Section Design Crack Width

[Anonymous]

I attempted to check a reinforced concrete section. After a few iterations, I noticed that the crack width under Quasi-permanent never changes and it considers the stress in the steel to always be 400N/mm2. I tried changing the load combinations, even removing all loads and analysing an empty load case, but the steel stress was still 400N/mm2.

Is there a way to control the stress in the steel for crack width calculations or the software only calculates the maximum crack width at section (steel) failure?

Thanks

[dave_geeves]

Hi @Anonymous 

It seems to me that you have the resistance field in the analysis form set to My,Mx or Axial rather than *Not Used*.

If you want to check the crack widths for a specific quasi permanent load case then you must set the load case to the correct one and the resistance to *not used* otherwise the program just calculates the crack width at the stress limit of the specified limit state - which will be the same no matter what load is applied.

If this is not the case then please send me your zipped up data file and I will investigate.

If this has answered your query then please mark my reply as a solution so that others can benefit.  Thanks

Kind regards

Dave

[Anonymous]

Hi Dave,

Thanks for the swift reply. When I changed the resistance to "not used" the output suggests that there will not be any cracking in the section. If so, where can I check the value of Mcr? I have uploaded the model just in case.

Thanks

Hristo

[dave_geeves · Accepted solution]

Hi Hristo,

If the most tensile stress is greater (more positive) than the tensile strength in the material properties then the section will not be cracked.

There is no direct way of determining the cracking moment but it can easily be found by manually manipulating the strains in the iteration control form.

First determine the strain that corresponds to the tensile strength:

Then open up the analysis form - setting both loadcase and resistance to *None* *Not used*.  then click on the "Control Iteration" button.

Firstly set the minimum strain to the calculated cracking strain.  Then change the max strain increment spin value to a sufficiently small value before using that spin button to change the max strain until the axial force is close enough to zero.  The resulting out of balance My moments is then the cracking moment (shown in the red box in the diagram below.

although is the process for calculating the MY moment (when the NA is horizontal) rhe NA can be set to any value (ie 90 for MZ moment) and the same process adopted.

If there is an applied axial load to consider then create a load case with the axial load and set this loadcase before entering the iterations control form.

I hope this has explained things ok and if so please mark the reply as a solution so that others may benefit. Thanks.

Kind regards

Dave 

[pedro_guerreiro]

Hi,

To make things clear: I just started to test ABD a few days ago, to see if it can be adopted to some projects in our company. 

Anyway, I tried to follow the procedure to determine the cracking moment as explained in the thread. I have a circular RC cross section, D=800mm, with 14Ø25 bars. Concrete fck=45MPa.

The strain for which the first crack will appear = -3.8/36.3 = -0.0001047

When the axial force is close to zero the balance moment is about 26kNm.

Making a rough calc I would expect a cracking moment of around 200kNm...

What I am ding "wrong"..? 

[dave_geeves]

Hi,

The problem is due to rounding in your arithmetic. The procedure only works if the section begins and stays uncracked uncracked.  The applied cracking strain  (-0.0001047) you have calculated is just over the cracking strain so the section starts and remains cracked.  If you change this to -0.0001046 you will see that the section starts uncracked, as there is a tensile stress in the concrete, and the resulting procedure then yields a cracking moments of around 207kNm

I hope this has helped to resolve your issue and if it has please mark my repky as a solution so that others may benefit.  Thanks

Kind regards

Dave Geeves

[pedro_guerreiro]

Hi,

Correct, I should have seen that from the stress diagram... Thanks!

One  other question, is it possible to get the cracking moment from the moment-curvature diagram?

The diagram does not show any sign of sudden curvature increase when 1st cracking occurs.

Thanks again. Regards!

[dave_geeves]

Hi,

No, I don't think it is possible to get the cracking moment from a moment curvature diagram as this diagram is for Ultimate Limit States only, where it is assumed that there are no tensile stresses in the concrete so the cracking moment in this case would be 0.0.  The discontinuities in the diagram show the yielding of the reinforcement where the reinforcement at different levels yield at different values of moment.

I hope this helps.

Kind Regards

Dave Geeves

[pedro_guerreiro]

Ok, thanks!

Best Regards,

Pedro