Monique Head

Evaluation of Combination Rules for Orthogonal Seismic Demands in Nonlinear Time History Analysis of Bridges

One of the parameters that directly affects the seismic demand on a bridge is the excitation angle of the ground motion. Since analyzing a bridge with all possible excitation angles is impractical, combination rules have been used for computation of seismic demands. Provisions and specifications recommend the use of combination rules for computation of displacements and elastic forces, but they do not clearly suggest any combination rules for nonlinear time history analysis of bridges. This paper evaluates the 100/30, 100/40, and SRSS combination rules for nonlinear time history analysis of bridges. The probability of underestimation is computed for each combination rule, assuming a uniform distribution for the excitation angle. Two cases are considered for the combination rules: In the first case, only the major components of the earthquake records are used; the second case consists of using paired acceleration time histories. Results show that in the first case, the probability of underestimation is more than the second case, but using the second case produces conservative results on average, which are not economical to use in the design. Among the combination rules in the first case, 100/40 rule has the smallest probability of underestimation of demands. Therefore, this paper suggests the use of the 100/40 rule with the major component of earthquakes for the nonlinear time history analysis of bridges.

Probabilistic Demand Models and Fragility Estimates for Bridges Elevated with Steel Pedestals

Probabilistic seismic demand models are developed for bridges elevated with steel pedestals by adding correction and error terms to commonly used models. Separate probabilistic demand models are developed for the force demand on steel pedestals and the shear and deformation demands on concrete columns. Nonlinear time history analyses on detailed, three-dimensional finite-element models are used to generate virtual experimental data. By applying a Bayesian updating method to the generated data, parameters of the probabilistic models and their correlations are estimated. Comparisons between the demands from the developed probabilistic demand models and the demands from their corresponding demand models without correction and error terms reveal that the developed probabilistic models provide more accurate and unbiased predictions of the demands of interest. As an illustration of the developed framework, fragilities are estimated for a two-span bridge. The results show that pedestals are more vulnerable in the longitudinal direction, and columns are more vulnerable in the transverse direction. A sensitivity analysis on the studied bridges shows that decreasing the pedestal height, increasing the length of the pedestal anchor bolts within the concrete bent, and increasing the concrete cover on the anchor bolts are the most effective ways to decrease the probability of failure.

Probabilistic Capacity Models and Fragility Estimates for Steel Pedestals Used to Elevate Bridges

Using steel pedestals has become an effective method for elevating simply supported bridges in the United States. However, a method is needed to estimate their lateral load capacity and probability of failure subjected to different levels of applied loads. This paper shows the development of probabilistic models for the lateral load capacity of steel pedestals. The capacity models consider the prevailing uncertainties, including statistical uncertainty and model errors due to inaccuracy in the model form or missing variables. The proposed capacity models are used to estimate the conditional failure probability (or fragility) of an example steel pedestal for given sets of lateral and vertical loads. Because of the discontinuity in the limit state function, Monte Carlo simulation is used to estimate the fragility. Results show that for the studied steel pedestal, increasing vertical load decreases the probability of failure subjected to lateral loads. To investigate the effect of different random variables on the results, a sensitivity analysis is also conducted for the example steel pedestal.

Modified Yield Line Theory for Full-Depth Precast Concrete Bridge Deck Overhang Panels

Full-depth precast deck slab cantilevers also referred to as full-depth precast concrete bridge deck overhang panels are becoming increasingly popular in concrete bridge deck construction. To date, no simple theory is able to estimate the overhang capacity of full-depth concrete bridge deck slabs accurately. Observations suggest that interaction between flexure and shear is likely to occur as neither alone provides an accurate estimate of the load-carrying capacity. Therefore, modified yield line theory is presented in this paper, which accounts for the development length of the mild steel reinforcing to reach yield strength. Failure of the full-depth panels is influenced by the presence of the partial-depth transverse panel-to-panel seam. When applying a load on the edge of the seam, the loaded panel fails under flexure while the seam fails in shear. Through the use of the modified yield line theory coupled with a panel-to-panel shear interaction, analytical predictions are accurate within 1–6% of experimental results for critical cases.

Experimental Performance of Full-Depth Precast, Prestressed Concrete Overhang, Bridge Deck Panels

The performance of a new full-depth precast overhang panel system for concrete bridge decks is investigated experimentally. In contrast to conventional cast-in-place deck overhangs, the proposed full-depth precast overhang system has the potential to speed up construction, reduce costs, and improve safety. Load-deformation behavior up to factored design load limits is first investigated. The panel is then loaded near its edge to examine the collapse capacity and the associated failure modes—particularly the influence of panel-to-panel connections that exist, transverse to the bridge deck axis. Comparative tests are also conducted with a conventional cast-in-place overhang system. When compared to the conventional cast-in-place overhang behavior, the experimental results show that the precast full-depth overhang introduces different behavior modes, largely due to the influence of the partial depth panel-to-panel connection, which reduces the capacity by some 13%.

