Crack width calculation according to eurocode

[Jummybear]

Hi,

I am a new user of ASBD and I have few questions about crack width calculations according to eurocode.

1. What is the difference between option "crack width Calcs" and "extreme face only"? I will bring an example.

- Only "crack width calcs" option selected. Max. crack width in this case 0,245 and crack location is bottom edge.

- both "crack width calcs" and "extreme faces only" selected. Max crack width is 0,319 and crack location is almost the same, bottom edge. 

Where this difference come from? 0,319 vs 0,245? Crack location is the same, why crack width is different?

2. According to help "To limit the bars used in calculation of the width, use the mouse to box around the required bars. Click once to start the box, then move the mouse and click a second time to finish the box, and the enclosed bars will be selected, and will change colour"

How and when should I use this option? I have quite heavily reinforced beam with bars in 3 layers. They are absolutely necessary for bending capacity and also for limiting cracks. Now, if I select only bottom layer of reinforcement my crack width become 0,134, looks very optimistic.

3. Last question. If I move all bars from side edges by 30mm "inside", then cracks remain widths remain almost the same 0,248 mm and 0,322mm versus 0,245mm and 0,319mm. Beam outer face is less reinforced right now and I assume that governing crack should move there but this doesn't happened, max crack location still remain to be on bottom edge... Any comments?

I went ahead and moved side bars again for next 30mm, so now they are moved totally by 60mm in comparison with initial model. The results are very strange. Crack width remain always 0,245mm!!! for "crack width calcs" only and with "extreme face only" option selected. Is it possible? Max. crack location moved to corner, which looks logical.

I will be grateful for any comments.

[Jummybear]

Anybody from support team here? 🙂

[dave_geeves]

Hi,

I am not from Autodesk support so I don't know the exact internal logic of the algorithms but I will try to explain my understanding.

To perform the calculations for each bar the program must establish the strain in the bar, the spacing to the adjacent bar and the cover from the bar to the face where the crack will occur.

The strain is relatively easily obtained from the strain compatibility analysis, but the spacing to an adjacent bar and the cover to these bars is open to interpretation, especially when several layers of tensile reinforcement are present together with bars along the side faces.  In this respect the program provides a method of restricting the number of bars to used in the calculations and also to restrict the faces of the beam that are used to determine the cover.  By default there are no restrictions so the calculations are carried out for all bars in the tension area and uses all faces to determine the appropriate cover.  The bar that produces the largest crack width together with the adjacent bar for the spacing and the location of the max crack are marked on the diagram.

By checking the "Use Extreme faces Only"  will limit the concrete faces at which crack calculations are performed to those faces intersected by a line through the reinforcing bar or tendon and perpendicular to the neutral axis.  So the side faces will not be used in the calculation of cracks.

As you have already mentioned you can select two bars to force the determination of spacing.

In this way your interpretation of the crack width parameters can be adhered to.

I hope this has helped.  It is also useful to look at the calculation "Results..." for each of your different cases to establish what parameters have changed and whether he programs interpretation is what you expected.

Kind regards

Dave Geeves