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Wood-Armer for Grillages

The Wood Armer equations are commonly used by engineers in the design of: 

  • Orthogonal reinforcement for RC slabs which are to resist a combination of moments (Mx, My) and a twisting moment (Mxy) 
  • Reinforcement for skew slabs (principle moment directions inclined to the reinforcement directions). The equations use Mx, My and Mxy obtained from a plate analysis [more on Wood-Armer with Plates].

To use the Wood-Armer equations for grillages, the bending and twisting moments obtained from the analysis have to be converted into "equivalent plate moments" in global directions. LUSAS carries out that conversion as well as the Wood-Armer calculation in post-processing i.e. for a "pre-determined field of moments" (Mx, My, Mxy).

An extra geometric property (effective width) has to be defined for the grillage members in order to compute the "equivalent plate moments". In the normal way the "effective width" will be the width of slab assigned to the member in question. Further information is given in the Help menu item:

Help Topics > Contents tab > Theory Manual > Chapter 6 > 6.2 Wood Armer Reinforcement

It should be noted that while general stress results on a grillage are "per member", Wood-Armer results, reported through the Print Results Wizard or the Values layer are "per unit width". Reporting Wood-Armer results via the Diagram or Contour layers is not valid.

An outline check may be carried out by any user on the grillage calculation within LUSAS Modeller by constructing a grillage and a plate model which can be compared. Allowance must be made in any comparison for the inherent differences in the analyses. Similarly a simple test will indicate that the Wood-Armer results, reported through the Print Results Wizard or the Values layer are "per unit width".

Other Wood-Armer related topics

Other Grillage modelling issues

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