COMPOSITE ANALYSIS AND
STRUCTURAL SIZING SOFTWARE
STRUCTURAL SIZING SOFTWARE
Shipping container design optimizations require practical and inexpensive solutions suitable for a cost driven market. HyperSizer is used here to optimize the sidewall of an International Standards Organization (ISO) intramodal cargo container. Structural components on the large (20'x8') panel are identified by optimization zones which represent convenient, manufacturable areas of stiffener tapers and fabric layups. An extensive series of strength and stability analyses were performed to evaluate different panel concepts and material selections in an attempt to find a better and lighter weight structural system. HyperSizer was able to identify a design five (5) times lighter than current in-service designs. MSC/NASTRAN buckling analysis was used to validate the explicit panel shear buckling analysis performed by HyperSizer.
Structural optimization of a 20'x8' ISO container sidewall was performed for the three loadings it must withstand to meet ISO specification. The first loading is a longitudinal racking load which induces partial picture frame shear. The second loading is a uniform distributed pressure of 0.6Pg which causes panel bending. P is the payload weight and g is gravity. The third loading is a uniform distributed floor loading of 2Pg, which induces in-plane shear similar to that of a short beam. In depth understanding of sidewall response upon the different loadings with different panel concepts is essential for optimization. All three loadings were analyzed with FEA as part of the automated structural sizing optimization and material tailoring process.
The ISO longitudinal racking test loading is intended to ensure that container designs have enough shear stiffness in the sidewall to maintain external dimensional preciseness so that corner fittings remain in proper location during handling and loading. This loading also requires a check of the material's shear strength and that the panel does not buckle in shear. In addition to shear, the floor loading condition causes compression which couples with the shear to cause compression - shear buckling interaction. This complex response is also handled with HyperSizer and buckling FEA eigenvalue analyses were performed using MSC/NASTRAN for verification.
The longitudinal racking loading causes load concentration at the corner fittings. This concentrated force can be effectively handled by tailoring the composite material, or reduced by providing stiffer corner posts. Analyses with corner posts that had varying stiffness were performed to quantify the effects of reduced concentrated forces. The optimum balance between corner post stiffness and panel type, material, and size are determined with HyperSizer's panel and beam optimization.
HyperSizer sidewall sizing and material optimization clearly shows that an 80% lighter design is possible for the sidewalls when they are made with another panel concept and an advanced composite material. The new weight is 105# versus the same sidewall made with steel at 545# (See Table 1). This considerable weight savings is based on extensive strength and stability optimization. HyperSizer was also able to automatically identify a design made from a significantly less costly composite material weighing 200# per sidewall. This less costly design also provides increased damage tolerance.
| Material | Weight |
|---|---|
| Original, corrugated steel design | 545 # |
| Composite with E-glass fiber | 200 # |
| Composite with graphite/carbon fiber | 105 # |
Table 1. HyperSizer computed weight savings.
Six structural components were identified for the ISO container sidewall. These components represent the optimization zones that HyperSizer uses to find the entire sidewall lowest weight and are regions which can be manufactured to different sizes and composite material layups.
Margins-of-safety were computed for each structural component. A negative margin indicates a failure of the structure and is unacceptable. For instance, the component may buckle, cripple, or be over stressed. A large positive margin-of-safety is undesirable because it indicates over-designed structure and generally excess weight. The ideal margin is just slightly positive. The sidewall margins vary from 0.0 to 0.098 with all components except one with margins less than 0.05 indicating structural efficiency.
Unit weight (lb/ft2) plots are useful for determining where on a structure weight is high and low. In the case of the sidewall, the unit weight varies from 1.13 to 1.55 with the heaviest area located at midspan close to the container endwalls. This is an area of panel shear buckling due to floor loading. The next heaviest area is at midspan of the sidewall center. In this area of the container, the dry cargo bears against the sidewall during ship rocking motions.
Container sidewall loading due to ship rocking
The lightest areas are at the top and bottom of the sidewall center where neither shear buckling nor panel bending are significant.