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Question About Joist 'Hanging' Loads


jaksprat's Avatar
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04-10-09, 05:20 AM   #1  
Question About Joist 'Hanging' Loads

I'm installing a boatlift that will hang from my boathouse ceiling. I am hoping that if I add a treated double 2x8 joist to hold the rear sling for my boat, it will be adequate to support the load. The front sling will hang from an 8x8 beam which I am not worried about. The rear sling will attach at 2 points on the new joist. The joist will have a 10' span and the sling cables will attach at 2 points, where each of them will be 1' from the end of the joist. My intuition tells me that the closer the load point to the end of the joist, the better the joist can handle it.

As for the load, the boat weighs 3,000 lbs, which includes motor, fuel and gear. The rear sling will carry roughly half of the load, which is 1,500 lbs. Therefore each of the 2 cables of the rear sling will have a 750 lb load on the joist. However, I'd feel better if the joist could handle about 1,100 lbs at each load point due to variances of boat weight distribution, and providing a safety margin.

So, will the double 2x8's be enough to support this load? Does anyone know where on the internet I can look up this kind of data for 'hanging' loads? All I can find are span tables for floor joists and decks.

Thanks,
Jack

 
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OhioDraft's Avatar
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04-10-09, 07:09 AM   #2  
1500# for a 10' span is excessive for 2x8's even considering the loading locations. If you are set on using lumber instead of steel, why not up it to 2x12's for a few extra bucks? Having said that, you are on the right track by including a factor of safety. You just need to increase the beam, and if it were my boat I would seriously consider steel, but hey i'm an engineer (in training), we're partial to it. That doesn't concern me as much. What I think you should keep in mind is that if this loading is not straight down as in, in the same plane as the beam, then the load will introduce torsion (twisting) and that will probably control the beam design. If that's the case you'll need bracing.

 
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04-10-09, 07:13 AM   #3  
Also, hanging 3000# from existing roof structure will certainly require an professional consultation. In my opinion, that is your biggest consideration.

 
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04-10-09, 07:42 AM   #4  
You don't need an engineer to tell you what you already know, and stated. The front sling will hang from an 8x8 beam which I am not worried about. I'm guessing this is a $10,000 boat give or take, so two $15 boards may be strong enough, but they don't inspire the confidence required. Put up a respectable beam and two similar posts within the wall and you are all set.

Bud

 
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04-10-09, 09:33 AM   #5  
Thanks for the comments guys. I appreciate your thoughts about beefing up the beam size, but I have limited headroom for the beam -- 8 1/2" at most. One thought is to rip 2x10's down to 8 1/2". The boatlift manufacturer says dbl 2x8's are OK for a 3,000 lb lift and dbl 2x10's for a 6,000 lb lift. I wanted a little extra since I was borderline for 2x8's. So I'm thinking dbl 2x8.5's(actual) might suffice.

Anyway, I'd have thought that this kind of load might be in some tables similar to span tables, perhaps showing for example, what dbl 2x8's could support at 1' intervals down the beam. Just a thought. Thanks for your inputs!

Jack

 
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04-10-09, 11:17 AM   #6  
Hi Jack, most data will deal with total deflection in the center of a span, rather than specific spots along the beam. Do you have room for three 2x8" side by side to give you that extra margin?

Also, how are you going to attach to the beam, bolt, sling, lag (not). Any holes you drill through will reduce the capacity.

Bud


Last edited by Bud9051; 04-10-09 at 11:19 AM. Reason: sp
 
sgtgerryf's Avatar
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04-10-09, 11:18 AM   #7  
Flitch Beam

Build a flitch beam which is a sandwich of steel plate between wood.

You'll have the extra strength you need and should take care of any potential twisting.

Just get a few voluteers to help you lift it up.

 
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04-10-09, 02:12 PM   #8  
Thanks for all the advice guys. I feel good about this now. I'll contact boatlift guy for final advice and know what options I have. They all sound good.

Jack

 
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