Sergio F. Breña

INCREASING FLEXURAL CAPACITY OF REINFORCED CONCRETE BEAMS USING CARBON FIBER-REINFORCED POLYMER COMPOSITES

A series of reinforced concrete beams strengthened in flexure using various carbon fiber-reinforced polymer (CFRP) composite systems were fabricated and tested in the lab to examine the effects of the strengthening configuration on the specimen behavior. The main aim was to find strengthening configurations to develop the strength of the composite laminates and preclude failure by debonding of the composite systems from the concrete surface. Results indicate that relying on the contact area between the composite laminates and the concrete surface is insufficient to eliminate debonding. Strengthening configurations involving techniques such as placement of transverse straps along the composite laminates or bonding the composites on the side surface of the specimens controlled debonding and provided a more ductile failure mode than placement on the bottom surface of the beams. Results of this investigation are intended to provide information required for the design of strengthening schemes of existing reinforced concrete bridges using composites.

Fatigue tests of reinforced concrete beams strengthened using carbon fiber-reinforced polymer composites

The use of carbon fiber-reinforced polymer (CFRP) composites has been identified as a viable technique for use in strengthening deteriorating or structurally deficient reinforced concrete bridges. This article presents the results of a study that consisted of repeated-load fatigue testing 10 reinforced concrete beams strengthened using two different types of externally-bonded CFRP composites. The results indicate that the bond between the composite laminates and surface of the concrete can degrade at load amplitudes corresponding to extreme load conditions for a bridge. The stress range applied to the reinforcing steel and maximum stress applied to the composite laminates were the controlling parameters that limited the fatigue life of the specimens under study. The authors conclude that an upper limit on stresses generated along the composite-concrete interface may have to be set during design to avoid premature debonding after a limited number of load cycles.

Experimental Behavior of Carbon Fiber-Reinforced Polymer (CFRP) Sheets Attached to Concrete Surfaces Using CFRP Anchors

Fiber-reinforced polymer (FRP) composite sheets have gained popularity as a viable strengthening technique for existing reinforced concrete structures. The efficiency of the strengthening system largely depends on adequate bond between FRP sheets and the concrete substrate. In recent years, techniques to anchor FRP sheets have been proposed in applications that have limited distance to develop FRP sheet strength. One promising technique consists of fabricating and bonding FRP anchors during the FRP sheet saturation and embedding them into predrilled holes in the concrete substrate. This paper presents experimental results highlighting the complex behavior between FRP sheets and anchors. The primary failure modes that the sheet-anchor system can experience are identified. The experiments identify the main variables that influence the FRP anchor-sheet system behavior. This research contributes to the needed experimental database that will aid in future development of design recommendations of this anchorage system.

Fatigue Behavior of Reinforced Concrete Beams Strengthened with Different FRP Laminate Configurations

Synopsis: This paper presents testing results of thirteen small-scale beams strengthened using carbon fiber-reinforced polymer composites tested under repeated loads to investigate their fatigue behavior. The beams were strengthened using different thicknesses and widths of composite laminates to identify parameters that would generate different failure modes. Two predominant fatigue failure modes were identified through these tests: fatigue fracture of the steel reinforcement with subsequent debonding of the composite laminate, or fatigue fracture of the concrete layer below the tension reinforcing steel (concrete peel off). Test results indicate that peak stress applied to the reinforcing steel in combination with composite laminate configuration are the main parameters that affect the controlling failure mode. Tests on large-scale components are required to verify the results presented in this paper.

Evaluation of Load Transfer and Strut Strength of Deep Beams with Short Longitudinal Bar Anchorages

This paper examines the behavior of struts and nodes in deep beams fabricated using short longitudinal bar anchorages. Laboratory tests were conducted of 12 deep beams in which the longitudinal reinforcement was anchored into the support using short straight bar anchorages. The shortest anchorage lengths provided were below 50% of those required by ACI 318-08 Chapter 12 provisions. Four different specimen groups were constructed using three different shear span-depth ratios (a/d) and two longitudinal bar sizes. Most of the beams failed by strut crushing after yielding of the main longitudinal reinforcement at midspan. Only those specimens with the shortest anchorage length in each group developed concrete splitting failures along the anchorage region. The effect of a/d and anchorage length on strut strength and load transfer mechanism observed in the tests is presented and discussed. Test results indicate that a significant portion of the applied shear force may be transferred through truss action even in beams with low a/d. Short anchorage length affected the load transfer characteristics of the deep beams. These findings can be used in better understanding the merits of different strut-and-tie models for design of deep beams.

