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Haselton C.B.,California State University, Chico | Liel A.B.,University of Colorado at Boulder | Deierlein G.G.,Stanford University | Dean B.S.,Walter P. Moore and Associates Inc. | Chou J.H.,Quincy Engineering Inc.
Journal of Structural Engineering | Year: 2011

This study applies nonlinear dynamic analyses to assess the risk of collapse of RC special moment-frame (SMF) buildings to quantify the seismic safety implied by modern building codes. Thirty archetypical RC SMF buildings, ranging in height from 1 to 20 stories, are designed according to ASCE 7-02 and ACI 318-05 for a high-seismic region. The results of performance-based seismic assessments show that, on average, these buildings have an 11% probability of collapse under ground motion intensities with a 2% probability of exceedance in 50years. The average mean annual rate of collapse of 3.110-4 collapses per year corresponds to an average of 1.5% probability of collapse in 50years. The study further examines the influence of specific design provisions on collapse safety. In particular, changes to the minimum seismic base shear requirement between 2002 and 2005 editions of ASCE 7 and variations in ACI 318 strong-column weak-beam (SCWB) design requirements are investigated. The study finds that the reduction in the minimum base shear, introduced in ASCE 7-05 and subsequently rescinded, dramatically increases the collapse risk of tall (long-period) frame buildings in high-seismic regions. An investigation of the SCWB requirements shows that the current ACI 318 provisions delay, but do not prevent, column yielding and the formation of story collapse mechanisms. An increase in the SCWB ratio above 6/5 (1.2) does not significantly improve performance of low-rise frame buildings but may reduce collapse risk for midrise and taller buildings. This study of modern RC buildings is contrasted with the collapse safety of older (nonductile) RC moment-frame buildings in the companion paper. © 2011 American Society of Civil Engineers. Source

Williams M.E.,Walter P. Moore and Associates Inc.
American Concrete Institute, ACI Special Publication | Year: 2014

In recent years concrete bridge structures in the USA have been experiencing varied levels of premature concrete deterioration due to alkali-silica reaction (ASR) and related condition delayed ettringite formation (DEF). While these deleterious reactions can affect various concrete bridge members under the right conditions, bridge columns can be notably more susceptible due to their unique exposure conditions and aggressive environments. The degree of deleterious reactions in concrete bridge columns is dependent on susceptibility of the aggregate, and on environmental factors, such as temperature, moisture, and external sources of alkalis. Temperature gradients are known to affect the rate and severity of the ASR expansion. Moisture gradients can be facilitated by high atmospheric humidity, exposure to weather, proximity to water spray from adjacent roadways, malfunctioning joint systems, and/or failed drainage systems, which collectively can provide sufficient conditions for ASR and DEF expansion. This paper suggests that a review of the aggressive environmental conditions at the bridge site can provide valuable insight into the occurrence and progression of premature concrete deterioration and provide direction as to a future course of action for maintenance and repair for concrete bridge columns. Repair procedures are provided dependent on the severity of premature concrete deterioration. Source

Mukhopadhyay B.,HDR | Dutta A.,Walter P. Moore and Associates Inc.
Water Resources Management | Year: 2010

Integrated water resources management at river basin scales and evaluation of effects of climate change on regional water resources require quantitative estimates of space-time variability of monthly discharges within a river network. This study demonstrates that such estimates, which can be called stream water availability, for regional river basins with meager or nonexistent gauge data, can be obtained by combining continuity models of hydrological processes, flow routing, and topology of the river basin. The hydrologic processes can be adequately modeled using high quality databases of hydrologic significance. A stream water availability model is presented for Upper Indus Basin (UIB) utilizing the most up-to-date datasets for topography, temperature, precipitation, net radiation, land cover, soil type, and digital atlas. Multiple datasets have been evaluated and the ones with best accuracy and temporal coverage have been selected for the final model. Upper Indus River and its major tributaries are highly significant in regional water resources management and geopolitics. However, UIB is a poorly studied and largely ungauged river basin with an area of 265,598 km2 and extremely rugged topography. Several factors, the chief ones being the challenging terrain and the trans-boundary nature of the basin, have contributed to this knowledge gap. Hydro-climatologically it is a complex basin with a significant cryospheric component. The spatial and temporal variation of the principal climatic variables, namely precipitation, net radiation, and temperature has been thoroughly accounted for in the development of a stream water availability model based on a process model coupled with a topologic model and a linear reservoir model of river flow routing. Model calculations indicate that there are essentially two hydrologic regimes in UIB. The regime that is truly significant in contributing stream flows, originates from the UIB cryosphere containing outstanding glaciers and snowfields. The other regime, generated from wet precipitation and melt water from seasonal snow covers is insignificant due to high rates of infiltration and evaporation in the semi-desert environment prevailing at elevations below perennial snow and ice covers. In general, the modeled stream flow characteristics match with the sparse discharge measurements that are available. Flow in the Indus considerably increases at its confluence with Shyok River and further downstream where other tributaries form the north join the main stem. At or near the outlet of the basin stream flow can vary from less than 800 m3 s-1 in the winter and spring to nearly 8,000 m3 s-1 in the peak summer and can persist to over 1,500 m3 s-1 in the autumn. The importance of snow and glacial melt in Indus River discharge is apparent and any global or regional climate change affecting the equilibrium line elevation of the snow fields in the Karakoram will have a profound influence on the water availability in the Indus. Estimates are made for per capita water availability in Ladakh and Gilgit-Baltistan territories, controlled by India and Pakistan respectively. Geopolitical significance and climate change effects are discussed briefly. © 2010 Springer Science+Business Media B.V. Source

Henkhaus K.,Walter P. Moore and Associates Inc. | Pujol S.,Purdue University | Ramirez J.,Purdue University
Journal of Structural Engineering (United States) | Year: 2013

Results from tests on eight full-scale RC columns with ties with large spacing (s>d/2) and 90 hooks are presented. The test results showed that increasing the number of displacement cycles and applying displacements along more than one axis decreased the maximum drift ratio reached before the columns experienced failure in axial compression. Test columns had little drift capacity beyond shear failure. The mean difference between the maximum drift ratio at axial failure and the drift ratio at shear failure was less than 1%. © 2013 American Society of Civil Engineers. Source

Lai Z.,Purdue University | Varma A.H.,Walter P. Moore and Associates Inc. | Griffis L.G.,Purdue University
Journal of Structural Engineering (United States) | Year: 2016

Concrete-filled steel tube (CFT) beam-columns are categorized as compact, noncompact, or slender depending on the governing slenderness ratio (width-to-thickness b/t or D/t ratio,) of the steel-tube wall. The current AISC specification recommends the bilinear axial force-bending moment (P-M) interaction curve for bare steel members for the design of noncompact and slender CFT beam-columns. This paper compiles the experimental database of tests conducted on noncompact and slender CFT beam-columns, and demonstrates the overconservatism of the AISC P-M interaction curve. This paper also presents the development and benchmarking of detailed 3D finite-element models for predicting the behavior and strength of noncompact and slender CFT members. The benchmarked models are then used to evaluate the fundamental P-M interaction behavior of CFT beam-columns, and the influence of material and geometric parameters such as the tube slenderness ratio , material strength ratio (Fy/fc′), member length-to-section depth ratio (L/D), and axial load ratio (P/Po). The parametric analyses indicate that for L/D ratios up to 20, the P-M interaction curves are governed by the relative strength ratio (csr=AsFy/Acfc′). The parametric analysis results are used to propose revisions to the current standard's interaction equations for designing noncompact and slender CFT beam-columns. © 2015 American Society of Civil Engineers. Source

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