Truss Type Research Articles

Aerodynamic stability of cable-stayed bridge with new Vierendeel-type girder

An aerodynamic study is described showing the stability of a new type of double deck cable-stayed bridge having both upper and lower flat, shallow, streamlined box girder decks connected by vertical members only. In long span, cable-stayed bridges the longer the span, the more important the aerodynamic stability of the structure becomes. Even though cable-stayed bridges evidence structural damping, the measure of the aerodynamic stability of such a bridge is whether or not a cross-sectional shape of the suspended structure is stable against flutter oscillation and vortex-excited oscillation. The experiments presented in this paper, therefore, focus on the aerodynamic stability against these phenomena in a partial scale model of a cable-stayed bridge of new design. The three aerodynamic components of force have also been measured. For comparison, a wind-tunnel experiment has been carried out using a partial model of a conventional suspended structure of a typical truss type double deck cable-stayed bridge. It is shown that the new system is superior to the truss type from an aerodynamic stability viewpoint.

The Westminster Hall Roof and Its 14th-Century Sources

The great angel roof of Westminster Hall, London, erected for Richard II in 1395-1396, remains the consummate achievement in medieval open-timber roofs. The framing, which has an exceptional single span just over 67 feet, is a multitiered construction that combines two major truss types: the hammer beam, and the arch-brace-and-collar roof. The architectural background of the Westminster Hall roof has generally been confined to precursors of the hammer beams that visually dominate the interior and are certainly a major aesthetic if not structural element. When the prominent role of the arch-brace and related decorative detail are recognized, however, a different set of prototypes emerges, principally the chapel at New College and the great hall at Windsor, whose connection with Westminster Hall is more immediate and compelling because of the documented relationships between patrons, craftsmen, and court. Finally, the interaction between late-14th-century carpentry and masonry charts a shift in stylistic values that is critical to understanding English Late Gothic.

The optimum height design of double-layer grids at the stage of preliminary design

Abstract This paper presents the results of a series of calculations for the estimation of the optimum height of space trusses at the stage of preliminary design. The procedure used herein does not require the use of a computer and relates to a specific type of double-layer grid with a predertermined ratio of the two main dimensions; however, it can be applied to other types of space trusses with different dimension ratios. The results are given in plots from which the optimum height can be estimated. Moreover a simple relation is suggested for the direct estimation of this height.

A TWO-PHASE OPTIMIZATION METHOD FOR MINIMUM-WEIGHT DESIGN OF TRUSSES

In this paper, a new twp-stage procedure for the minimum weight design of trusses is proposed. In the first stage (behavior phase) of optimization the behavior variables (joint displacements) are chosen as basic unknowns and the optimum displacement state of the truss or its searched region is found by using a maximum total strain energy criterion with linear programming. In the second stage (structural phase) the design variables (cross-section areas) are chosen as basic unknowns and the minimum weight design of the truss is obtained by using another linear program or a direct search technique. This method is effective for the minimum weight design of truss type structures with stress, displacement and geometric constraints. In most cases it can avoid the need for repeated iterations and structural reanalyses. Moreover, a minimum-maximum total strain energy criterion to select the optimal layout of structures is presented in this paper.

Strength of Nail Laminated Columns with Metal Plated Joints

ABSTRACT THE laminated columns tested in this study were manufactured with treated lumber spliced to untreated lumber with 16 gauge splice type truss plates and pneumatically driven nails having the approximate size and diameter of a 12 penny common wire nail. Two configurations and two lumber grades were studied. The lumber grades of the treated as well as untreated lumber were No. 2 KD Southern Pine and No. 2 KD Dense Southern Pine. The post configurations were a 2x4-2x6-2x4 combination intended to replace a solid-sawn 4x6 and a 2x6-2x8-2x6 combination intended to replace a solid-sawn 6x6. The nail laminated columns of this study rated well when compared to the allowable stress of a No.2 SR Southern Pine post which has an allowable bending stress value of 7584 kPa (1100 psi)

Reliability of Multiple 2 X 4's Used as Truss Lower Chords

ABSTRACT A procedure for investigating the relative safety of single and multiple lower chord member trusses is presented. The general approach is a Monte Carlo analysis using random strength parameters. A combina-tion of axial force and bending is considered and the reliability of the system is estimated in terms of a condi-tional probability of failure. The two different types of trusses analyzed are the glued plywood joint type and the metal plate type. Results indicate that differences in failure probabilities for the various cases are not signifi-cant and both single and double lower chord member trusses have about the same level of safety.

