Simplified Methods for Predicting the Pushover Response of a Hybrid Steel Braced Frame with Concentric and Eccentric Connections
Two methods to predict the pushover response of a hybrid steel braced frame that features concentric and eccentric connections are presented. The proposed hybrid frame configuration is intended for cold-formed steel (CFS) building structures and plays a dual role by providing gravity and lateral load resistance. The hybrid system features CFS columns for resisting gravity loads, tension only CFS straps connected concentrically to hollow structural steel (HSS) chords for resisting lateral loads, and prestressing strands connected eccentrically to the same HSS chords for further enhancing lateral load capacity. The first pushover response prediction method employs a nonlinear finite element modeling protocol that features only beam and truss elements. This method is presented as an alternative to 3D nonlinear finite element models, which require considerable modeling effort due to the multitude of the components included in the hybrid system, and the variety of interactions that would need to be defined. The second method includes closed form equations that allow the definition of the pushover curves using manual calculations and is appropriate for preliminary analysis. Formulations for predicting the initial in-plane system stiffness, and secondary stiffness are presented. Similarly, formulations for predicting the load to cause the first and second yield as well as ultimate lateral load capacity are provided. A closed form equation is presented to obtain the ultimate lateral displacement, which completes the definition of the pushover curve. The validation of the proposed pushover response prediction methods is conducted by first comparing responses from 3D nonlinear FEA to those obtained from the simplified FEA model. Similarly, predictions from the simplified FEA model are compared to those obtained from the closed form equations. Good agreements are noted in both cases. The influence of hybrid steel braced frame aspect ratio on system push over response is quantified. The contribution of each lateral load resisting element to lateral load capacity is presented as a function of aspect ratio. Finally, improvements in behavior and enhancements in lateral load capacity offered by the hybrid frame-wall compared to the traditional strapped wall system are demonstrated through monotonic pushover, cyclic pushover, and response history analysis.
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Flexural response of UHPC beams post-tensioned with internal unbonded
tendons: Experimental study
This paper presents the results of an experimental study that deals with the characterization of the complete flexural response of post-tensioned (PT) ultra high-performance concrete (UHPC) beams with internal unbonded tendons. A total of 6 PT UHPC beams were monotonically loaded to failure. The variables investigated include volumetric fiber content, non-prestressed reinforcement ratio, and loading configuration. Flexural response is characterized in terms of crack pattern formation, propagation, and localization as well as failure mode identification. The influence of the investigated variables on change in: strand strain, UHPC compressive and tensile strains, strand stress, moment capacity, curvature ductility and member level ductility is presented including explanations of observed phenomena. The measured flexural response and capacity are compared with the predicted response using two mechanics-based methods and a numerical method. Prestress losses are quantified and used to obtain the predicted response. UHPC material characterization is conducted to obtain the necessary
inputs for the prediction models. The measured to predicted ratio of flexural capacity and the corresponding COV for the considered prediction methods vary from 0.95 to 1.02, and from 6% to 11% suggesting good predictive capabilities. The use of nonprestressed flexural reinforcement in PT UHPC flexural members is recommended from the perspective of crack distribution, delay in the precipitation of flexural failure through crack localization and subsequent hinging, and member level ductility. |
Moment curvature response of composite UHPC filled hollow structural steel cross-sections
A mechanics based analytical method for obtaining the complete moment–curvature response of composite hollow structural steel (HSS) and ultra high performance concrete (UHPC) cross-sections is presented. Flexural response at the cross-sectional level is traced past the local buckling of the steel elements and up to the
exhaustion of UHPC and steel material capacity. The proposed method is validated using experimental data from seven flexural tests on composite HSS-UHPC beams. Flexural failure mode is classified as either a UHPC compression-controlled failure, UHPC fiber tension-controlled failure, or a balanced failure. Behavioral differences between the cross-sections that fall into these three categories are discussed in terms of flexural capacity, curvature ductility, and relative contributions of HSS and UHPC to flexural capacity. Flexural response of the composite HSS-UHPC cross-section is compared to that of the bare HSS cross-section. The enhancement in flexural capacity and curvature ductility, for UHPC fiber tension-controlled sections compared to the bare HSS cross-section, was 52% and 79%, respectively. For composite