Good afternoon, everyone, and welcome to the virtual learning course, The Basics of Bridge Design. My name is Brittany Harlow, and I will be moderating today's session. This session will be provided via both audio and web conference. This program will last approximately 45 minutes, followed by a Q&A session at the end.
If you have a question at any point during today's presentation, you may submit it via the Q&A button on the bottom of your screen. Please note that we will be submitting today's credit directly to AIA, and will send all attendees certificates of completion via email for AIA as well as PDH credit. And now, it is my pleasure to introduce our speaker for today, Gary Johnson, Director of Structure and Bridge Design at Timmons Group.
Take it away, Gary. All right. Thank you, Brittany, and I want to welcome to the Basics of Bridge Design brought to you by Timmons Group.
Before we get started here, just a little bit about ourself. Timmons Group is headquartered in Richmond, Virginia. We have 16 offices spread out across the U.S. with a staff of over 700 folks. We've been doing this for about 67 years and we're a full-service firm doing engineering, design, and technology. As Brittany said, my name is Gary Johnson.
I'm the Director of Structures and Bridge for the firm. I'm headquartered in Richmond, Virginia. I've been dealing with bridges for 27 years, doing all aspects that have to do with bridges from design, rehabilitation, demolition plans, erection plans, safety inspections, maintenance plans, pretty much anything to do with the bridge I get involved with.
Also doing conventional projects, as well as design-build projects, and clients that are state, federal, local, and private. And with all those different types of clients, they all have different parts of a project that they may find the most important. So it's interesting to work for different types of clients, gives an overall perspective. So what we're going to cover today is just some nuts and bolts type of thing about loading and forces. Going to get into bridge anatomy a bit, just talk about the different aspects of the bridge and different opportunities for different types of bridge structures.
Touch on pedestrian bridges and how they are a little bit different and run through some other design considerations that we do have that come into bridge design. then end up with aesthetics and a little bit of a summary and then we can have a Q&A session. So with that, some nuts and bolts. Well, we start with forces. That's what bridges are there to do is to move forces around.
You're probably familiar with the first five of these. You got tension, which is pulling on a member. Compression, which is pushing on a member.
Shear, which is trying to rip a piece apart in the course of shearing motion. Torsion, which is a twisting. which gets quite complicated in bridges.
And of course, bending, which you see the most of in bridges, what you think about. And the last one here is buckling. And the interesting component about buckling is that it really is a bending and compression acting together on a member. And this is a pretty big part of what we do when we design when it comes to either a column or a beam. And what it relates to is your unbraced length.
So, When you have a column or a steel section, it's not as easy as saying what is the capacity of that section because it has to do with what its length is and where is it braced. And this is in three dimensions, not just a two-dimensional drawing perspective. You need to look at it in all three dimensions.
Shown here on the right is a clip from an AISC beam table where on the Y-axis is its moment capacity and foot kips. And on the bottom is what's called its unbraced length. How often is it braced?
So I've highlighted a blue section there of a W24 by 84 steel beam. And that flat line up top says, if you brace it by seven feet or less, you get its maximum capacity of 840 foot kips. But as that unbraced length... increases, you can see how it drops down dramatically. And if your unbraced length goes from seven feet to 10 feet, it's actually a 10% reduction in the capacity of that beam.
So buckling plays a big part of the role of what we do and how we design. When it comes to materials, concrete and steel make up the vast majority of the bridges that we design. Concrete is great in compression. It does technically have tensile strength, but it's so low that in the bridge world, we assume it's zero. Steel is great in tension, and it's also great in compression when you're looking at it from a bracing component like we chatted about.
And wood's got the same properties. Great in tension, can be great in compression, and of course, cables are only good in tension. So pictured here is just a very simple bridge, what we call the simple span bridge, where you push down on the top.
and there's tension on the bottom and compression on top. And that is the forces in a very simple bridge. But compression is not always on top and tension is not always on the bottom. The Fourth Road Bridge in Scotland in the late 1880s had this. This was an aero structure, which is still in operation today.
When the designer wanted to show his plan for it, he came up with this idea of a sketch of showing how the tension and compression can actually hold up this larger structure. So if you look at it, the arms are in tension and these wooden poles or bats are in compression and that's how it works and on this type of a structure, tension's on the top and compression is on the bottom. This was of course 140 years ago, so my folks wanted to know if it still holds true, so we pulled it together and we could find out that these forces remain true to today and that's part of my bridge team there. So now we're going to talk a little bit about loads. So loads, dead load, live load, other loads.
