AD.2007-1001.1.9 Section Property Reduction in Analysis to Account for Cracking

Concrete design specifications recommend the use of cracked section properties for the analysis and design of concrete sections. Though the methodology to handle cracked section properties is non-linear in nature, i.e. the section capacities should be checked and modified depending upon the section forces the section is handling. The model should then be re-analyzed with modified reduced section properties and redesigned. This iteration should be continued until the forces in all sections designed are below the allowable limit of ultimate strength.

In STAAD.Pro this approach has been simplified as per the recommendations in the ACI-318: 2005 standard which suggests a user input of reduction factors for the individual members. Section 10.11.1 of ACI-318 has provided a list of suggested reduction factors of section properties dependent upon the nature of stresses the member is subjected to.

An additional sheet has been added to the Beam Specifications dialog to allow the reduction of section properties for analysis to be created and assigned. The method is identical to that for creating any other beam specification.

The Specifications dialog can be accessed from the General | Specifications Page in the Modeling Mode:

The reduction factor should be a fraction of unity; hence a factor of 0.5 defined for RAX will reduce a defined cross sectional area of 0.5 ft² to 0.25 ft².

The format of the command that is generated in the STAAD.Pro STD file is:

MEMBER CRACKED
<Member List> REDUCTION { RAX | RIX | RIY | RIZ} factor

Multiple factors can be assigned on the same line

Note: The reduction factor is considered only for analysis but not for design.