Existing Slab Footing FAIL when design using STAAD Pro

[engrAlan]

Can anybody help me what i missed?

I modeled an existing mat foundation of heat exchanger using plate element of STAAD Pro.
Slab Thickness is 1100mm reinforced with 28mm dia spaced at 200 center-to-center.
The provided steel is only 3.078sq.mm/mm with Moment cap = 1,131 kn-m/m (existing configuration).

When i modeled this foundation, STAAD computed the Maximum moment to be 1,798 kN-m/m with steel requirement of 5.10 sq.mm/mm which would then create problem because the existing structure fail, and this has been in the field for long time with no problem.

Our goal is to investigate the existing structure to accommodate additional load for a new equipment replacement which is 3.5% heavier than the original weight.

Attached is my STAAD model for reference.

Thank you.

 

[stpra]

Prior to attempting a large modelling of whole structure, have you verified your STAAD results from parallel "back of envelope" calculations.
I am sure that will guide you towards the results which you are looking for.
You have taken a value of subgrade modulus of 24 MPa/m. whether this parameter was same as originally assumed by engineer who has designed this mat?

STPRA

 

[engrAlan]

We cant find the previous set of calculations. However, I tried checking the slab capacity considering it as a doubly reinforced beam of 1m strip, it gave me a utilization ratio of 65%, is it acceptable to do it this way? as to subgrade modulus, we verified it to be the same as the previos one because we have the previos design basis but no calculation specific to the mat.

I think the previous calculation was done manually and no finite element analysis. anyway, thanks for your input STPRA.