Web openings and their deflection effect on beams
It is often required to let services pass through the web of beams. In such cases, the common solution is to provide the required number of openings in the web plate. Such an opening can have a circular or rectangular shape, depending on the amount, size and shape of pipes or ventilation or cable trays.
If the structural engineer has the freedom to position these openings along the beam, where to place them? What would be its effect on the deflection of the beam?
The effect of such openings on the deflection is more important when the length of the opening along the beam is increased. As circular openings are made with equal length and depth, they are usually less critical than rectangular openings.
The following demonstration will be made with a 6 meters long simple supported floor beam with a welded section.
Exposed to a linear load of 10 kN/m, the deflection at mid-span of the solid web beam is 4.6 mm.
Let’s assume that a 250 mm deep rectangular opening with a length of 400 mm needs to be provided on the web, at a distance of 300 mm from the left support.
Traditional analysis with beam finite elements
Consteel 7DOF beam finite elements are very powerful, but cannot consider natively such opening. The usual approach is to build a Vierendeel-type of model, by using additional beam elements with a T shape section „above” and „below” the opening. These additional beam elements are defined eccentrically to the reference line of the solid-web beam.
Eccentricities can be easily defined in Consteel using both smart and traditional link elements.
The deflection with this refined model will be equal to 4.8 mm.