Boomilever for 2013

[fanjiatian]

I definitely see how the tension and compression member are connected as a lapjoint. But if a vertical member was added, would it be possible to connect it as a lapjoint?

I'm stuck on how to make the bracings between the tension and compression members lap joints. If the tension members are parallel in my case, the tension members and the compression members would always be on a different plane so there'd be no way to connect the bracings as lap joints.

I think all of Aia's connections are lapjoints because her tension members narrow in towards the base.

[iwonder]

Ahh... those joints...

To connect those as lap joints you would have to cut a miter(of sorts) into the compression or tension members, which is probably not a very good idea. I think if you use those members you'd have to use butt joints... Maybe someone else could come up with a plan that doesn't involve cutting into either member(or possibly justify cutting a small notch).

Speaking of connections like that, I never understood them, I mean, I see that they exist to increase the rigidity of the compression member in that direction, but I always thought that putting a force on the tension member like that would take the member out of pure tension and allow it to fail much sooner than other members... is that true?

[_HenryHscioly_]

if the tension member end up meeting with just over 1.5cm between their outsides, and everything was straight, the "vertical members" would only need to be angled in 5degrees.
I would either sand some of the vertical member away to get the angle since it is so small of an angle, but I don't know how much that would weaken it unless you built a couple and tried.
If making the notch is no good, make a small triangular shim to fit in the angled gap.

I haven't tested any boomilevers before,
but maybe it is possible that people have tested and concluded those vertical supports may be able to reduce more weight from the compression part of the boom than they would add to the boom( their own mass + using slightly stronger tension members.)
Can anyone confirm this? but this would most likely vary and depend on the boomilever design.

[fishman100]

Ahh... those joints...

To connect those as lap joints you would have to cut a miter(of sorts) into the compression or tension members, which is probably not a very good idea. I think if you use those members you'd have to use butt joints... Maybe someone else could come up with a plan that doesn't involve cutting into either member(or possibly justify cutting a small notch).

Not always. You can glue the tension members inside the structure rather than on the outside. An example is the boom in the middle of the right-hand column (the structure directly above the one in the bottom right) in this image. I don't see how miters would be used here, especially since gluing the tension members inside automatically decreases the length of the connection base. However, doing this could cause the tension members to interfere with the loading block.

Also, I don't think using a butt joint on the tension-compression connection is a good idea.

[iwonder]

Oh, we were talking about a non-tower crane design and the added members in that... I would never think of using a butt joint at the distal end.

[SLM]

For the simple triangular design, the vertical member that connects the tension member to the compression member carries very little axial force. Well, theoretically speaking the applied load causes no axial force in that member, if the compression member is horizontal.

The (vertical) member mainly acts as a bracing. It restrains the vertical (up/down) movement of the free end of the compression member at the wall. Either the butt or the lap configuration can be used for connecting the vertical member to the compression/tension member without adverse consequences. Either type of connection, if done properly, provides adequate strength.

Here are a few possible ways to connect the members.

If the compression member is tubular:


If the compression member consists of two separated rectangular sections:


If the two compression members align vertically with the tension members:

[SLM]

...but I always thought that putting a force on the tension member like that would take the member out of pure tension and allow it to fail much sooner than other members... is that true?

True. But, you may not have any choice if you need to control buckling in the compression member using bracings. Axial force in a bracing is generally not large, compared with the force in the main compression/tension members. In most cases the force exerted on the tension member by a bracing is not large enough to cause failure, but, it is always a good idea to check it mathematically, or in our case experimentally.

[SLM]

...maybe it is possible that people have tested and concluded those vertical supports may be able to reduce more weight from the compression part of the boom than they would add to the boom( their own mass + using slightly stronger tension members.)
Can anyone confirm this? but this would most likely vary and depend on the boomilever design.

Axial force in the compression member is not going to be affected in any significant way by the presence of the vertical member at the wall. So, you will not be able to reduce the size of the compression member by placing a vertical member at the wall. However, in addition to restraining up/down movement of the compression member at the wall, the vertical member could influence rotation at the joints of the boom thereby having a significant (or measurable) effect on the deformation of the entire structure.

[_HenryHscioly_]

A vertical piece in the middle of the compression member connected to the middle of the tension member would help brace the compression member from buckling, but the force is not very big so may not affect the tension member that much? Did I interpret the posts you posted correctly?

Also, how would you place the loading block on a boomilever where the tension members are attached on the inside? I'm not really seeing how that would work.

[SLM]

A vertical piece in the middle of the compression member connected to the middle of the tension member would help brace the compression member from buckling, but the force is not very big so may not affect the tension member that much? Did I interpret the posts you posted correctly?


Also, how would you place the loading block on a boomilever where the tension members are attached on the inside? I'm not really seeing how that would work.

Yes, but not just one vertical member, more like this:


However, it may not be necessary to use bracings at alll if the compression member has adequate buckling strength on its own, if its cross-section has a large moment(s) of inertia.

If the tension members are attached on the inside of the compression member, I probably do something like this: