Bottom Reinforcement Research Articles

Serviceability of Bridge Deck Slabs with Arching Action

Although it is commonly known that bridge deck slabs have inherent enhanced strength due to the presence of arching or compressive membrane action, only recently has there been some acceptance of a rational treatment of this phenomenon for design and assessment purposes. To show the benefits of arching action, this paper presents the results of tests carried out on a reinforced-concrete beam and slab bridge in Northern Ireland that incorporated novel reinforcement type and position. The research was aimed at extending previous laboratory tests on 1/3-scale bridge deck edge panels. The measured crack widths and deflections were compared with the current code requirements. The full-scale bridge tests corroborated existing findings from scale model tests. The deflections in the tests panels, under a service wheel load of 25.3 kips (112.5kN), were independent of the amount of tensile reinforcement in the test panel. All of the test panels were uncracked under this service wheel load and all the test panels had much narrower crack widths than those predicted using current standards. The deck slabs with center reinforcement had strengths far in excess of the design ultimate loads and behaved in a similar manner to those with top and bottom reinforcement up to an applied load of 51 kips (225kN). This finding indicates that substantial economies in the amount of reinforcement could be made. By using the benefits of arching action on the strength of laterally restrained slabs, very low percentages of reinforcement are possible, which should decrease the likelihood of deterioration due to reinforcement corrosion.

Cyclic behaviour of interior post-tensioned flat plate connections

In high seismic regions, post-tensioned (PT) slab–column frames are commonly used to support gravity loads in conjunction with a lateral-force resisting system (LFRS) such as a core wall. The LFRS is designed to resist 100% of the design lateral forces as well as to limit lateral displacements to an acceptable level, whereas the slab–column frame must sustain the gravity loads under the expected (design) displacements. Given the relatively sparse data on the seismic performance of PT flat plate slab–column frames, cyclic tests of four interior PT slab–column connections were conducted. Primary test variables were the level of gravity shear at the slab–column connection and the slab tendon arrangement. Test results indicate that both the test variables strongly influence the cyclic behaviour of the PT connections, and that the use of slab bottom reinforcement at the slab–column connection was effective in resisting positive moment developed under lateral loading as well as improving the hysteretic energy dissipation capacity.

Optimal domains for strength design of rectangular sections for axial load and moment according to Eurocode 2

Domains associated with the optimal design of reinforcement for rectangular concrete sections according to the provisions of Eurocode 2 are established with respect to N – M coordinates using results obtained with Reinforcement Sizing Diagrams. The characteristics of the optimal reinforcement solutions are identified for each domain, analytical expressions for the domain boundaries are established, and a two-step solution procedure is presented for determining the top and bottom reinforcement required to resist an arbitrary combination of axial load and moment. Examples illustrate the design of columns, singly reinforced beams, and doubly reinforced beams. The two-step design approach provides a direct solution for optimal reinforcement and requires less computation than the Reinforcement Sizing Diagram approach. The optimal reinforcement solutions can result in a significant savings in reinforcement quantities.

포스트 텐션 플랫 플레이트 외부 접합부의 내진 거동

초록·키워드 목차 오류제보하기 등록된 정보가 없습니다. ABSTRACT1. 서론2. 기존연구3. 실험적 연구4. 실험결과5. 실험결과 분석6. 결론참고문헌요약

Three-dimensional finite element analysis to predict the effects of SAW process parameters on temperature distribution and angular distortions in single-pass butt joints with top and bottom reinforcements

Achieving adequate top and bottom reinforcement is important to minimize angular distortions in single-pass submerged arc welded (SAW) butt joints. This is achieved in the present work by using a reusable flux-filled backing strip and proper SAW process parameters without resorting to costly distortion mitigation techniques. The butt joints were made without edge (square butts) preparation. The process was also modeled by using three-dimensional finite element analysis by incorporating the top and bottom reinforcements into the modeling. Filler material deposition was also simulated. Temperature distributions and angular distortions obtained from the modeling closely matched with the experimental values. Thus, the cost effective experimental methodology established in the present work can be utilized for minimizing angular distortions in SAW square butts. The modeling methodology adopted can be used for predicting the angular distortions in SAW square butts with top and bottom reinforcements.