cross-sections that feature a balanced failure, the increase in flexural capacity and curvature ductility, compared to the bare HSS cross-section, was 37% and 35%, respectively. Closed form formulations are proposed to identify the flexural failure mode and predict flexural capacity. The influence of various parameters, such as HSS material grade, size of cross-section, and UHPC material characteristics on the complete flexural response of the cross-section is investigated. The average ratio and corresponding coefficient of variation (COV) of tested to computed flexural strength obtained using the proposed procedure was 1.06 and 21.28%, respectively. Similarly, the average ratio and COV of tested to calculated flexural strength obtained using the proposed closed form formulations was 1.07 and 22.18%, respectively. |
Punching shear capacity of nonprestressed UHPFRC flat Plates: Evaluation
of existing methods
An experimental database of 64 nonprestressed ultra high performance fiber reinforced concrete (UHPFRC) flat plates reported to fail in punching shear was generated and characterized. The selected plates featured round, end-hooked, and deformed steel fibers. All plates have uniform thickness with no shear reinforcement. Trends and correlations between the shear stress at failure and various parameters were investigated. The shear stress at failure was on average four times higher than the shear stress corresponding with the first crack. There was a notable positive correlation between nonprestressed flexural reinforcement ratio and shear stress at failure. Plate ductility at punching shear failure varied significantly. A total of 13 prediction methodologies were considered for final evaluation, 8 of which were developed for UHPFRC, 4 were developed for fiber reinforced concrete (FRC), and 1 was developed for high performance fiber reinforced concrete (HPFRC). The predictive performance of the various prediction models was evaluated using common statistical indicators, as well as a demerit point classification model. The average ratio of measured to predicted punching shear capacity ranged from 0.50 to 2.42. The corresponding coefficients of variation (COVs) ranged from 36% to 58%. Only five out of 13 prediction methodologies underpredicted, on average, the nominal punching shear capacity. The large COV suggests the need for more accurate prediction methods that capture fundamental phenomena observed during tests while being informed by parameters that are practical to obtain. Guidelines for casting methods that produce the desired distribution of fibers are essential in yielding a more predictable plate response.
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Punching Shear Strength of Post-tensioned UHPC Plates
A method to predict the punching shear capacity of ultra-high performance concrete (UHPC) plates posttensioned (PT) with internal unbonded tendons is presented. The method supplies simultaneously plate’s punching and rotation capacity by superimposing plate’s load rotation relationship and a rotation dependent failure criterion. The derivation of plates load-rotation relationship is based on engineering mechanics and is informed by a moment curvature relationship developed as part of this study. Criteria are presented for distinguishing between punching and flexural failures in numerical and prediction models. Validated nonlinear finite element models are used to create a database of punching critical posttensioned UHPC plates, which is used to derive a nonlinear regression-based failure criterion. The impact of column size, plate thickness, prestressed and nonprestressed reinforcement ratio, tendon configuration, prestressing force, and fiber content and characteristics on plate punching shear capacity is quantified.
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Time-Dependent Flexural Deformations in Composite Prestressed Concrete and Steel Bridge Beams: Part II - Comparison of Predictions
Time-dependent flexural deformations for composite and non-composite prestressed concrete beams and steel–concrete composite beams, which are subject to various loading and unloading events, are predicted based on several methods with varying levels of complexity. The considered prediction methods range from computational methods that provide curvatures at various points along the span as a function of time to widely used multiplier-based methods. The impact of various levels of analytical simplification on beam flexural deformation history is quantified. Pre-erection camber predictions based on various methods are compared with measured pre-erection camber values for 105 non-composite prestressed concrete beams. Similarly, predictions for the full flexural deformation history of six composite prestressed concrete and six steel beams are compared with test data reported in the literature. All computational methods considered result in similar predictions of pre-erection camber in prestressed concrete beams despite fundamental differences in the methods and despite gross simplifications in creep behavior used in some methods. However, when long-term flexural deformations at service and prediction of beam rebound are considered, the effect of simplifying assumptions for creep behavior becomes more significant. The quantification of such differences in predictions may inform decisions during the design phase regarding which method should be employed for a particular application.