CURVBRG: A program for analysis of curved I-girder bridges

A computer program for structural analysis of open girder (i.e. I-section) bridge super-structures is described. The program is based on a plane grid idealization, in which the effects of warping torsion are considered rationally, and in which structural components such as diaphragms and braced (truss type) cross frames are idealized by essentially exact procedures. The bridge may be of arbitrary shape in plan, the girders may be of variable cross section along their length, and changes in form of the structure as construction progresses may be considered. Deflections, stress resultants and stresses can be determined for static loads, support settlement effects and moving live loads. The structural idealization procedure and theory are explained, the features of a computer program developed to apply the method are described, and comparisons are made between the computed behavior and that observed in a model test.

Space Trusses with Brittle-Type Strut Buckling

Three experimental tests on nominally identical aluminum space trusses with constant section chords and webs have been compared with a simplified theoretical analysis. An idealized strut curve has been adopted to describe the brittle-type behavior of the compression chords. Collapse line patterns have been recognized for the type of truss considered; these patterns form the basis for the simple method of analysis proposed. The collapse line layout has a different form from that normally associated with slab collapse; the difference is attributed to the negligible torsional rigidity of the type of truss investigated. The analysis furnishes reasonable agreement with the experimental results, and draws attention to the importance of the strut characteristic in the post-ultimate range of behavior. The brittle-type strut characteristic leads to a situation where the theoretical estimate of the load at first yield may not be reached. Conventional elastic analyses may not be as safe as many designers assume for this type of truss.

Mobile Drilling Platforms

Abstract Types of modern drilling platforms are evaluated, including jack-up, column stabilized and such special types as single hull, catamaran, workover and wireline units. Some specific design procedures are given, including determination of dimensions, weights, stability characteristics and stresses. The relation of motion to stability is discussed and the advantages of model tests are mentioned. In Appendices A and B, an new method for calculation of righting moments by optical projection through a model is given as well as a convenient method for estimating wave forces. Introduction This paper discusses certain design aspects of the latest types of mobile drilling platforms. Earlier types of platforms, including types which are still being built as well as those which are no longer in favor, have been discussed in the literature. An attempt is made to give some of the basic design criteria as well as to make a comparison of advantage and disadvantages. Jack-Up Platforms General Features Jack-up platforms consist of a buoyant hull and three or more legs. To put such a platform in service, the legs are lowered to the bottom and jack pressure applied until a sufficient degree of bottom support is obtained, after which continued jacking will raise the hull to the desired clearance above the water line. The hull then serves as the drilling platform and is complete and self-contained in all respects. The legs may be individual in operation without any connection between them. Usually this type of leg will penetrate into the bottom to a fairly considerable depth. Buoyancy tanks or even enlarged footings may be located at the bottom of these legs. On the other hand, the legs may be connected at the bottom by a hull or mat. The bottom mat, while it does involve a substantial amount of additional steel, is very desirable on soft bottoms and has the advantage of making the bottom of the legs fixed-ended, thus reducing the bending moment in the legs to one-half of the value for individual free-ended legs. Determination of the air gap or distance from the water surface to the bottom of the upper hull is of major importance. Selecting this value too low could result in disaster. Having it too high will be very costly. One means of determining the over-all height is to take the designed mean low water level and add to it the following components:anticipated bottom settlement,storm and astronomical tide,crest height of wave above still water level, anda margin of 10 to 15 ft. The crest height of the wave above still water level may be estimated in the absence of definite oceanographic data at 55 per cent of the designed wave height. A typical example of this determination might be as follows: Mean low water 100 ft Anticipated settlement 4 ft Storm and astronomical tide 8 ft Crest height of 40-ft wave 22 ft Margin 10 ftTotal 144 ft In addition to the air gap margin, additional margin on the length of the legs should be provided in the event of greater settlement or the possibility of waves exceeding the design criteria. The legs may be of tubular or truss-type construction, according to the type of jacks to be used and the operating water depth. Tubular legs involve less labor cost per ton of steel but on large platforms they become too heavy and present too much projected area against waves. To keep the weight and projected area as small as possible with either type of legs, the use of alloy steel is indicated. For shallow water depths, the tubular type seems most economical; for great water depths, the truss type is best. The change-over point is probably somewhere near a height of 200 ft from sea bottom to the lower side of the hull. Most platforms of the jack-up type have the legs arranged to operate vertically in guides. In very deep water. to obtain a satisfactory resistance to overturning, this means that the legs sometimes must be spaced far apart. As a result. the bottom mat (if any) must then be quite wide, as must the upper platform. To overcome this, LeTourneau has provided hinges on the guides at the hull which permit the legs to tilt out to a fairly considerable angle. This permits a broad base at the bottom while keeping the upper platform at reasonable dimensions. JPT P. 1069ˆ