Valley Rafter Repair

[hemiv]

I'm working on a repair detail for a 2-ply 2x12 valley rafter in an approx. 130 yr old story building. The rafter is undersized and each ply is split at some knots near the mid span.

I think the best thing to do would be to replace or sister it, but that would require temporarily supporting the 2x10 jack rafters. The attic floor framing is unknown, but its span is pretty large and the slate roof is pretty heavy. I think shoring may require posts to go through the two floors and basement below.

The owner's rep wants to look at an option that doesn't include resupport of the jack rafters. I have an idea to run a channel toe down along the bottom of the valley rafter and connect it to the sides between each jack rafter via welded-on plate tabs and lag screws. The idea is to work like a beefed up flitch plate. My preliminary detail is attached.

I don't think I can get this to work as a composite member - the fasteners as shown will not develop horizontal shear as needed for composite behavior. Does anyone have another thought or a better approach?

 

[XR250]

If headroom is not an issue, why not put an LVL under it?

 

[hemiv]

Flat along the bottom of the rafter? The rafter has deflected enough that I think it will be impossible to lay an LVL flat along the bottom. But it may be an option, depending on the amount of deflection.

 

[EdStainless]

If the rafters are already cracked, I would cut spacers to fill the gap between them and then through bolt to hold your steel.
How much defection is there?
Could you start bolting at the top and work your way down (out) forcing things back into place?
As you get nearer the walls the attic floor should be better able to handle a bit of jacking force, maybe.

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P.E. Metallurgy, consulting work welcomed

 

[hemiv]

There is about 1/2" to 3/4" , I haven't been out to measure yet.

Yes, the spacer idea is on my mind. That would provide a load path for the rafter thrust, because as it stands now there isn't much.

And yes, if I settle on a repair like this I will have it progress from one end to the other to help take out some of the deflection.

Who knows, the attic floor may be sufficient, but that would involve a lot of field work and some demolition to really understand the whole system. The attic floor is supported by heavy timber girders on cast iron columns. And this is public building that hosts events, so any deflection or damage due to shoring is unacceptable.

 

[IFRs]

I'm guessing that this won't help?

http://www.eng-tips.com/viewthread.cfm?qid=501708

 

[XR250]

Flat along the bottom of the rafter? The rafter has deflected enough that I think it will be impossible to lay an LVL flat along the bottom. But it may be an option, depending on the amount of deflection.

No, full depth below it. Pad it out one or the other so they are the same width and fasten with 2x4 x 24" on each face every 24" or so. You could gap it a bit to accommodate the baked in deflection.

 

[hemiv]

IFR - I don't think in this application, but thanks for sharing! That's a good one to know about.

XR250 - Ah, got it. Space is a concern, I am not able to access or make new bearing points for something like this. My channel idea would work like a flitch plate, where the end fasteners would be designed to transfer the end reactions back into the main member very close to the supports.

 

[XR250]

If the near end portions of the existing valley are ok, then you could transfer the load in the LVL back into it.

 

[HouseBoy]

I don't see how the flat framed channel is going to work like a flitch plate beam. The vertical plates are onto "tabs" in between the existing rafters to facilitate connecting the the existing valley. Usually the flitch needs to be oriented upright in order to provide the needed stiffness.

What if.... Can you place some framing (like a multiple ply LVL beam) spaced just above the existing ceiling joists and then prop up off of the beam to support the existing valley?
Maybe you do that with two beams so you can brace on against the other (for stability) Or, use two props off of a single beam but oriented in a "V" shape.

 

[bones206]

I happen to be doing the same thing right now on a 130 year old hip beam. I went through a litany of ideas and settled on sistering with LVLs each side. I figure the contractor can shore the jack rafters on one side at a time. In my case it’s not a repair but a retrofit for the moder snow load.

I did look at a version of HouseBoy’s idea. I put some deep LVL beams in the ceiling plane and propped up to the hip beam at its third points. But I couldn’t get the stiffness of the LVLs high enough to draw enough load into the props. They just acted like medium-soft spring supports (in my RISA model). But for a repair situation I would think that would be fine since you are just trying to restore load path integrity.

 

[EdStainless]

Is there really a clear gap between the rafters?
If so then why not a steel plate welded to the channel up in that space?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[HTURKAK]

TLDR..

My question if the sketch posted scaled and the gap btw two ply 2x12 valley rafters continuous ..If so, you may consider to insert steel T section and connect to the 2x12 valley rafters with thru bolts.

If this is a viable option, you should consider to jack up the rafters so the Tee section would incorporate to resist.









Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[hemiv]

HouseBoy - Yes, I agree the channel might not be stiff enough. I could add some some angles. As for your other idea, it sounds like you're talking about building a sort of knee wall supported by a new beam in the plane of the floor joists. It would have to use the existing supports. One is a brick wall, which I'm not worried about, but the other is a heavy timber girder supported by cast iron columns. I would like to avoid the analysis of those historic materials, I just have no good feel for what they can support and I would have to remove some finishes to even get a look at the timber girde

bones206 - Yes, I would like to do this but I do not have a good way to shore the jack rafters. I know this is technically a means and methods thing, but contractors often come back to me for help with this stuff, and if something went awry during shoring everyone would, of course, look at me.

EdStainless & HTURKAK - No, that gap is not entirely clear. There are some nails that would have to be removed, and the gap varies quite a bit.

 

[HouseBoy]

I'm curious how long the valley is (in plan) and how much roof load is there (to have caused the existing valley beam to split.)

 

[hemiv]

It's about 18'. 375lb/ft DL and 750lb/ft LL over the two plies, but not uniformly distributed of course due to the nature of a valley rafter. The original slate roof was replaced with a much thicker clay tile roof. Slate is back on the roof now. It seems to me that the rafters were originally undersized and overloaded for quite some time. There are also some inconveniently placed knots, which is where the split occurred.

 

[hemiv]

It turns out I do have enough space for a multi-ply LVL underneath the damaged valley. Since the valley is deflected, I'm going to have shims installed between it and the top of the LVL to provide the vertical load transfer. I also want to put some steel flat bars on the sides to provide a redundant load path with positive attachment (so that it's not relying on shims forever), as well as lateral bracing.

XR250, you mentioned 2x4 at 24" o.c. - Is this primarily for lateral/torsional bracing of the LVL?

 

[Lindseyr]

It's crucial to thoroughly evaluate and develop a well-designed repair solution for an aging 2-ply 2x12 valley rafter in a 130-year-old story building, like my grandparents' old house. The rafter's undersized nature and splitting knots present significant challenges that require careful consideration and expertise. While options like using an LVL or spacers may work, any repair must not endanger the building's structural stability or cause further damage. Temporary support for the jack rafters may be necessary during the repair, requiring careful attention to avoid damage to the slate roofers and ensure safety for the building's occupants.