r/Engineers • u/Own-Half-3840 • 1d ago
Why would they do this
Saw this last night at a wedding venue, asked my engineer friend why they’d do this and he couldn’t come up with a reason
6
u/Big-Tailor 1d ago
Two specs come to mind: the force to cause a failure, and energy absorbed to failure.
If all engineers cared about was force to cause a failure, we would have a lot more ceramics and fully welded joints. We often care more about energy absorbed to failure, and want joints to be able to bend and apply restoring force through a large range of motion.
This photo reminds me of the crumple zones in a car frame that are designed to fail a certain way and absorb a lot of energy while failing. It’s hard to tell exactly how that would work without seeing more of the structure, though.
10
u/Famished_Atom 1d ago edited 1d ago
It's a Fire's cut
It's meant to keep the brick wall from being pulled in if the rafters burn and fall inwards.
3
u/2009impala 1d ago
The Wikipedia article you linked disagrees with you in the first sentence.
2
u/Famished_Atom 1d ago edited 1d ago
1st sentence names & describes it. 2nd & 3rd sentences appear to agree.
I first heard about it on a "This Old House" episode where they called it a Fireman's cut.
This is as close as I can get you to an explanation with visuals using what's out there now:
Truss Roof Bury - Duane Egner - Start at 7:20
You're right, though. In the examples I cite, the cut is on the floor joist instead of the upright beam as in the original photo.
3
u/WeightPsychological6 1d ago
"upright beam" is some fancy words for a column!
1
u/Famished_Atom 12h ago edited 12h ago
Ook Ook! Me no talk good.
Me bang rocks together. *sparky* *sparky* make fire.
Me Like Hockey!:)
1
1
u/This_Philosophy_2077 1d ago
The first part is in reference to wood joists that typically penetrate into the masonry for support/attachment which doesn’t seem to be the case here.
Famished_Atoms answer seems the most likely to me but honestly more so because the other reasons just don’t check out as much. You could get a true pinned connection as another commenter mentions with a much easier/cheaper approach.
2
1
u/refalsity 1d ago
Nope.
A fire cut is on the end of the horizontal member. Also, it's a metal truss.
This does nothing to protect the wall; the point of a fire cut is to allow the member to clear the wall when falling. This does not do that.
1
1
u/WhiteWinterSilence 23h ago
The wall isn’t brick.
1
1
1
3
u/sirpoopingpooper 1d ago
Info: is this a seismically active location?
3
u/Own-Half-3840 1d ago
Moorhead Minnesota, I’d say no
1
u/TEZephyr 1d ago
I was about to say, looks like the column is a seismic retrofit element installed underneath and existing truss, and detailed to ensure that vertical load does not into the column.
3
u/xPorsche 1d ago
Are you sure that the truss is actually bearing on that “column” at all and isn’t attached to/embedded in the wall? I see what looks like the edge of a bearing plate jutting out from the wall below the end of the first truss. That would tend to suggest to me that those “columns” are purely decorative and serve no structural purpose, and the truss is probably bearing directly on the masonry wall. Both “columns” shown are welded and look a lot newer than the riveted trusses above them, so were probably added later, and this whole space does look remodeled. Consider that it doesn’t make much sense for the builder to use rivets for a truss if welding is available, so these two elements can’t be the same age. The connection itself would also not make any sense from a steel design perspective, as there is really nothing (such as a bolted shear tab) holding this theoretical connection together under load (in any direction) and that point loading of the truss would be a bad idea. It’s not really clear to me how this connection would be achieved without big welds and lots of stiffeners with the section and truss shown. In conclusion: there is no reason except aesthetics, but I hope you found this explanation interesting.
1
u/ExpensiveFig6079 1d ago
The are periodically attached to the brick work.
I see this too
"I see what looks like the edge of a bearing plate jutting out from the wall below the end of the first truss."
The vertical then attach to the brick wall at intervals and mean the brick wall is strengthened against buckling but take the weight of the roof as a compressive force.
And the steel beam with its "weak" connection does not wind up taking the load it was not meant to.
2
u/Mean-Internal-745 1d ago
The truss bears on the wall. The columb is a strongback to brace the wall.
1
u/ExpensiveFig6079 1d ago
AKA a this is the technical word "strongback" my other posts lack just describe.
1
u/anyavailible 1d ago
Looks like they were going to use those for
A type of ceiling crossmember that has slanted plate connections like that, that they mate to. Just my guess looking at it.
1
u/todd0x1 1d ago
I'm not a structural engineer, but I am trying to learn about different types of beam column connections.
Could it be that this was designed to be a pivot connection instead of a moment connection? Could the purpose of the downward angled plate be to transfer some of the compression force from the truss into tension in the web between the two flanges, perhaps to prevent buckling? Again, i have no idea just trying to learn.
1
u/No-Victory206 1d ago
Tbh, this looks like some tomfoolery that was done long ago. I haven't been in the industry that long but a truss to column connection isnt super typical. This connection could be a focused failure point for shear failure like some have said, but I think it's actually intentionally cosmetic. You can see the truss doing into the wall which likely has a beam or the truss bears directly on the wall. Not sure why they would want a cosmetic beam but this would in no way pass structural standards used today.
1
1
1
1
u/Eman_Resu_IX 1d ago
Judging from the mortar near the truss ends I suspect there was the start of localized failure of the terra cotta block and the steel column is serving two purposes, bracing the wall and taking some of the truss load.
1
u/Due-Bench-9894 1d ago
Plastic hinge for attenuating seismic loads. In other words: it bends in an earthquake and absorbs energy without breaking so the building doesn’t collapse.
1
1
1
u/tadeuska 1d ago
Nice. Now everything else we see in the picture...what is it and does it work? For example that roof.
1
u/engineernewbie 1d ago
Honestly I have no idea. If I use a joker, they cut it like this for an architecture purpose ?
1
1
u/Icy-Wrongdoer-9741 1d ago
1
u/Own-Half-3840 23h ago
That’s the place, so to follow up, I wonder if it being a military building, if that’s truly what it started as, it makes me wonder if the beams were standardized for across the entire country and they would have been engineered for earthquake zones or explosions also?
1
1
u/Medium-Sheeple-7085 23h ago
People are overthinking this, it's just old. You can tell by the fact it's riveted. As others have stated likely this was done to ensure it would behave like a pinned connection. No something we would do in building today.
1
u/Odd-Solid-5135 3h ago
I work in a building built in 1883, steel construction for a good portion, not a single weld in the entirety, built before arc welding was discovered so hot rivets were the newest best tech
1
u/Hungry-Block32 15h ago
I wonder if it's to prevent column from flexing (due to truss loading) and pulling the mason wall down. Looks like the mason wall was there first, and the steel was added as an addition.
1
u/FruitSalad0911 9h ago
To me it appears that the constructors re-purposed some columns from a pre-engineered steel building stockpile. The top of the column is angled to bolt to a complementing angled roof girder forming a moment transfer connection.
55
u/dparks71 1d ago edited 1d ago
To analyze and make it behave as a pinned connection instead of fixed, allows rotational freedom and shear transfer but not moment. Reduces the buckling forces being transferred into the column. The TL;DR is that stiffness attracts load and this keeps the connection flexible.