The MEXE method is one of the most used for the assessment of masonry arch bridges (McKibbins, 2006, p. 252). The MEXE method is an elastic analyses theorem, in which the arch is assumed to behave in a linear elastic continuum. This method is based on the assumptions made by Pippard which are summarised below (PIPPARD, 1948)
Main Assumptions
- The arch is parabolic and has a span rise ratio of 4, the cross-section increases from the crown to the abutments in proportion to the slope of the arch. This assumption allows the load bearing capacity to determined from the geometry of the arch
- The back fill has no structural properties and merely serves the purpose of transmitting the loads to the arch structure.
- The density of the backfill is assumed to have the same unit weight as the arch ring, 2.24 t/m3
- A strip of arch with a width equal to 2 times the depth of the backfill is considered to give a load spread angle of 45o
- The critical load for the bridge is a knife edge load at the crown combined with the self-weight of the bridge with a tensile stress limit of 0.7 N/mm. This criterion is preferred to the middle third rule as experimental evidence suggests that the middle third rule was conservative.
The MEXE method tends to over estimate the capacity of short span bridges, and is only suitable for spans below 18m in length.
