Supporting electric hoist across ceiling joists/rafter ties

RatLabGuy

RatLabGuy

You look like a monkey and smell like one too
Joined
May 18, 2005
Location
Churchville, MD
Help me not destroy my roof system :D
I have an upcoming engine-pull project and debating instaling an electric hoist on a sliding rail to use like a gantry in the garage. Bonus is later I figure it could be used for pulling / holding 4runner / Bronco tops.
The problem is my garage is attached to the house which is a single-story 70s ranch, and the ceiling joists / rafter ties are only 2x4s at 24" spacing with a standard V-truss support.. Not really joists, just the bottom cord of a truss. I have full access from the attic but they were never really designed for weight bearing.
And of course the joists run front-back of the garage. The total span of joists is something like 27' wall-wall, of that 21' is open garage space so that means if I had a sliding run, it would probably be 6-8' (front-back) and overlap with the centerline where the joists are tied together.
I would tie it directly to a joist, but I have no idea if it could support the load and assume it would need to be tied laterally to the adjacent beams to spread the load.

Is this just a terrible I idea I should give up on? Default is to just get a rolling hoist and chaulk this up to more reasons for a better garage.
Rough guess is the max load I'd have on the lift, including weight of the hoist itself is ~500 lbs.
I don't even know where to start with the math.
 
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Sistering the joists is an option, but the load will also be supported by the wall so the wall studs should also be sistered. For 500 lb max I wouldn’t think it’s a bad idea, but I wouldn’t try to pull engine and trans in 1 go.

I’ve seen people hang hammocks to single studs in stick built apartments and if you know about cable tension it magnifies weight. You’re gonna want to make 2-3 of the “trusses” that are there completely sistered and the accompanying wall studs.

Other than hiring a structural engineer, you can always beef up what’s there

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Bottom chord is probably designed to support 5-10psf
 
Not saying this was a great idea but years ago I had to find a home for a chance buy on a Bridge Series 1 Knee Mill. That ended up being my parents garage where my dad built it with trusses at 24oc. I bought two 2x12x12' PT boards and slid them both up into the rafters to distribute the load across multiple chords. The mill weighs about 2300# and it did not cave in the roof when I used a come along with a strap over those 2x12s to lift it off my trailer deck to sit it on some pipes to roll to the back of that garage...
 
I needed something to work from in my old garage, so built a wood gantry. Used it to pull a few motors, heaviest I lifted (and probably should not have) was a 1500 lb generator.

10' ceiling, uprights were just over 11' apart.

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Chat GPT says:

Using Southern Yellow Pine (SYP) for your triple 2x10 beam over an 11-foot span will change the allowable stresses because SYP is typically stronger than Douglas Fir-Larch.



Updated assumptions for Southern Yellow Pine #2 grade:​


  • Allowable bending stress, FbF_bFb ≈ 1,200 psi
  • Modulus of Elasticity, EEE ≈ 1,900,000 psi (higher stiffness)


Step 1: Section properties (same as before)​


  • Thickness b=4.5"b = 4.5"b=4.5", depth h=9.25"h = 9.25"h=9.25"
  • Moment of inertia, I=296.63 in4I = 296.63 \text{ in}^4I=296.63 in4
  • Section modulus, S=64.1 in3S = 64.1 \text{ in}^3S=64.1 in3


Step 2: Calculate allowable moment​

Mallow=Fb×S=1200×64.1=76,920 in-lbsM_{allow} = F_b \times S = 1200 \times 64.1 = 76,920 \text{ in-lbs}Mallow=Fb×S=1200×64.1=76,920 in-lbs

Step 3: Calculate max center point load PPP​

P=4×MallowL=4×76,920132=307,680132=2,331 lbsP = \frac{4 \times M_{allow}}{L} = \frac{4 \times 76,920}{132} = \frac{307,680}{132} = 2,331 \text{ lbs}P=L4×Mallow=1324×76,920=132307,680=2,331 lbs

Final estimate:​


Maximum center point load on a triple 2x10 SYP beam spanning 11 feet ≈ 2,330 lbs



Additional notes:​


  • This assumes no deflection or shear limits.
  • Deflection limits (L/360 or L/240) might reduce allowable load.
  • Always check local building codes and consider consulting a structural engineer.
 
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