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Time Dependent Flexural Deformations in Composite Prestressed Concrete and Steel Beams: Part I – Prediction Methodology
A methodology will be presented to predict the complete history of flexural deformations in composite prestressed concrete and steel beams from inception to the end of service life, which will include a prediction of beam rebound and subsequent deformations when the deck is replaced. The methodology will use distinct creep curves for each loading event and will provide an efficient framework to account for the effects of differential creep, differential shrinkage, shrinkage-induced creep, loading time, concrete aging, prestress losses, and temperature gradients on composite and non-composite beam flexural deformation history. The method will be based on a time-dependent strain compatibility analysis, which will provide curvatures at various points along the span as a function of time to obtain the history of the deflected shape of the beam. Measured discrete and periodic flexural deformation data will be used to validate the methodology. The presented method will be used to quantify the influence of creep, shrinkage, modulus of elasticity models, temperature gradients, deck placement and replacement time, and time step generation method on the beam flexural deformation history. The results show that steel beams can rebound to their original position after the deck has been removed, the rebound in the prestressed concrete beams varied from 51% to 92%. In addition, the influence of the initial deck placement time had a marked effect on the short-term beam flexural deformations at service, and its influence diminished after 200 days for the cases that were considered in this study. In addition, the selection of models for creep, shrinkage, and modulus of elasticity at prestress release and 28 days had a marked effect on the beam deformation history, and the considered models for the variation in modulus with time resulted in similar predictions.
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Moment-Curvature-Deformation Response of Posttensioned UHPC Beams
A mechanics based analytical method for obtaining the complete moment-curvature-deformation response of ultra-high performance concrete (UHPC) beams posttensioned with internal unbonded tendons is presented. The proposed procedure does not rely on empiricism other than what is included in the assumed material constitutive models, and provides the means to determine the variation of curvature and deflection as the beam is loaded to failure thus providing an avenue to quantify ductility at the cross-section and member level. The influence of various constitutive models for the compressive and tensile domain of UHPC, prestressed and non-prestressed reinforcement ratio, loading configuration, and tendon profile on the complete beam flexural behavior is quantified. The most influential parameters for cross-section and member level ductility are the ultimate UHPC strain in tension and loading configuration, respectively.
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Out-of-Plane Buckling Strength of Free Standing Singly Symmetric Hollow Pinned Circular Arches
This study deals with the out-of-plane flexural torsional buckling strength of singly symmetric free-standing circular arches with a hollow cross-section. The behavior of such arches is investigated numerically in terms of the out of plane critical elastic buckling load and ultimate load carrying capacity when such arches are subject to a radial pressure that creates uniform axial compression. Prediction methodologies proposed by other researchers for flexural torsional buckling of arches with an I-shaped cross-section, as well as American, European, and Australian code provisions for flexural buckling of compression members are slightly modified and compared with numerically obtained critical elastic buckling loads and ultimate load carrying capacities. All investigations are conducted using validated shell finite element simulations. A parametric study is conducted to evaluate the influence of arch depth, wall thickness, subtended angle, arch radius, yield strength, initial imperfections, and residual stresses on the 3D elastic-plastic behavior of such arches. It is concluded that it is not necessary to use eigenvalue buckling analysis to obtain critical elastic buckling loads for improving capacity predictions and that such loads may be obtained using existing formulations with slight modifications. Additionally, existing code provisions for flexural buckling of compression members may be used with minor modifications to predict the out-of-plane flexural torsional buckling capacity of such arches. Also, prediction methodologies proposed by other researchers for flexural torsional buckling of I-shaped arches may also be used with slight modifications and resulted in the most accurate, consistent, and design appropriate capacity predictions.
