Existing Glulam Truss Connection

[Margana]

Hello,

I apologize beforehand if this is a redundant question. I have done a few years worth of work with wood design but haven't seen this before. I am doing an inspection of an existing glulam roof truss to verify the allowable live load capacity of the roof. Per my model, the wood truss itself seems quite strong, but the connections seem to be having problems (the connections max out at 60-70% of the strength of the wood). I would typically consider the first image to be a 4-bolt connection but I can't figure out what purpose the two perpendicular bolts serve.

I've heard of various types of specialty wood connections but haven't regularly seen them used. Does anyone have information on what the purpose is of these two perpendicular bolts and if they help the will increase the strength of the connection.

Thank you so much for your time.

 

[msquared48]

Look like pretty weird, incomplete, and unbalanced connections to me.

As for the two bolts, they seem different than the others. I am wondering if these were the bolts for a previous connection, and the other plates and bolts were added to supposedly strengthen the connection for some reason currently unknown.

Mike McCann
MMC Engineering

 

[hokie66]

I would think the transverse bolts are there as an effort to prevent splitting of the web members, like a clamp to hold the members together if they want to split along the bolt lines. As to how you would quantify the effect this has on connection capacity, I don't know.

I assume that you don't know what is behind the steel plates. Could shear plates or split rings have been used? If so, the capacity would be much greater than with the bolts alone.

 

[msquared48]

T me, the straps look like an after thought, and a very misplaced one considering what the bolts are and are not grabbing - one in the angled cross-grain and two for the parallel dies not equate in force.

As for the two bolts holding the members together, maybe, but I could not see any splits or member joint. Is there one?

Mike McCann
MMC Engineering

 

[Triangled]

Can't tell how deep your top chord is, but it appears the centroidal axes of your webs may not be aligning with that of the chord, to which I think mike is alluding.
When I see bolts without plates in trusses, I think "hidden" connection, as sometimes occurs for architectural purposes. But it's not occurring to me at this point what could possibly be hidden in your case, especially if the 3 or 4 web bolts through the steel are calc'ing adequately.
Also the end distance of the perpendicular bolt in the 4 bolted web is such that it likely achieves nothing, at least if the member is in tension. And for that matter, the third bolt from the bottom in your second photo.....is it even in wood?
Is that a butted joint in the top chord in photo 2? I've alway thought a nice advantage with Glulam in these sorts of trusses was long continuos lengths.
I would expect bigger spacings between bolt heads for shear plates or split rings.....but then again, we only have photos. But could be, as 66 reminds us. They are strong and not uncommon in these trusses, especially older ones it seems.
I am getting vibes of a poorly fabricated truss.
Just some thoughts.... G'nite.

 

[RARWOOD]

What you have is one of the strangest wood connections I have every seen. If you can post a diagram of the truss full length.

Regarding the bolts perpendicular to the steel plate, they are called stich bolts. There are different opinions on whether they are very effective. I knew one engineer who almost always used them.

The bolt that goes through the steel plates but not the wood may be to prevent buckling of the plate if under load reversal the member is in compression. With the splice in the top chord I wonder if the diagonals are intended to support the top chord through compression with the load being carried by bending of the bottom chord.

My opinion is the system was designed by someone who does not do a lot of wood design. My recommendation would be you get someone with more experience to look at it to give you some guidance. You might talk to your local glulam supplier or APA rep.