Investigation of the sliding behavior between steel and mortar for seismic applications in structures

The friction developed between a steel base plate and a mortar base contributes shear resistance to the building system during a seismic event. In order to investigate the possible sliding behavior between the base plate and the mortar, a shake table study is undertaken using a large rigid mass supported by steel contact elements which rest on mortar surfaces connected to the shake table. Horizontal input accelerations are considered at various magnitudes and frequencies. The results provide a constant friction coefficient during sliding with an average value of approximately 0.78. A theoretical formulation of the friction behavior is also undertaken. The theoretical equations show that the sliding behavior is dependent on the ratio of the friction force to the input force. The addition of vertical accelerations to the system further complicates the sliding behavior as a result of the varying normal force. This results in a variable friction resistance which is a function of the amplitude, phase, and frequency of the horizontal and vertical input motions. In general, this study showed a consistent and reliable sliding behavior between steel and mortar. Copyright © 2009 John Wiley & Sons, Ltd.     

Shake table study on an ordinary low‐rise building seismically isolated with SU‐FREIs (stable unbonded‐fiber reinforced elastomeric isolators)

This paper reports on the investigation of novel fiber reinforced elastomeric isolator (FREI) bearings, which do not have thick end plates, and are used in an unbonded application. Owing to the stable lateral load‐displacement response exhibited by the unbonded FREI bearings, the proposed bearings are referred to as stable unbonded (SU)‐FREIs. A shake table test program was conducted on a two‐story test‐structure having well‐defined elastic response characteristics. Compared with the results for the corresponding fixed base (FB) structure, the peak response values, distribution of lateral response throughout the height of the structure, and response time histories of the tested base isolated (BI) structure indicate that significantly improved response can be achieved. This study clearly indicates that SU‐FREI bearings can provide an effective seismic isolation system. Copyright © 2009 John Wiley & Sons, Ltd.     

Decentralized robust control of building structures under seismic excitations

Many of the control algorithms proposed for structures subjected to seismic excitations are based on a centralized design philosophy, such as the linear quadratic regulator (LQR) design. The information of all the states of the system is usually required in these methods to determine the control command. For applications involving large‐scale systems, it may be more convenient to design decentralized controllers that depend only on the information of the local states for control command calculation. In this study, a nonlinear decentralized robust control algorithm is proposed. The structural system is decomposed into several artificially uncoupled subsystems. The interconnections between adjacent subsystems are treated as uncertain but bounded disturbances to the subsystems. The controller associated with one subsystem determines the control command based only on the states of the local subsystem. Numerical examples of linear and nonlinear structural models are presented to demonstrate the effectiveness and robustness of the proposed controller. The traditional LQR design is used as a baseline for comparison. Copyright © 2007 John Wiley & Sons, Ltd.     

Seismic self‐centering steel beam‐to‐column moment connections using bolted friction devices

This paper first presents the force–deformation relationship of a post‐tensioned (PT) steel beam‐to‐column connection constructed with bolted web friction devices (FDs). This paper then describes the test program conducted in the National Center for Research on Earthquake Engineering, Taiwan, on four bolted FDs and four full‐scale PT beam‐to‐column moment connection subassemblies using the FDs. Tests confirm that (1) the hysteretic behavior of four bolted FDs is very stable, (2) the friction coefficient between the steel plate and the brass shim is about 0.34, (3) the proposed force–deformation relationships reasonably predict the experimental responses of the PT connections under cyclically increasing deformations up to a beam peak rotation of 0.05 rad, and (4) the decompression moments do not degrade as beam cyclic deformations increase. Copyright © 2007 John Wiley & Sons, Ltd.     

Seismic isolation by rubber–soil mixtures for developing countries

This paper proposes a promising seismic isolation method particularly suitable for developing countries, which makes use of rubber–soil mixtures. Apart from reducing the level of shaking in the horizontal direction, the distinctive advantage of the proposed method is that it can also significantly reduce the shaking level of vertical ground motion, to which an increasing attention has been paid in the earthquake engineering community. On the other hand, the use of scrap tires as the rubber material can provide an alternative way to consume the huge stockpile of scrap tires all over the world. Moreover, the low cost of this proposed seismic protection scheme can greatly benefit those developing countries where resources and technology are not adequate for earthquake mitigation with well‐developed, yet expensive, techniques. The proposed method has been demonstrated through a series of numerical simulations and a parametric study has also been carried out. Lastly, five important issues regarding the concept and feasibility have been discussed. Copyright © 2007 John Wiley & Sons, Ltd.     

