Connection between elements

[Alinalysis]

Hello

What type of connections are there between two structural elements, let's say for example connection between concrete column and beam.

In my concrete design course the assumption was that when analyzing a continuous beam the columns would act as pinned support, why we make this assumption ? is it just for making the analysis easier ? 

What are the other types of connection ? When using analysis software the bending moment value will be different because the columns are not action as pin support, it was said to me that the connection type is semi rigid, so what is semi rigid connection ? and how can i calculate the value of bending moment if the connection is semi rigid.

 

Thank you

[Ayesha]

On July 31, 2016 at 4:04 AM, Alinalysis said:

What type of connections are there between two structural elements, let's say for example connection between concrete column and beam.

There can be numerous kind of connections between two structural members. Generally speaking, connections are based on load transfer.

On July 31, 2016 at 4:04 AM, Alinalysis said:

In my concrete design course the assumption was that when analyzing a continuous beam the columns would act as pinned support, why we make this assumption ? is it just for making the analysis easier ?

If the connection transfers moment and shear, its a continuous connection. Assuming column as pin connection would be a case where there beam can only transfer vertical and lateral load in the column but not the moment. A real life example in concrete would be a beam that is connected to column through shear studs. The assumption made in your course is to make things easy as beam column connection are mostly moment connections.

On July 31, 2016 at 4:04 AM, Alinalysis said:

What are the other types of connection ? When using analysis software the bending moment value will be different because the columns are not action as pin support, it was said to me that the connection type is semi rigid, so what is semi rigid connection ? and how can i calculate the value of bending moment if the connection is semi rigid.

Rigid and semi rigid are ambiguous terms. The amount of moment transfered from beam into column depends upon relative stiffness of beam and column. That determines how much load gets transferred as moment, shear or displacement. 

 

 

[Alinalysis]

On 8/3/2016 at 3:15 AM, Ayesha said:

If the connection transfers moment and shear, its a continuous connection. Assuming column as pin connection would be a case where there beam can only transfer vertical and lateral load in the column but not the moment. A real life example in concrete would be a beam that is connected to column through shear studs. The assumption made in your course is to make things easy as beam column connection are mostly moment connections.

On 7/31/2016 at 1:04 PM, Alinalysis said:

So Let's say we have three storey frame, three beams per storey

 

 

In my concrete design course, I would take the first floor beams and draw my columns as pin supports like this

As you said this is just for simplifying right? So the way that it's being analysed is taking the whole frame and analyzing it, so there will be some moments acting on columns.

On 8/3/2016 at 3:15 AM, Ayesha said:

Rigid and semi rigid are ambiguous terms. The amount of moment transfered from beam into column depends upon relative stiffness of beam and column. That determines how much load gets transferred as moment, shear or displacement. 

 

And does reducing stiffness will reduce the moment transfer ? 

 

Thank you

[Ayesha]

3 hours ago, Alinalysis said:

In my concrete design course, I would take the first floor beams and draw my columns as pin supports like this

You might be doing it because the scope of problem is only to design beams.

3 hours ago, Alinalysis said:

And does reducing stiffness will reduce the moment transfer ? 

It might have some affect. You can check.

[Alinalysis]

18 hours ago, Ayesha said:

You might be doing it because the scope of problem is only to design beams.

22 hours ago, Alinalysis said:

What if I want to design a full building, in structural software by default the connection will be fixed.

 

I found this on steel construction webpage 

http://www.steelconstruction.info/design#Braced_frames

Quote

When the stability of the structure is provided by cores, or discreet vertical bracing, the beams are generally designed as simply supported. 

As far as i understood if we provide bracing to a steel structure, then we can consider the beams to be simply supported ? what about concrete structures

 

I am asking these questions because I'm confused, when I want to design a building using software, do I need to change the supports and connection of elements to pinned connection or leave them as default.

 

Thank you

[WR1]

On 8/5/2016 at 2:26 AM, Alinalysis said:

In my concrete design course, I would take the first floor beams and draw my columns as pin supports like this

 

In reality, load/moment transfer depends on relative stiffness of columns and beams.

When you model a whole building, the software takes care of relative stiffness automatically.

For manual calculations, there are various methods. For example in above image, replace the pin supports with actual columns and try solving this Moment Distribution Method by hand and compare the results.

Or make a computer example of above frame in elevation with actual columns and try reducing/increasing column sizes to see what are the effects.