Dead load is the weight of the bridge itself, utilities are the things that stay on it. Live load are loads that change, cars, trucks, special vehicles, pedestrians, and this thing called impact. And impact is not something running into a bridge, but it has to do with the bouncing of live load. because as you can imagine, when you jump on a bed, you can put a lot more load on that bed.
When it comes to our design of bridges, there's actually a 33% increase from just the loads bouncing as they ride on a bridge. And there's a whole sort of other loads that we look at. Wind, wind on live load.
If you have a truck and wind blows on that truck, the bridge needs to resolve that load. Seismic, of course, centrifugal forces, snow and ice. The major takeaway here is that thing I have up top, 1.25 dead load plus 1.75 live load. That controls most of what we do.
And what that means is we pretty much know what the dead lows are on the structure. So we increase those by 25% for an inherent factor of safety. But we don't know as much about the live load and what this bridge is going to hold in the future. So those are actually increased by 75% from our calculations.
And that's where we get some... some factors in there for the safety. Now looking a little bit more in depth with dead load, the timing of the load is important. It's not just the weight of the bridge itself, you add it all up and that's your load.
Because as you build a bridge, it gets stronger. Because you have, and then as the components come together, they work together. So we're looking at the capacity of the bridge.
in the different stages of construction, not just in its final condition. Shown here in the bottom left is a steel beam, which is the the dark black part, and the top part is a concrete deck. When it's totally constructed, this works together to form a T-beam.
That's called a composite action beam. So that's what we're designing to hold up the live load, which would be the traffic. But when you place the steel beam, it has to hold itself up.
Then when you go to cast the deck, that same beam needs to hold the wet concrete before it cures. So that is a major controlling point and often that controls our design, is the actual wet concrete on that steel beam. So in essence, the steel beam may end up being stronger than it needs to be for live load, just so then it can survive the construction. Shown here on the right is showing a composite beam versus a non-composite beam and and the strain arrangements between them.
And the takeaway there is it's much stronger with the deck on there, but you got to design the beam so it can withstand the wet concrete being placed. Now moving on to live loads. Essentially we design for trucks. That's usually the controlling load.
We got single trucks and tandem trucks. We look at lane loading, which is in the bottom right there, which is 640 pounds per linear foot. That's spread out over 10 feet. We also look at permit loads and emergency vehicles when we go to design the bridge.
And I can tell you this is it's extremely complex with how you add up the live loads and look at the different cases. We actually have another seminar that acts that's an hour long that just covers this second bullet here. So I'm not going to go into too much depth here, but there's dynamic load allowance, live load distribution factors, multiple presence factors and fatigue that we look at when we combine all the live loads.
But the takeaway there is, of course, we're controlling, we're designing for dead, which is the bridge itself, and we're controlling for live. And we often come across some special vehicles as well. We're actually working for a wind farm, working for a wind farm where they need to get these components to the site.
And the interesting component about that is they're massive loads, but they're also, the way the axles are configured plays a big role in our bridge design. And when we're checking bridges, which is called a load rating, which we'll get into toward the end of the presentation. But if you look here in the bottom left. you've got seven axles that are holding up this load.
One of the bridges we were checking wasn't even as long as that seven axle section. So the entire load can't even get on the bridge. So the point there is we look at the actual loads and the configurations and the axles in order to come up with whether a bridge can support a certain load.
And these major loads also have a little bit of fun with turning radius and actually planning to get the projects on site. In this image here, you can see in the bottom left, they actually had to clear a mound of earth in order to make the turn. And so that can get quite exciting too. Railings on bridges come into play as well to make all these pieces fit.
One other thing with live load, we always look at deflection, which sometimes controls design with vehicular bridges. And what we do is we look for the design truck. and we also look for the lane load and 25% of the design truck when we check for deflection.
With vehicular bridges, we often design for load and check for deflection. And our controlling parameter is the length of the span divided by 800. So a 150-foot span means we're allowed for it to deflect about two and a quarter inches. But when it comes to live load, things are a little bit different. Typically, when we're looking at a pedestrian bridge, we design for deflection and check for load because often deflection controls the design.
The pedestrian loads we often design for are somewhere between 65 and 100 pounds a square foot. You may ask yourself, why do we design for different loads of people? If you had a bridge that was in the middle of nowhere or perhaps on some sort of a trail, you could go on a lower level of 65 pounds a square foot. If you're designing a bridge that's outside of a sporting arena or a concert hall, where it actually may get that type of loading, you're more toward the 100 pounds a square foot.
