Bijan Khaleghi, Ph, D. P.E., S.E.

Transcription

1 0 Submission date: July, 0 Word count: 0 Author Name: Bijan Khaleghi Affiliations: Washington State D.O.T. Address: Linderson Way SW, Tumwater WA 0 INTEGRAL BENT CAP FOR CONTINUOUS PRECAST PRESTRESSED GIRDER BRIDGES Bijan Khaleghi, Ph, D. P.E., S.E. State Bridge Design Engineer Washington State Department of Transrtation Olympia, Washington Phone: (0) 0- Fax: (0) 0- khalegb@wsdot.wa.gov TRB 0 Annual Meeting

2 Bijan Khaleghi 0 ABSTRACT Ductile behavior of bridge systems is desirable under earthquake loadings. The ductility of precast prestressed girder bridges can be achieved by proper detailing of pier diaphragm through extended strands, column bars, and joint reinforcement. Extended strands at intermediate crossbeams are used to connect the ends of girders with diaphragms and resist loads from creep effects, shrinkage effects, and seismic sitive moments. The objective of extending the strands is to ensure that the tensile force carried by the strand can be transferred to the opsite side of the diaphragm. The use of ASTM A0 Grade 0 reinforcing steel with this higher tensile capacity can provide benefits to concrete bridge construction except where ductility is required by reducing member cross sections and reinforcement quantities. The use of high-strength steel reinforcement with high-strength concrete should result in more efficient use of both materials. This paper describes WSDOT procedure for the design of fixed pier connections of continuous prestressed girder bridges extended strands at fixed pier connections of continuous prestressed girder superstructures. TRB 0 Annual Meeting

3 Bijan Khaleghi INTRODUCTION Seismic design of precast concrete bridges begins with a global analysis of the resnse of the structure to earthquake loadings and a detailed evaluation of connections between precast girders and connections between the superstructure and the suprting substructure. Ductile behavior is desirable under earthquake loadings for both the longitudinal and transverse directions of the bridge. Further, the substructure must be made to either protect the superstructure from force effects due to ground motions through fusing or plastic hinging, or to transmit the inertial forces that act un the bridge to the ground through a continuous load path. Where traditional simple-span precast girders and prestressed girders made continuous in the deck for live loads and superimsed dead loads are suprted on dropped bent caps, there is an absence of monolithic action between the superstructure and the bent cap. The girder seats on the bent cap act as rollers or pinned connections. Consequently, while the transverse stability of multi-column bents with a continuous end-diaphragm provides good frame stability in the cap for the transverse direction of the bridge, stability in the longitudinal direction requires the column bases to be fixed to the foundation suprts. This requirement can result in substantial force demands on the foundations, particularly in areas of moderate to high seismicity. PERFORMANCE CRITERIA FOR PRESTRESSED GIRDER ORDINARY BRIDGES Designing for life safety means that significant damage can result. Significant damage includes permanent offsets and damage between approach structures and the bridge superstructure, and between spans at expansion joints, permanent changes in bridge span lengths, and permanent displacements at the top of bridge columns. Damage also consists of severe concrete cracking, reinforcement yielding and buckling, major spalling of concrete, and severe cracking of the bridge deck slab. These conditions may require closure of the bridge to repair the damages. Partial or complete replacement of columns may be required in some cases. For sites with lateral flow due to liquefaction, piles and shafts may suffer significant inelastic deformation, and consequently, partial or complete replacement of the columns, piles and shafts may be necessary. If replacement of columns or other comnents is to be avoided, a design strategy that produces minimal or moderate damage, such as seismic isolation or a control and reparability design concept, should be used. Designing for life safety means that significant disruption to service level performance is likely, resulting in the need for limited access to the bridge and ssible shoring requirements. More rigorous analysis and detailing should be considered for essential and critical bridges. PRECAST GIRDERS BEARING ON PARTIALLY PRECAST OR CAST IN PLACE BENT CAP The Global Seismic Design Strategies used by the AASHTO Guide Specifications for LRFD Bridge Seismic Design (AASHTO SGS) are based on the expected behavior characteristics of the bridge system and are commonly used for prestressed girder bridges. Type I ductile substructure with essentially elastic superstructure includes conventional plastic hinging in columns and walls, and abutments that limit inertial forces by full mobilization of passive soil resistance. Figure shows a typical Washington State Department of Transrtation (WSDOT) continuous precast prestressed girder bridge with lower a crossbeam meeting the requirements of Type I AASHTO SGS Seismic Design Strategy. The behavior is similar if bridge suprted on piles. TRB 0 Annual Meeting

