How to calulate Base shear

[Μηχανικος]

Hello everyone,

I am trying to determine the structural type of my structure in each directions X and Y by calculating the percentage of base shear resisted by walls and columns. This will allow me then to determine the behaviour factor q for ech diretion of the structure.

This is the procedure i followed.

1. Defined modal analysis parameters

2. Applied a seismic analysis using the elastic response spectrum (behaviour factor q not yet calculated) in deirections X and Y

3. Then i viewed the results from 

Tables -> Stories -> Reduced Forces -> for case Seismic EC 8 Direction X (see picture below) -> Story 2 results

As you can see from the results Fx to columns is 2420.41 kN

4. In order to double check this value i checked the Fx value on each individual column using the Results -> Diagrams -> NTM -> Fx Force (see the picture below) for case Seismic EC 8 Direction X  SRSS

When i sum up the max Fy on each column the total is 3085.77 kN which is nowhere close to results given by the Stories Table results.

So why does this occur?

Thanl you

[Μηχανικος]

In order to further check the results i decided to apply an Equivalent Lateral Force Method Analysis in order to determine the percentage of base shear resisted by walls and columns.

Here are the results 

Tables -> Stories -> Reduced Forces -> for case ELFM in Direction X (see picture below) -> Story 2 results

As you can see from the results Fx to columns is 7132.60 kN

4. In order to double check this value i checked the Fx value on each individual column as i did with the Elastic Resppnse Spectrum using the Results -> Diagrams -> NTM -> Fx Force (see the picture below) for case Seismic EC 8 Direction X

When i sum up the maximum force Fx on each column the total is 8380.86 kN which is nowhere close to results given by the Stories Table results.

Any suggestions? I would really like to hear how each one calculates their base shear so as to decide the type of structural system.

[Μηχανικος]

It seems like the exact same question was asked about 2 years ago and still hasnt been answered. see link below. I am afraid i will not get an answer either.

http://knowledge.autodesk.com/search-result/caas/discussion/t5/Robot-Structural-Analysis-General-Dis...

[dimogrec · Accepted solution]

Bar diagramms are refered to local coordinate system of bars

The reduced forces of the story table are refered to global coordinate system

So in your case FX of story table is an orizzontal force (shear) but FX on the diagramms NTM is the axial force (N).

(And FY and FZ are the shear (T) forces on bar diagramms)

So if you want to compare the results you have to compare FX of stories with FZ (or FY depending on bar coordinate system orientation) of diagrams for case EC 8 Direction X

I hope it's clear

[Μηχανικος]

Thanks a lot thats crystal clear, 

[Μηχανικος]

Hello dimogrec, I have followed your advice and now i get exactly the same resutls between the local analysis results on bars and global reduced values table.

Up to now i was able to compare the analysis results (shear force) of the columns but i cant figure out how to get the shear force results of the shear walls? I have tried the Results->Maps-> shear force but the results dont make any sense when compared to the reduced forces table (FX to walls). 

Can you please help me figure out how to get the shear force of the shear walls so that i can compare it with the reduced force table results, like i did with the columns 

[dimogrec · Accepted solution]

I suppose you are using walls with FE

In this case to compare the results you have to define core walls.

if you have a single wall somewhere in your building click on "Core walls" icon and add your wall as shown in the picture.

Do the same for any wall. Not all together in a single core 🙂

if you have a complex of walls (shape L or real core) you can decide to add all walls in a single core wall or separately as described above

After finishing defining the core walls go to Results->diagramms for buildings->Core walls  and Fy or Fx (this time the results follow the global coordinate system 😉 )

Don't you wish there was a proper user manual with some step by step examples for this software??

[Μηχανικος]

You are great thak a lot....

Who needs a user manual, weve got you 

seriously, for $6000, they need to give a manual with it.

[Artur.Kosakowski]

You may want to watch the recording of the yesterday's Robot webinar 

[Μηχανικος]

We have a webinar on walls? that sounds great. i am waiting for it to be available online

[Artur.Kosakowski]

it also inroduces AKN 

[dimogrec]

you're welcome Μηχανικος

Be patient my friend. I supose you're new in Robot, so you have to feel your self lucky because these days we have this forum where you can learn a lot of things and find answers to usual problems , many videos are already in youtube and the team (Artur,Rafal,pp2008 and others) except of supporting this forum are also making some very helpful webinars (I hope to see more).

Immagine starting using this sofware 8 years ago where there was no youtube videos no forum (so noone to share an opinion with) only some poor user manuals and some videos from the previous owner company. I can ensure you that it would be very hard and it would take much more time to learn using Robot so keep reading searching watching and sharing your opinions

Artur is there a link already about the yesterday webinar?

[Artur.Kosakowski]

Not yet, but the recording will be accessible from http://forums.autodesk.com/t5/robot-structural-analysis/robot-structural-analysis-webinars/td-p/5523...

[saclovitzky]

Where is it(Walls "webinar")?

[Μηχανικος]

You can find all webinars here

https://www.youtube.com/playlist?list=PLY-ggSrSwbZqow_60fiqJwS69mg1nQMzk