Effect of Stiffener Configuration on Bulkhead Modal Parameters

Optimization of bulkhead stiffener configuration has been an active area of research over the past decade, but no real practical solutions have been generated. This research investigates bulkhead stiffener configuration on a rudimentary level, by analyzing the modal parameters of three different stiffener configurations. Experimental data was used to validate the computational models of two modified bulkhead stiffener configurations. Operational boundary conditions were then applied to the computational models to assess the modal density of the modified bulkheads within the aircraft engine rotational frequency range. Removal of one horizontal stiffener reduced the overall stiffener mass by 12.2% without generating any modes within 4% of the engine rotational frequency. The inconsistencies of natural frequency changes due to stiffener configuration highlights the difficulty with applying generalized optimization approaches without a thorough understanding of the modes of interest. The results of this work suggest that the fundamental analysis performed herein is necessary to generate a complete understanding of the modal parameters of the bulkhead prior to performing in-depth optimization work.

Bidirectional Behaviour of Unstiffened and Stiffened Direct-Welded Connections for Square CFST Columns

This paper describes finite element analyses of two-way moment frame beam-column joint subassemblies constructed using steel I-shaped beams connected to square concrete filled steel tubular (CFST) columns. These are direct-welded connections with (i) no stiffeners, (ii) internal horizontal stiffener plates, (iii) vertical tube stiffeners, and (iv) the combination of (ii) and (iii). They were analyzed under one-way and two-way column lateral loading. It is shown that internal stiffeners increase the joint stiffness and strength by up to 80% and 60% respectively. The effect of bidirectional loading on the joint capacity is found to be minor. A design method is developed to predict the required stiffener capacity. The method is applicable for both one-way and two-way loading cases.

Modelling the Installation of Stiffened Caissons in Overconsolidated Clay

Design of suction caissons for installation in overconsolidated clay presents several geotechnical engineering challenges. These include (i) predicting the installation resistance and required ‘suction’ pressure, (ii) ensuring adequate skirt length to account for vertical plug heave, and (iii) accommodating the structural engineering stiffening requirements and their effects on the penetration resistance and plug heave. A suite of centrifuge tests in overconsolidated kaolin clay was carried out to investigate the effects of stiffener geometry on penetration resistance during direct jacking and suction installation. Three caisson geometries were compared: caissons with (i) no stiffeners, (ii) horizontal stiffeners only and (iii) both vertical and horizontal stiffeners. Results show negligible differences in penetration resistance between jacked and suction installation for each caisson type. The magnitude of soil heave within the caisson is seen to be highly dependent on the level of applied suction as well as on the volume of the stiffeners. Observations during and following testing indicated that minimal flow-round of the overconsolidated clay occurred for skirts with horizontal stiffeners. These included (i) linear penetration resistance profiles following penetration of the lowest horizontal stiffener, (ii) a wedge of clay observed only below the lowest horizontal stiffener following extraction, and (iii) unsupported plug heave heights following penetration. A comparison of measured data with back-calculated resistance factors suggests that current design methods adequately predict the measured penetration resistance assuming zero flow-round conditions, implying additional end bearing of the upper horizontal stiffener during penetration was negligible.