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Author Topic: Difference between concrete and metal bridges at the bottom  (Read 1152 times)


Big John

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Re: Difference between concrete and metal bridges at the bottom
« Reply #1 on: November 11, 2019, 11:07:48 AM »

The first one is stay in place deck forming, while other 2 have the forms after the deck hardened. It depends on the jurisdiction to determine which one to use.  IMO the removed forms make the bridge easier as the metal forming covers the bottom of the deck can hide any defects.
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tolbs17

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Re: Difference between concrete and metal bridges at the bottom
« Reply #2 on: November 11, 2019, 01:43:20 PM »

The first one is stay in place deck forming, while other 2 have the forms after the deck hardened. It depends on the jurisdiction to determine which one to use.  IMO the removed forms make the bridge easier as the metal forming covers the bottom of the deck can hide any defects.

And the first ones look like they are being built more nowadays. Because in this state, I can only find 2 overpasses that have the the forums after the deck hardened. Maybe the stay in place deck is cheaper?
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Duke87

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Re: Difference between concrete and metal bridges at the bottom
« Reply #3 on: November 11, 2019, 11:29:28 PM »

So firstly it helps a bit to understand some of the basic forces involved here. When you put weight on top of a beam, a slab, etc. that is only supported underneath in some places, it tends to want to bend downward in between its supports. You can see the effect of this yourself by taking a sheet of paper, holding it at both ends, and having someone else push down in the middle. Works the same with steel and concrete except it takes a lot more force to bend them.

Anyway, so as a result of this bending, the top of the beam/slab/etc. is getting pushed together ("compression", in technical terms) while the bottom is getting pulled apart ("tension", in technical terms).

One important fact about concrete: it's a lot better at withstanding compression than tension. Push it together and it takes a lot of force to crush it. Pull it apart and it crumbles under a lot less force.
Because of this, whenever concrete is used as a building material, it is always paired with some steel to carry that tensile force and prevent the concrete from crumbling.


The bridges where you're seeing a concrete surface on the bottom have steel rebar embedded within the deck.
Where you see corrugated steel on the bottom, this is known as a "composite deck" and that steel is not just a form for the concrete that was left in place - it is a substitute for embedding rebar within it, that corrugated steel is there to carry the tensile force at the bottom.

As for which is better, neither is inherently - both methods work. Any choice between one over the other may simply be a matter of which is less expensive at that particular time and place... or a preference by the designer or contractor as to which they are more familiar with.
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kphoger

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Re: Difference between concrete and metal bridges at the bottom
« Reply #4 on: December 11, 2019, 09:27:53 PM »

The bridges where you're seeing a concrete surface on the bottom have steel rebar embedded within the deck.
Where you see corrugated steel on the bottom, this is known as a "composite deck" and that steel is not just a form for the concrete that was left in place - it is a substitute for embedding rebar within it, that corrugated steel is there to carry the tensile force at the bottom.

I have a good friend who designs bridges for a living.  (He designed the 87th Street overpass on I-35 in Johnson County, KS.)  When I asked him this evening at church about "composite deck" bridges, he looked at me like I had centipedes crawling out of my nose.  When I explained the distinction you made, which I quoted above, he in no uncertain terms corrected me:  all bridge decks are constructed with rebar in them.  His words were, "When you see a bridge with steel or concrete on the bottom, it doesn't matter, there's rebar all over them no matter what."  In fact, he said, if you could come up with a non-iron solution for bridge construction, you could "save the world billions, maybe trillions of dollars".
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Duke87

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Re: Difference between concrete and metal bridges at the bottom
« Reply #5 on: December 12, 2019, 05:22:34 PM »

I have a good friend who designs bridges for a living.  (He designed the 87th Street overpass on I-35 in Johnson County, KS.)  When I asked him this evening at church about "composite deck" bridges, he looked at me like I had centipedes crawling out of my nose.  When I explained the distinction you made, which I quoted above, he in no uncertain terms corrected me:  all bridge decks are constructed with rebar in them.  His words were, "When you see a bridge with steel or concrete on the bottom, it doesn't matter, there's rebar all over them no matter what."  In fact, he said, if you could come up with a non-iron solution for bridge construction, you could "save the world billions, maybe trillions of dollars".

Perhaps it's worth clarifying - the steel on the bottom only substitutes for rebar which would otherwise have to carry the tensile load along the lower part of the beam. It does not substitute for all of the other rebar that is needed for other reasons so yes, there is still rebar in the structure. Just not as much of it.

This video gives a quick rundown of the concept. Note how towards the end it still shows a secondary frame of rebar going in place before the concrete covers it...
« Last Edit: December 12, 2019, 05:26:07 PM by Duke87 »
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kphoger

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Re: Difference between concrete and metal bridges at the bottom
« Reply #6 on: December 12, 2019, 05:39:49 PM »

I see that touted online related to building construction, not bridge construction.  Do you have any examples?
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SteveG1988

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Re: Difference between concrete and metal bridges at the bottom
« Reply #7 on: December 13, 2019, 06:54:53 PM »

I see that touted online related to building construction, not bridge construction.  Do you have any examples?

Fundamentally the same to be honest.
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mass_citizen

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Re: Difference between concrete and metal bridges at the bottom
« Reply #8 on: January 31, 2020, 04:01:57 AM »

Stay in place forms typically do NOT serve a structural purpose in bridges.  You may be thinking of concrete filled steel decks on movable bridges or orthotropic steel decks
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