Hysteretic behavior of exterior post-tensioned flat plate connections

The purpose of this study is to evaluate hysteretic behavior of exterior post-tensioned flat plate slab–column connections (PT connections) designed to resist only gravity loads. For this purpose, experimental studies were conducted using three approximately two-thirds-scale test specimens; two exterior PT connection specimens having different tendon layouts and one exterior reinforced concrete flat plate slab–column connection (RC connection) specimen. Quasi-static cyclic loading was applied to the specimen with a constant gravity load. All specimens had bottom bonded reinforcement around the column as required by ACI 352.1R-89. This study collects previous test results to draw a general conclusion for the hysteretic behavior of the PT exterior connections. This study observed that the tendon layout influenced the hysteretic behavior of PT exterior connections, which means that lateral drift capacity, dissipated energy, failure mechanism, and ductility vary with respect to tendon layouts. Moreover, this study shows that the amount of bottom reinforcement specified by the ACI 352.1R-89 is sufficient for resisting positive moments resulting from moment reversal under the cyclic loading. Shear strength of the test specimens is more accurately predicted by the shear strength equation considering average compressive strength ( f p c ) due to post-tensioning tendons than that without considering f p c .

A novel approach to estimating the bearing capacity stability of geosynthetic reinforced retaining walls constructed on yielding foundations

Yielding foundation conditions have been shown to adversely affect the stability and behaviour of overlying geosynthetic reinforced soil walls. To avoid serious problems and maintain a cost-effective design, careful consideration must be given to short-term stability. Previous research has shown that lengthening and stiffening the bottom reinforcement layer of the wall can increase the external stability, but the magnitude of this increase is not well understood. To provide insight regarding the potential benefit of lengthening and stiffening the bottom reinforcement layer, a numerical investigation is made of the plastic collapse mechanism due to bearing capacity failure of the foundation deposit for the case of a 6 m high geosynthetic reinforced retaining wall on a 10 m thick soft to firm visco plastic clay stratum. The calculated behaviour of the wall is compared with that from typical and novel design considerations for both a conventional reinforced wall and a wall where the bottom reinforcement layer has been extended and stiffened. A parametric study of the extended bottom reinforcement layer stiffness and interaction is reported, and the influence on the external stability is discussed.Key words: reinforced soil wall, soft yielding foundation, bearing capacity design, numerical analysis.

Strengthening of critically designed girders with dapped ends

Reinforced concrete (RC) beams with dapped ends are frequently found in bridge girders and precast concrete construction. A reduction in depth near the supports tends to produce a stress concentration and hence requires special analysis and detailing of any reinforcement used. Improper dimensioning and distribution of reinforcing steel can lead to undesirable cracking and failure mechanisms that need to be predicted in order that a proper strengthening system be applied. The experimental programme described in this paper incorporated 52 specimens with dapped ends; three main defects were intentionally introduced at the recess zone and 12 different strengthening techniques were applied. The defects included an inadequate development length of bottom longitudinal reinforcement at the dapped ends, and elimination of either horizontal or vertical shear reinforcement at the ends. External bonding of the steel angle at the reentrant corner, unbonded bolt anchoring, external steel plate jacketing, exterior carbon fibre wrapping and/or stripping were applied in order to determine the best strengthening technique. A strut-and-tie model was used in the analysis of such discontinuity regions and a strength enhancement index procedure was introduced. A comparative study was undertaken in order to evaluate the various strengthening techniques applied.

Strengthening of critically designed girders with dapped ends

Reinforced concrete (RC) beams with dapped ends are frequently found in bridge girders and precast concrete construction. A reduction in depth near the supports tends to produce a stress concentration and hence requires special analysis and detailing of any reinforcement used. Improper dimensioning and distribution of reinforcing steel can lead to undesirable cracking and failure mechanisms that need to be predicted in order that a proper strengthening system be applied. The experimental programme described in this paper incorporated 52 specimens with dapped ends; three main defects were intentionally introduced at the recess zone and 12 different strengthening techniques were applied. The defects included an inadequate development length of bottom longitudinal reinforcement at the dapped ends, and elimination of either horizontal or vertical shear reinforcement at the ends. External bonding of the steel angle at the reentrant corner, unbonded bolt anchoring, external steel plate jacketing, exterior carbon fibre wrapping and/or stripping were applied in order to determine the best strengthening technique. A strut-and-tie model was used in the analysis of such discontinuity regions and a strength enhancement index procedure was introduced. A comparative study was undertaken in order to evaluate the various strengthening techniques applied.

