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The results show a crack width of 0.491mm whether I have a moment or not. It appears to be calculating the maximum crack width when the concrete strain reaches 0.0035 at the face. I have attached a screen shot.
Crack AutoCAD Design Suite 2019 Crack
One thing I am noticing is that as I increase the moment, the section goes from uncracked to almost 0.5mm with no moment giving a crack width between 0 and 0.5mm. Do you know why this may be the case?
The section is uncracked because the tensile stress at the bottom of the section does not exceed the tensile strength of the concrete (fctm) which is specified in the concrete material properties (usually by default). Some users prefer to set this value manually to 0.0 (or a lower value) so that the section will always be cracked.
I have just built a section like yours and it appears that I have guessed the actual dimensions and properties correctly as I get exactly the same resistance moment as you show in your original post. I have created four Quasi-Permanent load cases with increasing My values (200, 250, 300, 400) and when analysing these for crack widths I get
The reason that I'm getting different crack widths to you is that I have assumed that all the load is permanent load when defining my load case and you probably haven't, If the elastic modulus is set to use an interpolated value (default) then the stress distribution calculated will probably be different.
My section is still telling me that the section is not cracked even though Axial is zero and I am applying an SLS moment which is approx 25% of the ULS capacity (i.e. it should crack). It is a 1250B x 1350D 30MPa section with 6DH25 top and bottom designed to AS5100. Moment applied is 870kNm with majority of this being permanent loading. Based on my work using other software to run the stress checks I am expecting a crack of circa 0.2mm.
As far as I was aware AS5100 does not have any specific clauses for checking the crack widths but just limits the minimum areas of tensile reinforcement and the maximum spacing. Maybe this has changed lately but I'm not aware of it. In Autodesk Structural Bridge Design we provided the calculations for crack widths using the methods derived in BS5400 (British Standards) for the AS5100 implementation, just for completeness, and these calculations are based upon the consideration of tension stiffening in the tension zone - and there is no way of switching this off. I tried to reproduce the section you have Modeled and get the same effect as you because the adjustment of the strain due to tension stiffening is greater than the strain at the tensile surface, hence it is deemed uncracked for the purpose of calculating crack widths.
The only thing I can suggest to get some crack widths is to change the design code to Eurocodes (this will create material that are equivalent to the AS5100 materials for these purposes) and you will need to define the load case again (using the "SLS Quasi-Permanent combination" in the data line). When this is analysed for crack widths, the calculations are done in a more rigorous way and as long as the minimum tensile strength is set to zero you will always get a calculated value if tensile concrete stresses are present.
Thanks for pointing this out as I don't use the AS5100 code much these days and I had forgotten this was the case. You are obviously using some documentation that gives some formulation for crack widths - possibly for the NZ transit code, which would be of interest to me to see if we can include this in the software in a later version so I would appreciate a copy of the calculations and the suitable reference.
I'm actually designing to NZ standards. My loads are governed by the New Zealand Transport Agency Bridge Manual (previous the transit manual) which is available online but the concrete design is performed to the New Zealand Standards for Concrete Design NZS3101-2006. I am finding that Structural Bridge Design (SBD) gives strength to AS5100 which is consistent with NZS3101 but it is the crack checks (and shear) that are difficult to make consistent with NZS3101. I've been experimenting because I do like the SBD moving loads and general interface and like the idea of an all in one tool.
Thanks for recommending the Eurocode outputs. I am finding that I can use this stress/strain output to do the quick NZS3101 crack checks. See attached extracts which show the NZS3101 crack check method and my calculation which uses Response2000 to find the required inputs. As you can see the crack checks are consistent when I use the strains which come from the Eurocode method.