There is a city in England called Durham. The central city section of this city was created a very long time ago when that area of England was almost wild, with little structure to the imposition of county-wide or nation-wide laws and controls to control and suppress outlaw activities. As such, (as was done for many other cities created in this type of environment), the city was put together to provide its citizens with protection and sanctuary. These constraints usually wind up making the city small, surrounded by relatively difficult terrain to traverse, and surrounded by a wall-type of city boundary. As the centuries go by and real civilization arrives, these previously valuable traits of the city begin to act to give it a historic patina, and a friendly close, everything pushed on top of itself quality…. but these very constraints so frequently act to limit any ability for the city to enlarge itself or enlarge what it can offer its citizens and its visitors and tourists. Suddenly the city is all cramped up and expansion gets really complicated.
The Wear River Bridge represents a way to add a friendly wide inviting open spacious sunlit avenue for the citizens, visitors, and tourists of Durham, England to meander through the original cityscape, out over the pedestrian bridge, and into the “new” inner city of Durham, located on the other side of the River Wear, which is a location where expansion can occur.
Here are some of the Sketchup drawing files that I created about the Wear River Bridge.
From a design viewpoint, I wanted to create a bridge that was open and expansive enough that, as people wandered about in the “original cityscape,” if they were to move to the area of this bridge, then they could continue their wandering up onto the bridge and over the river into the “new cityscape” in a manner where this transition to the “new cityscape” would be “friendly, unobtrusive, happy, airy, fun,” and very much as though one were on just “another byway” of the cityscape.
It seemed important to me that in order to achieve the design goals above, then the bridge needed to be wide, soft, bright, and solid. It needed to be exclusively for non-vehicular use and it needed to convey to those who were walking or bicycling on it an impression of being on a roadway or path and not up in the air over water.
I wanted park benches to be set out at intervals on the centerline of the bridge deck so that people could sit “up in the air over the river” and just take in the view, and the sounds, and the play of sunlight beams across the city and the river. I even wanted the new mothers with their toddlers in tow and babies in strollers to plan a “day outing” to the bridge and let the strollers bump along on the cobblestone of this bridge and take in the air and the light and the view without a care in the world.
Let’s see what these kinds of requirements will pull up for us as we put together this type of a pedestrian bridge.
Notice that the tubular steel supports under the bridge are anchored at locations wider than the bridge deck and lower than the bridge deck. The real purpose of these supports is to add stiffness to the deck so that the deck will not sway or undulate up and down or from side to side under the influence of wind or any possible coupling effects from people walking on the bridge and their steps (or the wind effects) getting coupled to any of the harmonic vibration frequencies of the deck. While these supports can also add some upward force to counteract the weight of the deck plus the people on the deck, these supports can also be kept small, understated, curved, graceful, and beautiful because the main support role for the deck of this bridge is via the suspension cables. This freedom from deck support constraints for the under body of the bridge gives me the freedom to try to add beauty to the overall visual impression of the bridge even from viewing angles that frequently aren’t considered very much.
The bridge will need enough width that its elevation off the water won’t be noticed unless one wants to go to the edge and look down. I feel 12 feet wide at the midpoint with expansion to about 18 feet wide at the ends of the bridge would do this. The roadway of the bridge must be pretty, solid, familiar, and inviting. Cobblestone fits this bill, with the provision that the protrusion of the bricks above the mortar must be about 1/16 inch or so, to give the impression of cobblestone without becoming a burden to bike wheels or stroller wheels, or people who may have some difficulties with walking.
The bridge needs to be solid enough not to give an impression of “swaying” or of being “up in the air” with “nothing beneath you.” So how could this bridge meet the rigidity and mass requirements listed above?
I would achieve these requirements by constructing the thickness of the bridge in a particular manner. The bridge would be constructed in 5 parts:
Part 1) the structures that support the bridge so that it will not sink into the ground. These will be micropile pylons.
Part 2) the concrete foundation structures that stand on the banks of the river (one foundation structure on each side of the river). These foundation structures support and hold up the rest of the bridge. These foundation structures are mostly concrete, but they are faced on all their surfaces with yellow-gold granite cut in a rough-cut fashion to give the impression (at least visually) that the foundation structures are constructed from granite blocks. The yellow-gold granite facing that is used on the surface of these concrete support structures where people will walk as they approach the deck of the bridge will not be as rough-cut as the rest of the foundation so that this surface will be suitable as a floor area for walking.
Part 3) a set of tubular structural steel elements that run beneath the deck of the bridge. These steel tubes are the lowermost portion of the span of the bridge.
Part 4) the “deck” of the bridge. This is the flat surface that people would walk and stand upon. This deck consists (starting from the lowermost portion of the deck) of 5-inch diameter support structural steel tubes, then aluminum pans that act to hold the mortar and bricks of the deck, then a layer of mortar 4 inches thick, then on top of this layer of mortar is another 4 inch thick layer of mortar that contains paver bricks laid in a cobblestone pattern.
