Iman Hajirasouliha

A simplified model for seismic response prediction of concentrically braced frames

This paper proposes a simplified analytical model for seismic response prediction of concentrically braced frames. In the proposed approach, a multistory frame model is reduced to an equivalent shear-building one by performing a static pushover analysis. The conventional shear-building model has been improved by introducing supplementary springs to account for flexural displacements in addition to shear displacements. The adequacy of the modified model has been verified by conducting non-linear dynamic analysis on 5, 10 and 15 story concentrically braced frames subjected to 15 synthetic earthquake records representing a design spectrum. It is shown that the proposed improved shear-building models provide a better estimate of the non-linear dynamic response of the original framed structures, as compared to the conventional models. While simplifying the analysis of concentrically braced frames to a large extent, and thus reducing the computational efforts significantly, the proposed method is accurate enough for practical applications in performance assessment and earthquake-resistant design.

Full-Scale Shaking Table Tests on a Substandard RC Building Repaired and Strengthened with Post-Tensioned Metal Straps

The effectiveness of a novel Post-Tensioned Metal Strapping (PTMS) technique at enhancing the seismic behavior of a substandard RC building was investigated through full-scale, shake-table tests during the EU-funded project BANDIT. The building had inadequate reinforcement detailing in columns and joints to replicate old construction practices. After the bare building was initially damaged significantly, it was repaired and strengthened with PTMS to perform additional seismic tests. The PTMS technique improved considerably the seismic performance of the tested building. While the bare building experienced critical damage at an earthquake of PGA = 0.15 g, the PTMS-strengthened building sustained a PGA = 0.35 g earthquake without compromising stability.

General Seismic Load Distribution for Optimum Performance-Based Design of Shear-Buildings

An optimization method based on uniform damage distribution is used to find optimum design load distribution for seismic design of regular and irregular shear-buildings to achieve minimum structural damage. By using 75 synthetic spectrum-compatible earthquakes, optimum design load distributions are obtained for different performance targets, dynamic characteristics, and site soil classifications. For the same structural weight, optimum designed buildings experience up to 40% less global damage compared to code-based designed buildings. A new general load distribution equation is presented for optimum performance-based seismic design of structures which leads to a more efficient use of structural materials and better seismic performance.

Influence of Higher Modes on Strength and Ductility Demands of Soil–Structure Systems

Due to the inherent complexity, the common approach in analyzing nonlinear response of structures with soil–structure interaction (SSI) in current seismic provisions is based on equivalent Single Degree-of-Freedom systems (E-SDOF). This paper aims to study the influence of higher modes on the seismic response of SSI systems by performing intensive parametric analyses on more than 6400 linear and non-linear Multi Degree-of-Freedom (MDOF) and E-SDOF systems subjected to 21 earthquake records. An established soil-shallow foundation-structure model with equivalent linear soil behavior and nonlinear superstructure has been utilized using the concept of cone models. The lateral strength and ductility demands of MDOF soil–structure systems with different number of stories, structure-to-soil stiffness ratio, aspect ratio and level of inelasticity are compared to those of E-SDOF systems. The results indicate that using the common E-SDOF soil–structure systems for estimating the strength and ductility demands of medium and slender MDOF structures can lead to very un-conservative results when SSI effect is significant. This implies the significance of higher mode effects for soil–structure systems in comparison with fixed-based structures, which is more pronounced for the cases of elastic and low level of inelasticity.

A new hybrid method for size and topology optimization of truss structures using modified ALGA and QPGA

AbstractModified Augmented lagrangian genetic Algorithm (ALGA) and Quadratic Penalty Function genetic Algorithm (QPGA) optimization methods are proposed to obtain truss structures with minimum structural weight using both continuous and discrete design variables. To achieve robust solutions, Compressed sparse Row (CSR) with reordering of Cholesky factorization and Moore Penrose Pseudoinverse are used in case of non-singular and singular stiffness matrix, respectively. The efficiency of the proposed nonlinear optimization methods is demonstrated on several practical examples. The results obtained from the Pratt truss bridge show that the optimum design solution using discrete parameters is 21% lighter than the traditional design with uniform cross sections. similarly, the results obtained from the 57-bar planar tower truss indicate that the proposed design method using continuous and discrete design parameters can be up to 29% and 9% lighter than traditional design solutions, respectively. through sensitiv...

