Structures Congress 2018: Bridges, Transportation Structures, and Nonbuilding Structures

Being only the third installation of spliced concrete girders in North Texas, the authors will highlight the unique aspects of these systems, and share some of the knowledge gained and experiences learned from involvement with this high-profile, complex long-span bridge design and construction project. Six long-span continuous concrete post-tensioned spliced girder bridges cross the Trinity River within the Horseshoe project, each with multiple spans of least 250 feet. These bridges span a long-planned, but as-yet unrealized chain of recreational lakes within the river levees, as well as many bike and pedestrian trails. The interrelationship of design and construction for these girder systems are explored and explained.

Many steel bridges built prior to mid-1980s are affected by distortion induced fatigue cracks, caused by out of plane bending of girder web gaps. Welded connections between longitudinal girders and lateral cross beams or diaphragms are most susceptible to cyclic out-of-plane deformations, resulting in significant second order fatigue stresses. Existing design procedures do not provide recommendations on how to directly estimate the magnitude of these stresses. Research using finite element modeling along with experimental field monitoring was conducted at the University of Minnesota Twin Cities to develop formulas to estimate web gap stresses. This paper presents an efficient correlation-based approach to determining the magnitude of the distortion induced fatigue stresses with limited fatigue analysis of few connections, while accounting for all bridge geometric characteristics. Linear regression analysis was used to establish statistically based relationships between web gap distortions and fatigue stress ranges. This approach was successfully implemented in the IH-345 viaduct rehabilitation project in Dallas, Texas, and resulted in significant saving in computational time without sacrificing the level of accuracy in fatigue stress calculations.

Residual stress-based treatments are proven to be an effective, reliable, and economical means for extending the service lives of steel bridges. When these treatments are applied, a compressive residual stress field is introduced near the surface of the treated weld toe, which delays, or even arrests, the growth of small fatigue cracks. Recently, the fatigue life increase resulting from impact treatments has been acknowledged in the International Institute of Welding (IIW) recommendations by explicitly permitting an increase in the structural stress class due to the treatment of non-load carrying welds from FAT-100 up to FAT-125. This presentation summarizes a comprehensive research program and case studies that were undertaken with the goals of examining the fatigue performance of treated highway bridge welds in the high cycle domain under simulated in-service loading conditions and developing S-N design curves for stress design of treated weld toes. In particular, the concept of using S-N design curves with m = 5, instead of the traditional m = 3, for the structural stress design of treated weld toes are introduced in this paper.

This study presents a practical way to include deep foundation elements, such as drilled shafts and soil effects (through p-y, t-z, and q-z curves), in the modeling of bridges. Superstructure, substructure, and foundations are modeled using finite elements. Shafts are supported by a collection of horizontal (p-y) and vertical (t-z) springs along pile length; at the tip of the shaft a vertical spring (q-z) is included. The incorporation of the nonlinearity of p-y, t-z, and q-z curves is solved by an iterative process using secant stiffness. Our studies have shown that this is a simple process requiring only a few iterations. These iterations are performed using linear analysis with typical finite element software in conjunction with spreadsheet processing. The procedure has been used in the analysis and design of White River Bridge an 18-span bridge in a seismic region with large scour, and for several bridges of the California High Speed Rail project where liquefaction was included. The study includes a range of foundations to incorporate both single-shaft and multi-shaft foundations, and describes a process that can also be applicable to buildings and other deep foundations.

Infrastructure risk due to fire has been well documented. In particular, bridges in the urban environment are susceptible due to low overhead clearances on overpasses, existing simple spans with no redundancy, narrow lane widths, and general alignment congestion. Although the risk is significant, the current AASHTO Bridge Design Specification does not include provisions for design due to fire conditions. As such, this paper considers alternatives for improving the fire resistance of steel bridges using both traditional and non-traditional fire protection methods. In this paper, a total of eight parameters are investigated: (1) global section factor (W/D ratio) of bridge girder cross-section; (2) thickness of the flange; (3) thickness of the web; (4) steel material specification; (5) concrete slab width; (6) concrete slab thickness; (7) thickness of intumescent paint; and (8) thickness of SFRM. It is shown that the temperature domain performance can be significantly increased by (1), (3), (7), and (8) while the other parameters are less effective. The recommendations from this research can be readily implemented in both bridge design and retrofit scenarios.

Precast, prestressed concrete has long been used in bridge construction. The production of prestressed concrete members involves the use of special prestressing equipment, requiring a prestressing bed and skilled labor, which eventually increases unit cost and production time. Thus, prestressing is uneconomical for short span members unless the same unit is used repeatedly. Often, camber-related issues pose challenges for designers, fabricators, and contractors. Additionally, prestressed concrete members are prone to long-term loss of prestressing forces due to creep, shrinkage, and relaxation. The prediction of long-term prestress losses is, in general, cumbersome and by no means accurate. This study explores a new concept of replacing prestressed concrete structures with non-prestressed ultra-high-performance fiber-reinforced concrete (UHP-FRC) structural members. UHP-FRC has a very high compressive strength (generally greater than 20 ksi (138 MPa)), a flexural strength of up to 3 ksi (21 MPa) or higher, great durability, low permeability, and exceptional compressive ductility with a maximum useable compressive strain of 0.015–0.03. The maximum useable compressive strain, εcu, specified in the ACI 318 building code and AASHTO LRFD Bridge Design Specifications are limited to 0.003 for conventional plain concrete with little ductility and a maximum compressive strength of about 15 ksi (103 MPa). Since the maximum useable compressive strain of UHP-FRC is 5 to 10 times that of plain concrete, this allows the designer to provide more reinforcement and, hence, increase the flexural capacity of members. The higher amount of reinforcement also increases the cracking resistance and decreases the deflection of UHP-FRC flexural members due to the smaller steel stresses. The very high modulus of rupture in UHP-FRC allows no cracks under service load, which was the purpose of prestressed concrete when invented several decades ago by Freyssient and others. This research investigates the flexural and shear behavior of UHP-FRC beams, reinforced with grade 60 steel (ASTM A615/A615M) and grade 100 steel (ASTM A1035/A1035M) bars. Experimental outcomes have shown promising results by retaining high stiffness up to very large loads, which can effectively control the deflection. UHP-FRC beams also have high initial cracking strengths with a very high modulus of rupture of approximately 3 ksi (21 MPa). The authors extended these experimental results to propose a new modified deck bulb tee (DBT) girder, which has nearly the same cracking resistance as typical prestressed DBT but without camber. This would facilitate accelerated construction of such bridges. Potential application of this proposed concept of replacing prestressed members with non-prestressed UHP-FRC members offers efficient design and construction practices by eliminating the issues with prestressing.

