Chapter 12 - Pseudo-Dynamic Tests and Analysis on Semi-Rigidly Jointed Steel Frames

Eight prestressed T-shaped beams were tested using the cyclic load test (CLT) method as proposed by ACI 437-12 followed by the ACI 318-11 monotonic (24-hour) load test method. The objective of the study is to assess the ability of these methods to evaluate damage in prestressed concrete (PC) beams. The test matrix included both pristine beams (subjected to no prior loading) as well as beams that were cracked and artificially predamaged using accelerated corrosion techniques, impressed current, and wet/dry cycles, prior to load testing. Deflections, crack widths, and slipping of the prestressing strands were recorded during the load tests. The load at which the monotonic test was conducted was chosen to be greater than the service load of Class U PC members, which does not allow cracking. This ensured that at the time of the monotonic load test the specimens were significantly damaged. However, the acceptance criteria associated with this test methodology were still met. Only one index in the CLT acceptance criteria (deviation from linearity) identified the condition of the specimens. The deviation from linearity index is found to correlate to the opening and widening of cracks.

Pipes can be damaged by strong external forces such as earthquakes. In particular, gas leaks and interruptions to water supply can cause serious secondary damage. This is one of the damage factors caused by earthquakes in urban areas. Bellows-type expansion joints for pipes are flexible and have low rigidity and good adaptability to relatively large deformations. Therefore, bellows-type expansion joints can be applied as seismic separation joints to improve the seismic performance of pipes. In this study, seismic performance considering low cycle fatigue was estimated for bellows-type expansion joints. Monotonic and cyclic loading tests for bending were performed on 80A 3ply bellows- type expansion joints manufactured by laminating three layers of 0.3 mm stainless steel plates. In the cyclic loading test, the multi-step increasing amplitude cyclic loading test to which the displacement history is amplified stepwise was applied and a constant-amplitude cyclic loading test considering various amplitudes was used to estimate the low-cycle fatigue life. In the case of the constant- amplitude cyclic loading tests, low cycle fatigue behavior was observed for 1/4 amplitudes of leakage displacements of monotonic loading tests.

Effectiveness of steel bracing system installed with fully mechanical interfaces is experimentally demonstrated through a series of monotonic and cyclic loading tests. Also, substructuring pseudo-dynamic earthquake response tests are performed on a possible situation of a two-story braced frame upgraded by the proposed bracing system. An earthquake record and theoretical impulses are adopted as input excitations. The results show that an impulsive excitation acts more stringently on the occurrence of brace breaking.

For mooring systems, the relationship between chain axial resistance and confining stress affects the tension transfer of embedded mooring chain, and an effective width parameter Et was adopted to reflect the complex geometry of the chain. However, the relationship in carbonate sand is fully understood. Designed as an element test, monotonic and cyclic loading tests were conducted to investigate the variation of chain axial resistance with confining stress under different conditions in South China Sea carbonate sand. Peak effective width parameter and secant coefficient were particularly analyzed to describe the gradual mobilization. The results show that the Et value in South China Sea carbonate sand has a mean increase of 15% compared with Pingtan quartz sand, which makes the chain harder to pull into the carbonate sand. The irregular particle shape may contribute to the higher axial resistance. In cyclic tests, the peak post-cyclic Et-m is larger than that of monotonic test under lower confining stress but is smaller under higher stress. This paper provides some references to chain profile prediction in the carbonate sand.

In recent years, both pseudo-dynamic and real time heterogeneous testing with dynamic substructuring — hybrid testing — have gained significant popularity for their applicability to testing several types of nonlinear structures/systems. In a hybrid test, a heterogeneous model of the emulated system is created by combining a Physical Substructure (PS) with a Numerical Substructure (NS) that describes the remainder of the system. Nevertheless, an efficient implementation of this technique requires overcoming certain problems, e. g., proper dynamic substructuring, reduction of external forces and actuator delay compensation. This paper presents a pseudo-dynamic test campaign undertaken by the University of Trento, Italy, on a typical full-scale industrial piping system subjected to earthquake loading in order to investigate its seismic performance. Some challenges faced during the implementation are shown and strategies adopted to overcome these problems are illustrated. Experimental activities will be described and performances of different components of the piping system, i.e., elbows, tee-joints, bolted flange joints and straight pipes under earthquake loading with the presence of an internal pressure of 3.2 MPa will be presented and commented.

