Analysis Loads Cracked vs Uncracked Design

[Anonymous]

Hi, 

I have seen posts regarding this topic before but no clear explanations and I was wondering if someone could once and for all reply to the following please? This is regarding the results - cracked vs uncracked please see below:

As far as I understand it:

1) When a model is built in Robot, it will by default assume uncracked sections. Hence, forces, moments, reactions and deflections are for uncracked sections

2) When a member is designed in the design module (column, slab, wall or beams) reduced sections will be analysed and designed, more accurate deflections are accounted for and cracking reinforcement is provided. However the loads that is designs for are for 'uncracked' sections as per item 1) This is particularly observable in slab design. 

However,

I would like to see cracked section building deflections which will be greater than uncreacked sections

3) Since reduced sections have been manually assigned, forces, moments, reactions and deflection now account for 'cracked' sections as expected

4) Generally forces are lower withing walls and increased within members such as columns and this makes sense as the forces are redistributed according to the changed stiffness. And the results make sense. 

5) When i design a slab (with reduced section) - within the design module the section is once again reduced as observed looking at deflection

6) Based on item 5) is this also true for columns, walls and beams? And if so is it safe to design the buildings as such?

The last point is critical as the forces are significantly redistributed. I just want to make sure Robot is designing the members correctly. However it seems vague to me as there is no clear answer could you help? I have attached the model and example analysis output. 

@Rafal.Gaweda @Artur.Kosakowski @mustafahesenow 

[Anonymous]

@Anonymous

Some helpful resources to answer your question:

1. Cracked versus uncracked deflections: https://knowledge.autodesk.com/support/robot-structural-analysis-products/troubleshooting/caas/sfdca...

2. Rather technical explanation, but I took my time to get through it and found this to be EXTRAORDINARILY insightful, regarding how Robot gets deflection results for RC slabs: https://knowledge.autodesk.com/support/robot-structural-analysis-products/learn-explore/caas/CloudHe...

In other words, if you're intentionally modelling your slabs with modified sections to account for cracking, I believe you're doing it wrong if you are then using the RC design modules. But if you're modifying your panels to account for cracking and just checking deflections using the results module / deflection maps, then !perhaps! you're getting a reasonable answer? (see item 1.)

[jpo82]

I am also interested about deflections in design modules. How does Robot determine deflections in Eurocode based design? Which load case does robot use? All user given SLS combinations or doess Robot some how filter charsteric combinations for certain use and quasi-permanent cases for some other?

I am asking because if I understand EC2 code correctly factor shown in equation 7.19 should be calculated according characteristic combination and deflections and crack widths usually according quasi-permanent actions. Is it possible to have this kind of "cherry picking" in Robot?

[Artur.Kosakowski]

Hi @jpo82

Robot 'selects' the EC2 SLS combinations defined as QPR.

 

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