Structural Performance of Bridge Approach Slabs under Given Embankment Settlement

Soil embankment settlement causes concrete approach slabs of bridges to lose their contact and support from the soil. When soil settlement occurs, the slab will bend in a concave manner that causes a sudden change in slope grade near its ends. Meanwhile, loads on the slab will also redistribute to the ends of the slab, which may result in faulting across the roadway at the ends of the approach slab. Eventually, the rideability of the bridge approach slab will deteriorate. The current American Association of State Highway Transportation Officials code specifications do not provide clear guidelines to design approach slabs considering the embankment settlements. State Departments of Transportation are spending millions of dollars each year to deal with problems near the ends of approach slabs. To investigate the effect of embankment settlements on the performance of the approach slab, a three-dimensional finite element analysis was conducted in the present study, considering the interaction between the approach slab and the embankment soil, and consequently the separation of the slab and soil. The predicted internal moments of the approach slab provide design engineers with a scientific basis to properly design the approach slab considering different levels of embankment settlements. A proper design of the approach slab will help mitigate the rideability problems of the slab.     

Design Guidelines to Accommodate Potential Upgrading of Elevator Systems

Upgrading of elevators is carried out in the interests of maintaining building quality and attaining market rental income. Difficulties experienced in the upgrading of elevator systems indicate a need for development of guidelines to be considered in the initial design of buildings to facilitate upgrading of elevator systems in the future. The focus of this paper is on the development of design guidelines for the potential upgrading of elevator systems in high-rise buildings. A case study as to the difficulties encountered in upgrading an elevator system is included. Some types of upgrading and replacement now possible are reviewed. Responses from 92 design professionals (architects and engineers) in nine countries—namely, the United States, Japan, Canada, Australia, United Kingdom, Singapore, Spain, Saudi Arabia, and Kuwait—to a survey on the upgradeability of elevators have been summarized in the form of guidelines for use by building designers.     

Multilevel Formwork Load Distribution with Posttensioned Slabs

Formwork and the associated shoring represent a significant proportion of the costs associated with the construction of multilevel concrete structures. To minimize these costs, a limited number of formwork and shoring sets are recycled up the structure as construction progresses, eliminating the need for a new set of formwork and shoring with each new slab. When a slab is posttensioned using draped tendons, slab lift occurs as a portion of the slab self-weight is balanced. The formwork and shores supporting that slab are unloaded by an amount equivalent to the load balanced by the posttensioning. This produces a load distribution through the structure that is inherently different from that of a conventionally reinforced slab. This paper presents two design methods suitable for modeling the multilevel formwork process for posttensioned slabs: A modification to the simplified analysis method and a finite element model—both techniques will be of immediate use by industry practitioners and of interest to researchers examining the load distribution phenomenon. The paper also summarizes the findings of one of only a few research projects in which actual shore loads were monitored during the construction of a multilevel posttensioned building, which is used to validate the proposed design models.     

Mechanisms of Seismically Induced Settlement of Buildings with Shallow Foundations on Liquefiable Soil

Seismically induced settlement of buildings with shallow foundations on liquefiable soils has resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate postliquefaction reconsolidation settlement in the free-field. A series of centrifuge experiments involving buildings situated atop a layered soil deposit have been performed to identify the mechanisms involved in liquefaction-induced building settlement. Previous studies of this problem have identified important factors including shaking intensity, the liquefiable soil’s relative density and thickness, and the building’s weight and width. Centrifuge test results indicate that building settlement is not proportional to the thickness of the liquefiable layer and that most of this settlement occurs during earthquake strong shaking. Building-induced shear deformations combined with localized volumetric strains during partially drained cyclic loading are the dominant mechanisms. The development of high excess pore pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced ratcheting of the buildings into the softened soil are important effects that should be captured in design procedures that estimate liquefaction-induced building settlement.     

