topic Re: Unsatisfactory Load Capacity of the Column in Robot Structural Analysis Forum

Unsatisfactory Load Capacity of the Column

Μηχανικος — Mon, 26 Jun 2017 15:18:03 GMT

I am designing a 2-storey RC Frame house and most of the columns get the error "Unsatisfactory Load Capacity of the Column" when designing the reinforcement.

one of the columns is 600x250mm, after the error I tried to increase the size of the column (within the provided reinforcement module) to 600x600mm and again i get the same error. ( i did not recalculate the analysis)

i even increased the column to 800x800mm and again i get the same error.

I also tried 900x900mm and it worked, well i have never seen such a column in a house or even a 5 storey RC frame so something is certainly wrong here.

can someone help me?

below is the design that actually worked..... excessive you say???

The column highlighted is the one being designed no.5

Diagrams below are shown for both the column being designed and the column above

Loadcase: Code combinations

This is the shear force on the column on the Z direction FZ

This is the shear force on the column on the Y direction FY

This is the axial force FX

This is the Bending Moment MY

This is the Bending Moment MZ

Re: Unsatisfactory Load Capacity of the Column

Μηχανικος — Mon, 26 Jun 2017 20:05:09 GMT

I tried increasing the dimensions of the column only on one side to 900x250mm and now it works.

i will try the same with all columns and see the end result.

Re: Unsatisfactory Load Capacity of the Column

Artur.Kosakowski — Tue, 27 Jun 2017 07:53:28 GMT

Hi @Μηχανικος

My understanding is that you expect the column to be of a smaller size. If so, from my experience usually the issue is not with the RC Design but with the values of internal forces you get for a column. Are they as you expect them to be or larger? Perhaps you should look at the value of the base shear for seismic analysis and compare it to the static force procedure (assuming you defined the response spectra one) or change the supports from fixed to pinned.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

Re: Unsatisfactory Load Capacity of the Column

Μηχανικος — Tue, 27 Jun 2017 08:07:14 GMT

Hi,

Instead of changing the support from fixed to pinned dont you think its better to just decrease the stiffness of the fixed support, lets say for example to 80% of the current stiffness.

But you are right though, in real life the supports even in RC frames are never fully fixed.

Re: Unsatisfactory Load Capacity of the Column

Μηχανικος — Tue, 27 Jun 2017 08:29:27 GMT

Hi,

I will check the base shear of the design response spectrum against the equivalent static analysis and let you know

Instead of changing the support from fixed to pinned dont you think its better to just decrease the stiffness of the fixed support, lets say for example to 80% of the current stiffness.

But you are right though, in real life the supports even in RC frames are never fully fixed.

Re: Unsatisfactory Load Capacity of the Column

Artur.Kosakowski — Tue, 27 Jun 2017 09:58:05 GMT

Hi,

Instead of changing the support from fixed to pinned dont you think its better to just decrease the stiffness of the fixed support, lets say for example to 80% of the current stiffness.

This is going to be your decision

Re: Unsatisfactory Load Capacity of the Column

Μηχανικος — Tue, 27 Jun 2017 10:00:12 GMT

Yes ofcourse, i will need to do some research on that. maybe other experienced professionals can share their ideas.

Re: Unsatisfactory Load Capacity of the Column

Μηχανικος — Tue, 27 Jun 2017 11:37:49 GMT

I just had a thought, eurocode8 suggests reducing the stiffness of the elements by 50% for seismic analysis due to cracking.

Which i did, Dont you think that is reasonable to assume that the stiffness of the supports also reduces during an earthquake due to cracking therefore it should be considered in the model?

Re: Unsatisfactory Load Capacity of the Column

Μηχανικος — Tue, 27 Jun 2017 11:46:24 GMT

I just had a thought, eurocode8 suggests reducing the stiffness of the elements by 50% for seismic analysis due to cracking.
Which i did, Dont you think that is reasonable to assume that the stiffness of the supports also reduces during an
earthquake due to cracking therefore it should be considered in the model?

