Wood Truss - Improving strength of Nailed Member Joints

[MarshfieldTimC]

I recently purchased a 30' x 40' (unfinished) pole shed, and I would like to add a ceiling and insulate it (along with the perimeter walls). The shed was built (in approx. 1990) with 6x6 posts spaced 8' apart, with wood trusses (spanning the 30' width) attached to the posts. The trusses are apparently home-made (built from scratch by a carpenter) and were not 'engineered'. All wood members are attached to each other using nails. I want to improve/strengthen the wood member joints, as I was told that the nailed joints are a likely weakness. Does anyone have experience with something like this? {For example, 2x6 cross member attached to a 2x6 bottom chord with 5 nails - Can I improve the joint strength by bolting these joints together (drilling holes, using flat steel plates, etc.)?}
All comments appreciated.

 

[KootK]

Are the truss members in the same plane and connected with plywood gusset plates. Or are the members place side by side and through nailed?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JStructsteel]

Can you post some pictures, and dimensions of the members? without extensive analysis, it will be hard to say if ok.

 

[MarshfieldTimC]

Thanks for the reply.
The truss members are not in the same plane (no gusset plates) - they are side-by-side (overlapped) and are nailed together. Sample photo attached.

 

[MarshfieldTimC]

I attached a drawing and another photo.

 

[KootK]

1) get some help from a local structural engineer if you're not one yourself. It may even be the case that no reinforcement is required, particularly if there are currently no signs of distress.

2) I'd be reluctant to pound the joints full of additional fasteners without having performed a detailed analysis. You run the risk of doing more harm than good.

3) A relatively safe way to add meaningful capacity to the joints is to double up members such that you've got three coming into each joint (one sandwiched) and your fasteners working in double shear. The added material will be expensive unfortunately.

4) there's an excellent chance that the heel joint where the top chord meets the bottom chord will be the critical thing here. There, the chords probably are in the same plane, right? If so, how are they connected?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MarshfieldTimC]

Per KootK's questions:
1. I am not a Structural Engineer. I do not see any signs of distress.
2. Agreed.
3. Doubling members is not my preference, due to cost and effort.
4. At the heel joint, where the top chord meets the bottom chord, this joint is also side-by-side (overlap). The 2x10 top chord butts against the 6x6 post, with the 2x6 bottom chord on top of the 2x10. I attached a picture of this also.
Thanks for the continued interest.

 

[BAretired]

Is the shed in a location likely to sustain a significant snow load?

The latest photo brings up a new problem, namely the possible rollover of purlins bearing on the top chord of the trusses. Solid blocking between purlins would prevent rollover.

A review by a local engineer is recommended.

BA

 

[KootK]

Seeing the elevation sketch:

1) Peak joint is unconventional and likely to be working hard.

2) if the chords are lappped, that means that the webs probably don't sit 100% cleanly against the chords, right? They tilt a bit off vertical?

3) Are the kneee braces your lateral load resisting system?

I'd estimate that your top chord to bottom chord connection will see upwards of 10,000 lb of force needing to be transferred. You appear to have about 14 nails there, each good for about 100 lb give or take. If your goal is to make this work by the book, the service of a structural engineer is likely to be required. I've got nothing but love for the DIY spirit but there's a pretty big gap in need of closing here.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SLTA]

What is the intended use of the shed? If it's for "living space", according to the code, or for space where people are going to spend a lot of time, then yep - get a structural out there to help. If it's just for a shed, then look for visible damage and move on. After adding the blocking.

Please remember: we're not all guys!

 

[MarshfieldTimC]

Thanks again for the replies and continued interest. Here's more info:
The location of the shed is central Wisconsin, and snow loading is a concern.
Presently, there's no blocking between the 2x4 purlins.
The peak joint of each truss is secured by a (nailed) 2x6 'gusset plate' on one side only.
The lapping of cross members and chords appears to be tight. At the outer heel joint, which is lapped also, the top chord does not bear on the bottom chord.
There are no other plates or braces (besides those shown on the truss drawing).
The intended use of the shed is for a shop area, not for living space, but it will be heated for year-round use.
Finding a local Structural Engineer may be difficult for this area.
Thanks again for all the comments.

 

[rdubs]

The shed has survived since 1990, so I wouldn't worry about snow loading, etc. The purlins are likely okay if there is no distress - it is common to install them on-edge without blocking in post-frame construction. The LFRS is via roof and wall diaphragm action. Your problem is confined to load that you add to the bottom chord.

 

[Jerehmy]

Central Wisconsin gets a good amount of snow, 40 to 50psf ground snow load.

I can't believe there haven't been issues. It has a low slope, but there's no way that roof carries a lot of snow. Those trusses are at 8' on center. They cannot carry 7-10 kips. That chord connection cannot hold 5kips, nevermind 15 kips.

I just looked through your recent snow storms, some dump 24-36 inches at once. I can't see this roof holding that much snow. I think the shed has gotten lucky for 27 years.

I'm curious, I'd look through your local weather history, maybe weather underground, and see what the biggest storm was since 1990 and how much snow it dumped.

 

[sekwahrovert]

I agree with those on this forum who have expressed concern with the overall strength of the existing trusses. The potential problem is not with the proposed addition of a ceiling and insulation--that won't make things much worse than they already are. I suspect that the slippery metal roof has caused snow to slide off of the roof, even though the slope isn't that great. You can find plenty of engineers and engineering firms licensed to practice in Wisconsin. The company I work for is licensed there and we employ one WI-licensed PE.

 

[MarshfieldTimC]

Well, after reading through all the comments and replies, I don't know if I should invest anything into a finished interior or not. Maybe I should not store anything in there over winter, as it sounds like the roof could collapse with moderate snow.
The consensus appears to be that the trusses need to be improved, mainly due to the member/chord connections. I assume the individual members (2x10, 2x6, etc.) are sufficient. I will likely try to hire a Structural Engineer to help me with this (if I can find one).
When I started this thread, I was searching for comments on how to improve/strengthen the nailed joints/connections. I still think that could be a solution, or at least an improvement. If anyone can offer comments (or suggest reference books/manuals) on how to improve/strengthen these nailed joints I would appreciate it.
Thanks again for all the replies.

 

[atrizzy]

Based on these photos, I'd be more worried about your lateral capacity than your vertical capacity.

 

[AKSherpa]

What is the 1 x 6 overlap on the bottom chord in the drawing referring to?

 

[BAretired]

I believe that is the bottom chord splice. Presumably one 1x6 on each side (I hope).

BA

 

[HouseBoy]

I'm wondering if this was designed for Alabama weather (just a hunch).
Wind loading is a concern too but still, if it is an agricultural storage building the requirements would be light.
No mention of embedded depth of 6x6 posts.
Lack of finishes make deflections much less noticeable too.

Details look like early generation pole building.
Much development of design standards has occurred in the past 25 years.

 

[MarshfieldTimC]

The 1x6 overlap on the bottom chord refers to the bottom chord splice - there are 1x6's nailed on each side of the 2x6 bottom chord members. The 2x6's are butted end to end at the center of the span.
There should not be much wind loading, as the site is surrounded (three sides) by trees that are taller than the shed roof. The exposed side would be subjected to winds out of the south (not the predominant direction), and are directed to the gable end.
Embedment of the 6x6 posts is unknown. Soils in this area are heavy clay/silt, prone to frost heave at shallow depths. There is no evidence of the 6x6 posts heaving, so I suspect they are embedded at least 5-feet.
Would cabling across the 30-foot span (top of 6x6 post to top of 6x6 post) help withhold the lateral load(s)?