Part 5) a set of suspension cables that run from one concrete foundation block out across the river to the other concrete foundation block. These cables provide support to the span of the bridge. They are connected to the span of the bridge in only one place. This connection point is at the midpoint of the span. These cables are in the air essentially above the deck of the bridge. These cables are kept elevated off the concrete support foundations by concrete-steel reinforced towers that are 18 feet tall. These cable support towers are faced with black granite blocks cut in a rough-cut fashion so that (at least visually) the towers appear that they are fashioned out of black granite blocks. The color black is chosen for these towers as a visual reminder of the long history of the Durham and Northern England area serving England as the source of England’s coal.
Beginning with the lowermost portion of the bridge deck, I would begin the span by placing a “parallel set” of structural steel tubes as the base.
(note: these tubes are curved and arranged in space so that they have a beauty to their course through space. This means that technically they are not really a “parallel set” because the tubes are not completely straight)
These steel tubes would run the entire 300 foot length of the bridge. Since a 300 foot “run” of a structural steel support tube would be exceptionally expensive both to acquire and to transport to the bridge site, then another technique would be used. I would divide the tubes into 50 foot sections.
As one goes across the bridge, one starts at the concrete base on the riverbank, moves 150 feet across the river, and at this point, one would be halfway across the bridge and directly above the middle of the river. One would travel another 150 feet further and one would then be on the concrete base at the other bank of the river.
Using tubular steel supports that are 50 feet long, then one would need a set of 6 supports to travel the entire 300 foot span of the bridge across the river.
The first supports would be the ones that touch the concrete bases. These supports would be 50 feet long. They would be 3 feet in diameter their end that touched the concrete foundation structure and would taper to 2 feet in diameter at their other end.
The next supports (again 50 feet long) would begin at 24 inches in diameter and would taper to 18 inches at their other end.
The final supports (again 50 feet long) would be 18 inches in diameter for their entire length. With these three types of supports, one set of 3 (at 50 feet long each) starting on one side of the river and another set of 3 starting on the other side of the river, we would have a tubular steel support truss that runs the entire 300 feet of the span of the bridge.
Remember, the bridge is 18 feet wide where it contacts the concrete bases, and I would use 6 runs of these tubular steel structural supports. The 3 foot diameter tubes where they rest upon the concrete support bases would be spaced about 2 feet apart from each other. The bridge deck is 18 feet wide where it contacts the concrete foundations. These support steel tubes are curved laterally as they approach the concrete foundations so that the horizontal width of their span is about 30 feet. This means that these support tubes are not actually under the deck of the bridge at the areas of the deck that are near to the concrete support foundations.
The 4 structural tubes are curved so that they are completely under the deck of the bridge at the middle of the bridge’s span. Since these support tubes are 18 inches wide at this location, and since the deck’s horizontal span here is a width of 12 feet, this means that these 18 inch support tubes will be spaced 2 feet apart at the midpoint of the span of the bridge.
Placed above these larger structural steel tubes would be a set of tubes that act as the direct support platform for the materials of the deck. These tubes would again be structural steel. They would be 50 feet long and 5 inches in diameter. Six of these tubes are needed in order for them to run the entire 300 foot span of the deck. There would be a total of 9 sets of these tubes running essentially in parallel down the entire length of the deck.
I went into Sketchup and “hid” the deck for these renderings so that these 5 inch diameter support tubes are easier to see.
I went into Sketchup and lowered the opacity of the deck so that you can see these 5 inch support tubes as they lay in place under the cobblestone of the deck.
The cobblestone surface of the deck consists of paver type bricks set into mortar. There needs to be a structure to hold the mortar, both when it is soft and has not set so that the pavers can be laid into the mortar, and also to act as a support foundation for the cobblestone mortar deck of the bridge after the mortar is set. This mortar foundation is described below.
Resting on these tubes is an aluminum tray constructed of sets of aluminum “pans” that are the width of the bridge and have a run of about 10 feet. This means we would need 30 of these pans.
Poured into the pans would be a concrete roadway base about 4 inches thick, with paver type bricks in a cobblestone pattern laid onto this roadway base, so that if one measured from the bottom of a person’s foot as it rested on the cobblestone downward toward the aluminum pan, then the distance from their shoe to the aluminum pan would be 8 inches. That is a 4 inch paver set with mortar around it and then 4 more inches of mortar under the paver, with the aluminum pan under that, and beneath the aluminum pan is the set of 5 inch structural steel tubes running the entire 300 foot length of the deck.
I feel these construction details would give the bridge the correct size and solidity parameters as discussed above.
