16′ LVL floor beams – sizing and piles
I’m about to order a Helical Pile installation for my detached shop and need to confirm the number of piles needed.
It’s a simple (if tall) 12′ x 16′ with 10′ high walls and a 5/12 gable roof. 2×6 framing, 24″ oc. The insulated floor assembly is 11 1/4″ thick (2×12 joists, 24 oc) and will have the structure’s beams incorporated into the assembly.
I intend to use 2-ply beams rimming the 4 sides each built up with pairs of 1 3/4 x 11 1/4 1.9E LVLs (https://tinyurl.com/mtmcveut).
My calcs passed Weyerhaeuser’s ForteWeb for 2.0E. They list nothing for 1.9E but I do not have access to 2.0E LVLs.
I’m planning on 1 pile per corner. If I have to add center piles on the 16′ sides it will cost me about $1000 more.
I’m in Minnesota (snow country, despite this winter’s drought).
So, should I trust ForteWeb and the tables or add the central piles?
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Nobody here can or should confirm the accuracy of your engineering; there is too much liability involved. I can say that on the occasions when I have used Forte, it has aligned pretty well with what I get calculating longhand. (I have a BS in structural engineering and have done most of my own engineering for the homes and renovations I design for 20+ years, but I am not licensed and I work with licensed engineers regularly for more complicated projects.)
Does Forte's snow load match your local requirements? They vary greatly. Just within 50 miles of me, they are between 50 psf and 140 psf.
What did you use for live loads? Shop equipment can be minimal, or very heavy. I have a bouncy floor in my shop and I don't like it.
Be aware that helical piles offer less resistance to lateral forces than more traditional foundation types, due to the small cross-sectional area. It depends greatly on the soil type, and also how far above grade the piles extend. In most cases they shouldn't extend more than about 6" above grade without lateral bracing, and if you have soft soil--sandy, for example--I would want more than the piers for lateral stability.
Your local lumberyard should be able to get you 2.0 or 2.1 MOE LVLs. The difference between 1.9 and 2.0 MOE is a 5% reduction in stiffness. That may or may not be noticeable. There is also a difference in the ultimate strength before breaking--called the extreme fiber in bending--but that is rarely an issue for projects of this scale.
Thanks Michael.
I don't recall what I used for those loads, but will double-check.
You're building a shop...
Without confirming your engineering, your proposed floor, built for utility purpose, is flimsy. Yes, use intermediate footings. And why piles? Joists centered at 24 inch centers- are you going to use full thickness 2x flooring? I'd be concerned about floor deflection between the joists otherwise. In fact, I'd consider framing a shop floor with 12 inch centers versus 16.
The math for designing floor systems is pretty straightforward, and based almost entirely on deflection under load. Joist spacing, joist size and subfloor details can all be changed and result in a floor that performs almost identically. The difference would be in vibration, but that's not something to worry about with sawn joists and this scale.
So I'm good with 24" oc?
I figured the 2x12 blocking at 4' oc throughout would reduce or eliminate vibration.
It saves me $250, which I know is not a big deal over the total cost of the structure.
I was planning to keep the height of the piles at 6" above grade.
It depends entirely on:
The loads you have to handle--i.e., snow loads;
How heavy the equipment in your shop will be--in a small shop like that they probably aren't large, but if you want a commercial-duty tablesaw, jointer and planer, that's going to be different than cutting on sawhorses with a skil saw;
What your tolerance is for deflection--code minimum for living space is the length of the joist divided by 360 (L/360), but the longer the joist, the bouncier that feels. I use L/480 for most things and L/720 in fancier bathrooms and kitchens.
Deflection is how much the floor sags under load. Vibration is when you hit a harmonic frequency while walking across the floor. Vibration is rarely an issue when using solid-sawn joists because they vary in density; it's more of an issue with engineered joists.
Blocking does little to nothing; its purpose is to keep joists from rolling under load, despite common assumptions. If the joists and floor sheathing are undersized, blocking does help share the load between joists, and it's also where a lot of floor squeaks occur. If the joists and floor sheathing are properly designed, the blocking doesn't help.
Thanks Michael. That helps.
My calling it a 'shop' is a little ambitious. There will be a chop saw, but nothing too heavy.
I found the factors I used for the floor joist calculations.
- Floor Live Load: 50 psf
- Floor Dead Load: 20 psf
- L/480
- 2x12 Southern Pine #1
- 24 oc
- Simpson joist hangers on each end (into the 2-ply LVL rims)
- Blocking every 48"
ForteWeb passes (though I'm not sure what the 'Accessory' option means in the Spans & Supports tab)
Running those numbers through AWC's Span Calc (https://tinyurl.com/8m44h2j6) shows a 13' 2" maximum span.
I then changed my lumber to MSR 2400. That shows a maximum span of 15' 10"
Am I missing something?
Going with intermediate footings on the long sides.
Piles because of cost, labor, and time. Frost line at 44" down.