Help me understand Truss Design Calcs?

[Angelfire]

Hiya Folks,
I'm currently gearing up for insulation and drywall but have decided I think I want to install a trolley/hoist/rail to handle light loads (mostly the top for my Jeep but certainly not more than say 500lbs). Anyway, my rail would have to run parallel to the roof trusses so I want to put up lumber or steel above the bottom chord spanning multiple trusses to spread the weight. The rail would hang from this added lumber or steel. I have my truss design but aside from one note that says "This truss has been designed for a 10.0 psf bottom chord live load nonconcurrent with any other live loads". What does this mean? I'm really just trying to figure out how many trusses to span with this but this line has me a bit confused. Attached is the truss design. I appreciate any insight.

Cheers

 

[readhead]

It is designed for drywall and some lights. That's it. On a truss designed like yours any collateral loads will usually be designed to hang from the top cord.

 

[Angelfire]

That makes sense. Thanks very much!

 

[GMCGarage]

Hiya Folks,
I'm currently gearing up for insulation and drywall but have decided I think I want to install a trolley/hoist/rail to handle light loads (mostly the top for my Jeep but certainly not more than say 500lbs). Anyway, my rail would have to run parallel to the roof trusses so I want to put up lumber or steel above the bottom chord spanning multiple trusses to spread the weight. The rail would hang from this added lumber or steel. I have my truss design but aside from one note that says "This truss has been designed for a 10.0 psf bottom chord live load nonconcurrent with any other live loads". What does this mean? I'm really just trying to figure out how many trusses to span with this but this line has me a bit confused. Attached is the truss design. I appreciate any insight.

Cheers

10 PSF Bottom chord DL designed for.

 

[The Tool Tyrant]

Take into consideration that the truss is designed for the major loading to occur above the top chord, which then distributes the load through the webbing to the bearing walls.
The bottom chord is 'normally' designed for 'uniform' loading as in a drywall ceiling.
The problem with your idea lays in the fact that the load is not uniform and if you are only spanning the bottom chords, you are now loading the gang nails in a fashion that they weren't designed or calculated for...ie, you are relying on the shear loading of the gang nail to keep the bottom chord from ripping away from the webbing.
If on the other hand, you had a 4x6 (just for example) spanning across (let's say 4 trusses) above the plywood, then you would be utilizing the load path that the trusses were designed for.

 

[wssix99]

^ Right. You could get away with more if you were going perpendicular to the trusses. Parallel is a disaster unless you could come up with some wild rig to distribute loads on the top of the trusses instead of the bottom.

If you really want to have this, then your best bet would be to get a special steel beam or steel truss fabricated that you can put up behind the drywall to attach your trolley to. This would distribute the loads back to the walls and skip your existing wood trusses and wood structure.

^ This is probably an expensive proposition, but what luxury isn't? If you aren't up for this as a luxury item for the garage, a portable hoist that rolls on the floor will be less expensive and more versatile. (You can also get models that break down for storage.)

 

[Angelfire]

Thanks folks for the replies and information. This wasn't a "have to have this' but more of a thought about getting it in before I close up the walls/ceiling just in case. I suppose I could hang it from the top chord but am not really enamored with that idea. So I'll probably skip it and hope I don't wish I had done something down the road
Cheers.

 

[jack stand]

Is the truss plant local? It might be worth a call to them. They may have a fairly simple solution like selling you 3 more to "sister" them to one of the existing or maybe sheeting each side of the truss with several layers of 3/4 ply wood? You just don't know if you don't ask. I would guess that the lawyer/liability side of them may just blow you off, but then......
I've had situations on the job where something just didn't work out as planned and a few of the truss bearing points changed, they (the truss Co.) gave us their fix.

 

[Angelfire]

I might give them a buzz on Monday to see what they say. If it's something simple that I can do in the field then it still might happen. But if not, then I'll move on. At this point, putting in sisters really wouldn't work unless I tear out a bunch of wiring and such which is really not worth it IMO. Thanks for the idea, I'll give them a shout.
Cheers.

 

[Angelfire]

Hiya Folks. I've reached out to the truss mfg for any advice they may have. We'll see what they come back with. But I stumbled across these brackets and got to thinking it might be possible to hang everything from the top chord without too much trouble:

http://i1337.photobucket.com/albums/o665/Rockcam/8427d90df639e7b919a9c6713c254cd8_zpsn4ofy8nb.jpg


Or possibly run unistrut along the bottom side of the chords (perpendicular to trusses) every 2 ft. then hang the trolley track from these? I don't like this idea as it would basically be putting all the weight on screws that could pull out whereas the bracket above would be mount through the truss itself.

Just curious to hear what folks think about this?

Thanks much.

