Reduction in core wall stiffness to account for cracking

[rnuttall4QL4R]

Hello,

I am having issues with Robot accurately analysing the additional deflection that will occur when the concrete core walls on my 30-storey tower crack. The core walls are modelled as 450thk throughout, slabs modelled but with no columns as a means of distributing lateral wind loads applied to the slab edges to the core walls, pinned linear supports at base of walls. 

I have tried manually reducing the moment of inertia within the "Walls" settings to 0.5xIg. This does not effect the deflections of the core at all, even when reducing to 0.1xIg. I have manually changed the E value of the concrete within the Job Preferences, changing 50% cracked to E=15GPa. This increases the deflections.

Please can you review the above and let me know the correct way of carrying out this lateral deflection analysis.

Thanks,

Richard 

[Simau]

@rnuttall4QL4R 

To my knowledge, the reduction of the modulus by half only concerns seismic cases, see  Eurocode_8  4.3.1(7)
The horizontal displacement downwind is calculated at SLS combination with no change in the modulus

[rnuttall4QL4R]

Thanks for this and noted on your points. 

We carried out further checks and the reduction in the modulus of inertia factor only affects the stiffness of the element for in-plane bending, which is useful for horizontal elements (slabs) but does not change the stiffness of vertical elements. 

To model the reduction in stiffness of core walls through cracking effects, the E value of the concrete should be reduced within the Job Preferences. 50% cracked is a reasonable value for SLS checks, which gives a corresponding E value of 15GPa.

[Simau]

@rnuttall4QL4R 

If your goal is to design slabs keeping into account cracking, you may use the reinforcement type. No need to reduce the modulus value.