Design and ultimate behavior of RC plates and shells

An iterative numerical computational algorithm is developed to design a plate or shell element subjected to membrane and flexural forces, which is based on equilibrium considerations for the limited ultimate state of the reinforcement and cracked concrete. Equations for capacities of top and bottom reinforcements in two orthogonal directions have been derived. To verify the design algorithm on the element level several experimental examples are designed. Nonlinear inelastic analyses are performed with the designed examples using the Mahmoud-Gupta’s computer program to show the adequacy of the design equations. The calculated ultimate strengths are from 3 to 18% higher than the ultimate strength obtained from the test results, except in one example. On the global structural level, a design is performed for a hyperbolic cooling tower to check the design strength to verify the adequacy of the design algorithm. Based on the ultimate nonlinear analysis performed with the designed reinforcement, the analytically calculated ultimate loads exceed the design ultimate load from 26 to 63% for analyses with various amounts of tension stiffening effect. Even though the ultimate loads are dependent on the tensile properties of concrete, the calculated ultimate loads are higher than the design ultimate loads for the cases considered. This shows the adequacy of the design algorithm developed, at least for the structures studied. The presented design algorithm for combined membrane and flexural forces can be evolved as a general design equation for reinforced concrete plates and shells through further studies involving the performance of many more designs and analyses of different plate or shell configurations.

Response of Slab Bridges Before, During, and After Repair

Continuous reinforced concrete slab bridges rely on reinforcing steel bars near the top of the deck over the piers to carry negative moment. Transfer of forces in these bars may be jeopardized by deterioration and repair procedures that involve variable depth removal of deteriorated concrete around the bars. Partial or full loss of continuity could overstress the bottom reinforcement. Truckload testing of three bridges with various levels of damage was conducted before, during, and after repair in an attempt to quantify the level of loss of continuity and to examine the effectiveness of repair in terms of increasing the load transfer and enhancing the overall stiffness. Test results show loss of stiffness during repair but increased stiffness after completion of repair. The continuity was found to be lost during repair, and the slab dead load positive moments may be increased by as much as 50%. After repair, the continuity was restored, and the live-load distribution was essentially unaltered. For the test bridges, the redistribution of dead-load moment to the positive-moment zones did not appreciably affect the overall bridge rating factor. The amount of moment redistribution may be controlled through planning of repair steps.

Holistic behaviour of concrete buildings in fire

This paper discusses various modes of structural behaviour of a concrete building when subjected to a fire, based on observations from a full-scale test. Although some data were lost during the test, the available results and observations presented provide a valuable insight into the holistic behaviour of concrete buildings when subjected to fire. The tested building was constructed using elements formed from normal and high-strength concrete and was designed for 60 min fire resistance, using the UK Design Code. High-strength concrete was used for the columns within the fire compartment and since it has previously been shown that this type of concrete is susceptible to spalling, polypropylene fibres were added to the concrete mix during construction to alleviate the problem. Both the UK and European codified design methods suggest that concrete spalling within the fire compartment should have been nominal and could effectively be ignored during the design. However the test showed that spalling of the floor slab was extensive and exposed the bottom steel reinforcement. Although concrete spalling considerably reduced the flexural strength of the slab, collapse did not occur. This could be attributed to the slab behaving in compressive membrane action, which is currently not considered in codified design methods. The test also showed significant lateral displacement of external columns due to thermal expansion of the heated slab. The main observations from the test show that designers will need to understand the behaviour of entire structures in fire, to ensure that premature collapse will not occur.