It needs to be noted that:
While the structural steel tubes have enough strength to give some support to the deck of the bridge,
since the bridge has suspension cables running above the bridge, then
it is not necessary for the structural steel tubes of the bridge to support the entire weight of the bridge roadway and all the people that could possibly be standing on the bridge at any one time.
The bridge does have cables that run above the bridge roadway and are terminated in the concrete bases on each side of the river. These cables have sufficient strength to act as support structures for the roadway. These cables are one of the defining points of the bridge. The cables are not strung using a catenary type shape. The “pure” catenary shape goes too high in the air if it is used for a 300 foot span. Instead, excess tension is placed on the wires so that they assume a curve that runs in a smooth and pleasing form that is elevated 2 feet above the middle of the bridge and is elevated 18 feet above the level of the bridge roadway at the concrete bases.
The only requirement for this support cable curve specification is that the cable must have sufficient tensile strength to withstand the tension on the cable that brings it to this curve shape, and to also withstand the tension on the cable from its work as a support structure for the roadway of the bridge.
I should note that, as you look at the images above, its clear that the suspension cables are not located vertically over the deck of the bridge.
Since this bridge certainly starts out giving an impression of a suspension bridge, then I think I need to address just why the cables are placed in their locations. To start, I would say, from simply a visual perspective, the cables look much better as they run in a position lateral to the edge of the deck.
I think moving the suspension cables laterally also improves the sightlines available to the public as they stand on the bridge and look out over the river. By “sightlines”, I mean that the people on the bridge would be seeing the geography of the environs around the bridge, instead of seeing the suspension cables.
Finally, having these cables anchored laterally and curving in toward the center of the bridge allows them to act to help stop lateral swaying or undulations of the bridge, either from wind effects that would “pump” resonant vibration frequencies of the bridge, or from lateral “resonant pumping type” forces that the people on the bridge may create, especially if a large group of these people (either on purpose or by coincidence) all made a lateral movement at the same time.
For the more mechanically inclined, I feel this more lateral positioning of the suspension cables creates a kind of mental pause.
This is in the sense of the person saying (to themselves): “Hold on a second,” “this is a suspension bridge, so how is this suspension working exactly?”
“Shouldn’t the suspension cables be over the deck?”
“And how are the suspension cables doing their work of suspending the bridge deck, when there’s nothing going from the cables to the deck, except that one connection point at the center?”
I feel this bridge is like an optical illusion in the sense that the person involved begins to have a realization that their first impression (when more carefully thought through) is not actually possible. So, my feeling is that this bridge is a kind of “mechanical illusion”. The truth is it’s not really a suspension bridge.
Note that if one split the bridge directly at the midpoint of the deck and removed one whole half of the bridge, and then one put a concrete support foundation under the bridge deck at the place that used to be the middle of the bridge, then one would have a perfectly functional bridge that was now 1/2 as long as it used to be.
This means that this 1/2 span of the bridge (150 feet long) is perfectly capable of supporting itself as a standalone structure without the need for any suspension cables.
The real story of this bridge is that it is an:
underbridge truss stabilized and supported span.
The truss is made of:
- curved tubular steel elements which withstand the compressive stresses of the weight of the bridge deck and the people on the deck, and
- linear or straight tubular steel elements that run directly under the deck to withstand the tension stresses that are created by the weight of the deck and the people on the deck.
What we have actually done is used a mechanical type of “trick” to solve the problem of what exactly supports the center of the bridge. We made the truss structure light and small to keep it beautiful, and we curved the truss to enhance its beauty. Unfortunately these effects to enhance the beauty of the truss have an effect that the truss becomes too weak to support the entire 300 foot span of the deck.
This modification of the support truss allows us to put beauty into the bridge but it does create a problem.
If the truss structure of the bridge cannot support the entire span of the bridge, then what will keep the center of the span from falling down into the river?
Obviously, as is done with so many bridges, the engineers simply construct a concrete foundation out in the middle of the river and the center of the bridge span rests on this.
We don’t want to to do this. It (a concrete center foundation) is visually boring, it wrecks the flow and beauty of the bridge. It goofs up the way that the bridge expresses itself as curves connected to curves connected to curves, etc. And from an environmental impact viewpoint, I doubt permission will be given to place a structure in the river.
So we just didn’t put a concrete foundation under the center of the bridge span. Remember that a concrete support foundation is nothing more than a structure that provides an upward force to the bridge that acts as a counter to the downward force acting on the bridge from the bridge deck’s structural weight and the weight of the people on the deck.
Another perfectly reasonable method to supply an upward force is to place a suspension cable above the deck and use the strength of the cable to place an upward force on the deck.
So we have a “ghost” foundation at the center of the span of the bridge deck. The foundation is there, it keeps the center of the deck from moving down, but it’s the cable that does this, not any physical structure that is under the bridge deck.
