Dr. Núñez-Castellanos, Eduardo

In this numerical research, the variation of cyclic behavior of beam-to-box column connection was studied. Dimensional and load conditions were parametrically evaluated with the goal of assessing the applicability and use of this biaxial moment connection according to the Seismic Provisions, such as bending strength and rotation capacity, secant and tangent stiffness, dissipated energy and strong column-weak beam relationship. A total of 83 different models of 3D connections were developed using in ANSYS software with the load at the top of the column. Results show a cyclic behavior not controlled by axial load. However, the variations of clear span to depth beam ratio caused degradation of the strength, secant and tangent stiffness as well as in dissipated energy. The 80% of plastic moment of beam and rotation at 4% interstory drift were reached for all models analyzed according to criteria established in AISC 341. Finally, the configurations designed with low levels of axial load are controlled by the design of the web panel zone shear, while configurations designed for high levels of axial load are controlled by the strong column-weak beam criterion.

Nonlinear static analysis is a validated tool for the seismic evaluation of existing and new structures, specifically for reinforced concrete buildings. In order to assess the performance of reinforced concrete frames designed according to the new Venezuelan seismic code, configurations of low-, medium-, and high-rise concrete buildings are subjected to 20 different load patterns considering the nonlinear behavior according to FEMA P695. A total of 140 concrete frame models were analyzed using modal response spectrum analysis and nonlinear static pushover analysis. The parameters considered for analyzing the models were the response reduction factor (R), the overstrength factor (RΩ), and the ductility factor (Rµ). The results showed a performance controlled by ductile failure mechanisms in low-rise models unlike combined failure mechanisms with columns with plastic hinge in high-rise models. Reduction factor values between 4 and 14 were obtained. In addition, the pushover curves were affected by the load patterns; therefore, it was necessary to identify the representative patterns, refusing the rest of the patterns. A statistical adjustment was performed using a log-normal distribution. The strength reduction factor specified in the new Venezuelan code was higher than the values obtained for the 95% confidence levels according to the distribution assumed in the reinforced concrete frames models. Finally, the strength reduction factor more representative is R = 4.

In this research, the influence of global slenderness and sliding pallets factor on the seismic design of steel storage racks are assessed. Variations in span length, the height of storage levels, live load, and percentage of live load considered in the seismic mass are studied for different levels of seismic zone and soil type. The models were designed according to the Chilean Code NCh2369. Subsequently, a global sensitivity analysis was developed to analyze the influence of each studied parameter in the seismic design response in terms of fundamental period, drift, and base shear from a response spectral analysis approach. A total of 12000 simulations were performed. Two-hundred additional models were performed to evaluate the variation of seismic mass in the structural response. Results indicate a significant influence of live loads and seismic mass on steel racks designed for soft soils and unbrace conditions. The stiffness also modifies the performance of the racks, mainly in models using braces in the down-aisle direction and cross-aisle direction. In these cases, the seismic mass factor does not have a strong influence on structural response in comparison to the global slenderness.

The properties of fibre reinforced concrete were studied under monotonic loads according to Japanese, North American and European codes. Two different types of SIKA polypropylene fibre were with different dosages. One hundred and eighty tests were performed, obtaining resistance to compression, tension, bending, toughness, and energy absorption in the hardened state. The results show that the addition of fibres affects the workability of the concrete mix. The increase in fibre dosage does not affect compressive, tensile, or flexural strength. However, the failure changes from brittle to more ductile, allowing it to reach residual strengths of 50% of the maximum reached and 200% deformation. The dissipated energy increased with increasing fibre dosage. The performance achieved by both fibres was similar, although the optimal dosage was 6 kg/m3 for type A fibres and 8 kg /m3 for type B. Therefore, the replacement of flexural reinforcing steel in the foundation slabs of one-story buildings can be performed if adequate dosages are used in compliance with established analytical procedures for industrial floor design.

This research evaluates the effectiveness of using a roller-type base isolation device with tensile strength in reducing the dynamic response of industrial steel storage racks. These were subjected to a seismic input acting separately in both directions of the structure. The seismic record obtained from the earthquake that occurred in Llolleo, Chile, on 3 March 1985, was used as input. This earthquake was scaled in the frequency domain, adjusting its response spectrum to coincide with the design spectrum required by NCh2745. In the calculations of this spectrum, the most hazardous seismic zone (zone 3) and soft soil (soil III) that amplifies the effect of the low frequencies of the earthquake were considered. These frequencies are the ones that have the most affect on flexible structures such as high racks and systems with base isolation. Numerical time-history analyses were performed in fixed base racks and base isolation racks. In both cases, the models include semi-rigid connections with capacity for plastic deformation and energy dissipation. Parametric analyses were carried out considering the most relevant variables, using an algorithm programmed in MATLAB software. The maximum relative displacement, maximum basal shear load, and maximum absolute floor acceleration were considered as responses of interest. The results showed the effectiveness of using the base isolation device by reducing the absolute accelerations between approximately 75% and 90%, compared to the same fixed rack at its base. This makes it possible to reduce the vulnerability of the stored load to overturn under the action of a severe earthquake.

