EWPAA Structural Plywood and LVL Design Manual - Engineered ...

AdhesivesBeams relying only on an adhesive to connect the flange and web components must achieve a reliablestructural bond. The only adhesives with proven structural durability and reliability are the Type A phenolicadhesives. To achieve a reliable bond with these adhesives requires good control over the bonding variables.Typically, beams with adhesive only flange/web bonds require factory controlled conditions to achieve qualitybonds. The advantage of glued beams is they become a completely integrated unit with no slippage betweenthe flanges and web, resulting in a stiffer beam. Glued I-beams with plywood web and LVL flanges arecommercially available.NailsThe simplest method to fabricate plywood webbed beams is to nail the flange/web connections. Nailsmust be flat head structural clouts. Smaller diameter nails at closer spacings are preferable to larger diameternails widely spaced. The use of a structural elastomeric adhesive in conjunction with nails, is not amandatory requirement, but it is good practice as it helps to limit nail slip and increase beam stiffness. Hotdipped galvanized nails should be used in areas of high humidity or mildly corrosive environments orwhere preservative treated plywood or timber are used as beam components. The availability of suitablemachine driven flathead nails should also be considered, but if used, should not be overdriven.8.3 Design of Nailed Plywood Webbed Box Beams - MethodologyThe design method for nailed plywood webbed box beams presented in this chapter follows the limit statesdesign methods detailed in AS 1720.1 Timber Structures Code and the design methodology set out in theEWPAA Design Guide for Plywood Webbed Beams. Formula for the design of C and I plywood webbedbeams can be found in the EWPAA Design Guide for Plywood Webbed Beams. The plywood webbed beamis analysed using transformed section methods and allowances made for the effects of nail slip.TABLES TABLE A8.4(a) and TABLE A8.4(b) provide initial guidance for selecting a beam configuration basedon span/depth and depth/width ratios and beam stiffness. Essentially the process for designing a nailedplywood webbed beam has the following steps:1. Select an initial beam trial size based on(a) Span/Depth (L/D) ratio(b) Depth/Breadth (D/B) ratio(c) Beam deflection approximated from bending deflectionAppendixTable A8.4(a) &Fig A8.2Appendix TableA8.4(b)Total deflection, ∆ τ in a nailed box beam is the sum of the bending deflection (∆ b ),shear deflection (∆ s ), and nail slip deflection (∆ ns ) : ∆ τ = ∆ b + ∆ s + ∆ nsTypically, shear and nail slip deflection comprise 50% to 100% of the bendingdeflection. (Note: In heavily loaded, deep beams, the percentage may be higher).That is:∆ τ is approximately in the range 1.5 x ∆ b to 2.0 x ∆ bTherefore a trial beam size can be selected from an estimate of total beam deflectionbased on the expected bending deflection. Bending deflection can be calculated fromactual load conditions. Or, as done in the worked example, conservatively estimated fromthe beam flexibility tables, by determining the deflection of a simply supported, single spanbeam subjected to a central unit point load.Appendix TableA8.5 or auniformlydistributed unitload (AppendixTable A8.6).73