And just for the enjoyment of the pedestrians, we actually have a more stringent deflection criteria, and that's the length divided by 1,000. So that same 150-foot span now can only deflect one and three-quarter. inches.
So that's it with some loads and some forces. Let's get into the bridge anatomy. When you take a biennial inspection course, this is one of the diagrams they show you about what the different components of a bridge are. And also I want to point out here that what's shown here where it says girder is a continuous girder going from abutment to abutment.
It's not in individual ones. But for the sake of this discussion, I'm going to divide this up into three sections. We've got the area above the red line we call the superstructure, the area between the red lines we call the substructure, and then below that is the foundation.
So those are just going to be the three components that you're going to hear that we're going to group this stuff together. But we're going to start off in the superstructure. The one component I want to talk about is the span arrangement when I said that this is a continuous structure. And what I mean by continuous structure is there are simple spans and continuous spans. And simple spans are what you think they are.
They're as simple as they can get. Each span works independently. This top image right here shows three simple spans lined up.
Generally, even though a truss is shown there, in general, the structure depth is about 5% of its span length. 100-foot span is going to be about 5 feet deep. But when you go continuous, the spans actually work together and they hold each other up. And in doing that, you can really increase your efficiency. And now the same 100-foot span is now maybe three to four feet thick.
So if you have vertical clearance issues, and there's a whole other lot of reasons to go continuous, but you can actually get a more slender structure going with continuous spans. Also, your main span, and in this instance, the second span here, your spans adjacent to it should be about 70% of your main span. And in this kind of arrangement, you can have a wider opening.
So if you look at in this... Overall bridge has the same length, but the continuous span can have a much larger center span in case you need to cross over something with a little bit more significance. And that is one of the things that we look at. If you don't follow the 70% rule and your approach spans are shorter than that, it's very possible that you can get uplift at the abutments at the end of the bridge, which in general is not a desirable scenario. You can deal with it, but...
Really, the 70% rule is the best way to go when you're laying out a bridge, if you can get away with it. So going a little bit more in depth with simple versus continuous, you have to do with where is this tension? Where is this compression that we spoke about?
Right. The top is the top sketch up there is the simple span, just like we spoke about with compression on top, tension on the bottom. But if we do a two span continuous beam and a three span continuous beam. like shown here, you can see that now you've got tension on the top and compression in the bottom and vice versa as well. And what's neat about this is there are then parts of a beam that go from compression to tension and from tension to compression.
And those points are called your points of contraflexure. So in these instances that's about here. Okay and it does move a little bit whether it's dead live or dead load or live load so it does move as the loading changes on the bridge. But the neat part about this is where tension and compression come together, you don't have moment. And by not having moment, that's where we like to put our splice points.
Therefore, the splices can be as small as possible. So if you ever go down a road and wonder why splices are offset, that is the reason why. That's the point of contraflexure.
And you can see that in the bottom left here. And that allows for the longer span to be over the railroad with the shorter spans holding up. that larger span.
So now that we come over the different types of span arrangements, we're going to start literally from the ground up and start and talk about our different foundation types. So the first type is going to be a spread footing and that's as simple as it gets. This is where you've got your load coming down which is a P and then you've got your Q which is your capacity of of your bearing strata pushing back up and you size the footing based on the load. Which shown here on the right is actually the Huguenot Bridge in Richmond, Virginia.
And we were lucky enough to have rock that was a very high quality up high, which means it was not deep at all. I believe this rock can hold 10 tons of square foot. So spread footings were actually holding up this pretty significant structure. It is keyed in a bit to the rock to get some lateral capacity, but that's about as simple and as best way as you can go in order to have a... bridge foundation.
Piles would be the next component of how to have a foundation system. And there's two main types of piles. You have end bearing and you have friction piles.
So on the bottom left here, you see end bearing piles is you have your load coming down on your pile and you want to get it down to your strong rock, which is the darker color on the bottom there. So what you're doing is you're just using this as a major... big column just to get the load all the way down. The other way to do it is to have a friction pile, which is you're pushing down on this pile and the friction of the earth adjacent to this pile gives you your capacity. So if you have better soils that are shallow, you can go ahead and do that.
Piles can also get capacity from end bearing or friction, but those are the two major ways. And piles can be made out of concrete or steel. Shown here on the right is my plant stand in my office.
