This section includes key notes and assumptions made for the British Standard design of tees and angle sections.

Classification checks

For axial compression and bending both the web and flange (Leg 1 and Leg 2) are classified as Class 1, Class 2, Class 3 or Class 4 and the worst of the two is the resultant classification for that cross section.

The rules from Table 11 and 12 of BS 5950-1:2000 apply for the classification of these sections.

Axial tension check

Section 4.6 of BS 5950 is used for this design check.

Axial compression check

Section 4.6 of BS 5950 is used for this design check.

Shear check

Section 4.2.3 of BS 5950 is used for this design check.

Moment check

Section 4.2.5 of BS 5950 is used for this design check.

Moment capacity for Class 4 slender sections:

Class 4 sections are designed as Class 3 effective sections.

Hence, additional moments are induced in the member due to the shift of the centroid of the effective cross-section compared to that of the gross section when under axial compression only.

Thus:

ΔM_Ed,x = e_x × F_c

ΔM_Ed,y = e_y × F_c

Where:

F_c is the max compressive force in the span.

For tees and double angles e_x = 0. Hence, total minor design moment = minor design moment.

Where:

e_x and e_y = the shift of the centroid of the effective area A_eff relative to the centre of gravity of the gross cross section

e_x = abs(cx_new – c_x)

e_y = abs(cy_new – c_y)

So finally, a total moment is obtained for which the moment design check is performed:

M_{total x} = Abs(M_Ed,x) + Abs(ΔM_Ed,x)

M_{total y} = Abs(M_Ed,y) + Abs(ΔM_Ed,y)

Single angles - asymmetric sections:

Single angles with continuous lateral – torsional restraint along the length are permitted to be designed on the basis of geometric axis (x, y) bending.

Single angles without continuous lateral – torsional restraint along the length are designed using the provision for principal axis (u, v) bending since we know that the principal axes do not coincide with the geometric ones.

ΔM_u = ΔM_x × cosϑ + ΔM_y × sinϑ

ΔM_v = -ΔM_x × sinϑ + ΔM_y × cosϑ

Note that when principal axis design is required for single angles and the classification is Class 4, all moments are resolved into the principal axes (total moment in the principal axes u-u and v-v).

Combined bending and axial check

Section 4.8.3 of BS 5950 is used for this design check.

For Class 3:

Abs (F_c / A_gP_y) + abs (M_x,Ed /M_cx) + abs (M_y,Ed/W_el,min,y) ≤ 1.0

For Class 4:

Abs (F_c/A_effp_y) + (abs (M_x,Ed) + abs (ΔM_x,Ed)) / M_cx + abs (M_y,Ed) + abs (ΔM_y,Ed)) / M_cy ≤ 1.0

Note that total moments are used when the section classification is Class 4.

Lateral torsional buckling check

Section 4.3 of BS 5950 is used for this design check.

In the case of a beam with continuous lateral torsional restraint along its length this check is not performed. The lateral torsional resistance is considered adequate.

For beams that are unrestrained, a Lateral torsional buckling (LTB) check is required, either:

• In its own right check for LTB, clause 4.3, and B2.8 for tee sections and B.2.9 for angle sections in BS 5050-1: 2000.
• As part of combined buckling, clause 4.8 “Members with combined moment and axial force”, 4.8.3.3, for single Angles I3 and I4 sections

This check is not performed when bending exists about the minor axis only

Combined buckling check

Single angles:

Clause I.4 - For beam with continuous lateral torsional restraint or for equal single angle sections with b/t ≤ 15ε a combined buckling check is performed according to clause I.4.3 - the simplified method.

For any other case clause 4.8.3.3.1 is used with the moments being resolved into the principal axes u-u and v-v. Two formula are provided in clause 4.8.3.3.1, both are checked

Tees: