Lower Lateral Strength Research Articles

Lateral performance of cold‐formed steel OSB‐sheathed stud wall panels

AbstractOriented Strand Boards (OSB) are currently one the most widely used cladding options in cold‐formed steel (CFS) stud wall panels. The sheathing generates significant resistance to lateral loads owing to its diaphragm stiffness, which is however typically not fully exploited in design. This paper aims to investigate the lateral inplane behaviour of CFS OSB‐sheathed stud wall panels and provide insight into their failure mechanisms and seismic characteristics. Detailed finite element (FE) models of CFS OSB‐sheathed wall panels were developed, featuring material nonlinearity, detailed connection behaviour and geometric imperfections, and their accuracy was validated against previously conducted experiments. The validated FE models were then employed in a parametric study to investigate the effects of key design variables, including the OSB and CFS element thicknesses and the board configuration, on the structural performance parameters and seismic characteristics of the walls. The results demonstrate that using thinner OSB sheathing resulted in lower lateral strength, initial stiffness and energy dissipation, but increased the ductility and ultimate displacement by up to 40%. It was also shown that the presence of horizontal/vertical seams significantly reduces the lateral load capacity and initial stiffness.

Experimental investigation on cyclic in-plane behavior of URM walls retrofitted with AFRP

URM buildings have been the most common constructions globally, although they are weak because of their low lateral strength against seismic loads. The seismic response of URM structures is highly dependent on the in-plane and out-of-plane behaviors of their load-carrying walls. As in-plane behavior, the shear failure has been known as the most brittle failure mode of URM walls. Although the monotonic response of the walls has been investigated widely, there is relatively less knowledge about their hysteretic response as well as the impact of seismic retrofitting on it. Considering the brittle nature of the in-plane behavior, the Aramid Fiber Reinforced Polymer (AFRP) sheet as a relatively soft member of FRP products was chosen as the retrofitting choice in this study. As an experimental study, cyclic lateral loading was applied to the bare and AFRP retrofitted masonry wall specimens in a displacement-controlled manner. To study the shear behavior of the AFRP retrofitted masonry wall specimens, the specimens were externally retrofitted with AFRP sheets in two different configurations; fully and diagonally wrapped. The load-deformation behavior of the walls during the cyclic loading was investigated using hysteresis diagrams. The test results revealed that AFRP retrofitting could improve the shear strength by about 75% and 81% for fully and diagonally wrapped specimens, respectively. Also, the integrity of URM walls was enhanced without altering their lateral stiffness. Moreover, the fully-wrapped specimen exhibited a deformation capability of about 37% higher than the diagonally-wrapped one. Furthermore, the failure of the fully-wrapped specimen was controlled by toe crushing of the masonry wall, while the rupture of the retrofitting sheet caused the failure in the diagonally-wrapped specimen.

Cyclic analyses of corrugated steel plate shear walls

SummaryCorrugated steel plate shear wall (CoSPSW), which consists of a steel boundary frame and a corrugated steel wall panel with the corrugation in the horizontal or vertical direction, is a new type of lateral load‐resisting system in the family of steel plate shear walls. Compared with the unstiffened special plate shear walls (SPSW), CoSPSW would have greater elastic buckling capacity and more resistance to the gravity loads transferred to the wall panel or neatly avoid them, depending on the corrugation direction. The main research contents of this paper is on the cyclic behavior of CoSPSWs and comparison with SPSWs. Nonlinear push‐over and cyclic analyses were conducted on a group of CoSPSW and SPSW models, and parametric studies were performed with different panel and frame configuration, as well as gravity load effects. It turns out that CoSPSWs with deep corrugation have higher lateral stiffness, lateral strength, and energy dissipation than SPSWs, whereas CoSPSWs with shallow corrugation have higher lateral stiffness and ductility, but lower lateral strength than SPSWs. For all cases investigated, CoSPSWs have stable hysteric curves with no almost pinching, and they are much less sensitive to the influence of gravity loads or weaker boundary frames, compared to SPSWs.

