Stiffeners design

BENDING STIFFNESSES, (EI) (EI)y POLAR BENDING STIFFNESS, (El)p TORSIONAL STIFFNESS, (GJ)  

Most of what has been described so far for stiffener design involves shape and size of the stiffener. Those issues involve selection of the type of stiffener, H-shaped cross section, blade, hat-shaped, etc. as well as the specific dimensions and material makeup of each stiffener element. Other obvious factors in the design of a stiffener include how far apart we space them, at what orientation we place them, and, perhaps most obviously in connection with what we addressed in Section 7.3, out of what material we make the elements. As you saw in some of the previous sketches for stiffeners, we are able with a composite stiffener to use different materials in different places very easily and to essentially optimize our materials usage so that the stiffening comes out to be as good as we can possibly make it. 

The unstiffened panel is generally designed by sizing the maximum in-plane dimensions of the panel and its minimum thickness to resist buckling. Then, the panel area dimensions can be reduced, and the thickness can be increased in the stiffened panel optimization process. 

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Stiffener design parameters and some design considerations for stiffeners. Finally, we will examine a new concept for stiffening composite structures, namely orthogrid. 

Further contrast between metal and composite stiffeners is revealed when we examine the objectives and characteristics of stiffener design. For a metal stiffener of uniform or even nonuniform thickness, we attempt to maximize the moment of inertia of the stiffener in order to maximize the bending stiffness of the stiffener. Those two factors are proportional to one another when we realize that the bending stiffness of metal stiffeners about the middle surface of the plate or shell to which they are attached is... 

Standard shapes for composite stiffeners are not likely to occur for most aerospace applications. There, the value and function of the structure warrant optimizing the stiffener design. In contrast, for more everyday applications such as scaffolding, stairways, and walkways in chemical plants, competitive pressures lead to a situation where compromises in stiffener efficiency are readily accepted (overdesign) in order to achieve lower cost than would be associated with optimum design. 

For cost reasons, several package suppliers also use an alternative to the heat spreader/stiffener design, namely a single piece lid design to act as a rigid support and provide heat transfer at the same time. This design is easier to manufacture and also reduces assembly steps (See Fig. 5830 for more details). 

Another variation of the standard heat spreader/stiffener design is the use of just a stiffener alone. This is usually for bare die applications where the wattage of the chip is so high that the thermal resistance between the chip and the heat spreader is unacceptable. In such cases, the stiffener is retained for stiffness and robustness, but the heat spreader is removed. 

Spheres, spheroids, and toroids use steel or concrete saddles or are suppoi ted by columns. Some may rest directly on soil. Horizontal cyhndrical tanks should have two rather than multiple saddles to avoid indeterminate load distribution. Small horizontal tanks are sometimes supported by legs. Most tanks must be designed to resist the reactions of the saddles or legs, and they may require reinforcing. Neglect of this can cause collapse. Tanks without stiffeners usually need to make contact with the saddles on at least 2.1 rad (120°) of their circumference. An elevated steel tank may have either a circle of steel columns or a large central steel standpipe. Concrete tanks usually have concrete columns. Tanks are often supported by buildings. 

Frequently cost savings for cylindrical shells can result from reducing the effective length-to-diameter ratio and thereby reducing shell thickness. This can be accomplished by adding circumferential stiffeners to the shell. Rules are included for designing and locating the stiffeners. 

Abrupt changes of section cause poor flow and differential shrinkage, giving sink marks (Fig. 28.11 - you can find them on the surface of many small polymer parts), distortion, and internal stress which can lead to cracks or voids. The way out is to design in the way illustrated in Fig. 28.12. Ribs, which are often needed to stiffen polymer parts, should have a thickness of no more than two-thirds of the wall thickness, and a height no more than three times the wall thickness. Corners are profiled to give a uniform section round the corner. 

Fig. 28.15. Rim design. The left-hand design is poor because the large bending moments will distort the rim by creep. The right-hand design is better the bending moments are less and the ribs stiffen the rim.

Stiffened panel designs. Stiffened panels are exterior fairing panels that are attached on three or four sides. They carry little to no structural loads they are designed primarily to take air loads. Early designs using simple riveted concepts were heavy and prone to rapid sonic fatigue. 

For the composite spoiler design, the bottom is a variable-thickness skin on one side in Figure 1-33, but with composite materials that construction is not difficult. We do not have to chem-mill a composite material to change its thickness. All we do is stop building up the material in layers in the middle, but continue to build it up at the sides. That s a very natural process for composite materials and does not involve a costly machining operation. Instead of machined extruded stiffeners, a honeycomb core is placed on the inside of the laminae. That honeycomb... 

We usually must go beyond the simple concept of a monocoque or single-thickness skin for whatever structure we design. That is, we must usually consider the bending stiffness, and, to achieve structural efficiency, we often must stiffen a structure in some manner. We will first address the terminology of stiffening and how it is used. Then, we will consider the types of stiffeners that could be used. Next, an important issue that arises in the design of stiffeners is whether the stiffener has an open- or a closed-cross section. Then, we will address some of the... 

Suppose we want to analyze the stresses in the two stiffeners. The geometry of the sandwich-blade stiffener is actually more complicated and less amenable to analysis than is the hat-shaped stiffener. Issues that arise in the analysis to determine the influence of the various portions of the stiffeners include the in-plane shear stiffness. In the plane of the vertical blade is a certain amount of shear stiffness. That is, the shear stiffness is necfessary to transfer load from the 0° fibers at the top of the stiffener down to the panel. In hat-shaped stiffeners, that shear stiffness is the only way that load is transferred from the 0° fibers at the top of the stiffener down to the panel. Thus, shear stiffness is the dominant issue in the design. And that is why we typically put 45° fibers in the web of the hat-shaped stiffener. 

Another issue that turns out to be very important for the sandwich-blade stiffener, but not at all important for the hat-shaped stiffener, is shear in the vertical web. Not shear in the plane of the web, but shear in the plane perpendicular to the web. This transverse shear stiffness turns out to dominate the behavior or be very important in the behavior of the sandwich blade, but simply is not addressed at all in the hatshaped stiffener. You can imagine that the transverse shearing stiffness would be more important in the sandwich blade when you consider the observation that the sandwich blade is a thick element and the hatshaped stiffener is a thin element. That is, bending and in-plane shear would dominate this response, whereas transverse shear, because the sandwich blade is thick, can very easily be an important factor in the sandwich blade. For both stiffeners, appropriate analyses and design rationale have been developed to be able to make an optimally shaped stiffener. 

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