Influence of Reinforcement on the Behavior of Concrete Bridge Deck Slabs Reinforced with FRP Bars

This paper presents the results of an experimental study to investigate the role of each layer of reinforcement on the behavior of concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars. Four full-scale concrete deck slabs of 3,000?mm length by 2,500?mm width and 200?mm depth were constructed and tested in the laboratory. One deck slab was reinforced with top and bottom mats of glass FRP bars. Two deck slabs had only a bottom reinforcement mat with different reinforcement ratios in the longitudinal direction, while the remaining deck slab was constructed with plain concrete without any reinforcement. The deck slabs were supported on two steel girders spaced at 2,000?mm center to center and were tested to failure under a central concentrated load. The three reinforced concrete slabs had very similar behavior and failed in punching shear mode at relatively high load levels, whereas the unreinforced slab behaved differently and failed at a very low load level. The experimental punching capacities of the reinforced slabs were compared to the theoretical predictions provided by ACI 318-05, ACI 440.1R-06, and a model proposed by the writers. The tests on the four deck slabs showed that the bottom transverse reinforcement layer has the major influence on the behavior and capacity of the tested slabs. In addition, the ACI 318-05 design method slightly overestimated the punching shear strength of the tested slabs. The ACI 440.1R-06 design method yielded very conservative predictions whereas the proposed method provided reasonable yet conservative predictions.     

Design Recommendations for Externally Restrained Highway Bridge Decks

A severe maintenance penalty exists for composite multibeam highway bridges in areas where the use of deicing chemicals is prevalent. This maintenance concern is largely due to the corrosion of embedded steel reinforcing bars and the attendant concrete degradation in the deck slab. Transverse steel straps, placed below the concrete slab, eliminate the deleterious effects of corrosion on the concrete. Further, the straps can be designed to provide the restraint necessary to promote the development of internal arching in the concrete slab in response to a concentrated load. A design procedure is presented for an externally restrained highway bridge deck. The method has been developed, based on the Canadian Limit States design philosophy, considering both the strength and serviceability requirements. It is demonstrated that the ultimate strength requirements dictate the external restraint requirements, and a numerical example of the design procedure is given.     

建(构)筑物渗漏水浅析

通过对部分建(构)筑物渗漏水工程(如屋面漏水、地下工程漏水、厕浴间漏水、外墙渗水)的调查、分析,发现其主要原因为:防水设计不合理,材料质量无保证,施工质量差。建(构)筑物渗漏水影响建(构)筑物外观和使用功能;严重者导致钢筋锈蚀、混凝土发生碱骨料反应,受力系统承载力降低,危及主体结构安全。为此,应规范防水工程设计、施工,加强防水工程的监理和质检监管力度,增大防水工程的投入,采用防水新材料、新技术、新工艺等措施防治建(构)筑物渗漏水。     

Concrete Slabs Reinforced with FRP Grids. II: Punching Resistance

The use of fiber-reinforced polymer (FRP) grid reinforcement for concrete slabs has been investigated, considering the behavior of the slabs in one-way bending and under concentrated loading. The behavior under the latter loading type will be considered in this second part of a two-part paper. From the performed punching tests and the analysis, a fairly strong interaction between shear and flexural effects was noted for most of the tested slabs. For the FRP-reinforced slabs with an increased reinforcement ratio or an increased slab depth (needed to fulfill the serviceability criteria in bending), the punching strength was similar to or higher than the tested steel-reinforced reference slabs. For most slabs, slip of the bars occurred resulting in higher deflections at failure. The calculation of the punching failure load according to empirical-based models (from different codes), a modified mechanical model, and an analytical model is evaluated.     

Experimental Study on the Performance of Approach Slabs under Deteriorating Soil Washout Conditions

