Sign Convention of shear on a Vertical Beam.
Sign Convention of shear on a Vertical Beam.
(OP)
With respect to a truss there are vertical members, horizontal members and diagonal members.
I understand the “shear displacement” for the diagonal and horizontal members; but, can not resolve the shear displacement for a vertical member. Specifically, which end is considered the “left side” for positive shear.
Consider a vertical beam connected to Node A at the top and Node B at the bottom. If positive shear is applied to Node A with respect to Node B how does Node A displace with respect to Node B? To the right or to the left?
I understand the “shear displacement” for the diagonal and horizontal members; but, can not resolve the shear displacement for a vertical member. Specifically, which end is considered the “left side” for positive shear.
Consider a vertical beam connected to Node A at the top and Node B at the bottom. If positive shear is applied to Node A with respect to Node B how does Node A displace with respect to Node B? To the right or to the left?






RE: Sign Convention of shear on a Vertical Beam.
Positive shear applied at Node A does not indicate anything about displacement of A with respect to B. For example, a simple span beam may have positive shear at A and negative shear at B but neither support is displaced.
BA
RE: Sign Convention of shear on a Vertical Beam.
Dik
RE: Sign Convention of shear on a Vertical Beam.
Hey dik, might you have a link to the International convention? That might be worth a look-see.
RE: Sign Convention of shear on a Vertical Beam.
Dik
RE: Sign Convention of shear on a Vertical Beam.
A shear force, when applied at one end of a beam, is considered positive if it would create an imaginary clockwise moment at the opposite end of the beam.
RE: Sign Convention of shear on a Vertical Beam.
BA
RE: Sign Convention of shear on a Vertical Beam.
Dik
RE: Sign Convention of shear on a Vertical Beam.
Z axis pointing up. X axis pointing right. Y axis pointing away.
Then, for simple span beam on X axis, gravity load is negative. Reactions are positive. Bending moment is positive. Torsion is negative for gravity load.
BA
RE: Sign Convention of shear on a Vertical Beam.
If you have a simple beam I don't understand how torsion is created in the beam from gravity. Can you explain this in a little more detail?
RE: Sign Convention of shear on a Vertical Beam.
Torsion results from gravity load acting at an eccentricity. If the eccentricity is positive, (positive Y), torsion is negative since the load is negative. If the eccentricity is negative (negative Y) the torsion is positive.
That is the way I believe it should be but dik may wish to comment.
BA
RE: Sign Convention of shear on a Vertical Beam.
BA
RE: Sign Convention of shear on a Vertical Beam.
RE: Sign Convention of shear on a Vertical Beam.
If Node A is at the top and B at the bottom, it would be better to rotate counterclockwise so that A is on the left and B on the right.
BA
RE: Sign Convention of shear on a Vertical Beam.
1. How does the force of gravity become eccentric to induce torsion?
2. Additionally, how significant is gravitational torsion with respect to a beams total load capacity?
RE: Sign Convention of shear on a Vertical Beam.
Dik
RE: Sign Convention of shear on a Vertical Beam.
Can be very significant. A steel WF or I beam is weak in torsion and may fail at a load much lower than it could carry if the load was concentric. A square or round hollow section is strong in torsion, so its capacity may not be significantly affected if the eccentricity is minor.
BA
RE: Sign Convention of shear on a Vertical Beam.
Dik
RE: Sign Convention of shear on a Vertical Beam.
It has been a while since I wrote a frame program, so I'm sure to be missing a few details, but that is roughly the way I remember it.
BA