Answer: Two types of cracked-section analysis are available, including:
Cracked-section analysis is run in SAFE using either of the following two methods:
Sustained portion, in which long-term cracked analysis considers the sustained loading from DEAD, SDEAD, and a portion of the LIVE load. Creep and shrinkage are included only in this sustained portion of analysis because these effects are only applicable under sustained loading.
NOTE: Short-term concrete modulus = Elastic concrete modulus Ec(to)
NOTE: Long-term concrete modulus = Age-adjusted concrete modulus Ec(t,to), given as:
For example, assume that 25% of the LIVE load is sustained. Analysis proceeds as follows:
Case 3: Long-term cracked analysis (with creep and shrinkage) for permanent load with long-term concrete modulus is given as DEAD + SDEAD + ΨLLIVE, in which ΨL = 0.25
The value of total long-term deflection is then the combination of Case 3 + (Case 1- Case 2). The difference between Case 1 and Case 2 represents the incremental deflection (without creep and shrinkage) due to non-sustained loading on a cracked structure.
The procedure indicated above results on total long term deflection over time. Most engineers simply check this values against ACI 318 Table 9.5(b), since this will always result in safe and conservative design. In order to remove portion of dead load deflection occurring before attachment of nonstructural elements, the following procedure can also be used:
Case 4= Cracked analysis for permanent load with short-term concrete modulus is given as DEAD + ΨDSDEAD, in which ΨD = percentage of super imposed dead load present before attachment of non structural elements
Or Case 4= Cracked analysis for permanent load with long-term concrete modulus creep and shrinkage is given as DEAD + ΨDSDEAD, in which ΨD = percentage of super imposed dead load present before attachment of non structural elements, and say using a creep factor for 3 months.
The value of total long term deflection to occur after attachment of nonstructural elements is then the combination of Case 3 + (Case 1- Case 2)- Case 4.
We recommend this method, though an alternative is available, described as follows:
SAFE can use user-defined reinforcement to compute cracked-slab deflection. For this option, select Run > Reinforcing Option for Cracking Analysis, then select User Specified Rebars in Reinforcement Source. Select Draw Slab Rebar from the vertical menu on the left. Note that rebar must be added in both the tension and compression regions for the entire slab since the software will only use the user-defined reinforcement, and not use the reinforcement design.
During nonlinear cracked-section analysis, SAFE estimates deflection using a moment-rotation curve as described in the reference which follows: