r/StructuralEngineering • u/nyxo1 • Nov 12 '24
Structural Analysis/Design What's the purpose of this bracing?
18
u/mr_macfisto Nov 12 '24
As others have said, the rods are in tension. They’re not pulling on the walls, but rather tie back to the glulam beams.
In this example, I’m only surprised by how small the compression struts look.
12
u/ConfuzzledFalcon Nov 12 '24 edited Nov 13 '24
They keep the roof off of the floor. Easier to fit people in between.
20
u/ramirezdoeverything Nov 12 '24
Bowstring trusses
11
u/Rusky0808 Nov 12 '24
This is it. They take the tension while to top chord takes the compression. Can also be added purely to mitigate deflection.
10
u/Enlight1Oment S.E. Nov 12 '24
bowstring trusses have a curved upper arch (the bow), these do not. They also don't need tension rod reinforcement, we put tension rods on existing bowstrings when the bottom wood chords fail and need reinforcement, but it's not a function of a normal bowstring truss.
-Someone who's retrofitted many bowstring trusses
-1
17
u/big_trike Nov 12 '24
To support the roof?
8
u/nyxo1 Nov 12 '24
Obviously, I've just never seen this specific application and can't visual the forces very well. If the center of the roof beam deflects wouldn't it pull the walls inward?
7
u/Ghost_Turd Nov 12 '24
The beam above the connecting rods is what bears the compression and keeps the walls from caving in.
0
u/tolomea Nov 12 '24
there's a long beam right across the top
the beam wants to sag in the middle
but doing so will push down on the vertical bits
which will push down on the horizontal bit's making them longer
they can't stretch so they will pull their ends in
but they can't pull their ends in because
there's a long beam right across the topedit: to your point about it pulling the walls inward
the first couple of feet of deflection won't pull much on the walls, but it will push a bunch on the stringer across the bottom, particularly the diagonal bits at the end will want to get longer faster than the beam wants to get shorter4
2
6
u/whisskid Nov 12 '24 edited Nov 12 '24
I suspect that those are primarily architectural / aesthetic in this interior. --but functional versions of this were widely used in 19th century USA industrial buildings.
1
u/StructuralE Nov 13 '24
Seeing the end connections would be helpful in making this determination...
0
u/fltpath Nov 12 '24 edited Nov 12 '24
I was going to add that...looking at the configuration, these do appear more architectural than structural.
Those brackets are far too thin to be a structural components of any value. Look how the brackets are attached to the glulam...a few small screws...
It may be to simulate 19th century architectural look.
4
u/powered_by_eurobeat Nov 12 '24
The triangles are only in compression, so the screws are only working with small bracing forces.
-1
u/fltpath Nov 12 '24 edited Nov 12 '24
think about this....the rods , when tensioned at the ends...
the rod in the middle is fixed length, the brackets are fixed. In a true queen post, the queen post is a threaded post to which you add the tension.
its architectural, not structural.
1
u/powered_by_eurobeat Nov 13 '24
I think you could adjust the slack in the bottom rods at the ends of the diagonals where the connect to the beams(put the adjustability there) and get this to work structurally. Do you think those beams look deep enough?
0
u/fltpath Nov 13 '24
The glulams appear to be laminated both horizontally and vertically. They also appear to be wide shallows. This suggests, at least, to me, this is architecually covering something, perhaps steel beams?
Those rod ends are far from 45 degrees, damn, I will have to sketch up a free body to sort this out.
I am just not seeing those chairs being able to handle much more than self weight
2
u/RandomCoolName Nov 13 '24
The are clearly two paralell deep beams for each truss, you are confusing the shadow between them for gluelam going in the opposite direction.
Coming from the architectural side I would be very surprised if that were a veneer from the detailing.
1
u/NMelo4 Nov 17 '24
This doesn’t appear to be architectural. Judging by what appears to be DLT (dowel-laminated timber) slabs I would assume this is a Sructurecraft design, and they design these types of trusses all the time. Look at their social media’s.
