Web openings (Composite beams: BS 5950)

Guidance on size and positioning of web openings to BS 5950

As each web opening is added it is checked against certain geometric and proximity recommendations taken from SCI Publication P068.

We advise you to comply with the following positional recommendations for web openings:

Web openings are designed using the bending moment and vertical shear values at the side of the opening where the moment is lower,
Openings should preferably be positioned at the mid-height of the section. If not, the depth of the upper and lower sections of web should differ by not more than a factor of two,
Openings should not be located closer to the support than two times the beam depth or 10% of the span whichever is the greater,
The best location for any opening is between 1/5 and 1/3 of the span from a support in uniformly loaded beams, or in lower shear zone of beams subject to point loads,
Openings should be not less than the beam depth, D, apart,
Unstiffened openings should not generally be deeper than 0.6D or longer than 1.5D,
Stiffened openings should not generally be deeper than 0.7D or longer than 2D,
Point loads should not be applied at less than D from the side of the adjacent opening.

Note: Dimensional checks - The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udls and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment or is pre-defined by the service runs.

Note: Adjustment to deflections - The calculated deflections are adjusted to allow for the web openings.

Circular openings as an equivalent rectangle

Each circular opening is replaced by equivalent rectangular opening, the dimensions of this equivalent rectangle for use in all subsequent calculations are:

d_o' = 0.9 * opening diameter

l_o = 0.45 * opening diameter

Properties of tee sections

When web openings have been added, the properties of the tee sections above and below each opening are calculated in accordance with Section 3.3.1 of SCI P355 (Ref. 10) and Appendix B of the joint CIRIA/SCI Publication P068 (Ref. 5). The bending moment resistance is calculated separately for each of the four corners of each opening.

Design at construction stage

The following calculations are performed where required for web openings:

Axial resistance of tee sections
Classification of section at opening
Vertical shear resistance
Vierendeel bending resistance
Web post horizontal shear resistance
Web post bending resistance
Web post buckling resistance
Lateral torsional buckling
Deflections

Design at composite stage

The following calculations are performed where required for web openings:

Axial resistance of concrete flange
Vertical shear resistance of the concrete flange
Global bending action - axial load resistance
Classification of section at opening
Vertical shear resistance
Moment transferred by local composite action
Vierendeel bending resistance
Web post horizontal shear resistance
Web post bending resistance
Web post buckling resistance
Deflections

Deflections

For both non-composite and composite beams without openings the deflection analysis includes the effect of shear. For composite beams this is conservative because it uses the shear area and shear modulus of the bare beam.

The deflection of a beam with web openings should* be greater than that of the same beam without openings due to two effects,

the reduction in the beam inertia at the positions of openings due to primary bending of the beam,
the local deformations at the openings due to vierendeel effects. This has two components - that due to shear deformation and that due to local bending of the upper and lower tee sections at the opening.

The primary bending deflection is established by 'discretising' the member and using a numerical integration technique based on 'Engineer's Bending Theory' - M/I = E/R = σ/y. In this way the discrete elements that incorporate all or part of an opening will contribute more to the total deflection.

The component of deflection due to the local deformations around the opening is established using a similar process to that used for cellular beams which is in turn based on the method for castellated beams given in the SCI publication, “Design of castellated beams. For use with BS 5950 and BS 449".

The method works by applying a 'unit point load' at the position where the deflection is required and using a 'virtual work technique to estimate the deflection at that position.

For each opening, the deflection due to shear deformation, δ_s, and that due to local bending, δ_bt, is calculated for the upper and lower tee sections at the opening. These are summed for all openings and added to the result at the desired position from the numerical integration of primary bending deflection.

Note that in the original source document on castellated sections, there are two additional components to the deflection. These are due to bending and shear deformation of the web post. For castellated beams and cellular beams where the openings are very close together these effects are important and can be significant. For normal beams the openings are likely to be placed a reasonable distance apart. Thus in many cases these two effects will not be significant. They are not calculated for such beams but in the event that the openings are placed close together a warning is given.

* The above technique for calculating the deflection of a beam with web openings does NOT include shear deflection due to the primary bending. Consequently, if the shear deflection component is more significant than that due to openings, it is possible that the reported deflection for a beam with web openings is less than that reported for the same beam without openings.