Drawing a Bridge in Autocad

Extruding 3D Solids – Methodology

An overview

The goal of this methodology is to provide the AutoCAD user with practical guidance and a how-to in modelling a structure, such as a bridge, which has complex 3D geometry. Often the design of highways complicates bridge geometry when a road alignment has both horizontal and vertical curves. This is accentuated when trying to model a box girder for example, in 3D, with a road alignment containing both horizontal and vertical curves.

When attempting to model such a structure we are confronted by what seems to be an AutoCAD limitation. AutoCAD is unable to develop a polyline with curves in two planes, which is required to have a true representation of a horizontally and vertically curved road alignment. If it were possible for AutoCAD to import a non-coplanar curve our job would be made all the more easier but as it stands we seem to be stuck.

Although drawing a true non-coplanar curved alignment in AutoCAD is impossible, we can however draw a 3D polyline that closely resembles the alignment by creating or importing a faceted string, made of closely spaced straight lines. With this workaround we'll be able to extrude a cross section along the 3D polyline.

The following examples provide two ways which this can be done. One method is to extrude a solid and the other is to loft a solid. The pros and cons of each method will be discussed and hopefully a clearer understanding of how each method works will be gained.

The example is of a real world interchange on the north coast of NSW, which possessed some interesting and challenging geometry. The interchange consisted of a North and South bridge arranged in a ring and I've chosen the south bridge for this particular tutorial.

The two methodologies are for the same bridge and the geometry is both horizontally and vertically curved.

Method 1 – Extrude

One of the first things to do is set up a 2D cross section which can be copied and then pasted into the 3D model. I've used a general arrangement cross section of the South Bridge and made a region out the concrete outline (see above image).

I've also set up the 3D model with a 3D polyline connecting the end points of vertical lines representing the reduced levels of the control line, spaced at 250mm along the control line MCR0 as shown below.

Once we have the model set up with the 3D control line data we can then go ahead and copy the concrete outline, using the intersection of AC and MCR0 as the base point. We'll then be prepared for the next step.

The 3D polyline will be used as the path to extrude the cross section along.

Now that the preliminary set up has been finalised the next step is to paste the 2D cross section into the correct position. The radial lines construction lines shown below help us to manipulate the UCS so we have the correct orientation to paste the section in with.

Using either UCS Object or 3 point UCS the UCS needs to be aligned so the X-axis is parallel to the first radial line and the Y-axis is parallel to the first vertical line.

I've used UCS object to select the first radial line which then changes the orientation of the UCS so X is aligned with that line. Then I've rotated 90 degrees about X so the Y-axis is aligned with the first vertical.

Rotate about X – UCS toolbar shown above.

The rotated UCS shown above.

Then paste the 2D cross section we copied earlier as shown in the above image.

Now it's a matter of moving the cross section up to the correct elevation taking care to use the appropriate base points, which in this case are the end points of the vertical line at the start of the deck. The end result of moving the section should look like the following image.

Move cross section to correct elevation shown above.

The next step is to then perform the extrusion.

I like to type "EXT" at the command line but other options are available, like the extrude button on the modelling toolbar or the 3D modelling ribbon.

AutoCAD prompts to " Select objects to extrude" , so select the cross section.

Once the cross section is selected AutoCAD then prompts to " Specify the height of the extrusion or [Direction/Path/Taper angle]:"

In this instance we type P for path and then select the 3D polyline as the path to extrude along.

Then presto AutoCAD does its magic and an extruded solid appears. Something like the following image should appear on your screen:

Wireframe of extruded section shown above.

Shaded extrusion above.

To check our newly created model a radial section needs to be cut at some random point along the control.

I've picked a random radial line and used it to set up my UCS in the exact same way as I did prior to pasting the 2D cross section into the model.

Then it's simply a process of using the section command ( SEC at the command line ) and selecting the extruded deck and using the XY plane option to cut the section through.

The following image shows a section through the 3D model along one of the radial lines.

Cut a section through the 3D model above – The picture is low res but you can make out the outline of the section cut in the middle of the deck, barely but it is there.

If we then copy and paste the cut section back into the drawing we used to create the 2D cross section we'll be able to compare the result and ascertain whether our model is correct.

Having pasted the cut section back into the GA drawing we can see that our model is not quite correct. One end is lower than it ought to be and the other is slightly higher.

It should be an exact match if we are to have any confidence when using our 3D model to prepare detailed design documentation.

I believe the reason why the model is different is because the way AutoCAD assembles the extrusion. I think AutoCAD makes adjustments when extruding by rotating the section as it extrudes. Perhaps by using tighter intervals when setting up the 3D polyline, the adjustments AutoCAD makes will be somewhat less pronounced but invariably the model will be out by some margin. However small the margin is, it is not the desired output we would like to use in our documentation.

In conclusion extruding is a relatively easy and quick tool to model with, however geometry which is curved and non-coplanar will produce unpredictable results and the final output may be distorted to an unacceptable level.

For simple geometry it is a fantastic tool but for geometry possessing curves in horizontal and vertical planes the results are unlikely to be satisfactory or desirable. Although extrude in this example didn't provide us with the results we were looking for, the extrude function may still be useful and have a more suitable application as a visualisation and conceptual analysis tool.

Depending on the project, extrude may still function satisfactorily and be within our constraints and construction tolerances. With closer experimentation, in particular the spacing of 3D polyline straights and length of extrusion a more favourable result might be achieved.

Extruding – Pros and Cons

Pros

• Quick and easy to use.
• Minimal initial setup required.
• Fantastic application for coplanar objects.
• Good visualisation tool.

