Quantity calculation inside Revit

[Duncan Lithgow]

Can anyone help me with some links explaining how Revit calculates quantities?

My specific problem just now is in a project where we are providing the bill of quantities used for the tender process. So they have to be as accurate and predictable as possible. Most of the quantities are pulled directly from the project using Dalux Project (beta software, but quite good for our purposes, here's a link: http://dalux.com/dalux-project/ ). Some of the quantities we are rolling together into one quantity and adding it manually to the BOQ. What I'm seeing is that the quantities for a material in an exterior wall are significantly (up to 25%) smaller than the area of the wall itself. That's the exterior cladding of a wall with a smaller area than the wall itself. It ought to be the other way around. The difference is more than I can find a reason for in the model or in my tests.

Just some links would be great at this stage, then we can discuss what's going on...

[Craig_L]

Duncan I find this strange, as I have used Revit in the past for volumes and "basic" quantity take-offs and have tested this against real life values at the end of the project we were 1% off the actual values. This may be an issue of how your external software is interpreting the data, I have no experience with the one you mentioned here. If the walls are "composite" this may be giving you errors as your external program may be including the "void" area in your wall as part of the volume. Without looking at how the element was modelled it becomes speculative for me to say much more than that. In addition any IFC export will rely on your materials defined in Revit being accurate. If these densities etc are incorrect then of course you will get false results. (Under the manage tab in revit go to "materials" and check that particular elements properties and data)

Quantity take-offs are easy to do in Revit - I would suggest making some basic "template" quantity schedules that report exactly what you want and using these to verify the values you are getting in your other program as a starting point. You can add formulas to your schedules to create columns that compile calculations of other reportable data etc also - this does get slightly more complicated but its not really that difficult to do, and once you have your template versions set up you will never need to touch these values again. You can also export these to Excel etc for use in other softwares and this may be a more reliable method for you assuming your external software is misinterpreting composite elements.

http://knowledge.autodesk.com/support/revit-products/learn-explore/caas/CloudHelp/cloudhelp/2015/ENU/Revit-DocumentPresent/files/GUID-F8270A42-CA71-46C7-B145-85EC2CB8E4A1-htm.html

[Duncan Lithgow]

Hi Craig, thanks for your answer

This problem is actually occurring inside Revit, not in the other software (which doesn't support takeoff for materials)
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Do you know of anything documented about how Revit makes these calculations? Particularly the different between Type Schedules and Material Takeoffs.

[Craig_L]

Ah ok...I see. Yes, this is known. I will direct you to this blog rather than try to reinvent the wheel for you in my explanation. In short - the schedule (as opposed to the take off) will give you a surface area for all of the surfaces of that element.

http://revitclinic.typepad.com/my_weblog/2009/10/material-takeoff-area.html

Read the comments section below in that blog also, as he elaborates on what appears to be directly your problem.

Hope this helps.

[Duncan Lithgow]

Thanks for trying to help Craig. This seems to be a very poorly documented aspect of Revit's behaviour! Even reading that and other similar pieces on the net I'm unclear which Types are calculated in which way. Please let me know if you have other links to reading on this subject.

[Craig_L]

Hi Duncan, I am actually getting more and more confused as to exactly what you need. I would simply say that almost anything is possible to schedule for quantities. If the values you are getting are not really correct, I would simply create a new column that calculates volumes simply and use that value after I verify it was what I wanted. Height x length x thickness will give you what you are after, so its a fairly simply formula. If the value is consistently 25% lower than it should be simply add another column to give you what you need by adding the missing amount... I understand it *should* be giving you reliable values I dont understand why its not without having a look in how you have set up your schedules...sorry I am one of those hands on kind of people that needs to look further if the simple solutions dont work out.

[Duncan Lithgow]

Hi again Craig, sorry about the delay.

I've created a file which I hope illustrates my confusion. Here's the main point:
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The RVT 2015 file is attached.
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From the example file, here are my questions:
* Why is the Reported area of Wall 1 apparently calculated at the exterior face? Is that normal? I is not affected by the Wall Location Line.
* Where does the Reported area of Wall 2 come from?
* Why are my lengths broken? I have remembered to set Length to Calculate totals in the Schedule formatting.

As the file shows I can set Revit up to calculate the area more as I expect it, but I need to understand Revit's default behaviour before I start making manual interventions all over the place.

This is as far as I've got documenting this. Next is the understanding Material areas...

I hope you, or someone, can offer some insights into what is going on.

[Craig_L]

Hi Duncan, your attachment doesn't seem to be showing.

[Craig_L]

http://www.inmotionhosting.com/support/website/error-numbers/406-error

Web browsers make a request for information from the server. When this happens, it sends an Accept header. This tells the server in what formats the browser can accept the data. If the server cannot send data in a format requested in the Accept header, the server sends the 406 Not Acceptable error.

[Duncan Lithgow]

Hi Duncan, your attachment doesn't seem to be showing.

They should be showing now.

[Craig_L]

My first very quick impression is that the culprit is the "composite" wall family.
I broke this wall up into two separate elements each wall, and got different results.
Let me know if this is closer to what you expect.
I have included the revit file back also so you can see what I did with the schedule (which was not much)

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[Duncan Lithgow]

Now I'm back from summer holidays, time to get back to AUGI...

Craig, it sounds like you're focusing on the materials quantity, which is how I started this thread. Right now I'm not even understanding how the object areas work. They don't seem to follow any of the dimensions I would expect, not centerline, outer face nor inner face! Madness! So what is Revit using for these calculations?