Author Topic: i beam vs. 2x8s  (Read 4405 times)

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Offline Paperdave

  • Posts: 1
i beam vs. 2x8s
« on: June 09, 2016, 07:07 PM »
Good evening, I am new to this. I am looking to replace doubled 2x8s with a 4 inch I beam. The span is 13 feet. Can someone assist me with the specs I need on the I beam? It needs to be no more than 4 inches in diameter.

Thank you!

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Offline ear3

  • Posts: 3594
Re: i beam vs. 2x8s
« Reply #1 on: June 09, 2016, 07:17 PM »
Welcome.  Post might be more appropriate for "Members Project" or "Home Improvement" subforums (no worries though), but could you give us more specific info about the application you're talking about?
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Offline bkharman

  • Posts: 1983
Re: i beam vs. 2x8s
« Reply #2 on: June 09, 2016, 11:44 PM »
Good evening, I am new to this. I am looking to replace doubled 2x8s with a 4 inch I beam. The span is 13 feet. Can someone assist me with the specs I need on the I beam? It needs to be no more than 4 inches in diameter.

Thank you!

You really should talk with a structural engineer local to you. They will give you all the details you need to know.

Not something to take lightly.

Cheers. Bryan.


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Offline tjbnwi

  • Posts: 5701
  • Cedar Tucky Indiana
Re: i beam vs. 2x8s
« Reply #3 on: June 09, 2016, 11:50 PM »
I assume these are load carrying 2x8's. Not going to happen.

As Brain said-go see a structural engineer.

Tom

Offline Cheese

  • Posts: 4920
Re: i beam vs. 2x8s
« Reply #4 on: June 10, 2016, 02:02 AM »
I'm a huge fan of working in alternative construction materials. I consistently substitute aluminum for wood because it can give you enhanced physical properties while allowing for a thinner/smaller visual profile. That being said, you really need to be able to work through the stress/strain numbers yourself or better yet, chat with a structural engineer and have him work the numbers for you. Also, the coefficient of thermal expansion is something that you have to keep in mind if you are working with aluminum.

Offline Kev

  • Posts: 7651
Re: i beam vs. 2x8s
« Reply #5 on: June 10, 2016, 03:15 AM »
That's an interesting first post for a forum like this!

I'm really curious why you'd choose the FOG to ask this question.

Unless you can determine the load at the various points on the beam, know what's acceptable for deflection, etc you are far safer engaging a professional. Further, a lot of people (regardless of whether they choose the right type of member for the job) come horribly unstuck at the installation point.

Please be very careful.

... and [welcome] to the FOG

Offline Holmz

  • Posts: 4010
Re: i beam vs. 2x8s
« Reply #6 on: June 10, 2016, 04:18 AM »
...
Not something to take lightly.
...

 [wink]

Are you looking for stiffness or strength?

You can probably find a table for loads for 2x8s, and two of them = a 4x8.
The centre 4x4 of the 4x8 does not do much for the stiffness, compared to the outer parts.

A 4x4 for the top and bottom requires no web, so I suppose a 2x4 would require a 4" web, but to account for the lost centre a 6" web should be about the same stiffness.

https://en.m.wikipedia.org/wiki/Bending_stiffness

Offline rizzoa13

  • Posts: 582
Re: i beam vs. 2x8s
« Reply #7 on: June 10, 2016, 04:55 AM »
Why don't you recess the current doubled 2x8s into the ceiling above? You already know that a doubled 2x8 will carry the load so there's no sketchy business with trying to spec a different beam.

That said its a simple job for a carpenter but could be well beyond the skill level of a diy'er so assess whether your capable before taking anything down.

Offline albertisakov

  • Posts: 6
Re: i beam vs. 2x8s
« Reply #8 on: June 11, 2016, 11:43 PM »
not enough info what you carrying?
1 floor 2 floors 3 load bearing?
hows the footing? 
from a typical stand point you can carry that with few LVLs and a microlam posts (4x4)(for a load bering wall)
from here on down its get technical, psf , live vs dead load leaving area vs bedroom etc this is how you calculate
if you do the work on you own make sure you do it right and do the research first or it can cost you twice more
if not more.
here in mass this is the min guide line hope this help you

http://www.mass.gov/eopss/docs/dps/inf/780cmr/780055.pdf

 you sure always want to exceed it when you can

good luck
« Last Edit: June 11, 2016, 11:51 PM by albertisakov »