A guide to: Motorway bridges

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StruSoft Fredrik L
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A guide to: Motorway bridges

Postby StruSoft Fredrik L » 11 Aug 2016, 17:07

In this guide we will take a glance at how to model a motorway bridge including moving loads.

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The purpose might be to design a new bridge or to investigate if a bridge can handle a certain combination of loads, or in Sweden to classify it in the three different BK’s (swe: bärighetsklasser). This guide will be based on a few scanned drawings so the user will not be able to use the external reference function, and everything has to be made by hand.

Structure

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The first step is to define how big the model will be. It is good to use something like a column or a wall as a reference point. In this case we will use the center column line and build up the model around it.

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Draw a few lines to indicate the center line of the columns, and then just a few guidelines to get the measurements correct of the distance between the columns and the span of the bridge. Then place the columns along the line in their proper position, The material in this old bridge is set to K300, a concrete material used in the old Swedish code, and is very similar to C25/30, creep and shrinkage will be set to approperiate values, according to the current code, EuroCode (EN1992-1). The length of the columns can initially be set to the default value (3.0m), but will later be stretched out to the correct length. The two lines about 10m offset from the column line are the positions of the retaining walls.

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Next optional step is to set some axes to view the model in. Let's start with three axes, one for the line of columns, and two for the retaining walls.

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Define the two retaining walls and model them along axis 1 and 3 (for this example). Thickness 400mm, if the thickness varies along the height it would be good to model the wall as a standing plate, since it is possible to change the thickness to be a varying thickness for plates. The height can be stretched/cut to suit the foundation and the bridge deck.

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Since the level of the foundation might vary between axis 1, 2 and 3, we now have to define what level the different objects should start on (bottom point of a column is consedered to be the start point and the top is the end point)

For this example the wall in axis 1 has:
  • Bottom level at +10.70
  • Top at +16.10
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Axis 2:
  • Bottom level at +10.95
  • Top at +16.42
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Axis 3:
  • Bottom level at +12.00
  • Top at +16.74
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Due to the spanlength the bridge deck will be sloped, but still linear between the walls. So, since the levels in this example doesn't really affect the windload or soil properties then we will just assume that the global z-level=0 in the model will be set to the lowest of the foundation levels (+10.70).

Axis B will have to have the columns trimmed in the bottom by 10.95 - 10.70 = 0.25m.
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And the wall in axis C needs to be trimmed 12.00 - 10.70 = 1.3m
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The bridge deck will be placed on top of the walls and columns, but the level has to be decided first. The bottom of the deck in one axis is about +16.10, it isn't even along the axis since in the middle of the deck it is a bit higher due to the inclination to drain the water. In this example we will assume that the height is even anyway. The deck is about 1m thick, but we will model it so the walls and columns end in the bottom of the deck.

In the middle the position of the deck is a bit higher due to the bridge is sloping upwards by 32‰. This means that over the columns the level will be +16.10 + 0.032 * 10000 = +16,42
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...and the level at axis C top-level will be:
+16.42 + 0.032 * 10000 = +16,74

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The bridge deck is 1m thick, so create a plate that has that thickness, material C25/30, use polygon-function to snap the plate on the outer points of the walls and outer columns (be aware that all points has to be in the same plane, and that the plane can be sloped like in this example). Copy the bridge deck to either side of the bridge, so that the moving load has possibility to start and end. It will also be easier to set a plane to create or place the truck/vehicle on the slab. Under the plates on either side of the bridge we place a rather soft surface support so that this support wont take som much load from the bridge structure. (Spring stiffness ~=40.000kPa/m)

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Insert one of the wings as a wall with the angle 45 degrees away from the bridge direction

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Stretch it down by select the bottom corners and stretch it down to the bottom of the retaining wall (step 1). Then stretch the wing corners to match the wanted geometry (step 2 and step 3).

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Copy, rotate and adjust this wing to fit on all four corners. Add line support (groups) under the walls and point support (groups) under the columns, all with their suitable boundary conditions.

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To finish of the structure we will add the edge beams and a slab on each side with variable thickness between the bridge deck and the edge beam. The easiest way to create the small edge slab is to copy the big bridge deck and then cut it (or stretch it). Then change the thickness by using Variable thickness-command.

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The beam used for this example is a 250x400 concrete section, insert it without any excentricity, but feel free to change the Physical alignment to match the visual of the drawing. This alignment has nothing to do with the calculations, only visual appearance when it comes to concrete objects.

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Loads

In order to place the loads on the road correct, due to the slope, we set the UCS aligned with the road. Since the bridge deck and the part of the motor way before and after have the same slope we select the UCS-function (Alt+U) and use the setting 3 points to change the working plane to be inline with the road.

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The vehicle travelling over the bridge in this model is not pre-defined so it has to be added to the vehicle library. The definition for the vehicle in this example is taken as a emergency vehicle from the Swedish Transport Administration code TRVK Bro 11 (publ. nr 2011:085).

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In order to draw it easily in the model, the road has been divided into lanes and with a initial line showing the path, drawn as regular lines. The shown point loads from the code will be converted into surface loads to reduce the strain on individual points and spread out the load as expected from the tyres in real life.

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80kN per axis ie. 40kN/tyre, gives a surface load of 40 kN / (0.6 m * 0.2 m) = 334 kN/m².

Create a load case (in this example called Vehicle), this is where we will put the loads before adding it to the library. After placing the loads in the model, start the Moving load-command and select Vehicle.

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The function works in several steps. First you select (box/crossing-select if you have more than one load) the loads. Second is to define the local coordinate system of the vehicle. Origin can be set between the front wheels, second point is the direction of the vehicle (pointing forwards, green arrow in picture above) and the third point should be perpendicular to the direction (so that the blue z'-arrow points up). To finish the command a window will pop up where you can review the loads and name the vehicle.

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The vehicle and the load case will now disappear, and the vehicle stored in the library for future reference/use. The reason it is removed is so that there won't be double loads when applying the moving load (the definition of the vehicle AND the vehicle moving). So in order to apply the moving load we now start the Define-command. Define what load case the vehicle-path should be known as, increase the number of steps or set it as a step length. In the Vehicle-tab, select the vehicle we defined earlier (Fire truck)

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Draw a path from one side of the bridge to the other. It is possible to draw a polyline and let the vehicle follow that path, it doesn't have to be a straight line. This example shows a 2+2 bridge (2 lanes in each direction)

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If you want, repeast the Define-command, but remember to rename the vehicle to not get a error message complaining that the name is already taken.

Several moving loads on the same bridge:

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Each set of moving loads has now got its own Load group (set of loads that can not act at the same time). To add the dead load of the bridge to the calculation we start by definig a Load case with "+Structural dead load".

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Add a load Group and assign the load case to it.

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... and finally before making the calculation, generate the load combinations.

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Tips:
  • To save a bit of calculation time, it might be good to not use too many steps, since 4 trucks with 25 steps will generate A LOT of different positions/combinations. (in this example: more than 25*25*25*25*3 = almost 1.2 million load combinations, that will take days to generate)
  • Let the truck start just before the bridge and end so that last axis just has passed over (less load cases to combine)
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Fredrik Lagerström
Technical Support and Developement
StruSoft AB

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