Bullen Consultants in Durham UK, have been using the SAM-LEAP5 integrated bridge analysis and design software for the past twelve months on a number of bridge design and assessment schemes. These vary from a 15 span 620m long composite steel/concrete viaduct, using a ladder deck construction in Doncaster, to a number of assessments of multi span simply supported pre-stressed / post-tensioned concrete bridges on the A1 Darrington to Dishforth DBFO project for the Highways Agency.

The scheme involved the Category III check of the viaduct which consists of a two girder, steel composite structure comprising 15 continuous spans carrying two 7.3m carriageways with 1.2m verges and a 1.8m central reserve supported on twin reinforced concrete columns at the piers. All of the pier structures were supported on piled foundations.

The single continuous deck has a steel composite construction with two main girders of 2.45m continuous depth with cross girders spanning between at 3.75m centres. Participating formwork spanned longitudinally between the cross girders to support the in-situ concrete deck construction. P6 parapet units and P2 parapet copings were formed in concrete and followed the curve of the deck.

Spans varied with an average span length of 44.3m. The maximum span was 47.9m and the minimum was 23.9m. The viaduct crossed numerous obstacles including the River Don and the East Coast Main Line Railway.

The bridge superstructure was checked using a SAM-LEAP5 grillage to model the deck. Due to the horizontal layout of the viaduct, the structure was split into two separate models with two of the spans from each model overlapping in the middle. This first grillage had a horizontal alignment on a radius of approximately 400m with the second grillage modelling the straight section. This allowed Autoloader to be used to assess the viaduct.

Autoloader was used to check for worst case loading throughout the differing span lengths. The structure was checked to full HA and 45 units of HB loading along with 2 No. AIL vehicles up to 69m in length. Alignment definitions were used to ensure the AIL's followed the road profile around the curved model.

Finite element models of the cantilevers to the P6 parapets were also used to check the cantilevers for local and global effects.

As part of the A1 Darrington to Dishforth DBFO Project there was a requirement to assess the existing structures that are to remain under the scheme to meet current loading requirements. Ten structures on the scheme were of pre-stressed / pre-tensioned concrete construction and were assessed using SAM-LEAP5 to BS5400 specifications. The bridges were modelled using the SAM-LEAP5 graphical user interface throughout for both model geometry layout and definition of the pre-stressed / pre-tensioned beams.

The bridge layout was defined using the graphical tools for creating the design lines, carriageways, span-end lines and support conditions from which the grillage meshes were easily produced. The pre-stressed beams defined included details for reinforcing bars and pre-stressing tendons. The beams were graphically associated with the grillage model and section properties used for analysis were calculated automatically.

Analysis of each bridge deck was carried out to determine the critical bending moment and shear force diagrams for the members in the grillage model using the optimal load placing tools in SAM-LEAP5. The results of the analysis were transferred to the integrated pre-tensioned pre-stressed beam module where the beam sections were checked in accordance with the BS5400 assessment codes.

This resulted in a substantial saving in time by avoiding data transfer between programs. "A typical simply supported 15m span pre-stressed bridge that would have normally taken 4 to 5 days to analyse and obtain design results can now be done in ½ a day", said Paul Musgrave, Principal Engineer at Bullen Consultants, Durham.