Section 4: Alignment
Anchor: #i1001880Overview
The subject of alignment covers horizontal and vertical curvature of the profile and/or station line and the cross-slope of the deck surface.
Anchor: #i1001890Background
In the early days, highway engineers were satisfied with bridges that were straight, square, and relatively flat. It gradually became evident that bridges could handle other types of alignment, although with considerably more complexity in the details. Presently, curves, skews, variable widths, and crown rollouts are normal. About the only alignment that is not compatible with bridges is the spiral curve. Spirals are still used occasionally for highway alignment, but they are usually approximated by three centered circles for use in bridge framing.
Anchor: #i1001900Current Practice
Highway alignment follows the guidelines given in the TxDOT Roadway Design Manual. Bridge alignment conforms to these alignments and is usually a “given” on the preliminary layouts.
Anchor: #i1001912Design Recommendations
Horizontal Curvature. Horizontal curvature up to 5 degrees on wide bridges and 10 degrees on narrow connection structures can be expected. Curves up to 20 degrees have occasionally been used on “button hook” ramps and turnarounds. Horizontal curvature of beams, with the exception of pan form girders, can be handled gracefully in cast-in-place structures, but these have not been economical in Texas for many years. The preferred support system is precast prestressed beams. Since the beams must be straight, overhang width to the curved deck edge may limit the span length. Figure 4-3 shows this relationship. If the curvature/span length combination exceeds the capability of the deck slab, the span must be decreased or other measures must be considered, such as the use of curved steel girders.
Vertical Curvature. Extreme vertical grade can cause construction problems but seldom influences structure type. Grades over 5 percent call for extra care during concrete placement. The concrete tends to flow downhill during finishing operations. Thicker slab spans are more sensitive than deck slabs. Elastometric bearing details for prestressed concrete beams require special consideration for grades over 5 percent, Extreme vertical curvature can seriously affect forming methods for deck slabs on prestressed beams, especially if precast concrete deck panels are used. Crest curves cause extra deck depth in the middle of the span. Sag curves cause extra depth at the ends. If the deck slab is cast on removable forms, this extra depth can be accommodated in the haunch depth over each beam. Even so, it is necessary to set the haunch depth carefully to avoid construction problems.
Figure 4-3. Guidelines for Horizontal Curvature Using Prestressed Beams (Online users can click here to view this illustration in PDF.)
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Based on AASHTO Slab Grade 60 Reinforcing *Adjust to 1/4 pt. of Flanges for Steel Beams |
Gradeline. Current practice is to set haunch depths that will keep the top of beam at or below the bottom of the slab. Extra reinforcing is required if the haunch depth exceeds 3.0 in. (75 mm). If precast concrete deck panels are used, the problem is more critical. Special grading details may be required to accommodate tall haunches. Bearing seat elevations may require lowering for sag curves. Variable camber in prestressed beams aggravates the problem. It may not be possible to cover all these variations in the design stage. Contractors have become accustomed to adjusting the gradeline after taking elevations on the tops of the erected panels.
Cross-slope or crown for bridges is 1 percent minimum, 2 percent desirable. If the structure is more than two lanes wide, the outer lanes are usually sloped 2.5 percent to facilitate drainage. Cross-slopes can transition into superelevations as much as 8 percent on curved structures. Superelevation above 5 percent can cause problems with concrete placement the same as steep grades. If such deck slopes cannot be avoided, the construction engineers should be alerted to the possible need for special concrete placement requirements. Superelevation also affects clearances between deck slab and beam, especially when precast concrete panels are used. Superelevation creates an apparent sag vertical curve along the prestressed beam, which is a chord to the curvature.
When vertical curvature and superelevation exist, and the effect of beam camber is added, drastic measures may be required, especially with sag curves and panel deck construction. All of this can be accommodated, but extreme caution should be exercised and detailed geometric computations made.
Roadway Design System. Deck dimensions, beam framing, bearing seat elevations, web cutting, and bent locations must be accurately calculated to fit the prescribed alignment. This calculation is the responsibility of the bridge designers. The Roadway Design System (RDS) is a geometric computer program, originally developed in Texas and formerly used nationwide. The program has several bridge oriented capabilities for slabs, beams, and girders and is used exclusively by the Bridge Design Section for prestressed concrete beam spans on curves. The more important bridge routines in RDS are the following:
- SLAB. Computes and tabulates edge dimensions and areas for deck slabs of all configurations. Slab edges can be plotted.
- SLEL. Will produce a tabulation of distances, surface elevations, bottom of slab elevations, and bottom of slab plus dead load deflection along the boundaries of the slab.
- FOPT. Computes and tabulates framing dimensions for beam spans or continuous girders according to one of several programmed options. Framing diagrams can be plotted.
- BMGD. Will produce a tabulation of surface elevations, bottom of slab elevations, and bottom of slab elevations plus dead load deflection along the centerline of each beam.
- VCLR. Computes vertical distance from a roadway surface to chorded beam lines. It is used to calculate vertical clearances and to check beam haunch within a span.
Contour plotting is also available in the program. Refer to the Roadway Design System Manual for further details.
There is a company that maintains the RDS program for national use. However, the Information Systems Division of TxDOT has performed most of the maintenance of the program’s bridge commands in the past few years. The program has also been made compatible with metric dimensions. Consultants should be careful to use the most recent version of the program.
Other Software. IGRDS, a computer software roadway design system, is an AASHTOWARE product available from AASHTO.
A new geometry program, called GEOPAK, is being used in most highway applications. It is very useful in performing the usual highway plan functions, though the company has not yet been able to provide good bridge routines. If there are significant bridges in a project, the alignment file must be duplicated in the RDS format for use in bridge framing.
One problem with RDS is that the roadway surface must be defined by radial cross-slopes from only one profile grade line. In varying roadway widths, where ramps are converging or diverging, it may be necessary to adjust cross-slopes at close intervals along the main profile grade line to provide a smooth transition to the ramp grade and avoid edge profile problems. Contours can be used to advantage in this situation.
Superelevation Transition. Superelevation transition across a varying width roadway can cause unsightly lines on the outside railing. This usually occurs where ramps enter or leave the main structure. Relative grades between the two also have an influence. Highway engineers are better able to work out this problem, but it appears often to be overlooked or loosely handled. It is recommended that bridge engineers consider this situation carefully before setting cross-slopes for framing computations. Contour plots and a plot option of SLEL can be useful in these considerations.
Under certain conditions, a combination of superelevation transition and vertical curvature with a constant roadway width can cause sags or humps on the outside of the bridge. Both are unsightly, and sags can pond water on the roadway surface. This problem is usually corrected by highway engineers, but it would be advisable for the bridge designers to verify the outside lines by contours or pavement edge profile plots.
Superelevation transitions can have an adverse effect on beam haunch. This effect can be minimized by starting and ending a transition at a bent. Figure 4-4 shows the built-in and recommended optional methods for handling superelevation transition in RDS.
Figure 4-4. Superelevation Transition According to Roadway Design System (Online users can click here to view this illustration in PDF.)