By lowering the embankments the cost of the earthworks may be reduced, but the resulting reduction in the construction depth may cause the deck to be more expensive. The economic implications of raising or lowering any approach embankments should then be considered. Constraints The construction depth available should be evaluated. The preliminary design usually settles the appearance of the bridge. preliminary design stage the sizes of individual members are finalised at detailed design stage. Decisions about form and materials are made at b. The structure must be pleasing to look at. At preliminary design stage it means choosing the right types of material for the major elements of the structure, and arranging these in the right form. The structure must make minimal demands on labour and capital it must cost as little as possible to build and maintain. Detailed design cannot correct faults induced by bad preliminary design. The ideal structure must not suffer from local deterioration/failure, from excessive deflection or vibration, and it must not interfere with sight lines on roads above or below it. This is more significant at detailed design stage as generally any sort of preliminary design can be made safe. It must be capable of carrying the loading required of it with the appropriate factor of safety. The ideal structure must not collapse in use. Consideration of each of the ideal characteristics in turn will give some indication of the importance of preliminary bridge design. Decisions taken at preliminary design stage will influence the extent to which the actual structure approximates to the ideal, but so will decisions taken at detailed design stage. Preliminary Design In selecting the correct bridge type it is necessary to find a structure that will perform its required function and present an acceptable appearance at the least cost. BS 5400 Pt.4 - γf3 is used with the load effect so Mult = 1.1 x 2300 = 2530 kNm. γfL = 1.50 (Ultimate limit state - combination 1) Design HA loading for a metre width of deck : W = 1.5 x 8.66 = 12.99 kN/m KEL = 1.5 x 32.88 = 49.32 kN Maximum mid span Bending Moment with KEL at mid span = Mult Mult = (12.99 x 342)/8 + (49.32 x 34)/4 Mult = 1877 + 419 = 2300 kNm Note: Use of γf3 BS 5400 Pt.3 & Pt.5 - γf3 is used with the design strength so Mult = 2300 kNm. α2 = 0.0137 α2 = 0.0137 = 1.0 Note: For loaded lengths less than 20m the load is proportioned to a standard lane width of 3.65m, i.e. Knife Edge Load = 120 kN (per notional lane) Cl. Loaded length = 34m W = 336(1/L)0.67 kN/m (per notional lane) W = 31.6 kN/m (per notional lane) Cl. Clause 6.2 Problem: How do you work out the HA loading and bending moment for a bridge deck ? Example: Carriageway = 7.3m wide Deck span = 34m (centre to centre of bearings for a simply supported single span) Design for a metre width of deck : Cl.3.2.9.3.1 Number of notional lanes = 2 Notional lane width = 7.3/2 = 3.65m Cl.
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