And my plant stand there is made up of an HP 14 by 117 pile that was cut off to a certain height. So that was a piece that was left over. So when you see HP 14 by 117, the H means it's an H section. That's kind of obvious.
The P means it's a pretty beefy section because it's a P for pile. So it's made to be driven into the ground. 14 means that's the... depth of the pile, the depth of the section, and 117 means how many pounds of linear foot it weighs. So, my plant stand weighs 150 pounds because it's just over a foot long.
So, with piles and how we get them in the ground, we can drive them or we can vibrate them in. So, with impact, you could just literally just drop a weight on it over and over again, which is shown here on the right, until the pile goes in the ground. You can also use a vibratory hammer, which is shown on the upper left here, to actually essentially shake them into the ground. And that has to do with the geotechnical strata that you have and the best way to do that.
We also sometimes put them in vertically to get that vertical load and we sometimes batter them, which is just put them at an angle and then that gives us a bit of a lateral load and a lateral capacity. You don't see as much as the battered piles anymore. They're a little bit more expensive to put in but we do often come across them and we do sometimes use them in our design.
Drilled shafts are another type of foundation system that we can use. They act like very large piles, but it's actually just you drill a hole. You can see in the bottom left here. I've had these get as big as nine feet in diameter.
You then fill it with reinforced concrete, and then you have a very stout column. And what's nice about this is we'll talk a little bit later about scour. And drilled shafts do great in scour. because they're braced because they're so big.
But that's just another type of a foundation system that you'll see. One of the more common ones are going to be your spread footings, your piles, and your drilled shafts. Moving up, literally, we'll move on to the substructure components and what goes into them. Starting with abutments. In Virginia, there's really five types of abutments that VDOT likes to use.
And there's a flow chart that is It's not as complicated as it looks, but there's a flow chart into what kind of abutment to use. The takeaway here is VDOT wants, and I totally agree with, the best type of abutment is a fully integral abutment and then going down in these different types and ending with a conventional abutment. So we're going to look at a couple of these now.
A fully integral abutment is the beams go right into the abutment. There are no bearings. It's all tied in together.
The thermal movements are taken by the movement of piles that are embedded in the earth. So there's no way for water to get intruded into the bridge and they can last much longer. So they're a little bit more expensive to construct, but they last a lot longer and don't require maintenance or as much maintenance as other types of abutments.
This bridge here is one we just inspected this month and it's, I believe, just over 10 years old. And it's still in showroom condition. This thing is in absolutely great shape and this is why we go ahead and use fully integral abutments.
The next best one is a semi-integral abutment when you don't have all the parameters when you can use a fully integral abutment. If it gets kicked out because of a skew or it's over a long or its entire span length or the movement it needs to take on or geotechnical considerations where you can't use steel piles, you use a semi-integral, but the concept is pretty similar. And then the last resort is a conventional abutment. And the reason conventional abutments are at a last resort is they always have a joint that's inherent in their design.
So shown here on the right is a joint called a compression seal, and they require maintenance, they leak. We had the... the luxury of inspecting this bridge in the rain this month and as you can see those joints are leaking and while it may not seem like a big deal, water and bridges don't do that well together, especially when they're steel, especially when you're in an environment where you put salt on the roads in the wintertime.
Salt water will then corrode your beams, you then end up with a maintenance issue. This bridge was built at the same time as that one I showed you before that I said was like in showroom condition. This bridge is still in really good shape, but it needs to have its joint replaced and it may have some B-man issues that are going to pop up if that joint is not replaced soon. So that's why that is the the last choice for an abutment type.
So another part of the abutments are wing walls. They're typically cast in place. or they're mechanically stabilized earth walls, MSE.
There are a few other types, but those are the two major ones. When it gets to the wing walls, which is the piece of getting from the ground up to the bridge, that's more of a wall than it is a bridge component. So I'm not going to touch on it too much here. Hopefully some of you have seen my webinar on retaining wall design where I cover these types of components.
Moving up to piers. Really four types of piers out there that we use. First one in the upper left is wall pier, which is essentially just like it sounds like.
It's just a wall that holds up the beams. These are good in scour conditions where you won't have floating debris get stuck. between columns.
It's also very simple to design. They're also great if you have strata that is very strong up high because it's a great way to spread out your load. Multi-column on the upper right, the kind of self-explanatory there.
On the bottom left is a hammerhead where you have one shaft coming down and then you have a transverse concrete beam of sorts to hold up the beams. You see these in more urban environments where you you want the room below the bridge. And also if you have to have a drilled shaft, like we chatted about, having this column go right down to a drilled shaft can be efficient.
