Cracking FAQ

This page is devoted to frequently asked questions (FAQ) related to Cracking.

On this page:

Why do linear and nonlinear long-term cracked-analysis results differ?

Extended Question: Since my load pattern, coefficients, and all other parameters are the same, why would I not get the same results?

Answer: The principle of superposition does not apply during nonlinear analysis, therefore results will be different between linear and nonlinear applications. Additional information on long-term cracked-section analysis is available through the Cracked-section analysis article.

Should stiffness modifiers be applied to floor slabs?

Answer: Stiffness modifiers need not be applied to floor slabs unless special conditions exist, such as when shear walls are close to one another and the shell object acts as a coupling/link beam. However, this is rarely the case in that floor slabs typically do not contribute to the lateral capacity of the system. In fact, ACI prohibits such behavior in high seismic areas because flexural contribution from floor slabs may increase lateral stiffness to an unconservative extent. Floor slabs should however behave as diaphragms to transfer in-plane forces.

Can I overwrite the resultant design slab reinforcement to reduce deflection?

Answer: Yes, various options are available for the redefinition of design reinforcement.

To begin, select Run > Cracking Analysis. Here, rebar from finite-element design is the default setting, which may be revised using either of the following methods:

Use the Quick Rebar Top/Bottom option to specify a fixed amount of reinforcement throughout the slab.
Use the Draw > Draw Slab Rebar option to model the reinforcement as required. Use the User-Specified Rebar option to analyze slab deflection.

What slab reinforcement is used in nonlinear analysis?

Answer: Cracked analysis uses rebars as defined in reinforcing source via run>cracking analysis options. One of the following three reinforcement sources may be selected:

a) User specified, user needs to add rebar in both tension and compression regions as applicable to the entire slab as program will not use design reinforcement along with user specified reinforcement.

b) FEM based, design rebars will be used as reinforcing source

c) Quick top and bottom tension reinforcing is self-explanatory.

Minimum reinforcing ratio used for cracking analysis applies regardless of the option selected for reinforcement source. The program uses the largest ratio between minimum and any of the options selected above a), b) or c).

To what demand does SAFE design the reinforcement?

Extended Question: Does SAFE design the reinforcement to match demand parameters, or does it exceed these values by a certain margin? Further, is there a way to specify reinforcement such that demand is 85-90% of capacity?

Answer: SAFE designs the reinforcement according to ultimate demands, as is common in practice. Resistance factors could be modified through Design > Design Preferences to provide for a lower demand/capacity (D/C) ratio.