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lens-like beam

lens-like beam

lens-like beam

1 - what is the correct term for this type of beam (lens-like beam is direct translation of italian term)?
2 - any special design rules/standards or just structural beam design ?


RE: lens-like beam

That's pretty neat. Not sure if there's a specific term for it in English - the bridge guys will have to speak up there.

Looks to me like this is a truss, though. Cantilevers come off of the abutments and are supported by the green towers and tension members. Then the "lens" is simply supported at the ends of the cantilevers. You have a top chord (cambered slightly up) and a bottom chord (cambered waayyy down) and web members (vertical pipes/round HSS, maybe?). I see several thin wire diagonals, but I can't tell if they're in the plane of the main truss or just for lateral stability (acting as bridge diaphragms).

Design rules in the states would be captured in AASHTO.

RE: lens-like beam

This type of truss arrangement is sometimes referred to as a "lenticular" truss, because of the lens-like shape. These often have symmetrical top and bottom chords, while the bridge pictured may be closer to an inverted arch bridge with a cambered top chord. For an example of an inverted arch bridge, check out the Kahoma Street Bridge on google.

RE: lens-like beam

Interesting structure, but I just wish the two systems were better integrated. Suspension bridge and truss. Why couldn't the tension "cable" continue around the bottom chord of the truss.

RE: lens-like beam

glass99 - agreed, I think hybrid structural systems like that need to be carefully detailed to achieve visual harmony, else they will look like a frankenstein's structure.

Kipfoot - That is a beautiful bridge!

RE: lens-like beam

This is not a lenticular truss, although it is outer shape is the same as one. This appears to lack web members, other than the vertical struts, so I'd call it an Arch of some type, say reverse arch or underslung arch. One engineering item of note: Under uniform loading this type of structure would be very strong, but a point load in the outer thirds of the span would create some significant bending moments in the chord members since there are no "truss" diagonals. That's why a lower cable in this instance wouldn't help much for that.

RE: lens-like beam

Quote (TrussBridgeboy)

but a point load in the outer thirds of the span would create some significant bending moments in the chord members

yes that's why suspension bridges have stiffening trusses. Its got a pretty sturdy top chord of the lense which should be able to do that job.

Its a shame bc when you get license from a client to do something different, you really should drive the idea all the way home.

RE: lens-like beam

My boss designed this bridge. I came in to work at the tail end of its construction. At the time it was designed, it was only one of two of its kind in the world. The other one being in Japan. It is an inverted concrete arch bridge, similar in behavior to the OP's original post but different in many ways. Concrete vs steel. Post-Tensioning Tendons vs Suspension Cable. Different way to dead end the tendons/cables at the end of the bridge.

It's hard to classify the OP's bridge with a name, but it is not a lenticular truss. It is not a truss at all. Maybe a hybrid suspension/inverted arch. I can't tell if the suspension cable is strung through the span of the arch but... Either way, the deck load is carried by the vertical compression struts to the inverted arch where the load passes across the plane of the deck to the suspension cables.

I really like the bridge the OP posted! It's beautiful, elegant, and you can tell that someone put a lot of thought and effort into its design.

As far as the OP's original question for what standards to use when designing these type of bridges? There is no cook book recipe that will teach you how to design like this. This comes from years of experience and a very solid foundation in structural mechanics theory. AASHTO will only help guide you with the more bread and butter common type structures. To design something truly unique takes a unique Engineer that is dedicated to his craft and is willing to take risks. Not for the faint of heart.

RE: lens-like beam

@strctpono: my quibbling about detailing aside, its still an interesting and novel structure which is worth all of us drawing inspiration from. More engineers should have such cojones.

RE: lens-like beam

Apologies for suggesting this was a lenticular truss, I intended that to address the "lens-like beam Italian translation" description. It seems there's agreement that OP's bridge can be classified as an inverted arch. I'm still not certain on what makes this not a truss, other than no diagonals, but could this be called a Vierendeel truss? I guess it's wrong to call it a truss because it wouldn't be designed using truss assumptions. I did some searching, and found more on OP's bridge. That segment can be seen in this picture being lifted and placed as a single stable structural unit:
Also, It turns out that the chords are actually glulam instead of steel! I think also that the overall structural system could be called a cantilever bridge, like the Firth of Forth bridge.

@STrctPono - Isn't that the Kahoma Street Bridge? That's very cool that you got to work on it. I have to disagree with you about OP's bridge being beautiful and elegant. The visible use of three different structural materials make it look disorganized. This impression is worsened by the fact that the structural system itself is a hybrid. Still this is a very interesting bridge, and it's obviously well worth discussion.

I don't believe that OP's bridge has suspension cables at all, but that makes me wonder: what is the difference, beyond semantics, between an inverted arch bridge, and an underslung bridge such as the Living Bridge at the University of Limerick? Can you get away with cables for the bottom chord if your top chord/deck is stiff enough?