So you're not going to want to put a wall pier on a drilled shaft, but if you need to have a drilled shaft or a few drilled shafts, a hammerhead may make sense. And then on the bottom right is what's called a straddle bend, which of course straddles a road. And this one here is just west of Richmond near Short Pump where 295 and 64 meet. And just based on the extreme skew of the overpass ramp going over 64, it was too long to span. And there wasn't enough room for a pier, so they needed to do a straddle bent that actually spans over.
Typically, your piers are made out of concrete, but they can be made out of steel. For that straddle bent, the columns are made out of concrete and that piece going transversely is actually a big steel box. Moving up to superstructure, first thing I want to talk about is approach slabs.
While not technically part of the superstructure, it is up at that point. And what this is, is at the end of a bridge, which is shown here on the right, there is often what's called an approach slab. which spans from the bridge to the roadway that's approaching. And the reason for that is when you are building a bridge, you're increasing grade.
It's just inherent in what you're doing. You're going to be filling up. So when you fill up, you have to compact the earth. And what's shown here in the bottom left is a sheep's foot roller, but there's all different types of rollers out there. So they will be compacting the earth as this is being built up.
But the bridge has to go in first before the fill. of course. So when they pull up to the bridge, they can't get close enough with the big equipment.
So you have to use more hand equipment, like it's shown in the middle there. And in doing that, it doesn't get compacted as well. So over time, that earth that was not compacted as well would then subside. And in doing that, you then get what everyone can see or feel is that bump when you go onto a bridge or off a bridge. So a relatively new...
way to take care of this in the life of bridges as a whole, it's relatively new, is to have an approach slab that actually spans over this part of earth that probably was not compacted as well, and you just can't get it as well compacted. The deck is the part of the bridge that you drive on, which is typically concrete. They can be timber, but typically they're concrete. It has primary steel, which is going transversely. which spans between the beams.
And then you've got secondary steel which goes longitudinally along the bridge which helps distribute the load. And the steel can be made black which is just your standard reinforcing steel, epoxy or stainless steel or other corrosion resistant material. The bridge shown here on the right is down in North Carolina that uses epoxy steel. VDOT does not use epoxy steel, so different states have different feelings on the best type of steel to be used in bridge decks. And again, we talked about composite and non-composite action on these beams.
See in the bottom left, those little pieces of metal sticking up? Those are what are called shear studs, which actually make the connection between the beams and the deck itself to become composite. Nearly all bridges that are built now are composite.
beams. We do come across some remediation ones. We're doing one in Charlottesville right now that actually has a non-composite deck that we're doing a deck replacement on that we're going to actually turn it into composite and actually increase its capacity. But typically, most bridges do come with composite action beams.
So this deck, you need to form it up somehow. A lot of times we're using stay-in-place forms, which are those metal pieces you see there on the right. The dark brown are actually the beams below. So this actually spans between each beam to hold up the wet concrete.
You can also use precast panels in instant or in place of these stay-in-place forms. The part on the left made out of wood is the overhang. And that has like a little angle of... what's called an overhang buck below it that holds that up.
So because you're going to see that part and there has to be some formwork associated with that, that wood is temporary. You place the concrete. Once it cures, you pull down the wood from below.
But the metal stay-in-place forms, they'll stay in place. And you can also see on these beams there are no shear studs. Sometimes the beams come with shear studs on them. Sometimes they're installed in the field.
If they're prefabricated and then they show up to the field, a lot of times they're bent. So there is a push right now to actually have them installed in the field. So we did talk a little bit about joints and we like to avoid them, but sometimes you do have to have joints. Especially on major crossings where you have a lot of movement, you do have to have joints.
So the types that you see most often is modular joints, which is in your top left, which is a series of smaller joints put together. finger joint on the upper right, which just looks like interlocking fingers. You can get about a foot of movement out of these.
Then the bottom left, my favorite, is a silicone joint when you have to have the joint, which is essentially a two-part epoxy of sorts that gets poured into the joint. And then on the bottom right is a compression seal like the one I showed before that leaked. The tough part about the compression seals is If a part of it goes bad, you have to replace the entire piece of the joint. And if you need to maintain traffic over four to five lanes and you've got two feet of a bad joint, it can really increase the cost from a maintenance of traffic.