Strengthening existing concrete columns using fiber-reinforced polymers (FRP) in the form of longitudinal near-surface-mounted (NSM) bars has not gained that much attention due to the possibility of premature crushing and/or buckling of NSM bars. This paper investigates a hybrid system using both NSM and wrapping methods to prevent the premature failure and extend the contribution of NSM bars due to extended strain of confined concrete. A total of 21 plain concrete cylinders (150 mm x 300 mm) were prepared, strengthened, and tested to characterize the performance of the hybrid system. Multiple glass FRP (GFRP) bars (#4) were mounted in 20 mm by 20 mm surface grooves, and unidirectional basalt FRP (BFRP) was used to wrap the specimens. The specimens were instrumented with multiple strain and displacement gauges and loaded under uniaxial compression. It was shown that the wrapping system effectively prevented the premature failure and extended the contribution of NSM bars, significantly.

Early stage objective information relating to the structural integrity of civil infrastructures such as bridges is critical for design verification and infrastructure management. Creating structural models validated with the collected structural response data from an instrumented bridge develops a comprehensive database for reliable performance prediction of the bridge under various environmental and traffic demands. In this paper, the newly reconstructed Memorial Bridge is considered as a case study to develop such a finite element (FE) model. The typical challenge associated with the bridge structural response is the behavior of the non-instrumented components which are addressed using a fully detailed three-dimensional finite element model, where all components are represented by shell elements. This detailed model is considered as a benchmark to assess a more efficient multi-scale model that uses a combination of shell and beam elements. This paper presents a comparison of the structural response of the multiple scale models and remarks on the accuracy of a full shell model and the practical application of a beam element model. This comparison includes computational effort consideration, an objective of the analysis.

Integral abutment bridges (IABs) in Illinois can be subjected to a wide range of seismic hazard, particularly due to the proximity of the southern region of the state to the New Madrid Seismic Zone. The lack of expansion joints in IABs places a greater emphasis on the interaction between abutment piles and surrounding soil during seismic events. The present research incorporates soil-structure interaction into an IAB model that also comprehensively represents other critical components in order to determine overall seismic behavior of a three-span steel IAB in southern Illinois. The IAB is analyzed under three soil conditions-one based on actual soil property data in southern Illinois, and two representing realistic stiff and soft soil boundaries for southern Illinois. Results demonstrate the effects that soil conditions have not only on the foundation piles, but also on the IAB as a whole.

A pedestrian bridge linking Moser Hall and Cushwa Hall at Youngstown State University (YSU), Youngstown, Ohio, was built almost 40 years ago. All three hammerhead pier caps supporting the bridge deck have experienced severe corrosion at the bottom. This corrosion has led to spalling of concrete at multiple locations and thereby exposed the reinforcing steel bars to open air facilitating more corrosion. An investigative study was undertaken to identify the causes of corrosion in these substructure units and to propose some viable remedies to this problem. The Schmidt hammer and profoscope were used to approximate the concrete strength, reinforcing steel bar locations, and clear covers. Reviews of previous research suggest that substructure corrosion can likely be carbonation-induced. Although carbonation-induced corrosion is a slower process in a dry environment and mostly occurs in older structures, it was predicted that the corrosion in those pier caps was due to the carbonation of concrete. Carbonation reactions take place on the outer surface of concrete and gradually penetrate through the concrete cover resulting in a low pH in the concrete. A full-scale structural analysis revealed no major reduction in strength of the pier caps. Physical inspections were conducted to ensure no major deficiency exists in the corroded concrete and steel reinforcement. After considering various repair methods, patching with fiber-reinforced concrete or low-slump dense concrete coupled with hydrophobic coating, and cathodic protection at damaged locations were suggested as the best remedial measures to protect the substructure from further deterioration.

Cracks in concrete shorten the life of bridge decks by providing easy access for deicing chemicals. This paper investigates the use of Type-K cement as a partial replacement of Portland cement to potentially reduce cracking in bridge decks due to drying shrinkage. The first portion of the investigation consisted of large-scale laboratory experimentation conducted through the use of 2 m x 3 m experimental concrete bridge deck models. The second phase of the research focused on monitoring a newly constructed bridge deck located in Peoria, Illinois. The research involved both experimental and field investigation in order to provide a better understanding of how strain changes under service conditions, and to compare and contrast the measured strains within the large-scale experimental deck. The results showed that Type-K cement expands during early-age hydration producing compressive stresses in the concrete and reduces the onset of cracking in bridge decks.

Precast pre-stressed box beam bridges are extensively used in short and medium span bridge design throughout the United States. Recently, skewed geometry and heavy pre-stressed box beams were developed. Field inspection of such beams indicated most of them experience vertical and inclined cracks within end regions of the box beam. The range of crack widths was within the limit that is required to be repaired before being used in the bridge construction. The depth of cracks was found to be very close to the tie rods and beam’s reinforcement. Field observation also indicated that cracks were more intensive at the acute corner than the obtuse corner. The beams with cracks form a durability concern, especially when the water and chemical agents filtrate through the cracks and reach the tie rods and strands. Therefore, a three-dimensional nonlinear finite element model was developed to simulate the behavior of the box girders after releasing strands. Case study girders surveyed during site visits were used for modeling and validating finite elements. The beams were modeled with actual material properties, geometry, and strands cutting sequence. The finite element results were validated with measured data in terms of crack locations and upward deflection. Two of the most appropriate crack-controlling techniques found in the literature were evaluated with finite element analysis: modification of beam design and debonding strands. The results of this study showed the modified girder design technique significantly minimize cracks at the top of the end zones if two strands with full pre-stressing forces were added around the vertical centerline of the top flange. A 35% level of strands’ debonding technique appeared to be effective in minimizing the maximum tensile strain at the beam’s ends. Debonding exterior strands was more effective on reducing tensile strain than debonding interior strands. The box beam with two strands at the top flange is recommended for implementation by skewed box beam design.