Preparation and Characterization of Ir Coating on Mo Network with a W Bond-Coat

A bi-layered Ir/W metallic coating was deposited onto the Mo network via a magnetron sputtering method. Results show that the sputtering W bond-coat is able to enhance the binding energy between the sputtering Ir coating and the Mo substrate, and virtually neither delamination nor peeling off is observed for the as-prepared coating. In addition, the interdiffusion between the W bond-coat and the Ir coating or the Mo substrate is not apparent. The dense and adherent Ir coating is identified to have a polycrystalline structure with (111) orientation. According to the three zone model suggested by Movchan and Demchishin for sputtering metallic coatings, the microstructure of the bi-layered metallic coating is consistent with the Zone 1 features. In fact, the bi-layered coating has very low lateral strength and inclines to crack under tension

REDUCTION OF SEISMIC RISK IN VENEZUELAN SCHOOLS

The Cariaco earthquake in 1997 led to the ruin of four school buildings and the loss of 23 lives. The failures were the result of conceptual deficiencies in the structural system, typical of the seismic design of the past decades: Very low lateral strength and stiffness, low energy dissipation capacity, insufficient shear resistance and the presence of short columns. On the other hand, the collapse of two buildings was also influenced because they were built in a seismic zone with twice the intensity specified in the construction plans. The schools identified as Old Type and Box Type, similar to the schools that collapsed in Cariaco, can be found all along the country. Using linear dynamic and non-linear static analysis, the seismic risk at which they are exposed was determined in the different zones of Venezuela. Old Type schools are exposed to unacceptable risk and need to be retrofitted, even in moderate seismic zones as it has been shown in one school in Arenales where an earthquake of moderate intensity endanger the stability of the school. On the other hand, only Box Type schools located in the high hazard zones need to be retrofitted. A national plan for the mitigation of seismic risk in existing schools and for the construction of new earthquake resistant schools is proposed.

An alternative approach for the seismic rehabilitation of existing RC buildings using seismic isolation

SummaryUsually, buildings with seismic isolation are designed to comply with an operational building performance level after strong earthquakes. This approach, however, may limit the use of seismic isolation for the seismic rehabilitation of existing buildings with low lateral strength or substandard details, because it often requires invasive strengthening measures in the superstructure or the use of expensive custom‐made devices. In this paper, an alternative approach for the seismic rehabilitation of existing buildings with seismic isolation, based on the acceptance of limited plastic deformations in the superstructure under strong earthquakes, is proposed and then applied to a real case study, represented by a four‐storey RC frame building. Nonlinear response‐time histories analyses of an accurate model of the case‐study building are carried out to evaluate the seismic performances of the structure, comparing different rehabilitation strategies with and without seismic isolation. Initial costs of the intervention and possible (future) repair costs are then estimated. Based on the results of this study, values of the behavior factor (i.e. response modification factor) higher than those adopted in the current codes for base‐isolated buildings are tentatively proposed. Copyright © 2015 John Wiley & Sons, Ltd.

Tests on cyclic behavior of reinforced concrete frames with brick infill

SummaryThis paper reports the results of cyclic loading tests performed on four specimens consisting of reinforced concrete frames with brick infill walls. The brick infill is pre‐laid, followed by the cast in‐place RC columns and beams. Test parameters include the height‐to‐length ratio of the brick infill wall and the mortar compressive strength. Test results reveal that the in‐plane lateral strength of brick infill wall is related to the fracture path. The fracture path for brick infill walls with large height‐to‐length ratios includes bed joints, cross joints, and vertical splitting of bricks. As a result, the lateral strength of this type of brick infill wall is larger. In comparison, the fracture path for brick infill walls with small height‐to‐length ratios only passes through joints, which is the reason why they have lower lateral strength. Mortar with higher strength improves the lateral strength of brick infill wall.In addition to presenting experimental observations in detail, this paper compares the test results with those obtained from existing methods for assessment of seismic resistance. Comments and recommendations are offered with respect to the capabilities of the assessment methods in predicting stiffness, strength, and ultimate deformation capacity of brick infill walls. Copyright © 2015 John Wiley & Sons, Ltd.