Comparative Experimental Performance of Bridge Deck Slabs with AFRP and Steel Precast Panels

AbstractFull-depth precast concrete panels expedite the construction process, enhance the safety and quality controls, and reduce the on-site labor requirements for bridge deck slab applications. However, corrosion-induced deterioration of conventional steel during the lifetime of the structure is a serious concern affecting the durability and serviceability of the deck panels. Although replacing conventional steel with fiber-reinforced polymer (FRP) bars has become more prevalent over the past few decades to overcome corrosion issues, there is still need for a comprehensive experimental study to investigate the constructability and structural performance of FRP concrete bridge deck slabs with precast panels at full scale. In this paper, a full-scale bridge deck slab consisting of full-depth precast panels reinforced and prestressed with aramid fiber reinforced polymer (AFRP) bars is experimentally investigated in terms of constructability and overall structural performance. It is then compared to a simil...

Computational Modeling of Aramid Fiber-Reinforced Polymer Prestressed Girder in Composite Action with Bridge Deck

Aramid fiber-reinforced polymer (AFRP) tendons, which are inherently corrosion-resistant, can be used to replace steel prestressing strands in bridge girders to enhance bridge sustainability. Despite ongoing experimental research, there is a lack of uniformity and consistency in testing procedures, definitions of material characteristics, and results. Therefore, a robust computational model is needed to perform a refined nonlinear analysis of full-scale AFRP prestressed girders. This paper presents the development of a computational model to numerically evaluate the flexural behavior of an AASHTO (American Association of State Highway and Transportation Officials) I-girder (Type I) prestressed with AFRP tendons in comparison to its conventional prestressing steel counterpart. Numerical results match experimental test data with a high degree of accuracy and reveal that an AASHTO I-girder prestressed with AFRP meets service and strength limit states. Numerical results also show that the deflection equation in ACI 440.4R overestimates the maximum deflection of the AFRP prestressed girder.

Compound Shear-Flexural Capacity of Reinforced Concrete–Topped Precast Prestressed Bridge Decks

Modern concrete bridge decks commonly consist of stay-in-place (SIP) precast panels seated on precast concrete beams and topped with cast-in-place (CIP) reinforced concrete. Such composite bridge decks have been experimentally tested by various researchers to assess structural performance. However, a failure theory that describes the failure mechanism and accurately predicts the corresponding load has not been previously derived. When monotonically increasing patch loads are applied, delamination occurs between the CIP concrete and SIP panels, with a compound shear-flexure mechanism resulting. An additive model of flexural yield line failure in the lower SIP precast prestressed panels and punching shear in the upper CIP-reinforced concrete portion of the deck system is derived. Analyses are compared to full-scale experimental results of a tandem wheel load straddling adjacent SIP panels and a trailing wheel load on a single panel. Alone, both yield line and punching-shear theories gave poor predictions of the observed failure load; however, the proposed compound shear-flexure failure mechanism load capacities are within 2% accuracy of the experimentally observed loads. Better estimation using the proposed theory of composite SIP-CIP deck system capacities will aid in improving the design efficiency of these systems.

EXPERIMENTAL INVESTIGATION OF FULL-DEPTH PRECAST OVERHANG PANELS FOR CONCRETE BRIDGE DECKS

Performance of a new full-depth precast overhang panel system for concrete bridge decks is investigated experimentally. In contrast to conventional cast-in-place deck overhangs, the proposed full-depth precast overhang system has the potential to speed up construction, reduce costs, improve construction safety and improve durability of the bridge system. To validate the capacity of the proposed overhang system, quasi-static monotonic tests are conducted on two precast overhang double-panel specimens and a conventional overhang. Factored design load limits are applied to the overhang to investigate the load-deformation capacity. Each overhang is then loaded near its edge to examine the collapse capacity and the associated failure modes– particularly the influence of panel-to-panel connections for the full-depth precast panels. When compared to the conventional cast-in-place overhang behavior, the experimental results show that the precast full-depth overhang introduces different behavior modes, largely due to the influence of the partial depth panel-to-panel connection. This reduces the ultimate carrying capacity by some 13 percent, but in spite of this reduction, behavior is satisfactory with a high margin of safety above the AASHTO LRFD design loads.

Environmental Effects on Material and Bond Durability of CFRP and AFRP for Prestressed Concrete Bridge Applications

Within the transportation community, steel is one of the most widely used construction materials; however, in most operating environments it corrodes, often resulting in a loss in capacity or serviceability. Fiber-reinforced polymers (FRP) have become a viable alternative to steel and continue to gain attention due to their high tensile strength and durability. To further the knowledge base of FRP, this paper focuses on the bond durability of unidirectional pultruded FRP bars—made with carbon (CFRP) and aramid (AFRP) fibers—in concrete after exposure to thermal fatigue and saltwater environments, for the purpose of understanding the performance of FRP for prestressing applications. Cube specimens containing AFRP, CFRP, and 2205 stainless steel (control) were exposed to these environments and then subjected to direct pullout tests to evaluate bond strength. Thermal fatigue caused the greatest reduction in bond capacity for all bar types, with CFRP retaining better performance than AFRP.