This paper presents an innovative precast through-girder system that involves using ultra-high performance concrete (UHPC) in connections. This can be adopted in shallow bridge structures and achieves both enhanced durability and simplified construction. The superstructure proposed includes precast concrete through-girder segments that are spliced longitudinally through post-tensioning. It also includes precast prestressed double-tee transverse beams that span between the through girders. An important feature of this system is that it uses UHPC at the transverse joints between adjacent transverse beams, as well as at the longitudinal joints between the transverse beams and the through girders. The use of UHPC allows the joint widths to be minimized and eliminates the need for longitudinal post-tensioning across the transverse joints, which simplifies and accelerates construction. Further, using UHPC enhances structural performance overall significantly and reduces maintenance costs.

Rotation of exterior girders during bridge deck construction is an issue that many Departments of Transportation (DOTs) around the U.S. are facing. The transverse bracing systems are commonly used to resist the torsional moments resulted from the deck construction loads. This study focuses on the bracing systems including diaphragms and cross-frames, and it targets the different methods used for connecting the diaphragms to the steel girders. Finite element (FE) models are developed to assess the differences among these connecting methods. The results showed that for bridges with diaphragms, using welded connections generally reduces the rotation. On the other hand, using bolted or welded connections in bridges with cross-frames has minimal effect on the rotation. It can be concluded that the effective height of the connection member is the major factor that affects the rotation of the exterior girder due to construction loads.

This study focuses on creating mechanistic thermal and structural analysis models for determining thermal stress and cracking in massive concrete structures when alternative, unconventional, or innovative concrete mixtures are considered for design. Current study, funded by the Georgia Department of Transportation, investigates thermal modeling techniques, applying theory to practice, in order to control cracks caused by hydration heat. Thermal cracking caused by hydration heat is detrimental to the long-term durability of large infrastructure such as bridge seals, foundations, and piers. In the proposed analysis, environmental conditions such as convection, weather variation, and solar radiation are considered in a multi-physics model. The thermal model will be validated by an experimental program. Subsequently, a structural model is designed to read in a nonlinear temperature profile and perform a nonlinear cracking analysis of structures based on the boundary and environmental conditions of a bridge seal structure. It is concluded that the proposed evaluation process is efficient and robust in evaluating the effect of innovative and unconventional concrete mixtures that are optimized to control the maximum temperature and temperature variations on mass concrete structures. It is also concluded that the experimental and analysis procedure is highly efficient to validate and streamline the thermal and structural evaluations without a loss of accuracy.

The Albion River Bridge consists of a timber deck, a steel truss superstructure, approach timber trusses, four-legged R/C piers for the steel trusses, timber-braced towers to support the approach timber trusses, and R/C pile caps with R/C or timber piles. The 2013 condition ratings of the deck, superstructure, and substructure are “good,” i.e., 7 out of 9 based on the FHWA National Bridge Inspection Standards, which makes the bridge “structurally not deficient.” However, the latest (2015) Structure Inventory and Appraisal Report by Caltrans has incorrectly assigned a rating of 3 out of 9 to the substructure, and declared the bridge “structurally deficient.” This paper discusses the studies conducted by the author to preserve the bridge. The efforts by the author and the Albion community have resulted in the listing of the bridge, in May of 2017, on the National Register of Historic Places of the U.S. Department of the Interior.

Unmanned aerial vehicle (UAV)-enabled bridge inspection has gained more interest over recent years among bridge owners, researchers, and stakeholders due to inefficiency and high cost of conventional access inspection techniques. A large number of deteriorating bridges can be efficiently inspected using UAVs equipped with various sensors. In fact, some departments of transportation (DOTs) (e.g., Minnesota DOT) in cooperation with research institutions have investigated the effectiveness of UAVs as a cost-efficient bridge inspection alternative. Based on the findings from the projects done by the DOTs in their states, this paper is intended to demonstrate the effectiveness of a UAV by inspecting a timber arch bridge in the State of South Dakota (SD). The bridge inspection using UAV was completed based on multiple analyses of high-resolution images and videos recorded from the UAV. Further, the use of a pixel-based damage quantification methodology provided a quantifiable value for the observed damage. The visual results obtained from the UAV-based bridge inspection were compared to those from the past inspection reports from SDDOT. The comparison of results demonstrated the ability of the UAV to identify damage. It is expected that this emerging technology will supplement routine bridge inspections conducted with conventional methods.

Structural health monitoring (SHM) has become interchangeable with strain gages, displacement transducers, and accelerometers, among other sensors, and the complex and extensive set of data that most think one needs a Ph.D. to understand. The challenge of extracting actionable information from complex structural response data is certainly not trivial and is an ongoing pursuit in the SHM community, both professional and academic. Traditional SHM metrics include load distribution, degree of composite action, modal parameters, movement system performance, among many others. However, this view of ‘structural health’ has been changing recently due primarily to a shift in how the end-users of SHM systems envision how it will provide value to their duty to maintain operable structures. Specifically, bridge managers are looking to SHM to provide value to operations and maintenance managers and to integrate within agency asset management systems. This paper aims to discuss how SHM can inform not only traditional structural health metrics but also operations and maintenance metrics. A case study will also be presented to demonstrate how an in-service SHM system has been integrated into a bridge agency’s asset management software platform.

Bridge inspections are important to proactively ensure that structures are safe for use by the traveling public. Accessibility challenges during inspections often necessitate the use of special equipment, such as under-bridge inspection vehicles, which may increase both cost and risk of inspections. Under-bridge inspection vehicles, however, cannot be used on all structures, such as load restricted structures. The objective of this research project is to develop and demonstrate a small unmanned aerial vehicle (sUAV) that can be used to perform bridge inspections that would typically require the use of under-bridge inspection vehicles. The proposed sUAV system builds off of previously conducted studies, which have faced challenges with insufficient GPS signal to fly underneath a bridge. By combining information from an optical flow sensor and a range-finding sensor, the proposed system offers a method of safely and efficiently operating underneath a bridge where a GPS signal may be denied. Additionally, the sUAV can capture images of the undersides of bridges using a top-mounted gimbal and a high-resolution camera. This paper provides an account of the methods and results of several demonstration sUAV bridge inspections. It then discusses the advantages and ongoing challenges associated with using sUAV systems to supplement bridge inspections. Finally, recommended applications for sUAV bridge inspections are provided, including periodically replacing the use of under-bridge inspection vehicles for inspections on bridges in good condition, and inspecting structures with load restrictions, especially post-impact, which may prohibit the use of under-bridge inspection vehicles.

