To Build a Bridge Part V: All Walled Up

A bridge, by its nature, is always going over something.  The vast majority of the time, this means that the height of the road approaching the bridge is artificially elevated.  That's true for water features or other natural crossing, but it is in particular, true for "grade separation" bridges.  Bridges where the point is to run one road over another.

Typically, the road surface beneath is lowered a bit by excavating out, while the above road surface is raised by adding "fill", which is just a slightly fancier word for soil (which itself is just another word for dirt, but as a geotechnical professor always told me: "Call it soil, no one is going to pay you to play with dirt").  However you do it, these kind of elevation separations require some kind of transition between the higher elevation of the soil above and the lower elevation of the road below.

This bridge (by Hoover Dam) has natural rock walls holding up the roadway leading onto the bridge.  Most situations require some kind of engineering to create a grade separation.

One solution is to just grade the soil down from high to low.  Which basically just means sloping the soil from high to low.  The old Greenfield bridge was this way, and some states, including Wisconsin, prefer this solution.  However, there are some draw backs to it.  Though it makes that transition cheap (essentially just the cost of some dirt) it takes up a lot of space and the bridge then has to span over all that extra distance.

Notice the span at the end.  Underneath that span, the "grade", meaning the surface of the earthwork there, slopes up from the road below to meet the road above.  In this case the slope is steep enough that concrete was placed on top to keep the soil from shifting.  The bridge needed to add an extra span just to go over this sloping soil.

The absolute steepest soil is typically allowed to slope upward is a 2:1 grade.  Meaning that for every two feet you move horizontally you can only go one foot up.  At this steep of a slope, you need to pour concrete on top of the slope to keep it from moving.  If it's just soil (no concrete or other binders) then 2.5:1 or 3:1 is where you need to be.  When you're in tight quarters, or just want to save costs on the bridge (by not making it span these extra distances) then you can save space by adding in a retaining wall.

The name plate for the wall on the West side of the Greenfield bridge.  All states have some way of designating the structures they build.  Here the 'R' refers to "R"etaining wall, and the numbers indicate the region it's built in as well as a unique identifier that's assigned pseudo-randomly.

Retaining walls, meaning walls that keep, or retain, the earth from spilling over on top of whatever is beneath them, come in two, general varieties.   The first is a "cut" wall, this is when the earth is already as high as you want it to be, so you cut a wall into it and then remove the soil in front of the wall.  Greenfield has such a wall on its East side, though sadly I wasn't able to get any good pictures of it.

Building a cut wall, in this case a "soldier pile" wall.  When they've finished placing it, they'll excavate the soil in front of those steel piles: that's the"cut" portion of the wall.

At Greenfield, it was a "tangent pile wall".  This means drilling holes into the soil and filling those holes with concrete and rebar.  The holes are drilled tangentially to each other, i.e. touching each other, thus the name.  The bridge will sit directly on top of the concrete shafts, and so the wall is designed to resist both the force of the soil behind it and the force of the bridge on top.

The West wall at Greenfield is a "fill" wall.  A fancy looking pattern is cast into the facing, but at it's structural simplest, this is just a collection of flat, precast, concrete panels all stacked up next to each other.

The second kind of wall is a "fill" wall.  Which is exactly what it sounds like: you put a wall where there isn't enough soil and fill it with soil.  Most such walls, at least here in the States (and my guess is this is true just about everywhere), are "MSE" walls.  With "MSE" standing for Mechanically Stabilized Earth.  The way it works is you take a panel (typically a 5'x10' or just 5'x5') that's around 5"-7" thick and attach straps to the back of it.  The straps can be just standard rebar, or the can be a specially designed plastic netting.  Their main purpose is to engage the soil that's going to go behind the wall.  Or in other words make it so that when the panel gets pushed out by the weight of the soil behind it, it pulls on the soil, whose weight stops it from actually going forward.  In other words: the soil anchors itself.

A panel with straps is placed with no soil behind it.  Then a "lift" (or layer) of soil is placed on top of the straps and compacted.  It needs to be compacted because it will settle eventually and you want to make sure there are no gaps, no strange settlements, and that the final surface of the soil doesn't change elevation.  Then, when that panel has been filled up, you place another panel on top of it and put down some more lifts.

Because space is at a premium both from an availability and cost perspective, transportation-related walls tend to be straight.  But nothing says you can't slope them.

Like the bridge, the soil you want around the wall itself is sand to ensure drainage (an actual drain is placed along the back of all these walls, leading to an inlet somewhere that connects into the storm drains).  But the wall has additional excavation behind the actual, stabilized part, and that can be just about any kind of soil.

