Connection load transfer

[Parrapit]

Hi Guys

I have a question and seems I get a lot of uncertain answers from other engineers not on this forum.

So if I could ask please try not to be vague.

If you design a beam member as pinned supported at both the ends and you use a fixed connection and not just a normal pinned would that moment be transferred to the underlying support or will the beam resist the moment as it was meant to do and the connection is mere a formality to connect the beam so it does not hang in the air.

I do understand you will have a little bit of a moment on the support at the bottom as the support is not always loaded eccentrically right in the middle.

 

[rowingengineer]

If the connections are truly fixed then the moments will distribute to both the beam and connections.

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[dimvoly]

You'll get some moment transferred, it depends on what kind of connection. As an example, if you have a cleat that is on the side of an I beam, that has three bolts, all the rotation is transferred into the I beam. However the I beam can't handle torsion well so you'll idealize it as a pin support.

If you for example use a fixed support like a moment end plate going into a column, then that is a different situation. This is definitely a fixed support and should be analysed as such.

I hope this was some help, your problem sounded a bit more complicated than I might have interpreted.

 

[Parrapit]

Let me put it this way if you could design a beam that is simply supported and you over design it massively and you could get a beam so stiff that the deflection in the beam is 0. Now you design a connection that is a fixed connection, let say the beam is a massive I section and the span is like 2m and it carries maybe 10 KN/m.

The way I see it the beam should be able to resist that moment without transferring it into the fixed connection.

But that is the question does it transfer the load anyway into the connection and then into the support although there is almost no movement and the beam would be able to resist the moment without the need of a fixed connection.

 

[johnbridge231]

I would add the following:
The connections and members should not only be design against imposed load (for example momenets, shear forces etc), but against the deformations that are calculated by the analysis.
For example, a pinned connection means that a rotation will take place at the end of the pinned member. Will the connection be adequate to safely carry that rotation? Furthermore, connections are classified to nominally pinned, semi-rigid and rigid. So, during the connection design, one must also confirm that the assumed member dof-releases in the analysis are valid.


Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[samnac]

Rotation/deflection is related to rigidity. The more rigid the member is the less rotation it allows. I think a shear connection is likely to be modeled for the case of neglegeable deflections.

 

[JAE]

Why are you asking this question?

There is no such thing as an infinitely rigid connection or beam.

And in any case, the stiffness of the beam has no affect on the end moment of a fixed end support.
It is always wL^2/8 at the end - the term doesn't include EI in the derivation of the end moment.

If you get a very short beam, with very high depth, then deep beam theory would kick in and the analysis would be different - but with that kind of beam, a "fixed end" wouldn't probably make sense either.

Again - why the question? What is this for?

 

[Parrapit]

All I want to know is in all cases, long beams with loading that deflect more or short beams that does not deflect at all or almost nothing, will the moment always transfer to the connection if it has a fixed connection and then transfer to the support whatever it may be a post or a column etc.

Or

if the beam is stiff enough (not stiffness as in EI) but strong enough that there won’t be any deflection movement in the beam, so that it won’t transfer the moment to the fixed connection cause the beam handles it like a simply supported beam which has no moment on the edges only it is fixed.

So basically like I said if you design a beam as simply supported but now on site you make a fixed connection to a column that was not designed to handle the fixed moment will it transfer that moment?

 

[Parrapit]

If you put it in terms of numbers.

You have your simply supported beam, max bending moment would be in the middle, of let’s say 30KNm now you designed your beam according to that and your deflections is like 1mm.

So if you make the connection a fixed end connections you would normally now have a moment on the connections and the supports on either side but the design would yield a much smaller beam that only needs to handle for arguments sake 10KNm and on the fixed connections the other 10KNm on either side of the supports which gives you your total of 30KNm.

What if the beam can handle the 30KNm moment with ease will the moment still be transferred to the connections and ultimately to the supports or will the load be transferred to the fixed connection as if it was a simply support?

 

[JAE]

Yes, per my post above, the end connection would still take moment, no matter what size of beam you use.

 

[Parrapit]

Thanks Guys

 

[RFreund]

Will the connection transfer moment? - Yes as long as the connection is 'stiff enough'.
How much moment? This depends ratio of column stiffness to beam stiffness. "stiffness attracts force" If you have a massive beam framing into a tooth pick the beam will take the majority of the moment (remember the moment distribution method, among others?). So if the beam is 'infinitely rigid' compared to the column (not sure how to design that, but...) I suppose the only moment transferred would be the eccentric shear at the end of the beam.

Also while I have a feeling your referring to concrete, take a look at "Type 2 Connections" or "Wind Only Moment Connections" or "Flexible Moment Connections"(all for steel structures).


EIT

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