Previous Topic

Next Topic

Book Contents

Book Index

IBC 1604.3 - Deflection Load Combinations

Notes d and f, in the IBC Table 1604.3 that indicate that deflection analysis should have different load combinations, in certain cases, than the other design criteria (shear moment, etc) are implemented in Sizer. You can control whether these provisions are to be applied to your member checkboxes in the Load Input view.

Use L + 0.5D for deflection (Note d)

Note d to Table 1604.3 says that limits for the D + L the load combination listed are for the creep component of dead load plus live load, so that for dry service conditions 0.5D can be used as the dead load component, and for wet service conditions or for CLT 1;0 D can be used.

When this checkox is checked then the deflections for the load combinations D + L, D + Lr, and D + L + Lr are calculated with the dead load component multiplied by 1.0 for CLT and 0.5 for all other materials. All other load combinations, such as D + 0.75 (L + S), remain unchanged; the full dead load is used for deflections in those combinations. The reaction, shear, and bending are not affected for any load combination.

The live roof Lr load type is mentioned in the note, but not in the table heading.

Note that the dead load, including creep, for long term deflection is 1.5 D for dry service conditions and 2.0 D for wet service and CLT, so this checkbox offers you a choice of designing for long-term deflection or designing for short term deflection + dead load creep.

Currently the program renames the setting to Use L+ 1.0 D when CLT is selected, however, the input is disabled for wet service conditions. Sizer does not currently allow for the use of this option with wet service conditions.

Component and Cladding Wind Loads (Note f)

Note f to Table 1604.3 says

The wind load is permitted to be taken as 0.42 times the ‘component and cladding’ loads for the purpose of determining deflection limits herein”

Thus for those wind loads that are component and cladding loads, as opposed to design of members that are part of the main wind force resisting system, you are afforded the option of applying 0.42 (0.6 x 0.7) rather than 0.6 as a load factor to the wind load type in all combinations containing wind loads.

Examples of these types of members are roof joists and wall studs that are directly impacted by wind pressures.

When the setting for this is checked, the deflection results for all load combinations containing a wind component are calculated with a 0.7 factor multiplied to the factor(s) that is currently applied to the wind component (0.6 multiplied by either 0.75 or 1 for combinations with other loads).

Other results including reactions, shear, and bending are unaffected.

See Also

International Building Code (IBC)