Deflection Review: Code Updates Bring Changes to Windload Requirements
Glazing systems designers in the past have used a windload reduction factor cited in section 1604.3 of the International Building Code (IBC) to allow for a reduced windload to check the deflections of glazing systems. The 2018 IBC provides the reduction in footnote F under Table 1604.3 and states:
“The windload shall be permitted to be taken as .42 times the component and cladding loads or directly calculated using the 10-year mean return interval wind speed for determining deflection limits in Table 1604.3. Where framing members support glass, the deflection limit therein shall not exceed that specified in Section 1604.3.7.”
The .42 reduction consists of a .6 factor to convert from ultimate loads to service loads combined with a .7 factor to convert loads from the 50 year Mean Return Interval (MRI) t he 10-year MRI (.6 x .7 = .42). The above footnote referenced by the IBC also points to Section 1604.3.7 for design of framing members supporting glass. This section states:
“The deflection of framing members supporting glass subjected to .6 times the component and cladding windloads shall not exceed either of the following:
• 1/175 of the length of span of the framing member, for framing members having a length not more than 13 feet 6 inches.
• 1/240 of the length of span of the framing member + ¼ inch, for framing members having a length greater than 13 feet 6 inches.”
From the above section of the IBC, referring to framing supporting glass, it looks like the framing design for glazing systems should not include the .7 factor to convert wind speed to a 10-year MRI. The section does not state that the .7 should apply and does not use the .42 combined factor. Although the 2018 IBC states this, some still follow older code references that do not distinguish between framing supporting glass versus other more ductile cladding components.
Design Details
The IBC specifies that the design windloads are according to ASCE 7 Minimum Design Criteria for Buildings and Other Structures. The wind speed maps given are based on a 50-year MRI with the probability of exceedance ranging from 1.6% to 15%, based on the building’s risk category. In other words, the more risk associated with the loss of building
function, the less probability of exceeding that wind speed in the 50-year time-frame. ASCE 7’s Commentary also gives an additional map with the 10-year MRI wind speeds which is associated with a 99% exceedance probability. This wind speed map is not mentioned or shown in the regular part of ASCE 7 or the IBC but in the Commentary supplement. In other words, using the 10-year MRI wind speed for design essentially guarantees that actual windloads on the designed framing will exceed the deflection limitations given by the IBC. Deflection of the framing members with large glass openings will almost certainly exceed what the insulating glass units (IGU) and seals were designed by the manufacturer to accommodate. This could contribute to IGU seal failures that occur before their expected life.
Chapter 24 of the IBC is associated with requirements of glass and glazing systems. Section 2403.3 Framing provides deflection limits of L/175 of the glass edge or ¾ inch, whichever is less, for framing to qualify as firmly supporting edges of glass. The wording in this section refers to loads as specified in IBC Section 1605.3.1 Basic Load Combinations which in turn indicates a factor of (.6W) or .6 times wind. We can see from these two sections that the code does not intend to further reduce the design wind pressure to a 10-year MRI by an additional .7 factor applied because it is not listed. If a designer includes this additional reduction, the frame may no longer be considered “firmly supporting” the edge of glass as indicated in Chapter 24 of the IBC at the full design wind load.
Glass and Framing Deflection
Another indicator in Chapter 24 is in Section 2404.1, “Vertical Glass.” In this section, the design of glass and insulating glass is determined by ASTM E1300 with the windload a .6 load factor on the wind pressure specified in Chapter 16 (.6W). It doesn’t mention allowing additional reduction of windload to check deflection of the glass. ASTM E1300 does not allow for further reduction of the windload to check deflection of the glass. The deflection of the glass and framing are important constraints due to the construction of the IGU, and the impact deflection has on their usable life. I contacted the International Code Council, which governs the IBC, for clarification on the subject, noting:
“Section 1604.3.7 Framing supporting glass indicates that deflections of framing members supporting glass subjected to .6 times ‘component and cladding’ windloads shall not exceed … This appears to disallow a reduction of .7 included in the note f (.70 x.60 = .42) reduction for loads associated with a 10-year mean return interval.”
The ICC responded via email:
“The IBC Section 1604.3.7 does not intend the application of a .7 reduction for framing members that are supporting glass.”
According to this version of the IBC, the .6 reduction (a conversion from ultimate loads to service loads) is the only reduction allowed for the glazing community for design windloads. Per this code, designers should no longer use a .7 factor (nor the .42 factor when combined [.6 x .7]) when checking deflection of glazing system framing members.
Stewart Jeske is president and owner of JEI Structural Engineering in Kansas City, Mo.
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