Robot SA software RC beam design

[leeo123]

HI,
I'm fairly new to robot structural analysis and I am trying to design a RC beam. I am having problems with the service load combinations...if I set all my combinations to strength there is no problem but when I allocate a combination "service" I get ridiculous answers.
For example a 7m beam 600x400 deep it would give 5N16s bottom with all cases at "strength" and about 20mm defl. If one combinations was changed to "service" it would give 8N40 bottom to satisfy required bottom steel component.
I know with SA software if you put in rubbish you get out rubbish. Any ideas on what I'm doing wrong

 

[rapt]

Do the check by hand and see what the answer should be!

And check with Robot Support, they should know their software!

And have you checked how deflections are calculated, regarding cracking and long term efefcts? The 20mm might be meaningless in reality!

 

[leeo123]

Thanks rapt,
As I'm learning the software I'm always checking the results. I'll try the robot support and see how I go.

 

[nicam]

Hi leeo123,

So I assume you use 1.2DL + 1.5LL for ultimate and
2.4DL + 1.26LL for service.

if so you don't get the software to check strength when your using a service combination. only check deflection.

So the ultimate combo for checking strength i.e designing the bars and use the service combo for checking deflection only.

BTW I have not used this software

 

[handex]

Having never used the software before, how does ROBOT calculate deflections? Is it just elastic deflections or does it try to allow for cracking and shrinkage/creep?

 

[leeo123]

thanks guys...The problem is solved
Handex....The software checks for cracking and increases the amount of steel to control cracking for serviceability states. This option can be turned off. This seems to have been the reason why I was getting large amounts of reinforcement as the base crack limit was initially set too high.

nicam...Robot differeniates between ultimate and service loads/combinations so when you assign it to either strength or service it will only use it for either capacity or cracking/deflection etc.

once again thanks for your help