Seismic Performance of Retrofitted Reinforced Concrete Bridge Pier

ACI Structural Journal, Nov/Dec 2005 by Schoettler, Matthew J, Restrepo, José I, Seible, Frieder, Matsuda, Ed

The test protocols for both phases of testing were divided into two stages: force-controlled and displacement-controlled. Force-controlled stages were used to calculate elastic stiffness and determine the idealized lateral yield force H^sub y,L^. Upon completion of the force-controlled stage, a displacement controlled stage was conducted until failure occurred. The force controlled stage is referred to as Stage 1 whereas Stage 2 indicates the displacement-controlled loading sequence. To summarize the loading protocol, Table 1 identifies the investigations conducted for the scaled testing of Bent P16.

Stage 1 was force controlled to levels of 0.5H^sub y,L^ and 0.75H^sub y,L^. Figure 6(b) plots the load step number versus loading level for Stage 1. The value H^sub y,L^ is defined in Phase I as the lateral force in the longitudinal direction that develops the nominal strength of the pile group due to pile cap rotation, based on measured material properties. For Phase II, H^sub y,L^ is defined as the lateral force corresponding to the development of the nominal flexural strength at the base of the column, calculated with measured material properties. The load step versus displacement for Stage 2 shown in Fig. 6(c). The longitudinal yield displacement Δ^sub y,L^ for Stage 2 was defined as 4/3 times the displacement measured at 0.75H^sub y,L^. Displacements of μ^sub Δ,L^ = ±1.0, 3.0, -2, ±1.5, ±2.0, ±3.0, and ±4.0 were intended in Stage 2 until failure.

DESIGN OF TEST MODEL

Dimensional analysis

The main differences between the as-built bent and the test specimen were scaling and grade of reinforcement used. The as-built bent was constructed primarily with Grade 40 (276 MPa) reinforcement, whereas the test unit was constructed predominantly with Grade 60 (414 MPa) reinforcement. The difference of reinforcement grade was due to difficulties in sourcing deformed Grade 40 reinforcement. To meet similitude laws, the test unit design accounted for the higher strength steel. This was accomplished by varying bar spacing and bar area creating equivalent stress levels in the test specimen to those found in the actual bent.

Similitude laws were met by designing the 1:2 scale test specimen to match Bent P16 using specific nondimensional and normalized ratios (refer to Table 2 and 3 for comparisons). These ratios are denoted by the shaded areas in Table 2 and 3. To meet similitude laws, a process of comparing nondimensional ratios between the as-built bent and test specimen was implemented for each reinforcement grouping. An appropriate bar diameter was selected and spacing adjusted to match the nondimensional and normalized ratios.

The nondimensional ratios for the test model found in Table 2 and 3 were matched as closely as possible to Bent P16. The nondimensional ratios are found to have large deviations because the grade of reinforcement is not taken into account. Table 2(a), however, shows an error of 2% between the normalized values. Similar results are obtained in Table 2(b), 2(d), and 3(a). Differences are noted, however, in the pile cap reinforcement layer and retrofit overlay, refer to Table 2(c) and 3(b). The normalized ratio of the bottom pile cap layer is noticeably higher than required by similitude law. This was done intentionally to provide a capacity protected member (see the following "Construction" section).