Calculations of a beam with a post below
Calculations of a beam with a post below
(OP)
If anyone could shed some light on what is happening in thses calcs below it would be greatly appreciated.
I have a load on a beam that is pinned at one end (the right) and a post Im setting at 3' from the left side. The values turn out as expected: (see image below)

When I add a 2nd post below the numbers drastically increase and one side reverses:

Does anyone know the theory behind this and/or why this happens?
I have a load on a beam that is pinned at one end (the right) and a post Im setting at 3' from the left side. The values turn out as expected: (see image below)

When I add a 2nd post below the numbers drastically increase and one side reverses:

Does anyone know the theory behind this and/or why this happens?






RE: Calculations of a beam with a post below
Where is the second post?
--
JHG
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
What is your diagram showing? Is that deflection, or stress? If it is bending stress, it completely makes sense to me.
--
JHG
RE: Calculations of a beam with a post below
The reality of the structure probably won't be so severe. If you replace the vertical stiffness of each boundary condition with a spring based on the AE/L of the post, I think the results will start making a little more sense. Though you will get something in-between the two results you have shown.
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
why doesn't the curve (showing displacement ?) continue past the LH support ?
as a sanity check, look at the two loads separately and superimpose (to get the combined loading shown).
calc by hand (so you don't rely on canned s/ware) ??
Quando Omni Flunkus Moritati
RE: Calculations of a beam with a post below
title says "post" is that meant to mean something restraining the beam from displacing ? if that is what the 141.4 force is meant to be doing; well, it isn't ... if it was there'd be zero displacement at the force (not the large +ve displacement shown)
Quando Omni Flunkus Moritati
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
The resolution of the diagram is poor. There seems to be some sort of blocky thing at the left end. Maybe it is doing something.
--
JHG
RE: Calculations of a beam with a post below
The structure when its built may not know it was supposed to do what your moment diagram was showing. If this was wood for example, the outer left side post would see no load at all as the beam deflected it would rotate up and off the outer left post. If it is all pinned connection steel, then you will get a large uplift load at the outer left post.
RE: Calculations of a beam with a post below
when you add another post nearby, the question to me is can the original post react a tension load ? (or is the beam just sitting on top of the post ? maybe this is the cantilever referred to ? (ie the couple between the two LH supports) ??
what is the curve showing ? the first pic could be either moment or displacement, the 2nd looks like neither ? there could be zero deflection at the LH support, but +ve deflection at the added post ? there could be zero moment at the LH support, but +ve moment at the added post ?
the two LH reactions are clearly working as a couple, the total of the three reactions is the same as the original. But the 2nd (added) one seems to function differently to the first, based on the curve results.
Quando Omni Flunkus Moritati
RE: Calculations of a beam with a post below
This does not appear to be a practical structure. It looks like output from some sort of analysis software. It is too neat to have been done by hand. Had they done it by hand, the problem probably would have been obvious to them.
This could be a student post. It could be someone playing with an unfamiliar piece of software.
--
JHG
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
The two closely spaced pin supports are acting together to form a fixed support. Not realistic with shoring towers, but that is why models have to be understood, not just accepted.
RE: Calculations of a beam with a post below
It's essentially trying to act as a fulcrum about your new post.
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
now the real world is going to say the the deflection of the (simply supported) beam over this very short span is likely to be very small and the real world is less than infinitely stiff, and the supports will deflect under load (one stretching, one complressing) so this large couple won't appear in the real structure.
if you can, use finite stiffness to represent the supports, something like AE/L.
Quando Omni Flunkus Moritati
RE: Calculations of a beam with a post below
I think the OP didn't ask for the moments on the cantilever in the printout.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Calculations of a beam with a post below
RE: Calculations of a beam with a post below
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Calculations of a beam with a post below
What you have done in your calculation is input a model where the beam has zero load, and pinned the 2 shoring towers at the left end of the beam, able to resist downward and upward forces without any vertical displacement whatsoever, and then essentially applied all the beam load including its own self-weight.
RE: Calculations of a beam with a post below
This is a steel joist?! Well, then you definitely do not want a negative moment. And at least you are looking at the moment diagram and red flags are going up.
If for whatever reason there needs to be two shoring posts, I would use a transfer beam between the two and have one bearing point for your joist. And by design, the joist needs to be shored/supported at the seat unless you modify the joist.