This paper presents the capabilities and limitations of high-definition (HD) imaging and infrared thermography (IRT) using unmanned aerial vehicles (UAVs) for bridge inspections. Regarding HD imaging, this study shows the potential to detect the required minimum, 0.1 mm, width of cracks using simulated cracks at a distance of 1-3 m from the bridge. As for IRT application with UAVs, the effect of vibration caused by the drone was investigated. This paper concludes that the hovering flight of drones was extremely stable and IRT with UAVs can provide reliable data for bridge inspections. If IRT is used without hovering flight, the effect of flying speed must be investigated since it might also affect the IRT results. This paper shows great potential of image-based technologies with UAVs for bridge inspections without any traffic disturbance as a complimentary approach to current practices.

Because of continued depletion of natural resources and the impact of carbon emissions on the environment, it is essential that buildings and bridges perform optimally with minimal damage when subjected to atypical loading demands. Today’s building codes are focused primarily on life-safety, not on performance. With the support of research and practice, codes of the future are expected to specifically address minimum expected levels of performance and perhaps will require structures to behave with limited damage when subjected to extreme loadings such as caused by major earthquakes. Friction historically has been used as a means of controlling behavior of structures through devices such as seismic isolators (e.g.: friction pendulum isolators) at the bases of buildings but rarely, if ever, has friction been used to control behavior directly in building frames. Friction can be used to additionally damp structures as well as to allow the dissipation of energy through heat. What is most important about the use of friction devices is that devices offer the possibility of protecting associated structural elements from damage. Pin-Fuse seismic systems have been under development for several years and one system is currently under review by the AISC’s Connection Pre-qualification Review Panel (CPRP). These systems include devices incorporated into building frames to dissipate energy during earthquakes, supplement structural damping, to allow structures to remain essentially elastic in major seismic events, thus enabling them to remain in service after such events. This paper will present the latest breakthroughs relating to the performance of non-metallic friction materials in the devices, proposed applications for the devices in buildings both new and retrofitted, a summary of full-scale testing relating to pin-fuse device and component testing performed to date, and planned future testing.

Deteriorating bridges require maintenance and repair activities to extend their service life and maintain a satisfactory performance level. Additionally, bridges located in highly corrosive environments, due to severe chloride exposure or marine environmental effects, may require more frequent maintenance actions during their life-cycle. These maintenance actions can lead to indirect costs associated with traffic delays and environmental effects, in addition to the direct cost of maintenance actions, which can significantly increase the life-cycle cost of the bridge under consideration. Accordingly, these indirect effects should be considered for the proper life-cycle management of deteriorating bridges. In addition, climate change may affect the performance of bridges vulnerable to flood and corrosion. This paper discusses probabilistic approaches applicable in optimizing maintenance activities of deteriorating bridges while considering climate change effects, direct maintenance costs, and indirect impact arising from maintenance and repair actions. Uncertainties associated with the various stages of the life-cycle management are discussed. The optimum maintenance strategies can be established through an integrated framework which minimizes the total life-cycle cost of the bridge under investigation while maintaining the desired performance level throughout the service life.

There is an increasing importance of the topic of the extension of life-cycle of existing structures and infrastructures. This fact is corroborated by different reports, e.g. the 2013 ASCE Report Card for America’s Infrastructure. Three important elements are required in the extension of the life-cycle of structures and infrastructures: (i) the availability of repair/strengthening techniques for the specific problem under consideration; (ii) assessment of the effectiveness of the strengthening method; and (iii) implementation of strengthening techniques in design standards following semi-probabilistic formats. Concrete is the most used construction material in the world; as a result, among those structural elements in need of repair/strengthening are reinforced concrete (RC) columns. Utilization of FRP jackets in the confinement of RC columns has received large attention from both the academia and practitioners. In this paper we discuss a number of the issues related to this technique in the light of the three aforementioned requirements.

Current industry trends in bridge load ratings acknowledge both the benefits of refined ratings and the added expense. Refined ratings are used sparingly in part because of the added level of resources needed produce and evaluate analytical models. This additional effort consists of critical broad engineering decisions such as selecting a modeling approach as well as more specific and laborious tasks like manipulating a finite element analysis program via a graphical user interface (GUI) to build and analyze a model. By separating the broad engineering decisions and passing them as input into a series of automated algorithms that carry out model building, analysis, and results extraction, the refined rating process can be made much more efficient and consistent. The biggest challenge of automating this process is balancing the flexibility and robustness of the process with the amount of engineering effort required to develop the necessary input. The objective of this paper is to demonstrate the value that an automated approach can bring to a refined load rating. Automation also makes a process completely repeatable and more transparent than traditional refined analysis. This paper discusses the refined rating process, as well as the challenges and benefits of automating this process for multi-girder bridges.

Most civil engineering structures are designed using justifiably conservative models with the goal of achieving life-safety. The use of conservative models during design and safe construction practices creates reserve capacity enabling structures to last well beyond their design life. Understanding structural behaviour can enhance the effectiveness of asset-management tasks such as retrofit or replacement of these structures. With advances in sensing and computing technology, it is now possible to interpret vast amounts of measurement data to evaluate structural performance. In this paper, the application of a sequence-free iterative structural identification framework on a steel-concrete composite bridge is presented. The iterative nature of decision-making is illustrated through varying conditions during the service life of the bridge, in this case increased traffic intensity. The engineer utilises new information as it becomes available to update knowledge of structural behaviour and thus update predictions of reserve capacity.

This study investigates the vulnerability of coastal bridges along the Atlantic coast of Georgia and develops a fragility curve which describes the probability of failure such as deck unseating in terms of hurricane categories. Improved resiliency largely provides environmental benefits such as reduced construction, demolition, and/or replacement materials. This study is focused on the design phase of life and recommendations for bridge bearing connections between superstructure and substructure systems. Dowels are often used in bearing connections because they are easy to install and protected from corrosive coastal environment. They also enable the rapid construction of precast bridges, which minimize environmental impacts of construction activities. It is concluded from this study that anchor bolts are generally more effective in providing vertical uplifting resistance to hurricane-induced forces although they are more susceptible to corrosion damage. It is recommended that stainless steel bolts and precast bridge systems including dowels used in conjunction with external restrainers which optimize the sustainability performance of coastal bridges be considered.

