Thursday, January 26, 2012

Revit Inconsistent Units

Whilst searching the forums trying to get my head around why Revit is always telling me I have inconsistent units in the calculated values in my schedules, I came across this little beauty on the Revit Forum

It really does hit the nail on the head, and is well worth printing out and keeping hold of, thanks to Klaus Munkholm for sharing this one!

....."Inconsistent Units ! Probably THE error box in Revit that have caused me the most grief, over the years I´ve spent hours and hours on trying to work around it. But once I got the hang of it, and not least once I understood WHY Revit would throw that error box at me, it´s not that bad. Hope some of you will find this helpful.

Let´s begin with some basic algebra:

<100 mm * 100 mm = 10,000 mm²>

So if you have a Length parameter, and try to use a (Length * Length) formula, you´ll get the " Inconsistent Units" error, because (Length * Length = Length Squared). Only way around is to neutralize the units on at least one of the parameters in the formula. The easiest way to neutralize a unit is by dividing by 1 (one):

<100 mm * (100 mm / 1 mm) = 10,000 mm>

Why? Because <100 mm / 1 mm = 100> (unit less) and <100 mm * 100 = 10,000 mm>

And another example:

<100 mm * 100 mm * 100 mm = 1,000,000 mm³>

So again, if you need to multiply 3 length units in a Area or Length parameter, you´ll need to neutralize the units as above.

<100 mm * 100 mm * (100 mm / 1) = 1,000,000 mm²>
<100 mm * (100 mm / 1) * (100 mm / 1) = 1,000,000 mm>

So far, dividing by 1 (one) have been a success, but in some cases it´s necessary to multiply by 1 (one) instead. An example: You want to multiply two Number parameters into a Length parameter:

<100.0 * (100.0 * 1) = 10,000 mm>

Another common situation for the "Inconsistent Units" error, is when quantifying costs in schedules

In the schedule you already have "Area" and "Cost" but want to add a Calculated Value "Total Cost":

<(Area / 1) * Cost = Total Cost>

There´s probably a gazillion other possibilities, but I hope that this explains why we are getting the Inconsistent Units Error, and how to work around it...."

Tuesday, January 17, 2012

Revit Structure Schedules: The Basics and Beyond

The aim of this post is to highlight the benefits that can be gained from leveraging the information held within the database of a Revit Structure model. Using the intelligence of the software to provide quantitative information about a building’s design using Revit’s scheduling capability can provide invaluable assistance to clients and contractors alike, allowing much tighter control of project budgets when considering scheme costs and allowing tenders to be much more accurate and thus competitive in difficult market conditions.

The Basics
Most Revit users reading this article will by now be familiar with the basic scheduling tools within the software. The ability to automate the scheduling process for elements such as columns, beams and foundations; tagging the elements and scheduling the mark, type, count, etc. considerably speeds up the traditional ‘draughting process.’

Using the parameters that are already embedded in the system, additional information can be added to schedules to further add value to your documents (when using Revit for information production). Concrete volumes can be added to foundation, column, beam and floor schedules.

This information can be further broken down within the schedule using the sorting / grouping tab of the schedule properties, enabling the user to easily schedule concrete volumes by floor level within a building for example, or the concrete volume of columns by storey, rather than providing just an overall total.

Concrete Volume Schedule Cautions
There are some basic modeling principles that users need to be aware of when scheduling concrete volumes within Revit. It is easy for the inexperienced to model elements in a ‘lazy’ manner that causes them to intersect each other as opposed to forming neat geometric joints.

On paper this method of assembly may appear to be satisfactory, however, when you start to interrogate the reported volumes within the schedule, discrepancies are found if the model has been constructed in this manner. The concrete volumes reported in FIG04 are incorrect, and should actually be showing the values shown in FIG05.

The reason for this is because the concrete wall is intersecting the column and the geometry has automatically joined. This has resulted in a miscalculation of concrete volume for the individual elements. Whilst the reported overall concrete volume of the structure will be the same, the breakdown of the various elements to show different grades of concrete is where the problem will arise. The solution sounds simple; model each element correctly! However, in practice this might prove to be harder than it seems.

Beyond the Basics - Parameters
There are two types of parameters that can be added to schedules; ‘Project parameters’ which can be added within the project environment and appear in schedules but cannot be tagged, and ‘shared parameters’ which are built into the families and can be scheduled and tagged.

