National Academies Press: OpenBook

Integral Steel Box-Beam Pier Caps (2004)

Chapter: Chapter 3 - Interpretation, Appraisal, and Application

« Previous: Chapter 2 - Findings
Page 19
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Application." National Academies of Sciences, Engineering, and Medicine. 2004. Integral Steel Box-Beam Pier Caps. Washington, DC: The National Academies Press. doi: 10.17226/13773.
×
Page 19
Page 20
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Application." National Academies of Sciences, Engineering, and Medicine. 2004. Integral Steel Box-Beam Pier Caps. Washington, DC: The National Academies Press. doi: 10.17226/13773.
×
Page 20
Page 21
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Application." National Academies of Sciences, Engineering, and Medicine. 2004. Integral Steel Box-Beam Pier Caps. Washington, DC: The National Academies Press. doi: 10.17226/13773.
×
Page 21
Page 22
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Application." National Academies of Sciences, Engineering, and Medicine. 2004. Integral Steel Box-Beam Pier Caps. Washington, DC: The National Academies Press. doi: 10.17226/13773.
×
Page 22
Page 23
Suggested Citation:"Chapter 3 - Interpretation, Appraisal, and Application." National Academies of Sciences, Engineering, and Medicine. 2004. Integral Steel Box-Beam Pier Caps. Washington, DC: The National Academies Press. doi: 10.17226/13773.
×
Page 23

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

19 CHAPTER 3 INTERPRETATION, APPRAISAL, AND APPLICATION The overall understanding of the behavior of integral pier connections between steel I-beam superstructures, steel box- beam pier caps, and concrete columns was gained from the questionnaire, literature survey, and results of the analytical and experimental studies. This understanding was used in determining the feasibility of the integral connections tested and in developing a design methodology for the integral pier caps. This design methodology is presented below. Proposed design provisions and commentary written in the format of the AASHTO LRFD Bridge Design Specifications were devel- oped based on the proposed design methodology. The proposed design provisions and commentary are presented in Appen- dix H provided on the accompanying CD-ROM. A detailed design example using the proposed provisions is presented in Appendix I provided on the accompanying CD-ROM. Some of the observations from the analytical and experi- mental studies were extrapolated to accommodate expected variations in geometry and site conditions that were not directly included in the studies. This extrapolation is based on the design experience of many of the research team members and sound engineering judgment. 3.1 FEASIBILITY The experimental studies demonstrated the feasibility of connecting a concrete column integrally to a steel box-beam pier cap. They further demonstrated that the bond between the concrete placed inside a box-beam pier cap and the lon- gitudinal column reinforcement is sufficient to develop the overstrength column moment and no further connection is required for the column reinforcement. The test results also indicated that shear connectors installed inside the box-beam pier cap are sufficient to transfer the column moments from the concrete to the pier cap. 3.2 PROPOSED DESIGN METHODOLOGY 3.2.1 Method of Analysis Based on the analytical studies conducted as part of this project, the following observations were made: • The maximum moments and shears in the girders cal- culated using a conventional line girder computer pro- gram that does not take into account the effect of the inte- gral connection to the substructure compared well with the forces determined using a 3-dimensional finite ele- ment model of the bridge. The location of the maximum positive moments in the girders was different in the two cases; however, the shift in the maximum moment sec- tion location was relatively small to affect the design. • The live load moments in the single column pier from the 3-dimensional model compared well with the moment calculated using a 2-dimensional frame representing the bridge in elevation. The section properties of the mem- bers representing the superstructure were taken equal to the section properties of the entire bridge cross section. • The maximum live load torsional moment in the pier cap and the maximum live load moment at the top of the col- umn are produced by different load cases. The maximum torsional moment is produced when opposite halves of the two spans next to the pier cap are loaded (see Figure 16a). The maximum live load column top moment is produced when the longer of the two adjacent spans is loaded (see Figure 16b). In the prototype bridge, the maximum live load torsional moment in the pier cap on either side of the column, produced from a load case sim- ilar to that shown in Figure 16a, is approximately equal to the maximum live load moment in the column at its top, produced from a load case similar to that shown in Figure 16b. For the case of maximum torsional moments in the pier cap, the column moments and the rotation at the column top and at interior girder intersections with the pier cap are rela- tively small. This results in the exterior girders transferring most of the pier cap torsional moment. The maximum seismic torsional moment in the pier cap on either side of the column may be taken equal to one-half the column overstrength moment. For a two-span bridge with two girders on either side of the column, the interior girders and the exterior girders transferred 64 and 36 percent of the torsional moment, respectively. Assuming that 40 percent of the torsional moment on either side of the column is trans- ferred by the exterior girder and the remaining 60 percent is transferred by the first interior girder is expected to result in adequate design accuracy.