In-Plane Behavior of BFRP Sheets Bonded to Damaged RC-Brick Masonry Walls with Opening

FRP strength technique can increase the lateral strength of masonry walls, but the effect of the presence of pre-damage in the walls before retrofitted has not been studied. In this study, the experimental results from two half-scale RC-brick masonry walls with opening retrofitted with BFRP composite strips are presented. One wall was initially damaged in shear loading up to its maximum strength, and then repaired with BFRP sheets; another one was directly strengthened with BFRP sheets in the same strengthening configuration. All the walls were subjected to cyclic in-plane shear loading up to failure. Compared to the strengthened walls, the repaired masonry wall has almost the same failure mode and FRP strain rule, and slightly lower lateral strength and deformation capacity as well as energy dissipation capacity.

A case study on the P–Δ effect on inelastic seismic behaviour of a bridge pier

The P–delta (P–Δ) effect imposes higher ductility demand during earthquakes, which may not be adequately catered for in the prescribed values of response reduction factors (Rμ). The present work investigates the P–Δ effect on inelastic seismic response of slender bridge piers through a case study. Displacement ductility demands and overturning moments of a typical pier are computed both with and without the P–Δ effect using non-linear dynamic analysis and pushover methods. Response history analysis is carried out for four hysteresis behaviours: elasto-plastic, strength degrading, stiffness degrading, and combined strength and stiffness degrading behaviour. The study shows that the P–Δ effect at the plastic stage is a progressive phenomenon and depends on strength rather than on stiffness. The conventional practice to restrict sway based on first-order analysis to prescribed values for containing the P–Δ effect is ineffective. By using a case study, the research provides a methodology for arriving at acceptable Rμ values to prevent occurrence of any instability due to the P–Δ effect arising from undue low lateral strength.

Simplified non-linear seismic displacement demand prediction for low period structures

The prediction of non-linear seismic demand using linear elastic behavior for the determination of peak non-linear response is widely used for seismic design as well as for vulnerability assessment. Existing methods use either linear response based on initial period and damping ratio, eventually corrected with factors, or linear response based on increased equivalent period and damping ratio. Improvements to the original EC8 procedure for displacement demand prediction are proposed in this study. Both propositions may be graphically approximated, which is a significant advantage for practical application. A comparison with several other methods (equal displacement rule, EC8 procedure, secant stiffness and empirical equivalent period methods) is performed. The study is based on non-linear SDOF systems subjected to recorded earthquakes, modified to match design response spectra of different ground types, and focuses on the low frequency range that is of interest for most European buildings. All results are represented in the spectral displacement/fundamental period plane that highlights the predominant effect of the fundamental period on the displacement demand. This study shows that linearized methods perform well at low strength reduction factors but may strongly underestimate the displacement demand at strength reduction factors greater than 2. This underestimation is an important issue, especially for assessment of existing buildings, which are often related with low lateral strength. In such cases, the corresponding strength reduction factors are therefore much larger than 2. The new proposals significantly improve the reliability of displacement demand prediction for values of strength reduction factors greater than 2 compared to the original EC8 procedure. As a consequence, for the seismic assessment of existing structures, such as unreinforced masonry low-rise buildings, the current procedure of EC8 should be modified in order to provide accurate predictions of the displacement demand in the domain of the response spectrum plateau.