This paper presents two analytical models, Model Fiber and Model RS, which predict the seismic performance of two repaired specimens using a carbon fiber-reinforced polymer (CFRP) shell and epoxy anchored headed steel bars to relocate the column plastic hinge. In Model Fiber, distributed plasticity considering bond slip is assumed to be concentrated in a plastic hinge length of the nonlinear beam-column element instead of the whole length of the element; this was implemented using the BeamWithHinges element in OpenSees. In Model RS, concentrated plasticity was considered using a non-linear moment rotational spring located at the repaired cross-section. A sectional moment-curvature analysis is performed, based on damaged steel properties and considering bond slip, to obtain the moment-rotation relationship, which is assigned to the non-linear rotational spring. Both analytical models proposed in this paper include low-cycle fatigue of longitudinal steel bars and bond-slip effects. Numerical simulations show that the analytical results, in terms of hysteretic response, cumulative hysteretic energy, and bending moment-rotation, agree with the experimental results.

Bridges are one of the most important lifeline structures such that loss of functionality of the network system is directly related to the social and economic losses experienced following an earthquake. Due to the potential of strong aftershocks to cause further damage to mainshock damaged bridges, accurate evaluation of the structural performance during a real seismic sequence is essential. This paper investigates the seismic damage of a reinforced concrete (RC) skewed bridge subject to a sequence of earthquake ground motions. Fragility curves are developed for both with and without consideration of the effects of aftershocks on the seismic vulnerability of the bridges for two cases: original bridge and retrofitted bridge by FRP. Results of this study have revealed that the effect of aftershocks on damage states appears to increase the seismic vulnerability of RC bridges. In addition, the strategy of retrofit will significantly affect the seismic response and therefore fragility of bridges.

Fiber-reinforced polymer (FRP) bars are gaining increased use in the construction market as a replacement to steel reinforcement for concrete structures. However, their use as compression members is limited due to their mechanical properties, typically low flexural rigidity, so that local buckling may occur at relatively low stress levels. Their premature rupture in RC columns could be eliminated by enhancing the columns’ response elastically and concentrating damage to the externally installed, replaceable post-tensioned structural fuses. This paper is part of a larger numerical and experimental investigation to develop and validate design methods for a rocking Aramid fiber reinforced polymer reinforced concrete column. The numerical study, implemented using the Extreme Loading for Structures (ELS) software, consists of a total of thirteen columns. The columns were subjected to axial load followed by cyclic lateral load up to failure to examine the effectiveness of the rocking connection and overall response. The effect of key variables such as longitudinal bar ratio, structural fuse sizes and tie spacing was investigated. Ductility and energy dissipation gains of the rocking AFRP RC columns were compared to that of the conventionally reinforced steel column, and expected performance of the rocking connection was achieved.

In recent years, the concrete-filled fiber reinforced polymer (FRP) tube (CFFT) system has been widely studied as a durable and cost-effective alternative design for bridge columns. This system has been formally recognized by AASHTO for application as load-carrying components of bridges. This system eliminates the need for scaffolding, framework, and form removal that leads to cost savings and simplified construction. While a CFFT system with no rebar reinforcement is attractive in its ease of construction, the inadequate energy dissipation capabilities limit its use as structural components in areas subjected to extreme loadings such as earthquakes. Research has shown that a lightly reinforced CFFT system provides the energy dissipation required for resisting extreme events. A novel hybrid concrete-filled FRP tube (HCFFT) column system is proposed to combine the ease of construction of conventional CFFTs with energy dissipation characteristics of lightly reinforced CFFT columns. The HCCFT column system consists of a hybrid metal-nonmetal FRP tube filled with unreinforced concrete. The hybrid FRP shell is manufactured by adding layers of longitudinal steel fibers during the filament winding process. The hybrid FRP combines the energy dissipation capability and ductility of continuous steel fibers with the excellent strength-to-weight ratio and re-centering capabilities of glass fibers. The polymeric resins improves the durability of the columns. The manufacturing process for the hybrid shell is presented. To evaluate the performance of the system, moment curvature analyses were conducted using material properties obtained from experimental tests. The material properties used in the analysis were evaluated on a component level. The material properties were applied to the column model and validated with compression data for a 152.4 mm (6 in) HCFFT. The validated model was then used to conduct moment-curvatures analyses on (16 in) diameter HCFFT columns to characterize the axial load-bending moment relationship. The axial force-bending moment interaction curves are presented for various steel fiber reinforcement ratios. This data is expected to help development of design guides for the HCFFT column system.

Precast post-tensioned column is a novel structural system that can be used as bridge piers. In such a system, the column does not have a moment connection with the foundation, therefore the rocking mechanism combining with post-tensioned force provides excellent deformation and re-centering capacity. The columns can be comprised of multiple segments or a single segment. This study examines the dynamic behaviors of post-tensioned columns under seismic excitations. The equations of motion are developed using Lagrange’s method and solved using fourth-order Runge-Kutta method. The effects of impacts, tendon stiffness, initial post-tensioning force, and characteristic of applied loadings on the dynamic behaviors are investigated. Results show that the vibration period increases with the increases in rotational amplitude. The increase in the tendon stiffness increases vibration frequency. Under sine and cosine pulse with the same amplitude and period, the rocking responses are different in terms of rocking amplitude. The use of post-tensioning tendons can help to reduce rotational amplitude and delay the initiation of rocking.

Fatigue cracking of rib-to-deck welded joints is a critical issue compromising the service life of orthotropic steel decks (OSDs). A new thickened edge U-shaped rib (TEU) was proposed by researchers to enhance the fatigue performance for rib-to-deck joints, but few research related could be found. This study evaluated the fatigue performance of rib-to-deck welded joints in OSDs with thickened edge U-ribs (TEUs) through fatigue tests. A total of seven rib-to-deck specimens were tested, including four specimens with TEUs and three specimens with conventional U-ribs (CUs). The fatigue strength of specimens was compared using both the nominal stress and hot spot stress methods. The results were plotted against the FAT 100 and FAT 125 S-N curves. In terms of the nominal stress range, all the four data points of the TEU specimens are above the FAT 100 curve, while only one of the three data points of the CU specimens is above the FAT 100 curve. A similar trend was found in terms of the hot spot stress range with respect to FAT 125 curve. Meanwhile, the average fatigue strength of TEU specimens was 20% higher in terms of the nominal stress range and 24% higher in terms of the hot spot stress range than that of the CU specimens. Thus, the specimens with TEUs demonstrated higher fatigue strengths than those with CU. Finally, two full-scale orthotropic steel deck specimens were tested to assess the feasibility and advantages of the TEU over the CU in field applications. The fatigue life of the rib-to-deck welded joints with TEU was increased by 105% (from 4.2 M to 8.6 M cycles), proving the effectiveness of TEU in enhancing the fatigue resistance of these welded joints. Overall, the use of TEUs enhanced the fatigue strength of rib-to-deck joints. The research results have been incorporated in the relative specification in China, and the TEU has been used in several engineering projects.