In the U.S. bridge design practice, an approach slab is commonly provided to facilitate a smooth transition from the highway pavement to the bridge deck. Maintenance of bridge approaches often necessitates the repair or replacement of approach slabs owing to damage from heavy traffic loads, washout of fill materials, and settlement of the approach embankment. Approach slab damage because of embankment settlement is considered a more common problem and has been extensively investigated in the literature. In this paper, performance of the approach slab degraded by void formation underneath the slab is examined by load testing. Full-size approach-slab specimens were tested under increasing magnitude up to four times AASHTO HS20-44 design truck loads. The test matrix included four slab specimens with the following details: (1)?conventional steel reinforcement representative of current California design; (2)?steel reinforcement replaced by a double-layer pultruded fiber-reinforced polymer grating; (3)?steel reinforcement replaced by glass fiber-reinforced polymer rebars; and (4)?incorporation of steel and polyvinyl alcohol fibers in the concrete mix and removal of top longitudinal and transverse steel. Results indicated that the slabs show satisfactory performance under standard HS20-44 design truck load. Tests also revealed that these slabs exhibited similar performance in terms of stiffness, deformation, and crack pattern when fully supported, but registered noticeable difference in performance under deteriorating soil washout conditions. The fiber-reinforced concrete slab in general showed the best crack control and the smallest deflection and end rotation among the four slabs.     

Designing Buildings for Maintenance: Designers' Perspective

The degree to which the design of a building embraces maintenance considerations has a major impact on its performance. A survey was conducted of the largest 211 building design firms in the United States to investigate the relationship between design practices and maintenance considerations. The findings shed light on the extent to which maintenance issues are considered when designers specify building materials and service equipment; the level of designers' knowledge in maintenance-related issues; the degree to which design personnel is exposed to training in maintenance-related matters; the extent to which designers consult property managers and maintenance consultants; the relative importance of maintenance issues to other design factors; the level of difficulty in cleaning, inspecting, repairing, and replacing various building components; and the magnitude and frequency of maintenance-related complaints that designers receive from clients and tenants. The performance of buildings is likely to be enhanced if designers are cognizant of maintenance-related matters.     

Settlement of Mine Spoil Fill from Water Infiltration: Case Study in Eastern Kentucky

Mine spoil valley fills are a by-product of mountaintop removal mining in the Appalachian coal mining region of the United States. These fills often result in large expanses of relatively flat land covering thousands of acres, which can be used for commercial or industrial development. However, this material is susceptible to damaging settlement, and highly publicized failures of structures built on mine spoil fills have led to reluctance on the part of investors to develop these areas. A key settlement mechanism in mine spoil is water infiltration. Percolating water slakes the shaly, angular spoil material at interparticle stress points, leading to excessive deformation and settlement. A lumber processing facility in Hazard, Ky., is an example of a structure that sustained serious damage as a result of settlement caused by water infiltration. A forensic site investigation of the facility revealed that excavation of existing surface mine spoil beneath the building footprint removed the low-permeability crust that forms on the top of mature mine spoil fill deposits. The removal of the crust allowed the infiltration of surface water. This, coupled with the unique configuration of the storm water drainage system at the facility and surface water drainage toward the building, led to differential settlement up to 1:120 (vertical:horizontal) and angular distortion up to 1:150 over a period of several months. Foundation underpinning was performed to remedy the situation. For future development on mine spoil sites, recommended mitigating measures include presaturation of the mine spoil, design of drainage systems to adequately convey surface water away from the building, and use of geosynthetic barrier layers to prevent infiltration of surface water into the mine spoil beneath the structure.     

Performance-Based Design Approach in Seismic Analysis of Bridges

The seismic design requirements used in the United States are based on the recommendations of the American Association of State Highway and Transportation Officials. These requirements are primarily based on the importance of the structure, the level of deformation imposed on the structure, soil conditions, and the ductility of structural members, especially piers and supports. In the performance-based design approach, the design is primarily focused on meeting a performance objective, which is in line with a desired level of service. Currently, the effort toward implementing the performance-based design approach in buildings is under way in the United States. The seismic performance criteria for buildings have been established and reported by various organizations. It seems that at least three levels of performance, ranging from “fully operational” to “near collapse” can be used to meet the postearthquake conditions, safety, usage, and occupancy for the varous levels of service expected from all types of structures. In this paper a critical evaluation of these performance criteria and their relevance to highway bridge design, in conjunction with the current design practice, is discussed. Various types of designs such as those based on strength, deformation, nonlinear behavior, and energy, which can be used to meet the specified performance levels in seismic design of highway bridges, are also discussed in the paper. Examples of real applications of the method in highway bridges are reviewed. Furthermore, the procedure by which the performance-based method has been implemented in these example cases is described and discussed in the paper.     