Many situations, if not most, aren't clearly in a "cut" or "fill" category.  Maybe there's a soil layer that goes up 8' but you need 20'.  In those cases, and in fact in every case, it is function followed by cost that determines what kind of wall you use.  The actual function may just be to hold up soil, but it could be it also has to allow utilities to travel behind it or bear the weight of a structure.  If that's the case, sometimes more expensive walls are used because the cheaper versions aren't as functionally flexible.  For a fill wall, there has to be a lot of excavation behind the wall itself (remember the 2:1 thing?  That holds true for temporary, construction situations as well, though contractors will try to get away with 1.5:1 or sometimes even 1:1 slopes).  There's not always room to do that, especially for very tall walls.

After function is determined and met, cost drives wall selection.  There are many situations where the wall is in an entirely cut condition, meaning the existing soil level is already at or above what will be needed in the final condition, but a MSE (fill) wall is used.  MSE walls are really cheap, and so any chance to use them is jumped at.  It seems a bit silly to excavate all this soil to build it and then fill it back up,but that is often cheaper than building a cut wall.  So Greenfield had one cheap wall (MSE) and it also had the most expensive wall type we use: the tangent wall.  A nice mix.

To Build a Bridge Part IV: A Firm Foundation

As I mentioned before, the bridge is (in a very general sense: you skip around a lot) designed from the top down.  But it's built from the bottom up, and while the bridge-using public doesn't see it the foundation takes a fair bit of design.  This is where geotechnical engineers get into the game: they're the ones that calculate the strength of the soil and rock in the area.

Any project, once it's decided about where the bridge/wall is going to be, starts with some soil borings.  This essentially means take a drill rig and drilling into the earth some specified distance.  Every so often (1.5-5 feet it typical, depending on whose ordering it and how deep you are) a soil sample is taken and analyzed.  The main property being tested is the strength, which is inferred from multiple different measurements.

For a wall, a 30' (for those not "in the know", an apostrophe represents feet and a quotation is inches, so 30'-6" is 30 feet and 6 inches) boring is typical.  For a bridge, 100' is not uncommon.  Some footing types even require deeper borings, but that can get expensive quickly as equipment that goes that deep isn't cheap.

Piles sticking out of the ground for the West abutment foundation.  The piles have an estimated length they're supposed to be driven, but you don't stop until you hit a certain level of resistance.  Thus, most piles end up with a lot of extra length on them (you can see where the top of wall ends, the piles are supposed to be at about that elevation) and are cut off when it comes time to build the foundation around them.

 A "footing" is the part of the bridge that transfers the force from the structure to the soil.  A pile is a structural element that goes deep into the ground.  Above you can see steel, tube piles sticking out of the ground where the West abutment will go.  They terminate somewhere well beneath the surface.  The estimated lengths to "drive to capacity" are around 70' here.

Pile driving is exactly what it sounds like.  You take some amount of pile, stick it into the ground, and then continue to pound on it until it drives in deep enough to take the load.  Essentially you just put a pneumatic hammer on top of it and turn it on.  The hammer measures the resistance, and when you get enough you turn it off and start on the next pile.  They don't start with 70'+ long piles, they're much shorter.  As the pile starts to get close to running out of length, they pause the process and weld another length of pile onto the current one.

Drilling a concrete shaft for the pier footing

 But piles aren't the only way to support a structure.  This bridge actually started off with a "shallow foundation".  That essentially means something where you aren't drilling or driving, or doing more than excavating 5-10' down into the ground.  The original design called for a mat footing underneath all those columns that are going to be at the pier.  This essentially means digging out a place to put a 4'-5' deep concrete pad (as I recall, it was about 12' wide and spanned the whole, 136' of the bridge width from side to side).  That way, the weight from the bridge is supported entirely by pressure from the soil right there.

But we had to make a last minute change.  There's a 60" sewer pipe directly beneath the pier foundation.  Originally that was going to be grouted up (filled with a cementitious material: essentially turning it into a long, cylindrical rock) as it's old and a new one is coming to replace it.  But just before the project was bid, some guys in the utilities world decided they wanted to keep the pipe in use for another couple of years before closing it off.  We weren't sure the pipe could take the extra stress that would come from loading the soil on top of it (our shallow foundation) so we switched foundation types.

The edge of the wall can be seen in the foreground.  Most walls do not directly support the bridge, instead the force is transferred down behind the wall to the soils deep beneath it.

 To ensure accurate placement and sufficient strength, we went to drilled shafts.  These are shafts (here they're around 3.5'-4' in diameter) that are drilled into the ground to a specified depth (around 60-70').  Because nothing is being driven, you can't cut out early if you achieve capacity at a shallower depth than what's estimated.  Once the shaft is drilled it's filled with rebar and concrete, and you build the footing on top of it.