The effect of crown architecture on dynamic amplification factor of an open-grown sugar maple (Acer saccharum L.)

Tree failure may cause significant economic and societal disruptions in urban environments. A better understanding of the relationship between branches and stem as they affect the dynamic response of decurrent trees under wind loading is needed to reduce the risk of tree failure. Finite element (FE) models were used to identify the parameters that primarily impact tree response. A base model was developed using data from a sugar maple (Acer saccharum L.) located in Belchertown, MA, USA, from which parametric models were subsequently developed. Confidence in the base model was gained by comparing the natural frequency of this tree with experimental results. Results from a parametric study incorporating changes in eight different tree parameters (stem diameter, slenderness ratio of branches, number of branches, damping ratio, branch attachment heights, branch attachment angles, branch azimuth angles, and elastic modulus) are then presented to help identify critical model properties that affect the dynamic amplification factor (Rd) of the tree. A single parameter was varied in each model while keeping others unchanged from the base model. Parameters with the greatest effect on Rd included stem diameter, number and slenderness of branches in the crown, elastic modulus of stem and branches, and damping ratio. Thus, it may be possible to use pruning to alter crown architecture to reduce the risk of tree failure.Tree failure may cause significant economic and societal disruptions in urban environments. A better understanding of the relationship between branches and stem as they affect the dynamic response of decurrent trees under wind loading is needed to reduce the risk of tree failure. Finite element (FE) models were used to identify the parameters that primarily impact tree response. A base model was developed using data from a sugar maple (Acer saccharum L.) located in Belchertown, MA, USA, from which parametric models were subsequently developed. Confidence in the base model was gained by comparing the natural frequency of this tree with experimental results. Results from a parametric study incorporating changes in eight different tree parameters (stem diameter, slenderness ratio of branches, number of branches, damping ratio, branch attachment heights, branch attachment angles, branch azimuth angles, and elastic modulus) are then presented to help identify critical model properties that affect the dynamic amplification factor (Rd) of the tree. A single parameter was varied in each model while keeping others unchanged from the base model. Parameters with the greatest effect on Rd included stem diameter, number and slenderness of branches in the crown, elastic modulus of stem and branches, and damping ratio. Thus, it may be possible to use pruning to alter crown architecture to reduce the risk of tree failure.

Performance of Conventionally Reinforced Coupling Beams Subjected to Cyclic Loading

The behavior of coupled-wall buildings primarily depends on the performance of coupling beams. Existing coupled-wall buildings designed and built prior to the 1970s contain coupling beams reinforced with horizontal and vertical bars. Seismic evaluation of these older buildings is commonly performed using nonlinear static or dynamic analyses techniques. Results from nonlinear analyses depend on the quality of backbone curves that characterize the behavior of the individual coupling beams. Backbone curve construction must, therefore, focus on reliable estimates of the strength and deformation values used to characterize the behavior of the structural component. This paper presents experimental results of four conventionally reinforced coupling beams designed to exhibit different behavior modes during cyclic loading. The effect of different amounts of longitudinal and transverse reinforcement on behavior is highlighted. The measured shear strength of the beams is compared with values calculated using existin...