4 Bijan Khaleghi Figure : Typical WSDOT Continuous Prestressed Girder Bridge During a seismic event, the moment rapidly changes with the cyclic behavior. On one side the moments are additive, while a relatively smaller and constant sitive moment occurs on the opsite side. This distribution is reversible depending on the direction of the earthquake force and is intensified by vertical ground motion in larger events. More rigorous analysis and detailing should be considered is case of sites subjected to seismic vertical acceleration. Structures which are skewed in plan are subjected to in-plane rotation toward the obtuse corners due to lateral seismic forces. Fuses in the form of concrete shear keys may be required in moderate to high seismic regions. This concept is illustrated in Figure. Girders and deck slab are continuous at piers with girders framed into the pier diaphragm. Such structures are thought to exhibit behavior as a continuous superstructure with a fixed moment resistant connection to the substructure. This connection concept is commonly used by WSDOT for bridges in moderate and high seismic zones. Figure : Pier with Girders Framed into the Pier Diaphragm Plastic hinges form before any other failure due to overstress or instability in either the overall structure, or in thefoundation, or both. Plastic hinges are permitted only at locations in columns where they can be readily inspected and repaired. Superstructure and substructure comnents and their connections to columns that are not designed to yield are rather designed to resist overstrength moments and shears of ductile columns. The plastic moment capacity for reinforced concrete columns is determined TRB 0 Annual Meeting

5 Bijan Khaleghi using a moment curvature section analysis, taking into account the expected yield strength of the materials, the confined concrete properties, and the strain-hardening effects of the longitudinal reinforcement. Capacity-protected members such as bent caps, joints at top and bottom of column, and integral superstructure elements that are adjacent to the plastic hinge locations are designed to remain essentially elastic when the plastic hinge reaches its overstrength moment capacity. The superstructure is designed as a capacity protected member. Any moment demand caused by dead load or secondary prestress effects in case of continuous tendons over piers is distributed to the entire width of the superstructure. The column overstrength moment, in addition to the moment induced due to the eccentricity between the plastic hinge location and the center of gravity of the superstructure, is distributed to the spans framing into the bent on the basis of their stiffness distribution factors. This moment demand is considered within the effective width of the superstructure. POSITIVE MOMENT CONNECTION AT PIER DIAPHRAGMS The procedure used to calculate the required number of extended strands is described in this section. Calculations assume the development of the tensile strength of the strands at ultimate loads. Strands used for this purse must be developed within the short distance between the two girder ends. The design moment at the center of gravity of the superstructure, M CG is calculated using the following equation: Where: top M = plastic overstrength moment at top of column, ft-kips Base M = plastic overstrength moment at base of column, ft-kips h = distance from top of column to c.g. of superstructure, ft L c = column clear height used to determine overstrength shear associated with the overstrength moments, ft This moment is resisted by the bent cap through torsion. The torsional capacity of the bent cat shall be investigated. The torsion in the bent cap is distributed into the superstructure based on the relative flexibility of the superstructure and the bent cap. Hence, the superstructure does not resist column overstrength moments uniformly across its width. To account for this, an effective width approximation is used, where the maximum resistance per unit of superstructure width of the actual structure is distributed over an equivalent effective width to provide an equivalent resistance. The equivalent width concept is illustrated in Figure. For concrete bridges, with the exception of box girders and solid superstructures, this effective width can be calculated as follows: Where: M CG = M top + top Base ( M + M ) h L () B eff = D c + D s () D c = diameter of column c D s = depth of superstructure including cap beam TRB 0 Annual Meeting

6 Bijan Khaleghi 0 0 Total number of extended straight strands, N ps, needed to develop the required moment capacity at the end of girder is based on the yield strength of the strands: where: A ps area of each extended strand, in. f py d N M SIDL K φ ps = [ M sei K M SIDL ] 0.φA D S yield strength of prestressing steel, ksi distance from top of slab to c.g. of extended strands, in. moment due to SIDL (traffic barrier, sidewalk, etc.) per girder span moment distribution factor strength reduction factor for flexure K A B eff D C Figure : Effective Superstructure Width for Extended Strand Design CONTINUITY OF EXTENDED STRANDS K B ps Continuity of extended strands is essential for all prestressed girder bridges with fixed diaphragms at piers. Strand continuity may be achieved by directly overlapping extended strands as shown in Figure a, by use of strand ties as shown in Figure b in case of curved superstructures with corded precast girders, by the use of the crossbeam ties as shown in Figure along with strand ties, or by a combination of all three methods. f py d () B eff K A D C TRB 0 Annual Meeting