Vulnerability analysis of RC buildings with wide beams located in moderate seismicity regions

A significant number of short-to-mid height RC buildings with wide beams have been constructed in areas of moderate seismicity of Spain, mainly for housing and administrative use. The buildings have a framed structure with one-way slabs; the wide beams constitute the distinctive characteristic, their depth being equal to that of the rest of the slab, thus providing a flat lower surface, convenient for construction and the layout of facilities. Seismic behavior in the direction of the wide beams appears to be deficient because of: (i) low lateral strength, mainly because of the small effective depth of the beams, (ii) inherent low ductility of the wide beams, generated by high amount of reinforcement, (iii) the big strut compressive forces developed inside the column-beam connections due to the low height of the beams, and (iv) the fact that the wide beams are wider than the columns, meaning that the contribution of the outer zones to the resistance of the beam-column joints is unreliable because there is no torsion reinforcement. In the orthogonal direction, the behavior is worse since the only members of the slabs that contribute to the lateral resistance are the joists and the façade beams. Moreover, these buildings were designed with codes that did not include ductility requirements and required only a low lateral resistance; indeed, in many cases, seismic action was not considered at all. Consequently, the seismic capacity of these structures is not reliable. The objective of this research is to assess numerically this capability, whereas further research will aim to propose retrofit strategies. The research approach consists of: (i) selecting a number of 3-story and 6-story buildings that represent the vast majority of the existing ones and (ii) evaluating their vulnerability through three types of analyses, namely: code-type, push-over and nonlinear dynamic analysis. Given the low lateral resistance of the main frames, the cooperation of the masonry infill walls is accounted for; for each representative building, three wall densities are considered. The results of the analyses show that the buildings in question exhibit inadequate seismic behavior in most of the examined situations. In general, the relative performance is less deficient for Target Drift CP (Collapse Prevention) than for IO (Immediate Occupancy). Since these buildings are selected to be representative of the vast majority of buildings with wide beams that were constructed in Spain without accounting for any seismic consideration, our conclusions can be extrapolated to a broader scenario.

Reducing Seismic Risk of School Buildings in Venezuela

School buildings have frequently collapsed during earthquakes. The 1997 Cariaco earthquake led to the ruin of four reinforced concrete school buildings that were built several years ago. Failures were the result of structural deficiencies: short columns and low lateral strength, stiffness and energy dissipation capacity. Seventy percent of Venezuelan schools are in high-hazard regions; about 1,000 are similar to the collapsed schools. With the purpose of developing a national risk-reduction program, the expected seismic performance of two typical schools was evaluated: one representing schools built 50 years ago (Old-type) and one representing schools built 20–30 years ago (Box-type). These were analyzed utilizing nonlinear pushover techniques and compared with the inventory of schools in Venezuela. Old-type schools were found to need retrofitting in moderate- and above-seismic zones, and Box-type schools in higher zones. Practical retrofitting is achieved with the addition of auxiliary structures to support the seismic loads, leaving the existing structures to support only the gravity loads. This effort has led to a national program. The initial phase, surveying approximately 28,000 existing schools, has begun.

A PROCEDURE FOR ESTIMATING INPUT ENERGY SPECTRA FOR SEISMIC DESIGN

In this paper, the damage potential of an earthquake ground motion is evaluated in terms of the total power of the acceleration of the ground motion. By assuming an appropriate spectral shape for the input energy spectrum, and using the well-known Parseval theorem for evaluating the total power of a random signal, the peak amplification factor for the equivalent input energy velocity spectrum can be determined. It is shown that the peak amplification factor for the input energy spectrum depends on the peak-ground-acceleration to peak-ground-velocity ratio and duration of the strong motion phase of the ground motion. Values for the equivalent input energy velocity amplification factor vary from about 2 to 10 for most of the recorded ground motions used in this study. Although a considerable scatter of data is observed in this study, the peak amplification factor predicted by the Fourier amplitude spectrum of the ground acceleration provides a fairly good estimate of the mean value of the peak input energy compared to that determined from inelastic dynamic time history analyses, particularly for systems with high damping and low lateral strength. The peak amplification factor derived in this paper provides a more consistent approach for estimation of seismic demand when compared to an earlier empirical expression used for the formulation of duration-dependent inelastic seismic design spectra, even though only a slight difference in the required lateral strength results from the use of the new formula.