Boomilever B/C

[Cougars]

Our team started to look at mounting blocks more closely... Someone correct me if my observation is incorrect. The so called center line of the J hook is about one cm higher than the horizontal line that the J hook is mounted thru the back board. The 15cm (C div) horizontal contact depth line in now 16 cm below the block mount center line. Anyone else see this? Not that this is a problem, just something to consider...
http://www.soinc.org/sites/default/file ... lation.pdf

I think you are incorrect. 3b says "measured below the center of the hole(s) for the hook", not "of" the hook. So drill the hole in the support wall and measure down 15cm, no greater then that I think.

[baker]

Our team started to look at mounting blocks more closely... Someone correct me if my observation is incorrect. The so called center line of the J hook is about one cm higher than the horizontal line that the J hook is mounted thru the back board. The 15cm (C div) horizontal contact depth line in now 16 cm below the block mount center line. Anyone else see this? Not that this is a problem, just something to consider...
http://www.soinc.org/sites/default/file ... lation.pdf

I think you are incorrect. 3b says "measured below the center of the hole(s) for the hook", not "of" the hook. So drill the hole in the support wall and measure down 15cm, no greater then that I think.

The J hook, hooks up. Look at the picture in the link. The radius of the J hook is above the upper line even though the threaded screw part of the hook goes thru the center line of the line. When you start to make your mounting block you'll see what I mean... Either way, just an observation..

[Balsa Man]

baker is correct.

I just tested my boomilever. It broke at the center of the compression members at 6.2 kg. The buckling strength of each boxbeam compression member is about 1kg and I braced them every 8 cm. if the force is 31 kg, then each compression member only needs to carry 10.5 kg. I really don't understand why my 1 kg buckling strength compression member with 6X bracing (6^6=36) failed that early.

Which way was it braced? And how did it fail? I'm guessing it was braced horizontally and it failed by bending up or down vertically? Did you have any kind of tension-compression brace?

That's a good guess, I think, iwonder. When jma says the buckling strength is 1k, that makes me think symmetrical cross section- square; same buckling strength in vertical plane (let's call it the x-plane), and the horizontal (let's call it the y-plane). Bracing between only improves things in the y-plane. There was lots of discussion on this last year; worth reviewing.

[Balsa Man]

The effect on forces is actually a bit larger (with the centerline of the T=member going thru the center of the....J-hook circle. See my post, previous page. The centerline intersects the wall at about 16.4cm...

If that's how high the centerline sits in regards to the wall, how far from the wall is it? I know that the hook itself is 2.5 cm from the wall before the actual hook part, for lack of a better word; however, the length will have to include part of the actual hook part and it will most likely not sit at exactly the middle part of the hook meaning the distance won't be the maximum length of the hook from the wall, if I'm not mistaken.

Sorry for the double post- I tried to copy tsom's into last message, but it didn't work.
The easiest way to work out your basic geometry is careful drawing to scale. Get a 1/4" j-hook, lay it out so the inside end of the hook is 2.5 cm from the wall. That's the key locational info in the rule. Then trace the outline. You can also google the specification number, and get exact dimensions

[baker]

The effect on forces is actually a bit larger (with the centerline of the T=member going thru the center of the....J-hook circle. See my post, previous page. The centerline intersects the wall at about 16.4cm...

If that's how high the centerline sits in regards to the wall, how far from the wall is it? I know that the hook itself is 2.5 cm from the wall before the actual hook part, for lack of a better word; however, the length will have to include part of the actual hook part and it will most likely not sit at exactly the middle part of the hook meaning the distance won't be the maximum length of the hook from the wall, if I'm not mistaken.

Sorry for the double post- I tried to copy tsom's into last message, but it didn't work.
The easiest way to work out your basic geometry is careful drawing to scale. Get a 1/4" j-hook, lay it out so the inside end of the hook is 2.5 cm from the wall. That's the key locational info in the rule. Then trace the outline. You can also google the specification number, and get exact dimensions

We bought two, one for the test table and the other for layout on our paper for the geometry, just like you mention.. Less than a buck for each

[jma]

I just tested my boomilever. It broke at the center of the compression members at 6.2 kg. The buckling strength of each boxbeam compression member is about 1kg and I braced them every 8 cm. if the force is 31 kg, then each compression member only needs to carry 10.5 kg. I really don't understand why my 1 kg buckling strength compression member with 6X bracing (6^6=36) failed that early.

Which way was it braced? And how did it fail? I'm guessing it was braced horizontally and it failed by bending up or down vertically? Did you have any kind of tension-compression brace?

There were no tension-compression braces in my boomilever. The braces were between 2 compressions members. I didn't see any bending (watched back the recording several times); the compression members just snapped in half.

[iwonder]

That's fine, sometimes the failures are instant. So when you brace a member, the bracing, for the most part, only helps in the direction is was braced. Since your braces were between the two compression members, the side to side buckling strength of the member was very large, but the up and down buckling strength wasn't helped by the bracing (I'm talking about how it fails, if it snaps in a plain parallel to the floor (side to side) or perpendicular to that (up and down)). Try either increasing the vertical height of your member (rectangular cross section) or bracing to the tension members. Keep in mind that some designs don't accommodate tension-compression bracing, since the tension members aren't always in line with the compression member.

[UQOnyx]

Is there a way to calculate how many lateral bracings are needed?

[jma]

That's fine, sometimes the failures are instant. So when you brace a member, the bracing, for the most part, only helps in the direction is was braced. Since your braces were between the two compression members, the side to side buckling strength of the member was very large, but the up and down buckling strength wasn't helped by the bracing (I'm talking about how it fails, if it snaps in a plain parallel to the floor (side to side) or perpendicular to that (up and down)). Try either increasing the vertical height of your member (rectangular cross section) or bracing to the tension members. Keep in mind that some designs don't accommodate tension-compression bracing, since the tension members aren't always in line with the compression member.

Thanks. Btw, I'm not sure if I understand "tension members & compression members aren't in line" correctly. If the compression members are 5.2 cm apart and the tension members are 5.2 cm apart at the tension-compression connection & 2 cm at the base, does that mean the compression & tension members aren't in line?

[iwonder]

Yup, exactly what I meant.