Thursday, June 14, 2012

DWG Files and Funky Snapping

Importing survey files has always been problematic with Revit. Two things are fundamental issues: large file extents and distance from origin.

Geometry that is far apart will annoy Revit, geometry more than 20 miles in extents is the current "line in the sand". Just how precise this 20 mile consideration is a little hard to pin down. There must be some specific logic or algorithm the application uses but I've tried numerous times without success to arrive at something consistently true.

For example I've tried to draw squares and cubes, even circles and spheres to test the extremes and predict the trigger of the error messages I've written about before. As soon as I think I have something I can predict I try it again and get a slightly different result. My general rule of thumb is to try hard to make sure that I don't have extents that exceed a sphere that is 20 miles in diameter.

The other issue is when the data in the CAD file is small enough in extents but really far away from the file's origin, according to the WCS which is all Revit really pays any attention to. I can have a relatively small project site survey but far from origin and end up with Revit using Auto - Center to Center to place it. That's not really a problem though. We can just tell Revit how to resolve that with the Specify Coordinates at Point tool.

Now as for the title of this post, Revit hollers when our file is "too big". The warning tells us that this situation may affect the graphic depiction of information on screen.

This manifests itself in snap icons that appear to miss the element we are attempting to reference in the CAD file. It also shows up when you use the Pick tool to place lines and the sensitivity of the tool seems offset somewhat from where we see the line.

With Revit 2013 they've added an option in the Import dialog that says "Correct lines that are slightly off axis". This is a bit of spin unfortunately because this addition means it acknowledges that Revit has been altering them before, all along. Part of the solution to the funky snapping is resolved with 2013, don't check this option.

With Revit 2012 we can still avoid it but we have to import the DWG files a little differently. The trick is to let Revit place them using Auto - Center to Center and then move files, however many you need to use, into place manually. If we try to use the trick to Specify Coordinates using one file first and then import the rest using "Auto - by Shared Coordinates" it will trigger something internally that in turn triggers the snapping issue. That trick generates a message that these files aren't sharing coordinates but the end result is that they land in the correct location because shared coordinates are matching their own understanding of the WCS.

Here's a summary of steps, sort of...

Examine your survey file and make sure it isn't too large.

If it is you need to reduce it so just the really relevant part of the site is what you import. I realize that there may be situations that challenge this assumption but if you create two files from one that provides two that are small enough to avoid the error message about extents then you can still bring them in, position them manually and then specific coordinates.

Once you've resolved the survey file checking, pick a spot in each file that can be used as a common reference benchmark. You'll need it to move them into position in Revit. Write down what the coordinates are for this benchmark.

Now you can import the files into Revit. Use the Auto - Center to Center option. In Revit 2013 don't check the "Correct lines" option. You don't have this option in 2012 but this process will still work.

Move the files into position keeping them all near the Revit project file origin. Now you can use the Specify Coordinates at Point (SCaP) tool to match up Revit with your benchmark in the cad file. Unclip the Survey Point and move it to the benchmark location. Fire up the SCaP tool and enter the matching coordinate values.

Revit now understands your real world situation. You should find that snapping to the imported file elements is not showing the graphical glitch.

Important, there is one more thing to avoid, don't create a master survey file that uses attached Xref's to combine several files together into a single file that gets imported into Revit. Doing this will trigger the snapping issue again. It does it in 2012 and 2013 regardless. I believe that the Xref's mark the origin somehow in the master file which creates an unavoidable file extents problem which brings the snapping issue back to life.

Easy?? Wish it were easier!

Wednesday, June 13, 2012

Room Separation Lines - Agida for MEP?

Via email the other night, he writes...(edited a little)

We use Revit MEP. I’m doing some research on Room separation lines, as to how and why architects use them, and when is appropriate. The problem I am consistently running into is that room separation lines divide spaces as well. These divisions, while relevant to the architectural model, are ruining the possibilities of creating gbXML files without extensive work on the part of the engineer to remove the room separation lines, and then repair the damage done to the room names and numbers.

