Deflection

* For most residential applications the I-joist industry recommends a minimum floor joist live load (LL) deflection criteria of L/480. RedSpec™ allows the user to specify a more stringent deflection criteria to provide for a more discriminating occupant. Ceramic tile floors require additional consideration.

Deflection is defined as the initial deformation of a product under load. Deflection is a function of load, product cross-section, material variability and moisture content. Creep is the amount of additional deflection that occurs over time and is usually expressed in terms of percentage of initial deflection. All wood products creep. Creep is influenced by each of the issues impacting deflection. Changes in moisture content often impact creep more significantly than the actual wood moisture content. Considering the rates at which moisture can move in and out of different sized members, small cross-sections are impacted by creep more severely than large cross-sections.

Under typical light loads, in dry controlled environments, one can expect deflection to increase over time by approximately 50% more than the initial deflection. As noted in the "Wood Handbook" (Forest Products Laboratory, 1999), "Ordinary climatic variations in temperature and humidity will cause creep to increase. An increase of about 50℉ in temperature can cause a twofold to threefold increase in creep. Green wood may creep four to six times the initial deformation as it dries under load."

I-joists have not been evaluated for creep performance under wet or humid environments. The primary concern is the OSB web. The United States Forest Products Laboratory (FPL) evaluated plywood and OSB for creep due to flat bending under various environments for six months. Under dry conditions OSB had creep deflections 20% greater than plywood. Under wet or cyclic (50% to 85% RH) conditions, OSB creep deflections were three times larger than under dry conditions. Also creep deflections appeared to have increased at a constant rate, which implies long term (> 6 months) creep deflections may be much greater than found in dry conditions.

Deflection and creep are a function of the following factors:

• Initial instantaneous calculated deflection, which is a function of load and cross section.

• Variation from average MOE. Occasionally material may be from 10% (LVL) to 20% (MSR and Glulam) below published values used for initial deflection calculations.

• Variation in stiffness due to moisture content.

• Dry products under load will typically experience creep, which will increase initial computed deflection by approximately 50% over time. This increase is cumulative with any increase due to material variability or moisture content. Creep occurs relatively quickly and at a decreasing rate. Most creep will occur within the first week under load.

• Creep deflection for saturated products dried under load can exceed 3x initial deflection

• Products subjected to varying temperature or moisture contents will experience additional (sometimes dramatic) creep deflection.

• Small cross-sections (i.e. I-joists) are more prone to creep.

• Under wet or humid conditions, OSB web creep deflections are significant.

Design loads are set such that creep and deflection are typically not a problem. Keep products dry and install them in applications where they remain dry and within normal temperatures. Let wet products dry before installation and application of load. Remember that final deflection will likely exceed computed deflection as a result of variability, moisture content and creep. This is normal for all wood products.

Composite Stiffness

Floor and ceiling sheathing attached to a member increase the bending stiffness relative to a "bare" member. The method of attachment also affects the stiffness, with a glued-and-nailed system offering the best composite stiffness.

RedSpec™ presents the method of sheathing attachment on the Serviceability tab. This is available only for floor members, and only for I-joists and open-web trusses. All roof members and all beams are designed as bare members, with no composite stiffness. Ceiling sheathing and concrete topping do not contribute to composite stiffness in the RedSpec™ analysis.

I-Joist: Composite stiffness is considered in the deflection analysis. The stiffness is calculated in accordance with WIJMA's Establishing Prefabricated Wood I-Joist Composite EI. Assumed panel properties are based on OSB span-rated sheathing panels per the 2005 NDS (IBC ASD) or per the CSA O86-1 (NBC LSD).

Open-Web: Stiffness modification due to composite action is similar to the I-joist method, assuming the steel webs do not contribute to member EI. The modified stiffness is used to adjust the deflection resulting from the finite element analysis.

Beam: Composite stiffness is not considered.

Shear Deflection

Deflection in a member is due to the accumulated axial and shear strains in the materials. The method of accounting for shear deflection varies for the different member and material types.

I-Joist: Shear deflection is included in the analysis in addition to the bending deflection. Shear stiffness is a published property of the member.

Open-Web: Shear deflection in a truss is due to axial strain in web members, so the term is not strictly applicable. Strain in chords and webs all contribute to deflection of the truss as part of the finite element analysis.

Beam: Shear deflection of LVL is taken into account by adjusting the modulus of elasticity of the material, which is sometimes called "true E." Shear deflection of glulam and sawn lumber beams is an integral component of the unadjusted MOE, or "apparent E."