Deflection Criteria

Deflection criteria due to gravity loads are described here. These criteria can be specified on the The Member Info Tab. However, the software will only use deflection limits that are equal to or more stringent than minimum building code requirements.

Members will deflect or flex under load. The amount of flex depends on the magnitude of the load applied and the stiffness of the member. Deflection criteria are a set of deflection limits that apply to floor and roof members and are defined by building codes. These deflection limits are intended to ensure user comfort and to prevent excessive cracking of finish materials. The improved deflection limits built into our software may be more restrictive than the minimum values required by building codes. Field experience has proven that these improved limits provide better framing systems and more satisfied customers.

For IBC, the deflection limits defined in Section 1604.3.1 shall be used unless more restrictive deflection limits are required by a referenced standard for the element or finish.

For NBCC, the deflection limits defined in Division B, Section 9.4.3.1 shall be used unless more restrictive deflection limits are required by a referenced standard for the element or finish.

Cantilever Deflection

Spans and overhangs are individually checked to verify that they meet or exceed deflection requirements.

Cantilever deflection limits are twice the span deflection criteria for both live load and total load deflection. For example, if the span live load deflection limit is L/360, the cantilever live load deflection is limited to L/180.

• Short cantilever deflection limit: Larger of twice the span deflection criteria or 0.2".
• Upward cantilever deflection warning: When upward deflection at a cantilever exceeds 0.4".

Creep Deflection for Treated Products

Deflection calculated by ForteWEB for comparison to IBC deflection limits are conservative relative to IBC requirements for all wood products.

 

For Parallam Plus PSL, the calculated total load deflection has been further increased to mitigate potential serviceability issues due to long-term loading under variable moisture conditions. However, ForteWEB does not calculate long-term deflection as outlined in the National Design Specification (NDS) for Wood Construction. Where deflection under long-term loading must be limited, it is recommended to use a creep factor of Kcr = 1.5 for dry use and Kcr = 2.0 for web use in accordance with NDS procedure.