In the example below, project parameters have been added to the family allowing the column schedule to show the various loads on each column: Dead, Imposed, Wind, Shear Major and Shear Minor. Using an external plug in programme, Microsoft Excel files can be imported and exported. This enables the user to easily update the loads in the Revit schedule, using values exported from the analysis package, resulting in all the information being stored within the Revit database.

Shared parameters could be considered to be the back bone of the Revit platform and the key to producing data rich Building Information Models. Shared parameters are stored independent of the family or project environment, and can be used in all families (standardizing the naming of parameters within your library). Once added to families, these can be called up in tags and schedules (speeding up the information production process).

In the example below, shared parameters have been built into a parametric base plate family. These parameters have been scheduled to call out the base plate dimensions, No of bolts, bolt size and dry pack thickness. Additional parameters could be added to the bolt family and shared to the baseplate family to include additional data such as bolt lengths.

Beyond the Basics – Reinforcement Estimates
The default fields available within the Revit schedules (for concrete) allow for the scheduling of estimated reinforcement volumes assuming the reinforcement has been modeled in the project. This figure can be converted into an estimated reinforcement weight by using a calculated value in the schedule.

The idea of using Revit to provide accurate reinforcement estimates could be viewed by the user to be quite a labor intensive process, requiring all elements within the model to have the reinforcement modeled to suit the preliminary reinforcement design.

However, the use of the reinforcement extensions provided by Autodesk as part of the subscription advantage pack will speed up this whole process. Using this extension columns, beams, foundations and walls can be quickly and accurately detailed with the designed reinforcement, producing accurate schedules to assist with pricing.

Beyond the Basics – Steel Tonnage
The ability to calculate steel tonnages from a Building Information Model not only assists the tender process but in such a competitive market can prove invaluable when undertaking value engineering on a scheme.

By adding a shared parameter into the steelwork families to allow the W value (Mass per metre kg/m) to be scheduled, the member’s tonnage can be easily calculated using the basic formula; Mass per metre * Cut Length / 1000 (for metric tonnes)

This method is applicable to all steelwork, although it is worth noting with column schedules there is no ‘cut length’ field, so the parameter to use in this instance is length.

Tonnage schedules can be broken down by element, by floor level, etc. using the sorting / grouping tab in the schedules dialogue box. It is often worth sorting the elements by type mark and having this field set as a ‘hidden field’ under the formatting tab of the schedule properties.

Custom framing members such as cell beams with different top and bottom web and flange sizes can also be scheduled, providing you have the relevant information relating to its mass, and add that information into the family as a shared parameter.

Being able to accurately schedule the steel weights alongside the member utilisation ratios can help to identify inefficiencies in the frame design and provide an opportunity to refine the design and make savings. In today’s highly competitive market, innovative use of the software in this manner may just provide the cutting edge that results in a winning tender for the team.

Moving Forward
Structural analysis and design packages have the ability to export plans, elevations, sections and 3D views showing the utilization of members such as framing. Traditionally any members with a utilisation ratio greater than 1.0 would be shown red, whilst members with utilisations less than or equal to 1.0 would be coloured according to their actual utilisation based on a graduated colour chart typically turning deeper shades of blue as the members become increasingly redundant. These colours are usually set within the program and cannot be changed.

In order to promote efficiency in design, clients are beginning to request graphical confirmation of the structural utilisation of the design detailed on the construction issue drawings. Through the application of colours to the individual members (based on their level of structural usage), any areas of excessive structural redundancy in the frame will be apparent and allow clients to consider design modifications to improve efficiency of use.

Through the intelligent use of schedules and filters within Revit, structural members can be highlighted using any chosen colour scheme based on their levels of utilisation stored as parameters within the families.

Using an external plug in program to manipulate data transfer between the design package and Revit, utilisation parameters in the member families can be populated at the click of a button based on the results of the structural design. Thus construction drawings can be easily and quickly ‘converted’ into structural efficiency drawings following each design revision, until such time as the client is satisfied and the design frozen.

This method provides clients with a means checking that they are not paying for an inefficient design, and because they are able to visualize this in a 3D environment, they can gain a better understanding compared with traditional 2D.

In summary, schedules should be considered as;

- an extremely valuable and powerful feature of Revit.
- a way of providing additional (quantitative) information at little extra effort.
- an opportunity to gain a competitive advantage to tendering and value engineering.

To gain most benefit from scheduling, careful consideration needs to be given to;

- the information required and parameters added to families accordingly.
- sorting / grouping data in a logical way.
- the way your Revit model is constructed

Finally, not only are schedules useful for quantification purposes, they provide a powerful way of linking Revit to external software packages.