Design of Big Openings in Beams and Downstand Beams

Technical Article

Since the ultimate limit state of beams in the area of openings is affected, particular attention should be payed to this. In general, small openings can be sufficiently covered by adapting the beam structure to the openings. For big openings, it is necessary to consider and model the area separately.

Possibilities in RFEM

Since a direct consideration of openings in beams and downstand beams is currently not possible, a solution must be found with the existing tools. An input in RF-CONCRETE Members is not possible so that it is necessary to use surfaces here.

Modeling downstand beams can be performed with surfaces without difficulty. With the possibility to evaluate internal forces from the surface calculation with result beams as member internal forces, it is then possible to perform member design.

Modeling

The concrete beam or downstand beam is modeled as surface model instead of, as usual, a member beam or rib. Afterwards, a result beam is modeled in this surface in which the internal forces from the surface calculation are integrated over the desired area.

Figure 01 - Structure

The same action takes place, as shown in Figure 02, for the smaller cross-sections around the openings. The respective cross-section of the integrated surfaces is assigned to the result beams as cross-section.

Figure 02 - Integration Parameters Result Beam

Internal Forces

For the design of the result beam in RF-CONCRETE Members, the internal forces in surfaces are converted to member internal forces. Only then will it be possible to evaluate the internal forces in surfaces meaningfully.

Figure 03 - Internal Forces

Design

The member design is performed as usual in RF-CONCRETE Members. The reinforcement areas are defined according to the member internal forces in the result beam and have to be adjusted if necessary to create the transitions between main member and openings reasonably. Due to the calculation of the chords in the openings as compression and tension member, rebar diameter or reinforcement content often differ.

Figure 04 - Optimization of Reinforcement; Top: Without Adaptations, Bottom: Adjusted Longitudinal Reinforcement

Further Considerations in Case of Big Openings

In addition to the reinforcement determined in RFEM, further details have to be considered for the reinforcement of openings. The anchorage length of the reinforcement above and below the opening should be selected in such a way as to sufficiently anchor the concrete strut at an inclination of concrete strut of 45 °. Furthermore, it is necessary to provide a suspended reinforcement at both ends of the opening to transfer the arising transversal tension forces. Their calculation can be performed, for example, according to booklet 566 or 599 of DAfStb (German Committee for Structural Concrete). Both methods are presented in [3].

Reference

[1]  Manual RFEM. Tiefenbach: Dlubal Software, February 2016. Download
[2]  Manual RF-/CONCRETE Members. Tiefenbach: Dlubal Software, October 2017. Download
[3]  Hegger, J.; Mark, P.: Stahlbetonbau aktuell 2016. Berlin: Beuth, 2016

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