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Sizing for a residential steal beam.

Sizing for a residential steal beam.

Sizing for a residential steal beam.


As a mechanical engineer I have a tendency to question things but structural engineering is not my expertise.  I hired an architect for a full house redesign but I think his beam calculation is off.   The current beam is W8x15 with a single column which I'm looking to remove.

Single floor (bedrooms)
Span length = 18ft
Tributary width= 10ft (2x10's on 16 oc)

Using the AISI Residential Steel Beam Span tables and a 40psf for the live load and a 20psf for the dead load I'm coming up that a W8 x10 beam will be sufficient.   The architect is saying I need a W8x48 beam which seems to be a little overkill.  

Am I looking at this right or am I just dreaming about not having to change out the beam since I'm doing the work myself.

Thanks for the help,

RE: Sizing for a residential steal beam.

Looking just at strength and deflection, I'm coming up with heavier than an 8x10 (that is a very light beam), but lighter than an 8x48.  Is it only supporting one floor?  It is likely helping to support the roof as well.
Additionally, I don't do residential work so it's possible there is a more stringent deflection criteria.
Finally, vibrations should be considered.  You would hate to get woken up from a good sleep because your kids are jumping in the next room, right?

RE: Sizing for a residential steal beam.

Would you like an architect to comment on your mechanical design?

I would suggest asking him, if he works for you, you should be free to ask and he should be able to justify the choice of beam size.

RE: Sizing for a residential steal beam.

I did talked to him and he said that's what he always specs for this type of situation.  I also asked him if he had any calculations and or codes that show that is the required beam size to which he said no.


RE: Sizing for a residential steal beam.

How is the architect qualified to design steel beams?

RE: Sizing for a residential steal beam.

Sorry Steve, I did not know that you already talked to him. His reply seems unacceptable to me, but I am not sure you can do a lot. After all it is him signing the drawings, is he not?

RE: Sizing for a residential steal beam.

I wouldn't let an architect design any structure in my house!

RE: Sizing for a residential steal beam.

You should make sure you are accounting for all of the possible loads.  Are you considering any attic live load?  From ASCE 7:

attics with out storage = 10psf
attics with storage = 20psf

Also, for a new beam in an existing house I am pretty strict with deflection.  The limits I use are referenced against total load deflection.... not live load.   The last thing I want is the plaster to crack or the doors to not work because there is a deflection problem.

RE: Sizing for a residential steal beam.

First of all, architect have nothing to do with structural calculation. Hire another structural engineer while telling the first designer that you failed to provide me the design as requested.

RE: Sizing for a residential steal beam.

I'd probably use a W10x22 - LL delf is .28'' with total load defl at .43''.  While this meets code - it would not be good for a tile or marble floor.  Also, your dining room cabinents loaded with glasses might tinkle a bit when you walk by.  

Beam comes in at 39% - very conservative - but defl is the problem....

If you really want little deflection - try a W12x22 --- less than .33'' total deflection - but your clear basement ceiling height might suffer.  Or use two or three sill plates to raise the level.

RE: Sizing for a residential steal beam.

Assuming sheetrock is attached below, the deflection limit is l/360 (LL) and l/240 (DL+LL); which commonly controls the design.  In the IRC 2003, the deflection limits are shown in Table R301.7 and in the IBC 2003, Table 1604.3 (I just happen to have the 2003 on my desk, but they haven't changed).  

At a quick glance it looks like you're closer, but a little light.  Although, to have him redesign it, he will likely defer to an engineer.  You would then need to pay the engineer to design the beam; which would likely not save much expense, unless there are numerous beams to save on bulk.  

For example, a W8x48 will run you about $688 at the plant and a W8x18 would cost about $258; which will save you $430.  A nice chunk of change, but an engineer will probably charge you a minimum of $300 dollars to design and seal it, so your down to $130 savings and your Architect is not very happy with you.
Although there appears to be alot of waste, I would probably tell you to leave it alone.  Just my two pennies.
Best of Luck.

RE: Sizing for a residential steal beam.

You say that your tributary width = 10'.  This would typically indicate that you have floor joists spanning 10' on each side of the beam, is this correct?  This is not a very large area to be overly concerned about vibrations.  Using 40 psf LL and 20 psf DL, assuming no roof load, and using a LL deflection criteria of L/360, I would use a W8x18 beam.  This is assuming that you positively attach a 2"x nailer to the top flange of the beam and your floor joists are fastened to this nailer so that you have a "continuously" braced beam.

RE: Sizing for a residential steal beam.

I agree with Miketheengineer too.  It;s not all about load, moment and shear.  You have to consider deflection limits to.  It drives the size in many situations.  Looks like yours too.  I would limit the live load deflection to at least L/360, L/600 with tile floors.

Mike McCann
MMC Engineering

RE: Sizing for a residential steal beam.

Yes, the floor joists are 2"x10", 10ft on either side.  The beam is only supporting two bedrooms in which case I believe the LL can be reduced to 30psf.  The roof is a double hiped with no connectios to the ceiling joists. I'm trying to error on the high side using 40psf LL with a 20psf DL.  The current beam is a W8x15 or W8x18. The flange is 5-1/4 but the flange thickness only measures .25in.

There is currently a nailer but my plan is to through bolt two 2x8 lam beams on either side of the beam and use joist hangers so I can recess the beam into the ceiling.

Thanks for the help,

RE: Sizing for a residential steal beam.

A W8x28 gives you a deflection of about .8-.9 inches, this is way too much.  Deflection is the issue, you can't use the existing beam.  A W14x22 gives you almost a half inch of deflection . . too much.

He's acutally real close if not on with what he says (if you're concerned about deflection, which you should be).

Your existing beam deflects much more about 3/4 inch if you use it, too much.  I would never attempt to use it augmented with 2x8 glulams.

Get rid of the architect, if he cannot show you calculations, he's no good.  It's not that hard, I'd hate to think it but now you have to question if he knows the residential code in your area and what else is he winging.

You also have to question his judgement and common sense.

Get someone who knows what they are doing.

The 2x8 lams won't help.  Spend some time and look up a flitch plate, which is what I think you want to do (sort of).  They require fastening all along the beam to transfer forces properly between the steel and wood (through bolts).

You can check with your local code folks and ask them to show you residential plans (already approved) where the second floor is the only loading and the span is 20 feet.  They will show you as they are public records.  Explain your concern without naming the architect.

He's probably right on because once along time ago he paid an engineer to do the calculation but the calcs are marked with the date, owner and engineer who did the calcs.

Good luck.


RE: Sizing for a residential steal beam.


I won't even try to give you yet another opinion about an appropriate size for your beam, but am sure you are confused and dismayed by the variation.

Pay particular attention to the comment by SteelPE about installing a new beam in an existing house, and consideration should be given to preloading the beam by wedging against the floor joists.

You should get a better answer by hiring a structural engineer who is responsible for the design rather than depending on an architect or this forum.

RE: Sizing for a residential steal beam.

One more note.

Regardless of the cost (i.e. low fee), the engineer doing this calculation SHOULD make a site visit.

Anyone who does not is not taking it seriously.  They should also be taking their own measurements and photos.

They should provide you with the calculations if you ask them.

Be there when they visit, you'll learn alot.

Good luck,

RE: Sizing for a residential steal beam.

This will be my last comment: promise.

You have doubled the load on the columns at either end of this beam, they are probably currently a couple of 2x4's.  They will probably need to be increased.

Construction of this type is governed by the state building code, possibly with supplements locally.

New code was issued for NY State on 1 Jan 08.  Other states are different.

Good luck,

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