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Comparison of wood beam to steel ibeam

ntatcu's picture

Comparison of wood beam to steel ibeam (post #215131)

I have a wood beam in the basement of my house whch has concerned me since bought is. The deflection is noticible and I would like to replace it with a steel ibeam.

the existing beam is (4) 2x8s with a 13' span. I am proposing a standard W6x15 steel ibeam to replace it.

If I am correct, the steel beam will have a significantly higher load capacity from the existing wood beam. That said, I am clearly not a structural engineer and I am hoping that someone more qualified can confirm that this is accurate and give me an idea of how the two compare in load capacity.

Not knowing anymore than I do (post #215131, reply #1 of 9)

Not knowing anymore than I do about actural loading and conditions and assuming that that the existing lumber beam is supporting it's maximum allowed / designed load, I think that you will still have a noticable deflection with the steel beam.  But don't take my word for it.  I'm not there.

You really need to have someone (like an engineer type person) look at your situation.

I had a similar situation. I (post #215131, reply #2 of 9)

I had a similar situation. I replaced three 2x10's that had severe termite damage and noticible deflection with the equivalent in LVL lumber figuring the lvl's would be far stronger than the original  dimentional lumnber. And wouldnt you know it the darn LVL's deflected under the load. I ended up adding flitch plates to the outside of the LVL's (not ideal they should be sandwiched in the middle) 

I would definitely have an engineer provide calcuations and a written recommendation it will cost you some but you get what you pay for.

Odd.  I replaced 3 2x14s (post #215131, reply #3 of 9)

Odd.  I replaced 3 2x14s with two 13" LVLs (which were 2 1/4" thick, IIRC) and got significantly less deflection.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

How long are the floor joists (post #215131, reply #4 of 9)

How long are the floor joists that the beam is supporting?

Is the a wall above the beams putting in addtional loads for second floor or roof?

Look at the span tables for the beam you want to use, with the structural arrangment you have.

If there is considerable deflection on the wood beam, and it is not damaged, then the beam is seriously undersized.  Replacing with a steel beam, which will be stiffer, may not solve the problem if it too, is undersized for the load.

Check the span tables.

Are you experiencing some (post #215131, reply #5 of 9)

Are you experiencing some noticeable effects such as doors not closing property, cracking wall finishes or bouncy floors.

How much deflection are you talking about? 

If it is not over say 5/8", that is well within allowable for that beam span.

If it ain't broke, don't fix it/

I appreciate everyones' (post #215131, reply #6 of 9)

I appreciate everyones' responses. Is it not possible to give me a comparison of the two beam's load carrying capacities simply supported and evenly loaded for the span indicated?

Let me put on my professional (post #215131, reply #7 of 9)

Let me put on my professional engineers hat...

It is possible to give you the stiffness and strength ratios of the wood vs steel beam.

That would be irresponsible of me. 

You are really asking if the steel beam you chose is adequate for the job. Comparing the relative stiffnes of the two beams is not addressing the real question, which is ' What is the required size of a replacement beam to carry the loads in the building, with sufficient stiffness.

The correct process is the determin the required beam size according to your local building codes. That will be the minimum required, regardless of what is there now or the relative stiffness of the two beams.  If you want a stiff floor, go one size up from what the building code requires.

You can get an engineer to calculate the required size, or you can use span tables from the building code.  Span tables are easier and less expensive.

Changing the beam is a structural modification to the house.  You will likely need a building permit.  Talk with your local Building Department.  They will have the codes and span tables there if you do not have other access to them.


Catmandeux is spot on! (post #215131, reply #9 of 9)

Mr Catmandeux is correct!

Please answer a few questions. 1) Is the offending beam in the basement?  If not, where is it (in the first floor...the second floor)?  2) If the beam in in the basement (supporting the first floor and, possibly, more), does the basement /crawlspace have a concrete floor that is reasonably sound?  3) Are there lots of obstructions around the beam? 

I think your first concern should not be about the relative stiffness/bearing of beam materials. Rather, you should be looking at ways to remedy your situation without damage to the house! For instance, all that you may need to do is prevent further sagging. You MIGHT be able to straighten the beam with one or more steel jackposts. There are other issues you must contend with. For instance, you could crack plastered walls above the beam as you jack up, remove the beam, or otherwise try to remedy the problem. In sum, you will need to take several factors into consideration. Relative beam strength may not be at the top of the list. Last but not least, installing a new beam can be tricky...and expensive.

Mel Fros

Go buy a 12 ft long piece of (post #215131, reply #8 of 9)

Go buy a 12 ft long piece of 1/8" thick 6" wide steel, should be only about $20.00

Drill 5/16 (NOT 3/8" or even 11/32) holes 3/4" in from each edge and 1.5 inch from each edge, stagger teh 3/4" and 1.5" holes,  Near center of beam space the holes 18" apart lengthwise, then progressively place them closer together towards the ends till the spacing is only 5" at the ends.  e.g 10" from center, 28" from center, 44",  56" , 66",  71".    

Tack nail the 3" wide steel on the BOTTOM edge of the 2x and put a jack under the steel, jack the beam up until about 1/8" to 3/16 higher than flat (or until you hear palster crack in any wall above).  Flitch beams (steel on the side of the 2x8s) are a waste of steel.

Then, starting from the center, BOLT (not nail) the steel plate to the existing 2x8s with 48 total 2-1/2" long lag bolts.  Remove jack.

Search for comosite beams if you want to know how that works.  Deflection will be about 1/8 to 1/4 of the present deflection depending on how tight the 5/16" lag bolts fit the holes (sloppy = more deflection).  Strength about doubles.

Once you find online calculation for lateral shear in built up beams, you may be able to decrease the # of bolts if yu do the calculations, I just did the above #s in my head. 

PS:  buy a cheap $50 HF drill press if you dont have a drill press, drilling 48 holes is LOTs faster with a drill press.