Robot Structural Analysis Forum

[#1]

Hi all,

I'm surprised that I didn't found any post regarding this issue,

can someone give me some explanation about the hight Abmin for top reinforcement for Rc beam.

I am using RSA 2015 SP5 with ACI 318M-11. I modeled the slab as material orthotorhy in order to try to simulate One way-joist slab.

the beam dimesion is 70x30 cm.  

beam.png ‏275 KB

abmin.png ‏217 KB

model.png ‏297 KB

[#2]

I assume you mean the peek of the Amin visible on the diagram. It is due to applying Amin provisions of ACI for elements under pure tension (it is in the location where bending moment is zero and tension force exists). If you consider the axial force as small enough to be neglected just exclude it from the analysis and run reinforcement design for pure bending. 

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[#3]

ACI 10.9.1

When N (axial force) > 0.1 * Ac * fcd

Asmin = 1%*Ac

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.

[#4]

Dear Arthur, 

I very appreciate your quick reply.

-can you please clarify your above statment since ACI 318-11 section 10.9.1 is the below. 

-Second, it is not resnable to get a hight axial load since it a Small span with only DL and LL forresidential buildings. Can this be caused by other factors?

[#5]

I'm sorry but I'm not sure what I shall calrify 

Ast > 0.01Ag = 1%*Ag

For the value of the axial force in the beam - it is as calculated for a model of the structure you created. If you consider it as being excessive you should check why this happens and possible modify your model before running the design of the RC Beam. 

[#6]

0.01x70x30=21 cm2 

robot gives me 17.58  cm2.

and this code is for compression members. 

can this huge axial load is due to wind load?(although its not significant?)

[#7]

Could you attach the model please?

[#8]

kindly note that I tried to remove the SW underneath the beam and change the conection type between column and beam but I Still got the same issue 

Block C BEAMS.rtd ‏2620 KB

[#9]

Please disregard the first model,

there was a roof slab plane issue that i fixed,

please note the attached one

Block C BEAMS.rtd ‏3856 KB

[#10]

Having your model (thank you for atatching it ) it was possible to determine what drives Amin for thsi beam. You should look at ACI 10.6.4 provisions. 

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.

[#11]

Thank you for your reply,

thought,clause 10.6.4 does not verify the results showing.

Is there any case that we still have the issue raised in the thread blow ?

https://forums.autodesk.com/t5/robot-structural-analysis/abmin-how-rsa-calculte-this-value/td-p/4475...

[#12]

From the thread you refered to:

Thanks for solve this in the last service pack for RSA 2015. 

RSA 2015 SP5

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.

[#13]

I had the same issue and when I updated my version to SP5 it was solved for bottom steel reinforcement,

is there any chance that the issue it is still existed for this case of top reinforcement and We didn't notice it since we rarely face it (i.e. a beam linearly on top of a wall).

it doesn't make sence to have this huge area of of top reinforcement for a beam supported linearly on top of a rc wall.

please bear with me, 

kindly find attached simplified model that shows what I mean about Ab min for top steel, can u clarify why robot gives me this results? 

beam top Ab min.rtd ‏1204 KB

beam test.png ‏174 KB

[#14 · Accepted solution]

I assume you mean the peek of the Amin visible on the diagram. It is due to applying Amin provisions of ACI for elements under pure tension (it is in the location where bending moment is zero and tension force exists). If you consider the axial force as small enough to be neglected just exclude it from the analysis and run reinforcement design for pure bending. 

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.

[#15]

Dear artur ,can you just clarify for me where accordin to what provision of ACI 318 robot consider this value of As min?