But if you have a silicone joint like the one on the left, you can just cut out the part of the joint that went bad and replace the only point that is bad. So there's a lot of betterments, a lot of reasons why to go silicone over compression. but they're often used and you can take out a compression seal and put in a silicone and vice versa. But it is a lot easier to go to silicone than it is to compression. So how about the beams themselves?
What are they often made out of? Of course the vast majority are concrete and steel. You can have timber and you can have actually prefabricated trusses and such that we'll touch on.
There are other types I'm not going to talk about today, but you can have FRP, fiber resin polymer. You can also have composite components and as well as plastic bridges. There's actually beams out there that are made out of plastic.
But when you have a concrete beam, here are the parameters that kind of go into it. Typically, they have even span lengths. Because they're actually simple spans, the beams themselves that are precast beams that are popped on, you often have the same span length that is repeated over and over again. And they're actually shorter because... these beams are much heavier, so you end up with shorter spans versus steel spans.
They are quite durable. You get the precast quality of them being actually fabricated in a shop, so they tend to have lower initial cost and lower life cycle costs. But they're not good in curvature. If you do have to have a curve on a bridge, you can use concrete beams, but they are what's called corded, which means they're just a series of straight spans that are actually at, each one is at an angle and then you can actually take the curvature component and make it by actually curving the deck above. So in in the last 10 to 15 years concrete beams have essentially won out over steel beams if they meet these parameters but there are times when you do want to go to steel beams.
And steel beams, we talked about that balanced span length, that if you have an arrangement like shown here, where you need a longer span and then shorter spans, that balanced span configuration can give you a very efficient design. They're almost always now fully continuous. Of course, there's a lot of bridges out there that aren't. They're great in curvature. Overall, they're lighter as compared to concrete, so you can get longer spans.
They have a thinner cross section. So if you have a tight vertical constraint, steel can often help out with that. They are initially more expensive, but that can change. There was a time 20, 25 years ago when they were.
much cheaper than concrete, but it's just based on materials. Concrete, like I said, it has been cheaper recently. And they can have a higher maintenance because of the painting that is required. There are types of steel that's called weathering steel that don't require as much painting, but that in general, maintenance on steel costs a little bit more than maintenance on concrete. Timber is always an option as well.
And the US Forest Service does have guidelines. The smaller spans, when you're looking at these, they're almost like a deck with the way that they're pulled together. But they can actually be some major spans where you can actually go up to about 60 feet with the beam that's called a glulam beam, which is actually a built-up section made out of wood.
Prefabricated bridges are out there as well. If you want to go that route or when you go that route, you've got to look at what type of what are your deflection components that you want to carry and what is the weight, what is going to be used in the bridge to get these things designed. The things I will caution about this is often things that are overlooked with prefabricated bridges are the substructure components and the design of that, the foundations, the deck up above.
With the foundations and the abutments, the global stability analysis needs to be looked at outside of just the prefabricated bridge. delivery costs, inspection and testing, and a lot of times the actually installation of the bridge may or may not be included with prices that are pulled together. So those are components to look out for with prefabricated bridges.
But in the right spot, they are wonderful bridges. The one in the upper photo there is at Longwood University, which was a great use of a prefabricated truss. Of course, there are a lot of other very complex bridges that are out there, but I was just sticking with the more simplistic ones that are out there as part of this course.
With pedestrian bridges, a lot of the same components, or almost all of the same components we talk about apply, but there are some other components to look at. Often you want to increase clearance over a pedestrian bridge. The reason for this is, generally, they're narrower and they're lighter. So if you have an over-height truck hit a vehicular bridge, the mass of the bridge itself will sustain itself where it will stay up or it probably will stay up.
But pedestrian bridges are so much lighter that an over-height load could actually take down the entire bridge. So a lot of times you're looking to provide 16 foot 6 for clearance as opposed to 14 foot 6 that you would get on a road. A lot of times you need fences so people can't throw things out from being on the bridge down to an aero track or to a road below.
We often have kick plates, which is so if there's little pebbles and people are walking, that can't fall down on traffic below. Wind loads get pretty significant on these just because they are lighter structures and wind can actually control a little bit more. Of course, you need to get the access and the ADA and the ramps that lead up to the structure as part of that design. And again, like we talked about, they have much tighter deflection controls that often control design.
So with all of that, there are some design considerations that we look at when we do design a bridge. From the road itself, what's the vertical clearance we need? What's the horizontal clearance? Checking cross slopes to make sure we don't have a pinch point.
And does this bridge need to be widened in the future? And if so, how does the low cord and the cross slope affect that? We often come across railroad crossings. Typically, we're providing 23 feet of clearance for a railroad crossing. We also have horizontal clearances.
