Location of pin support on beam
Location of pin support on beam
(OP)
Hello everyone,
I'm looking at the design of beam that is pinned-pinned beam with a uniform load distribution.
The beam is built-up from a thin web with two extrusions fastened to the upper and lower edges of the web to form the beam caps.
The beam will be assembled to the rest of the structure by two pins.
My question is regarding the location of the two pins relative to the neutral axis. If the pins are located on the neutral axis, then they would just see the vertical reaction load. But if the pins were located in the upper chord of the beam, would they not see additional horizontal shear components equal to M/h (h = Distance betwene beam chords)? What if the location of the pins were such that they were above the upper beam chord?
Hope the problem description is clear enough. I look forward to hearing the opinions on this design detail.
I'm looking at the design of beam that is pinned-pinned beam with a uniform load distribution.
The beam is built-up from a thin web with two extrusions fastened to the upper and lower edges of the web to form the beam caps.
The beam will be assembled to the rest of the structure by two pins.
My question is regarding the location of the two pins relative to the neutral axis. If the pins are located on the neutral axis, then they would just see the vertical reaction load. But if the pins were located in the upper chord of the beam, would they not see additional horizontal shear components equal to M/h (h = Distance betwene beam chords)? What if the location of the pins were such that they were above the upper beam chord?
Hope the problem description is clear enough. I look forward to hearing the opinions on this design detail.






RE: Location of pin support on beam
RE: Location of pin support on beam
Thanks civilperson, that is what I was looking to confirm.
RE: Location of pin support on beam
pinning the ends of the beam makes it as close to ideally pinned as you're going to get, abit surprised you're doing this instead of clipping the web of your beam onto the frame web where i assume you are. pinning on the neutral axis is "ideal" but not IMHO truly Necessary.
RE: Location of pin support on beam
RE: Location of pin support on beam
The construction of the beam is irrelavent at this point, because I'm working out the loads on the pin so that I can properly size it.
The poor sketch shows three beams. The first beam has the pinned supports at the NA. The second has a the pinned supports along the upper edge of the beam, and third has the pinned supports far above the beam.
Now assume there is a distrubuted load, causing the beam to bend (up or down doesn't matter). In case 2 and 3, I would recon that there is a significant axial force (horizontal force) incurred because motion of the outer most point is restricted (usually they would have some relative motion if the pins were on the NA).
RE: Location of pin support on beam
I agree with civilperson. From a theoretical standpoint, the pins located above the neutral axis will see some tension due to the rotation of the ends of the beams about the neutral axis.
Whether or not it will see it in practice is a different matter. Probably not, as the oversize of the holes to allow for fit-up would likely allow the minor movement to occur at the top of the beam without affecting the pins.
RE: Location of pin support on beam
as to the axial force in the beam, i think if the beam is pinned on the neutal axis, then it's "properly" pinned. if the supports are off the neutral axis then a horizontal load will develop (opposite at both ends) due to the rotation of the end face of the beam about the neutral axis. however, i think this'll amount to roughly 0.5 of a gnat.
a question back to noor, how is the UDL being developed ? we're questioning minor (IMHO) details about the end restraint but i wonder if the beam is attached to the fuselage ? (which would obviously change everything)
RE: Location of pin support on beam
RE: Location of pin support on beam
Miecz, you're correct about the rollers, but in this case the pinned supports are rigidly constrained (not shown in the diagram), and the pins are close tollerance. You're right that in this configuration the pins would see additional horizontal loads even if they are on the NA. Like you say, the FBD can be drawn such that there are vertical members from the NA to the pins, and with both vertical and horizontal reaction loads at both pins. The problem is then statically indeterminate.
Thanks to everyone who contributed to this post, and helped me sort out the real issues with this problem.
RE: Location of pin support on beam