# Thread: Sketch Up - checking a design destined to fail?

1. ## Sketch Up - checking a design destined to fail?

Here is a 1st phase of a concept for a 2 person bench - part of a set of 4 to sit around a big round table.

Sure looks to me like it will fall over. Is there a way to test this in SketchUp?

Extra info: The outside width of the bench is 46" X 17 3/4" tall x 15" deep. The overhang of the top relative to the line drawn at the base is 7 3/4".

I'm certain I need another leg or that I need to reposition those as drawn.

It sure would be fun and useful to know how to check a design for real world stability
. Any ideas?

Screen shot 2013-01-15 at 10.00.14 AM.jpg Screen shot 2013-01-15 at 10.00.42 AM.jpg

2. Originally Posted by Sam Murdoch
Here is a 1st phase of a concept for a 2 person bench - part of a set of 4 to sit around a big round table.

Sure looks to me like it will fall over. Is there a way to test this in SketchUp?

Extra info: The outside width of the bench is 46" X 17 3/4" tall x 15" deep. The overhang of the top relative to the line drawn at the base is 7 3/4".

I'm certain I need another leg or that I need to reposition those as drawn.

It sure would be fun and useful to know how to check a design for real world stability
. Any ideas?

Screen shot 2013-01-15 at 10.00.14 AM.jpg Screen shot 2013-01-15 at 10.00.42 AM.jpg
Well, you need to find the center of mass of the top. Someone has probably written a plugin to do that. If the center of mass is outside of the legs, it will fall over. Even if it doesn't fall over, it looks iffy enough that you need to consider what happens if you put any kind of weight on the front. Again, it's a center of mass problem. It's really not difficult to calculate this at all. If looking for a plugin, look for:

-center of mass
-center of gravity
-centroid (good enough because you have a simple shape with constant density)

They will all tell you what you want to know.
Last edited by John Coloccia; 01-15-2013 at 10:38 AM.

3. Sam, there is indeed a plugin to calculate the CofG. I'll get it to you ASAP.

4. Looking at it, I would agree, it needs a leg under that convex apron.

5. You don't need software to see that will tip in a heartbeat... drawing a line from one leg to the other, you can see it is basically inline with the center of mass. That leaves no stability.

6. SketchyPhysics is a plug in that will likely illustrate the obvious: the bench will be unstable if somebody sits on the center of the arc.

7. If you use SketchyPhysics, however, be aware that it screws with classes and can make other plugins fail to operate correctly. It is one of a handful of known problematic plugins which is why I didn't suggest it.

8. Well as you all know this is a 1st phase drawing. It has been modified through today's efforts, with a center leg and now in the context of the whole (3 more benches with the round table) I am making it more shallow, more curved and longer. The 5th leg looks great by the way. I have the plugin that Dave sent and will put it to the test, though as you all realized - we don't need a plugin to tell us this won't work. The plugin will tell me how easily it will fall over though and that's worth knowing just because...

9. Looking at the pictures I'm not sure we can say with any certainty where the center of mass. The structure and the weight of individual components does play a role (light top, heavy bottom). Regardless, this piece would rather unsafe, unless attached to the floor.

10. Originally Posted by Darius Ferlas
Looking at the pictures I'm not sure we can say with any certainty where the center of mass. The structure and the weight of individual components does play a role (light top, heavy bottom).
True, but some basic assumptions can be made... even if the bench legs are made of heavy steel and the top of balsa wood, a typical human would easily make it wobble like a weeble. Sometimes engineering design isn't about the hard calculations, it's that nagging thought in the back of your mind that says "This just looks dangerous"

11. Originally Posted by Sam Murdoch
Well as you all know this is a 1st phase drawing. It has been modified through today's efforts, with a center leg and now in the context of the whole (3 more benches with the round table) I am making it more shallow, more curved and longer. The 5th leg looks great by the way. I have the plugin that Dave sent and will put it to the test, though as you all realized - we don't need a plugin to tell us this won't work. The plugin will tell me how easily it will fall over though and that's worth knowing just because...
I don't see your latest design but going back to the original post, I believe you can get a stable chair with 4 legs. Just move the back 2 legs around the curve some which moves them closer together but also out beyond the center of mass.
Last edited by Brian Tymchak; 01-16-2013 at 11:20 AM. Reason: clarification about which design I was referring to

12. Here is the 5 leg version - Won't tip under load. I agree with Brian that I could just move the back legs in towards the curve and that might be stable enough but I like this 5 leg thing . Of course the client will have the final word.

Screen shot 2013-01-16 at 5.21.55 PM.jpg Screen shot 2013-01-16 at 5.22.32 PM.jpg Screen shot 2013-01-16 at 5.22.44 PM.jpg

13. Sam, it looks good. Nice images, too.

14. Member
Join Date
Apr 2009
Location
Lancaster, PA
Posts
273
The potential problem with moving the back legs is that part of the seat will now overhang the legs and the bench could then tip if someone were to sit on the overhanging portion. The variables here matter, of course. Bench weight, overhang distance and weight of the subject sitting on the overhang. Given the size of the bench and needing to design for a person at least 300 pounds (probably more) a small overhang could be disastrous.

I think the 5 leg option looks right. The only way I see around 5 legs is to drastically straighten out the curve, which may or may not be possible given the size of the table and the number of benches.

Originally Posted by Sam Murdoch
Here is the 5 leg version - Won't tip under load. I agree with Brian that I could just move the back legs in towards the curve and that might be stable enough but I like this 5 leg thing . Of course the client will have the final word.

15. Contributor
Join Date
Apr 2008
Posts
1,374
If you wrap a string around feet, no portion of the seat should protrude past this footprint by an inch or two.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•