Sunday, July 14, 2013

To Build a Bridge Part III - Coming Down and Going Up

Jackhammers demolishing the deck of the old, Greenfield bridge
The first step to building the new Greenfield bridge is getting rid of the old one.  The new bridge is going to be bigger, and built to an updated code, but there was already a bridge there.  It will be down for about 6 months while the new one is put up.  This is fast, but does not really fall into the realm of "accelerated bridge construction" (ABC).  I just means the crews will be working a little harder, or with a few more people on staff to get it done.

A look at the equipment and the bare girders behind it with the deck stripped off.
Because the bridge crosses an interstate, they have to work fast.  Demolition can only happen with traffic beneath shut-down.  So the state gave the contractor two night closures in which they had to demolish the entire bridge.  They can clear up the rubble and cart off the old material after it's down, but following two nights of work: there should be nothing standing over the interstate.

After night one: the girders over the interstate remain as well as the piers.  The old deck with its rebar is sitting in the foreground waiting to get carted off.
But with it down, the question at hand is: what will replace it?  Bridges can be all sorts of different lengths, heights and widths.  As discussed before this kind of geometry is set by roadway engineers who try to fit the bridge into the minimum necessary space.

Most bridges people know, or think about when asked are long, impressive looking ones.  These are often referred to as "signature" bridges.  A signature bridge is one in which a lot of extra money is put in to make something unique and beautiful.  They can be shorter if they're in a prominent location and the powers that be want a specialized look (an example from my neck of the woods is the Milwaukee art museum bridge).  Some bridges can't help but be signature-style bridges due to their immense length.

Most bridges, however, are not signature bridges.  They are designed to be as cheap as possible, with maybe 1% of cost added to make them look vaguely appealing to the public.  That aesthetic improvement typically coming by adding relief patterns to the columns and staining the whole thing what are deemed to be pleasing colors.  Thought is given by the designer to how things will look, but that isn't likely to be a controlling factor unless the cost and construction considerations of the choice are essentially zero.  And that tends to only be true for details that the public is unlikely to notice.

The old, steel girders with shear studs that used to connect them to the deck.
When the bridge type is chosen for a new bridge there are various different factors.  There's the overall length of the bridge (from abutment to abutment: essentially the distance you're not on ground) but the more important value is span length.  The distance between substructure units.  Designing a bridge that is is a mile long is easy if there are no spans longer than, say, 100 feet (from a structural perspective, it's still a pain to produce plans for all that bridge).  A mile long span is essentially a signature span by definition.  There are, in fact, only three bridges in the world with spans of a mile or more: it's expensive and complicated.

The shear studs are designed so that when the girders bend, the concrete above would bend with it, thus you can take advantage of the added strength of the deck when designing the girders and save money on steel.
The geometry of the bridge is also important.  "Skew angle", meaning the angle between the direction of the bridge and the angle of the substructure units can be important if it's very large.  Here's an example of a highly skewed bridge; notice the non-perpendicular angle between the girders running across the river and the abutments supporting them on either side.  The farther off of 90 degrees it is, the bigger the skew.  Geometry would also refer to the bridge curving horizontally over its length.

The metal beams in between the girder (the smaller steel connectors that connect the bigger ones) are there for construction only.  They keep the girders from tipping over before the deck is placed.  Once the deck is there they serve no purpose, and can actually have negative consequences due to various fatigue considerations.  But pulling them out is hard and expensive so normally they're left in for the life of the bridge.
When spans are greater than 30 to 50 feet, and less than about 120 feet, every state will have a preferred bridge type.  In fact, up to around 250 feet most states are likely to have prefered types that change with the span length.  But the sweet spot is 50-120 feet, because that's the range that most highway bridges fall in so there are a lot of them.  And making them all (or as many as possible) the same type can save a lot of money.  Bridge selection (what kind of bridge to use) is almost always driven by cost.

Most states use prestressed, concrete girder bridges in this range.  A few with big steel industry in their state will use steel girders.  Wisconsin does not have a big steel industry so concrete girders are used in this range.  All bridges are made out of steel and concrete.  There are a lot of fancy materials out there, but nothing can compare to steel or concrete for their strength-to-price ratio.  Other materials are used in small capacities (like using rubber to seal joints to keep water out) but structurally it's always concrete and steel.

The steel is cut using a flame-cutter.  This is essentially an acetylene torch that you can blow compressed air through.  You heat the metal, and when it turns liquid, blow it away: cutting through whatever piece you're working on.  The flame-cut ends are visible here.
When someone says a bridge is "steel" or "concrete" they're referring to the superstructure.  The part of the bridge that actually does the work of carrying the massive load across the span.  The old Greenfield bridge that is shown here being demolish is steel, the new one will be concrete.  No bridge is actually all concrete or all steel.  If it's concrete it needs steel reinforcing bars (rebar) or it can't take the load.  If it's steel, it will still have a concrete deck and substructure.

Each material has different ways it can be used.  Very short bridges are normally "slab" bridges.  Essentially the deck is made strong enough to carry the whole load without any girders beneath.  Then you get to traditional girder bridges like both the old and new Greenfield bridge which have these girders running the length and supporting the deck above.

As the spans get longer, the girder shapes change.  A typical girder is shaped like an 'I' (see the above pictures), a thin 'I' if it's steel, and big, overweight 'I' if it's concrete.  Longer spans, or ones with horizontal curves (and some very highly skewed bridges) will switch to "box" or "tub" girders.  When you start to get above 250 feet you start to get into fancier and fancier systems, moving away from girders entirely.

But Greenfield isn't fancy.  It will be wide, but it's a work-a-day bridge and so it will use, as much as possible, all the standard, WisDoT features of a normal length bridge.

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