11.4Members Subjected to Bending

11.4.1 Bending Stresses

The actual bending tensile and compressive stresses, $f_{b t}, f_{b c}$ at working (service) load moment, $M_{s}$ of a bending member shall be less than or equal to the permissible bending stresses, $f_{a b t}, f_{a b c}$ respectively, as given herein. The actual bending stresses shall be calculated as:

$f_{\mathrm{bc}}=M_{\mathrm{s}} / Z_{\mathrm{ec}}$ and $f_{\mathrm{bt}}=M_{\mathrm{s}} / Z_{\mathrm{et}}$

The permissible bending stresses, $f_{\mathrm{abc}}$ or $f_{\mathrm{abt}}$ shall be the smaller of the values obtained from the following:

• a) Laterally supported beams and beam bending about the minor axis:

1. Plastic and compact section

$\cdot f_{\mathrm{abc}}$ or $f_{\mathrm{abt}}=0.66 f_{y}$

2. Semi-compact sections $f_{\mathrm{abc}}$ or $f_{\mathrm{abt}}=0.60 f_{y}$

• b) Laterally unsupported beams subjected to major axis bending:

$\begin{aligned} f_{\mathrm{abc}} & =0.60 M_{\mathrm{d}} / Z_{\mathrm{ec}} \\ f_{\mathrm{abl}} & =0.60 M_{\mathrm{d}} / Z_{\mathrm{et}}\end{aligned}$

• c) Plates and solid rectangles bending about minor axis: $f_{\mathrm{abc}}=f_{\mathrm{abi}}=0.75 f_{y}$ where
• $Z_{\mathrm{cc}}, Z_{\mathrm{ct}}=$ elastic section modulus for the cross section with respect to extreme compression and tension fibres, respectively;
• $f_{y} \quad=$ yield stress of the sect; and
• $M_{\mathrm{d}} \quad=$ design bending strength of a laterally unsupported beam bent about major axis, calculated in accordance with 8.2.2.

11.4.2 Shear Stress in Bending Members

The actual shear stress, $\tau_{\mathrm{b}}$ at working load, $V_{\mathrm{s}}$ of a bending member shall be less than or equal to the permissible shear stress, $\tau_{\mathrm{ab}}$ given below:

Actual shear stress, $\tau_{\mathrm{b}}=V_{\mathrm{s}} / A_{\mathrm{v}}$

The permissible shear stress is given by:

• a) When subjected pure shear $\tau_{\mathrm{ab}}=0.40 f_{y}$
• b) When subject to shear buckling (see 8.4.2.1. $\tau_{\mathrm{ab}}=0.70 V_{n} / A_{v}$ where
• $V_{\mathrm{n}}=$ design shear strength as given in 8.4.2.2 (a), and
• $\begin{aligned} A_{\mathrm{v}} & =\text { shear area of the cross-section as given } \\ & \text { in } 8.4 .1\end{aligned}$

11.4.3 Plate Girder

Provisions of 8.3, 8.4, 8.5, 8.6 and 8.7 shall apply, for the design of plate girder, except that the allowable stresses shall conform to 11.4.1 and 11.4.2.

11.4.4 Box Girder

In design of box girder the provisions of 8.8 shall apply, except that the allowable bending stresses shall conform to 11.4.1.