The paper describes how a severely damaged pedestrian suspension bridge located in a remote valley in the Nepal Himalaya was strengthened by repurposing the existing main cables and anchorages to support expected pedestrian and yak loads. A repair scheme was designed and implemented over a seven day period during a service learning trip in the summer of 2016. The strengthened bridge functions as a cable-stayed bridge without compressing the deck. Surveying showed that the main cable tensioning operation raised the deck three inches at midspan which reduced loads on the abutment anchor bolts. Any additional live loading would go directly into the modified structural system and not overload the deck or bolts.

This paper discusses the field investigation and analytical methodology developed to validate flame ducts at space rocket launch facilities. To be commercially viable, commercial space programs are using re-launchable spacecraft and current launch complexes. These launch complexes were originally designed for different spacecraft with different launch characteristics such as launch pressure and temperature time histories. Essential parts of a launch complex are flame ducts that are used to deflect the rocket's exhaust and debris away from the rocket. While these flame ducts were often designed and constructed to accommodate a range of rockets, it is desired to use existing flame ducts for newer, larger rockets located closer to the inlet of the flame duct. Under such conditions, it is necessary to perform field as well as analytical investigations to validate the adequacy of the existing flame duct.

Per the request from a client, S&L performed a comprehensive dynamic evaluation to determine the dynamic characteristics of existing fan foundations for evaluating their suitability to support new variable speed fans. The dynamic evaluation includes two parts: field dynamic test and finite element analysis. The field vibration test was conducted to determine the fundamental frequencies and dynamic stiffness of existing fan foundations. Based on the field test result, the finite element analysis was performed to evaluate the response of new fan foundation systems.

The selection of concrete mix design types affects the performance and constructability of petrochemical projects. There are many concrete mix design types that may be required for industrial projects including: controlled low-strength material, cellular concrete, self-consolidating concrete, high performance concrete, and chemical-resistant concrete. Each of these mix designs has unique properties and each has a specific use for petrochemical projects. Petrochemical structures have distinctive loading and durability demands due to their exposure to harmful chemicals and interaction with large mechanical equipment and piping systems. Revamp and retrofit projects should be done while the facility is in operation to minimize business interruption. If concrete work has to be done during a shutdown then the work must be done in a short duration. Specifying the proper mix design, therefore, is critical to the performance and constructability of a petrochemical structure.

Implementation of sustainability methods in the design of industrial structures can be mutually beneficial to both industrial facility owners and the environment. Industrial facility owners benefit from sustained production with minimal business interruption over the course of the life span of the facility, while the environment benefits from a reduction in both the demand of natural resources and the disposal of contaminated structural materials. Business sustainability is improved for industrial facilities by making efficient design decisions in the beginning that extends the design life of industrial structures and minimizes the interruption from environmental loads such as wind, earthquakes, snow, and ice. Performance based design methods can be used to determine the desired functionality of a structure beyond the life-safety requirements of the code when subjected to such events. Sustainability benefits to the environment can be achieved by minimizing and limiting the natural resources required for the retrofit and repair of industrial structures. Such revamp work requires a large amount of natural resources because it is necessary to work around existing operating facilities. Business sustainability approaches for the design of industrial facilities can result in a conservation of natural resources which provides benefits to our environment.

Steam-turbine-generator (STG) and combustion-turbine-generator (CTG), are at the “hearts” of power generations. They are heavy, yet very precisely-manufactured, rotational vibrating machineries, running either at 50 or 60 Hz (25 or 30 Hz for nuclear power plants). Normal operations of such equipment demand minimum dynamic amplification effects from supporting foundations, to avoid long-term deteriorations of critical mechanical components (bearings, shafts, rotors etc.). The foundations, often called turbine pedestals due to their massive appearances, must be designed to withstand all applicable load scenarios, including stringent stiffness and vibration criteria, stipulated by machine manufacturer and industrial standards, under cyclic, dynamic unbalance forces. In practice, structural engineers follow manufacturer's foundation requirement documents and applicable codes or standards to perform the design. However, manufacturer requirements do vary, and code or standard provisions are often subjected to interpretations when applied to such foundations. Thus, many practitioners have developed their own “proprietary” or "customized" approaches, based on own experiences and preferences, some of which are conflicting. The ASCE Energy Division recognized the need for a report that can provide practitioners with clearer instructions, guidelines, and recommendations for TG foundation design. To this end, the task committee for “Turbine Generator Foundations” was formed in April 2014, whose work is now published in Task Committee for Turbine Generator Foundations (2018). This paper summarizes the work of the task committee. Note: All figures in this paper are from Task Committee for Turbine Generator Foundations (2018).

The vibration response of turbine-generator foundations under the effect of normal machine unbalance loads is important for the long-term operation of combined cycle power plant facilities. In this paper, a case study of the forced-vibration analysis of a combustion turbine generator (CTG) foundation is presented, wherein the effect of three (3) different soil modeling approaches on the vibration response of a foundation supported on a shallow layer of structural fill situated above in-situ hard rock is investigated. A three-dimensional (3D) finite element model of the CTG foundation is developed using ANSYS and is subjected to harmonic loads at critical machine frequencies. The soil characteristics are modeled using different approaches, starting with the computation of frequency-dependent soil stiffness and damping coefficients using conventional methods, followed by the determination of frequency-dependent impedance function of the spring-dashpot system by explicitly modeling the site-specific soil and foundation profile in SASSI2010. The final approach explicitly models the structural fill layer using linear elastic 3D solid elements in ANSYS along-with the structural model. The results show that the conventional approach generally results in larger critical vibration amplitudes compared to the other methods, for which results match reasonably well. However, in some cases, the conventional approach may yield unconservative results. The case study highlights the importance of appropriately capturing soil-structure interaction (SSI) effects in the vibration response of turbine-generator foundations for non-uniform soil profiles. The approaches discussed in this paper can be used to supplement the conventional method, gain confidence, and mitigate the potential risks associated with long-term plant operability.