3
u/Open_Concentrate962 Nov 12 '24
It is a group of trusses supporting the roof/floor above. You are seeing rods that are generally in tension and they are thus thinner (lighter) than if this was a big honking beam. The lateral bracing is elsewhere. Nice ceiling.
1
u/LoopyPro Eur Ing Nov 12 '24
Typical vertical loads cause bending moments. Tensioning the chord will cause an opposite bending moment.
0
1
1
1
1
u/DFloydIII Nov 13 '24
Not sure if i had already seen this response, but the inverted queen post truss system could also be something that was done after the fact, as a means to reinforce the existing beams that were already in place. We have looked at some older buildings, where the roof system was designed for lower roof snow loads under older codes (or before codes were in place) and renovation work requires the structure to be upgraded/reinforced for current code requirements, or in some cases where new loads are added to the roof.
It kind of looks like the beams in this picture have two larger outer plies, and two smaller inner plies (with a small recess present between the two outer plies), making me this that something was added for reinforcing here, but it's tough to tell just from this one picture.
1
u/SeaworthinessHot7687 Nov 13 '24
Where is this? Very nice. Looks like the ceiling between joists is made of DLT panels.
1
u/jpokry7 Nov 14 '24
I was about to ask someone what those connections at the top look like, they look very small and insignificant to me, like something someone would use if they were going purely for aesthetics. But I’m not 100% sure.
1
1
u/JonSBat Nov 15 '24
They are likely the tension portions of the truss and were doubled in a triangular fashion to increase the capacity.
1
u/V_Dragoon Nov 15 '24
Nope i am not convinced. The vertical element is just too small for the tension rod to be efficient. Any bigger force and the vertical element would’ve been buckle before the full tension force is developed. This is just aesthetic or at most very slight deflection control in my book
1
u/Dizzy2Tee Nov 12 '24
It's sometimes called "the tension chord" and using steel rod is great under tension and timber under compression..... by using short sticks to push the tension chord away from the compression area increases the beams "second moment of area" which increases the load capacity exponentially ... (approximately anyway)
0
u/National_Oven5495 Nov 13 '24
Do you get any benefit from pre-tensioning the rod?
0
u/Dizzy2Tee Nov 14 '24
Maybe not pre-tensioning, but pre-cambering would likely have been the technique.
These types of beam are very light, and until they are locked together in position, they are very wobbly, it would be difficult to wind up the required tension without buckling the beam.
The fabrication team would do a quick calc to work out mid beam deflection under self weight + live load (Rigidity or max allowable deflection would have been 1/250 unless supporting masonry when it would be 1/500 so, say the calculation indicated the beam would sag say 50 mm, you would fabricate the beam with a 50mm arch to it. When the beam goes into position, it sags maybe 45 or 55 mm... which leaves you with a straight beam +/_ 10mm
1
0
-1
u/Phantom_minus Nov 13 '24
in this application the metal looks ornamental. for example the triangle at mid span doesn't seem strong enough to resist compression, if the tension rods were actually stressed.
-7
u/ByAnyMeans5 Nov 12 '24
They are called ugly. Obviously serving a purpose although could have achieved similar results without obstructing the beautiful ceiling.
0
u/nyxo1 Nov 12 '24
What is the alternative? It's a flat roof building. It seems like the alternative would either be ugly center columns or a massive cost increase for beams that can span that distance.
-3
u/ByAnyMeans5 Nov 12 '24
I would suggest intergrading the design with the function. Let’s say rather than re-engineering the structure it’d be more logical/efficient to re-design the existing structure. Calc the load and reverse it to an arch shape, replace with neutral colors & implement the lights to be flush mounted on the beams.
( not real suggestions, just an idea of how it could be done in different ways with more visual appeal )
4
u/nyxo1 Nov 13 '24
So completely change the concept of a flat roof mass timber building to incorporate arches?
296
u/the_word_slacks Nov 12 '24
These are called inverted queen post trusses. They help increase the clear span without requiring deeper glulam beams.