2001 ASD Supplements - unprotected PDF - American Wood Council

GL-8ALLOWABLE STRESS AND STIFFNESS3.1 GeneralThe allowable stresses and mean moduli of elasticityof glued laminated timber are affected by the layup ofmembers composed of various grades of lumber as wellas the direction of applied bending forces. As a result,different design values are assigned for glued laminatedtimber used primarily in bending, Table 3.1, and primarilyin axial loading, Table 3.2. The design values are usedin conjunction with the dimensions provided in Tables 9.1(Western species) and 9.2 (Southern Pine) of Section 9,but are applicable to any sizes of structural glued laminatedtimber when the appropriate modification factorsgiven in Section 4 are applied. It is noted that the layupcombinations given in Tables 3.1 and 3.2 are those mostoften used by designers. For other layup combinations thatare also available, refer to APA EWS Technical Note Y117,AITC 117, or NDS-2001.3.2 Allowable Stress and Modulus of ElasticityAllowable properties are given in Table 3.1 for bendingabout the X-X axis (see Figure 3.1). Althoughpermitted, axial loading or bending about the Y-Y axis(also see Figure 1) is not efficient in using the glued laminatedtimber combinations given in Table 3.1. In suchcases, the designer should select glued laminated timberfrom Table 3.2. Similarly, glued laminated timber combinationsin Table 3.2 are inefficiently utilized if the primaryuse is bending about the X-X axis.The values for allowable stresses and moduli of elasticitygiven in Tables 3.1 and 3.2 are based on use undernormal duration of load (10 years) and dry conditions (lessthan 16% moisture content). When used under other conditions,see Section 4 of this Supplement for adjustmentfactors. The allowable flexural stresses are based on membersloaded as simple beams. When glued laminated timberis used in continuous or cantilevered beams, the allowableflexural stresses given in Column 4 of Table 3.1 shouldbe used for the design of stress reversal (when compressionzone is stressed in tension).It is noted that horizontal shear values for glulam arebased on full scale bending tests. The results of these testssupported values that are 10% higher than the tabulatedvalues. The tabulated values were arbitrarily reduced toallow for checking in the shear critical zone of up to 10%of the width of the member. If the designer does not wantto include this arbitrary allowance for checking, the tabulatedvalues can be increased by 10%. It is noted thatchecking seldom occurs in the shear critical zone of glulambeams.Figure 3.1XYYX - X Axis LoadingLoading in the X-X andY-Y AxesXYXXY - Y Axis LoadingY3.3 Straight-Tapered End Cuts on the CompressionFaceStraight-tapered end cuts on the top of a beam aresometimes used to improve drainage, to provide extra headfor downspouts and scuppers, to facilitate discharge ofwater, and to reduce the height of the wall. Table 3.3 providesallowable stresses and mean moduli of elasticity forglued laminated timber with straight-tapered end cuts onthe compression face. The allowable stresses are providedfor bending, F b, and compression perpendicular to grain,F c⊥ , and replace the allowable values provided in Table3.1 when tapered end cut members are used.APA – The Engineered Wood Association