BRIDGE DESIGN MANUAL

Miscellaneous Design Miscellaneous Highway Structures Design

8.3.4 Bridge Approach Slabs

A. General

Bridge approaches typically experience two types of settlement, global and local. Global settlement consists of a consolidation of the underlying natural foundation soils, and is evidence of possible long-term differential settlement between the bridge structure and the bridge embankment. Local settlement consists of compression of fill materials directly beneath the approach pavement, and is evidence of possible embankment consolidation within the upper 10 to 20 feet of the bridge embankment. It is the combination of global and local settlements adjacent to the bridge end piers that forms the characteristic “bump” in the pavement at the bridge ends. The purpose of the bridge approach slab is to significantly reduce local settlement and to accommodate global settlement by providing a gradual transition between the roadway and the bridge deck.

Bridge approach slabs are required for all new and widened bridges, except when concurrence is reached between the Materials Laboratory Geotechnical Branch, the Region Design Project Engineer Office, and the Bridge and Structures Office, that approach slabs are not appropriate for a particular site. In accordance with WSDOT Design Manual Chapter 1120, the State Geotechnical Engineer will include a recommendation in the geotechnical report for a bridge on whether or not bridge approach slabs should be used at the bridge site. Factors considered while evaluating the need for bridge approach slabs include the amount of expected settlement and the type of bridge structure.

B. Bridge Preliminary Plan

All bridge preliminary plans shall show approach slabs at the ends of the bridges. In the Notes to Region in the first submittal of the Preliminary Plan to the Region, the designer shall ask the following questions:

1. Bridge approach slabs are shown for this bridge, and will be included in the Bridge PS&E. Do you concur?

2.     The approach ends of the bridge approach slabs are shown (a) parallel to the back of pavement seat (b) normal to the survey line (the designer shall propose one alternative).
          Do you concur?

3.     Please indicate the pavement type for the approach roadway.

Depending on the type and number of other roadway features present at the bridge site (such as approach curbs and barriers, drainage structures, sidewalks, utilities and conduit pipes) or special construction requirements such as staged construction, other questions in the Notes to Region pertaining to the bridge approach slabs may be appropriate.

Special design conditions exist when the skew of the bridge pavement seat is greater than 30 degrees, and for wide roadway widths (including bridge widenings with or without existing bridge approach slabs). The preliminary plan shall include details showing how these conditions are being addressed for the bridge approach slabs, and the designer shall include appropriate questions in the Notes to Region asking for concurrence with the proposed design.

BRIDGE DESIGN MANUAL

Miscellaneous Design Miscellaneous Highway Structures Design

C. Bridge Approach Slab Design Criteria

The current standard plan for bridge approach slabs is based upon the following design criteria:

1. The bridge approach slab is designed as a slab in accordance with Section 4.6 of the AASHTO LRFD Bridge Design Specifications – U.S. Units, Second Edition 1998
and current interims (Strength I loading, IM = 1.3, no skew).

2. The end support is assumed to be a uniform soil reaction with a bearing length that is approximately 1/3 the length of the approach slab (25/3 = 8 feet).

3.     The Effective Span Length (S_eff) is assumed to be:
          25 feet - (25 feet/3)/2 - 10 inches = 20 feet

4.     Longitudinal steel reinforcing bars do not require modification for skewed applications.

5. See Bridge Design Manual Section 9.3.2 for a slab reaction applied to the pavement seat that assumes S_eff = 20’-0”.

6. The maximum skew accommodated by the standard design is 30 degrees.

Design circumstances that vary from the above design criteria may require revisions to the details shown in the standard plan.

D. Bridge Approach Slab Details in Bridge Plans

The bridge approach slab shall be shown in the PLAN view of the LAYOUT sheet. The set of Bridge Sheets shall also include the two sheets of approach slab details (Bridge Design Manual Detail Sheets 8.3-A13 and 8.3-A14), modified as appropriate to match the site conditions of the bridge. Additional details to address special roadway features and construction requirements (such as approach curbs and barriers, drainage structures, sidewalks, utilities and conduit pipes, stage construction, wide roadway width, and skew greater than 30 degrees) should also be included.

Bridges for local agency projects may reference Standard Plan A-2 in place of Bridge Design Manual Detail Sheets 8.3-A13 and 8.3-A14.

The bridge approach slab width shall be curb to curb of the outside bridge barriers (traffic barrier or pedestrian barrier). This means, if sidewalks and interior barriers (such as traffic-pedestrian barriers) are present, special details will be required in the Bridge Sheets to show how the sidewalks and interior barriers are connected to and constructed upon the bridge approach slab.

BRIDGE DESIGN MANUAL

Miscellaneous Design Miscellaneous Highway Structures Design

Approach slab anchors installed at bridge abutments shall be shown in the Bridge Sheets. For bridges with semi-integral type abutments, this can be accomplished by showing the approach slab anchors in the End Diaphragm or Pavement Seat details. For bridges with L type abutments, this requires adding the detail shown below to the end pier Bridge Sheets. The approach slab anchor for bridges with L type abutments shall be a #4 bar at one foot spacing, bent as shown, with 1’-0” embedment into both the pier and the bridge approach slab. This bar shall be included in the bar list for the bridge substructure.

For bridges with skews greater than 30 degrees, the approach end of the bridge approach slab shall be normal to the roadway centerline. Also for bridges with skews greater than 30 degrees, additional steel reinforcing bars shall be added to the acute corners of the bridge approach slabs, as shown in the detail below. The flared corner steel shall be 10 feet long, epoxy coated, and placed on top of the top layer of steel.

6. Precast Approach Slabs:

Bridge approach slabs for skewed bridges of significant width (three lanes or more) may have the approach end of the bridge approach slab stepped or staggered to reduce the size of the slab or to accommodate stage construction.

Semi-integral type abutments should not be designed for contributory expansion lengths greater than 200 feet (point of fixity to joint) because large expansion movements and highly skewed joints may require compression seals larger than the 2-1/2 inch compression seal shown on the Standard Plan. See Bridge Design Manual Section 8.4.1.

Bridge deck surface drainage shall be carried off and collected beyond the bridge approach slab. Drainage structures such as grate inlets and catch basins shall be located in accordance with Standard Plan A-4 and the recommendations of the Hydraulics Office. If bridge deck surface drainage must be collected at the back of pavement seat, the drainage structure shall be a bridge grate inlet (See Bridge Design Manual Detail Sheet 8.4-A4).

BRIDGE DESIGN MANUAL

Miscellaneous Design Miscellaneous Highway Structures Design

All costs in connection with constructing bridge approach slabs are included in the unit contract price per square yard for “Bridge Approach Slab”, including steel reinforcing bars, approach slab anchors, concrete, and compression seals. For the Contractor’s information, the total quantity of epoxy-coated and plain steel reinforcing bars shall be listed in the bar list sheet for each bridge.

E. Bridge Approach Slabs for Bridge Widenings

Adding bridge approach slabs to existing bridges that are being widened may require modifications of the existing bridge ends. When the existing bridge end does not provide sufficient support shelf length (ten inches minimum) any existing substandard pavement seat shall be removed, and a replacement pavement seat shall be constructed. The Bridge Design Engineer on a site-specific basis may modify this requirement. Pavement seat details conforming to Bridge Design Manual Detail Sheet 8.3-A15 shall be added to the Bridge Sheets.