Trusses

Trusses over weir restricting vapor disengagement from downcomer in high pressure system. Premature flooding. Design error. 

From strength of materials one can move two ways. On the one hand, mechanical and civil engineers and applied mathematicians shift towards more elaborate situations, such as plastic shakedown in elaborate roof trusses here some transient plastic deformation is planned for. Other problems involve very complex elastic situations. This kind of continuum mechanics is a huge field with a large literature of its own (an example is the celebrated book by Timoshenko 1934), and it has essentially nothing to do with materials science or engineering because it is not specific to any material or even family of materials. 

Typical diameters are up to 150 ft. and machines have operating torques to 750,000 fir-Lbs. The Bridge - or Beam or Truss - spans the diameter of the tank and supports the drive and rake mechanisms. The underflow is removed from the discharge cone at the bottom center. 

The diameters on these machines are over 400 ft., with operating torques to 2,400,000ft.-Lbs. The stationary center pier supports the drive and rake mechanisms. The truss extends from the center pier to the tank periphery supports walkway, power lines and feed lasunder. 

FIGURE 13.29 Average plume flow rate at roof truss level for a tapping operation. 

Some graphite-epoxy structures can be tailored to have a zero coefficient of thermal expansion, a big advantage for large antennas that must pass in and out of the sun, yet maintain dimensional stability for accuracy of pointing the signal. For example, a graphite-epoxy truss is used to stabilize and support the Hubble Space Telescope. 

The fundamental objective in this section is to describe the factors and procedures to select the right material for a specific structural application. The right stuff for a material, as for a fighter pilot or an astronaut, is a complex combination of characteristics. To select the proper material requires being able to characterize and evaluate various composite materials (or metalsl) and to compare their attractive characteristics with the behavioral features required for a particular structure. Finally, a materials selection example of a space truss design problem will be addressed. 

Figure 7-21 Space Shuttle with a Truss Being Erected...

The problem must be simplified considerably to permit solution in the context of this book. Suppose an equilateral triangle is subjected to some loads in the vertical direction as in Figure 7-22. A load P of 100 lb (445 N) is applied to the top joint, and that load can go in either the downward or upward direction (in the diagram, not in space ). This truss must take its reversible load with, say, a factor of safety of two against whatever event would cause it to fail. What material, size, and weight of truss element would you select to satisfy the design requirements that include building the structure for the lowest cost ... 

In order to evaluate one of the issues that is very pertinent to this material selection, the cost to get the truss elements up into space must be known. In 1985, a Shuttle flight cost 90 million. If the Shuttle is capable of carrying a payload of 60,000 lb (27,000 kg), then for every... 

What is important for this space truss problem depends on which of the various technical issues influence the design. Is stiffness an issue Is strength an issue If so, why Is buckling an issue Can fatigue be a problem Or corrosion Thermal expansion or joints Those factors are listed in Figure 7-23. 

This space truss is seemingly an unusual example, but space is now a part of our everyday lives. Thus, there are many space examples in which you must expect to use composite materials because they are the least-expensive design solution. The raw material cost is not even close to the bottom-line cost. The raw material cost has something to do with the bottom line, but the ordering of material choices that you get based on raw material cost does not mean anything in comparison to the ordering of the actual bottom-line costs. 

A frame is a structure with at least one member that supports more than two forces. Members of a frame may support lateral as well as axial forces. Connections in frame need not be located at the ends of the members. Frames, like trusses, are designed to support loads, and are usually motionless. A machine also has multiforce members. It is designed to modify and transmit forces and, though it may sometimes be stationary, it always includes parts that move during some phase of operation. 

Derrick A semipermanent structure of square or rectangular cross-section having members that are latticed or trussed on all four sides. This unit must be assembled in the vertical or operation position, as it includes no erection mechanism. It may or may not be guyed. 

For purposes of this specification, stresses in the individual members of a latticed or trussed structure resulting from elastic deformation and rigidity of joints are defined as secondary stresses. These secondary stresses may be taken to be the difference between stresses from an analysis assuming fully rigid joints, with loads applied only at the joints, and stresses from a similar analysis with pinned joints. Stresses arising from eccentric joint connections, or from transverse loading of members between joints, or from applied moments, must be considered primary stresses. 

Trusses may be the best solution for very high-imposed loads. Frame action with columns is not possible with trusses. Although trusses are generally the lightest form of roof construction, they may be the most expensive due to high fabrication cost. A combination of lattices or lattice and truss may form a sawtooth roof profile for incorporation of north lights. 

Space trusses. Three-dimensional space trusses utilizing proprietary nodal joints can achieve substantial two-directional spans. Typically, they are too expensive for normal industrial buildings. 

Laidlaw, R. A. and Pinion, L. C., Metal Plate Fasteners in Trussed Rafters Treated with Preservatives of Flame Retardants-Corrosion Risks, IS 11/77, Building Research Establishment (1977)... 

In the Forth, Severn and many other suspension bridges, zinc coatings have an important function. The whole main structure is of steel and has been zinc-sprayed on the external surfaces, while the main cable and hanger ropes have been coated by continuous hot-dip galvanising. Case histories of galvanised multi-truss bridges cover more than 30 years. 

Since much of the wood in common use today for joinery, including external window frames and roof trusses, is redwood, such as Pinus sylvestris, preservative treatment is very necessary. However, whilst it is known that little corrosion of fasteners can take place in dry wood (<15 Vo moisture... 

In this section we discuss the idea of degrees of freedom of motion of simple structures, with emphasis on space frames and trusses. These simple structures are often easily understood and exhibit simple and predictable behavior. They can also be structurally efficient and thus make a valuable group of structures interesting in astronomy. 

Figure 7.6. When a force is applied to a bone of uniform structure (a), the structure adapts by the feedback mechanism shown in Figure 7.5 and forms a nonuniform structure to carry the load efficiently (b). The resulting structure resembles the familiar design of bridges and other man-made trusses (c).

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