Custom Rebar shapes and missing parameters

[semi]

HI everyone,

When changing a rebar shape to a custom shape that doesn't exist in the library some rebar parameters go missing?

In our Bar Bending Schedule we've been using the parameter "Length of Each bar" for years now to calculate the length of every rebar.

This parameter seems to be missing when we make a custom rebar shape.

For example:

Our standard rebar parameters look like:

While a customised rebar looks like this (Rebar Shape 1):

We can't use this at all + the measurements are totally wrong

[ovidiu_paunescu]

Hi @semi 

If you sketch a new shape, not all shared parameters are automatically added. For example, Length of each bar is a parameter added to shapes in the UK library, to compute the length as specified by BS 8666.

My recommendation, after you are happy with the geometry of the bar in the project to edit the family and configure what parameters are showing which lengths.

• To do this, select the bar, edit family 
• Select each dimension and add the expected parameter, A, B, C
• Add a default value to all parameters assigned to dimensions 
• Add the "Length of each bar" by clicking new parameter, and selecting the shared parameter definition for UK rebar shapes (by default this txt file is located next to the rebar shape families in your UK library)
• Define the formula for length of each bar
• Load into project 

See page 13 of the handout from my AU class for more info about defining rebar shapes

https://www.autodesk.com/autodesk-university/class/How-Make-Most-Rebar-Detailing-using-Revit-2022#ha...

[semi]

Yeh, I know how to solve this workaround, I was just wondering why? How is this user-friendly?

[ovidiu_paunescu]

Well, the "Length of each bar" uses formulas defined by the British standard, and is only added for shapes in the UK library. For any new shapes that parameter needs to be configured.

Those formulas may contain some simplifications and roundings, so there wouldn't be a straight forward way for the system to automatically define formulas for any geometry that you can sketch.

Even if you produce the new shape by starting from an existing one that has it (add hook to one shape), which would keep the parameters, the formula still needs to be edited manually. 

I guess the other point is about which letters are assigned to which segments. The system defines the constraints of all segments, plus some orthogonal dimensions, and uses the A, B, C parameters in order for all segments. So, to report the lengths as expected, some configuration of the family may be required (e.g., I would like a certain segment to be C and another to be F, while the ortho dimension should be E).

An alternative is the Bar Length, which is precisely computed for the rebar shape, whatever geometry it has. One option is to use that.

[semi]

I didn't know Length of each bar was british only, that explains it.

Is there some information I can find online with an overview of all bar systems used in Revit? 

Without knowing we're using the UK version while we are located in Belgium. I don't think we are using the european standards then?

[semi]

Hmm, looking at the US libraries, I do find all the same extra segment lengths and importing that shape into my project also gives me wrong total length...

The lengths are inajustable and the total length of this example should be 194+16+16=226, while the parameter says 220??

[ovidiu_paunescu · Accepted solution]

Unfortunately, I don't know such online resources.

In your example, you are adding the segment lengths and not accounting for the fillets at the corners. This is an approximate formula, which isn't very far off. 

The Bar Length is computed precisely. This is why there is this option of adding other parameters with formulas.

For example, from the same BS 8666, there are two shapes like that and the formula for 24 is more approximate, while the one for 21 is more precise (and subtracts the bending radius and 2 bar diameters)

There is also a note stating this fact.

[semi]

If I would calculate the mass of the rebars this way and I send these to the fabricator that would be an error I can't think off...

[ovidiu_paunescu]

They are taken into account, meaning you subtract something due to their presence (see formula from my previous post). Of course writing the formula like that (A+B+C) for that bar, is conservative and ensures there is enough length to create those bends. 

[semi]

Even my bending detail says its wrong:

This is the result of the same bar

[ovidiu_paunescu]

If you can, add a sample model with this exact bar and I can have a look.

[ovidiu_paunescu]

Anyway if you apply the formula, in this case you get the exact length:
160 + 1940 + 160 - 2*12 - (7*12)/2 = 2200 mm

[petrlanik]

So the calculated parameter "Length of bar" that is common for all rebar is calculated as exact length in center of rebar, while the "Length of each bar" parameter is British only and have separated formula for predefined rebar shapes only?

For the stirrups the segment length is measured on outside of the stirrup and so the "Length of bar" parameter changes logically with bar diameter (893mm for R14 and 930 for R8)

What about more dificult shapes, where the bends are on outer and inner side like this one

Can you provide some sketch, where exactly is the length of the segment measured, because in the family file, the bar shape is only represented by line.

In CZ and GE the used software for rebar usually gives outer measures as for stirrups in Revit(or let the user select how to measure the segments). It also allows to create the german style BVBS file for bending machines, is anything like that planned in development?

And I would have two more questions concerning segment length parameters in rebar shape family files.

Why as in other types of families the change of parameter value has no effect on actual shape?

And what are the rules to assign values of one segment length parameter to other parameter, for example A=B/2, in the family of stirrup I have values D=B, but the parameter D is not used so there is no problem with shape, but once I use the parameter there is a problem with the shape that dimension can not driven by parameter equation. I can assign the Length of stgart hook parameter, but no parameter from dimensions. Would I have to create separate parameters in other category to be able to assign them to segment length dimension parameters? But I suppose it would not let me read the values of segment length anyway.

[ovidiu_paunescu]

Hi @petrlanik 

The segment lengths are measured to the exterior. In the properties palette, the lengths are shown non-rounded, and in brackets, rounded according to the reinforcement rounding options.

When defining a formula for the bar length, all the segments are evaluated to the exterior, as shown in the properties palette.

Regarding how they are measured, we have the following situations:

If the angle is equal to 90 deg, then the dimension is tangent to the adjacent segment

If the angle is below 90 deg, the dimension is tangent to the fillet

If the angle is above 90 deg, the dimension is measured from the intersection of the exterior of the segments joining in that corner.

There are two dimension references (visible when hovering the cursor over the corner) that you can use to add dimensions.

For a shape with adjacent bends above 90 deg and in opposite directions, the distance is measured on the diagonal between the exteriors of the segments. If you dimension from the previous two references, the difference will be quite small.

For each rebar, the segment lengths are reported and can be used through schedules and API to extract data for any kind of format. Of course the bar geometry itself can be obtained through API as well.

Please take a look at our public roadmap for further information about our current plans https://www.autodesk.com/blogs/aec/roadmap/revit-structure-roadmap/?redirected=1

The rebar shape family doesn't flex like other families. It is used to define the shape geometry and constraints. 

The dimensions and their parameters define which segments, orthogonal dimensions or angles are controlled and reported by the parameters. 

Right now the only formula you can define is an equality formula for params, for example A = B for the opposite legs of a stirrup, to create a square stirrup. Other formulas are not supported. You can check the rebar shape status control in the ribbon, to see if the shape is correctly defined and can be loaded into the project.