20 multi-column bents, the stiffness of the member rep- resenting the substructure will be taken equal to the sum of the stiffness of all columns in the bent. – The transverse moments in the substructure columns may be determined using 2-dimensional frame analy- sis of the columns and the pier cap. The loads trans- mitted from the girders to the substructure should be applied as concentrated loads at girder locations. – The live load torsional moment in the pier cap on either side of the column of a single-column integral pier may be taken equal to maximum live load moment acting at the top of the column determined using 2-dimensional frame analysis as described above. This moment may be assumed constant along the full length of the pier cap. – The maximum torsional moment in the pier cap due to seismic loading may be taken equal to one-half the column overstrength moment at the top. The exterior and the first interior girders may be assumed to trans- fer 40 and 60 percent of the torsional moment due to seismic load to the pier cap. – In most cases the column of a single-column pier will be located at mid-width of the structure. This was the geometry used in this study. However, site geometri- cal constraints may require offsetting the column from the mid-width of some bridges. The simplifications provided above are recommended to be used if the column is offset by no more than 10 percent of the bridge width. The 10-percent limit is an arbitrary limit based on engineering judgment. • For other bridges, 3-dimensional refined analysis should be used to determine the forces acting on the compo- nents of both the superstructure and the substructure. 3.2.2 Design and Anchoring of the Column-to-Pier Cap Connection The following were observed during testing of the two test specimens: • The column design forces may be determined using the AASHTO LRFD Specifications (1) supplemented by any specific requirements of the owner agency. Accord- ing to the AASHTO LRFD Specifications, column design moments and associated shears are taken as the largest calculated forces from all applicable strength limit states and the extreme event limit states. For the extreme event limit state that includes seismic forces, the maximum col- umn moment is determined as the lesser of the moment calculated from elastic analysis and that based on plastic hinging of the column including consideration of the col- umn overstrength effects. • Filling the pier cap compartment directly above the col- umn (see Figure 17) with concrete and extending the col- For a bridge with more than two girders on either side of the column, it is rational to expect that the percentage of the tor- sional moment transferred by the girders is highest for the inte- rior girder and is lowest for the exterior girder. It can also be rationally expected that the interior girder in a bridge with two girders on either side of the column transfers more moment than that in a bridge with a larger number of girders. In the absence of analytical studies on bridges with more than two girders on either side of the column, the proposed distribu- tion, which is based on results of a bridge with two girders on either side of the column, is expected to yield conserva- tive results for bridges with more girders. Based on these observations, and considering the limita- tion of the study, the following recommendations are made: • For I-girder superstructures connected integrally with the substructure the following approximations may be used: – The girder forces may be determined ignoring the effect of the integral connection (i.e., conventional line girder computer programs may be used for the analy- sis and design of the girders with integral connections). – The longitudinal moments acting on the substructure columns may be determined using 2-dimensional frame analysis. The section properties of the frame members representing the superstructure and those representing intermediate piers or bents should be taken equal to the flexural stiffness of the full cross section of the bridge and the flexural stiffness of the pier column, respectively. In the unlikely case of using Integral pier cap Span 1 Span 2 > Span 1 Loaded areas (a) Loading for Maximum Torsion Integral pier cap Loaded area (b) Loading for Maximum Column Moment Figure 16. Live load cases for maximum pier cap torsion and maximum column moment.

umn longitudinal reinforcement into the pier cap pro- vides adequate anchorage to the column. The length of the column longitudinal reinforcement above the bottom flange of the pier cap should be sufficient to develop these bars. If needed, the bars may be extended through the top flange of the pier cap into the deck slab. • At high levels of inelastic deformation, the column longi- tudinal reinforcement in the first test specimen appeared to have not been fully confined at the point these bars passed through the bottom pier cap flange. At this point, the column spiral reinforcement was stopped at either side of the pier cap flange plate and was anchored by two extra turns of the spiral as recommended by Sritharan et al. (15) which is more than the one and a half turns required by the AASHTO LRFD Bridge Design Specifications (1). In the second test specimen, the spiral was anchored using 21 two extra turns and the ends of the spiral bars were bent toward the center of the column and were provided with a seismic hook. The column longitudinal reinforcement in the second specimen did not lose confinement until the end of the test. Unfortunately, the second specimen failed at a lower level of inelastic deformations than the first specimen because of the loss of bond between the longi- tudinal bars and concrete. It was expected that the perfor- mance of the confinement reinforcement in the second specimen would exceed that in the first specimen because of the extra anchorage provided. • Providing shear studs inside the pier cap (see Figure 17) to transfer the column axial load and moments to the pier cap provided a satisfactory load path. The moment used to design the connection should be taken as the column top design moment, including consideration of the over- A A Transfer horizontal shear between column and cap beam Section A-A ML MT Carry shear from beams to column produced by MT Carry shear from beams to column produced by ML. Also carry shear from axial load since this is the most direct load path to the column from the beams. diaphragm girder Figure 17. Shear studs in the integral connection.