Controls on sediment evacuation from glacially modified and unmodified catchments in the eastern Sierra Nevada,California

The degree of glacial modification in small catchments along the eastern Sierra Nevada, California, controls the timing and pattern of sediment flux to the adjacent fans. There is a close relationship between the depth of fan‐head incision and the pattern and degree of Late Pleistocene catchment erosion by valley glaciers; catchments with significant glacial activity are associated with deeply incised fan heads, whereas fans emerging from glacially unmodified catchments are unincised. We suggest that the depth of fan‐head incision is controlled by the potential for sediment storage during relatively dry ice‐free periods, which in turn is related to the downstream length of the glacially modified valley and creation of accommodation through valley floor slope lowering and glacial valley overdeepening and widening. Significant storage in glacially modified basins during ice‐free periods leads to sediment supply‐limited conditions at the fan head and causes deep incision. In contrast, a lack of sediment trapping allows quasi‐continuous sediment supply to the fan and prevents incision of the fan head. Sediment evacuation rates should thus show large variations in glacially modified basins, with major peaks during glacial and lows during interglacial or ice‐free periods, respectively. In contrast, sediment removal from glacially unmodified catchments in this type of setting should be free of this effect, and will be dominated instead by short‐term variations, modulated for example by changes in vegetation cover or storm frequency. This distinction may help improve our understanding of long‐term sediment yields as a measure of erosional efficiency. Copyright © 2008 John Wiley & Sons, Ltd.     

Pseudo‐dynamic test of a full‐scale CFT/BRB frame—Part II: Seismic performance of buckling‐restrained braces and connections

This paper is Part II of a two‐part paper describing a full‐scale 3‐story 3‐bay concrete‐filled tube (CFT)/buckling‐restrained braced frame (BRBF) specimen tested using psuedo‐dynamic testing procedures. The first paper described the specimen design, experiment, and simulation, whereas this paper focuses on the experimental responses of BRBs and BRB‐to‐gusset connections. This paper first evaluates the design of the gusset connections and the effects of the added edge stiffeners in improving the seismic performance of gusset connections. Test results suggest that an effective length factor of 2.0 should be considered for the design of the gusset plate without edge stiffeners. Tests also confirm that the cumulative plastic deformation (CPD) capacity of the BRBs adopted in the CFT/BRBF was lower than that found in typical component tests. The tests performed suggest that the reduction in the BRB CPD capacities observed in this full‐scale frame specimen could be due to the significant rotational demands imposed on the BRB‐to‐gusset joints. A simple method of computing such rotational demands from the frame inter‐story drift response demand is proposed. This paper also discusses other key experimental responses of the BRBs, such as effective stiffness, energy dissipation, and ductility demands. Copyright © 2008 John Wiley & Sons, Ltd.     

Parametric study of ductile moment‐resisting steel frames: A first step towards Eurocode 8 calibration

A parametric study of 13 608 ductile moment‐resisting steel frames designed according to Eurocodes 3 and 8 is performed. A flowchart for the evaluation of the seismic‐resistant capacity of the designed frames is developed based on the N2 method. The design structural overstrength, ductility supply, plastic redistribution parameter, supply reduction factor and performance ratio of the frames are analysed. We determine that the frames have performance ratios higher than 1, mostly due to high values of design structural overstrength, showing that the seismic supply produced by the restraints of Eurocodes 3 and 8 is always higher than the seismic demand. Copyright © 2008 John Wiley & Sons, Ltd.     

Estimation of surface roughness (z0) over a stabilizing coastal dune field based on vegetation and topography

Accurate knowledge of the surface roughness and the resultant wind speed are important for many applications, such as climatic models, wind power meteorology, agriculture and erosion hazards, especially on sand dunes in arid and semi‐arid environments, where vegetation cover is scarce. In this study we aimed at quantifying the effects of vegetation cover and topography on surface roughness over a stabilizing dune field on the southern coast of Israel. Forty‐six wind measurements were made at various distances from the coastline, ranging from 10 to 2800 m, and z₀ values were calculated from the wind measurements based on the ratio between the wind gust and the average wind speed. We estimated vegetation cover using the soil adjusted vegetation index (SAVI) from Landsat satellite images for the upwind sector at various lengths, ranging from 15 to 400 m, and based on digital elevation models and differential GPS field measurements we calculated the topographic variable of the relative heights of the stations. z₀ values were positively correlated with the winter SAVI values (r = 0·87 at an upwind length of 200 m) and negatively correlated with the relative height (r = ?0·68 at an upwind length of 200–400 m for the inland dune stations). Using these variables we were able to create a map of estimated z₀ values having an accuracy of over 64%. Such maps provide a better understanding of the spatial variability in both wind speed and sand movement over coastal dune areas. Copyright © 2007 John Wiley & Sons, Ltd.