Concrete tanks are used in high seismic regions for water, chemical storage, and treatment. These tanks may be reinforced or prestressed concrete structures with rectangular or round shapes and may locate above or underground. Based on governing codes, such as ACI 350-06, ACI 373, the tanks should have strength, water tight, durability for cracking, and corrosion control. ACI 350.3 also provides equations for seismic forces of rectangular and circular tanks. However, in a design practice, we may still have issues that ACI codes don’t cover, such as tsunami events, and uplifting requirement of underground tanks. The section forces of critical load combinations are difficult to be found unless a 3-D computer model is created. After we have a computer model, we may still doubt what is the impact of soil sub-grade on the base, side soil spring for underground tanks because these data have a very big influence on dynamic properties of tank system, such as the fundamental frequency. We may wander, without these data, if the seismic analysis of tanks per ACI 350.3 is still reliable. Modeling of the complex liquid-structure and soil-structure interaction are required for a reliable design. In this study, the codes and phenomenon of concrete liquid tanks in high seismic regions will be reviewed and summarized, and the seismic analysis based on ACI 350.3 will be illustrated. The finite element methods of pseudo-static seismic analysis and responds analysis following original Houser’s assumption of ACI 350.3 will be presented.

Since the 1970s, traditional curtain wall engineering has used allowable stress design (ASD) level wind loads as its basis for design. Most curtain walls in this country have been engineered to resist wind loads based on a 50 year mean recurrence interval (MRI) for both strength and serviceability limit states. Hospitals and other buildings of high importance have been engineered to 100 year MRI wind loads. These design loads match the intended design life of the building and are reasonable for use in analyzing the structural adequacy of the framing systems that comprise the building envelope. At the same time there is much documented research to suggest they might be overly conservative for serviceability criteria. It has been argued that basing deflection criteria on an event that might occur only once in the life of the structure is too stringent. The International Building Code (IBC) “permits” a reduction to wind load for determination of deflection limits of exterior walls in Table 1604.3. This reduction approximates a reduction to a 10 year MRI for deflection analysis. However, with today’s high performance wall systems that are required to provide a very tight filter against the effects of the exterior environment as well as resist strong winds; the designer must be astutely aware of the limitations all components within the wall system as well as a wide variety of cladding materials. There is a trend toward engineering the framing members in high performance wall systems for reduced wind load without full consideration of all related serviceability considerations. This paper addresses serviceability limits for exterior wall systems and their design wind loads.

Industrial equipment can be elevated and supported in structures or at grade on foundations depending on equipment operations, process requirements, or construction methods used in modularization. It is required to consider each stage in the life of the equipment–including transportation and installation, operating loads and temperature changes, process upset conditions, environmental loads, and maintenance requirements–for the design of supporting structures. The support types and connections for equipment can be arranged differently because of the varying operating requirements for each piece of equipment. Structure behavior can be complicated for a group of equipment interconnected with mechanical systems, which may be supported on elevated structures, at grade or on separate structures connected by large size pipes or ducts, particularly for those equipment and mechanical systems supported by tall structures. It is essential to coordinate among engineering disciplines and vendor suppliers to achieve successful equipment supporting system design. This paper discusses design considerations for equipment support structures. Considerations include interactions between equipment and supporting structures, structural framing system selection, analysis model types, structural rigidity, design loads, and framing connections. Case studies of analysis modeling, brace layout comparison, connections between equipment and structural supports, load paths from equipment to supporting structures, structure natural frequencies, and structure to foundation connections are included and discussed.

Performing structural repair work on the North Slope of Alaska presents obvious logistical and environmental challenges. Geotechnical concerns must also be considered given the nature of permafrost and the effects of varying subsurface soil conditions. Many structures on the oil fields of the North Slope are comprised of large modules housing various types of industrial equipment. The equipment often generates significant amounts of heat resulting in design temperature differentials of up to 200 degrees Fahrenheit between the inside of the module and the outside artic air. This temperature differential along with several other factors result in unique deterioration mechanisms throughout the buildings. Before implementing repairs to address structural deficiencies, identifying the root cause of the damage is of critical importance. This paper is targeted at structural design and inspection professionals and will focus on the assessment of a specific module located on the North Slope with a deteriorated roof system. An on-site field investigation was initially performed to confirm as-built drawings and identify areas of corrosion and damage that could result in a reduction of structural capacity. During the site investigation, multiple buckled diagonal braces were noted throughout the structure’s braced frames, leaving it vulnerable to design level wind and seismic events. The forensic efforts undertaken to identify the root cause of the bracing failures, the evaluation performed to assess the criticality of the damage, and the repairs designed to restore structural integrity will be discussed. Ultimately, through the detailed site investigation and data evaluation, the root cause of the damage was identified and an appropriate repair strategy is currently in process to mitigate potential instabilities resulting from the failed lateral bracing members.

Industrial structures in oil refineries face unique challenges. Aside from the highly corrosive environments typically found within refinery processing units, the equipment supported by the structures can pose serious risks of fire. For example, many vessels containing flammable hydrocarbons operate in excess of 900 degrees Fahrenheit (482° C). Furthermore, no new major refineries have been built in the United States for over 30 years and many structures are nearing or exceeding the end of their design life throughout our nation’s refining infrastructure. All of these factors are contributing to a widespread need for structural rehabilitation. Another issue that is becoming more prevalent throughout the industry is damage caused by fireproofing deterioration on structural steel members. In the event of a fire, steel structures need adequate protection from high heat until emergency responders arrive. Protection is generally provided by concrete cast in place around critical structural steel members. This method provides adequate fire ratings; however, if not applied and maintained correctly, concrete fireproofing may result in significant structural deficiencies and safety hazards in the long term. This case study will focus on the rehabilitation of a fluid catalytic cracking (FCC) unit at a Midwest refinery and is targeted at structural design, analysis, and inspection professionals. A structural assessment of the 12-story braced frame structure was performed in response to a connection failure resulting from corrosion due to moisture trapped in the mastic fireproofing material. Because of the size of the structure and the extents of deterioration, a systematic approach was required to develop a long-term rehabilitation strategy. A detailed field inspection of the structural members was performed in conjunction with the development of a full structural model. In pairing the results of the field investigation with the utilization ratios obtained from the model, a practical rehabilitation strategy was developed.

