Reinforcing Attic Roof Trusses for Flooring and Storage

[antivenom]

I'm converting the attic above my garage for light storage by adding flooring and a folding aluminum attic ladder. Also, I have an electric hoist I plan to install to haul storage items up there so I don't have to awkwardly carry them up the ladder. The attic area is about 20x20. The trusses are a fink with a raised heel(I think that's the correct terminology), 24" OC. They are designed for 10psf on the bottom cord and 20psf live load on the bottom cord in the center between the two tallest webs. I've attached the truss specs.
For the flooring I'm using 3/4 OSB T&G ([URL unfurl="true"]https://www.lowes.com/pd/23-32-CAT-PS2-10-Tongue-and-Groove-OSB-Subfloor-Application-as-4-x-8/1000065715[/url]). I've had to cut it into ~2' wide strips to get it up there easier.
Should I strengthen the trusses?
Specifically around where the attic ladder is installed and where I'll put the hoist. I haven't really figured out the best way to permanently mount the hoist yet. The attic ladder is ~50lbs. Link to attic ladder Link to hoist The hoist capacity is way over what I will ever lift up there. I don't plan to lift anything over ~75lbs.
Will sistering 2x4's the length of the bottom cord on each side of the attic ladder add any strength or will that just add weight?
Should I add gusset plates to any of the joints?
Also, I have discovered that one of the trusses has a split in the W5 web at the bottom, what would be the best way to repair it?
I can take some pics of that later if it would be helpful.
Thanks in advance!

 

[phamENG]

Generally speaking, adding a light attic floor over the area designed for 20psf live loading shouldn't be a problem. The lift may be an issue. It really doesn't matter what you plan to do with it, it's a question of what it can do and what the next person to own the house can reasonably do with it.

Given the damage to a member and your plans to install a lift, you should consult a local structural engineer to inspect it and determine what needs to be done.

 

[r13]

The record drawing is very well detailed and interesting. I don't see the 20 psf load, but the 10 psf dead load for the bottom chord. Also there is no live load allowance for the bottom chord, nor the top chord besides the snow load. Suggest to engage a structural engineer for assistance.

 

[EdStainless]

The trick with the stairs inst how much they weigh, it is the loads when you are climbing them with a 40# box. You may want to reinforce that area a bit more.

Do yourself a favor and put a 2x4 or 2x2 along the inside face of W3 & W5 about a foot up. This will keep stuff from getting pushed off of the floored section.

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P.E. Metallurgy, consulting work welcomed

 

[phamENG]

retired - that's a standard pre-fab wood truss calculation sheet. There is a live load for the bottom chord, you just have to follow the asterisk(*).

We are in agreement about engaging a structural engineer, though. Prefab wood trusses are not to be trifled with - they are optimized and can be sensitive to modification of any kind.

 

[r13]

Yeah, missed that. What it means "in areas where a rectangle 3-6-0 tall by 2-0-0 will fit"? I think they might have placed the live load uniformly in the mid-span as OP's claim, but a funny note.

 

[jayrod12]

They mean that if there isn't a minimum of 3'-6" between top of bottom chord and the web/chord member above, they didn't apply the load there. So they are expecting that the storage is contained to only the areas with more than 3'-6" head clearance. Believe me when I say they will not have designed that entire centre section of bottom chord for it. So be mindful of that fact when you're putting down your plywood.

 

[antivenom]

Maybe I wasn't clear, I'm putting flooring across the whole area, not just the center section.

If I move, I'll be taking the hoist with me, so I'm not too concerned about a future owner lifting an engine(or something else equally heavy) with it. Since I am adding easier access to the area, it is a potential problem with someone overloading the area. I guess if I ever move I'll have to put up some big caution signs indicating loading limits. But I can cross that bridge when I come to it.

The 20psf live load is listed in the notes section in 6). Also, the rest of the bottom cord is rated for 10psf live load as indicated in section 5). I don't know why they listed all that in the notes section...

As far as mounting the hoist I was thinking of mounting it on a trolley in some unistrut channel, so that once I lift something, I can move it over to a floored section. I'm thinking of mounting the unistrut channel to a 2x6 and running the 2x6 perpendicular to the trusses. The 2x6 with the channel would be supported by vertical 2x6s on each side, notched for the horizontal 2x6. One vertical 2x6 on the house side, which is a doubled girder truss with a 2x10 bottom cord. The other vertical 2x6 would be supported by the floor.

I may contact the truss manufacturer, not sure if they will be interested in helping me with this. I guess otherwise I'll have to find a structural engineer to take a look.

 

[LittleInch]

That's a weedy looking truss to me.

20 foot spans of a 2 x 4 won't do much for anyone.

Your flooring alone takes up 3-4 psf.

