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Belfast truss reinforcing

Belfast truss reinforcing

Belfast truss reinforcing

(OP)
Client has a 60ft span Belfast truss with a 7ft rise. Not a system a lot of people are familiar with, took some digging on my part to identify it. (see attached) The building was built circa 1918. The trusses bear on top of columns with beams supporting shed roof framing on both sides.

About 20 years ago, a tie-rod system was added to the bottom chord of the truss to increase the LL capacity of the truss to get close to a warm roof with an unobstructed slippery slope (current condition).

Jump to present day, client wants to insulate the roof. Unbalanced snow loads in this case are more than double the current capacity.

I looked at the possibility of further reinforcing the truss or sistering a truss next to the existing one. We can't just add a truss midspan due to the capacity of the existing beam.

I'm at a loss as to how to accomplish the construction. The plant is in operation, but is fairly open. How do you construct a top chord with an 81'+ radius? Or if the answer a bowstring truss, dbl howe or dbl fink truss with blocking up to the existing purlins? The plant produces wood products, so client is thinking maybe this is a project they will construct.

Thoughts other than "forget it it can't be done" smile

Thanks in Advance.

RE: Belfast truss reinforcing

I'm not clear on the spacing of the trusses or the purlin system between them but if it is substantial, it seems that a new truss or glulam bowstring arch beside each existing truss would be the way to go. If the spacing is close, any decent gang-nail truss company can design a girder with a pseudo-curved top chord, with field splices if necessary to get it in there. Adding capacity by fastening things to such a complicated truss would be difficult to design & justify the design.

On the other hand, are you sure it needs it? I've analyzed patented trusses from the 1930s-40s on hockey arenas, 100-110' spans, that numerous parties said should be replaced and it turned out they were actually better than anyone really imagined. Some of the trusses were laminated up out of 1x10s, if you can imagine.

RE: Belfast truss reinforcing

(OP)
The trusses are 20 feet O.C.

I've attached a sample photo at an end bearing condition.

Most of the members are ok at the increased load, but the splices and connections are questionable, especially on the bottom chord which is nearly at capacity for bending. There is also a problem with excessive shear in the top chord near the ends.

The stresses in the webs are minor but there are a few that are cracked or have been notched.




RE: Belfast truss reinforcing

That's a beautiful truss, even if not necessarily the most efficient. 2 hockey arenas that I analyzed with Hipel trusses (there are apparently only 5 left in Ontario) had both had previous misguided reinforcing, and connections turned out to be the only real issues with them. Even the connections that appeared extremely underdesigned didn't actually show any distress, but we added bolts anyway just to sleep better. I think the biggest problem is in really modelling them properly,it's way beyond my capabilities but I had 2 young guys from another engineering firm doing it for one of the buildings, and they started out shaking their heads at the trusses, but ended up with a new respect for them and for older buildings in general.

All that aside, 20' is a pretty fair span for wood purlins, and from the looks of that photo, there's no way you're going to sister the trusses. Can you increase the shear capacity of the top chord with dowels or screws? Check out SFS Intec.

RE: Belfast truss reinforcing

(OP)
I'm no spring chicken anymore, but I did manage to model the truss. winky smile With the complicated geometry, it wasn't easy.

The top chord is 2 layers of (5)-1.625x3.5 timbers laid flat. My concern with plating the top chord is the delicacy of placing any through bolts or lags and the potential to compromise the strength of one of the 2xs with a misplaced hole.

I haven't been able to determine how the layers are connected or how they are spliced. Some of the connections are a mystery.

Also, some of the trusses have had webs added to the exterior of the truss to supplement some that were charred in a fire. (this keeps getting better, right?)

I've done flitch plates in the past, I guess I am not sure how I would increase the shear capacity of the top chord with dowels.....

RE: Belfast truss reinforcing

Could you do load test on one to see if it has the capacity for the new snow load?

RE: Belfast truss reinforcing

(OP)
That wasn't an option that had crossed my mind. Unfortunately, we couldn't risk a failure, we would have to build a replica and test that.

I'm in a holding pattern right now while my client evaluates costs vs payback.

RE: Belfast truss reinforcing

I didn't realize that the top chord was 2 built-up members, but even so dowels could increase shear capacity if ply-to-ply or even the shear capacity of the species is insufficient. I think from the sounds of things, (fire damage?, yes, I saw that in one arena too and it wasn't pretty) you have a lot of considerations to deal with and maybe not enough budget. It's fun, though, isn't it? Until the crows come home to roost.

RE: Belfast truss reinforcing

You can increase shear strength with glued and screwed plywood, LVL.. plates on both sides. Or with inclined long screws with full thread. Is it longitudinal shear that is the problem or splitting of the wood?

RE: Belfast truss reinforcing

I would add new columns and new trusses to simplify the whole guessing game instead of messing around with the existing trusses. Obviously cost will be an issue.

RE: Belfast truss reinforcing

(OP)
They decided that the cost, with everything said and done, the payback just wasn't there. Oh well.

RE: Belfast truss reinforcing

Those connections will probably be split rings or shear plates. You should be able to figure out the diameter with a probe from the top/bottom. Cleveland Steel (TECO) had design resources last time I checked.

Sry for ninja edits: Also you will want to find the engineering for the previous retrofit (tension rod, plates, saddle, bolts). It looks like that will be in the way of any attempt at sistering.

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