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A Flexural Design Methodology for Composite Heterogeneous and Homogeneous UHPC Bridge Beams Prestressed with Bonded Strands
A non-iterative flexural design methodology for composite, heterogeneous, and homogeneous ultra-high performance concrete (UHPC) bridge beams prestressed with bonded strands is presented. One key feature of the proposed methodology is the development of closed-form equations for calculating strain in concrete at the most extreme compression fiber at the ultimate limit state, εc, as a function of various parameters. Separate formulations for predicting εc are provided for homogeneous and composite cross-sections. The predicted concrete strain, together with the maximum usable tensile strain for UHPC, εtu, at the most extreme tension fiber are used to calculate cross-sectional curvature and the distribution of strains and stresses. Force equilibrium is then used to determine the depth to the neutral axis and the nominal moment capacity of the beam. The flexural failure mode for the majority of beams considered is a fiber tension controlled failure. From a beam flexural strength perspective, the compressive strength of the deck or top flange for composite and heterogeneous beams, respectively, does not need to exceed 28 MPa. Any excess compressive strength will remain either unutilized or will result in marginal or negligible increases in moment capacities. The magnitude of the cracking strength of UHPC plays an important role in the contribution of UHPC to the moment capacity of the beam and determines whether this contribution is higher or smaller than that provided by the prestressing strands. The strand stress at the ultimate limit state, fps, varied from 1688 MPa to 1743 MPa and was past the linear elastic branch of the assumed stress-strain curve. The parameter that had the highest influence on fps was εtu. The proposed methodology is validated using test data as well as results from validated nonlinear finite element models and strain compatibility analysis.
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A Flexural Design Methodology for UHPC Beams Posttensioned with Unbonded Tendons
This paper provides a framework for predicting the flexural behavior of ultra-high performance concrete (UHPC) beams posttensioned with unbonded tendons. A mechanics based phenomenological model is presented to predict flexural capacity, and a set of equations that can be used to predict strand stress at the ultimate limit state is proposed and considers how the nonlinear domain of UHPC in tension affects flexural behavior. It is demonstrated that predictions based on the proposed equations and the presented flexural design methodology are in close agreement with results obtained from validated numerical simulations. The strand stress at ultimate is expressed as a function of neutral axis depth, effective depth of tendon, tendon length, loading configuration, loading pattern, plastic hinge length, maximum usable UHPC compressive and tensile strain, and shape of the stress-strain curve of the tendon. The influence of tendon area, mild steel area and yield stress, specified UHPC compressive strength and tensile strength, as well as beam cross-sectional dimensions are captured indirectly through the calculation of the neutral axis depth. The flexural design methodology is presented in terms of the failure mode observed when the considered specimens reach their ultimate load carrying capacity. The failure mode is characterized as either a fiber tension controlled failure or a UHPC compression controlled failure. The change in strand stress at the ultimate limit state, Δfps, the strand stress at the ultimate limit state, fps, and the nominal moment capacity, Mn, of 221 UHPC posttensioned beams obtained based on the proposed methodology are compared with results obtained from validated numerical models and it is demonstrated that average predicted values are within 5% of computed ones and the coefficient of variation is not greater than 17%.
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Alternative Approaches to Predict Shear Strength of Slender RC Beams Strengthened with Externally-Bonded FRP Laminates
The method provided in ACI 440.2R-17 for predicting the shear strength of reinforced concrete (RC) beams strengthened with externally bonded fiber-reinforced polymer laminates (EBR) gives conservative estimates of shear capacity due to its inability to capture the variation of β and θ. This paper presents three iterative as well as a non-iterative approach to predict the shear capacity of RC beams strengthened with EBR with more accuracy and improved appropriate safety. The first iterative approach is based on the combination of a slightly revised version of the Modified Compression Field Theory and the model proposed by Chen and Teng. The second iterative approach is based on the combination of the Simplified Modified Compression Field Theory and the model proposed by Chen and Teng, and the third iterative approach is based on the combination of a revised version of AASHTO’s model with the approach proposed by Chen and Teng. The non-iterative approach is developed using Monte Carlo Analyses using the first iterative model as the target function and includes two simple closed-form equations to capture the variation in β and θ. All proposed approaches are compared with the ACI 440.2R-17 method in terms of accuracy and appropriate safety. A global sensitivity analysis is performed to assess the influence of various parameters on the shear capacity of RC beams strengthened with EBR using various approaches. A database of 172 RC beams strengthened with EBR is compiled to appraise the predictive capability of the proposed models and to demonstrate the improvement that they offer regarding accuracy and appropriate safety.