7 Bijan Khaleghi a: Overlapping Extended Strand b: Strand Ties Figure : Overlapping Extended Strand and Strand Ties 0 Figure : Lower Crossbeam Ties The following methods in order of hierarchy are used for all precast girders for creating continuity of extended strands:. Direct extended overlapping strands are used at piers without any angle int due to horizontal curvature and for any crossbeam width. This is the preferred method of achieving extended strand continuity. Congestion of reinforcement and girder setting constructability is considered when large numbers of extended strands are required. In these cases, strand ties may be used in conjunction with extended strands. TRB 0 Annual Meeting

8 Bijan Khaleghi Strand ties are used at piers with a girder angle int due to horizontal curvature where extended strands are not parallel and would cross during girder placement. Crossbeam widths are greater than or equal to six () feet measured along the skew. While it is preferable that strand ties be used for all extended strands, if the region becomes too congested for rebar placement and concrete consolidation, additional forces may be carried by crossbeam ties up to a maximum limit as specified in equation () below. For crossbeams with widths less than six () feet and a girder angle int due to horizontal curvature, strand ties are used if a minimum of eight () inches of lap can be provided between the extended strand and strand tie. In these cases, the strand ties are considered fully effective. For cases where less than eight () inches of lap is provided, the effectiveness of the strand tie is reduced prortional to the reduction in lap. The () inches lap, used for ½ inch and 0. inch diameter strands, has not been tested but is commonly used in the practice and the adequacy has been verified through FEM models. All additional forces not taken by strand ties must be carried by crossbeam ties up to the maximum limit as specified in equation () below. If this limit is exceeded, the geometry of the width of the crossbeam is increased to provide sufficient lap for the strand ties. The area of transverse ties considered effective for strand ties development in lower crossbeam should not exceed: Where: A ps Area of strand ties, in n s f py f ye A s = Number of extended strands that are spliced with strand and crossbeam ties Yield strength of extended strands, ksi A Expected yield strength of reinforcement, ksi ps f f py ye n s The above equation is driven from the strut and tie model considering the -dimensional effect and conservative engineering judgment. Two-thirds of A s is placed directly below the girder and the remaining part of A s is placed outside the bottom flange width. The size of strand ties is the same as the extended strands, and is placed at the same level and proximity of the extended strands. Extended strands at intermediate crossbeams are used to connect the ends of girders with diaphragms and resist loads from creep effects, shrinkage effects, and seismic sitive moments. These strands are to be developed within the diaphragm between the two girder ends at piers. The objective of extending the strands is to ensure that the tensile force carried by the strand can be transferred to the opsite side of the diaphragm, thus ensuring an adequate tensile load path for joint forces. The use of an extended strand that overlaps with the strand from the opsite girder or the use of strand ties will facilitate development of strands within the diaphragm between two girder ends. It should be noted that extended strands are not prestressed. The extension is provided by a lap splice between the strand ties and the extended tensile strands. Because the strand strength is much higher than that of deformed bars, strand ties are recommended. The number and the size of the strand ties is the same as that of the extended strands that are being developed with strand ties. Circular strand anchors of two and three-quarters (¾) inchesø by one () inch should be installed at four () inches from the opsite girder end. Circular strand anchors are required for ease of construction. The strand extension beyond the face of strand anchors is limited to one () inch. () TRB 0 Annual Meeting

9 Bijan Khaleghi 0 Additionally, the top of the ties in the lower crossbeam are considered to contribute to the extension of the strand tensile force into the diaphragm. This force transfer takes place through non-contact lap-splice (strut and tie) action between the extended strands and the lower crossbeam ties. The top of lower crossbeam is affected when extended strands are developed by a compressive strut forming between the strand anchors and crossbeam ties. The designer is to calculate the number of crossbeam ties as shown in Equation (), which assumes that one-half of the non-overlapping strand force is transferred from the strand anchors to the lower crossbeam ties through strut action. It is also assumed that only two-thirds of the lower crossbeam ties need to be placed directly below the girders within the width of bottom flange since the affected zone on crossbeam ties is larger than the width of bottom flange. The tie bar size may be increased to meet the design requirement but preferably, the spacing should be kept the same as the crossbeam stirrup spacing for ease of construction. Ties may be bundled, if necessary. The tie bar size should not be less than the crossbeam stirrups bar size. JOINT PERFORMANCE FOR SDCS C AND D Moment-resisting connections for prestressed girder bridges in SDCs C and D are designed to transmit the maximum forces produced when the column has reached its overstrength capacity. A "rational" design is required for joint reinforcement when principal tension stress levels become excessive. The amounts of reinforcement required are based on a strut and tie mechanism similar to that shown in Figure for a hammerhead pier crossbeam. 0 Figure : Strut and Tie Model of Intermediate Diaphragm EXTENSION OF COLUMN REINFORCING INTO CROSSBEAMS For precast prestressed girder bridges in SDCs C and D with fixed diaphragms at piers, all column longitudinal reinforcement should be extended into the cast-in-place concrete diaphragm on top of the crossbeam. For bridges in SDC B with fixed diaphragms at piers, column longitudinal reinforcement can be terminated at top of lower crossbeam. Column longitudinal reinforcement can be terminated at top of lower crossbeam in all SDCs if analysis shows that plastic hinging will not occur at the top of column under the design earthquake. TRB 0 Annual Meeting