This paper develops a novel dry connection utilizing high‐strength bolts and introduces the corresponding low‐rise precast wall panel structure system. To investigate the seismic performance of the structure system with full bolt connections, monotonic loading tests of the connection joint and cyclic lateral loading tests of three full‐scaled precast shear walls are both conducted. Based on the test data, axial and shear mechanical models of the connection are given. Meanwhile, experimental results show that the failure mode of the connection is dominated by anchored rebar ductile rupture, and the precast structure system presents a stable energy dissipation capacity and a good seismic ductility. The numerical model of the precast shear wall is then developed and validated by the cyclic loading test. Also a simplified calculation method to predict the lateral strength of the precast shear wall is proposed. According to the calculation results, the distance between the center of the connection and the edge of the shear wall is suggested to be 150 mm, while the wall thickness is recommended to be 120 mm or 150 mm. Finally, a three‐story precast wall panel structure is employed to assess the collapse performance of the proposed precast structure system by using the presented numerical model. The results indicate that the proposed structure system with full bolt connections has high stiffness and high seismic resistance against collapse.

In this study, a new technique for seismic retrofitting via the attachment of an “external steel reinforced concrete frame” (ESRCF) system was developed to strengthen medium-to-low-rise reinforced concrete (RC) buildings. Two methods (bolting and welding) were developed to connect existing RC frames and external strengthening elements; these methods are technically and practically suited to various construction conditions. The retrofitting method developed in this study can be used to perform seismic strengthening construction, while residents continue to live within the building. The method is categorized as a “strength design approach” implemented via retrofitting, allowing the lateral ultimate load capacity of RC buildings, whose failure mode is shear, to be increased easily. Test specimens were designed based on an existing RC building in Korea lacking seismic data and then strengthened using the ESRCF system. Pseudodynamic and cyclic loading tests were conducted to verify the effects of seismic retrofitting. In total, four RC frame specimens were prepared: one nonstrengthened control specimen for the cyclic loading test, one nonstrengthened control specimen, one specimen strengthened with a welded ESRCF system, and one specimen strengthened with a bolted ESRCF system for the pseudodynamic test. The earthquake response behavior with use of the proposed method, in terms of the maximum response strength, response displacement, and degree of earthquake damage, is compared with a control RC frame. The test results indicated that both the bolting and welding methods used for connecting the existing RC frame to the ESRCF effectively increased the lateral ultimate strength, resulting in reduced response displacement of building structures under large-scale earthquake conditions.

Seismic retrofitting techniques using an energy absorption segment for steel bridge piers

Testing of a transfer plate by pseudo-dynamic test method with substructure technique

The effectiveness of seismic retrofitting of multi-storey multi-bay reinforced concrete (RC)-frame buildings by converting selected bays into new walls through infilling with RC was studied experimentally at the ELSA facility of the Joint Research Centre in Ispra (Italy). A full-scale model was tested with the pseudo-dynamic (PsD) method and consisted of two four-storey (12 m tall) three-bay (8,5 m long) parallel frames linked through 0,15 m slabs with the central bay (2,5 m) infilled with an RC wall. The frames were designed and detailed for gravity loads only and were typical of similar frames built in Cyprus in the 1970s. Different connection details and reinforcement percentages for the two infilled frames were used in order to study their effects in determining structural response. The results of the pseudo-dynamic and cyclic tests performed on the specimen are presented, and conclusions are drawn.