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Web Compression Buckling Capacity Prediction for Unstiffened I-Sections with Opposite Patch Loading
Two web compression buckling capacity prediction methods are introduced for unstiffened steel I-sections subject to opposite patch loading applied to the flanges. The methods are generally posed as a function of loaded width to web depth ratio, and are applicable for opposite patch loading applied at the interior of a wide flange section or at the end of it, where the web has a free edge. The proposed methods include three parts: 1) an expression for predicting the squash load, 2) an expression for predicting the elastic buckling load, and 3) a resistance function. The squash load is calculated using an empirically derived effective width concept based on observations at the ultimate load from an extensive experimental database and validated numerical simulations. Web slenderness is defined as the square root of the ratio of the web squash load to the web critical elastic buckling load. The critical elastic buckling load is defined consistently with that obtained with a plate buckling energy solution for patch loading on infinitely long strips and considers the shortened web buckling half-wavelength resulting from flange rotational restraint provided to the web. The methods are validated with existing experimental data and shell finite element collapse simulations, and are shown to be more accurate and more widely applicable than current American Institute for Steel Construction (AISC) Specification provisions.
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Development of high performance concrete deck configurations for movable bridges
Four high performance nonproprietary concrete deck configurations for movable bridges
Four high performance nonproprietary concrete bridge deck configurations are presented for Louisiana’s movable bridges as an alternative to traditional steel grid decks, which have exhibited durability problems. These concrete decks are as light and as deep as the traditional steel grid decks and meet the maximum weight limitation of 0.96 kN/m2 imposed by the capacity of the mechanical systems that operate the movable bridges. The four concrete deck configurations feature unique nonproprietary concrete mixtures that possess high strength and low
unit weight. The development of each concrete mixture is presented. All reinforcement is corrosion resistant and consists of glass fiber reinforced polymer (GFRP) bars and a two-way carbon fiber mesh. Several nonlinear finite element analyses are performed to simulate the behavior of all four concrete deck configurations from the onset of loading to failure and to ensure that the developed deck configurations meet AASHTO’s load and deflections requirements. AASHTO’s ultimate load demand is met regardless of whether the deck system is made continuous for live loads. Two deck configurations meet AASHTO’s deflection requirements when continuity for live loads is established. The failure mode of the concrete deck panels is dominated by shear. The presented deck configurations offer the departments of transportation various feasible options and thus more flexibility for to how to address problems related to the deterioration of steel grid decks using locally available materials, and provide guidance as to what experimental testing to perform in the future. |
A High Performance Concrete Deck for Movable Bridges using Ductal
Louisiana has approximately 160 movable bridges, mostly in the southern part of the state. This places Louisiana among the states with the highest inventory of movable bridges in the nation. The typical deck systems in these movable bridges are steel grids. Records show that steel grids have had maintenance issues. An alternative ultra-high/high performance concrete (UHPC/HPC) bridge deck system is proposed for Louisiana’s movable bridges. This system consists of precast waffle slab deck panels, which are reinforced with glass fiber reinforced polymer (GFRP) bars as positive moment reinforcement, and a two-way carbon fiber reinforced polymer (CFRP) mesh as top reinforcement. Several validated nonlinear finite element analyses were performed to simulate the behavior of the precast panels from the onset of loading to failure. It is concluded that the precast concrete waffle slabs provide a viable alternative to steel grids by supplying load capacities that surpass service level and ultimate level load demands and deflection capacities that are within code specified limits.
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Development of Alternative Concrete Bridge Superstructure Systems for Short and Medium Span Bridges
Two new concrete bridge superstructure systems for short and medium span bridges are presented. The investigated systems consist of adjacent hollow precast concrete beams with and without concrete topping. The proposed configurations are compared with traditional adjacent box beam and decked bulb tee systems for spans that range from 24.4 m to 45.7 m. It is demonstrated that the proposed system that features concrete topping (PS2) is lighter than the adjacent box beam system, requires fewer strands, provides shallower superstructure depths, and exhibits lower cambers for spans equal to 24.4 m and 30.5 m. PS2 addresses the reflective cracking problems manifested in adjacent box beam systems by shifting the location of the longitudinal connections to the bottom flange and away from traffic loads. The system that features no topping (PS4) is slightly heavier than the decked bulb tee system, but features shallower superstructure depths, requires fewer strands, and exhibits lower live load deflections and camber. Transverse bending moments demand in PS4 are reduced compared to the decked bulb tee system due to the smaller span provided by the two web supports. Live load distribution factors (LLDF) for PS2 and PS4 can be conservatively estimated using AASHTO provisions for adjacent box and decked bulb tee systems, respectively.