For example, imagine open library stacks with dedicated study areas. In the architectural file, they used room separation lines as a rectangle to create rooms and tags for these study areas. They were also used to define a corridor along one side of the stacks (but still in the same open area). For MEP these are really all part of one big room. We won't need to add dedicated air terminals to these areas, nor special lighting just for these areas. So any of the analysis tools in Revit using spaces will be wrong, as most of them use an average over the entire area, (CFM/SF, Average illumination levels/SF etc..). Further, we are unable to accurately or efficiently create a gbXML file for use in Green Building Studio, HAP, Trace, etc. This complication is greatly hampering many of the benefits of using Revit for engineers.

I’m not saying that room separation lines should never be used, there are perfectly good places to use them. Rooms or spaces that are actually going to be treated as a separate room, not a room with in a room.

I initially thought that architects were using these (in addition to the obvious) to check for programming requirements, and code compliance for areas and such but it turns out a lot of architects are using the area tool for those purposes. They were however using them for the room finish schedules.

From a program standpoint, I was surprised to find out that while room separation lines can be assigned to a workset and that workset can be unloaded (through a linked file) from the MEP model. However the Room Separations still existed. We just couldn’t see them. And even if that did work, we engineers would still need to correct/edit the space names and numbers manually now since the spaces wouldn’t match the rooms (Space naming tool wouldn't work).

He closed wondering if I could offer any advice, ideas or a solution.

The essence of my response via email:

HVAC Zones are meant to combine spaces into larger more coherent collections for engineering purposes. The example given above for the library is pretty common place, need to know how much area is dedicated to carrels as well as a corridor within the open space of the library. From the HVAC engineer's perspective irrelevant but not from the client's or the architect trying to satisfy them.

If room separation lines are "off the table" then the architect would end up using something else like actual walls and hiding them. They might end up asking Autodesk to make more elements room defining? Area plans could help document such things but then floor plans would be "ignorant" of what these kinds of "areas" are, no tagging possible, just "dumb text". Not ideal either. My greatest concern about room separation lines is when they are used as "band-aids" to try to fix room area issues, where rooms aren't bounded properly and not generating area values.

Regarding the workset comments. Worksets unload information but that doesn't mean Revit isn't aware of the elements that are assigned to the unloaded workset. When we unload a workset the walls are still there and that means so are Room Separation lines.

Any comments from readers??

Tuesday, June 12, 2012

Measure Between Two References

Saw a post at asking how best to go about finding out what the perimeter of a given building is. I was a little late to the party there and I was mulling over what to write about tonight...

The catch, the building is a linked file. Not much of a catch but there is no equivalent floor in this case. If there was a floor there I'd just check the properties of the floor. Technically if a linked floor matched the perimeter of the building you can still see properties of the linked element if you use the TAB select technique to grab a hold of the element in the link.

...or you could sketch your own a floor around the perimeter pretty quickly. or you could use the area tools. or you could draw a property line. or you could....

Use the Measure Between Two Reference (MbTR) AND check the Chain Option! This will give you a running total as you pick additional points along the perimeter of the building. Here's a quick video demo.

For my money, I'd probably favor a floor. That way when the person who asked you to find out says something like this, "Hey what was that perimeter value again? Sorry I didn't write it down" You can just check the properties of the floor instead of using MbTR again, or flinging your scale at them.

Monday, June 11, 2012

Detail Level Ignored with Unlocked Wall Layers

We can unlock a layer or layers in a wall to allow "pulling" them up or down separately from the rest of a wall. This makes it possible to create a brick veneer that drops below a level some distance or pull the stud layer up farther, like this.

To make this possible you need to edit the wall's structure and select a layer to unlock, like this: 1 - Click Modify, 2 - Select the edge of a layer, 3 - Unlock the padlock

Interestingly, in Ceiling Plan views only, when we take advantage of this we lose the ability to show just two lines with Detail Level: Coarse.

I suppose it makes sense somewhat, since they are now distinctly separate but the hope was that we could still see just two line in Coarse. Seems to work fine in plan views though.

Friday, June 08, 2012

Revit MEP Electrical Calculations

Martin Schmid has been sharing some deeper insight into the "magic" behind electrical calculations that Revit provides. Every engineer I've met is, not too surprisingly, skeptical about trusting yet another software for these answers. There has been data, in help documentation, to support the results but not quite as deeply as the last three posts combined. Check them out!

Why do I get different Voltage Drop results with Revit compared to the NEC?
To compute Voltage Drop, why does Revit use the formula VD=(L*R*I)/1000?
Why does the Revit Wire Impedance Factor table have separate sections for Three Phase and Single Phase?