Multi-girder composite bridge — Initial sizing


Table of AASHTO LRFD Bridge Design Specification 8th Edition recommends:

where \(L\) is the span length.


Clause 2.4.1 of WSDOT Bridge Design Manual recommends the following span/depth ratios:

Situation Span/Depth Ratio
Simple span 22
Continuous span 25
Variable depth girder at mid-span 40
Variable depth girder at intermediate pier 20


SCI’s recommends the following proportions:

Element Size
Slab thickness 250mm
Girder centres Up to 3.8m
Girder depth Span/20 to Span/30
Top flange width Minimum width 350 mm
Top flange thickness Top flange width / 16 in sagging zones. Up to 50mm in hogging zones.
Bottom flange width Girder depth / 2
Bottom flange thickness Up to 50mm
Webs Minimum 15 mm at mid-span. Up to 25mm at supports.
Web to flange welds 6-8mm at mid-span. 8-10mm at supports.
Shear studs 19mm diameter x 150mm high. Rows of 3 at 300 mm centres at mid span increasing to rows of 3 or 4 at 150 mm centres at supports.