Steel buildings with tubular columns showed a satisfactory performance during the Honshu (2011) earthquake, unlike steel buildings in the 1994 Northridge and 1995 Kobe earthquakes, where welded moment connections showed damage in their joints. In this research, a lateral joint using a hollow structural section (HSS)-beam and HSS-column subjected to cyclic displacement was performed. Three large-scale specimens were tested and a numerical model was calibrated, reaching a good adjustment. Later, several configurations of beams and columns were evaluated using finite element (FE) models from the numerical model previously calibrated. A flexural resistance higher 0.80 Mp at 0.04 [rad] was obtained for all cases studied. The ductility factor in the 3 specimens was lower than 2.5, therefore a non-ductile behavior was controlled in the connection. This aspect is very important although a 0.8 Mp at 0.04 [rad] was achieved. Finally, the typical welded moment connection can be improved using the bolted moment connection, which allows the concentration of inelastic incursion in the beam compared with the welded solution. However, a non-ductile behavior derived from local buckling in flanges of a tubular beam can affect the seismic performance.

The 2008 Wenchuan-China earthquake showed the importance of considering the bidirectional seismic action as a cause of failure in column hinge mechanisms. Subsequently, the large 2011 Tohoku-Japan earthquake revealed that Special Moment Frames buildings, made of tubular columns (Hollow Structural Section or Built-up Box Section) and rigid connections with I-beams, did not suffer serious damage. However, only the ConXtech® ConXL™ moment connection has been prequalified according to the (American Institute of Construction) AISC Seismic Provisions for use with tubular columns and the rest of connections do not consider biaxial resistance. The research reported herein investigated the cyclic response of box-columns joints, connected to I beams using the four-bolt extended endplate connection, subjected to bidirectional bending and axial load on the column. To conduct the study, complex nonlinear finite element models (FEMs) of several I beam to box column joint configurations were constructed and analyzed under cyclic loading using the ANSYS software. The results reveal that the failure is concentrated in the beams of all joint configurations except for the columns with axial load equal to 75% of the column capacity, where a combined failure mechanism is achieved. The energy dissipation capacity of joints with a greater number of beams is lower than joints with fewer beams. The bidirectional effect of the seismic action and the level of axial load must be considered to avoid the formation of a column-hinge fragile failure mechanism also the behavior exhibited by 3D joints is more realistic than 2D joints according to real structures.

This paper presents a numerical study on the seismic performance of end-plate moment connection between I-beam to HSS (hollow structural section) column stiffened by outer diaphragms (EP-HSS). In previous experimental research, this moment connection showed a satisfactory performance according to requirements established in Seismic provisions. However, one type of joint was studied and bidirectional and axial loads were not considered. In this since, several configurations representative of 2D interior joints and 3D interior and exterior joints in a steel building were modeled and subjected to unidirectional or bidirectional cyclic displacements according to protocol in seismic provisions. Firstly, a similar joint configuration was calibrated from experimental data, obtaining an acceptable adjustment. The assessment of seismic performance was based on hysteretic curves, failure mechanisms, stiffness, dissipated energy, and equivalent damping. The results obtained showed a ductile failure modes for 2D and 3D joint configurations with EP-HSS moment connection. The axial load has no significant effect on the moment connection. However, it affects the column strength due to the increase of the stresses in the column wall. Compared with 2D joints, 3D joints reached higher deformations even when a similar number of beams is used. The external diaphragms to the column panel zone provided rigidity in the joints and no degradation of slope for each loop in load/reload segment for elastic loop; therefore, curves without pinching were observed. All inelastic deformation is concentrated mainly in the beams. A moment resistance above 80% of the capacity of the beam at a drift of 4% is achieved in all joints. From the results reached, the use of EP-HSS moment connection with hollow structural section columns is a reliable alternative in seismic zones when steel moment frames are employed.

Brittle failure mechanisms can affect the seismic performance of structures composed of intersecting moment resisting frames, if the biaxial effects are not considered. In this research, the bidirectional cyclic response of H-columns with weak-axis moment connections was studied using numerical models. Several configurations of joints with bidirectional effects and variable axial loads were studied using the finite element method (FEM) in ANSYS v17.2 software. The results obtained showed a ductile behavior when cyclic loads are applied. No evidence of brittle failure mechanisms in the studied joint configurations was observed, in line with the design philosophy established in current seismic provisions. However, beams connected to the column minor axis reached a partially restrained behavior. Joints with four beams connected to the column exhibited a partially restrained behavior for all axial load levels. An equivalent force displacement method was used to compare the hysteretic response of 2D and 3D joints, obtaining higher deformations in 3D joints with respect to 2D joints with a similar number of connected beams. Consequently, design procedures are not capable of capturing the 3D deformation phenomenon.

On May 31st, 2019, a tornado hit the city of Talcahuano, Chile, generating significant damage to structures and leaving one person dead. The objective of the present paper is to report on damage to structures in Talcahuano. A preliminary survey was performed by the Municipality of Talcahuano and covered the entire affected area with a cellphone web application used to report the severity and distribution of damage. A more comprehensive damage survey was conducted in the Brisa del Sol neighborhood in the Medio Camino area by the UCSC team to assess the damage distribution within an area with well-defined and homogeneous building typologies. The results of the field surveys showed that the tornado behaved as a skipping tornado and that most damage to houses consisted of wall opening damage, roof sheathing failure, and wall cover removal (EF0), followed by partial roof removal(EF1). It was noticeable that self-built systems (house additions) were more damaged than original houses, which may be explained by the fact that such structures do not always meet minimum building standards. It is recommended that field surveys conducted by municipalities and the Ministry of Social Development considertypical damage types rather than just categories such as minor, moderate, or major. Finally, it is recommendedthat the feasibility of implementing mitigation measures such as stricter wind load provisions and dual-objective tornado design philosophy in the Concepci´on-Talcahuano area be analyzed.