Holistic behaviour of concrete buildings in fire

This paper discusses various modes of structural behaviour of a concrete building when subjected to a fire, based on observations from a full-scale test. Although some data were lost during the test, the available results and observations presented provide a valuable insight into the holistic behaviour of concrete buildings when subjected to fire. The tested building was constructed using elements formed from normal and high-strength concrete and was designed for 60 min fire resistance, using the UK Design Code. High-strength concrete was used for the columns within the fire compartment and since it has previously been shown that this type of concrete is susceptible to spalling, polypropylene fibres were added to the concrete mix during construction to alleviate the problem. Both the UK and European codified design methods suggest that concrete spalling within the fire compartment should have been nominal and could effectively be ignored during the design. However the test showed that spalling of the floor slab was extensive and exposed the bottom steel reinforcement. Although concrete spalling considerably reduced the flexural strength of the slab, collapse did not occur. This could be attributed to the slab behaving in compressive membrane action, which is currently not considered in codified design methods. The test also showed significant lateral displacement of external columns due to thermal expansion of the heated slab. The main observations from the test show that designers will need to understand the behaviour of entire structures in fire, to ensure that premature collapse will not occur.

Forensic studies of a large cover‐plate steel moment‐resisting connection

AbstractA large‐size, single‐sided cover plate connection composed of a built‐up BW36×670 column and a W36×359 rolled beam section was tested in a custom‐made test fixture using the SAC Phase I loading protocol. A trapezoidal plate was used as top flange reinforcement and a rectangular plate was used as bottom flange reinforcement. The test specimen failed catastrophically at a story drift angle of 1.7% and a beam plastic rotation of 0.009 radian by fracture through the beam top flange and beam web. An ABAQUS nonlinear finite element model of the connection was prepared and analyzed. The ABAQUS data indicate that the likelihood of brittle or ductile fracture at the observed crack initiation site is very low. Fractographic and microstructure studies were conducted after testing. The fractographic analysis of the failure surface showed that the fracture initiated at the toe of a fillet weld that joined the top cover plate to the beam flange. The microstructure analysis around the crack initiation site revealed laminations in the base metal at the crack initiation site. Copyright © 2002 John Wiley & Sons, Ltd.

Cyclic behavior of HPFRC-repaired reinforced concrete interior beam-column joints

A large number of old buildings have been identified as having potentially critical detailing to resist earthquakes. The main reinforcement of lap-spliced columns just above the joint region, discontinuous bottom beam reinforcement, and little or no joint transverse reinforcement are the most critical details of interior beam column joints in such buildings. Five 1/3-scale interior beam column joints representing these critical reinforcement details under seismic loads were constructed and tested under cyclic lateral loading simulating seismic excitation. These specimens behaved weakly such that they attained low load carrying capacity, small energy dissipation, and failed in diagonal shear in the joint region. After testing, the specimens were repaired using high performance fiber reinforced concrete (HPFRC) jacket, all around the joint column regions, and retested up to failure. Higher load levels were attained, more ductile behavior was achieved, substantial energy dissipation was observed, slower stiffness degradation was also noted. Failure modes were transformed from brittle shear in the joint diagonals to a ductile one in the beam region through plastic hinge formation in the beam maximum moment sections. Based on the findings of this study, the strengthening technique used promises to provide a cost effective, long-term repair and retrofit solution that can be implemented in the field.

Concrete Bridge Decks Constructed with Fiber-Reinforced Polymer Stay-in-Place Forms and Grid Reinforcing

A description is given of laboratory testing, initial test results, and continuing research to develop design procedures and plans for construction of a two-span highway overpass on US-151 in the state of Wisconsin using only a fiber-reinforced polymer (FRP) reinforcement system in the concrete bridge deck. The use of FRP reinforcing is being pursued to increase the durability of bridge decks and to reduce unit cost and time. The unique aspect of the new bridge is that it will use FRP stay-in-place formwork, placed over precast concrete I girders, to serve as formwork and as the bottom transverse reinforcement of the bridge deck. In addition, a heavy-duty prefabricated FRP grid will be used for the top layer of concrete reinforcing. Full-scale prototype laboratory testing is being used to develop design recommendations including effective distribution widths. From the spring to fall of 2003, a construction productivity study comparing the FRP deck construction sequence with that of identical adjacent steel reinforced deck will be conducted. Both bridges will be field load tested before opening to traffic. The FHWA Innovative Bridge Research and Construction Program is supporting the research and construction.

