Design of composite beams for buildings T-beams with partial encasement or with web openings

The first part of this article describes the calculation of a T-beam with a slab above and connected to a steel section.

In this second part, we first present the calculation of partially encased beams, i.e. with reinforced concrete poured between the footings of the steel section, and then the calculation of beams with webs featuring large openings, commonly used today.

In most cases, partially encased beams are simply joists supported at their ends, for which the determination of bending moment and shear force does not present any particular difficulties. However, there may also be cases of partially-covered continuous beams, such as floor edge beams. In such cases, the applicable global analyses need to introduce a few restrictive conditions compared to the same analyses used with common T-beams. Obviously, when a continuous beam is made with a partial coating, the risk of instability due to tilting can be greatly reduced, thanks to a certain lateral support of the bottom flange.

On the other hand, for T-beams with wide web openings, the risk of tilting would theoretically be increased by the greater lateral flexibility of the steel web; but to our knowledge, there are currently no examples of continuous beams with wide web openings close together in the area of negative bending moments. For this reason, we assume that the values of the positive bending moment and shear force in this type of beam are known, particularly at the open web section. The spacing between two adjacent openings is chosen to be sufficient to avoid premature failure of the web upright between these openings. It should also be pointed out that the theoretical expressions are developed for mono-symmetrical steel beams, the current trend being to manufacture open web beams with a bottom flange that is significantly stronger than the top flange.

For the sake of clarity, the examples discussed here follow on from the examples in the first part of this text, i.e. the article [C 2 561] , which explains their numbering.

The final chapter contains a glossary of all the technical vocabulary used in this article.