RESISTANCE FACTORS FOR ECCENTRICALLY BRACED STEEL FRAMES PROPORTIONED USING CAPACITY DESIGN

Components proportioned using Capacity-Based Design have resistances that exceed the forces that can be generated by adjacent elements, particularly ductile elements in seismic-load- resisting systems. This thesis investigates the reliability indices for compression braces in Eccentrically Braced Frames (EBF) proportioned using Capacity-Based Design. The primary research objectives are: (1) to develop statistical models for link overstrength due to strain hardening and probable yield strengths; (2) to examine, using these models, the reliability indices obtained using the current Capacity-Based Design provisions for EBFs in CSA SI6-09 (CSA, 2009); and (3) to calibrate the resistance factor for a proposed new overstrength model to replace the link overstrength criteria in CSA SI6-09. The statistical models for link overstrength account for the effect of the normalized link length, the plastic link rotation, the ratio of ultimate to yield strength and the normalized web stiffener spacing and are determined using a database of 77 EBF tests performed by others. The statistical model for higher-than-nominal yield strength is determined using a database of 7717 tests of coupons from Class 1 sections presented by Schmidt (2000). Reliability indices,/?, are computed using the First Order Second Moment (FOSM) method. The reliability indices for the current code provisions vary from 1.30 to values above 5.0 for typical ranges of link parameters. A new design equation is proposed that provides much more uniform reliability. Compression braces designed using the proposed design equation require the resistance factor 0 be decreased to 0.75 from the current value of 0.9 to achieve acceptable reliability levels

Evaluation of seismic displacement demand for unreinforced masonry shear walls

Unreinforced, non-engineered low-strength brick masonry structures comprise a large percentage of buildings in the Himalayan region and have been extensively damaged in recent earthquakes. Due to the high seismic hazard of the region and the inherent vulnerability of non-engineered masonry structures, a seismic assessment of masonry construction in this region is imperative. In this study, a suite of strong ground motions is developed using data from major Himalayan earthquakes. Using a mechanistic-based procedure for predicting the monotonic load envelope which identifies limit states of cracking, strength, and collapse using stress-based criteria, a hysteretic model was calibrated to experimental data of unreinforced masonry shear walls. Nonlinear time history analyses are performed on the validated single degree of freedom models of two unreinforced masonry walls. The analytical results correlate well with observed damage to masonry structures in Himalayan earthquakes. Peak ground acceleration of ground motion is observed to be the key parameter influencing displacement of walls. A linearly increasing trend is observed between the PGA and the observed displacement up to a PGA value of 0.1g. A weak correlation is observed between displacement and ground motion frequency parameters

Finite element analysis of unreinforced masonry walls with different bond patterns

Masonry is the oldest building material, yet it is also the least understood due to the non-linear and composite nature of masonry, which consists of brick units, mortar, and unit-mortar contact. In this paper, the response of a two-dimensional masonry wall with a window opening subjected to an in-plane lateral pushover loading is simulated by varying the interface properties of brick such as crushing, elastic, cracking, and shear properties. The simplified micro-modeling technique with the Engineering Masonry model for bricks and linear stiffness properties for the interfaces in the bed and head joints is employed to investigate the geometric nonlinear behavior of the masonry wall. The pushover curves obtained from the numerical simulations indicate that there is a significant influence on the lateral load response of the wall due to elastic, crushing, and shear parameters while the cracking parameters have less impact on the ductile capacity of the structure. Moreover, the study is also extended to examine the effect of bond patterns such as English, Stretcher, Flemish, and Header bond with varied aspect ratios of 1,1.5 and 0.75. In all four bond patterns, it was observed that the walls with lower aspect ratios exhibited higher strength. Further, in comparison to the other bond patterns, walls with the Flemish bond pattern demonstrated higher strengths at both lower and higher aspect ratios

Numerical simulation of soft brick unreinforced masonry walls subjected to lateral loads

Unreinforced soft-brick masonry structures comprise a large portion of buildings in seismic prone regions of India. Recent earthquakes have extensively damaged such structures. Experimental and numerical studies on soft brick unreinforced masonry structures are scarce in literature. In this paper, numerical simulation of soft brick unreinforced masonry panels subjected to lateral loads is performed. At the panel level, the masonry structure is modelled using the micro modelling approach within a finite element framework. Monotonic load envelope curve is generated through a pushover analysis of the finite element model. The in-plane failure modes are identified, and the effect of aspect ratio and axial stress on failure is examined. Cyclic load analyses are performed on the finite element model to understand the complex hysteretic behaviour of unreinforced soft-brick masonry walls

Correlating Peak Ground A/V Ratio with Ground Motion Frequency Content

Peak ground A/V ratio has been used an empirical parameter to estimate ground motion frequency content and categorize ground motion suites for performing nonlinear time history analyses of structures. Ground motions are usually classified into three subjective categories: low A/V, intermediate A/V, and high A/V to reflect low-, moderate-, and high-frequency contents, respectively. However, the relationship between A/V and frequency is very complex as the A/V ratio depends on faulting processes, distance from source to recording station, and local geological conditions. Frequency content of earthquake waves is represented by single-frequency parameters, such as mean period, Tm , and predominant period, Tp . In this paper, the relationship between A/V and frequency parameters is explored. Linear regression analyses are performed on data obtained from three major earthquakes. Regression analyses indicate that the predominant frequency content of the ground motion exhibits little or no correlation with A/V ratio. The A/V ratio may be used as an empirical parameter to obtain an estimate of only the mean frequency content of the ground motion