I “see” this as a “mechanical illusion.” Why not?
Creating a cable that has these tension specifications is not that difficult. The need is for a cable that has a tensile capacity of 500 thousand pounds force. Two cables are run, so that the concrete bases must be able to supply a counterforce to each of the cables.
This means the cable anchors must be able to withstand 1 million pounds force (2 x 500 thousand) on each side of the bridge. This can be achieved by the traditional method of burying a support in the ground. But, I wanted a different “look” for the bridge.
I feel that people are influenced by the objects of construction that surround them. These include houses, buildings, bridges, etc. I feel that people are affected by the mass of objects of construction. I feel that truly massive construction has a definite “feel” and “effect” on people. I feel with massive construction, as long as it has symmetry and does not project oppression or a “threatening pose,” that people enjoy “big mass.” They enjoy the “feel” of big mass and they enjoy the way that “big mass” seems to “lay down onto” its surroundings a very specific “feel” and “ambience.”
I cannot say that I can prove this; it’s just what I feel.
So, I wanted to let concrete foundations of the bridge to be given enough mass that they themselves represented the “mass” that would supply the 0ne million pounds force against the cables supporting the bridge roadway. Since these foundations consist of a cube of concrete resting on a set of concrete “trusses” with these concrete “trusses” resting on in-ground pylons, then the mass structure that needs to weight 0ne million pounds would be:
the weight of the concrete cube plus the weight of all the concrete “trusses” that this cube rests upon.
If one wants this, then, with typical concrete, one needs a concrete “cube” about 30 – 40 feet on each side.
I didn’t want these concrete cubes to be too “oppresive” to the river and its environs, so I investigated how to do this. It turns out that one can sink support pylons into the riverbank, then put standard vehicular roadway type concrete trusses on these pylons, then put a 40 foot concrete 0ne million pound weight cube on these trusses without any real problems and without use of any “new” or “unproven” technology.
The pylons to support this will be micropile type. I include a small highlight of a United States Department of Transportation discussion paper on the topic of micropile support pylons.
Here’s the website for the whole micropile discussion:
1.2 MICROPILE DEFINITION AND DESCRIPTION A micropile is a small-diameter (typically less than 300 mm (12 in.)), drilled and grouted non-displacement pile that is typically reinforced. A micropile is constructed by drilling a borehole, placing steel reinforcement, and grouting the hole as illustrated in Figure 1-1. Micropiles can withstand relatively significant axial loads and moderate lateral loads, and may be considered a substitute for conventional driven piles or drilled shafts or as one component in a composite soil/pile mass, depending upon the design concept employed. Micropiles are installed by methods that cause minimal disturbance to adjacent structures, soil, and the environment. They can be installed where access is restrictive and in all soil types and ground conditions. Micropiles can be installed at any angle below the horizontal using the same type of equipment used for the installation of ground anchors and for grouting projects.
page 104 of this document details a micropile support that by itself can support 276000 pounds weight. These are the micropiles that I have drawn in under the bridge concrete “trusses.”
Page 105 of this document details how it is possible to take the parameters of an particular design of a micropile and perform calculations on these parameters to obtain the load of compression that would be appropriate for that particular micropile.
This seems to me to be friendly enough to the riverbank. All that would be done to the river environment in the soil would be the pylons, and one would need only about 24 of them on each side of the river.
Final result is a cobblestone “roadway” out over the river. The support cables are only 3 feet above the bridge roadway for the middle 220 feet of the span of the bridge and would run just exactly as a guardrail would run. They would be close enough to the bridge roadway that they would not interfere with the sightlines off the bridge.
I will note that in the Google Earth image drawings I have placed a “causeway” next to one of the support foundations of the bridge. This “causeway” is needed so that people can move from ground level up onto the area of the deck of the bridge. As drawn this “causeway” appears to be concrete and it is massively thick. There is no need for it to be this thick and I don’t think it should be concrete.
Much more in keeping with the bridge would be for this “causeway” to be a series of essentially giant “steps.” These “steps” would be 20-30 feet wide and 5-6 feet deep. The elevation between the steps would be about 6 inches. I would actually place 10 inch diameter structural steel tube elements on the ground running in a graceful curve from ground level up to the top of the concrete foundation structure. I would attach the “steps” to these structural steel tubes, and I would have the steps be aluminum pans that would contain a mortar and cobblestone brick paver surface so that the walking surface of these “steps” is exactly like the walking surface of the deck of the bridge.
Unfortunately, I simply ran out of time, so I put in the concrete “causeway” that is shown basically to act as a place holder.
When I get the “giant cobblestone step causeway” done, I’ll add it in.
Below is the SketchUP 2016 drawing file of the bridge above