RCDC - Beam Design with ACI 318M-14 - Crack Width Calculation

Question

Dear Bentley team, 
 I encountered an issue regarding crack width calculation in RCDC with ACI 318M-14 code, described as follows: 
 Refer to the outputs attached below. The considered beam is B5 and Zone is Top right. 
 When beam is designed with permissible crack width set as 0.25mm, then actual crack width with 1 layer of bar with 3 no. of 13 dia bars is coming out to be 0.1539mm which is within the limit. 
 Now for the same design if I restrict the permissible crack width to 0.2mm, then also it should satisfy the design (as 0.1539mm < 0.2mm). But when I reduce the limit to 0.2mm, RCDC is increasing the rebar to 2 no. of layers with 3 no. of 13 dia bars in first layer and 2 no. of 13 dia bars in second layer. 
 Changing only the permissible crack width limit should not ideally affect the actual crackwidth calculation. But it is somehow dependent on that and leads to increase in rebar. Please help in rectifying the issue. 
 Please find attached RCDC file, and two output sheets with maximum crack width set as 0.25mm and 0.2mm for your reference 
 . PDF PDF FMO_P01_01(02.12.2021)-NV-2-Beam-1-0 m.rcdx.7z

Abhijeet Gundaye · Answer

Hi we have investigate this issue. The Ast required for the Crackwidth should not change if permissible Crackwidth value is changed. following are our findings, We have observed that the number of rebar to be provided in single layer have been changed to 3 for 200mm width beam. By default value was 2. if you maintain the rebar numbers as 2 on this form it will give the same design output when the permissible Crackwidth is 0.25mm and 0.2mm. We have some internal design flow to check whether the Crackwidth is satisfied with highest diameter if it fails for the preferred rebar arrangement and diameter. For 200mm width it has considered 2 rebars for initial checking of Crackwidth with 13 diameter. 
 When user changes the rebars numbers on above form, in few cases it showing Ast required without optimization. In this case Ast required is changed as for 2 rebars and 13mm diameter initially it failed in Crackwidth if permissible limit is 0.2mm. it passes if the permissible Crackwidth is 0.25mm. When it failed in case of 0.2m, Ast has increased till it satisfied. With this Ast final detailing of the reinforcement is done. We need to improve the internal logic when user provides the more / less number of rebars for given width. We will take this as a defect in RCDC and we will try to resolve in near future.

Abhijeet Gundaye · Answer

Hi 
 it is not always 2 rebars used. The Number of rebars used are depend on the width of the beam. More the width higher number of rebars assumed. As rightly said by you, in both the cases crack-width is satisfied with provided reinforcement. In any case the final Crackwidth is checked against the final reinforcement provided in the beam which includes the number of rebars and diameters. So, if beam is passed it will always show the pass in crack-width. 
 As a software we have a internal logic check whether beam is passed for higher diameter with available rebar numbers. this is just to check that if any case if beam fails for higher diameter with maximum reinforcement (total Ast) it should be reported as failed. This is just performed to avoid multiple iterations in the design as software. after performing this, we always check the Crackwidth for the final reinforcement arrangement . if it fails we increase the Ast or diameter to satisfy the Crackwidth check. So user will never get the incorrect design. The issue here , it is not optimizing the reinforcement for available rebars from user. Higher the numbers lesser would be the crack-width. This is caused the issue as final detailing is done with rebars (lesser c/c spacing) Hope this clarifies your doubt.