2017 Archie Seminars


2017 Archie Seminars: Scotland (21 August), Bristol (29 September), London... [read more]

New BoMs and a word of warning


Somewhat belated but the last two BoMs for 2014 are atNovember, WhangareiandDecember,... [read more]

August 2014


Bridge of the Month up to date again [read more]

New file


Seems that the demo uploaded was the wrong one. Sorry. now... [read more]

New Download + Bridge of the Month


DOWNLOAD Version standalone now available for... [read more]


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The live loads specified in BD21 are modified by transfer of load from one axle or wheel to another and by distribution of loads through the fill and road surfacing. These modifications are discussed below.


Load transfer may be induced by dynamic or static effects. The important dynamic effects are centrifugal action and dynamic impact from the spring system. The static effect considered is lift off of axles on hump backed bridges.


Impact is the effect of springs being compressed due to undulation in the road surface. A loaded lorry does not respond quickly to the changing force from the springs. Thus the springs can be compressed by an uneven road profile and that compression may take some distance to work out of the system by raising the body of the vehicle. The impact allowance is 1.8 and is applied to one axle only. It is necessary to determine which axle produces the most severe effect.

impact diagram

Centrifugal effects

Centrifugal effects are caused by the vehicle swaying sideways as it turns. there is no mention of a need to deal with horizontal transverse forces in the arch as a result of such turning. We believe that the clauses of BD21 combine in such a way as to make the application of centrifugal effects unnecessary. This is not true of the railway code

centrifugal diagram

Lift off

Liftoff is only likely to happen on bridges with a very pronounced curvature, and where a long lorry with a wide wheel spacing is used. BD21 asks the engineer simply to decide whether liftoff is likely. Lift off is quite a rare phenomenon, requiring quite a sharply curved vertical alignment. Bridges with a long straight grade entry and exit and a sharply curved crown may induce liftoff as the tractor descends one side of the bridge while the trailing wheels are still climbing the other. Even then a long vehicle will be needed. When a hump bridge arises in a long flat road, liftoff may happen as one trailer axle sits on top of the hump.

lift diagram

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