Re: Unsatisfactory Load Capacity of the Column

Artur.Kosakowski — Tue, 27 Jun 2017 13:11:12 GMT

Hi @Μηχανικος

As Poland is not in the seismic zone I have limited experience in this area. I hope somebody else will give you some good advice.

Re: Unsatisfactory Load Capacity of the Column

Μηχανικος — Sat, 15 Sep 2018 15:22:29 GMT

Hi, its been a long time but i never found the answer to this question, my writing is in red

@Artur.Kosakowski wrote:
Hi @Μηχανικος

1. My understanding is that you expect the column to be of a smaller size.
1. Yes I expect the column to be of normal size. Obviously there is an issue if such a large section is required.

2. If so, from my experience usually the issue is not with the RC Design but with the values of internal forces you get for a column. Are they as you expect them to be or larger?
2. So if the problem is about the internal forces, why do i get this issue? why there is an issue with the internal forces and what is usually the solution?

3.Perhaps you should look at the value of the base shear for seismic analysis and compare it to the static force procedure (assuming you defined the response spectra one) or change the supports from fixed to pinned.
3. Why did you ask me to compare the base shear of the Seismic (Equivalent Lateral Force Method) with the Seismic (Design response spectrum)

Re: Unsatisfactory Load Capacity of the Column

Artur.Kosakowski — Mon, 17 Sep 2018 08:38:30 GMT

Hi @Μηχανικος

2. If so, from my experience usually the issue is not with the RC Design but with the values of internal forces you get for a column. Are they as you expect them to be or larger?

2. So if the problem is about the internal forces, why do i get this issue? why there is an issue with the internal forces and what is usually the solution?

The results you obtained are for a model as defined. If e.g. it is created in such a way it behaves differently than expected (e.g. missing releases or supports; ignored instability, wrong seismic analysis parameters etc. ) then the column size and its reinforcement will also be different than expected. As you should know the estimated values of forces in the column the question are the obtained values in this range?

3.Perhaps you should look at the value of the base shear for seismic analysis and compare it to the static force procedure (assuming you defined the response spectra one) or change the supports from fixed to pinned.

3. Why did you ask me to compare the base shear of the Seismic (Equivalent Lateral Force Method) with the Seismic (Design response spectrum)

To check if the results of the seismic analysis are in the expected range.

Re: Unsatisfactory Load Capacity of the Column

Μηχανικος — Mon, 17 Sep 2018 09:16:39 GMT

2. "The results you obtained are for a model as defined. If e.g. it is created in such a way it behaves differently than expected (e.g. missing releases or supports; ignored instability, wrong seismic analysis parameters etc. ) then the column size and its reinforcement will also be different than expected. As you should know the estimated values of forces in the column the question are the obtained values in this range?"

2. The results of the external forces are reasonable, however the results of the internal forces given in the RC Design Module -> Provided Reinforcement -> Column Results Tab are excessive.

could this be due to the layout and behaviour of the structure during an earthquake?

3. "To check if the results of the seismic analysis are in the expected range."

3. Yes the results of the seismic analysis are in the expected range. Below are the checkings i make to ensure that the analysis and input loads are correct.

1. Selfweight:

First i check the total selfweight of the frame (given by robot in Stories Table -> Values -> for the selfweight Load Case) against hand calculations.

Checking 1 is OK.

2. Dynamic Mass:

Then i check the dynamic mass (given by robot in Stories Table -> Values -> for the Modal Load Case) against hand calculations.

Checking 2 is OK.

3. Seismic Base Shear:

Then I check the seismic base shear acting on the columns & walls (given in robot in Stories Table -> Reduced forces -> for the Seismic Load Cases in X & Y directions) against hand calculations.

I also check the shear force given by Robot in Results -> Diagrams for bars

Checking 3 is OK.

Re: Unsatisfactory Load Capacity of the Column

Artur.Kosakowski — Mon, 17 Sep 2018 09:37:29 GMT

Hi @Μηχανικος

So my understanding is that the forces in the column are correct (as expected). If so, could you attach your hand calculations for the indicated column from Robot model (size and reinforcement) for this set of forces co that we could try to look for differences?