Effective Stiffness/Crack Coefficients for tilt up panels

[jdelacruz]

I am in the process of designing a number of tilt up panels and I have a question about the "correct" initial effective stiffness/crack coefficients to use for out of plane service and ultimate load combinations. ACI 10.10.4.1 states that the effective stiffness shall 0.70Ig for walls in compression, for uncracked condition and 0.35Ig for cracked. Furthermore, in the commentary, the code suggests to start with 0.70Ig and if cracking failures occur to drop down to 0.35Ig. In the office and in research I have done through reading articles and research papers, historically an effective stiffness/crack coefficient of 0.1Ig and/or 0.2Ig has been used. I am trying to decide which set of crack coefficients to initially start with. The differences in these values can produce a wide range of results. I am also wanting these values to use in spWall in their properties box that pertains to these values. I am asking what is typically for light to moderate axial loads. I am somewhat leaning towards using the code specified coefficients. Thanks for any replies.

 

[IDS]

You should certainly check for the code values.

Then consider the possible consequences of the structure having less than the nominated stiffness, and if the consequences are not acceptable then you should do a check using a lower bound stiffness value, which for a cracked section under long term load is likely to be much less than 0.35EIg.

If you don't know what the possible consequences are you will need to do the lower bound stiffness analysis anyway. I'd suggest calculating a stiffness value, allowing for creep, shrinkage and loss of tension stiffening, rather than using a standard multiplier, since the stiffness will be highly dependent on the reinforcement ratio, position, and ratio of design moment to cracking moment.

If there are large short term loads you might need to look at an upper bound stiffness value as well.

On the other hand if you are confident that lower or higher than expected stiffness will not be a problem you can just stick with the code requirements.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[spats]

What you need to be using is Section 14.8 of the ACI318 code.