Structural Engineers: Can an aluminum I-Beam span 25 feet?

[Mindy Jensen]

Can a 25 foot long 6"x4"x.210" 6061 aluminum association I-beam can span a distance of 24 feet? It is for the top of a pergola and both beams would be supporting about 200 pounds of rafters and lattice on top.

[Brian Dance]

@Mindy Jensen

I'm a licensed structural engineer from Atlanta, GA. I have a few questions about your situation:

1. What is driving the depth of the beam? A 12" deep beam (11-7/8" LVL) for this application would be better, as you typically want about 1/2" of depth for each foot of beam span for a purgola. For a floor or roof beam a good rule of thumb is an inch of beam depth per foot of beam span. 

2. If the lattice you're adding to the top has the possibility of trapping snow, you'll need to account for a snow load. Also wise to think about perhaps if you sell the house, will the new owner add some lattice up there that will trap snow. I doubt you'd be liable if they did that and it failed, but better to not even have to ever worry about it. 

3. I did a quick back of the envelope calc on the beam and with only the weight of the beam itself and an additional 20 pounds per foot (very minimal load), it will deflect over 1". 

4. I like the LVL option more than the aluminum option. And with the alum. option, you'd need to add a nailer plate to attach the cross-beams to, which would increase the depth of the beam. If you REALLY want aluminum, I would go with an alum. box beam. 

5. For a local PE to look at the plans, size the beam properly, and even draw a few connection details (the most important piece) you're probably looking at $500-$800. He can even come look at it after it's built and if it's not done correctly, will have more "power" to get the contractor to redo the work. 

Hope this helps. Happy to answer any other questions.