Jakehall wrote:I've read through most of this and found it all very useful and interesting, but there is one thing I haven't seen addressed (and if it has I missed it), but what are the average distances you guys have been using between your two long trusses on a tension boom? I am looking for a starting point, so even if you've nothing to back up your chosen distance, it'd be a useful starting point. Thanks!
iwonder wrote:What do you mean by distances between the two long trusses? I assume you mean the members in tension(It's not technically a truss, to my knowledge)? Aia's boomilever guide(the link should be here somewhere, it's on the wiki) has a lot of good starting information, and it talks about the spacing between the tension members, if that's what you mean. I'm not sure why this would vary from boom to boom, it seems like the spacing Aia talks about is pretty much accepted as what you want...
Hmmm…, and I thought he must be referring to the compression member(s). Correct, common vocabulary would be helpful.
Don’t think anyone’s going to be able to say anything about “average” spacing (whether tension or compression members)- just what they’re using.
So, assuming the question is on the compression members, and we’re talking two compression members and two tension members….
The obvious/common distance apart is 5cm (outside edge to outside edge), so they support the load block (as in Aia’s boom guide). You
certainly don’t want them any wider apart than that. Closer together has the (weight) advantage of shorter bracing pieces between them-bracing to shorten effective column length/strengthen buckling strength in the horizontal plane-see earlier discussion on this). The limit on this is getting the tension members to clear the load block; you want the tension members to join the compression members at the centerline of the load block.
Figuring out how best to get needed clearance for tension members at the distal end takes you to how you do the wall attachment. There are two…..apparent options; running both from one bolt, or running them from two bolts.
For a reason I’ll get into in a second, if you go the two bolt direction, you want to use the 10cm spacing you get from using the center and one of the ‘side’ bolt holes,
not the two side holes (which are 20cm apart). As with virtually all design options, there are tradeoffs- pros and cons, and you want to make those tradeoffs in ways that maximize structural efficiency-least weight to get you the strength needed.
The apparent advantage of two T-members from two bolts is stability against side-to-side sway in the loading process. What are disadvantages?
First, they need to be a little longer than they would need to be if they came from a single bolt- that means a little heavier- going out to the side bolt holes (20cm apart) would mean even longer/even heavier.
Second, that additional angle to the side means the tension force goes up a bit, compared to T-members running from a single bolt; not a lot, but a bit, and that means stronger- denser, heavier wood; an additional weight increment.
Third, the angle to the vertical plane of the compression member needs to be dealt with by using an ‘attachment wedge’- a triangular piece; one side glued to the compression member, one side, at an angle to the compression member side, to which the t-member is glued. Both of those glue joints are in shear loading. On the side of the ‘attachment wedge’ that joins the compression member, the shear is parallel to the grain. On the side of the ‘attachment wedge’ that joins the T-member, the shear is at an angle to the grain- a bit ‘cross-grain.’ As seen in the article on wood properties that I posted a link to earlier, shear strength in wood is maximum when parallel to the grain- as the angle to the grain increases, shear strength falls off; the more cross-angle there is, the stronger the parallel to the grain shear strength needs to be, to provide enough shear strength at a cross-grain angle, which means higher density, which means higher weight.
These factors, taken together suggest advantages to a single bolt approach. Lengths are shorted, tension loads are lower, and attachment wedges strong enough to carry cross-grain shear loading aren’t needed. For reasons discussed before, you want the T-members to run as close to the edge of the washer as possible- so ¾” separation. Given that, that means C-member separation that gives you outsides ¾” apart, or insides ¾’ apart.
Hopefully, this gives you some food for thought- an understanding of the tradeoffs you have to play with.