Steel/Concrete composite section - Crack width calc

[Anonymous]

Is it possible to calculate concrete crack widths in a steel/concrete composite section in the section module of ASBD?

Regards

Chris

[david.geeves · Accepted solution]

Hi Chris,

There is no direct method for doing this but it should be possible indirectly by considering a general stress strain analysis on the composite section with the applied service limit state loads applied as ultimate limit state loads, which will provide you with the stresses and strains at the top and bottom of the cracked concrete..  In a second section you can consider the slab on its own and apply service limit state forces and moments to this section that will create the same strains as in the composite analysis.  With this load case applied you will be able to carry out a standard crack width calculation at the service limit state (ie not a general stress strain analysis any more.

Using ASBD 2016 will allow both sections to be analysed in the same project file (.sst file).  To determine the equivalent forces in the concrete slab (to give the correct strain plane) it is necessary to analyse the section and force the iterations to fail to converge, which may be difficult (but the analysis for combined Mz and axial capacity with a large ratio - say 1000 often does the trick)  In this form the max and min strains can be entered, together with the NA angle and the out of balance forces and moment will give the desired equivalent loads.  One thing to consider is that we are assuming that the concrete will be cracked so ensure the tensile strength specified in the material properties is set to zero.

Using ASBD 2017 allows direct access to the iterations control form so it is not necessary to get the slab analysis to fail to get the equivalent forces and moments

I have included a short video for 2016 which shows this process which can be downloaded form here

I hope this is useful

Kind regards