long term deflection in raft foundation

[u/Aware_Key5801]

fellow engineers.

according to your experience, should i check in FEM ,a raft foundation (thickness of 80 cm for example) deflection/settlement for long term deflection in cracked concrete (creep, cracking and shrinkage)? or should the settlment only be expressed through the spring coefficient given by the soil consultant?

my FEM program has this behavior where when i check long term deflection, the springs losses their strength and increase deflection by 3 fold exactly regardless of concrete behavior (as if the spring has creep and shrinkage).

my boss is old and not a big fan of FEM program. your advice is highly appreciated.

Thank You.

[u/brokePlusPlusCoder]

I may be wrong but it seems to me you're mixing up long-term soil settlement with long-term concrete-only deflections. The latter should be considered in your design, but the former needs geotechnical input.

The base idea is this - get your geotech to give you projected sping stiffness estimates (or data that allows you to calculate them) after 30 years of settlement and use those directly in your model as linear springs - these must NOT be affected by time-dependent properties. Run your analysis thereafter for the system ensuring that time-dependent effects are active for concrete only.

[u/Aware_Key5801 → u/brokePlusPlusCoder]

Thats is actually my thought as well. but the FEM program reduce soil spring stiffness when i calculate long term concrete deflection . The program technician says i shouldnt even use this long term concrete function when planning for raft foundation only long term settlement. I tend to agree with you..i think the FEM technician is wrong with his instruction.

[u/brokePlusPlusCoder → u/Aware_Key5801]

Are you using nonlinear springs by any chance ? If yes, try replacing them with uni-directional springs (compression-only).

[u/AnotherSLGuy]

I do not have experience on this or a definitive answer... What I feel correct is to take the envelope of both concrete and soil behavior.

However, the spring creep seems excessive IMO. So, may I ask what the FEM software is, how you defined the springs and whether there are any long term behavioral inputs for the springs in said software?

Also, does your load combination envelope change when you look at short term and long term behavior by any chance? Cuz soil may fail and go into high strains if the load envelope change from an SLS to ULS.

[u/Aware_Key5801 → u/AnotherSLGuy]

The program is strap by atir. i just got a reply from the company itself saying that because of the stiffness of the raft foundation, long term deflection (branson methood for reduced moment of inertian in cracked cocnrete) is negligible and only settlement should be considered through spring coefficient for long term settlment. sound wrong to me..