Bending Moment in bar
Bending Moment in bar
(OP)
Am I right in thinking the BM in the bar shown is the resultant load R multiplied by the perpendicular distance to the opposite pin?. I suppose it's similar to a spreader bar but the few calcs I've seen for spreader bars haven't taken bending into account and I'm sure there must be some.





RE: Bending Moment in bar
=====================================
(2B)+(2B)' ?
RE: Bending Moment in bar
Moment in the bar is zero, the wires act as ties and the centre bar is acting as a srut.
desertfox
RE: Bending Moment in bar
Ted
RE: Bending Moment in bar
RE: Bending Moment in bar
P horizontal wire = P/2
P diagonal wire > P/2 = P/2/sin(theta)
(theta measured to the vertical)
Pspreader = (P/2/sin(theta))*cos(theta)
RE: Bending Moment in bar
RE: Bending Moment in bar
The bar reacts the vertical component of the applied force, so the compression in the bar will be:
P/2 x sin O
Where O is the angle in degrees. Beacuse both angles are the same (45 degrees) then both values should be the same.
If you imagine taking the bar away, you will hopefully be able to picture the horizontal wires wanting to come together - this is caused by the forces which are putting the bar into compression.
RE: Bending Moment in bar
I will try and wipe it from my memory banks.
RE: Bending Moment in bar
In the case you have above the load in the horizontal wire P/2 is different to the load in the 45 degree wire P/(2 cos 45).
RE: Bending Moment in bar
RE: Bending Moment in bar
RE: Bending Moment in bar
for me the diagonal wire is balanced by compresison in the spreader and tension in the hoizontal wire ... the two react components of the diagonal load. The spreader has one wire for the load, and another for the support.
if it is a continuous wire, and there's a roller at the ends, then i think your picture holds. i think there's a physical problem keeping the spreader in place, it's perfectly neutrally balanced ...
try this.
imagine you have two diagonal wires holding the load, both in tension. now you come along with a prop and push it into the wire, bending it. the prop supplies the force R to the wire, balanced by the tenisons in the wire ... the tensions are the same (as the wire is continuous). rotate the prop away from the resultant and yes there's bending in it, but there's also a problem with it slipping along the wire.
RE: Bending Moment in bar
Until it buckles.
RE: Bending Moment in bar
Ted
RE: Bending Moment in bar
but, assume the load is supported by two wires, so that the wires have constant tension.
now take a prop, which has a roller (or a nicely radiused end) and bend the wire. The tension in the wire is still constant, the prop bisects the angle between the deflected wire portions (as shown in the pic above).
now assume you've done this to both wires. it seems to me that you could have a bowed spreader, the spreader would take the shape of an arc. it is clearly a very "ropey" sort of structure, the ends of the spreader are just begging to slip out, it is a severely bent column ('cause it'll be in compression), but there would be bending at the mid-point.
RE: Bending Moment in bar
Until it buckles.
RE: Bending Moment in bar
RE: Bending Moment in bar
RE: Bending Moment in bar
Mick
Roll on Saturday