Form: Live Load Reduction Factor
Click the Design menu > Live Load Reduction Factors command to access the Live Load Reduction Factor form. Use the form to specify live load reduction preferences. Note that for live load to be reduced, it must be defined as a reducible type live load.
Important Note: ETABS applies live load reduction to frame objects (and links) and wall-type shell objects only. It does not apply live load reduction to floor-type shell objects.
Note: Live load reduction can be overwritten on an element-by-element basis in each of the design postprocessors (see the Design menu > {Steel Frame, Concrete Frame, Composite Beam, Composite Column, Steel Joist, Shear Wall} Design > View/Revise Overwrites commands) .
The Live Load Reduction Factor form has the following areas:
Live Load Reduction Method options. Choose the formula or method, if any, to be used for live load reduction:
No Live Load Reduction option. When this option is selected, no live load reduction is performed even if static load cases have been defined as reducible live load type load cases.
Note: ETABS calculates the tributary area for a frame, shell or link element from the floor-type shell objects that load the element. If no floor-type shell object loads a particular element, the tributary area for that element is calculated as zero by ETABS.
Tributary Area (Based on Design Code) option.
ASCE7-95 option. When this option is selected, the influence area live load reduction method based on Section 4.8.1 of the ASCE 7-95 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
AI |
= |
Influence area for the element, ft2. The influence area for a column is taken as four times the tributary area. The influence area for a beam, brace or wall is taken as two times the tributary area. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
ASCE7-05 option - When this option is selected, the influence area live load reduction method based on Section 4.8.1 of the ASCE 7-05 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
AI |
= |
Influence area for the element, ft2. The influence area for a column is taken as four times the tributary area. The influence area for a beam, brace or wall is taken as two times the tributary area. AI must be greater than 400 ft2. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
ASCE7-10 option - When this option is selected, the influence area live load reduction method based on Section 4.7.21 of the ASCE 7-10 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
AI |
= |
Influence area for the element, ft2. The influence area for a column is taken as four times the tributary area. The influence area for a beam, brace or wall is taken as two times the tributary area. AI must be greater than 400 ft2. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
AS/NZS 1170.1-2002 option - When this option is selected, the live load reduction method based on Section 3.4.2 of the AS/NZS 1170.1:2002 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
A |
= |
Sum of all area supported by the element, m2. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
Chinese GB 50009-2001 option When this option is selected, the live load reduction method based on Section 4.1.2 of the Chinese GB 50009-2001 Standard is used. The basic formula is as follows:
The total stories beyond the design element |
LLRF |
1 |
1.0 |
2 to 3 |
0.85 |
4 to 5 |
0.70 |
6 to 8 |
0.65 |
9 to 20 |
0.60 |
Over 20 |
0.55 |
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
The RLLF factor is limited to Minimum Factor values (see bullet below).-
Eurocode 1991:2002 option -When this option is selected, the influence area live load reduction method based on Section 6.3.1.2(11) of the Eurocode 1991:2002 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
n |
= |
Number of stories (> 2) above the loaded structural element. |
|
= |
0.7 in accordance with EN 1990, Annex A1, Table A1.1. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
Indian IS 875-1987 option - When this option is selected, the live load reduction method based on Section 3.2.1 of the IS:875(part2) - 1987 Standard is used. The basic formula is as follows:
Number of Floors (including Roof) |
LLRF |
1 |
1.0 |
2 |
0.9 |
3 |
0.8 |
4 |
0.7 |
5 to 10 |
0.6 |
Over 10 |
0.5 |
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
Hong Kong COP 2001 option. When this option is selected, the live load reduction method based on Section 3.7.3.1 of the Hong Kong COP 2011 Standard is used. The basic formula is as follows:
Number of Floors (including Roof) |
LLRF |
1 |
1.0 |
2 |
0.95 |
3 |
0.90 |
4 |
0.85 |
5 |
0.80 |
6 |
0.75 |
7 |
0.70 |
8 |
0.65 |
Over 8 |
0.60 |
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
KBC 2009 option - When this option is selected, the influence area live load reduction method based on Section 0303.4.1 of the KBC 2009 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
A |
= |
Influence area for the element, m2. The influence area for a column is taken as four times the tributary area. The influence area for a beam, brace, or wall is taken as two times the tributary area. A must be greater than 36 m2. |
The RRLF factor is limited to Minimum Factor values (see bullet below).
