

Subject:
Strength *ESTIMATE* of 2X4 Wood Beam vs I beam made from three 2X4s
Category: Science > Physics Asked by: jpsjpsga List Price: $10.00 
Posted:
02 Oct 2006 13:13 PDT
Expires: 11 Oct 2006 08:20 PDT Question ID: 770229 

There is no answer at this time. 

Subject:
Re: Strength *ESTIMATE* of 2X4 Wood Beam vs I beam made from three 2X4s
From: barnecaga on 03 Oct 2006 06:19 PDT 
"comparative strength (for equal deflection??)" doesn't really make sense, at least not to me. you might get more action on your question if you clarified what you would like compared. the stress and deflection under the same load for (3)2x4's in an ibeam vs. a single 2x4? the stress and deflection under the same load for (3)2x4's in an ibeam vs. (3)2x4's side by side? the stress under a particular load for (3)2x4's in an ibeam vs. the stress under a *much* smaller load, which causes the same deflection in a single 2x4? (3)2x4's properly nailed together are roughly 1314 times as stiff as a single 2x4, i.e. the ibeam deflects about 93% less than a single 2x4. cab 
Subject:
Re: Strength *ESTIMATE* of 2X4 Wood Beam vs I beam made from three 2X4s
From: barnecaga on 04 Oct 2006 04:12 PDT 
that wasn?t an answer, just a free comment. if that answered your question, the good news is i saved you $5, the bad news is some google researcher?s kids are going to bed hungry tonight. i wonder if the good karma and the bad karma cancel each other out? :) for a given span and a given type of wood, a (3)2x4 ibeam can theoretically carry 1314 times the load to get the same deflection as a single 2x4; 78 times the load to have the same maximum bending stress; but only about double the load to get the same shear stress. luckily, shear stress is only a concern for short spans, and wouldn?t control for a 14 ft span. keep in mind these increases assume a perfect connection between the 2x4?s. depending on the actual load, species of wood, span of the beam, etc., you might not be able to nail them frequently enough to get the entire increase. the nailing can?t be determined without this additional info, so if you need it you could repost the question and maybe feed a researcher?s kids after all. if you don?t need that info, then glad to be of help. cab 
Subject:
Re: Strength *ESTIMATE* of 2X4 Wood Beam vs I beam made from three 2X4s
From: barnecaga on 05 Oct 2006 17:36 PDT 
hi john, first off, it looks like this is your first time asking a question on google answers, so i should have explained better before. i?m not a google researcher, just a commenter and member of the peanut gallery. i can?t get paid for the answer, only researchers can do that. comments are free, but worth what you pay for them; you can?t really have the same level of confidence in a comment as in an answer. on the other hand, it?s hard to beat free. since i?ve already waded into this, i?ll finish it up for you. if we?re making guestimates, we can throw out complications like shear (which hardly ever controls) and nailing (which i don?t have enough information to consider). leaving these issues aside, it?s not that hard. i?ll give you the equations, then the answers, and you can figure out the steps in between if you want to. I = moment of inertia = stiffness of shape for deflection E = modulus of elasticity = stiffness of material for deflection S = section modulus = resistance of shape to bending P = point load L = span of beam w = distributed load Ibeam: I=72.95in^4, S=22.45in^3 2x4: I=5.36in^4, S=3.06in^3 Point load: Deflection = PL^3/(48EI) Bending stress = PL/(4S) Distributed load: Deflection = 5wL^4/(384EI) Bending stress = wL^2/(8S) E, P, w, and L are the same for all cases. I and S vary. so we can just set deflections or bending stresses equal to each other, and we can solve for how much longer the ibeam can be for the same deflection or bending stress. Point load: Deflection: Ibeam can be 2.39 times as long as 2x4 for same deflection under same P. Bending stress: Ibeam can be 7.34 times as long as 2x4 for same bending stress. Distributed load: Deflection: Ibeam can be 1.92 times as long as 2x4 for same deflection under same w. Bending stress: Ibeam can be 2.71 times as long as 2x4 for same bending stress. just remember nailing limitations haven?t been considered, and could bring some of these numbers down quite a bit. enjoy. cab 
Subject:
Re: Strength *ESTIMATE* of 2X4 Wood Beam vs I beam made from three 2X4s
From: myoaringa on 05 Oct 2006 17:57 PDT 
CAB, I really don't understand anything you have posted, so perhaps I shouldn't comment, but wouldn't it add to the strength if the 2x4s are glued as well as being nailed (screwed?) together? I recognize that gluing may be beyond DIY capability. Just a thought, Myoarin 
Subject:
Re: Strength *ESTIMATE* of 2X4 Wood Beam vs I beam made from three 2X4s
From: barnecaga on 05 Oct 2006 20:20 PDT 
good point myoarin, when i say "nailed", you should interpret that as "nailed, screwed, glued, or some combination of those". my point was that to get them to act compositely with each other, you need to think about their connection, not just the shape. cab 
Subject:
Re: Strength *ESTIMATE* of 2X4 Wood Beam vs I beam made from three 2X4s
From: barnecaga on 08 Oct 2006 13:17 PDT 
glad i could help. cab 
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