[Alinalysis]

9 hours ago, Rana said:

Or make a computer example of above frame in elevation with actual columns and try reducing/increasing column sizes to see what are the effects.

Thanks, will do this and post the results

[Ayesha]

For braced frames, the beams are intentionally kept pinned as you don't need to have a moment connection for load transfer as brace takes care of it.

 

[Alinalysis]

I'm back and I've done the example 

So I created three cases

 

1) beam 20X50, columns 30x40

 

 

2) same beam size, columns 1x1 and I got results as a simply supported beam , M = 30x5^2/8 = 93.75

 

3) continuous beam with three columns 1x1 but got a an unexpected result (maybe I'm wrong?)

 

 

 

 

Edited September 3, 2016 by Alinalysis

[Ayesha]

I am not clear about ur question. Mind explaining?

[Alinalysis]

7 hours ago, Ayesha said:

I am not clear about ur question. Mind explaining?

I created a simple model to see how does the properties of column would affect on the support conditions.

 

When increasing to size of columns (increases stiffness?) the column will carry more moment.

When decreasing the size the carrying moment will decrease, until it gets to zero.

So does that mean when increasing the size it will increase the stiffness of the moment and the column will act as fixed support ? 

And when decreasing the size the column will act as hinge ? (I guess that's why I got that shape for bending moment in the third picture)

[UmarMakhzumi]

Hi Alinalysis,

Instead of labelling conditions as pinned or fixed please consider them interms of load path. Your observations reflect that for case of extremely  stiff beams (Moment of Inertia of beam >>  Moment Inertia of Column), relatively speaking, there would be no rotational restraint at end of beam and therefore no moment, which should be the case. Hope this answers your question.

How about having super big  cross-sectional columns and small cross-sectionall beam, would it affect your result? Can you guess the result without running the numbers.

Thanks.

[Alinalysis]

8 hours ago, UmarMakhzumi said:

Hi Alinalysis,

Instead of labelling conditions as pinned or fixed please consider them interms of load path. Your observations reflect that for case of extremely  stiff beams (Moment of Inertia of beam >>  Moment Inertia of Column), relatively speaking, there would be no rotational restraint at end of beam and therefore no moment, which should be the case. Hope this answers your question.

How about having super big  cross-sectional columns and small cross-sectionall beam, would it affect your result? Can you guess the result without running the numbers.

Thanks.

Hello

I would assume that the column will not allow rotation therefore there will be moment developed

[UmarMakhzumi]

Yes, that would be correct for columns stiffer than beam.

Thanks.

Edited September 5, 2016 by UmarMakhzumi

[Alinalysis]

19 hours ago, UmarMakhzumi said:

Yes, that would be correct for columns stiffer than beam.

Thanks.

Thanks you that cleared some concept for me

May I ask another question, 

Is there any difference in analysis between modeling a building in 3D using any FEM software, and modeling one frame only, or modeling as separate elements (and just considering the analysis for gravity loads)

To make it clear, we have three story building, now let's model it using a 3D software.

Ok, now if we split the building into frames, we can see beams and columns.

Now split the frame into separate elements, Continuous beams with pinned supports only.

Is there going to be difference between these three modelings with respect to analysis results which will affect the design results.

 

Thank you

[UmarMakhzumi]

Yes, you can do like that. The difference in results would not be significant if both conditions are done appropriatley.

Thanks.

[WR1]

What happens if, supports in your example are very flexible as compared to slab or beam, so it behaves as simply supported.

If the support stiffness is huge (rotational restraint) as compared to the member, it will be a continuous frame.

Yes you can do separate analyses and combined together as long as you respect the equilibrium, comparability of deformations, constitutive relations.

[Alinalysis]

17 hours ago, UmarMakhzumi said:

Yes, you can do like that. The difference in results would not be significant if both conditions are done appropriatley.

Thanks.

 

14 hours ago, Rana said:

What happens if, supports in your example are very flexible as compared to slab or beam, so it behaves as simply supported.

If the support stiffness is huge (rotational restraint) as compared to the member, it will be a continuous frame.

Yes you can do separate analyses and combined together as long as you respect the equilibrium, comparability of deformations, constitutive relations.

 

Thanks to all of you I have an idea now how to look at the support condition.

Now last question, in the three analysis models 3D, 2D, and continuous beam with pinned supports, if I made a condition for analysis in each model which could give me different design moments, will the design still be economical and safe for each type ?

 

Thank you