A series of bridge models (1:2.5 scale) will be tested in the European Laboratory for Structural Assessment (ELSA) of the Joint Research Centre of the Commission of the European Communities (CEC). This activity is part of a pre-normative research project in support of Eurocode n. 8 under the Human Capital Mobility Programme of the CEC aiming at the improvement of design procedures for bridges in seismic regions. The structures will be tested pseudo-dynamically using the substructuring technique; the piers are tested physically whereas the deck is simulated on-line analytically. Results from these tests will certainly provide useful data for the calibration of analytical models and consequently to assess and improve the current seismic design procedures for bridges. The study herein reported intends to contribute to the success of the programme focusing mainly on the influence of the connections between the piers and the superstructure (the interface between the physical and the analytical structural parts) on the seismic response. Moreover, results from dynamic non-linear analyses are compared with the results from the numerical pseudo-dynamic (PSD) test simulation. In addition, a short description of the substructure technique in PSD testing, as implemented in ELSA, is made.

The bond-slip behavior of H-shaped steel embedded in UHPC under reversed cyclic loading

Triangular-plate added damping and stiffness (TADAS) devices are reliable metallic energy dissipaters for seismic upgrading used in design and retrofitting of civil structures. Conventional TADAS devices are designed with closed-ended slots. In this study, a modified form of the TADAS device is proposed with open-ended slots in order to reduce the manufacture cost, facilitate the assembling and avoid abrupt stiffness increase. Cyclic and monotonic loading tests are then conducted to investigate the mechanical characteristics of the modified TADAS devices with regular Q345 steel and low-yield point LY160 steel triangular plates. The test results show that although the hysteresis performances are stable, the cyclic hardening behavior is different between the TADAS specimens with different grades of steel. The TADAS specimen with LY160 triangular plates exhibits more significant overstrength behavior than the one with Q345 triangular plates in cyclic loading, which is unsuitable to be described by the classic Bouc-Wen model. Therefore, a modified Bouc-Wen model is proposed to describe such overstrength behavior. It is shown that the modified model is able to simulate different extent of overstrength behavior in cyclic loading, based on which the cyclic hardening behavior of the TADAS specimen with LY160 triangular plates can be well described.

In our previous study, great increases of hysteretic damping and initial slip resistance of timber joints were attained by applying axial pretension to the steel fasteners. To evaluate the effectiveness of this method, 1-year stress-relaxation measurement was carried out. Nine prestressed joints were prepared and three of them were restressed after 3 and then 6 months after the initial prestressing. All joints were exposed to indoor conditions, and relaxation of the pretension was regularly measured from time-dependent decreases of axial strain of the bolts. After measurement, the joints were subjected to cyclic and monotonic loading tests until failure. The average ratio of residual stress to the initial prestress after 1 year was about 0.23 and 0.66, respectively, for joints without restressing and those with restressing. A simulated stress-relaxation curve developed from the four-element relaxation model predicted 3% of the initial stress after 5 years. Without a regular restressing program, the initial prestressing effect therefore must be considered negligible. However, about 20% of the pre-stress level can be reasonably assumed if restressing is carried out annually. This small residual stress was found to introduce suffi cient frictional damping to signifi cantly increase the equivalent viscous damping ratio of the joints.

The cracking of concrete flanges in the negative bending moment region has always constrained the development of steel-concrete composite continuous beams. The proposal of steel-concrete composite-laminated beam is one of the effective ways to solve this problem. As a new type of structure, the research on its mechanical properties is not yet sufficient. In order to study the residual mechanical properties of composite-laminated beams after cyclic load tests, static load tests were conducted on one composite-laminated beam specimen, and cyclic load and final failure static load tests were conducted on three composite-laminated beam specimens. The experimental results show that the fatigue failure mode of the composite-laminated beam was characterized by uplift-restricted and slip-permitted connectors failure, and the steel beam was still in an elastic state at this time. Compared with the static load specimen, the stiffness of the specimen that failed in the cyclic loading test decreased by about 30%, the bearing capacity decreased by about 10%, and the residual bearing capacity and ductility were basically stable, without affecting the normal operation of the specimen. Therefore, the new structure not only has good static performance, but also has good fatigue resistance.