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

Length of Bearing of Member (mm) 12 25 50 75 125 150 or moreValue of k7 1.85 1.60 1.30 1.15 1.05 1.00TABLE 6.4: Factor for length and position on bearingk 9AS1720.1-1997clause 8.4.6– Strength sharing modification factor for grid systemsclause 2.4.5k 9 = 1.0 for LVL used in parallel systems clause 8.4.7For bending members k 9 applies in two different scenarios, i.e. for:• combined parallel systems, does not apply to LVL because it is treated as solid sawntimber. However, it has all of the attributes, since it is made from parallel elementsrigidly connected forcing them to deflect the same amount.AS1720.1-1997 (Clause 2.4.5) defines a parameter n com which is the effective numberof parallel elements shown in FIGURE 6.8 which combine to form a single member andfor which n com = 4.0;• discrete systems, which applies to, e.g. LVL joists sheathed with plywood causing loadsharing between joists in the system. Effectiveness of the load sharing is dependentupon the joist spacing and the stiffness of the plywood interconnecting the joists asshown in FIGURE 6.8. The number of members involved in the load sharing is definedin Clause 2.4.5 as n mem . In a normal plywood sheathed floor system n mem = 3 wouldbe usual for a floor of 5 or more joists.FIGURE 6.8: Parallel and grid systemsk 11– Size FactorThe tensile behaviour of timber modelled on brittle fracture mechanics indicates a higherprobability of finding a flaw (eg naturally occurring characteristic such as a knot, split, gum vein,etc.) leading to a brittle fracture in higher volume members. The k 11 size factor is used toaccount for this increased probability of finding a flaw in larger volume, tensile members.AS1720.1-1997clause 8.4.7LVL members are modified for size effects as follows:(a)(b)For bending k 11 = 1.0 for member depths up to 300mm. For member depth greaterthan 300mm k 11 = (300/d) 0.167For tension parallel to grain k 11 = 1.0 for member depths up to 150mm. Formember depth greater than 150mm, k 11 = (150/d) 0.16746