10 Bijan Khaleghi 0 0 Careful attention should be given at the preliminary plan stages to ensure that column reinforcement does not interfere with extended strands projecting from the end of precast girders. Cases of column longitudinal reinforcement with the extended strands and bars are shown in Figure a, b and c. these details have successfully been used in recent WSDOT bridge projects. In case of interference, column longitudinal reinforcement obstructing the extended strands should be terminated at the top of the lower crossbeam, and should be replaced with the equivalent full height stirrups extending from the lower to upper crossbeam within the effective zone. The effective zone is defined as the width D c +D s for columns without headed bars and D c + D s for columns with headed bars, where D c is the column width or diameter, and D s is the depth of lower crossbeam. Headed bars are only used if the depth of the lower crossbeam is less than. times the tension development length of column longitudinal reinforcement. a) No interference b) Partial Interference c) Interference 0 Figure : Column Longitudinal Reinforcement Interference with Extended Strands HIGH STRENGTH REINFORCEMENT The use of steel with this higher capacity could provide benefits to concrete bridge construction by reducing member cross sections and reinforcement quantities, leading to savings in material, shipping, and placement costs. Reducing reinforcement quantities will also reduce congestion problems leading to better quality of construction. Reinforcement conforming to ASTM A0 Grade 0 may be used for WSDOT bridges in Seismic Design Category (SDC) A for all comnents. For SDCs B, C and D, ASTM A0 Grade 0 reinforcing steel should not be used for elements and connections that are prortioned and detailed to ensure the development of significant inelastic deformations, for which moment curvature analysis is required to determine the plastic moment capacity of ductile concrete members and expected nominal moment capacity of capacity protected members. ASTM A0 Grade 0 reinforcing steel may be used for capacity-protected members such as footings, bent caps, oversized shafts, joints, and integral superstructure elements that are adjacent to the plastic hinge locations if the expected nominal moment capacity is determined by strength design based on the expected concrete compressive strength with a maximum usable strain of 0.00, and a reinforcing TRB 0 Annual Meeting

11 Bijan Khaleghi 0 0 steel yield strength of 0 ksi with a maximum usable strain of 0.00 for #0 bars and smaller, and 0.00 for # bars and larger. The resistance factors for seismic related calculations are taken as 0.0 for shear and.0 for bending. ASTM A0 Grade 0 reinforcing steel should not be used for transverse reinforcement in members resisting torsion. The applicability of AASHTO SGS is limited to grade 0 ksi reinforcing steel. Since the suitability of ASTM A0 Grade 0 ksi reinforcing bars for ductility and confinement has not been tested, the applicability should be limited only to non-ductile elements. CONCLUSIONS The ductility for precast prestressed girder bridges can be achieved by proper detailing of pier diaphragms through extended strands, column bars, and joint reinforcement. The following conclusions can be made:. Extended strands at intermediate crossbeams are used to connect the ends of girders with diaphragms and resist loads from creep effects, shrinkage effects, and seismic sitive moments.. For SDCs C and D with fixed diaphragms at piers, all column longitudinal reinforcement shall be extended into the cast-in-place concrete diaphragm on top of the crossbeam.. For bridges in SDC B with fixed diaphragms at piers, column longitudinal reinforcement can be terminated at top of lower crossbeam.. Column longitudinal reinforcement can be terminated at the top of the lower crossbeam in all SDCs if analysis shows that plastic hinging will not occur at the top of the column under the design earthquake.. The use of ASTM A0 Grade 0 reinforcing steel with this higher capacity can provide benefits to concrete bridge construction by reducing member cross sections and reinforcement quantities. REFERENCES. AASHTO Guide Specifications for LRFD Seismic Bridge Design, nd Edition, 0.. Bridge Design Manual, Publication No. M-0, Washington State Department of Transrtation, Bridge and Structures Office, Olympia, Washington, 00.. Shahrooz, B, Miller, R, Harries, K, and Russell, H Design of Concrete Structures Using High- Strength Steel Reinforcement, NCHRP Rert, Washington D.C. TRB 0 Annual Meeting