Coker structure is a heavy industrial reinforced concrete construction in a refinery that supports a delayed coking unit, consisting of multiple coke drums, on an elevated platform. The process temperatures inside a coke drum can reach up to 480 °C (900 °F) and thus subject the supporting concrete structure to extreme thermal gradients. Due to these high temperatures in conjunction with mechanical stresses, the areas of concrete surrounding the coke drums deteriorate over a period of time and in most cases before the lifetime of the concrete structure. The deteriorated reinforced concrete elements needs to be addressed before they lose structural capacity which may lead to a structural failure. Thus, it is important to develop an effective repair strategy for the restoration of the Coker structures giving at most importance to the safety during construction and reliability of repairs to extend the lifetime of the structure. A Coke drum can reach up to a weight of 2.7×10⁶ kg (6000 kips) under its full operating condition. Considering the size of these drums, it is very critical to design a phased repair solution for the repair of Coker structures when they are deteriorated over the course of their service life. A phased repair solution is developed by analyzing the structure by redistribution of drum operational weight and wind force on the drum on to the structure’s non-critical members which are repaired in the subsequent phases of construction. The construction of the repair and the effectiveness of the repair to develop the structure’s original strength depends on the ability if the repair to transfer the planar stresses between the existing concrete and the new repair material. A repair mechanism to transfer these stresses will be discussed as a part of the best repair approach for concrete sections.

Existing criteria for protection of occupants in government owned and/or occupied facilities in the U.S. is largely based on estimates of damage limits for conventional façade structures and materials such as concrete, brick and concrete masonry, and glass. These damage limits are commonly based on extent of damage as a function of lateral deformation or ductility and the extent to which that deformation requires subsequent repairs or allows re-use. This approach works well for new construction where modest strength and mass increases can provide sufficient capacity at minimal cost so as to not exceed deformation limits. For existing buildings and historic facades, the approach can be onerous and inefficient, since retrofits to large sections of the façade may be required. This paper describes initial efforts to define blast performance requirements for existing buildings in terms of occupant hazard based on injury potential rather than on deformation limits. Existing injury data and simplified façade failure and debris kinematics and analytics are used to relate applied blast load to actual occupant hazards. The approach provides information to building owners and stake holders that can provide much more efficient and cost effective extents for façade retrofits. The retrofits required are also more efficiently localized to lower levels and higher threat locations near the building. The approach is currently being employed in assessments of existing structures throughout the U.S. where the supplemental hazard information can assist owners and stakeholders in funding and schedule decisions for retrofits.

The dynamic response of machine foundation system depends on several factors such as (1) the soil dynamic properties, (2) the geometric properties of the foundation, (3) the amplitude of the applied dynamic loads, and (4) the frequency of the exciting dynamic force. The main goal of machine foundation design is to keep the foundation response within a specific limit of response in order to enable a satisfactory operation of the machine. If the foundation response exceeds this limit, the foundation will adversely affect the performance of the machine and may damage the machine internals, or cause it not to function properly. Furthermore, the excessive vibrations will impose additional stresses on the machine resulting in an increased unbalance loading and thus leading to increased dynamic loads on the soil-foundation system. This paper presents the results of the dynamic analysis of a four-cylinder compressor foundation. The original design of the foundation was performed in the early 1960s and ignored the effect of the soil in the response of the foundation system, thus, the foundation has been suffering from excessive vibration. The foundation block supports a four-cylinder dress-rand compressor, suction and discharge bottles, a crank and driving motor with a total weight of approximately 219 kips. The results of a three- dimensional finite element model of the soil–foundation system were used to determine the dynamic response of the soil-foundation system and to assess the foundation response under the applied dynamic loading imposed by the compressor crank. The dynamic analysis is performed by: (1) performing eigenvalue analysis of the foundation block, considering the effect of the soil-foundation interaction to determine the soil-foundation natural frequencies and modal participation factors, and (2) performing forced response of the foundation under applied crankshaft unbalance load to determine the forced response amplitude of the soil-foundation system.

This paper explores the buckling behavior of a typical aboveground storage tank (AST) under wave impacts. A finite element model is first developed and validated against experimental results to determine the hydrodynamic pressure time histories on the AST from hurricane-induced waves; the characteristics of a severe wave impact are derived from numerical simulations of hurricanes in the Houston Ship Channel. The pressure time histories are then applied on a separate finite element model of the AST and its internal liquid, considering geometric and material nonlinearities. Dynamic buckling analyses are performed using the Budiansky-Roth criterion and compared with simpler buckling analysis procedures. This comparison is performed for two different internal liquids, gasoline and crude oil, and three internal liquid heights. Results indicate that the dynamic effects from wave impacts are not significant and could be reasonably omitted to estimate the critical load. Simpler static pressure models and computationally cheaper static buckling analysis methods provide, for practical purposes, an adequate estimate of the buckling strength of the AST. However, dynamic buckling analysis may still be required if the objective is to assess the post-buckling behavior and deformation of an AST under wave impacts, rather than only to estimate the critical load.

Offshore wind turbines under combined wind and wave loading suffer from excessive vibration which damages the structural integrity and causes potential collapse of the wind turbines. In this paper, a three-dimensional pendulum tuned mass damper (3d-PTMD) is utilized to mitigate the bi-directional vibrations as well as harvest the kinetic energy. A linear electromagnetic energy harvester consisting of magnets and coil assemblies is connected with the pendulum to convert the kinetic energy of the pendulum into electricity. An analytical model of an offshore wind turbine coupled with the 3d-PTMD is established. To examine the performance of the 3d-PTMD in mitigating vibrations in the fore-aft and side-side directions, misaligned wind and wave loading is considered. For comparison, two linear tuned mass dampers are deployed in the side-side and fore-aft directions. A 5 MW baseline wind turbine model is utilized to examine the performance of the 3d-PTMD. Results show that the 3d-PTMD outperforms the traditional TMD in mitigating the bi-directional vibrations induced by misaligned wind and wave loading. In addition, considerable amount of energy can be harnessed using the energy harvester.