Factors affecting strength of elements designed using strut-and-tie models

The authors have made an interesting contribution to theexperimental study of strut-and-tie models. However, thediscussers would like to address some aspects in this study:1. The discussers have reviewed several publicationslisted in the References section of the paper regarding thefundamental concepts of strut-and-tie models. An importantparameter of these modes is the concrete effectiveness factorthat depends on the strut type, reinforcement’s arrangements,and so on, but the authors did not consider this parameter in theexplanation of their modeling. The concrete effectivenessfactor ν is an essential parameter that needs to be insertedinto the development of the plasticity theory. The bestagreement between theory and experimental data is obtainedby the appropriate choice of ν, but the authors did notprovide the value used in thei r study. The authors statement,“The strength of the nodes, struts, and ties was calculatedusing procedures in Appendix A of the 2002 ACI Code(ACI Committee 318 2002)” is very unclear because thisstrength depends on the quite a lot of parameters and thiscode provides several expressions to calculate theconcrete effectiveness factor.2. The authors adopted the strength reduction factor φ =0.75 to find the ties armors, which is an inadequate and conser-vative approach for this type of research. The abundance ofarmors in several regions of th e beams is corroborated by thelow strain measured in the several ties. All specimens haveunusual reinforcement arrangements. The secondary armorson Specimens 1A and 1B certainly are responsible for thegreat discrepancies among theoretical and experimentalresults. This fact is corrobora ted by the authors’ approximateprocedure to estimate the contributions of these secondaryreinforcements, substantially reducing these differences.3. Specimens 2A and 2B were designed for an ultimateload with very complicated models, and this is not the basicidea of the strut-and-tie model approach. Instead of verycomplex modeling for these specimens, it would be moreinteresting to use the simplest steel reinforcement arrange-ments, similar to what is currently done for reinforced concretedeep beams with openings.4. All four models shown in Fig. 2 of the paper arecomposed by superimposed models, but this information isnot given by the authors. Strut-and-tie modeling is a rationaland simple method for analysis and design, but for Models 2Aand 2B, it is not true. These two models are very complexand inappropriate for an engineering design.5. All three possible different D regions are cataloged inJennewein and Schafer (1992), where the expressions forforces, angles of the struts, and ties are given for each Dregion, and it is a waste of time trying to compare withanother special truss model for D regions near the openings,which is the case of models of Specimens 2A and 2B.6. The authors failed to explain the theoretical considerationsabout the models. It would be interesting to know the values ofthe struts angles adopted in the analyses. Another considerationthat requires a better explanation is the optimization of themodels. Further precise information of the models conception isnecessary—for example, struts lengths and widths, nodedimensions, types of the basic models that are superimposed,and details about nonlinear analyses undertaken.7. The discussers believe that it is impossible to check thetheoretical values given in Tables 2 and 3 of the paper, andseveral topics of the paper are confusing. Therefore, thediscussers would greatly appreciate if the authors couldprovide some complementary information about the research.

Evaluation of a Noncomposite Steel Girder Bridge through Live-Load Field Testing

This paper presents the field evaluation of a damaged noncomposite steel girder bridge that is part of the interstate highway system in Vermont. This bridge is typical of late 1960s construction and spans over a two-lane state highway near the town of Weathersfield, Vermont. The superstructure contains three-span continuous girders supported on abutments at the ends and on RC multicolumn interior bents. Strains were measured during live-load testing that was conducted to better understand the bridge behavior. The field results were compared with results from finite-element models created using common engineering assumptions. In addition, the load-distribution characteristics of girders that were damaged by an overheight truck traveling on the state highway under the bridge were evaluated. The results indicate that alternate load paths were developed within the bridge superstructure as a result of damage from the truck impact. For the loading magnitude applied during the load tests, evidence of composite action was observed and participation of curbs on the response of the bridge was noticed. Bridge skew and partial restraint generated at the supports also contributed to differences in the observed and calculated responses. These nondestructive load testing results were used to provide confidence on the load-carrying capacity of the bridge and to avoid costly bridge closures and detours.

Load Testing and Modeling of Two Integral Abutment Bridges in Vermont, US

Abstract Load testing of two instrumented single-span integral abutment bridges (IABs) in Vermont, US, was conducted using loaded multiple tandem dump trucks. Data was used to validate finite element (FE) models of each structure. Field data and FE model results were compared to gain an understanding of the distribution of live load on completed integral bridges and resulting stresses and deformations in structural components. The superstructure of the two bridges consists of a concrete deck supported on steel I-girders that are cast integrally into concrete abutments at the bridge ends. Abutments are supported on H-piles oriented with their weak axis resisting deformations in the direction of bridge alignment. The spans of the two bridges are 43 and 37 m, with girders connected to the abutments perpendicularly and with a skew angle of 15°. Correlation of measured data, such as girder strains, pile strains and deflections, abutment displacements, and backfill pressures, with FE model values are presented. Important considerations in the interpretation of field data are presented, including evaluations of the degree of composite action in girders, effects of thermal fluctuations over the course of testing and challenges faced when using field data directly to compute live load distribution factors.