When performing an analysis of a concrete frame

[GalileoG]

Sorry for this very amateurish question, my experience has been almost entirely with structural steel but have recently started getting exposed to reinforced concrete design.

My question is the following: How can I account for cracking that would take place in my reinforced concrete members during the analysis process? I imagine I would have to reduce the gross moment of inertia by some code determined factor.

CSA A23.3-04 CLAUSE 10.14.1.2 has a table that states 0.35Ig for beams shall be taken and 0.70Ig for columns shall be taken. Are these the value to be used for a first-order linear elastic analysis?

Is my understanding correct?

Thank you,

Clansman

 

[JAE]

Clansman,

First off - I am in the US so I don't know about the CSA code but ACI 318 has the similar 0.35Ig and 0.70Ig values as you state.

In ACI 318, you have two choices:

1. Second Order Analysis - You can model the structure taking into account material non-linearity, cracking, lateral drift, member curvature, shrinkage, creep, and foundation interaction.

All this is basically saying you have to offer your first born to the structural gods to get it done right. We've modeled concrete frames using a second order analysis and calculating effective moments of inertial (Ie) of columns and beam - or going conservative and using cracked I values.

In this option, we would included intermittent nodes up the length of the columns (say at 1 ft spacing) to included P delta effects of the column curvature (P[δ]) as well as the lateral drift P[Δ] effects.

2. Magnified Moments - This is using a first order analysis with a model using 0.35Ig and 0.70Ig in the model and then calculating moment magnification factors ([δ]) to apply to the moments to mimic the second order effects that you could get with Option 1 above.