 

[readhead]

The bottom cords are out of bounds for anything but drywall. The bottom cord is a tension member and not designed for anything more than light 10 lb collateral load.

 

[johninct]

Sorry for the rant but I am tired of people thinking that trusses or any other type of structural engineering is no big deal! People spend years of studying and years of actual experience just to take a Professional Engineering exam. And YES, their profession does have life or death consequences. Some of the questions I have seen are like asking "my foot hurts, what prescription do I need?"

 

[GMCGarage]

Hiya Folks. I've reached out to the truss mfg for any advice they may have. We'll see what they come back with. But I stumbled across these brackets and got to thinking it might be possible to hang everything from the top chord without too much trouble:

http://i1337.photobucket.com/albums/o665/Rockcam/8427d90df639e7b919a9c6713c254cd8_zpsn4ofy8nb.jpg


Or possibly run unistrut along the bottom side of the chords (perpendicular to trusses) every 2 ft. then hang the trolley track from these? I don't like this idea as it would basically be putting all the weight on screws that could pull out whereas the bracket above would be mount through the truss itself.

Just curious to hear what folks think about this?

Thanks much.

Some quick analysis the truss should handle that 500lb load globally, as long as its not got the full live load, i.e. dont lift **** when you have a foot of snow.
The local connection at the truss though could be in jeopardy, looks like 6' panel points, so they would be 120lbs each. What this means is, the plate they sued at the web/top chord interface may only be designed for that much, not 500lbs.

Also how do you hang it from teh top chord and not hit the bottom?

 

[Angelfire]

The bottom cords are out of bounds for anything but drywall. The bottom cord is a tension member and not designed for anything more than light 10 lb collateral load.

Yes, we've established that. I was referring to the top chord above. Thanks.

Sorry for the rant but I am tired of people thinking that trusses or any other type of structural engineering is no big deal! People spend years of studying and years of actual experience just to take a Professional Engineering exam. And YES, their profession does have life or death consequences. Some of the questions I have seen are like asking "my foot hurts, what prescription do I need?"

Which is precisely why I've reached out to my truss mfg and their PE to get a definitive answer. Regarding your foot, start with aspirin and work up from there

Some quick analysis the truss should handle that 500lb load globally, as long as its not got the full live load, i.e. dont lift **** when you have a foot of snow.
The local connection at the truss though could be in jeopardy, looks like 6' panel points, so they would be 120lbs each. What this means is, the plate they sued at the web/top chord interface may only be designed for that much, not 500lbs.

Also how do you hang it from teh top chord and not hit the bottom?

Thank you for this. As for missing the bottom, the brackets that would attach to the top chord actually stand it off the face of the chord by about an inch or so so the down rod would miss the bottom chord.

Anyway, thanks for the education folks. I'm going to see what the PE comes back with but it's definitely looking like a no go on this which is perfectly fine.
Cheers.

 

[lakeroadster]

Hiya Folks,
I'm currently gearing up for insulation and drywall but have decided I think I want to install a trolley/hoist/rail to handle light loads (mostly the top for my Jeep but certainly not more than say 500lbs).

Cheers

Leave the existing trusses alone. 1/2" drywall with insulation will eat up 70% of your 10 PSF Dead Load Rating. Weights of Building Materials

All is not lost however.. I made a hoist for my Jeep Hardtop. Go to post 311 here.... It's a Jeep Thing... Hard Top Hoist

 

[Angelfire]

Thanks for the post on your hoist for the jeep top. I may build something mobile but for now was just focused on getting something into the ceiling before drywall so haven't given any thought to the mobile option. At this point, unless the PE that designed the trusses comes back with something that will work, then I'm going to skip the in ceiling plan.
Cheers.

 

[lakeroadster]

Thanks for the post on your hoist for the jeep top. I may build something mobile but for now was just focused on getting something into the ceiling before drywall so haven't given any thought to the mobile option. At this point, unless the PE that designed the trusses comes back with something that will work, then I'm going to skip the in ceiling plan.
Cheers.

It's not mobile.. it's an independently supported structure installed above the lower truss chord, not supported by the truss.

 

[GMCGarage]

Thanks for the post on your hoist for the jeep top. I may build something mobile but for now was just focused on getting something into the ceiling before drywall so haven't given any thought to the mobile option. At this point, unless the PE that designed the trusses comes back with something that will work, then I'm going to skip the in ceiling plan.
Cheers.

What he did is a pretty good idea, and its hidden like you want it to be.

 

[Angelfire]

It's not mobile.. it's an independently supported structure installed above the lower truss chord, not supported by the truss.

I know yours is not mobile. I'm just saying that I would probably build a mobile one vs trying to fish 10-12' sticks of 2x6 into my trusses. All my wiring is done etc.... so very difficult to get anything of size up there.