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Calculations of a beam with a post below

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?

RE: Calculations of a beam with a post below

namorim,

Where is the second post?

--
JHG

RE: Calculations of a beam with a post below

The images are identical.

RE: Calculations of a beam with a post below

Your pics look the same.

RE: Calculations of a beam with a post below

(OP)
Sorry here it is

RE: Calculations of a beam with a post below

namorim,

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

When you place two "perfectly rigid" boundary conditions so close together you tend to get behavior like that. Essentially, you're resisting the cantilever moment with a force couple between those very closely spaced columns.

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

The cantilever has little to do with it. You mostly have a 2-span continuous beam, and the first span (left) is so small compared to the second span, the large uplift at the first (left) post creates a large downward reaction at the second post. Yes it is a force couple but if you remove the cantilever, I don't think the cantilever has very much effect.

RE: Calculations of a beam with a post below

i think he's added an up load very close to the LH support.

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 "cantilever" ... no cantilever here.

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

To me, the curve is moment, and is inverted compared to usual USA convention.

RE: Calculations of a beam with a post below

Also moment is not shown along the cantilever for some reason.

RE: Calculations of a beam with a post below

Quote (AELLC)


Also moment is not shown along the cantilever for some reason.

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

Take a step back from this moment diagram- what kind of structure is this where you have two posts next to each other? What material and what type of connection?

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

ok, the support on the LH end is a post ... why no curve to the left of this support ?

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

Quote (a2mfk)


Take a step back from this moment diagram- what kind of structure is this where you have two posts next to each other? What material and what type of connection?
...

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

(OP)
Thanks for all the responses, I just can't make logical sense of whats going on here. I've marked up my image to further explain the situation and some questions that were being asked. Thanks again for everyone's input.

RE: Calculations of a beam with a post below

(OP)
@drawoh Yes this is RISA, yes I know how to use the software, no I'm not a student. You can input this into any other software, like force effect and it has the same results. I have done the final calcs by hand but I'm wondering if anyone knows the logic behind it, as I can't make sense of it. Thanks you

RE: Calculations of a beam with a post below

Use compression only springs (you will need to play with the stiffness) instead of rigid supports for the two at the left side. The two rigid supports so close together are restraining uplift at the cantilever, thus the funky results.

RE: Calculations of a beam with a post below

I'm not sure why you can't make sense of it. The two supports are simply holding the small 0.5 ft. section of beam level and the beam length to the right is trying to rotate/deflect downward and the beam to the left is also doing the same. So there is a large negative moment (65.5) next to this pair of "grips" which would be similar if you changed node N3 to fixed. As you move to the left grip node (N2 I believe) the moment decreases to that of the pure cantilever - the moment just left of node N2 is independent of the rest of the span.

RE: Calculations of a beam with a post below

JAE is entirely correct.
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 a pretty simple system. Ask yourself where this 132,000# is coming from or going to.

It's essentially trying to act as a fulcrum about your new post.


RE: Calculations of a beam with a post below

Yes - JLNJ's point shows just how inefficient a short lever arm can be.

RE: Calculations of a beam with a post below

your question "the question is ..." is simply that the two point are contraining the beam from deflection, principally making the beam straight between them, when the beam would prefer to rotate (if the support was a single pinned connection). your model has two infinitely stiff supports very close togther.

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

You have to include the posts into the model!

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

I disagree that you have to model both shoring towers like the OP did, unless the one on the left is capable of taking tension. I would just combine the two as one support, or just use one tower like on the right end.

RE: Calculations of a beam with a post below

I assume that the post will take tension but I don't know why it is there. It may be that he needs it for the load. If the both posts are modeled, they might both see a compressive load despite the moments. I simply don't know enough about the structure.

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

Now that you say there will be 2 shoring towers, 2' apart, then the shoring will have to be done in a manner to insure approximately the same downward gravity load in each. An experienced builder can do that. Those RISA calculations don't represent that for obvious reasons.

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

I must be the only person who zoomed in until I could read the diagram. Next time, please, attach your printout as a separate image so we can easily read it.

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.



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