RE: lens-like beam

Couldn't really be a Vierendeel truss without web-chord connections capable of transferring substantial moment.

I hear the aesthetic argument for continuing the stay/suspension cable into the middle segment, but that defeats the key benefit of the cantilever span system -- constructability. You build the cantilever arms balancing out from the piers, then lift the completed suspended span to complete the bridge. Beautiful system to solve bridge spans without effective access. (Not that it seems to be a particular issue here).

just call me Lo.

RE: lens-like beam

I posted the 1st photo just because of the shape (I didn't know about "lenticular truss beam", new for me and not available in Italy as far as I know). Sorry for my misunderstanding, but now I realize a world of structures similar by shape but not by design criteria. Very interesting : I appreciate Your comments. Thanks a lot

RE: lens-like beam

Quote (HSA_PA_EIT)

Also, It turns out that the chords are actually glulam instead of steel!
Timber! Very 2020. Negative carbon footprint too. Great photo.

Is this a footbridge?

Quote (Lomarandil)

You build the cantilever arms balancing out from the piers
Is there any condition that would have put the bridge into uplift? If not the central portion could have been installed with a cable bottom chord in exactly the same way.

I imagine the design was also a function of the limitation on the available length of glulams. I imagine also that whoever was in charge was pretty excited about glulam. It looks like it was built in the 80's when that technology was new and hot.

RE: lens-like beam

I must be retarded or something because I cannot seem to find any information on the bridge when I google it. What's the bridge name? I am curious as I want to read more about it.

Given that it is wood, relatively narrow, and that railing does not look like it is rated for vehicular traffic my assumption is that yes, this is a pedestrian bridge.

After looking at it further, I agree with HS_PA_EIT that those do not appear to be standard suspension stays. Looks like it might just be structural steel tubing. But I do think that the arch is steel sandwiched with glulam wood. If you look at the original photo, there is a green streak that runs on the top of the arch for the full length. Maybe it's to aid in the attachment of the steel compression struts but I think that they are using that steel as the tension element of the arch span.

As far as aesthetics... it's a subjective topic and everyone is going to have a different opinion.

And Yes HS_PA_EIT, that is Kahoma Stream Bridge. 360 ft single span concrete highway bridge on a curve with a massive amount of PT tendons in that inverted arch slab. When stressed, that bridge lifted itself off the sand jacks.

RE: lens-like beam

@STrctPono - I had to do an image search for it. I'm not sure on the name, but it seems to just be called "The Calore River Bridge". I found this paper that talks about it, but it leaves me with more questions than answers: http://support.sbcindustry.com/Archive/2010/june/P...

I'm not sure if that green streak is structural or some flashing mentioned in the paper. They also mention cables, so that could be what that is?

Agreed on the aesthetics, and to be fair it's grown on me. It being wood with green paint, it reminds me of a playground or cabin/camping structure.

That's so cool. I am very jealous of that experience. I can't imagine what the calcs were like for the bridge.

@Lomarandil - I think I see what you mean about cantilever system, this way they could sort of drop in the central span section. It seems to suggest in the paper I linked above that they diverted the stream prior to construction.

@glass99 - Nice to see it used for something that big. I believe it is a footbridge on a bike trail

RE: lens-like beam

That’s a really neat bridge. My hat’s off the design and building teams. The top green steel (or whatever it is) on the lower arched member is probably just basically top flashing, to keep the water and junk off the wood. Anything which really tried to impart some tensioning at that level would need significant hold down hardware, or it would just rip up off of the glulam member. Posttensioning cables would have to be internal or under the glulam member to have a constructive affect, without ripping upward. Continuous posttensioning wouldn’t work too well either, since it would tend to lift the middle span right off its end bearings if it imparted any real serious posttensioning. The two cantilevers show a pretty clever use of tension tie-backs to increase the canti. cap’y. Otherwise, that is canti. construction, “Gerber beams,” by another name, which we used all the time in warehouse roof framing schemes.

The sad thing is that that type of structure, bridges, many civil structures and the like, are some of the few types of structures where the Structural Engineer get to express their aesthetic design ability and structural imagination, without the dictation of others who think that they are the only people who have any taste in design world. Within reason, I don’t much care about the color of the brick, wall or floor tile, etc., the Arch’s main effort, but I sure do care that our structures perform as intended and don’t fall down on anyone. So, despite the common thinking, our contribution is fairly important, and often really complicated by the ignorance and egos of others. The plumbing, heating and electrical ought to work too, and those professionals are excepted to work miracles too, but they suck hind tit also, when it comes to design fees.

RE: lens-like beam

HS PA EIT : yes correct : bridge on "Calore" river, but only for bicycles and pedestrians. doc link is correct too

RE: lens-like beam

This is one form of cable stayed bridge, nice shape and design.

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