I was just wondering what would be the best method for modelling stiffening ribs on GRP (glass reinforced plastic) ducting. The ducting diameter is 1600mm and the spacing between ribs is approximately 1500mm.

My concern is that the wall thickness of the ducting was probably calculated according to BS7159:1989, which would allow for a lesser wall thickness due to the presence of stiffening ribs. If I divide the Caesar II model into sections, the one consisting of the typical ducting wall thickness and the other representing the thickness of the stiffening ribs, will Caesar II not calculate a stress in the duct wall away from the rib which is higher than that allowed for by the code? As a result would it be correct to reduce the stress induced due to vacuum (or external pressure) by a certain factor (or possibly reduce the applied vacuum)?

If not, how should one approach modelling pipes / ducts which have stiffening ribs?

Thank you for your time

Jason

1) Yes you could define different cross-sections, and build your model this way. However, it may be easier to just increase the density in a short element every so often to consider the weight of the ribs.

2) CAESAR II has no clue about stiffening, external pressure requirements, or buckling.

3) The flexibility calculations of BS-7159 are unaware of stiffening ring size/spacing.

Remember, pipe stress programs, like CAESAR II, use a 3D beam element formulation. This is essentially an infinitely thin stick, with properties such as length, diameter, thickness, modulus, and thermal expansion coefficient. The overall behavior of the system is obtained based on the displacements and rotations of the node points defining these elements. The flexibility stresses obtained are accurate as long as the overall system wide behavior is dominated by bending. Buckling and local element stiffening are beyond the scope of the 3D beam element.