Best way to span 26' with living above

[dablack]

Ok, my building is 26' deep and 52' long.  I want living area upstairs and would like to use rough cut lumber for all construction.  I have a perimeter footer and two footers running from front to back (26' long).  These two 26' long footers are 17' from the ends of the building. 

To span 26' I'm assuming I'm going to need I central beam running the length of the building.  I'm assuming that this beam will be supported in four places (the two gable ends and at the two 26' long footers).  That means the three sections of the beam will need to be 17.33' long. 

I'm assuming a 40psf live load and 10 psf dead load for the bedrooms and a bathroom upstairs.  I'm using souther yellow pine.  I'm thinking 2x8x13' long for the floor joist.  I haven't looked it up to see if I will need 16"oc or 24" oc. 

I'm more worried about the 17' long beam and the post that supports it.  The square footage supported by this beam is 26' x 17'.  So that is 442 sqft.  But the beam only take half of that.  The other half is split between the load bearing walls (correct?).  So we are talking 221 sqft and 50 psf.  So, 11050 lbs on the beam and 650 lbs per ft of length.  With pine (fiber bending stress of about 850psi) that gives me a 8x14 beam?  Is this correct?  Is that over doing it? 

How big does the post need to be to carry that load into the concrete footer?

Are my assumptions ok? 

thanks
Austin

[Jim_Rogers]

I've got to get outside and cut some lumber, but I took a few minutes and ran the numbers you provided through DonP's calculator for grade #2 SYP and this is what I got:



 

Hopefully we can read this drawing.

I have attached a pdf of it as well.

Jim Rogers

[Jim_Rogers]

Later on I'll look at the post size.

Jim Rogers

[dablack]

Thanks Jim,

The beam span tables I could find only went up to 13' long so I was having to guess at the 10x14.  I'm going to have to save my biggest pines for these beams.  Most of my pines only have a 15" diameter and that won't make it.  Maybe I could put in some braces to shorten up the span. 

Austin

[Jim_Rogers]

When we look at the size of the post needed to hold up a structure we look at two things.
One is the end grain of the post. As wood is very strong in compression this usually isn't a factor.

For example the beam is already a 10" wide beam so we'd use a 10" wide post. But if we made the post a 10 x 10 then that would be 100 sq inches. Each sq inch can hold up 525 lbs so that's 52,500 lbs that size post could hold up. Way too big.

We next look at the side of the beam, as it also has to hold up the weight/load. The side grain of the beam can hold up 375 lbs per sq inch.
So 5633.34 Lbs / 375 = 15.02224 sq inches.
So the post needs to be at least a 3x10 on the end. extra for margin of safety.
And the post in the center would need to be a 6x10 at least.
So that each beam can rest on 3x10 inches of the post.

Attached is another updated version of the drawing; stating this and also an end view of a log needed to make a 10x14 beam.

Jim Rogers

[dablack]

Jim,

That makes perfect sense.  Are the sq-in capacities specific for pine or are they a general rule of thumb. 

thanks
Austin

[Jim_Rogers]

The values I mentioned are specific for Southern Yellow Pine SYP. Every type of wood has their own values.
You have to look them up in the NDS (Nation Design Specifications) book, and see what they are.

Jim Rogers

[barbender]

Dablack, in a lot of my building I was always trying to make things work with what ended up being not big enough trees to start with. I'd look into shortening up your spans if possible. Or if you want a large open area (a living room for example) you would only need one large beam. My longest span was a 14'6", I ended up using a 12"x8" Red Pine timber that's what the numbers called for. If you need a 12" timber I'd say you'd need a log with a 14" TOP.

[Raider Bill]

I used engineered I beams to span 24x40. Not free like trees but really not that expensive and actually I was surprised at how little they cost.

[grweldon]

As mentioned in Jim Rogers drawing, a 2x8 at 13' span fails, most likely in deflection.  My calculations show that it passes (full 2x8... Jim probably used a dressed 2x8 in his calcs), but just barely with .395" deflection (50 PSF load) and 1/360th deflection is .425".  The fiberstress in bending is right at the limit as well.  These calculations assume 16" OC spacing.

I built a 24x24 cabin using 2x8s with a span of 11'6" and even though it passed the stress and deflection calculations, it is a bit spongy to walk on.  If you are going to be cutting your lumber for the floor joists (you said you were using rough sawn) I would use a full 2" x 10" joist.  This give you a safety margin of about 40% and a deflection of just over 3/16" (.202) at mid-span under load assuming 16" OC.

The "design for bending" calculator found here is a great tool and can be very helpful.  The design values I'm using are for #2 SYP, 850 PSI max fiberstress, 1.2 Million PSI MOE and 165 PSI max shear.  My calculations were a span of 153" (13' minus 1.5" at each end for bearing).  The wider the bearing surface, the better.  1.5" is a minimum.

Hope this helps some.

[dablack]

I used engineered I beams to span 24x40. Not free like trees but really not that expensive and actually I was surprised at how little they cost.

You are correct.  They aren't crazy expensive.  I'm coming up with about $2500 to do what I need to do with Ijoists.  The problem is that there is more to this than what I have shown.  I'm also having trouble with roof loads pushing out my 1' or 2' pony walls.  I'm balloon framing the 2nd floor at 8' and will have 11 or 12' exterior walls to give me as much head room upstairs as possible.  The solution to this is a ridge beam or trusses.  I can't afford trusses but a three piece beam is possible and can be cut from my trees.  Now I just need to figure out the size of the ridge beam and make sure my slab can handle that load.

thanks
Austin

[dablack]

As mentioned in Jim Rogers drawing, a 2x8 at 13' span fails, most likely in deflection.  My calculations show that it passes (full 2x8... Jim probably used a dressed 2x8 in his calcs), but it is a bit spongy to walk on. 
Hope this helps some.

Yes, I noticed that too.  I have never liked passing by the skin of my teeth.  Yes, I will be using 2x10s or maybe 2x8s 12" OC.  Not sure which way would be better right now. 

thanks
Austin

[dablack]

Ok, I'm having some trouble deciding what would be the appropriate live and dead load for the roof.  No snow load so I'm assuming a live load of 10 PSF.  For the dead load we are talking 2x8 roof rafters, fully sheathed with 1x material, a metal roof and sheetrock on the inside.  Also, what about strong winds?  We are at the top of the hill.  I want this thing to last. 

thanks
Austin

[dablack]

Looks like my roof is 45 psf.  15 for the dead load, 15 for live and 15 for extra wind.  So the ridge beam will be sized the same as the beam holding the 2nd floor.  Now I need to check the compressive strength of the 1st floor posts. 

[dablack]

From running the numbers like above, a 10x10 post will work on the first floor and a 6x10 post will work upstairs. 

Anyone have any thoughts. 

I'm going to have to start looking into how these 17'4" beams are going to be hooked together and how they will be secured to the posts.  Also, I'm assuming the upstairs post will go through the floor and sit on top of the two beams below it.  How will the upstairs post be attached to the down stairs beams?  I'm worried about uplift. 

thanks
Austin

[Jim_Rogers]

When I ran the numbers on the floor joists, full dimension 2x8, I used a value from the NDS book that may have been the wrong value. I'm so use to using the values from the timber framing section that I think I used those values.
I most likely should have used the values from the 2by section of the book.
Those values are different I think.
I don't have time now to run the numbers again, but I will do it sometime later on.

Jim Rogers

PS. You can always run metal straps from one post up through the floor to the side of another upper posts to hold the two together. And cover with some trim boards if they are going to show.