103 year old church building - structural assessment 2

[Sam165]

I recently finished a structural assessment on a church building that was built in 1917. The building is comprised of 55-foot long 9-ply wood trusses with 2x8 roof joists spanning 12-feet at 16-inches on-center perpendicular to the trusses. There are two specific deficiencies that I would like to pick your brain on:

1) There are a couple of 2x8 joists at each bay that are notched at the end bearing and this has caused the member to split parallel to the wood grain (see attached pictures). I ran a calculation (see attached) to determine the shear strength of the notched member and determined that I would need two LSL members sistered on both sides of the joist to reinforce it. What is the best way to reinforce this type of notch?

2) The truss members are attached with 1-inch thick plates (iron?) and 0.75-inch diameter through bolts. Many of the square nuts are missing and some of the bolts are too short or long to get a nut to tighten against the plate. There is one particular plate that has a hairline fracture that runs through the entire plate section. I'm thinking of reinforcing this plate with a steel plate that is placed in front of it and specifying replacement bolts for all areas that require them. I'm hoping they can get these old rusted bolts out to replace them. In addition, I was wondering if I can have them add a wood shim to the bolts that are too long so they can get the nut to tighten against that. What issues might that create?

Thanks for your help.

Sam

 

[phamENG]

Pretty nifty.

For the notch, I've seen light gauge straps used with some success. Simpson Coil Straps, cut to length, and fastened to the upper portion of the joist and looped down to create their own "seat" - it helps keep the joist from splitting along the grain there.

Since you'll be replacing bolts anyway, why not remove the fractured plate and replace it outright? I'm not a fan of hiding broken stuff, especially when it could continue to come apart and create a gap behind the plate that should be doing the work.

For places where you're not replacing the plate and the bolts are too long, use metal plate washers of whatever thickness you need - not wood.

 

[KootK]

I like screw reinforcement where it can be made to work.

I'm surprised that bending in the bolts works. At nine plies, the bolt diameter looks a little small-ish to me.

 

[canwesteng]

The bolts can't be original can they, if it was built in 1917?

 

[jimstructures]

Square head bolts are older than 1917. (By quite a bit, at least the Civil War (USA ), or older)

Jim

 

[Sam165]

@phamENG: I like the strap idea to avoid the joist splitting. As far as the fractured plate goes, I really did not want to alter the original construction unless absolutely necessary. The additional steel plate would act to stop any further separation in the existing fractured plate. Also, some of the bolts are 3.5-inches too long so that makes me lean towards a 2x4 shim as opposed to a steel one.

@KootK: I also like the screw idea, it will either be that or a strap and the two LSL gussets attached to both sides of the member. The bolts definitely looked like they were undersized for such a large truss, but it's been holding relatively well for 103 years.

@canwesteng: Yes, the bolts are original.

 

[Ron]

@jimstructures....square head bolts and nuts are still available and in use.

Agree with phamENG on strapping.

 

[IFRs]

Are the metal parts brittle castings? Should they all be replaced or overlaid with more ductile steel plates? Are the bolts also of inferior qualtiy? Should they be knocked out and replaced with fasteners of suitable quality? Yes it has stood the test of time but in the process corrosion and stress has damaged some of the structure which may lead to a cascade failure of adjacent joints. The safety of those under the roof is the number one concern.

 

[phamENG]

IFRs - agree with you completely on the safety of the occupants and the need to investigate those items. In my area, there is significant pressure on designers to maintain the "historical fabric" of the structure to the maximum extent practicable. That may be a consideration here as failure to do so can sometimes disqualify owners from getting tax breaks/credits or other assistance from historical groups. That said, I've had to stare them down across the table and tell them it didn't matter - the historical fabric was too far gone.

 

[IFRs]

I just never want to be in front of a judge saying "well, it was good for 103 years, so I figured it would be good for 50 more"

 

[Sam165]

@IFRs: My scope does not include assessing the material composition of the existing structural components. The church does not have the budget to go that route. I recommended that they have a preventative maintenance program in place to help identify any future areas of concern. Some of the bolts are deficient in regards to not being the correct length and showing some corrosion, but I might have them replace all of the bolts to be on the safe side. The only plate that was observed to have a hairline fracture was the one I included a photograph of in my original post.

phamENG: What do you think of sistering two LSL's on both sides of the notched rafter in addition to the strap? This is what is required to reinforce it for shear per my calc but I'm wondering if there is an easier way to reinforce it.

 

[phamENG]

I suppose it makes sense. How did you arrive at the allowable stress values for the existing member?

 

[Sam165]

Made an assumption of 135 psi for Fv which is conservative since wood that old is much stronger than what is available today.

 

[phamENG]

Not always. You could have a nice, old, dense piece of lumber with a giant knot at mid-span on the tension face. That IS true with base values for Southern Pine, but only visually graded stuff. You're in Arizona - what species are you working with? DF? I'm not familiar with west coast lumber value trends. Current shear value for Southern Pine ins 175psi, regardless of grade. DF is better than that.

That's your F_v, but what are you using for your F_v'? You're in the roof, so you may get a boost from the load duration (though at 100 years old, I can understand if you bring that back down to 1.0). How hot does that attic get? Would it be considered "sustained exposer to elevated temperatures up to 150F"?

 

[Sam165]

I’m currently in Texas, they usually use SYP, but I assumed SPF No.1/No.2 to be conservative. My F’v is the same value, didn’t want to risk taking Cd as 1.25. Again, trying to be conservative since I’m using assumptions. The attic will not get that hot here in Texas.

 

[phamENG]

Fair enough. I will say that assuming SPF is probably overly conservative. SPF may be readily available in the South now, but that's due mostly to modern transportation networks. The species that make up SPF grow mostly in Canada, though there are a few that grow on the north west coast of the US. 100 years ago those transportation networks weren't quite in place, and it's very unlikely that they didn't use something from the Southern Pine species group.

 

[jimstructures]

Ron,

Thank you for the reply to my post.

I am well aware that square head bolts are commercially available at the present time. I was responding to "canweseng's" question of if those could be original bolts, i.e. 103 years old? Those bolts could be original and I would assume they are original unless there is additional evidence one way or the other.

Jim,

 

[KootK]

There are some things that I don't love about the strap setup. I've tabled my thoughts in the sketch below for discussion.

What do you think of sistering two LSL's on both sides of the notched rafter in addition to the strap? This is what is required to reinforce it for shear per my calc but I'm wondering if there is an easier way to reinforce it.

1) As shown below, there may be a way to justify a lower amount of shear over the notched depth. I'm not sure if this is already baked into your calculation or not.

2) Detailed properly, the LVL sistering scheme would surely be the most robust approach as it requires no questionable assumptions about how much shear is or is not to be excluded from consideration when evaluating shear in the joist bearing extension.

 

[Ron]

@jimstructures....sorry, I noticed that after I posted.

As for the strapping, if the crack is glued (epoxy preferred), clamped and then the strapping is applied, it negates the issue of non-composite action. Have had good luck with epoxy injection of damaged/deteriorated timber sections.

 

[phamENG]

KootK - I've never considered the strap as a means to increase the capacity at the joint, just a means to keep the member from splitting any more (or in the first place, if needed). And even if you could justify transforming your V2 into a point load applied within d, you don't get to ignore it, just reduce it (by a factor of x/d). So the gains would be minimal.