Joist and Girder spans

[Ashful]

Converting an old three-bent (two bay) post and beam carriage barn to a work shop, and looking to install a second floor. Existing structure is three 20' bents on 15' spacings, so to keep joist span reasonable, they will be running parallel to the ridge as two 15 foot spans, riding on three girders perpendicular to the ridge.

Photos and sketches for reference:





Sketches of existing structure:



So, plan is a load-bearing framed wall on each end, supporting a new 2 x 10 rim joist, and a girder down the center, to carry the new floor joists, as sketched:



Total span of center girder is roughly 19 feet, minus whatever new jack posts are placed under the ends. The big question and/or goal, is eliminating the need for a lally column in the middle of the shop.

Floor width is 28 feet by my figuring, as two joist spans of 14 feet each. Standard residential girder tables (interior 2308.9.6) put the max span of four 2 x 10's at 8' - 9" with a 28' floor width, meaning I'd need a lally column mid-span. I know of no tables calling out a solid lumber girder which will span anywhere near 19 feet. Truthfully, I could easily cut the span down to 17 feet, by moving the jack posts (end columns) in 1 foot each (gives better room for the supporting piers, anyway), but even that seems to be off the standard tables.

Are there any structural guys out there? What span might four 2 x 12's get me on a 28 foot building width (single floor interior)? My father was a structural engineer, and I had some exposure to it, but I'm in over my head here.

Adding to the mix is that there's already a 6" x 6" DF beam in the location of that girder. Seems a 6 x 6 doug fir exceeds two 2 x 10's in Vr and Mr (a little short on E, though), so I'm hoping to keep it there, and sister the new members on either side.

Thanks!

 

[Beetle-Kill]

I'm not a structural engineer. That being said, I'd guess you're going to get a substantial floor bounce without that column. You could add a flitch plate with a sistered joist and thru-bolt it, but I would suggest the column. Or replace it with a stout W-beam and pack it out. A 3" sch. pipe column is only 3.5" OD, add an adjustable cap to it. It's pretty easy to get used to and work around. Plus you can use it as a work station. My chain grinder is mounted to one of the two I added in my garage, and I have nowhere near the room you have.

 

[Ashful]

Heh... this shop will be packed TIGHT! I have a small army of very large, industrial, antique woodworking equipment.

I was staring at the joist and girder tables tonight, and came up with "plan B". Rather than a 19' girder down the center (front-to-back) supporting 14' joist spans (side-to-side), I could just run the joists front to back. Depending on how I frame in the front and rear walls, the span would be roughly 18' - 6", which is doable with 2 x 12's placed 12" OC. Spanning the two 10 foot garage doors will be fun (a four 2 x 12 header over each!), further complicated by the fact that I don't trust the floor / footer in that area... but it's the beginning of an alternate plan, nonetheless. It also partially solves my issue of too-little clear headroom on the second floor.

 

[Beetle-Kill]

How much weight are you planning on the upper level?
You could add a piece of channel across the front to help with the rigidity of the existing beam.
I work for a steel fab. shop, we do things like this all the time, but with the engineers approval of course.

 

[Mr A]

I'm, not a structural engineer, but you can hire one for few hundred bucks to come out and come up with a design for exactly what you want to do. Then, you can hire just about any decent carpenter to build according to plan, or do it yourself. If you hire a contractor, they will probably call an engineer to tell them what to do, you will probably need an engineer's stamp for a building permit anyway.. Depending on what you already have, you may need to strengthen your foundation, probably not as your concrete foundation probably is already below the frost line, your foundation would crack from expansion of frozen ground if it wasn't.

 

[Ashful]

Thanks, guys. Permit is not an issue around here, still being mostly rural and township management being decidedly relaxed on most matters. Our code enforcement officer (a retired "gentleman" farmer and large property owner himself) usually advises, "don't ask, don't tell," on barn and garage projects such as this. That said, there's no reason to not do it right, structurally.

I'll have to ask around, and see if I can locate a structural engineer willing to take on such a small job. My father was a structural engineer, but aside for odd jobs for friends and family, he was always working on much (much, much) larger stuff. Not looking to make a mountain out of a mole hill, and it's always been my impression you could not call a structural engineer without spending at least four figures.

As to the weight, I've spent a lifetime working in (and on) old carriage barns. The weight that goes upstairs in an old barn is a function of what your senses tell you the structure can bear. Circular logic, I know. But, when you've always lived and worked in 150+ year old structures, you get a real good feel for floor bounce, and develop your own comfort level. I was planning to meet or exceed the standard 40/20 live/dead load tables (residential living spaces) for girder and joist sizing, and leave it at that.

