MSE Wall - LRFD Version

[Jalthi]

In the external design of MSE structures, a limiting eccentricity of L/6 is conventionally used in ASD method. [Reference: Page 92, MSE Walls and Reinforced Slopes Design and Construction Guidelines, March 2001 (FHWA-NHI-00-043)] As we know, the limiting value is derived from the ‘middle third rule’.

A latest version of this design manual was published in November 2009 with LRFD procedures (FHWA-NHI-10-024). This manual calls for a limiting eccentricity of L/4 (Page 4-23). Can someone explain the rationale behind this change?

The only reason I can think of is that in the ASD method eccentricity was determined before applying any factors. But in the LRFD method it is determined after load and resistance factors are applied. Is this the reason why limiting eccentricity is “relaxed”?

Thanks.

 

[ishvaaag]

A service level loading bringing the reaction on the base to the limit of the central nucleus of inertia means that even in ASD it is assumed that if earth push (the destabilizing party) was to be factored up and -as usual- gravity loads aren't allowed to be considered to grow proportionally (as stabilizing forces that are) then any such growth of loading would mean a resultant out of the central nucleus of inertia. LRFD would be then merely marking to what extent in one of such scenarios the eccentricity could go.