The ultimate load carrying capacity of concrete members with headed bars

Transcription

1 The ultimate load carrying capacity of concrete members with headed bars P G Bakir*, Istanbul Technical University, Turkey M H Boduroglu, Istanbul Technical University, Turkey 27th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2002, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 2m Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2002, Singapore The ultimate load carrying capacity of concrete members with headed bars P G Bakir*, Istanbul Technical University, Turkey M H Boduroglu, Istanbul Technical University, Turkey Abstract In certain parts of structures, such as beam-column joints, proper anchorage of longitudinal bars is very important. Codes require that standard hooks should be used to anchor longitudinal reinforcement within a joint, or in beams or columns, minimum embedment lengths are specified by the codes. Nevertheless, the use of standard hooks results in steel congestion, making the fabrication and construction difficult. Headed bars are widely advocated today among researchers due to the practicability they provide. Nevertheless, little information exists in literature for the design of headed bars. In this study, the authors have carried out several parametric studies on an experimental database consisting of concrete members with headed bars situated in the centre, corner and edge respectively. The factors influencing the ultimate load carrying capacity of concrete members are determined. It is apparent that the ultimate load carrying capacity will be much higher if the bar is a centre bar and the ultimate load carrying capacity of concrete members with headed bars will be much lower if the bar is a corner bar. If the bar is an edge bar, the ultimate load carrying capacity will take a value between the load carrying capacity of the centre bar and corner bar. Keywords: headed bars, ultimate load carrying capacity. 1. Introduction The transfer of force between concrete and reinforcing bars is one of the most important aspects of structural design. In situations where there is not adequate length for a straight bar anchorage, the bar is hooked with either a 90 or 180 degree hook. However, the use of standard hooks often leads to steel congestion in concrete members, resulting in fabrication and construction difficulties. This study aims at determining the factors that influence the ultimate load carrying capacity of concrete members with headed reinforcement. A new design equation is proposed for determining the ultimate load carrying capacity of concrete members with headed reinforcement. The proposed equation is applied on an experimental database consisting of tests of concrete members with headed reinforcement placed in the center, edge and corner. The basic test set up for shallow embedment pullout tests are shown in Figure 1. The tests used in the experimental database are the tests of DeVries [1]. The setup consisted of a concrete member with headed bars with a ratio of embedment depth-to-edge distance of less than 5. Devries et a/. constructed three large concrete blocks for these pullout tests. Each block was 530 mm deep with eight to eleven headed reinforcing bars cast within the block. The plan dimensions of the blocks were 1520*2900 mm. The bars were spaced with a centre-to-centre distance greater than three times the embedment depth to prevent overlap of the expected failure surfaces. Some of the notations and 163

3 variables tested are shown in Figures 2 and 3. Figure 1 a: Basic test setup for pull out tests for centre bars. t /\ \I -- Figure 1 b: Basic test setup for edge and comer shallow embedment pull out tests. t Figure 2a: Comparison of embedment length h.! and bonded length 4, for test set up. PVC Sheating 164

4 Figure 2b Edge distance and head parameters.. T c 1 Fig. 2b demonstrates the edge distance and the head parameters. The bonded length Ib for headed reinforcement is defined as the length along a deformed bar bonded to the concrete from the head to a critical section as shown in Fig.2-a. The minimum edge distance C 1 is defined as the distance from the closest edge of the concrete to the centre of the bar. In Fig. 2, all dimensions measured parallel with C 1 are denoted with the subscript 1, and all dimensions parallel with C 2 are denoted with the subscript Parametric studies The authors carried out a parametric investigation of concrete members with headed bars. The factors influencing the ultimate load carrying capacity of concrete members with headed bars are investigated. A design equation is proposed based on the parametric studies carried out on the experimental database. Table 1 shows the results. The proposed equation gives accurate predictions of the ultimate load carrying capacity of concrete members Influence of the embedment depth The relation between the ultimate load carrying capacity of concrete members and the embedment depth is shown in Figure 3 and Equation 1. Figure 3: The Influence of the embedment depth on the ultimate load capacity in concrete of headed reinforcement 100D00 (' I. l v = xOB167, R2 = i ISO 200 hd I center edge corner (1 ) 165

5 where P u is the ultimate load carrying capacity of concrete members with headed bars hd is the embedment depth It is apparent that as the embedment depth increases, the ultimate load carrying capacity of the headed bars increase. The embedment depth determines the size of the pullout cone and the area of the failure surface. It is thus anticipated that as the embedment depth is increased, the failure surface is increased and the resistance to loading is enhanced Influence of the head cross-sectional area The parametric study in Figure 4 clearly shows that ultimate load carrying capacity of concrete members with headed bars is related to the cross-sectional area of the head as shown in Eq. (2). P u :::::: (34.64 * Ahead ) (2) Figure 4: The influence of head area on the ultimate load capacity of concrete with headed bars..., [ ~~ ~~--~~ ~ y =34.643x ~~ ~ ~~~ ~.. ~ ~~----~--=-~ ~~ "'".,., : Head area 2.3. Influence of the location of headed bars The location of the bars has an important influence on the ultimate load carrying capacity of concrete members with headed bars. The ultimate load carrying capacities of edge bars, centre bars and corner bars are different. This is very clearly shown in Fig. 5. So a capacity reduction factor will be included into the proposed design equation to account for the location of the headed bars. This factor is the highest for the centre bars and the least for comer bars as expected. This is because, the area of the failure surface is the highest for the centre bars and the least for the corner bars. Figure 5: The differences in the ultimate load capacities of concrete with headed bars 2~00 ~ ~ z ::i Q center edge corner 2.4. The proposed design equation The proposed equation for predicting the ultimate load carrying capacity of concrete members with headed bars is shown in Eq. (3). p =P*h *(34 64 * A ) u d head (3) 166

6 where ~ = for edge bars ~ = for corner bars ~ =0.02 for centre bars hd =embedment depth Ahead =cross-sectional area of the headed bars The proposed equation is applied on the experimental database in Table 1. It is apparent that the suggested equation gives accurate predictions of the ultimate load carrying capacity of concrete members with headed bars. Table 1: The experimental database Researcher Location Specimen hd, mm fc, Mpa Ppred/Pactual center center DeVries centre T1B DeVries Centre T1B DeVries Centre T1B DeVries Centre T1B DeVries Centre T1B DeVries Centre T1B DeVries Centre T1B DeVries Centre T3B DeVries Edge T2B DeVries Edge T2B DeVries Edge T2B DeVries Edge T2B DeVries Edge T corner DeVries Corner T2B DeVries Corner T2B DeVries Corner T2B DeVries Corner T2B DeVries corner T3B average Conclusions A simple design equation is proposed to predict the ultimate load carrying capacity of concrete members with headed bars and shallow embedment depths. The present research has three results. First, the equation can account for the location of the headed bars. Second, the ultimate load carrying capacity of concrete members with headed bars is dependent on the embedment depth in the proposed model. Third the equation gives accurate predictions of the ultimate load carrying capacities. References [1] DeVries A.R., Jirsa J. And Bashandy T., Anchorage of headed reinforcement in concrete members with shallow embedment depths, ACI Structural Journal, September October