Cons

• Output may be distorted in more complex geometry applications.
• May not be a good tool to use when preparing a model for detailed design when the geometry is curved in two planes.

Method 2 – Loft

Lofting is a fairly recent addition in the AutoCAD repertoire of commands and is perhaps borrowed from other 3D Visualisation packages like Viz or Studio Max. Regardless of its origins, lofting is a function which offers an invaluable resource when it comes to building a 3D model and one worth learning in its own right.

Considering the extrude method of modelling our bridge deck produced inaccurate results we are left with limited options. However, since the introduction of lofting functionality in AutoCAD 2007 we have at least one more way to construct our deck.

Loft requires more preparation work and can be a lot more tedious to set up, which is illustrated in the following.

As we did previously we'll paste the 2D cross section into our model. However, in this case, instead of just one cross section, we'll want several cross sections and in this example I'll paste a cross section in at each of the 250mm spaced points. It is perhaps excessive and far fewer sections may suffice, but one can reasonably assume that the more cross sections available to loft with, the more accurate the model will be. (Tip: In AutoCAD 2010 and above it is best practice to create a closed polyline of the cross section to be pasted into the model, rather than regions, so that the loft will be created as a 3D lofted solid instead of a 3D lofted surface)

How many is enough? It depends on the work at hand and it's hard to say without testing a sample.

Because I've used a set spacing of 250mm in the previous example it'll be beneficial to stick with the same here so we can make a comparison between the two methods without introducing differing variables.

To illustrate the process with which this example will be based, please see the images below to get an idea of how the preparation work will eventually look like.

Paste sections with UCS in correct orientation as shown above.

Place a series of cross sections parallel to the radial lines as shown above. Although the above image shows only a few as an example, I've done the same thing for each of the radial lines. A tedious process and extra attention to detail needs to be exercised in order to make sure the cross sections are pasted in the correct location. Changing the line colours as you progress is a useful way to ensure you don't mistakenly miss a section or paste it in the wrong location. (Tip: Always remember to orientate the UCS into the correct working plane! The X and Y axes should be orientated in the same way you would draw in 2D – only it'll be in 3D )

Move sections to correct elevation as shown above.

Then move each cross section to the correct level; another laborious process and one fought with peril, with masses of lines designed surely to confuse, disorientate, confound and infuriate even the most studious and keen eyed practitioner of draftmanship.

Once all the hard work of preparing our lofting model is done, we are ready to loft the sections together and to do this we simply to type " loft" at the command line and then select each cross section. Careful attention to detail will be in order for this task also.

Just when we thought all the hard work was over we need to select each cross section in order of the loft. More eye crossing work – just what our eyes were begging not to endure one moment longer. It is easy to accidentally select or miss a cross section and is something we want to avoid. It may not matter since they are so many but paying close attention now will save some time later if we find by missing or reordering a section produces a corruption in output and we have to undo and start again.

Select the cross sections, making sure to select each section in the correct order of the loft.

Once we have all the sections selected and we hit enter some options are available and dialogue box appears with some further options. We want to use <Cross sections only> and make sure the smooth fit radio button is checked.

Loft settings dialogue box shown above.

The lofted section will look similar to the following image.

Wireframe loft above.

Shaded loft – with concrete material added for effect, shown above.

As we did with the extruded model we want to cut a section through the model to determine the accuracy and correctness of what we've just modelled.

Following the same procedure to cut a section and then paste it back into GA drawing for comparison.

Cut section through model shown in green above.

Pasted section in GA drawing comparing our model to the 2D cross section shown above.

After pasting the section into the GA drawing and overlaying it we can see that the result of our laborious efforts preparing a loft has produced a superior section than that of an extrusion. We can be confident that the model is accurate enough to prepare detailed design drawings from.

Lofting – Pros and Cons

Pros

• Smoother finish without messy iso-lines. (although they can be turned off)
• Can produce accurate non-coplanar curves.
• Good application for modelling complex geometry.
• Good visualisation tool.

Cons

• More preparation work is required.
• May produce strange results occasionally.
• Easy to become confused if lots of cross sections are used.
• Longer time required to achieve a result.

Conclusion

From the Interchange – South Bridge example, we've become familiar with various modelling techniques and gained knowledge in how both extruding and lofting work. Clearly lofting produced a superior result and for this example it is the preferred method to model a bridge with horizontal and vertical curves.

Even though lofting is exponentially a more labour intensive process, the results speak for themselves and if one is to make a comparative analysis, both methods used the same 250mm spacing along the control line. The extruded model introduced a distortion which was too high to be considered useful in preparing detailed design documentation, while the loft produced a result within drafting tolerance.

It's not to say extruding is a lesser tool because it is a powerful function in the appropriate application and can be used to advantage for a quick result. If a concept design required only basic visualisation as a requirement, then clearly extruding the deck would have been the preferred application to use. For a straight bridge extruding is ideal as there is no reason for AutoCAD to make adjustments by rotating a section around the curve.

For the most part there is no right or wrong method and one could even utilise surfaces if solids aren't required. Experimentation is often required and if one method works where another fails, then so much the better, but 3D modelling will require a number of different methods be utilised for maximum results.

In a real world scenario the best practice will be to sample different methods with higher or lower control line vertices on pieces of a model before committing to one method over another.

A final rendered view of the south bridge.

johnsonthiandes.blogspot.com

Source: http://www.bridgedrafter.com/cad-guides/autocad-creating-3d-bridge-deck/

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