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An investigation of AASHTO’s requirements for providing continuity in simple span bridges made continuous
AASHTO’s requirements for providing continuity in simple span bridges made continuous are investigated by performing a parametric study which consists of 140 time dependent analyses for various precast concrete beam shapes. The beam shapes considered include three precast concrete bulb tees (PCBT), two AASHTO type beams, and two Florida I-beams (FIB). Various beam spacing and span configurations are considered. A sectional analysis approach that employs the age adjusted effective modulus method for capturing creep effects is presented for calculating restraint moments. AASHTO models for creep and shrinkage are used to conduct the parametric study. For each case considered a minimum girder age for when continuity can be established is recommended such that the total restraint moment at the intermediate support is equal to or smaller than zero. The recommended minimum girder ages at continuity vary from 55 to 90 days for PCBTs, 55–70 days for AASHTO type beams, and 55–80 days for FIBs. The influence of fc′i, choice of creep and shrinkage model, and choice of analysis method on the magnitude of restraint moments is investigated. The specification of a higher fc′i is an effective technique to reduce the minimum required girder age at continuity. The magnitude of restraint moments appears to be highly sensitive to the selected creep and shrinkage model. The proposed analysis method addresses the shortcomings of other closed form formulations and results in restraint moments that are more sensitive to the girder age at continuity.
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Rehabilitation of Deteriorated Timber Piles using Fiber Reinforced Polymer (FRP) Composites
Louisiana has a large inventory of timber bridges in service. The timber piles in these bridges are succumbing to the effects of biological degradation that initiates in the wet-dry zones. Replacing these deteriorated piles is a costly process and in-situ repair of the piles with fiber reinforced polymers (FRP) is an economic alternative. An experimental program was conducted to evaluate the capacity of FRP strengthened deteriorated timber piles under axial loads with different lengths and depths of deterioration zone. A total of 11 monotonic tests were conducted. The investigated repair technique increases the capacity of damaged piles by 98% to 383% and enhances the capacity of undamaged piles by 3% to 22%. All failure modes were observed in the wooden portion of the pile outside the repaired region. Strain gage measurements indicate that the FRP shell is mobilized more when the annular void is smaller.
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Investigation of Web Post Compression Buckling Limit State and Stiffener Requirements in Castellated Beams
The research presented in this paper addresses the need for a design method to estimate the nominal capacity of castellated beams against concentrated loads. The limit state investigated is that of web post buckling due to compression loads. The purpose of the paper is to twofold: a) to investigate the limit state of web post buckling due to compression loads, and b) to quantify the enhanced capacity of the web post against concentrated loads when stiffeners are provided. Five castellated beam depths are considered which cover a wide range of the available depths. For each beam section three load cases are investigated: a) center of load aligns with the middle of web post, b) center of load aligns with the center of the hole, and c) center of load aligns with a point half-way between the center of web post and center of hole. For each load position two cases are considered; one without a stiffener and one with full height transverse stiffeners. Each case is investigated using non-linear finite element analysis to examine the behavior of the web post to failure. The efficiency of stiffeners to increase the resistance of castellated beams against concentrated loads is examined. For each investigated beam depth and stiffener arrangement, the loads that cause failure are noted. In addition, a simplified approach for checking the limit state of web post bucking in compression is proposed.
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First bucked mode shape for CB12x40
Deformed shape at failure for CB12x40
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Development of a Composite Concrete Bridge System for Short-to-Medium-Span Bridges
The increasing traffic demands on the bridge infrastructure present significant challenges for the Departments of Transportation within the framework of bridge construction and rehabilitation. It is therefore essential to address these problems with minimal interference with the motoring public. The inverted T beam system is a bridge system that provides accelerated bridge construction and replacement for short to medium span bridges. The system consists of adjacent precast inverted T beams finished with a cast-in-place topping. This research focuses on some of the main issues associated with such composite systems such as reflective cracking, composite action, appropriate live load distribution factors, stresses in the pre-tensioned anchorage zones and time dependent effects due to creep, shrinkage and thermal gradients. To address these issues, I have engaged in an analytical and experimental study to investigate different ways of connecting the precast inverted T’s transversely, optimize their profile, quantify time dependent effects, investigate shrinkage and creep properties of various mixes for the cast-in-place topping and provide insight about some of the assumptions that the structural engineer will make when designing this system. The goal of this research is to develop a bridge system intended to accelerate construction and enhance the resiliency of short to medium span bridges against traffic, environmental and time dependent effects.
