AASHTO LRFD Selection of Girder Depth and Spacing

We usually decide on the girder depth and girder spacing based on the span length and width of the bridge. Over the years, engineers have developed certain simple thumb of rules on decision making that gives a start to design.

Clear span length is usually measured as a distance between center line of bearings and is used effectively in design. Girder length is slightly longer than clear span length. The clear span length can reach up to 130 ft (40 m) for simply supported girders. The width of the bridge is based on the number of lanes carried by the bridge. The typical two-lane bridge can have a width of 45 ft (14 m) accommodating two sidewalks.

Girder spacing along the width is usually uniform. The spacing of the girders can be selected based on the selected girder’s depth. If a shallow girder depth is selected, the girder spacing will be close to each other than bridges with deep girders. Shallow girder depth’s may be used where vertical clearance below the bridge is critical.

In the example, simple thumb of rules are used to start the AASHTO-LRFD design for prestressed precast girders. The engineer first selects a girder depth/span ratio from where spacing can be set.

AASHTO LRFD Bridge Construction Specifications, 4th Edition

Bridge Design Flow Chart

Start Material Selection Design Basis Construction Stages Preliminary Girder Design Based on Span Length & Width Dead Load Live Load Service Limit Checks Preliminary Tendon Design Prestress Losses Concrete Stress Checks at Construction Concrete Stress Checks at Service Canber and Deflection Strength Flexure Strength Shear Interface Shear Design Deck Design Bridge Modeling