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"bridge in a backpack"

"bridge in a backpack"

RE: "bridge in a backpack"

Interesting. It's the idea of carrying concrete in by backpack that I don't like!
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: "bridge in a backpack"

(OP)
I'm still waiting for the day Legos become a viable building material.  Back then the biggest question was "should I use a round or square 2x2?"  Usually more colorful too.

RE: "bridge in a backpack"

I'm having trouble visualizing this.  They say the arches are "preformed from carbon fiber composite" and "inflated on site" and "cure within hours".  Elsewhere (mainly in the news article), they say the arches are "filled with concrete" - which led me to wonder how the heck they got the concrete inside the tubes.  I guess you could inflate an arch and then coat it with epoxy resin (or have it precoated), leaving it inflated for the resin to cure.  But, then how do you fill the tube with concrete without compromising the composite tube's strength?  Or is the concrete simply backfill over the top of the tube arch and sheet panels?  I think the latter, but the "filled with concrete" verbiage is misleading in that case.

RE: "bridge in a backpack"

I'm figuring you have to put an elbow and a standpipe on one end of the arch tube, and dump the concrete in at the top of the standpipe.  You need the gravity head to inflate the arch tube and provide enough internal pressure to make it self-supporting.

Yeah, saying 'bridge in >a< backpack' is misleading.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: "bridge in a backpack"

But, then you'd need a crane to put the concrete-filled arch into place, which they implied was not required.  And I'm not sure a concrete-filled carbon fiber sleeve (as opposed to hardened/cured carbon/epoxy tube) would be stable enough to hold the arch shape while the concrete cured.  Dunno, I may have to research it some more...

RE: "bridge in a backpack"

Hmm.  Careful re-reading of the article, and similar on the U of Maine composite center's site, the arches are shipped to site, then inflated and formed to arch shape in situ, and then rigidized (cured).  Subsequently they are filled with concrete (doesn't say how, and I can't see any external fittings at the top of the arches to allow for gravity feed, so I guess they pump it thru...how do they keep from having a void at the top of the arch I dunno...one more of life's mysteries...)

RE: "bridge in a backpack"

(OP)
Good find, it seems like this would work for smaller bridges but I can't imagine that those tubes would be able to span very far filled with concrete.

RE: "bridge in a backpack"

SKIAK, they do mention a girder design using some of the same principles which I'd think might enable larger spans.

I guess fundamentally they're using an exoskeleton like an insect rather than an endoskeleton like mammals etc.

I wonder how well the composite really protects the concrete, or if it degrades over time or at least if moisure can seap in somehow.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: "bridge in a backpack"

An exoskeleton, like reinforced concrete with the tension members on the outside.

I'd think a veneer of mortar would be appropriate, else the bridge is susceptible to damage by vandals.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: "bridge in a backpack"

btrue, I was thinking they filled it 'in place' either with a pump or maybe gravity flow as mike says.  http://www.maine.gov/mdot/transportation-research/pdf/pittsfield%20neal%20bridge.pdf

Quote:

The arches were manufactured from 12-inch diameter rigidized inflatable hollow carbon fiber composite tubular arches spaced at approximately 2-feet on center, erected into place and filled with concrete.

(originally posted after 14 Apr 09 13:44 post)

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: "bridge in a backpack"

(OP)
Damage is a good point.  I'm sure they could grout, get a fiber patch, epoxy?  Seems like the epoxy step could be problematic, getting the old to bond with the new... sounds similar to fixing my Coleman raft, I usually just ditch it and get a new one.  Heck at $500K a pop I might get one for my back yard... the bridge to nowhere...

RE: "bridge in a backpack"

I have some of the same misgivings about the exposed fiber tubes, but more due to debris impact from gully-washer floods and limbs/stumps/trees being washed down.

Kenat, yes, I read that too, but it wasn't clear at first whether the tubes were truly filled with concrete, or just backfilled/overburdened with concrete (i.e. left air-filled or hollow).  The photos and text in the original article weren't very clear on the sequence.

Then again, yes, I'm sometimes guilty of overthinking, and over-posting.  Debated red flagging my whole string of posts.  Then decided, nah, I'll add more noise with a reply to your post instead...

RE: "bridge in a backpack"

My post was originally in the middle of your diatribe winky smile but I screwed up the /quote and asked 'the management' to fix my typo.  However there was an issue and I was asked to repost, but now it's not in order ...