Part 1 - Investigation of the Effects of Transverse Bending in Inverted T-Beam Bridge Systems
The inverted T-beam system is a bridge system that provides an accelerated bridge construction alternative for short-to-medium-span bridges. The system consists of adjacent precast inverted T-beams finished with a cast-in-place concrete topping. This bridge system is intended to address reflective cracking problems manifested in short-to-medium-span bridges constructed with the traditional adjacent voided slab or adjacent box beam systems. When concentrated loads are applied to a bridge of this type, the bridge deforms as a two-way flat plate. This paper presents the results of an analytical and experimental investigation focused on the first inverted T-beam bridge in Virginia on US 360 over the Chickahominy River to study the relationship between transverse bending and reflective cracking. Transverse bending moment demands are quantified using a finite element model and compared to tested transverse bending moment capacities provided by several sub-assemblage specimens. The tested sub-assemblage specimens feature a combination of various precast inverted T-beam cross-sectional shapes and transverse connections. It is concluded that all tested specimens performed well at service load levels. The detail which features a precast inverted T-beam with tapered webs and no mechanical connection with the adjacent inverted T-beams and cast-in-place topping is the simplest and most economical.
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Transverse bending created due to wheel loads
Transverse bending effects investigated via sub-assemblage specimens
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Part 2 - Investigation of Time Dependent and Temperature Effects on Composite Bridges with Precast Inverted T-Beams
This study investigates time dependent effects on composite bridges with precast inverted T-beams. The analysis is performed for a two-span continuous bridge. This system provides enhanced performance against reflecting cracking because it offers a thicker cast-in-place topping over the joint between the precast members. An analytical study is performed to quantify the stresses generated as a result of differential shrinkage, creep and temperature gradient at various sections in both directions. At the cross-sectional level, an elastic sectional analysis approach using the age adjusted effective modulus method is used to perform the investigation. At the structure level the effects of uniform temperature changes, thermal gradients and differential shrinkage and creep are investigated and quantified in terms of axial restraint forces and restraint moments. It is shown that by paying attention to detailing and by selecting a mix for the cast-in-place topping that has relatively low shrinkage and high creep the potential for excessive cracking can be reduced and the longevity of the bridge prolonged. Results are presented and recommendations are made for strategies to reduce the magnitude of tensile stresses created as a result of these effects.
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Transverse cross-section of the bridge
Typical composite cross-section
Tensile stresses created due to temperature gradient and time dependent effects at the cross-sectional level
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Part 3 - Reducing Deck Cracking in Composite Bridges by Controlling Long Term Properties
Composite concrete bridges are widely used because they combine the advantages of precast concrete with those of cast-in-place concrete. However, because of the difference in shrinkage properties between the girder and the deck and because of the sequence of construction, the deck is subject to differential shrinkage tensile stresses. These tensile stresses may lead to excessive cracking. This paper demonstrates how the likelihood of deck cracking due to differential shrinkage can be reduced and how consequently the resistance of composite concrete bridges against time dependent effects can be enhanced by choosing a deck mix with low shrinkage and high creep. An experimental study on the long term properties of seven deck mixes is presented to identify a deck mix with the aforementioned properties. A comparison of three composite concrete bridge systems used for short-to-medium-span bridges is performed to identify the bridge system that is most resistant against time dependent effects. The mix with saturated lightweight fine aggregates appears to best alleviate tensile stresses due to differential shrinkage and the bridge system with precast inverted T-beams and tapered webs appears to be the most resistant.
