Shih Ho Chao

A Seismic Design Lateral Force Distribution Based on Inelastic State of Structures

It is well recognized that structures designed by current codes undergo large inelastic deformations during major earthquakes. However, lateral force distributions given in the seismic design codes are typically based on results of elastic-response studies. In this paper, lateral force distributions used in the current seismic codes are reviewed and the results obtained from nonlinear dynamic analyses of a number of example structures are presented and discussed. It is concluded that code lateral force distributions do not represent the maximum force distributions that may be induced during nonlinear response, which may lead to inaccurate predictions of deformation and force demands, causing structures to behave in a rather unpredictable and undesirable manner. A new lateral force distribution based on study of inelastic behavior is developed by using relative distribution of maximum story shears of the example structures subjected to a wide variety of earthquake ground motions. The results show that the suggested lateral force distribution, especially for the types of framed structures investigated in this study, is more rational and gives a much better prediction of inelastic seismic demands at global as well as at element levels.

Bond Behavior of Reinforcing Bars in Tensile Strain-Hardening Fiber-Reinforced Cement Composites

Bond between deformed reinforcing bars and concrete induces significant tensile stresses that lead to cracking in concrete due to its weak and brittle nature in tension. Contrary to plain concrete and conventional fiber-reinforced concrete, high-performance fiber-reinforced cement composites (HPFRCC S ) show strain-hardening response under tension and, thus, their use can lead to enhanced bond performance. Pullout-type tests comprising various types of loadings were carried out to investigate the influence of strain-softening and strain-hardening fiber-reinforced cementitious (FRC) composites on the bond strength and the bond stress-slip response of deformed reinforcing bars. Test results showed that the bridging effect provided by fibers in FRC composites after cracking can effectively provide post-clacking tensile capacity to the concrete matrix and limit crack width, thereby leading to enhanced bond resistance. HPFRCC specimens gave the best bond performance in terms of bond strength and stiffness retention capacity, as well as damage-control ability.

Seismic Behavior of Steel Buildings with Hybrid Braced Frames

AbstractThis paper presents the seismic performance of buildings with a hybrid bracing system in which buckling-restrained braces (BRBs) are used at the lower stories with the higher level of ductility demands, and conventional buckling-type braces consisting of steel hollow structural sections (HSS) are used in the upper stories with relatively smaller ductility demands to minimize the probability of their fracture under reversed cyclic displacements. This type of hybrid braced frame (HBF) could prove economical, especially for retrofitted buildings in seismically active regions. A series of nonlinear time-history analyses was conducted to investigate the seismic performance of 3- and 6-story buildings with the hybrid bracing system. The main parameters studied are the seismic design parameters, maximum interstory drift ratios, fracture response of the HSS, and the optimal deployment of the BRBs. The seismic performance of the HBFs was compared with conventional concentrically braced frames and buckling-...

Use of Double Punch Test to Evaluate the Mechanical Performance of Fiber Reinforced Concrete

Mechanical properties of fiber reinforced concrete (FRC) determined by material test methods can be used to ensure that the FRC mixture is batched properly and to give indications of performance if used in structural members. An ideal material test method for FRC should give low variability in the measurement of properties such as peak and residual strengths. ACI 318-08 uses results from the third-point bending test [1] as the performance criteria for FRC. Experimental evidence, however, has indicated that this bending type test method has potential problems in the reliability of determining the peak and residual strengths. The coefficient of variation of residual strength is typically very high and generally greater than 20%. The considerable scatter results make it difficult for quality control, particularly when such properties are intended to be used to estimate the strength of structural members. In addition, the complex test setup and the requirement of using a closed-loop servo-controlled machine often prevent its use in small laboratories. Other test methods such as the direct tensile test experience similar problems. As a consequence, these test methods are generally time consuming and expensive as they require more specimens to obtain reliable test data.

Stiffness-based design for mitigation of residual displacements of buckling-restrained braced frames

AbstractBuckling-restrained braced frames (BRBFs) are often used as primary seismic force resisting systems to achieve the desired seismic performance of building frames. However, significant residual displacements of these systems may cause concerns on their postearthquake performance, especially under long duration earthquakes and strong aftershocks. Although backup moment frames (MFs) along with the BRBFs, referred to as dual systems, can be used to mitigate their residual interstory drifts, there is no design criterion on how to determine the section sizes in the MF for this purpose. In the research reported in this paper, a simple mitigation technique was developed, which relies on a stiffness-based design in which the residual interstory drifts can be effectively controlled by increasing the elastic story stiffness using column sections with a higher moment of inertia. Nonlinear time-history analyses were carried out on low-to-high rise BRBFs for a set of 20 design basis earthquake (DBE) level groun...

A New Data Set for Full-Scale Reinforced Concrete Columns under Collapse-Consistent Loading Protocols

A series of eight full-scale reinforced concrete column tests was recently carried out at the NEES (Network for Earthquake Engineering Simulation) Multi-Axial Subassemblage Testing (MAST) site at the University of Minnesota as part of a National Science Foundation (NSF) NEES research program. The tests were conducted to address the shortcomings in the available database of reinforced concrete (RC) columns tested with large drift ratios under monotonic and cyclic loading protocols. The specimens were designed based on ACI 318-11 and featured two different cross-sectional dimensions, both larger than nearly all of the columns tested previously. They were subjected to several large displacement loading protocols, including a monotonic and a cyclic biaxial loading protocol. Also, to investigate the effectiveness of novel materials, one specimen was constructed with ultra-high-performance fiber-reinforced concrete (UHP-FRC). This paper presents a description of and potential uses for the data set that is made accessible via a digital object identifier (DOI) (data set DOIs: 10.4231/D33T9D65T, 10.4231/D3028PD2G, 10.4231/D3V97ZR8Z, 10.4231/D3QN5ZB62, 10.4231/D3KW57J3S, 10.4231/D3G44HQ9B, 10.4231/D3BC3SX4Q, and 10.4231/D36M3340C).