A lot of times when we're dealing with a new crossing of a railroad, like this one here in Charlottesville, there's plans for a third track to be installed. So we can't just put a pier right next to the track and think it's okay, so you need to negotiate with the railroad about what they need. It's often great to just span over the entire right-of-way for the railroad, but sometimes that's not possible.
And again, you got to look at that load cord and watch out for the cross slopes and leave enough room for them to reballast their tracks and not impede on that vertical clearance that they need. When we cross a waterway, we typically get the required opening from the hydraulic folks, and then we design the bridge around that. durability comes into play and mainly when it comes to scour is because we need to design this bridge for scour or protect against it.
In the bottom left, you can see that there's riprap there, which would be a scour remediation component that you can do to put in to protect against scour. But if you do have a scour component like the bottom right here, those piles are now not braced. So you, this bridge is essentially up on stilts and you need to make sure that those piles don't buckle.
So while your original design may have had them fully in earth where they're fully braced, in a scour condition, we do need to make sure that bridge does not fall down if scour does occur. When it comes to the construction itself, we look at all those different loads. We have construction equipment with work platforms on bridges that are put in.
When you're looking at the diaphragms, which are those the pieces of steel going between the beams there, that helps cut down on the unbraced length. We talked a bit about the wet comp. concrete and the non-composite capacity and that often controls our design. But we also have components like what you see here where there's a temporary cantilever condition where we may have uplift at the abutment and we need a design for that because that changes where the tension and the compression is in on these beams. And then when these bridges are before they're the slab is put on top they are light so wind loads during construction sometimes control.
We also conduct what's called load ratings, which is where we compare the loads that are going to be put on the bridge to its capacity. And this could be either in its perfect condition of being new or it could be in its degraded condition when we go out and do a safety inspection. And then we may have to use, if there's some degradation of the beams, that goes into our analysis.
We typically just look at the superstructure, which is the beams and deck. because that almost always controls, but in certain scenarios we do look at the substructure as well. And if we do run into an issue, that's when you see a posting. You may see those signs out there with the weight limit. That means a load rating has been done on that bridge and it needed special consideration for what trucks can actually use the bridge.
Camber and deflections is part of our design as well. Camber is we actually have the beams. pointed up and we do that because as the bridge gets constructed it gets heavier and it deflects more. So if we just started out with a flat bridge where we needed it to be as we add all the other components it would then sag.
So shown here on the top part is what we call a camber diagram which is an exaggerated scale of how the beam will look. So that curve is the way the beam will look when it shows up on site. It will then deflect a little bit down by the weight of itself then the concrete on it, and then it becomes stronger, and then we put other components on the bridge, and it comes down a little bit more.
So for an elevation view that you see down there below, the beams will show up to sight, and they'll have those humps to them that you can see when you look down the bridge. And we do that so then when all the components are added in, and all the deflections take course, and all the loads are on it, it is now in its final condition. But it is interesting is what is a final condition?
Because we have this other thing that we deal with, which is called creep and shrinkage. And what this is, is creep is movement under load, which is you may have a bookcase in your office where the shelves are sagged. That's creep. That is, you put load on something, over time it is just going to move down. If you take off that load, some of it will bounce back, but some won't.
There's also shrinkage. which is evaporation of water from the concrete. And that starts immediately and it doesn't go away. It just keeps getting, it just gets, it flattens out, but it never goes away.
But when you add these two together, you can actually have movement of bridges over time vertically that you may or may not want. Some states have us design our camber for five years out. Some want it perfect on day one.
So depending on what state we're in, it does affect what your camber is. And over time, when bridges get older, you can actually feel this. If anyone's ever driven down to the Outer Banks, that is the Wright Brothers Bridge and the bottom right, that bridge has been around a while.
Those beams have all creeped down. And now when you go over that, you get almost like a galloping effect as your car goes up and down. And you can mainly feel it a lot more on longer bridges with repetitive spans like that. bridge right there. Another component with bridge design is fracture critical members.
For something to be fracture critical it needs to be steel, intention, and non-redundant. Meaning if something happens to that component there's no other place for the load to go. It sounds really bad and it's not related to condition. It's inherent in the overall design.
And sometimes you just need to design it based on the condition of the bridge. Here is that straddle bent I showed you before. And that bottom flange right there is in steel. It's steel.
It's in tension and it's non-redundant. So that is a fracture critical member. So when we do safety inspections, we put a lot more emphasis on those components.
