Click for next page ( 23


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

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

OCR for page 22
23 Internal diaphragm Girder flange splice plate A A Bolts transferring torsion to pier cap Splice plate to girder connection Section A-A Figure 18. Girder-to-pier-cap connections. more liberal construction tolerances. However, where it of making the integral connection relative to other con- is desirable to eliminate the need to shore up the super- struction operations. structure, the integral connection may be made before Temperature changes after placement of the concrete of the superstructure girders are erected. In such cases, the integral connection cause displacements that produce tight tolerances on the orientation and elevation of the forces in both the substructure and, due to the integral pier cap are required to allow the superstructure girders connection, the superstructure. The movements associ- to be connected later without undue difficulty. Unbal- ated with temperature changes before the connection con- anced dead loads and the sequence of construction of crete reaches adequate strength may damage the bond the superstructure and pouring of the deck may develop between the pier cap and the connection concrete. To permanent locked-in stresses in the connection. These minimize these forces, measures to stabilize the temper- permanent locked-in stresses are required to be accounted ature of the girders during, and for adequate time after, for in the design. The designer needs to specify the time placement of the connection concrete should be applied.