Building Design and Construction Handbook - Merritt - Ventech!

10.26 SECTION TEN
multiplied by applicable adjustment factors (Art. 10.5). (The duration-of-load factor
does not apply to F c� for either solid sawn lumber of glulam timber.)
Limitations on compressive stress perpendicular to the grain are set to keep
deformations within an acceptable range. An expected failure mode is excessive
localized deformation rather than a catastrophic type of failure.
Design values for F c� are averages based on a maximum deformation of 0.04
in in tests conforming with ASTM D143. Design values F c� for glulam beams are
generally lower than for solid sawn lumber with the same deformation limit. This
is due partly to use of larger-size sections for glulam beams, length of bearing and
partly to the method used to derive the design values.
Where deformations are critical, the deformation limit may be decreased, with
resulting reduction in F c�. For example, for a deformation maximum of 0.02 in.
the ‘‘National Design Specification for Wood Construction,’’ American Forest &
Paper Association, recommends that F c�, psi, be reduced to 0.73 F c�.
10.7.5 Example of Design of a Glulam Beam
Standard beam formulas for bending, shear, and deflection may be used to determine
beam sizes. Ordinarily, bending deflection governs design; but for short, heavily
loaded beams, shear may control.
Design values for bending are tabulated in the supplement to the ‘‘National
Design Specification for Wood Construction.’’ These values should be adjusted for
service conditions (Art. 10.5). Section properties for solid sawn lumber and timber
and glulam members are listed in the supplement to the ‘‘National Design Specification
for Wood Construction,’’ American Forest & Paper Association, Washington,
D.C.
With the following data, design a straight glued-laminated roof beam, simply
supported and uniformly loaded: span, 28 ft; spacing, 9 ft c to c; live load, such
as snow, 30 lb/ft 2 ; dead load, 5 lb/ft 2 for deck and 7.5 lb/ft 2 for roofing. Allowable
design value for bending of glulam combination grade is 2400 psi, and for horizontal
shear is 195 psi, for modulus of elasticity E � 1,800,000 psi. These are
typical for a generic grade of glulam that can be manufactured using several different
species. Deflection limitation for total load is L/180, where L is the span, ft.
Assume the beam is laterally supported by the deck throughout its length.
With a 15% increase for load duration, such as snow, the allowable bending
stress F b becomes 2760 psi, and the allowable horizontal shear F V, 224 psi.
Assume the beam will weigh 22.5 lb/lin ft, equivalent to 2.5 lb/ft 2 based on a
9-ft c to c spacing. Thus, the total uniform load comes to 45 psf. So the beam
carries w � 45 � 9 � 405 lb/lin ft.
The end shear V � wL/2 and the maximum shearing stress � 3V/2 � 3wL/4.
Hence, the required area, in 2 , for horizontal shear is
The required section modulus, in 3 ,is
3wL wL 405 � 28 2
A � � � � 37.9 in
4F 299 299
v
2 2
1.5wL 1.5 � 405 � 28 3
S � � � 172.6 in
F 2760
b
If D � 180, the reciprocal of the deflection limitation, then the deflection equals