Strength of Plates of Rectangular Industrial Ducts

Large industrial duct system is often rectangular and consists of stiffened plates, where the plates along with stiffeners act to resist the pressure loads. Since the stiffeners are often closely spaced, the plate element between the pair of parallel stiffeners is often idealized as a long plate spanning between and fixed supported by those stiffeners. The internal pressure and serviceability limit determine the plate thickness and the stiffener spacing. Currently, the engineers determine the plate thickness and the stiffener spacing based on elastic large deflection plate theory in which bending and membrane actions both dictate the strength and deflection of the plate. This paper postulates that the plate design allowing for partial yielding may result in economical and efficient duct system. The objective of this paper is to establish relations between loads and stiffener spacing recognizing the available ductility and the true capacity of steel plates associated with large industrial ducts. Since, large displacement analysis of plates beyond yielding is quite complicated due to non-linearities, this investigation is based on finite element analysis of long plates made of elastic-plastic steel. The model uses a four-node nonlinear shell element based on Mindlin/Reissner plate theory. The parametric study considered plates having various slenderness values, and results were established for pressure-slenderness relations and out-of-plane deflection-slenderness relations for three cases namely; 0%, 16.5% and 33% of through thickness yielding of the plate. Design equations were established for the above three cases. Results show that approximately 55% and 91% increase in load carrying capacities when 16.5% and 33% yielding is permitted. However, such yielding results in 30% and 50% increase in deflections as well. Partially yielding plates can easily satisfy the serviceability limit states and lead to economical stiffened plate system for industrial duct. The paper presents design equations for industrial plates for three different design scenarios namely; 0%, 16.5% and 33% of through thickness yielding of the plate.