Need some help with dimentional and geometric constraints in dynamic block please

[kostastomp]

Hello everyone,

I am building a dynamic block which represents a complex shape stirrup reinforcement, comprising a diamond shape inner stirrup and a rectangular stirrup around it in a rectangular concrete section.

In reality the two stirrups are one piece, made from a continuous reinforcement bar formed into shape, hence the two hooks. In AutoCAD, I have broken the polylines in smaller segments in order to achieve a realistic visual effect where stirrup legs overlap each other.

I have set the top,left,right,bottom concrete covers and the width and height for the cross sections with dynamic parameters. Everything seems to stretch as I want, but the diagonal legs of the diamnd stirrup do not remain tangential to the rounded corners of that stirrup as I would like. I understand that the solution is to add some dimentional and/or geometric constrains, which I have tried in all possible ways I could think of, but I am not getting the desired result.  Just for clarification: the bend diameter of the rounded corners should remain constand after stretching the block (which it does at the current state of the block. it's only the tangent connection that doesn't work).

Any help, or advice would be greatly appreciated.

Many thanks for your time.

[chrismunt]

Hello,

This has been resolved in a previous post. You are on the right tracks with using constraints. I would suggest you read this post, download the resolved block and review it/reverse engineer it to suit your needs.

https://forums.autodesk.com/t5/dynamic-blocks-forum/dynamic-stretch-and-maintaining-circle-radius-fi...

You may also find this 1 minute tutorial on how to apply constraints useful too:

https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/How-to-create-an-a...

[j.palmeL29YX · Accepted solution]

Attached a first attempt how that can be done using parametric constraints. 

You can change the width (B1) and the height (H1) as well as the wire diameter (dw), the bend radius and other dimensions if needed.

[kostastomp]

Thank you so much! I wouldn't have done it without your help!

Some key take aways:
1) Even all the object snap types are enabled, dimensional constraints do not snap to all types. For instance, in case of arcs, they will only snap to start, mid and end. If you want other snapping points (such as arc centre) you have to force it with Shift+right click when inserting the dimensional constraint.
2) The "Coincident" geometric constraint has two modes. Either point and point (for instance two end points), or object and point. The latter is quite usefull.
3) The "Equal" constraint, when applied to arcs, it equalises the arc radius, not the length of the arc.

I attach my final version of the block in case anywone wants to explore it.

[kostastomp]

hi @j.palmeL29YX ,

Could you please explain the purpose of this coincident point shown below?

Did you apply it as object (the horizontal line) and point (the arc center) or somehow else?

And most importantly, what is its purpose?

I just applied it blindly, copying from your block, but I would like to know what I am doing.

Thanks

[j.palmeL29YX]

See what happens if you delete this coincident constraint: 

BTW: AutoCAD tries to avoid an "overconstraining", but in some rare situations it can be that you will find reduntant constraints (if you did not work careful enough  😉 ). It is not "nice", but in most cases they will not cause any problems. 
But - in the situation above the coincident constraint is necessary (or you have to add another constraint to fix the geometry in the correct horizontal position).

[kostastomp]

Thanks a lot, I think I understand it now.