bottom of the grid tubes was unconfined but was treated as confined concrete for

simplicity and because the strains in that area close to the neutral axis were small and

therefore the effect on the strength was minor. The unconfined region was divided into

one rectangular fiber layer while the confined region into eight rectangular fiber layers.

For the control beam case the compression zone was treated as an unconfined concrete

region and was divided into eight fiber layers.

 Some of the assumptions employed in this fiber model include perfect bond

between concrete and the reinforcing bars (strain compatibility), plain sections remain

plain (linear strain distribution), the area below the neutral axis was considered cracked

and was ignored in the force and moment calculation and monotonic loading and

deformation of the section.

 1" 0.75"




 S.... ........ Gd Tube
 c-" .A.
 ---- ------_ GrdTube



 _. _, _- __9 9.. 'iA

 ** *


 Unconfined concrete

 X Confined concrete

Figure 4-14. Tube beam cross-sectional fiber model