Análisis de los flujos en partida crítica

Based on the strength requirements, the maximum load that can be carried by the joint is controlled by: 1- maximum flexural capacity of the beam, 2- maximum shear capacity of the beam, 3- column flexural capacity, 4- column shear capacity, and 5- shear capacity of the joint. These factors are dependent on many variables: 1- composite action between the steel plates and the concrete, which depends on the efficiency of shear connectors in transferring forces between the concrete core and the steel plates, 2- strength of steel skin plates, 3- core strength, and 4- anchorage (bond) mechanism of the tension plate to the joint region.

By providing anchorage in three different ways, the enhancement ratio for the maximum load can be varied from 517% to 871% of the original double skinned specimen (Table 4-1). Adding normal steel reinforcing bars

(longitudinal bars and links) to both the beam and the column raised the maximum carried load from 10.5 kN to 54.3 kN. Adding the welded reinforcing bars to the beam plates and normal steel reinforcement in the column (longitudinal bars and links) enhanced the load to 62.7 in the first test and to 72.5 in the repeated test (about 15% difference). In the joint with extended beam plates the load increased to 91.5 kN.

It was expected that the welded bars would give higher resistance because the location of the added bars is larger than the normal reinforcement, which increased the lever arm of the tensile force.

The double skin beam-column joint with bars welded to the beam plates and normal steel reinforcement in the column was selected for the parametric study, i.e. to study the effect of the concrete’s compressive strength and the effect of steel fibres as well as to study its behaviour under a cyclic load. It can be said (Table 4-1) that increasing the concrete compressive strength (by using HSC) has no great effect on the maximum load, which can be attributed to the nature of the failure mechanism in the current SCS beam-column joint. The failure was mainly dependent on the initiation of stud failure, which highly affected the joint’s response. By comparing shear cracking in the joint region it can be concluded that improving the concrete compressive strength by using HSC reduced the number and width of cracks, especially shear cracks. This enhancement in the joint shear resistance coincides with the finding of Kim and LaFave (2007), Kularni and Patil (2103) and Roehm et al. (2015). The fibres maintain the integrity of the concrete by bridging the cracks. This observation can be confirmed by findings from Liew and Sohel (2010): “The

presence of fibres in the concrete increases the ultimate load carrying capacity of the beam”.

Figure 4-11: Load deflection for all tested SCS joints

Moreover, Figure 4-12 shows the increase in the load corresponding to the indicated percentage of maximum deflection of the joint cast with NC core. During initial load stages, it can be seen that the improvement in the load was pronounced (up to 82% improvement); on the other hand, the enhancement in the maximum load was very low in correspondence to the final stages in the NC joint, although other joints (SFC and HSC) reached greater load. This comparison reflects the effect of steel fibres on the ductility, as discussed in the previous paragraph.

Figure 4-12: Variation in Maximum Load with Concrete Type

The previous discussion has taken into account the flexural capacity of the beam; no shear failure has been identified in any of the tested joints, which reflects the efficiency of the presented design to resist shear stresses. The beam resists shear through two components, concrete and shear studs, as

0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 Lo ad ( kN )

Deflection Below Load (mm)

DSC-NO MODIFICATION DSC-NORMAL CAGE DSC-EXT PL DSC-WELD-1 HPC VF = 0.25% 0% 20% 40% 60% 80% 100% 40% 60% 80% 100% Ma x im u m L o ad o f NC Deflection Ratio Normal Concrete-1 Fibrous Concrete – Vf=1% High Performance Concrete -1

well as a low amount of resistance being produced by the dowel action of the steel plates.

The core of the joint has been reinforced by shear links spaced at 100 mm, as detailed in Chapter three of this thesis; these shear links increased the joint shear resistance, as indicated by Hamil (2000). Shear links enhance the joint region’s ability to resist shear by arresting the cracks by dowel action.

As reported by Pauletta et al. (2015), the maximum capacity of a reinforced beam-column joint is achieved after extensive cracking in flexural regions; moreover, Hamil (2000) showed that the joint can resist a further significant load after shear cracking has appeared. In the present study, all the tested specimens showed the same behaviour, i.e. increase in load capacity after severe cracking in the critical flexural regions and after initial shear cracking in the core region.

This behaviour can be attributed to the similarity in the resistance mechanisms of SCS beam-column joint and RC beam-column joints in the connection zone. The SCS joint exhibited very low strength without any strengthening, as presented in section 4.2 of this thesis; therefore, the structural behaviour of the SCS joint in flexural mainly depends on the welded bars that are used to provide anchorage and flexural resistance. Shear strength in the SCS joint is enhanced by using shear reinforcement in the core of the joint. In RC joints, flexural strength and shear resistance, in the same manner, depend on the flexural capacity of the beam as well as bond (anchorage) capacity in the joint, and the shear strength showed dependency on the concrete and presence of links in the core (Sharma et al., 2011).

Using steel fibre concrete improves the overall structural response and increases the load-carrying capacity of the RC beam-column joint (Campione, 2015; Liang et al., 2016; Abbas et al., 2014). The effect of steel fibres on the behaviour of the beam-column joints can be attributed to the role of steel fibres in improving: 1- flexural strength, 2- shear strength,3- ductility, 4- energy dissipation, and 5- fracture toughness (Shakya et al., 2012; Bischoff, 2003; Jo et al., 2001). In the present study, the SCS beam-column joint with SFC core showed better load-carrying capacity and increase in ductility, which coincides with the advantages of using SFC in RC beam-column joints.

Table 4-1: Comparison between the SCS Joints Joint Max. Load (kN) Cracking Load Pcrack (kN) Percentage of Pmax after providing anchorage Maximum Displ. max(mm) Based on Load Decreasing Maximum Displ. max(mm) Based on Max. Strain Stud failure Load (kN) Failure Mode No modification 10.5 8.9 100 27.7 - 9.9 -

With Normal Bars 54.3 10.5 517 91.8 - 53 -

With Extended Plates

91.5 39.1 -

45.5* 871 92.8 - Non -

All the DSC joints listed below have welded bars

Normal Concrete-1 62.7 13 595 67 - 62.7 PL- Ten.

Normal Concrete-2 72.5 13 690 50 28.7 67.9 PL- Ten.

Fibrous Concrete – Vf=1% 85.3 19.5 812 50 47 85.1 PL- Ten.

Fibrous Concrete – Vf=0.25%

69.7 11.5 -

16.63** 664 54 24 69.4 PL- Ten.

High Performance Concrete -1 67.8 23 646 56 30 64.9 PL- Ten.

High Performance Concrete -2 66.8 23.3 636 59.3 - 66.8 PL- Ten.

*: first crack appeared at the corner of the lower corner of the beam column intersection region under a load of 39.1 kN but the author believes it was because of the presence of the bolts. The second flexural crack appeared in the beam at a distance of 300 mm away from the column face under 45.5 kN.

**: due to a technical issue, the test stopped when the load reached 16.63 kN and the specimen was not cracked and, when the specimen was reloaded, the crack appeared under a load of 11.5 kN.