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.

[Nate Aden]

Good evening Mindy,

Usually if you call the supplier, they should have a table that they could send you that provides load capacities if they can't tell you right off the bat. I typically am working with suppliers of red iron beams, but I would imagine suppliers would still have something.

https://www.engineeringtoolbox... is a great free resource for information on different types of materials.

Hope this helps at least some!

- Nate

[Todd Rasmussen]

@Mindy Jensen

200 lbs each beam or total load is 200 lbs? Asking for a friend.

[Mark Fries]

@Mindy Jensen

Yes

[Mindy Jensen]

Originally posted by @Nate Aden:

Good evening Mindy,

Usually if you call the supplier, they should have a table that they could send you that provides load capacities if they can't tell you right off the bat. I typically am working with suppliers of red iron beams, but I would imagine suppliers would still have something.

https://www.engineeringtoolbox... is a great free resource for information on different types of materials.

Hope this helps at least some!

- Nate

I would think the manufacturer would have this info and willingly share, but they don't. The store said they've been trying to get this info forever, and the manufacturer is super cagey about it. (This for a gluelam) so I don't want to use them. 

Thanks for the link to that site. I'll check it out. 

[Mindy Jensen]

Originally posted by @Todd Rasmussen:

@Mindy Jensen

200 lbs each beam or total load is 200 lbs? Asking for a friend.

Total load

[David M.]

@Mindy Jensen

As a non-licensed engineer, at first glance I'd say "yeah, sort of."  200# loading of other rafters and what not is fine, especially spreadout.  I'm mostly more concerned about the end connections and in poor weather conditions (e.g. a strong summer squall or a hurricane) if it will stay up.  The mfg won't produce a "load rating chart" since there are too many load combinations to worry about.  Thats why you have engineers and architects.

From your other posts, are you trying to use this Al section in lieu of a wood gluelaminate piece?  If you know the latter will work, can you give me the info on that?  I can lookup (I don't think I can calculate it anymore) and compare the section properties and tell you how the Al piece compares.

[John Pearson]

Once you decide on materials and start construction,,DO NOT work on top of structure, it won’t support the live load of human.

[George W.]

200 pounds of rafters sounds a little light for a 24' span. What size rafters are you using? Also have you considered any additional weight of snow on that might accumulate on the lattice and the rafters? It wouldn't be much provably but not 100% sure. 

I'd seriously be careful and have a local engineer figure that out for you to avoid someone getting hurt. 

[Jerry M Cutrona]

Consult a structural engineer for other material choices. A 6" deep aluminum beam for this application doesn't sound promising.

[Mindy Jensen]

Originally posted by @David M.:

@Mindy Jensen

As a non-licensed engineer, at first glance I'd say "yeah, sort of."  200# loading of other rafters and what not is fine, especially spreadout.  I'm mostly more concerned about the end connections and in poor weather conditions (e.g. a strong summer squall or a hurricane) if it will stay up.  The mfg won't produce a "load rating chart" since there are too many load combinations to worry about.  Thats why you have engineers and architects.

From your other posts, are you trying to use this Al section in lieu of a wood gluelaminate piece?  If you know the latter will work, can you give me the info on that?  I can lookup (I don't think I can calculate it anymore) and compare the section properties and tell you how the Al piece compares.

We were also thinking of sistering two of these together: https://www.menards.com/main/building-materials/trusses-i-joists-engineered-lumber/laminated-veneer-lumber-lvl-strand-lumber/1-3-4-x-7-1-4-lvl-1-9e/1061006/building-materials/trusses-i-joists-engineered-lumber/laminated-veneer-lumber-lvl-strand-lumber/1-3-4-x-7-1-4-lvl-1-9e/1061161/p-1444438527854.htm

We'd have to get a treated version or wrap them to protect from the elements.

[David M.]

@Mindy Jensen

Well, trying to find mechanical properties on the laminated veneer lumber (LVL) is quite difficult as I was guessing.  But, I found a bit here:  
https://www.awc.org/pdf/education/mat/AWC-MAT230-Glulam-2hr-150530.pdf
The aluminum I-beam properties are well documented here:
https://www.aluminum.org/sites/default/files/ADM2010Errata2.pdf

In short, one can't make a definitive conclusion.  I assume you are posting your question as you are not looking for a "stamped" design, otherwise you would have paid an architect or structural engineer.  I can tell you the aluminum is far stronger than the gluelam data that I found:  the company presentation is reporting some strength value at 2400 psi while 6061-T6 is ~40,000 yield / ~45,000 ultimate.  The aluminum, as a material, should be much stiffer.  In the physical arrangement as an I-Beam, the aluminum is far more efficiently used but not as stiff with its bending modulus at 22 to 25 in^4.  I calculated the LVL beam to be 55 in^4.

As a whole, you may see the aluminum beams bending/deflecting more with the wind, but I doubt they will break.  But, presumably people will be around this so you really should have this properly designed.

Just another way to look at this...  If you pictured putting an aluminum extension ladder across the top, do you think it would fail?  The rungs of the ladder are analogous to your rafters.  ...Anyway, you get the idea.  Be safe.

[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.

[Eric Teran]

@Mindy Jensen definitely follow @Brian Dance advice. Everyone is talking about the beams but what about the posts? In your sketch only one post is above a footing. The other three are just hanging in between joists. How are those going to be attached? The loads from the beam will travel to the post to the joists to the footings. That is why structural engineers start at the top of a structure when doing their designs. They have to know all the loads to design the foundation correctly. I would make sure the base of the posts are attached correctly.

[Mindy Jensen]

Originally posted by @Eric Teran:

@Mindy Jensen definitely follow @Brian Dance advice. Everyone is talking about the beams but what about the posts? In your sketch only one post is above a footing. The other three are just hanging in between joists. How are those going to be attached? The loads from the beam will travel to the post to the joists to the footings. That is why structural engineers start at the top of a structure when doing their designs. They have to know all the loads to design the foundation correctly. I would make sure the base of the posts are attached correctly.

Ah, yes every support beam has its very own cement footing, they're just not in the drawing. My husband and I are doing this work ourselves and we just poured the concrete last weekend. This will be our third structure like this, but the first with the overhead beams. I'm 100% certain with the foundation, it's the overhead I want to double/triple check. 

[Andrew S.]

Adding 45 degree angled gusset supports 1', 2', or 3' (whatever doesn't protrude too much) out horizontally will stiffen it up.  Could also extend up the vertical posts a couple feet above the beam.  Then add cable with a turnbuckle towards the center of the beam to hold it up a little bit. 

[Patrick McCauley]

At a quick glance I do not think a 6" aluminum beam would work under deflection/bending. Along with the dead load of the framing and lattice, you need to take into account other forms of loading such as snow load, roof live load, and wind load to see which combination produces the worst load effects. It is also important to verify that connections to the posts are adequate for the required loading as member failure is not a catastrophic as connection failure. When in doubt, it's best to consult a licensed engineer.

[Sam Ray]

Hi @Mindy Jensen, could you share what you ended up doing this for your project? I'm in the similar situation like you and have 20-ft span to cover. I would really appreciate if your reply and/or any suggestions. Thank you so much.

Regards,

Sam