Crack Width Calculation (Negative Crack Width Determination)

[gab1414]

I am looking at designing a concrete structure containing aqueous liquids to BS 8007: 1987. After analysis of the crack width of my retaining basement wall I find that I have a negative crack width. I spoke to a water engineer who advised me that this simply means that there will be no cracking in the concrete. The negative value arises as the average strain at the soffit of the beam (εm) is negative since it is equal the strain at the soffit of the beam (ε1) minus the strain due to stiffening of concrete between cracks (ε2). Since ε2 is greater the design surface crack width is negative. Does anyone know of a way to overcome this issue or even know a decent explanation as to why this occurs?

 

[IDS]

It occurs because the crack width calculation method you are using is an approximation, and the approximation is not valid when the stress in the concrete is less than the cracking stress.

There are very many different methods of calculating crack widths, some of which will give "negative crack widths", and some won't. To check that your section really is uncracked (rather than an error in the calculation) I suggest you check the cracking moment of the section, taking acount of shrinkage and creep effects. If the section is exposed to the sun and long term crack widths are important you should also look at the effect of differential temperature.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/