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Type HA loading (Example)
Use of HA loading for buried structures.
How do you work out the HA loading and bending moment for a bridge deck ?
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 :
Number of notional lanes = 2
Notional lane width = 7.3/2 = 3.65m
Loaded length = 34m
W = 336(1/L)0.67 kN/m (per notional lane)
W = 31.6 kN/m (per notional lane)
Knife Edge Load = 120 kN (per notional lane)
α2 = 0.0137[bL(40-L)+3.65(L-20)]
α2 = 0.0137[3.65(40-34.0)+3.65(34.0-20)] = 1.0
Note: For loaded lengths less than 20m the load is proportioned to a standard lane width of 3.65m, i.e. 0.274bL = bL/3.65.
For a metre width of deck :
W = (31.6 x 1.0)/3.65 = 8.66 kN/m
KEL = (120 x 1.0)/3.65 = 32.88 kN
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.
BS 5400 Pt.4 - γf3 is used with the load effect so Mult = 1.1 x 2300 = 2530 kNm.
The formular gives high values for HA udl for small box culverts. Can this be reduced through the fill depth?
BS 5400: Part 2 should not be used for loading on buried structures. Reference is made in BD 37/01 which says that the design loading requirements for buried concrete box-type structures are given in BD 31/01 "THE DESIGN OF BURIED CONCRETE BOX AND PORTAL FRAME STRUCTURES".
BD 31 says that for cover depths exceeding 0.6m, the HAUDL/KEL combination does not adequately model traffic loading. In these circumstances the HAUDL/KEL combination shall be replaced by 30 Units of HB loading. Account also needs be taken of the single 100kN HA wheel load. Allowance is made for the wheel loads to be dispersed through the fill depth.
Last Updated : 20/11/2013
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