Still, seems pretty neat to me but I'd be interested to know about the durability issues.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: "bridge in a backpack"

I read it as another possibly cheaper way of doing formwork before the concrete is poured in. The composite will fall to bits over time leaving the circular arches unconnected to anything. These will in turn fall off sideways leaving a piece of corrugated steel dangling by hope alone to the concrete. The metal will then fall off leaving a wierd corrugated arch profile on the bridge. Water will get into the pores and inevitable voids in th corrugated profile and if there is any freeye thaw action at all the corrugations will fall off in chunks.

Looks like a design which saves money on construction and generates a whole load of maintenance problems 30-50 years on.

IMHO - Rubbish.

Can´t wait to see their new take on the wheel........

gwolf

RE: "bridge in a backpack"

Gulp! I wouldn´t drive over it!

RE: "bridge in a backpack"

"The composite will fall to bits over time leaving the circular arches unconnected to anything. These will in turn fall off sideways leaving a piece of corrugated steel dangling by hope alone to the concrete. The metal will then fall off leaving a wierd corrugated arch profile on the bridge. Water will get into the pores and inevitable voids in th corrugated profile and if there is any freeye thaw action at all the corrugations will fall off in chunks."

Guffaw.  Except that our current crop of highway bridges, especially in high road-salt-usage areas, pretty much follow the "rot away to the rebar" pattern in some 30 years.  While for the composite arches, the vinyl ester resin is potentially a moisture and salt barrier for the concrete, the assumption is that the barrier is not breached for some period of time (to justify their claims of 50+ year life).  One bum with a campfire under the bridge...bingo, no more composite.   

RE: "bridge in a backpack"

(OP)
Hmm... I do have my reservations about the lifespan of the composite, but isn't this stuff used in the aerospace field as well?  Seems like it should be a little more durable than you make it sound.  Don't they use carbon fiber laminate to repair or reinforce existing concrete structures?

RE: "bridge in a backpack"

"isn't this stuff used in the aerospace field as well?"

Yes, and the damage susceptibility is a real problem there; it is managed to minimize problems, and costs are (sometimes) traded off against the performance advantages of the lightweight properties of the materials.  But when a mechanic drops a wrench on a carbon fiber wing section, that section can quite readily develop hidden damage due to fiber breakage or delaminations.  A lot of time and money are spent inspecting parts made of composite materials to ensure they aren't falling apart, or about to.  They certainly don't park their expensive aircraft on roadsides.

RE: "bridge in a backpack"

(OP)
Thats a good point, if I owned an airplane I don't think I would park it on the side of the road either.  Although going down the road of campfires, I live in close proximity of several timber bridges.  I know that large wood members can resist fire, but I can't imagine that this carbon fiber construction would be more susceptable to fire than wood... right?  I guess I don't have a good grasp on carbon fibers.  I wonder if they put any thought into repairing these tubes.

RE: "bridge in a backpack"

SKIAK,

do you ski? I had carbon fiber ski poles once, when they broke you can see the carbon fibers held strong but they are held together by some epoxie media which splintered - possibly from the cold. I would guess they would just hold up well against a small fire...

how big of fires do bums normally have?

RE: "bridge in a backpack"

(OP)
Heck ya I ski.  Love it.  Although I don't think I have had a pole break, but maybe I'll start a bonfire this weekend and throw one in... for research purposes.

RE: "bridge in a backpack"

I was being a bit facetious about bums and campfires.  But weathering and impact damage would be my worries for a bridge 'round here.  We have big storms that send a lot of wood debris churning down streams.

RE: "bridge in a backpack"

(OP)
Sometimes its hard to tell... and I agree, I could see a problem with a large log ramming into the side of one of these.

All things considered (or at least most), I think its apparent that this type of construction has its limitations as with any.  I just enjoy reading about people "thinking outside the box."  Helps remind me that there is still a lot of creativity and ingenuity that goes on out there.

RE: "bridge in a backpack"

Agree with your last, Skiak.  It's a cool technology, and some simple tweaks could make it robust against the types of things we are quibbling about here.  Certainly the speed of construction and use of light(er) equipment make it a nice idea for emergency repairs, or road work well back in the boonies where heavier equipment might not be available or transportable over existing roads.

RE: "bridge in a backpack"

An interesting observation by JE Gordon is that there is no uniform 'best' solution for bridge structures, despite the rather obvious limitations on the general requirement. So, expanding the envelope of possibilities is a good move.


 

Cheers

Greg Locock

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