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Thread: Span tables for structural aluminum beams? Design/construction ideas?

  1. #1
    Associate Engineer
    Join Date
    Jan 2019

    Span tables for structural aluminum beams? Design/construction ideas?

    As a licensed GC in residential construction, I consulted building code span tables almost daily. The tables included 3, 4, sometimes 5 species of wood, 3 or 4 lumber grades, and values for floors, ceilings, flat roofs, shallow pitch roofs, and steep pitch roofs. I can find no such tables for aluminum structural members. If such a creature exists, please share a link.

    I am trying to design a solar carport with a shallow pitch roof, that utilizes the panels themselves as the roof surface - why build a roof to poke holes in it and cover it with plastic sheets that are warranted for 25 years? The concept is ultra-simple, like an oversized Erector Set: four 6x6 posts in the corners, an I-beam/L-beam header at the high end and low end, and the rafters resting on the header beams and set 39" apart, the width of each panel. Open span on the driveway side (east) is max 21'8" (measured horizontally) which is also the span for each rafter. The header beams would have a max span of 17' 8". Lower end cantilevered 9" beyond the outside of the poles/15" beyond the header beam. Upper end cantilevered 16"" beyond the outside of the poles/22" beyond the header beam. Total rafter length is max 25'. A third vertical post could be set midway on the closed (west) end of the carport for added stability.

    The panels are 39.4" x 66.3", 42 lb each and there will be 20 of them. I am estimating about 600 lb of edging, end caps, fasteners, wiring, etc. in addition to the weight of the panels, beams, and rafters. I envision using large dimension aluminum C-beams as rafters so that only one thru bolt is required to secure the panels on either side - an aluminum cap with bolt though it in a "T" config. I-beams could be an alternative (think boat trailer rails), even though they might require more drilling and more bolts.

    Additional details such as mounting to floor surface, cross bracing, etc. will be addressed as the plan evolves, but for now.......

    1) Is there a span table or something similar that I can use for material sizing?

    2) If no such document exists, what size primary components (rafters and beams) do I need? What rafter profile would give the best strength vs cost and size?

    Thanks for any insights, ideas, caveats, suggestions, etc. BTW, I have contacted three structural engineers, all of whom said they wanted to work on this project, none of whom ever made an attempt to visit the site.

  2. #2
    Associate Engineer
    Join Date
    Jan 2019

    Take 2

    After 2 years of trying to design the carport described above, I have abandoned the "solar panel as roof surface" concept in favor of an aluminum roof frame, with trapezoidal aluminum structural roof panels, with the solar array mounted on top in the standard configuration. In all that time, I have not been able to find any sort of span table for structural aluminum components. I CAN find such information for steel I-beams, but nothing for aluminum.

    It's like an elevated stage, held up in the four corners by posts, with an aluminum I-beam running up the rake on each side (N<->S), still 21' 8" open span. I imagine seven or eight C-beams running across the face of the slope, still 17' 8" span (E<->W) attached to the I-beams such that it is a flush roof frame system, slope ~1.6/12. Total weight of beams, roof surface, panel mounting rack and hardware, and solar panels is about 2850 lb, distributed evenly up and down, side to side.

    I understand that there are many other components that need to be sized appropriately, and a host of design details need to be considered, but I just want to know about how big these beams need to be. 6061 alloy in T6 temper is the most easily sourced material. Can I get some input, please? Thanks!

  3. #3
    Currently, there are no standardized span tables available for structural aluminum beams akin to those for wood. However, for your solar carport project, you can employ engineering principles to determine suitable beam sizes. Considering your design, aluminum C-beams or I-beams can serve as rafters. The selection should prioritize strength, considering the weight of the panels and additional loads. Given the 21'8" span, opt for beams capable of supporting distributed loads, factoring in cantilever lengths. Consult structural engineering resources or software for load calculations to ensure safety and compliance. For the best strength-to-cost ratio, evaluate various beam profiles, favoring designs capable of handling distributed loads efficiently. Since you've faced challenges with engineers, consider seeking recommendations from peers or reaching out to structural engineering associations for referrals to professionals experienced in aluminum structures.

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