12-06-2010, 01:07 PM

There are two types of cracked section analysis i.e., Immediate Cracked Deflection and Long-Term Cracked deflection accounting the creep and shrinkage effects.

In SAFE v12, cracked section analysis can be performed by using two different methods which are described as follows:

Method-1:

Apply a single load pattern in load case and start another case continues From State at End of Nonlinear Case.... For example, adding SELFWEIGHT, SIDL and LIVE load case for performing cracked section analysis creates the following three cases:

• Add SELFWEIGHT Load case using Nonlinear (Cracked) started from Zero Initial Condition.

Where SELFWEIGHT: Presents the own weight of structure itself.

• Add SIDL Load case using Nonlinear (Cracked) started from using From State at End of Nonlinear Case "SELFWEIGHT".

Where SIDL: Presents the additional dead load due to finishing … etc.

• Add LIVE Load case using Nonlinear (Cracked) started from using From State at End of Nonlinear Case "SIDL".

Where LIVE: presents the additional live load.

SELFWEIGHT load case predicts the cracking from zero initial condition when no load was present and computes the cracking due to application of SELFWEIGHT load pattern. Adding SIDL in other case staring From State at End of Nonlinear Case "SELFWEIGHT" uses the stiffness at the end of SELFWEIGHT load case and computes the additional deflection due to SIDL case. The reported deflection shows the total deflection due to SELFWEIGHT plus SIDL case. However, this method is unable to recognize the increase in SELFWEIGHT load deflection due to increase in cracking when SIDL load is added. Therefore this method is not recommended.

Method-2:

The recommended method by CSI SAFE 12's developers for computing cracked section analysis is to apply all load patterns in a single load case and use the Crack analysis or Cracked Long-Term Analysis Option as discussed below:

Immediate Cracked Deflection:

Apply all loads (i.e. SELFWEIGHT + SIDL + Live) in a single load and use the Crack Analysis option.

Long-Term Cracked Deflection:

The creep and shrinkage effects are only applicable for sustained type loading i.e., SELFWEIGHT, SIDL and a portion of LIVE load (in case of Warehouse, Stadium etc.) if applicable. The analysis is divided into two category cases i.e. a cracked section analysis which determines the incremental deflection due to non-sustain portion of LIVE load and the long-term cracked analysis which includes the sustained type of loading including creep and shrinkage effects.

A 25% Live load is assumed to be of sustained type in the example shown below:

Case 1: Short term load with short term concrete modulus (SELFWEIGHT + SIDL + Ψs LIVE) where Ψs = 1.0 (i.e. Crack analysis)

Case 2: Permanent load with short term concrete modulus (SELFWEIGHT + SIDL + ΨLLIVE) where ΨL = 0.25 (i.e. Crack analysis). Use ΨL = 0 if 100 percent of LIVE load is non-sustained type of load.

Case 3: Permanent load with long-term concrete modulus plus creep and shrinkage (SELFWEIGHT + SIDL + ΨLLIVE) where ΨL = 0.25 (i.e. Long-Term Cracked analysis with creep and shrinkage)

The long term deflection is the combination of Case 3 + (Case 1- Case 2)

The difference due to Case 1 and Case 2 represent the incremental deflection due to non-sustained loading without accounting creep and shrinkage for fully cracked structure.

Conclusion:

For Nonlinear Cracked Section Analysis for both Short & Long Term Conditions, It seems that SAFE 12 Doesn't store the tangent stiffness / deformation at the end of Nonlinear Case, Thus CSI SAFE 12's developers always recommend to use method-2 as mentioned above.

Hussein Rida.