NBCC95 option - When this option is selected, the live load reduction method based on Section 4.1.6.9(3) of the NBCC 1995 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
B |
= |
Sum of all area supported by the element, m2. B must be greater than 20 m2. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
NBCC2005 option - When this option is selected, the live load reduction method based on Section 4.1.5.9(3) of the NBCC 2005 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
B |
= |
Sum of all area supported by the element, m2. B must be greater than 20 m2. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
NBCC2010 option - When this option is selected, the live load reduction method based on Section 4.1.5.8(3) of the NBCC 2010 Standard is used. The basic formula is as follows:
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
B |
= |
Sum of all area supported by the element, m2. B must be greater than 20 m2. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
UBC-97 option - When this option is selected, the live load reduction method used is similar to that described in Section 1607.5 of the 1997 UBC. The basic formula used is:
RLLF = 1 - 0.01r (A - Amin)
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
r |
= |
Rate of live load reduction, 1/length2. The default value is 0.08 in 1/ft2 units. |
A |
= |
Tributary area for the element or reaction, length2. If A does not exceed Amin then no live load reduction is used. |
Amin |
= |
User specified minimum tributary area for the element or reaction, length2. The default for this item is 150 ft2. |
The RLLF factor is limited to Minimum Factor values (see bullet below).
User Parameters (per Section 1607.5, UBC 1997) - When this option is selected, the tributary area live load reduction method based on Section 1607.5 of the 1997 UBC is used. The basic formula is as follows:
RLLF = 1 - 0.0008(A - 150)
where,
RLLF |
= |
The reduced live load factor for an element, unitless. The RLLF is multiplied times the unreduced live load to get the reduced live load. |
A |
= |
Tributary area for the element or reaction, length2. If A does not exceed 150 ft2, no live load reduction is used. |
The RLLF factor cannot be less than the Minimum Factor value, which is described below.
Note that no check is performed to limit the RLLF based on Equation 7-2 in Section 1607.5 of the 1997 UBC.
User Defined Curves (By Tributary Area) option. Select this option and click the Define button to access the Live Load Reduction Factor Curves form. Use that form to specify live load reduction factor curves by specifying reduction factors by tributary area.
User Defined (By Stories Supported) option. Select this option and click the Define button to access the Live Load Reduction By Stories Supported form. Use that form to specify reduction factors depending on the number of stories supported by an element.
Minimum Factor options. The availability of the Single Story and Multiple Story edit boxes depends on the Method option selected. When available, use the edit boxes to specify the reduced live load factors (RLLF). One applies to elements receiving load from one story level only and the other applies to elements receiving load from more than one story level.
If the Use default minimum factors check box is checked, ETABS will use default values provided for these minimum reduced live load factors. Uncheck this check box to access the Single Story and Multi Story edit boxes and specify those factors. The default values for the minimum reduced live load factors for the three different live load reduction methods are as follows:
Tributary area method: 0.6 for elements with load from only one story level and 0.4 for elements with load from more than one story level.
Influence area method: 0.5 for elements with load from only one story level and 0.4 for elements with load from more than one story level.
User-defined method: 0.6 for elements with load from only one story level and 0.4 for elements with load from more than one story level.
Important Note about Design Forces: In ETABS the live load is currently only reduced for design forces that are used in the ETABS design postprocessors. Live loads are not reduced in the basic analysis output even if the live load is specified as a reducible-type live load when the static load case is defined and live load reduction is enabled using this command. Thus when live load reduction is enabled, it is possible that live load forces will be different for the exact same item in the basic analysis output and the design output because the live load is only reduced in the design output. Again, in the analysis output, it is unreduced.
Application area. The Application area of the Live Load Reduction Factor form reinforces the above important note. The only application available in the area is Design Forces. It cannot be changed. If live load reduction is not to be applied to design forces in the design postprocessors, select the No Live Load Reduction option in the Method area of the form.
Application to Columns options. For columns, the live load reduction can be specified to apply to the axial load only (choose the Apply to Axial Load Only option) or to all force components (choose the Apply to All Forces/Components option). By default ETABS assumes only the axial load component of columns receives the specified live load reduction. This is useful when it is necessary to reduce the axial live load but not the moment caused by live load in the column.
See also:
