Combined Axial Loading And Moment

Whether a built-up column is formed by gluing or by mechanical fixings, the design procedure is only applicable where the column is subjected to axial loading acting through the centre of gravity of the built-up section and to the cases where there are only small moments (e.g. due to self-weight) in addition to the axial loading.

Under such conditions, the design requirements for built-up columns are the same as those for solid sections detailed in 5.4.1(b). Because of the small moment condition permitted, lateral torsional buckling effects can be ignored and on the premise that both

A.ef and Xz will result in relative slenderness ratios exceeding 0.3, the strength equations to be complied with are:

gc,0,d + ^^ + km^^ < 1 (EC5, equation (6.23)) (8.38)

kc,y fc,0,d fm,y,d fm,z,d gc'°'d + km^^ + ^^ < 1 (EC5, equation (6.24)) (8.39)

kc,zfc,0,d fm,y,d fm,z,d kc,y =- 1 (EC5, equation (6.25)) (8.40)

where kz is obtained from equation (5.5b); acj0jd and fc,o,d are as previously defined and the bending functions are as defined in Chapter 4.

See Example 8.8.3. 8.6 EFFECT OF CREEP AT THE ULS

Where the compression members of a built-up column have different values of modulus of elasticity, and/or the joints in the column are made using fasteners rather than glue, due to creep behaviour the compression stresses within the elements of the built-up column will change with duration of load and change in moisture content. Under such conditions, as required by Clause 2.2.2 in EC5, when undertaking a first-order linear elastic analysis the effect of creep behaviour must be taken into account.

The analysis procedure in 8.4 relates to the derivation of stresses in the built-up column based on the stiffness properties derived using the mean value of the modulus of elasticity of the materials and the instantaneous value of the slip modulus.

When taking into account the effect of the redistribution of stress due to creep behaviour, EC5 requires that final mean values of stiffness properties as defined in be used. The approach gives an approximation to the true behaviour of the column and for this condition the final mean values of the relevant moduli are as follows:

(a) Modulus of elasticity:

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