Modelling of Earthquake Hazard and Secondary Effects for Loss Assessment in Marmara (Turkey)

This study proposes an innovative Earthquake Risk Assessment (ERA) framework to calculate seismic hazard maps in regions where limited seismo-tectonic information exists. The tool calculates the seismic hazard using a probabilistic seismic hazard analysis (PSHA) based on a Monte-Carlo approach, which generates synthetic earthquake catalogues by randomizing key hazard parameters in a controlled manner. All the available data was transferred to GIS format and the results are evaluated to obtain a hazard maps that consider site amplification, liquefaction susceptibility and landslide hazard. The effectiveness of the PSHA methodology is demonstrated by carrying out the hazard analysis of Marmara region (Turkey), for which benchmark maps already exist. The results show that the hazard maps for Marmara region compare well with previous PSHA studies and with the National Building Code map. The proposed method is particularly suitable for generating hazard maps in developing countries, where data is not available or easily accessible.

Performance-based optimisation of RC frames with friction wall dampers using a low-cost optimisation method

Friction-based dampers can be considered as one of the suitable passive control systems for seismic strengthening and rehabilitation of existing substandard structures due to their high adjustability and good energy dissipation capability. One of the main issues in the design of these systems is to obtain the magnitude of the maximum slip force and the distribution of slip forces along the height of the building. In this study, a practical performance-based optimisation methodology is developed for seismic design of RC frame buildings with friction energy dissipation devices, which allows for an accurate solution at low computational cost. The proposed method aims at distributing the slip loads of the friction dampers to achieve a uniform distribution of damage along the height of the building. The efficiency of the method is evaluated through the optimum design of five different low to high-rise RC frames equipped with friction wall dampers under six natural and six synthetic spectrum-compatible earthquakes. Sensitivity analyses are performed to assess the reliability of the method using different initial height-wise slip load distributions, convergence parameters and earthquake records. The results indicate that optimum frames exhibit less maximum inter-storey drift (up to 43%) and global damage index (up to 75%), compared to uniform slip load distribution. The method is then developed to obtain the optimum design solution for a set of earthquakes representing a design spectrum. It is shown that the proposed method can provide an efficient tool for optimum seismic design of RC structures with friction energy dissipation devices for practical purposes.

Nonlinear behaviour of reinforced concrete flat slabs after a column loss event

Previous studies have demonstrated that reinforced concrete flat slab structures could be vulnerable to progressive collapse. Although such events are dynamic, simplified static analyses using the sudden column loss scenario are often used to gain an indication into the robustness of the structure. In this study, finite element analysis is used to replicate column loss scenarios on a range of reinforced concrete flat slab floor models. The model was validated against the results of scaled-slab experiments and then used to investigate the influence of different geometric and material variables, within standard design ranges, on the response of the structure. The results demonstrate that slab elements are able to effectively redistribute loading after a column loss event and therefore prevent a progressive collapse. However, the shear forces to the remaining columns were 159% of their fully supported condition and increased to 300% when a dynamic amplification factor of 2.0 was applied. It is shown that this can potentially lead to a punching shear failure in some of the slab elements.

Analytical Study of the Seismic Performance of Steel-Braced Frames with Masonry Infill

AbstractSpecial concentrically braced frames (CBFs) are widely used as efficient lateral-load resisting systems in seismic regions. In this study, experimentally validated finite-element (FE) models are used to investigate the effects of masonry infill and gusset-plate configuration on the seismic performance of CBFs. It is shown that the presence of masonry infill can increase the initial stiffness and ultimate strength of CBFs by up to 35 and 52%, respectively. However, the frame-infill interaction imposes high plastic strain demands at the horizontal re-entrant corner of gusset plate connections, which may lead to premature failure of fillet welds under strong earthquakes. Whereas using tapered gusset plates can significantly increase the fracture potential at fillet welds, gusset plates with elliptical clearance of eight times the plate thickness can lead to up to 54% lower equivalent plastic strain demands at both gusset plate connections and brace elements. Although the effects of masonry infill are...

Bond of Substandard Laps in Reinforced Concrete Beams Retrofitted with Post-Tensioned Metal Straps

This paper investigates the bond-splitting performance of lapped steel bars in tension. Twelve reinforced concrete (RC) beams with substandard laps (lap length = 25 bar diameters) at midspan were tested in flexure. The variables examined include the bar diameter, concrete cover, and three different confinement conditions at midspan: 1) no confinement; 2) internal steel stirrups; and 3) external post-tensioned metal straps (PTMSs). The test results show that, in comparison to unconfined specimens, PTMS confinement enhances the bond strength of the lapped bars by up to 120%. Based on the results of this study, an innovative equation is proposed and validated to calculate the additional bond strength provided by PTMS confinement. The equation can be used for assessment and retrofit of substandard lapped bars in RC structures and should prove useful in practical retrofitting applications.