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1) Can you show me how Robot is using the Wood & Armer forces (Mxd*, Myd* etc.) to calculate required reinforcement? I am trying to verify this by hand using ACI-318 design code. I am using shell elements in panels and designing for bending + compression/tension.
I'm not sure if I understand you correctly. Basically the idea is to design reinforcement for combined axial load and bending moment for axial force in the direction of reinforcement plus bending of value equal to Mxx + Mxy
2) Specifically I would like to know how Robot uses the compression/tension forces (Nxr*, Nyr*, Nxs*, Nys*) when designing for bending + compression/tension. Would a tension force be divided between top and bottom reinforcement regardless of the bending moment?
Reinforcement will be design based on equilibrium of forces in the cross section.
3) When designing the top reinforcement does Robot assume there is zero reinforcement in the bottom face?
No, as the design of reinforcement is based on the equilibrium of forces in the cross section the compression reinforcement is included when exists.
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Artur Kosakowski
My understanding is that you try to compare results of your hand calculations with the area of reinforcement obtained in Robot for given value of bending moment and axial force (which are determined by w and A method). I'm afraid that with my limited knowledge of ACI I'll not be able to give you step by step description but I may try to find out what causes the difference if you send me the model (and indicate which center of surface element or node as well as which load case to focus on) and your hand calculations.
Artur Kosakowski
First of all I'd like to focus your attention to the fact that for the W and A method the in-plane forces as assumed as NXX and NYY on the direction of reinforcement rather than Nxx(WandA) and NYY(WandA). This is not a factor for this example but may be important for others.
As far as I can see you just calculated separately area of reinforcement for plain bending and tension/compression and then you add up the obtained values. This is not the same as the approach adopted in Robot which is to assume top and bottom reinforcement and verify the equilibrium of internal forces in the cross section. What it means is that the top reinforcement can be smaller as this part of the cross section is under compression due to bending and the bottom one can be smaller as there is the influence of the existing top reinforcement.
I think that the better approach for checking if Robot results are correct would be to manually verify if the areas calculated by Robot are sufficient enough to withstand combined action of axial force and bending. I hope this helps.
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.
Artur Kosakowski
Robot determines the equilibrium of forces in a cross section and calculates top and bottom reinfiorcement from the equations as below:
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.
Artur Kosakowski
Edward G. Nawy - "Reinforced concrete. A fundamental Approach"
Artur Kosakowski
I'm not sure if I'm able to give you step by step procedure as it is iterative soluition but try the following approach:
1. Define reinforcement as calculated by Robot for Ax(-) and Ax(+)
2. Verify if it can withstand Nxx + Mxx (W&A)
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.
Artur Kosakowski
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