Angled truss

[Grom]

Hello Forum

I have searched, but I could not find... I am interested in angled trusses. How do I calculate forces on two points of a truss, when the truss is angled?



Also, there is a problem of getting the right span between points on truss in order for deadhangs to be vertical, but that is not a problem, really, as if I know left and right deadhang length, I could easily calculate needed space between points on truss.

The main question is, how to calculate the forces on point A and on point B, if truss is angled at certain degree (alpha)?
I was thinking, if I am tipping a truss over, the higher end should bear less load. But that seems too plain...
Is there some literature on this, or does anyone know how to approach the problem?

I would appreciate your input.

[rinus bakker]

Hello Grom.
First: in your picture the assumption of "the higher end should bear less load" is absolutely mistaken!
Just keep-on-going with the rotating movement of that truss, increasing that slope.
At one point the lower chain will be completely slack in stead of carrying all the load!

It is relatively simple if you have just the truss, and it's center of gravity (CoG),
Parameters are eg.:
- the distance in height between roof-attachements and truss-attachement
- the distance in height between truss-attachements and truss-CoG,
- the horizontal distance of truss-attachement-points relative to the truss-CoG.
Thus:
when you have your support point over your CoG the higher hoist(s) will be carrying more-and-more the major part of the load.
This will get (considerably) more when the CoG shifts downwards as a result of the CoG of the added load, that is attached to the bottom chords of the truss.
When 'raking' a free-suspended truss, the CoG moves toward the higher points,
and certainly NOT to the lower points!

In case of guided load lifting (e.g. wind-ups) this is exactly the opposite.....
Than the lower lifter might be carrying most weight.
I have never seen (or used) a formula with all the parameters included,
but with 30 years of experience I have found that I can pretty much do without with a few of rules of thumb (- and a couple of load-cells in assembly and rehearsal periods, whenever I might be in doubt).
Any gifted mathemacian however, is very wellcome to get all parameters in a formula that gives a theoretical models as to represent all the real life situations.

[Grom]

Hello Rinus,

When we had to carry heavy (or light) cases upstairs, I always preferred upper end, since it was easier to carry...
When we erected towers on roof (Thomas, Litec) we attached one end of tower (which is in horizontal position) to base, and than start lifting the tower from the other end. The higher we got, the easier it was to lift it, since more and more weight was on base. But that was wrong approach...

I dig your point of shorter support taking more load. now it seems logical.

But I don't understand that completely, and I got this link http://www.vieweg.de/freebook/978-3-519-35025-5_l.pdf , and it is in German, so I am still doing the translation... Apperantly (on 'first' look), the angle is of no matter, but I could say more once I understand that German text...

Thanks for reply

[laserguy]

I think indeed ONE things needs to be done FIRST BEFORE anyone can create any formula: and that is: calculate the CoG and combine (vectors) all the forces of truss weight and the weight of the devices on the truss in that point.

[Grom]

I think indeed ONE things needs to be done FIRST BEFORE anyone can create any formula.

Ola LaserGuy,

I think, first thing for me to do is to buy two spring-scales. That should do the trick, right?