Portions of the uniform load considered to be applied to the overall product are assumed to act upon each member in a triangular or trapezoidal loading profile, or as a concentrated load. Formulas are given for calculating the bending moment for these loading patterns, and for both loads concentrated at the center of the member and for loads concentrated at any off-center point along it. Diagrams of 15 different example cases of load distributions upon window and door framing members, based on different framing configurations, are given—square, oblong, and oblique using two-lite, four-lites of equal size, four unequal lites, two lites with an integral transom, and two main lites below four-lite transom configurations. The proper treatment of load deflection is discussed for each different configuration.
Finally, formulas are provided for determining the maximum deflection of a framing member of a given length subjected to bending from a uniform load. This takes into account the previously calculated area moment of inertia and modulus of elasticity, as well as—in the case of a trapezoidal loading configuration—an appropriate moment coefficient.
The maximum permissible deflection of a framing member is typically expressed as a fraction of the length (L) of its unsupported span. Under NAFS, the total maximum deflection cannot exceed the limitation of L/175. Note, the calculated deflections cannot exceed those set forth in the referenced performance standard for products with framing spans longer than those of the tested version. Also, the span-to-deflection ratio of the calculated unit cannot be less than that of the tested specimen. This deflection is subject to the limitations imposed by AAMA TIR-A11, Maximum Allowable Deflection of Framing Systems for Building Cladding Components at Design Wind Loads, or ASTM E1300/CAN-CGSB 12.20, Standard Practice for Determining Load Resistance of Glass in Buildings. The glass edge is considered to be supported, as set forth in ASTM E1300 or CAN-CGSB 12.20.
To analyze profiles with complex multi-element cross-sections, the entire section should be divided into any number of simple subsections to calculate the section elements of each, tabulating the calculations for all subsections.
The analysis procedure for each subsection unfolds as follows:
- Record the width and depth of each subsection, which when multiplied together, yields the area of each subsection.
- Locate the center of gravity of each subsection and record its distance from its defined “0-0” axis.
- Calculate each subsection’s area moment of inertia about its own center of gravity.
- Total the results for all three subsections to provide the total area, first moment, and area moment of inertia of each subsection about its own “0-0” axis.
- Calculate the position of the neutral axis (“X-X”) of the entire profile and, in turn, the greatest distance from the neutral axis to the extreme fiber can be calculated.
- Calculate the area moment of inertia of the entire profile about the “X-X” axis.
- Calculate the section modulus based on the results indicated above.
Be aware: formulas are provided for all the referenced calculations in AAMA 2502.
An example five-subsection profile is given, showing the calculation of the area moment of inertia of each subsection about its own center of gravity and about the “0-0” axis for the complete profile. These are totaled from the tabulation to yield these moments for the entire profile. Formulas are given for calculating the moment of inertia of the entire profile about its defined neutral (“X-X”) axis and for calculating the overall section modulus.
The comparative analysis procedure in AAMA 2502 is especially suited for code jurisdictions where it is desirable to document the performance of each window and exterior door size to meet specific structural DP criteria. For window and door manufacturers, the procedure provides a uniform approach for dealing with different code jurisdictions and specific DP for each size of fenestration product openings.
Aaron Blom joined the Fenestration and Glazing Industry Alliance (FGIA) in 2021, and is responsible for conducting and developing the association’s professional education programs. He began his career in 2007, as a technical representative for an aluminum framing systems manufacturer, and represented a major fenestration producer, where he focused on the specification and sale of fenestration systems for monumental commercial construction projects. Blom also has worked for a multidivisional specialty subcontractor. He can be reached at ablom@fgiaonline.org.