This specialized equipment was driven out from the Rockies, once the driller has done it's work, a rebar "cage", pre assembled steel bars, will be lowered into the hole and concrete poured in to fill it up.

If you can place your structure directly on bedrock you'd love to do so.  But in Southeast Wisconsin that's just not feasible (it's way too deep to reach) so the geotechnical engineers will give you a graph that tells you expected levels of support for a given pile type and size versus the depth you take it to.  It's then up to the structural engineer to pick the foundation type and design the pile if necessary (steel piles aren't designed: just counted, but the reinforcing in the drilled shafts have to be designed).

When you've built your foundation, you fill up all the excess area with sand and get started on the rest of the substructure.  The sand has all sorts of fancy names and criteria but it's essentially just sand.  Despite the parable, sand is actually a pretty good material to build on, but more importantly: it drains water.  You take a lot of precautions to avoid having water just sit right next to your concrete footing.  The concrete doesn't mind (as long as it doesn't freeze and thaw too much) but eventually the water penetrates and hits the rebar.  This is bad.  So you put quick-draining materials everywhere and then make sure there's a piece of perforated drain pipe somewhere nearby to take all that water away from your structure and get it out of there.

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.

To Build a Bridge Part II - Where's it going to go?

The Greenfield bridge is part of a massive rebuild of the "Zoo Interchange" located at the Northwest corner of Milwaukee.  This, the Greenfield bridge, is one of the first projects out the door for the interchange and part of the reason is that it's greatly increased capacity will help mitigate traffic problems when the Zoo interchange itself shuts down for reconstruction.

The process that decides this involves a lot of people.  And some of it is politics which is not my forte, but most of it is engineering, and that is.  The road layout, the position of sidewalks, the width of the traffic lanes all factor into positioning the bridge.  Some people seem to think that we basically show-up, see a space, and throw a bridge in there.

This is not the case: the roadway is planned out with several considerations.  First, of course, is the size of the road.  Traffic patterns are predicted about 25 years in advance and total lanes needed to accommodate that traffic are estimated and (if in budget) provided.  Sight lines horizontally and vertically are checked to ensure that national codes are met.  These relate to the amount of time a driver needs to see an object of a given size and stop (or otherwise avoid it).  Time to merge or turn is calculated based on road design speed (which is different from the posted speed limit) and things like crash protection and shoulder size is determined based on estimated traffic speeds.

Every aspect of the bridge is laid out to the nearest 1/8th of an inch.  It's a bit of a surreal feeling to be sitting in a building, miles away, specifying where the edge of a concrete barrier will be to the nearest eighth of an inch.

Every bridge has pages of plans drawn up specifying every bit of concrete, every single piece of rebar, every bolt and patch of rubber.  Those plans are public record, but they aren't posted yet.  I do have a copy, but I'm not sure of what my legal right is to share them, so ... I'll show one page here.  From an old set.

One of what ended up being about 40 pages of the plan set

From the roadway engineering perspective, it's essentially a question of fit.  They want to accomodate 'x' number of lanes going in 'y' direction, passing over/under 'z' road.  The department of transportation for Wisconsin (WisDoT) already owns some amount of land, and while it's expensive, they can use eminent domain to buy more.  But the goal is to help out the area, not destroy homes or business, so an attempt is made to buy and clear as little land as possible.

The roadway engineer (which I'm not) will work with the structural engineer (which I am) to make sure that a bridge will fit in that area, and have enough space to support it's weight.  Position of columns, decks and girders are taken into account and a horizontal and vertical profile is created that is supposed describe all the driving surfaces in all three dimensions.

Once the geometry of the bridge is determined (and this is actually an evolving process, that goes back and forth over the course of the design, but in theory it's linear) the structural design starts.  In construction the bridge is of course built from the ground up.  But it's designed from the top down.  Meaning the function is considered first (function being the driving surface) and then what's needed to make that function is designed.

Once again the process is iterative, as the design of the substructure (the piers/columns supporting the bridge, abutments or supports at either end, and anything in the ground) may influence what has to be done on the superstructure (anything above the substructure).  Part of being a good engineer means being able to make good estimates for things you haven't designed.  For example, if I was told that the desire was to span about 100 feet, I'd guess that we'd need a superstructure that was 5 feet deep to do so.  A roadway engineer could then design her vertical profiles to ensure that there was enough clearance between the bridge and the road beneath it.  As I refine my design, I may discover that I only need 4 feet and 9 inches.  The choice can then be made to lower the bridge, or to leave the excess clearance in.

Some bridge terminology

But I'm getting ahead of myself.  While the design came before anything was done in the field, the field had to start off by clearing the old one.  The road was going to be widened but not re-routed, so the destruction of the old bridge needed to be done before a new bridge could be built.