Openings in Concrete Beams – Effects on Strength and Serviceability

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Transverse openings in concrete beams could be different in shapes and sizes as shown in Figure 1. Even though large number of transverse openings with various shapes and sizes might employ, but it is better to use circular shape to make rooms for service pipes, for example electrical supply and plumping, and using rectangular transverse opening through for accommodating rectangular ducts of air conditioners.

Figure 1 different shapes and sizes of transverse openings in concrete beams

In existing structures and buildings, creating transverse openings through beams for better utilization and services above beam soffit other than dead spaces could be detrimental potentially. Problems related to transversal openings through beams can be avoided provided that placements of utility services is planned and taken into account previously but this is not the case most of the times. Generally, there are two most common situations when drilling holes are required which are during utility installation of recently constructed buildings and in existing structures. In the former case because of contractor request who want to save money by rearrangement of pipe services that is not accounted for carefully in the design stage. These types of requests would endanger serviceability and safety of buildings. The latter is when structural performance of an old building is needed to be assessed. In this circumstances core sample of concrete is taken out for testing purposes after that the opening is filled with grouts that are non-shrinking. The effects of openings in concrete beams on serviceability and strength of the structure are discussed in this article.

Effects of Openings in Concrete Beams on Serviceability

Effects on Cracks and Crack Width

It is demonstrated that beams with opening near center stub showed similar crack pattern. Generally, flexural cracks are appeared ahead of shear cracks but this pattern is reversed in beams with openings. If opening size is increased, Loads that cause diagonal cracks and crack width will decline. Moreover, it can be claimed that crack width is larger in beams with openings than solid beams and exceed limitations of crack width irrespective of location and size of holes. From this it becomes quite clear that serviceability of cracks could extremely influenced by openings. Moreover, there are various techniques that are used to repair beams which are drilled for assessment purposes such as filling holes with unshrinking grouts and externally bonded fiber reinforced polymer plate. The latter method proved to be much more effective compare with the former. Beams improved with externally bonded method provide much higher crack control compare with grouting and even better than solid beams. Furthermore, opening locations in beams affect the diagonal cracks and changing locations could lead to narrowing crack width when openings are close to center stub.

Effects on Stiffness and Deflection

The stiffness of beams after cracking is considerably affected by openings and any increase in the size of holes will directly decline beam stiffness. In contrary, there are no obvious or clear influences of the opening location on the beam's stiffness. Openings in beams will lead to increase deflection without considering size and location of openings. Consequently, this could be detrimental for serviceability of the beams. It should be said that beam strengthened with externally bonded plate method entirely remove weaknesses that arise from openings. Additionally, grouting improvement might make some improvement but it is not near from the desired result.

Effects of Openings in Concrete Beams on Strength

Effects on Ultimate Strength

Increasing opening sizes and changing their locations from the center stub of the beam are obviously decreasing the ultimate strength of the beams. When the openings are located close to the beam supports to skip failure plane, the ultimate strength decreases compared with beams without openings by more than ten percent. This could be resulted from cutting one or more stirrups which decrease capacity to carry considerable amount of applied shear. In circumstances where the openings are drilled for estimating performance of old buildings, different methods for example externally bonded fiber reinforced polymer plate, grouting by using non-shrinking cement or any other way can be employed to regain the ultimate strength of the elements. Grouting might regain the beam strength to an extent, but it is around twenty percent less than original strength. Externally bonded FRP plate will be beneficial to regain the strength that has been lost as a consequent of openings. Finally, it is recommended to adopt a considerably large safety factor in the original design or utilizing a promising technique such as externally bonded FRP plate for regaining strength when an opening is made in an existing beam.