Cutting in two will have destroyed it's integrity. I've put this stuff up and a lot of its strength comes from the interlinking tongue and groove. This will visibly and noticeably bend along your cut lines as you walk on it or store anything. Once you board it out the temptation to load and stack things is irresistable and you'll get concentrations of load.

Some pics are always good.

Do you have any insulation up there? If not why not? That's usually 12" deep.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[phamENG]

This is the tradeoff with trusses. They're inexpensive and fast to put up, which saves the builder (and occasionally the first owner) of a house on construction costs upfront. And that's great, but after that you're pretty well stuck with what you have. Modifications and alterations are very difficult, and will cost a pretty penny in engineering. Those trusses are optimized to use the bare minimum of material. As LittleInch pointed out, that truss won't do much for anyone, but it will do enough - barely.

 

[oldestguy]

I'd go with 1/2" plywood deck and a ladder, no stairs. Ladder at one wall in center hinged at top if possible. Might flex when walked on, but won't fail. Keep loads near the verticals where possible.

 

[LittleInch]

I'd missed that this is above your garage, so if starts to sag a bit no one is going to complain too much(!)

In that case why don't you just strip the ceiling off (if indeed there is one) and then simply span inside your trusses with some bits of 2 x 10 or similar off the walls?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[antivenom]

Yeah, since it’s above the garage there is no insulation. There is a drywall ceiling, which I don’t want to remove.
I’ve already installed the attic ladder, with the hinge and major support point attached between points J and K on the bottom cord. The bottom of the ladder is supported by the floor and the top is on the header and screwed and nailed to the bottom cord I either side, so the load is distributed. And that’s the section designed for 20psf BCLL and 10psf BCDL. I attached a pic below of where the ladder is installed.

 

[MegaStructures]

Second LittleInche's suggestion. Are the walls CMU? If so span the walls with your own floor framing and you should have no problem with a reasonable load. If the walls are wood frame you may need to look a little closer at what kind of loads you are adding. Personally I wouldn't touch the trusses on my own house for reasons others have pointed out above.

Any idea why the wood split at W5 as you mentioned? Was there a collision at any point? Where is the splinter? Is it close to the gusset plate? Has there been any unplanned loading on this truss in the past?

 

[LittleInch]

OK, So boarded out already?

I think you need to be careful to store anything as close to the walls as you can and leave that middle bit free to walk up and down.

did you sister any of the joints or add strength to the gusset plates?

Just be careful not to park an expensive car underneath it.....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[antivenom]

The walls are wood frame. I have engaged the truss manufacturer to recommend a repair for the split web. As to how it happened, I have no idea. As far as I know there has been no collisions or loading in the past. Here's a pic of the split. [URL unfurl="true"]https://files.engineering.com/getfile.aspx?folder=96cb926f-fb8d-48e9-88b3-5b3195c28171&file=IMG_3555.jpeg[/url]


I would definitely prefer not to have to put in framing just for the floor, but I guess if the trusses are not up to the loading and strengthening them is either not possible or more work intensive then that's what I'll do.

I have some boards up there to walk around on, but not all boarded out.

Also, the 3/4" OSB I have would add 2.4psf dead load, since it's adding rigidity by connecting the bottom cords of the trusses, does it also add some load capacity?

 

[phamENG]

No, it doesn't. Refer back to the calc sheet and see the "Rep Stress Incr YES" in the second column from the left. That means they already accounted for the load sharing between adjacent trusses.

If you're talking about added rigidity as you you might find in a composite steel/concrete section or a concrete T, then no again. There's too much slip in the fasteners for that to work in a reliably quantifiable way. Does it happen? Sure. But to my knowledge nobody has come up with a method to account for it safely in design and analysis. You can add glue, but the stuff you'll find at home depot isn't reliable enough for that. There was another thread on that recently - the "wood glue" used for things like glulams and CLTs are specialty products that have to be installed under careful factory controls.

 

[antivenom]

LittleInch, why would it be better to store items closer to the outside walls, when the center section has a higher(20psf) live load capacity?

 

[BAretired]

The split in member W5 is definitely something to be concerned about. It carries a maximum compression of 104# or a maximum tension of 467# and it looks like the crack is headed toward the left edge of the member. If it continues to the edge, the truss loses all of its strength. The location of the crack probably results from shrinkage, together with the positioning of the truss plate. As built, the truss plate engages only half of the member. It would have been better to use a larger truss plate, one which engages all or most of the member.

Member W6 has more compression (246#) but less tension (196#). It has not cracked yet, but it is not very well connected at the bottom. The same condition seems to be true for members W3 and W4.

BA

 

[EdStainless]

My old boss would have strangled someone for plate placement like that. Of course he would have killed one of us for designing a joint like that first.

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P.E. Metallurgy, consulting work welcomed