Q. Show the Limitations of grillage analysis?

In designing the number of cells for concrete box girder bridges, in case depth of a box girder bridge exceeds 1/6 or 1/5 of bridge width then it is recommended to be designed as a single cell box girder bridge. Though if the bridge depth is smaller than 1/6 of bridge width then a twin-cell or multiple cell is a better.Though one should note that even for wider bridges with small depths, number of cells must be minimized since there isn't much improvement in transverse load distribution when number of cells of box girder is increased to three or more.

For structural analysis of bridges, grillage analysis that involves the structure to be modelled as a series of transverse and longitudinal elements that are interconnected at nodes is normally adopted. 

Grillage analysis suffers from the following limitations:

(i)  For coarse mesh, torques may not be identical in orthogonal directions. In the same way twists may differ in orthogonal directions.

(ii)  Moment in any beams is primarily proportional to its curvature only. Though moment in an element depends on curvatures in the beam's direction and its orthogonal direction. 

Grillage analysis can't be used to determine the effect of distortion and warping. Furthermore the effect of shear lag may hardly be assessed by using grillage analysis. By employing fine mesh of elements, local effects may be determined with a grillage. On the other hand, local effects can be assessed separately and put in the results of grillage analysis.