Shear Capacity of Reinforced Concrete Deep Beams

A mechanism analysis of shear failure of simply supported reinforced concrete deep beams is presented. Concrete and steel reinforcement are modeled as rigid perfectly plastic materials. The failure modes are idealized as an assemblage of rigid blocks separated by failure zones of displacement discontinuity. The shear strength of deep beams is derived as a function of the location of the instantaneous center of relative rotation of moving blocks. Minimization of the developed function gives the shear capacity of deep beams. Comparisons of the predicted shear capacity of numerous deep beams show good agreement with results obtained from experiments. A parametric study of main variables affecting shear strength of deep beams is conducted. The present model shows that the shear-span-to-depth ratio has more influence on the shear capacity than the span-to-depth ratio and as the former increases, the shear strength decreases. Increasing main longitudinal bottom reinforcement increases the shear capacity up to a...

Experimental investigation of the role of reinforcement in the strength of concrete deck slabs

To study systematically the role of each layer of steel reinforcement in conventionally reinforced deck slabs of girder bridges, a full-scale model was built of a 175 mm thick concrete deck slab on two steel girders with a center-to-center spacing of 2.0 m. The 12 m long deck slab was conceptually divided into four 3 m long segments, identified as segments A, B, C, and D. Segment A contained isotropic steel reinforcement in two layers, conforming to the requirements of the Ontario Highway Bridge Design Code (OHBDC). Segment B contained only the bottom layer of steel reinforcement. Segment C contained only the bottom transverse steel bars. Segment D contained only bottom transverse glass fibre reinforced polymer (GFRP) bars having the same axial stiffness, but 8.6 times the axial tensile strength, as those of the steel bars in segment C. Each segment of the deck slab was tested to failure under a central concentrated load, simulating the dual tire footprint of 250 × 500 mm dimension of a typical commercial vehicle. All segments failed in the punching shear mode. The failure loads for the four segments were found to be 808, 792, 882, and 756 kN, respectively; these failure loads are similar in magnitude to that of a 175 mm thick steel-free deck slab with steel straps having nearly the same cross-sectional area per metre length of the slab as those of the bottom transverse steel bars in the first three segments. The tests on the four segments of the full-scale model have confirmed that (i) only the bottom transverse reinforcement influences the load carrying capacity of a reinforced concrete deck slab and (ii) the stiffness of the bottom transverse reinforcement, rather than its strength, is of paramount importance.Key words: arching, deck slab, FRP, shake down, slab-on-girder bridge.

建設材料 鉄筋コンクリート内柱・梁接合部における通し梁主筋の降伏前の付着挙動の評価

In this paper using the available data on interior reinforced concrete beam-column joints tested by the authors, the evaluation method of bond behavior of beam bars before yielding passing through the joint has been proposed. Here the several influencing factors were analyzed and quantitatively evaluated. The following conclusions can be made:(1) The relationship between average bond stress in a joint region (oτav) and tensile strain of beam bars at the column face (oεst) was formulated for the single layered bar arrangement and for the outer bars of the double layered bar arrangement, in which the effect of the bar diameter to column depth ratio, concrete compressive strength, axial compressive stress levels of columns, and the ratio of top to bottom reinforcement areas were considered.(2) Applying the oτav-oεst equation, the relationship between average bond stress in a joint and tensile strain at the column face of the inner bars of the double layered bar arrangement (iτav-iεst) could also be evaluated by considering the deformation condition due to shear distortion and shear cracking widths of the joint panel.(3) The strain distribution of beam bars at the column face could be evaluated by considering the relationships of oτav-oεst and iτav-iεst.

Post-punching resistance of connections between flat slabs and interior columns

The paper reports tests designed to study the improvement in the post-punching resistance of slab–column connections by the provision of bottom bars passing through the columns and anchored in the slab. A series of model tests on slabs was used to developan expression for the strength limit governed by the slab concrete, while large-scale tests on abbreviated specimens were used to formulate the criterion dependent on the failure of the reinforcement. It is demonstrated that bottom reinforcement can be of significant benefit and that the resistance given by it can be assessed with reasonable accuracy.