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Stress contours due to differential shrinkage and shrinkage induced creep at the component level
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Part 4 - Investigation of Stresses in the End Zones of Precast Inverted T-beams with Tapered Webs
Short-to-medium-span composite bridges constructed with adjacent precast inverted T-beams and cast-in-place topping are intended to provide a higher degree of resistance against reflective cracking and time dependent effects compared to voided slab and adjacent box girder systems. This paper investigates the stresses in the end zones of such a uniquely shaped precast element. The transfer of prestressing force creates vertical and horizontal tensile stresses in the end zones of the girder. A series of 3-D finite element analyses were performed to investigate the magnitude of these tensile stresses. Various methods of modeling the prestressing force including the modeling of the transfer length are examined and the effect of notches at the ends of the precast beams is explored. Existing design methods are evaluated and strut-and-tie models, calibrated to match the results of 3-D finite element analyses are proposed as alternatives to existing methods to aid designers in sizing reinforcing in the end zones. It is shown that the magnitude of tensile stresses in the pre-tensioned anchorage zones depends on the eccentricity of the prestressing force. Recommendations for how to apply existing provisions and recommendations to such a uniquely shaped precast member are presented.
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Vertical normal stress contours at the end zones of precast inverted T-beams with tapered webs due to the transfer of the prestressing force
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Part 5 - Investigation of Composite Action in Bridges Built with Adjacent Precast Inverted T-beams and Cast-in-Place Topping
Short-to-medium-span composite bridges constructed with adjacent precast inverted T-beams and cast-in-place topping are intended to provide a higher degree of resistance against reflective cracking and time dependent effects compared to voided slab and adjacent box girder systems. This paper investigates the composite action between the unique precast and cast-in-place element shapes. A full-scale composite beam has been tested under different loading arrangements with the purpose of simulating the service level design moment, strength level design shear, strength level design moment and nominal moment capacity. To investigate the necessity of extended stirrups one half of the span featured extended stirrups whereas the other half featured no extended stirrups. It is shown that the system behaved compositely at all loading levels and that no slip occurred at the interface. In addition to measuring slip at various interface locations full composite action has been verified by comparing load displacement curves obtained analytically and experimentally. It is concluded that because of the large contact surface between the precast and cast-in-place elements, cohesion alone appears to provide the necessary horizontal shear strength to ensure full composite action.
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Test setup for investigating composite action
Illustration of potential failure planes
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Part 6 - Investigation of Live Load Distribution Factors for Composite Bridges Constructed with Adjacent Precast Inverted T-Beams with Tapered Webs
Short-to-medium-span composite bridges constructed with adjacent precast inverted T-beams and cast-in-place topping are intended to provide a higher degree of resistance against reflective cracking and time dependent effects compared to voided slab and adjacent box girder systems. This paper presents the results of an analytical and experimental study performed on the first inverted T-beam bridge in Virginia on US 360 over the Chickahominy River. The purpose of the investigation was to determine whether the behavior of this type of bridge is similar to that of adjacent voided slab or cast-in-place slab-span bridges. A finite element model of Phase I of the US 360 Bridge was created and the live load distribution factors were analytically determined. Live load tests using a stationary truck were performed on Phase I of the US 360 Bridge with the purpose of quantifying live load distribution factors and validating the results from the finite element analyses. The live load tests featured four transverse truck positions on the bridge. It is concluded that it is appropriate to estimate live load distribution factors using AASHTO provisions for cast-in-place slab span bridges.
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Live load testing of first implementation of precast inverted T-beams
with tapered webs Longitudinal strains in each beam, (a) Truck Position 1, (b) Truck Position 2
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Accelerating Bridge Construction Using the Full Depth Precast Prestressed Concrete Deck Panels
The Departments of transportation are continuously looking for new and innovative ways to accelerate new bridge construction and existing bridge rehabilitation to keep up with the demands of a fast paced modern life in which traffic volumes are increasing every day and people's time is becoming more valuable. Full depth precast prestressed concrete panels offer an alternative to cast-in-place concrete deck construction, which eliminates the need to construct and install formwork on site and therefore saves considerable time. The on-site activities are limited to the installation of the panels, grouting of the shear pockets, shear keys, haunches and placement of overlays if applicable. The full depth precast panels can be used for new bridge construction or existing bridge rehabilitation. One of the key design features of such a system, is providing composite action between the precast girders and the precast deck. This research project focused on investigating the shear strength at the interface of the precast girder and the precast deck panels for different types of shear connectors, grout types and haunch heights.