Experience with Self-Consolidating High-Performance Fiber-Reinforced Mortar and Concrete

This paper discusses how self-consolidating high performance fiber reinforced cementitious composites (SC-HPFRCC) combine the self-consolidating property of self-consolidating concrete (SCC) in their fresh state, with the strain-hardening and multiple cracking characteristics of high-performance fiber-reinforced cement composites (HPFRCC) in their hardened state. The paper introduces two different classes of SC-HPFRCC: concrete based and mortar based. They all contain 30 mm long steel fibers in volume fractions of 1.5% and 2%, and exhibit strain- hardening behavior in tension. These mixtures are highly flowable, non-segregating and can spread into place, fill the formwork, and encapsulate the reinforcing steel in typical concrete structures. Six concrete based SC-HPFRCC mixtures, with compressive strengths ranging from 35 to 66 MPa (5.1 to 9.6 ksi), were successfully developed by modifying SCC mixtures recommended in previous studies and using the available local materials. Spread diameter of the fresh concrete based SC-HPFRCC mixtures measured from the standard slump flow test was approximately 600 mm (23.6 in.). Strain-hardening characteristics of the hardened composites were ascertained from direct tensile tests. Three mortar based SC-HPFRCC mixtures with 1.5% steel fiber content were also developed and exhibited average compressive strengths of 38, 50 and 106 MPa (5.5, 7.2 and 15.3 ksi), respectively. Recent structural large scale laboratory applications (structural wall, coupling beams, panels etc.) made of SC-HFPRCC have demonstrated the applicability of these mixtures.

Local Bond Stress-Slip Models for Reinforcing Bars and Prestressing Strands in High-Peformance Fiber- Reinforced Cement Composites

This paper discusses previous studies that use pullout-type tests comprising monotonic, unidirectional cyclic, and reversed cyclic loads have shown that bond between reinforcing bars/prestressing strands and concrete can be significantly enhanced by replacing the conventional concrete with high-performance fiber-reinforced cement composites (HPFRCCs). This can be attributed to the fact that, compared to plain concrete and conventional fiber-reinforced concrete (FRC), HPFRCCs exhibit a strain-hardening response under tension up to large strains, thereby preventing the concrete from deterioration under bond action. Pullout test results provide the bond stress versus slip relationship that can be considered the constitutive property of the steel-to-HPFRCC interface. Since the post-cracking tensile stress and strain of fiber-reinforced cement composites are the fundamental characteristics that distinguish them from conventional concrete, the HPFRC tensile stress-strain response obtained from direct tensile tests was used to derive the local bond stress-slip models presented in this paper. This paper shows that the proposed models are more concise than previous models suggested for FRC and give good agreement with test results.

Crack opening evaluation and sustainability potential of highly flowable strain-hardening, fiber-reinforced concrete (HF-SHFRC)

Highly flowable, strain-hardening fiber-reinforced concrete (HF-SHFRC) has good workability in the fresh state, and it exhibits the strain-hardening and multiple-cracking characteristics of high-performance, fiber-reinforced cementitious composites in the hardened state. HF-SHFRC can be easily manufactured and delivered by ready-mix trucks for cast-on-the-job sites. Structural large-scale test results from several research programs also showed that HF-SHFRC is effective in increasing shear strength, displacement capacity, and damage tolerance in members subjected to large inelastic deformations. The results of two tests, a long prismatic tensile test with continuous reinforcement and an in-plane pure shear panel test, are summarized in this paper. Relative to conventional concrete, HF-SHFRC not only demonstrates much better mechanical performance, but also presents reduced crack potential and excellent crack width control. These characteristics of HF-SHFRC can further diminish the need for repairs, rehabilitation, and maintenance after extreme loading events and give infrastructure a longer service life, which will eventually lower the life-cycle cost.

Use of Steel Fiber Reinforced Concrete for Enhanced Performance of Deep Beams with Large Openings

Reinforced concrete deep beams are used as primary load distribution elements in various civil engineering structures. Large openings often interrupt the load transfer by concrete struts in these beams and cause a sharp decrease in strength and serviceability. Although the strength evaluation and reinforcement details around the openings are essential considerations, the ACI Building Code does not provide explicit guidance for designing these elements with openings. Strut-and-tie models are commonly used for strength evaluation and design of deep beams with openings. However, reinforcement detailing based on these models can be very complex and the failure of deep beams may be due to localized damages that could not be predicted by the strut-and-tie models. In this study, an experimental investigation was conducted on two concrete deep beam specimens with large single opening, namely, reinforced concrete (RC) and steel fiber reinforced concrete (SFRC), to evaluate their performance under monotonically increased load. The reinforcement detailing in the SFRC specimen was considerably reduced since the steel reinforcement bars were only used for the tensile longitudinal reinforcements and the boundary elements. Both test specimens had significantly higher strength than the designed load computed based on one assumed strut-and-tie model.