And that was all set and that came to light in 1967 with the collapse of the Point Pleasant Bridge in West Virginia that really started this whole look at fracture critical members. Now, I said that we look at that as part of inspection and bridge safety inspection is a large part of what goes into actual taking care of bridges. And I can tell you that every bridge in the U.S. that's open to traffic is safe. And I can say that.
because every bridge gets inspected at least every 24 months. And if it's more critical, like a fracture-critical bridge, it's every 12 months. And there are reasons even to go down to six months. But we do this to ensure that every bridge out there is safe.
That's the MLK Bridge in Richmond, Virginia right there that we inspected a few months ago. And we go all over the bridge, and there's a very regimented form that we pulled together in terms of showing that these bridges are safe. Factors that affect our bridge costs are curved alignments. Curved bridges are much more expensive than straight bridges, as well as skews due to acute corners.
So we try to avoid those when we're laying out alignments. And with bridge costs, typically bridges are costing $250 to $500 per square foot. Smaller bridges are more expensive per square foot just by economies of scale. And just as a point of reference, the Brooklyn Bridge cost $15 million to build, which is about $400 million today.
Neat component there is when those piers were built, they were the tallest structure in North America. So I think it's pretty neat. Wrapping up here with aesthetics.
I see aesthetics as in the eye of the beholder. My personal feeling is form should follow function. In the bottom left here, you can add pieces to an existing bridge to perhaps increase the aesthetics.
On the upper right is the Zakem Bridge in Boston, which is a beautiful cable stay bridge. But when your waterway is controlled by a bridge just downstream with a certain width, do you really need a bridge that long? I'm not saying there's anything wrong with that, but I think form should follow function with that.
So I'm just going to quickly run through a few examples of bridges. This is McIntyre Interchange in Charlottesville. I think it's a nice component of colors and lines and scale.
It also has form liners on the wing walls. You can see there to actually look like stone. I think the colors all match and it works together to be an appeasing looking bridge. We often have to do renderings as part of bridges to get... buy-in from the public.
This is Route 27-244 interchange in Northern Virginia, where there was just a tremendous amount of public involvement with this bridge. The existing bridge was an arch structure, but with the new crossing that couldn't fit. So an arch was painted on the beam and it actually worked. It actually looks and makes nice, but it's kind of good to show here how we have to do rendering sometimes.
in order to get the project off the ground. Southgate Drive Interchange in Blacksburg used components of some form liners, prefabricated arch on the facade with more standard beams behind it. Also they brought in a lot of landscaping and just some different colors there as well for a bridge that looks great. McIntyre Interchange, another one in Charlottesville, this is the railroad crossing I showed. showing the ramp structure, which can be pretty massive.
It's almost as big as a bridge itself. Again, using form liners and colors that kind of match, where you can actually get a railroad feel from a truss without it actually being an aeroad bridge. And also there was a desire to have weathering steel for this bridge, but we went with a more painted option to avoid the staining of the concrete below that you would see with weathering steel. Huguenot Bridge, another one in Richmond, I think just has the right scale and the right sizes and colors and pier shape to just be aesthetically pleasing.
So with that, I'm going to just quickly wrap up here that bridge design is a balancing act of a whole bunch of different components, be it safety. It's use, durability, performance, cost, and last but not least, aesthetics. So then it looks great. And with that, I'd love to see if anyone has any questions. Thank you, Gary.
Like he said, at this time, we will begin taking questions from all of our participants. Please use the Q&A tab at the bottom of your screen to submit your question. Brittany, I don't see any questions coming in, but up on the screen is my email and my phone number.
If you have any questions or any comments, I would welcome an email or you can reach out to me. I did get a question in here. How do you feel about the different specifications for bridge rebar, coated, uncoated, stainless steel?
Great question. VDOT used to be all about epoxy. steel and that was the thing to do maybe eight ten years ago and it was put into practice and in a lab it works phenomenal the issue is is if it gets nicked at all where that plastic coating which is the epoxy on it gets nicked it can actually be a riser where the degradation of the bar actually gets increased in that part so you have to be very careful to not nick the coating with epoxy steel. So VDOT is now going with stainless steel or corrosion resistant steel in all of their bridge decks. And I think it's going to pay off in 40 to 50 years from now when they don't need to replace the decks.
So that's a great question. And typically, if it's below the deck, we're using black steel, which is just your standard steel, unless it's in a spot that gets sprayed by... by water or salt.