TR.20.10 Member Property Reduction Factors

Concrete design specifications recommend the use of cracked section properties for the analysis and design of concrete sections. The methodology to handle cracked section properties is nonlinear 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, you can specify a set of reduction factors to be applied on the calculated section properties such as area, moments of inertia, and torsional constant. If you want to adopt this approach to account for cracking of concrete sections, refer to Section 10.11.1 of ACI 318 for a set of values to use for these reduction factors depending upon the nature of forces and moments the member is subjected to.

Note: The specifications in the AISC 13th edition manual suggest reducing the stiffness of the steel member during the analysis. In place of the MEMBER CRACKED command, the REDUCEDEI parameter may be used when the PERFORM DIRECT ANALYSIS command is used. See TR.37.5 Direct Analysis for additional information.

General Format

The format of the command is:

MEMBER CRACKED (CODE IS1893 2016)

{ member-list | group-list } REDUCTION *{ RAX f1 | RIX f2 | RIY f3 | RIZ f4 }

Each reduction factor value, f1 through f4, should be a fraction of unity.

Parameter	Description
RAX f1	Reduction factor in the axial area. `RAX` is not applicable for reduction factors for the IS1893 2016 code.
RIX f2	Reduction factor of the torsion constant (about the local x-axis).
RIY f3	Reduction factor in the moment of inertia about the local major axis (y-axis).
RIZ f4	Reduction factor in the moment of inertia about the local minor axis (z-axis).

This is a multiplication factor on the property value. It does not signify the amount by which the property is reduced, but, it is simply a value by which the unreduced property is multiplied. Thus, the calculated (or the user-specified value) of the property will be multiplied by the reduction factor to arrive at the value used in the analysis.

For example, a factor of 0.45 defined for RAX will mean that if the cross sectional area of the gross section is 0.8 ft², the value used in the analysis will be 0.8 × 0.45 = 0.36 ft².

Multiple factors can be assigned on the same line.

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

Code-Specific Reduction Factors

IS1893 2016 Clause 6.4.3 calls for a reduction of moment of inertia values by 0.35 for beams and 0.7 for columns of concrete members only while analyzing the structure for static seismic and response spectrum/linear dynamic analysis.

Note: Automated stiffness reduction requires a STAAD.Pro Advanced license.

Code-specific reduction factors will not affect non-concrete members. A separate stiffness matrix is generated for the use of IS1893 2016 static seismic and response spectra loads. For all other load cases, the analysis is performed using the unreduced stiffness matrix.

Tip: For a STAAD.Pro model containing multiple analysis commands, a different set of code-specific reduction factors may be used for each analysis command.

The CHANGE command has no affect or code-specific reduction factors.

Example

MEMBER CRACKED
1 REDUCTION RAX 0.35 RIX 0.40 RIY 0.45 RIZ 0.45

IS1893 2016 Example

START GROUP DEFINITION
MEMBER
_BEAM 1 4
_COLUMN 2 3 5 6
END GROUP DEFINITION
…
MEMBER CRACKED CODE IS1893 2016
_COLUMN REDUCTION RIY 0.700 RIZ 0.700
_BEAM REDUCTION RIY 0.350 RIZ 0.350