Building Robustness Research during World War II

This study reviews research carried out in the U.K. to understand and improve the robustness of buildings when subject to blast from high explosive bombs. The work concentrates on the performance of ordinary civilian buildings, with particular emphasis on multistory buildings framed in either reinforced concrete or structural steelwork. At that time, some of the data were used to enhance conventional building construction, principally on government buildings, and some were used to aid postwar hardened building construction. The two main U.K. researchers whose work is the basis of this paper (Professor Sir Dermot Christopherson and Professor Lord Baker) identified a number of building weaknesses that led to local or progressive collapse, including connections in steel-framed buildings, as well as detailing weaknesses in reinforced concrete constructions. This paper reviews these features, as well as those that added resilience to bomb damage, with particular emphasis to the use of masonry infill panels in framed buildings. Much of the information on building performance is relevant to today’s engineers engaged in the design of buildings to survive blast from terrorist attacks involving a vehicle-borne improvised explosive device.     

Finite Element Analysis of Concrete Pavements Over Culverts

Accelerated distress of Portland cement concrete pavements (PCCP) over structures such as culverts, pipes, and tunnels beneath roadways is a common occurrence. In this article, finite element analysis is employed to analyze the response of concrete pavements over such structures. The factors that influence the overlying pavement slabs include: (1) cover depth, (2) pavement slab thickness and length, (3) cement concrete elastic modulus, (4) foundation modulus, and (5) backfill soil modulus. The tensile stresses at the bottom and top of the slab induced by wheel loads are predicted. In the traditional pavement design only the tensile stress at the bottom of the slab is considered to be significant. However, this study shows that the tensile stress at the top surface of pavement slabs over culverts may also cause the concrete pavements to fail. A laboratory model was employed to study the mechanical characteristics of Portland cement concrete pavement slabs over culverts and to verify the theoretical analysis.     

Performance of Pile-Supported Bridge Approach Slabs

A large number of pile-supported bridge approach slabs in southeastern Louisiana were examined to identify the factors that affect their long-term performance. Design drawings and subsoil conditions at these sites as well as their traffic and maintenance records were compiled, and seven representative test sites were selected for thorough field investigation that included inspection of the approach slabs, bridge decks, bridge abutments, and roadway pavement. Field evaluation included walking profiler, falling-weight deflectometer (FWD), laser profiler, geodetic survey, soil borings, cone penetrometer, and nondestructive testing. Measurements made with the walking profiler agreed well with the geodetic survey. The FWD and nondestructive testing were effectively used to detect voids under the approach slab. Results of the study indicated that the current empirical methodology used by the Louisiana Department of Transportation and Development for design of pile-supported approach slabs yields inconsistent field performance. It was concluded that this inconsistent performance is primarily due to the differences in roadway embankment design and construction and in subsoil conditions, which in turn affect the negative skin friction (downdrag) loads imparted on the piles. Impact of other variables such as ramp type, speed limit, traffic volume, and so on was found to be insignificant. Results of the field study were used to develop a new rating system for approach slabs (IRIS) based on International Roughness Index (IRI) measurements obtained with the laser profiler.     

IDA Comparison of IBC-Designed and DDD-Designed Six-Story Light-Frame Wood Buildings

The current International Building Code (IBC) allows wood-frame (i.e., light-frame wood) construction to be three stories high or four stories high if sprinklers are included in the design. Several jurisdictions within the United States have opted to allow four and five stories if sprinklers are present. This paper presents a comprehensive numerical analysis of a six-story wood-frame building designed on the basis of the 2006 IBC methodology and a building designed by using the direct-displacement design (DDD) approach. The objectives of the design methods are different and are explained herein. Incremental dynamic analysis (IDA) was used to numerically investigate the response of the buildings on the basis of two criteria: the peak interstory drift and the building’s base shear capacity. Gypsum wall board was included in both models, but exterior finishes (e.g., stucco) were not.     

