Beam Opening

Very often, openings in beam are required to allow the passing through of E&M Services.

The addition of beam openings will impair strenght of the beam. General recommendation for size and location of beam opening could be found in PANP 230 published by the Buildings Department.

If large opening is required, the beam opening shall not be located near concentrated point load and the strength of the beam shall be checked as following:

(1) Global Check

(i) Determine the design moment(M) and shear(V) at the opening location.

(ii) Check if the opening will be extruded into the compression zone of the section.

(iii) If the whole opening is located within the tensile zone, then only local checking is required. Othrwise, the moment capacity of the reduced section shall be checked.

(2) Local Check

(i) The upper chord and lower chord of the opening will take up shear in accordance with their stiffness (ie, their height h₁ and h₂).

(ii) Shear of upper chord, V_up = V x h₁/ (h₁ + h₂) and Shear of lower chord, V_low= V x h₂ / (h₁ + h₂)

(iii) The Design Moment for Upper Chord, M_up = V_up x L/2, where L = width of opening. Similarly, the Design Moment for Lower Chord, M_low = V_low x L/2

(iv) Design the upper chord and lower chord with M_up, V_up, M_low and V_low.