Effect of Large Openings on the Behavior of Reinforced Concrete Continuous Deep Beams under Static and Repeated Load

. This paper aims to investigate the behavior of the existence of large openings on reinforced concrete continuous deep beams (RCCDBs) under static and limited cycles of repeated load. The experimental work included testing seven RCCDBs. One of these beams was solid subjected to static load, while the other beams having external shear spans, internal shear spans, and mid-span openings subjected to static and fifteen cycles of repeated loading. The range of the repeated loading was varied between a lower level and an upper level, equal to 30 and 70 percent, respectively, of the ultimate load of the beam subjected to static load. All test specimens were tested under five-point bending. For beams with large openings, 160×160 mm (which represents 40% of the overall depth of the section) openings were created symmetrically at various locations. Experimental results showed that the maximum ultimate loads of beams with large openings were decreased by about 36% for beams with large openings locating at interior shear spans compared to a solid beam. However, the maximum reduction in ultimate load of the beam with a large opening subjected to repeated load was about 6%.


Introduction
In structural engineering experience, reinforced concrete continuous deep beam (RCCDB) is a subject of great interest [1][2][3]. The deep beam is one with a clear span (lc) to overall depth (H) ratio that is less than or equal to four [4]. Figure 1 shows an example of continuous deep beams. Openings in the web of continuous deep beams are commonly required to facilitate important services like water supply, electricity cables, telephone, and computer network and provide functionality such as doors and windows. The presence of openings such as in tubes and cables reduces the total shear strength of such beams, resulting in an increased deformation at ultimate strength. [2,3,6,7]. Mansur and Tan [6] classified the openings in reinforced concrete (RC) beams as small or large. They recommended that an opening is considered large if the opening depth (ho) is greater than 25% of the overall depth of the section (H). Apart from that, the opening may be described as small.
Openings in RCCDBs were never specified in the American Concrete Institute AC1318M-19. As a result, there is a great requirement for a safe, precise, and consistent design technique. Additionally, the existence of openings produces interruptions or disruptions in the normal flow of stresses. Loading on offshore structures, highways, and many other structures is often repeated in nature. Extensive theoretical and experimental studies of simply supported deep beams with or without openings over the years, well-established approaches for serviceability design under static loads have developed. [2,3,[7][8][9][10][11][12][13][14][15]. However, the effects of repeated loading on RCCDBs are still rare and scarcely understood. Concrete structures subjected to repeated loading experience higher deflection comparing to those exposed to static loading. In this study, experimental investigations on the behavior of RCCDBs with large openings under five-point load have been carried out on continuous deep beams having large symmetrical openings subjected to static and limited cycles of repeated load. Figure 1. Photo picture of RCCDB [5].

Experimental Program
Descriptions of tested beams. The experimental work included testing seven RCCDBs under five-point bending. The tested specimens were 2500 mm overall length (clear span of 1190 mm), 400 mm overall depth of beam, 160 mm width of the beam, and a shear span of 595 mm as shown in Figure 2. The first specimen (CDB-Solid) was prepared without opening (as a reference specimen). Openings of dimensions 160×160 mm (representing 40% of the overall beam depth) were created in the remaining specimens. These openings were created symmetrically at the external shear spans, internal shear spans, and center of spans of the other specimens. The RCCDBs with openings were tested under static and repeated load. At the same time, beam CDB-Solid was tested under static load only. Table 1 summarizes the description of the tested beams.    Instrumentation and test setup. The test setup for beams with five-point bending and a shear span of 595 mm is shown in Figure 4. The load was applied with a 1000 kN hydraulic jack and measured using a load cell (2000 kN). Vertical deflections were measured using linear variable displacement transducers (LVDTs) for each span of beams. Three stages were involved in the application of repeated load. The minimum and maximum cyclic loads in the first stage were 30 percent and 70 percent, respectively, of the ultimate load of the similar beams under static load. The cyclic static load was applied until it reached 70% of the ultimate load, then it was unloaded to 30%. A total of fifteen loading and unloading cycles were completed. In the second step, the load was released to zero after fifteen cycles of loading. Static loading was applied to the beams until they failed in the final step.  . Test setup for typically tested beam.

Description of Experimental Test Results
Initial crack and mode of failure. The initial crack load was seen in the mid-span of each span of the continuous deep beam (i.e., flexural crack) for beam CDB-Solid during the experimental test. In the case of beams with large openings, the initial crack load was noticed at the opening corners and spread towards the loading and supporting points. Table 2 summarizes the cracking loads for tested beams. This table shows that the presence of large openings affects the behavior of continuous deep beams by decreasing the initial crack load compared to RCCDBs without opening (beam CDB-Solid). The percentages decrease in initial crack load were about 45, 58, and 30% for beams CDB-M-EO, CDB-M-IO, and CDB-M-MO, respectively, compared to beam CDB-Solid. These percentages were the same for beams subjected to limited cycles of repeated load.  Figure 5 shows the crack pattern for all tested beams. It was observed from this figure that the mode of failure was a shear failure for beam CDB-Solid. While for beams CDB-M-MO and CDB-R-MO the mode of failure was bearing failure at the top cord of openings. However, for beams with openings locating at external and internal shear spans (i.e., beams CDB-M-EO, CDB-M-IO, CDB-R-EO, and CDB-R-IO), the mode of failure was observed opening shear failure. Load mid-span deflection response. Figure 6 shows the total applied load (P) with mid-span deflection of all test beams under static load. From this figure, it can be noticed that large openings reduce the stiffness and ultimate load and increase the deflections within the entire range of loading compared to beam CDB-Solid. In addition, Figure 7 to 9 shows the total applied load with midspan deflection for a beam with various locations of large openings under static and limited cycles repeated load. From this figure, it can be noticed that the application of limited cycles of repeated load leads to a decrease in the ultimate load compared with beams under static load. Table 3 summarizes the test results of the seven specimens in terms of key parameters of load-deflection curves.   Figure 6. Load mid-span deflection for tested RCCDBs with openings under static load. Figure 7. Load mid-span deflection for tested RCCDBs with openings located at external shear spans. Figure 8. Load mid-span deflection for tested RCCDBs with openings located at internal shear spans. Figure 9. Load mid-span deflection for tested RCCDBs with openings located at mid-spans.

Conclusions
The following conclusions can be drawn from this research work.  Large openings in RCCBD lead to a decrease in the ultimate load and an increase in mid-span deflection.  From the experimental test, the existence of large openings within exterior or interior shear spans caused a reduction in ultimate load by about 26% and 36% compared with beam without openings. However, when opening located within mid-span region, the percentage decrease in ultimate load became about 23%.  For beams under limited cycles of repeated load, it was observed that the reduction in ultimate load was about 5, 6, and 4% for beams with opening locating at the exterior, interior, and midspan, respectively, compared to beam under static load.