Survey of Residential Foundation Design Practice on Expansive Soils in the San Francisco Bay Area

The San Francisco Bay Area, California has experienced significant population growth over the last 60 years. The design practices for residential foundations have evolved substantially over this period, as a result of improved geologic characterizations, better engineering understandings of foundation performance, building code changes, and project litigation. The majority of residential foundations are constructed on expansive soils and bedrock, with the primary movement as a result of swell due to wetting of materials in an arid environment. A survey of design practice of practicing geotechnical engineers in the bay area was conducted using a written questionnaire and telephone interviews to compile data regarding the most commonly used design procedures, design details, drainage recommendations, and construction monitoring practices. The results of this survey are compiled and presented in this paper. Three primary foundation systems were identified in the survey as being commonly used in the bay area—rigid footing grids, drilled piers, and mats/slabs. To illustrate problems that have occurred with each of these foundation systems, case histories are presented for recent bay area expansive soil projects for each of these three foundation types.     

Centrifuge Testing to Evaluate and Mitigate Liquefaction-Induced Building Settlement Mechanisms

The effective application of liquefaction mitigation techniques requires an improved understanding of the development and consequences of liquefaction. Centrifuge experiments were performed to study the dominant mechanisms of seismically induced settlement of buildings with rigid mat foundations on thin deposits of liquefiable sand. The relative importance of key settlement mechanisms was evaluated by using mitigation techniques to minimize some of their respective contributions. The relative importance of settlement mechanisms was shown to depend on the characteristics of the earthquake motion, liquefiable soil, and building. The initiation, rate, and amount of liquefaction-induced building settlement depended greatly on the rate of ground shaking. Engineering design procedures should incorporate this important feature of earthquake shaking, which may be represented by the time rate of Arias intensity (i.e., the shaking intensity rate). In these experiments, installation of an independent, in-ground, perimetrical, stiff structural wall minimized deviatoric soil deformations under the building and reduced total building settlements by approximately 50%. Use of a flexible impermeable barrier that inhibited horizontal water flow without preventing shear deformation also reduced permanent building settlements but less significantly.     

Survey of Window Retrofit Solutions for Blast Mitigation

Explosive attacks on buildings create a rapid release of energy in the form of a shock wave. Window glass is often the weakest part of a building, breaking at low pressures compared to other building components. Explosion-related injuries caused by glass breakage include eardrum damage, lung collapse, and penetration- or laceration-type injuries from airborne glass fragments which may lead to fatalities. As glass breakage may extend for miles for a large external explosion, reducing glass hazard is an important aspect of blast-mitigation for buildings. Blast-mitigating window products are in their infancy in the United States. Categories of available window retrofit solutions include: window film technologies, mullion and frame upgrades, catch systems, window replacement systems, installation of secondary windows, and/or a combination of these technologies. Installation of daylight film is the least costly but provides the lowest level of protection, while replacing windows is the most costly with the greatest gain in blast protection. Selection of the best window retrofit solution is usually based on a number of design constraints including blast criteria, building constraints—which can be broken down into type of building, maintenance and operability requirements, interior finishes, energy conservation requirements, lifecycle—and finally the retrofit cost.     

Issues in Building Maintenance: Property Managers' Perspective

A survey was conducted of the largest 230 property management firms in the United States to investigate their current maintenance practices. The findings shed light on property management firms' policies concerning outsourcing versus using in-house maintenance services; their use of preventive∕routine∕corrective∕deferred maintenance; the frequency of their meetings with tenants; their policies about cleaning, inspection, repair, and replacement; the way they select maintenance contractors; the severity of the maintenance-related complaints they receive from tenants; the extent to which they are involved in the design process of the buildings they subsequently manage; and the frequency with which designers come back to assess their buildings' performance after the projects are completed. Property managers should be cognizant of building users' concerns and they should also make designers aware of these concerns; the performance of buildings is likely to be enhanced if designers and property managers take action in the design and operation phases, respectively, in response to users' concerns.