent with the content of most truss design notes.
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