Bending Moment Design

This topic describes the calculation of the factored bending resistance for glulam as per CSA O86. 7.5.6.5.1

Bending Moment Resistance

The factored bending moment resistance Mr shall be taken as the lesser of

M_r2 = 0.9 F_b S K_X K_L

or

M_r1 = 0.9 F_b S K_X K_Zbg

Important Note: Because the equation for M_r effectively uses the lower of K_L an K_Zbg, and K_L has an upper limit of 1, the upper limit of K_Zbg = 1.3 will never be reached for a governing case, i.e. K_zbg's between 1 and 1.3 will never be used. This was not the intention when the procedure for K_Zbg was changed for CSA 086-14, and the O86 committee is currently reviewing this clause.

Sizer shows both K_L an K_Zbg in the Factors table of the Design Check output regardless of which one governs.

Lateral Stability Factor K_L

Refer to Lateral Support and to Beam Stability Factor KL and its subtopics for the calculation of K_L

Size Factor K_zbg

The size factor K_Zbg is calculated by

K_Zbg = [(130/b) (610/d) (9100/L)]^0.1 <= 1.3

where

b = width of the widest lamination in millimetres ( = member width for single-piece laminations)

d = beam depth in millimetres

L = beam length in metres for single span beams, or segment length from zero moment to zero moment for multi-span or cantilevered beams.

The lamination width b is input in beam view. The default values for each member width are in the database, taken from the CWC Wood Reference Handbook:

The program evaluates K_zbg independently for each span, and makes a separate determination of moment resistance vs. the maximum moment in each span using the K_zbg factor computed for that span.