Cut length in structural beams

[Garn]

Can anyone explain how the Cut Length is derived in a structural beam? I modified a beam family with a void at each end which I could then adjust the angle of so that the ends would be plumb when I put the beam in on sloped plane. In the RFA it works great but when I place it into a project the Cut Length is about 1000mm where it really should be about 3000mm. This happens regardless of whether the beam is placed on a slope or not. Only the extrusion in a 3d view is a problem, the stick line frame views in plan and elevation are ok.

I'm also puzzled as to how Revit calculates how far back to cut the beam from its connection to a column. In the beam RFA that I started with. there were 3 ref planes at each end, one to control the stick lines, one for the connection (controlled by the length value) and one to control the extrusion length. However I can't see what constraints exist between them. They appear to have a default distance of 152mm - 6"

[Garn]

hello out there ? any suggestions anyone?

[aaronrumple]

Holiday weekend... try Tuesday. ;-)

[Garn]

happy and safe holidays everyone - hoping you''ll come back well refreshed (and hopefully maybe with an answer to my question ...?)

[lev.lipkin]

Cut length property of the beam is computed as extent of beam geometry (extrusion after voids are taken out in this case) in projection on driving curve of the beam. In this example it will be distance measured along the curve of the beam from sharp corners of the beam at each end that are further away. In other words, if you take infinite extrusion what minimal length you need to cut to be able to fit actual beam in. Hope that helps.

[Garn]

Thanks Lev,

If I understand you correctly, the cut length is calculated from the diagonal measurement from the sharp corners. I'm not an engineer so I'm sorry that the rest of your explanation went over my head. I still cannot see how to fix that beam.

The problem is, if I draw my beam on an angle between two structural columns , see attached JPG, the stick view looks ok but the extrusion length is incorrect. I've done something I gues that forces the wrong length, but I recreated this thing quite a few times over and still cannot make it behave itself. Part of my problem I suspect is not appreciating/understaining the relationship of the 3 ref planes at each end of the beam , and I've probably unlocked something that shouldn't have been, but I still can't see what...

[lev.lipkin]

Cut length is computed not between points (corners), but along 'center' curve of the beam (in projection). Imagine you need to get piece of 'stock' steel that is cut perpendicular to the beam's curve of extrusion. (Then you can continue to cut that piece to get desired shape of the beam.) The length of this stock piece is 'cut length of the beam'.

When beam is joined to column, Revit repositions reference plane (of the family) to the 'face' of the column, and if extrusion in the family is constrained to that reference plane, the shape of the beam will change accordingly. Cut length will be computed from resulting shape of extrusion.

If you do not want extrusion of the beam to change depending on joins, create different family with extrusion constrained to reference plane of the stick line.

[Garn]

"If you do not want extrusion of the beam to change depending on joins, create different family with extrusion constrained to reference plane of the stick line."

Thanks Lev - I'll give that a go.