Web openings (Composite beams: BS 5950)
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:
do' = 0.9 * opening diameter
lo = 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 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, δs, and that due to local bending, δbt, 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.