Fmy

120kc

Fc,d Xef 3600kc

Fc,d 60kc for A.ef < 30 for 30 < A.ef < 60 for A.ef > 60

In the above, Fc d is the design axial load on the column and the other functions are as described in the text.

Where the built-up column is assembled using glued joints, the horizontal shear stress in the members at the glue lines shall be checked using conventional elastic theory and the shear force on the section shall be taken to be Vd. As described in 4.5.2, the shear strength of the members will depend on the direction of the shear force relative to the grain in each member.

8.4.3.1 Design criteria for built-up sections

For built-up sections, the design criteria to be satisfied to comply with the requirements of EC5 are as follows:

Under this condition the built-up section will not buckle, but will fail at the compression strength of the built-up members, i.e.

where:

• oc, o, d, i is the design compressive stress parallel to the grain in member i and:

-Li=i EiAi where Fc,d is the design axial load on the column, Ei is the mean value of the modulus of elasticity parallel to the grain of member i, and Ai is the cross-sectional area of member i .

• fc,o,d,i is the design compressive strength parallel to the grain of member i and:

where:

- kmod.i' is the modification factor for member i for load duration and service classes as given in Table 2.4.

- ksys is the system strength factor discussed in Section 2.3.7. When dealing with single column members, this factor is not relevant and will be taken to be unity.

- YM,i is the partial coefficient for material properties for member i, given in Table 2.6.

- fc.o.iM is the characteristic compressive strength of the member i parallel to the grain. Strength information for timber and commonly used wood-based structural products is given in Chapter 1.

If either Xrel z or Xrel y is > 0.3, the member can buckle and the design requirement will be as follows:

(i) ^reu > 0.3 and the built-up section is not symmetrical about the y-y axis. For sections that are not symmetrical about the y-y axis (due to geometry or a combination of materials), the axial load on the column, Fc d, must be less than or equal to the summation of the load-carrying capacities of the individual members of the built-up section bent about the respective member neutral axis parallel to the z-z axis of the built-up section.

(ii) ^reu > 0.3 and the built-up section is symmetrical about the y-y axis. Under this condition, the built-up section will buckle as a composite section and:

where:

• oc>o,d,i is the axial stress in member ias defined in equation (8.16).

• kczi is the instability factor for member i and is determined from the application of equations (5.4b), (5.5b) and (5.6), with the relative slenderness ratio, Arel,Zji, being:

n V E0.05,i where /c,o,k,i and E0.05,i are as previously defined and Xef,z is the slenderness ratio of the member about the z-z axis, obtained from:

0 0

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