r - The Hong Kong Polytechnic University

failure mode specimens, applying CFRP directly can’t provide enough confinement stress to increase frictional
force between the lap-spliced longitudinal reinforcements
From the other test results for seismic retrofit of circular RC bridge column using steel jacketing and CFRP
(Hwang and Hseih 1999; Hwang and Kuo 2000) concluded that the retrofit using steel jacketing is effective in
enhancing the seismic resistance of existing circular RC bridge column. With steel jacketing and CFRP
jacketing, the failure mode changes from flexural failure to the breaking of longitudinal bar in the bottom of the
columns, and the ductility and maximum lateral force have increased. For lap splice failure mode specimens,
using steel and CFRP jacketing can tremendously increase the confinement strength and ductility of bridges
columns. Also, the more layers of CFRP can obtain higher ductility. On the other hand, for shear failure mode
specimen, even though using steel and CFRP jacketing can also tremendously increase the confinement strength
and ductility of bridge columns, more layers of CFRP may not have higher ductility.
ROCKING EXPERIMENTS FOR BRIDGE PIERS WITH SPREAD FOOTINGS
Background and Objectives
After chi-chi earthquake, the design earthquake intensities of some area in Taiwan were shifted to a higher value;
thus a great number of bridges need to be retrofitted. As the retrofitting work leads to a higher plastic moment
capacity of the columns, the design force for the foundation needs to be increased based on capacity design. In
order to satisfy the stability check of a spread footing under the application of this plastic moment transferred from
the column base, some of the retrofitting works resulted in uneconomically large spread footings. Some newly
designed engineering practices also met the similar situation. According to previous design code in Taiwan, the
footing uplift involving separation of footing from subsoil is permitted only up to one-half of the foundation base
area as the applied moment reaches the value of plastic moment capacity of the column. The reason for this
provision is that rocking of spread footings is still not a favorable mechanism. However, recent researches have
indicated that rocking itself may not be detrimental to seismic performance and in fact can act as a form of seismic
isolation mechanism. In order to gain a better understanding of the problem of rocking and then to get more
confidence to update the seismic design code and seismic evaluation guidelines, two series of rocking
experiments were performed at NCREE.
Experimental Program
For the first series of experiments (Hung et al. 2008; 2010a), a total of three circular RC columns with spread
footings were tested. Using pseudo-dynamic tests and a cyclic loading test, these columns were subjected to
different levels of earthquake accelerations, including a near field ground motion. The focus of this experiment
was to investigate the rocking behavior of both lightly transverse reinforced columns and retrofitted columns in
order to clarify that if the widening and strengthening of the foundations to limit the rocking mechanism of spread
footing is necessary for the retrofit work. These three columns were named specimens A, B and C, respectively.
These columns measured 60 cm in diameter with a clear height of 3.4 m, were poorly confined and were
lap-spliced above the top of the foundation. The columns were reinforced with 26-D19 longitudinal reinforcing
bars, and were transversely reinforced with D10 perimeter hoops spaced 12.7 cm apart, corresponding to an
insufficient volumetric confinement ratio of ρ s = 0.0039. Other material properties for these test columns were as
follows: concrete compressive strength f c ’ = 278 kg/cm 2 ; yield strength of longitudinal reinforcements F y = 3840
kg/cm 2 . In order to investigate the rocking behavior of a retrofitted column with a ductility capacity that meets
the requirement specified by the design code, one of the test columns, specimen C, was wrapped with 6 mm thick
A36 steel plate jacketing with a length of 150 cm. During the tests, one as-built test column (specimen B) and the
retrofitted column (specimen C) were rested on a neoprene pad to allow the rocking to take place. Another as-built
column (specimen A) was constrained to the strong floor during testing to represent a benchmark test with fixed
base condition. The summary of the test sequence is shown in Table 4. In this table, TH1 and TH2 represent a
code compatible medium earthquake and a code compatible design earthquake, respectively. TH3 and TH4 are
the near-field ground motions recorded during Chi-Chi earthquake, but were scaled to have the same PGA of the
code compatible ones TH1 and TH2, respectively.
Fig. 3 illustrates the test setup. In the case where the rocking mode of the foundation was restrained (Fig. 3a), four
tie-down rods were placed through the foundation and anchored into the strong floor of the laboratory. In the case
where the rocking mechanism was considered (Fig. 3b), and the square footings were rested on a neoprene pad,
simulating a spread footing foundation in a stiff soil. By comparing the experimental response of the retrofitted
column with that of the as-built one, the interaction effect of the rocking on the ductility demand and the strength
demand of the columns was identified. A critical side effect of increasing the displacement response at the deck
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