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# Substructure Analysis of Pier Cap and Column: Factor of Reduced Moment of Inertia

## Substructure Analysis of Pier Cap and Column: Factor of Reduced Moment of Inertia

(OP)
Does AASHTO specify criteria for reducing moment of inertia when calculating deflections (cracked section properties) for elastic or inelastic analysis?

I know ACI 318-14 Table 6.6.3.1.1(a) permits 0.70Ig for columns and 0.35Ig for beams but not sure about AASHTO.

Also, how do designers account for deflections due to creep for RC? Currently, an eccentric cantilever hammerhead bent is being analyzed using leapbridge concrete.

Any help is appreciated.

Thanks,

### RE: Substructure Analysis of Pier Cap and Column: Factor of Reduced Moment of Inertia

Those figures are for sway analysis of structures.

I would be calculating the actual cracked inertia and allow for tension stiffening if you want to get relatively real results for your design situation.

You need to allow for both creep and shrinkage. They affect each other so do it properly you should calculate curvatures allowing for cracking, creep and shrinkage.

### RE: Substructure Analysis of Pier Cap and Column: Factor of Reduced Moment of Inertia

(OP)
Rapt,

Thanks for responding. As suggested, I will calculate the cracked moment of inertia to calculate long term deflections. How is strain computed for both shrinkage and creep? Is there a reference you suggest looking into?

Thanks,

### RE: Substructure Analysis of Pier Cap and Column: Factor of Reduced Moment of Inertia

Time Dependent Behaviour of Concrete Structures by Gilbert and Ranzi goes into it in detail.

### RE: Substructure Analysis of Pier Cap and Column: Factor of Reduced Moment of Inertia

Another excellent resource, out of North America, is the one shown below. Many US software packages base their deflection calculations on that work.

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