In my last barn, the floor width was 19 feet, and the girder span was 17 feet. We just sistered three 2 x 12's to the existing 6" x 6" x 17 foot girder on each floor (2 floors), and it has worked well since 1990, with no signs it won't be the same in 100 years. The floor would get a little bounce to it when I'd put something real heavy in the middle of the room, but that's what I'd use as my barometer, for when to stop moving weight upstairs.

 

[ironpony]

first I would get away from dimensional lumber and go to micro lams and TJI's your spans will be larger with less deflection. I would also be more concerned about footer/slab construction at your end point loads. also the walls carrying the new floor are probably weak points compared to the new structure.

 

[Ncountry]

I am sure a micro lam beam down the center can be found. I do not have the program but it seems Like the last one we did with a similar span was a trippled up 14" micro lam. Floor trusses are also made to span the full 28' also. Sometimes it is less expensive to buy the floor trusses than to engineer a beam and support it properly at both ends.

 

[Ashful]

Okay, guys. Putting a call into a structural guy tomorrow.

Thanks!

 

[Ncountry]

Okay, guys. Putting a call into a structural guy tomorrow.

Thanks!

I have a small truss co. Not far from me, they have all the engineers and programing etc.. to figure these things right up for 0$

Maybe you have one in your area.

 

[Mr A]

first I would get away from dimensional lumber and go to micro lams and TJI's your spans will be larger with less deflection. I would also be more concerned about footer/slab construction at your end point loads. also the walls carrying the new floor are probably weak points compared to the new structure.

Yea, good building starts with the foundation. If I was doing it my self I would dig around where you want o place a post, determine your existing concrete. 19 foot spans are routinely used with TJI joists. I can guess you have a lot a lot of concrete at the perimeter foundation by your location and the expected frost line. Can't go wrong with a 6X6 post. The lumber span tables should fill in the rest

 

[stee6043]

I just went through a very similar situation in my basement regarding load bearing walls/beams/etc.

The lesson I learned was this - your local lumber yard will be a HUGE help. I'm not talking HD/Lowes, I'm talking a true lumber yard. I walked into my local lumber yard after spending a few weeks trying to figure out how to properly support a floor (pre-structural engineer). The guys at the lumber yard knew exactly what I was doing and even called the LVL beam manufacturer while I was standing there to get their structural guys to recommend a beam member.

As stated above I'd ditch your plans for dimensional lumber and find a local yard that can supply LVL's. I bought some rather large units and they were only $5 a foot +/- (I used 3x so call it $15 per foot of finished beam). Having access to the manufacturers engineers was priceless and painless.

 

[Mr A]

Yes, engineered structural products are superior to dimensional limber

 

[Ashful]

Perhaps I need to give in on the LVL idea. I've always avoided putting new tech into old buildings... it just feels (and looks) all wrong to me. All of the beams added to my house and barn to date, have come out of the old 1770's barn, which was disassembled by the prior owner. Now having run out of that, I was looking at dimensional lumber as the next best option. Running a bead on the shaper down one lower edge of each joist or girder, goes a long way to keeping it looking like a proper old barn.

Re: foundation. My assumption is the foundation is not sufficient to carry any point loads. I believe the slab was poured as a monolithic, with a perimeter trench, although I do not know how deep. A load bearing wall sitting on my existing 6" x 8" sill beams is of less concern, as the load is well distributed along the length. But all new posts and columns (point loads) will be sitting on new piers, dug and poured by me.

 

[peakbagger]

Reusing old timber may feel right but if a code official or structural engineer gets a wind of it, they will either make you vastly oversize the beam or not allow its use. The problem with using old timber is that its not graded. The grade is used to look up the allowable stress which is then used as part of the calculations. If there isnt a grade stamp, someone has to make an assumption on the strength and since their license is riding on it, they will use a low value and factor of safety on top of that. With a grade stamp, they can look it up in the table and use the highest value.

 

[Ashful]

Good to know! Had not thought of the legality of it. I just look at a 150 year old DF beam without a knot or check to be seen anywhere within, and say it must be grade SS.

Would be interesting to see what they make of the large oak and walnut (yes... walnut) floor joists in my house! I've not seen a grading table for them... but they've held fine for 240 years.

 

[peakbagger]

Well if you can find a licensed lumber grader and he is willing to grade it, not a problem. Structure very rarely fail due to overloaded structural members, usually they fail due to poor detailing. A piece of wood will deflect a significant amount before it breaks.

In recent history timber framers had a tough time figuring out a good exterior wall system. Eventually many of them settled on stress skin walls. Of course soon thereafter the stress skin folks figured out they didnt need the timber frame, the stess skins worked fine for both the walls and the structure.