strength effects when analyzing load cases that include seismic loading. Based on these observations, the following recommenda- tions are made: • Girder spacing should be selected to eliminate the inter- ference of the girder flanges with the column longitudi- nal reinforcement extending into the pier cap (i.e., the girder flanges should be located outside the width of the columns). This is of particular importance in seis- mically active zones. In case one or more girders are placed within the width of the column, the column lon- gitudinal reinforcement bars within the width of the gird- ers need to be terminated below the bottom of the gird- ers. These bars should be ignored when determining the moment resistance of the column top. • The location of the two girders next to a substructure column should preferably be symmetric with respect to the column. • The intermediate diaphragms inside the box-beam pier cap divide the interior space of the beam into separate compartments (see Figure 17). A compartment bounded by the webs of the box-beam and two intermediate dia- phragms should be centered above the column and, in the unlikely case of multi-column bents, above each substruc- ture column. This compartment should then be filled with concrete to anchor the column longitudinal reinforcement. • The longitudinal reinforcement of the columns should be extended into the pier cap for at least the develop- ment length of the rebar. • At the end of the spiral on either side of the pier cap bottom flange, two extra turns of spiral bars or wire are required to anchor the spiral reinforcement. Bend the end of the spiral toward the center of the column for a distance equal to 12 times the spiral reinforcement diameter. • To ensure adequate confinement of the concrete in the connection region, it is necessary to extend the trans- verse reinforcement of the column in the integral con- nection region inside the pier cap. The transverse rein- forcement ratio inside the pier cap should not be less than the greater of the minimum transverse reinforce- ment ratio required by the specifications and one-half of that used outside the pier cap. • Shear connectors should be provided inside the concrete- filled compartment of the pier cap. These shear connec- tors are required to be designed to transfer the column forces to the pier cap. Column design moments should be converted into shear force acting on the shear con- nectors. The magnitude of the shear force may be deter- mined by dividing the column top design moment by the distance between the planes of the shear connectors assumed to transfer the moment to the pier cap. • Shear connectors sufficient to transfer the maximum shear in the column should be installed on either side of the pier cap bottom flange. 22 3.2.3 Connection Between the Girders and the Pier Cap Based on the procedures used in designing the test speci- mens and the successful performance of the specimens dur- ing testing, the following recommendations may be made. • The continuity of the girders over the pier cap may be provided through the use of girder flange splice plates that span the width of the pier cap and are connected to the flanges of the girders on either side of the pier cap (see Figure 18). The splice plates may be assumed to resist the full design moment in the girder. The design force of the splice plates and their connection to the girders may be taken equal to the design moment in the girder divided by the girder depth. • The connection between the webs of the girders to the webs of the pier cap may be designed to resist the max- imum vertical shear in the girders. The effect of the moments on this connection may be ignored when the flange splice plates are designed to resist the full moment on the connection. • The difference in a girder moment at either face of the pier cap is transferred to the pier cap in the form of tor- sional moment. This moment is transferred through the shear force acting on the connection between the girder splice plates and the pier cap. It is recommended that the connection between the girder flange splice plates and the pier cap (see Figure 18) be designed to resist a shear force equal to the maximum torsion transferred to the pier cap at the girder location divided by the depth of the girder. It also is recommended that the splice plates and the con- nection between the splice plates and the girder (see Fig- ure 18) be designed to resist a force equal to the girder moment at the face of the pier cap divided by the depth of the girder. 3.2.4 Box-Beam Pier Cap Design Once the design forces are determined, box-beam design provisions currently in the AASHTO LRFD Specifications (1) are sufficient to design the box-beam pier cap. Bolting spac- ing and clearance requirements in the specifications should also be satisfied. 3.3 CONSTRUCTION RECOMMENDATIONS Based on the information collected during this study, the following construction recommendations are made: • Making the integral connection after the steel members are erected and the deck slab is poured is the favorable construction sequence because it minimizes the dead load locked-in forces in the connection and allows for

more liberal construction tolerances. However, where it is desirable to eliminate the need to shore up the super- structure, the integral connection may be made before the superstructure girders are erected. In such cases, tight tolerances on the orientation and elevation of the pier cap are required to allow the superstructure girders to be connected later without undue difficulty. Unbal- anced dead loads and the sequence of construction of the superstructure and pouring of the deck may develop permanent locked-in stresses in the connection. These permanent locked-in stresses are required to be accounted for in the design. The designer needs to specify the time 23 of making the integral connection relative to other con- struction operations. • Temperature changes after placement of the concrete of the integral connection cause displacements that produce forces in both the substructure and, due to the integral connection, the superstructure. The movements associ- ated with temperature changes before the connection con- crete reaches adequate strength may damage the bond between the pier cap and the connection concrete. To minimize these forces, measures to stabilize the temper- ature of the girders during, and for adequate time after, placement of the connection concrete should be applied. Section A-A AA Girder flange splice plate Internal diaphragm Splice plate to girder connection Bolts transferring torsion to pier cap Figure 18. Girder-to-pier-cap connections.

Next: Chapter 4 - Conclusions and Suggested Research »
Integral Steel Box-Beam Pier Caps Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB’s National Cooperative Highway Research Program (NCHRP) Report 527: Integral Steel Box-Beam Pier Caps examines details, design methodologies, and specifications for integral connections of steel superstructures to concrete intermediate piers. The report also includes an example illustrating the design of the connection of the cap beam to the girders and column is also included.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!