Joint Eccentricity Research Articles

The effect of joint eccentricity on the distribution of forces in RHS lattice girders

This paper is concerned with investigating the effect of joint eccentricity on the local and overall behaviour of truss girders made from rectangular hollow section members. Three typical truss girders with identical general layout and comprising different joint eccentricities are analysed. Different numerical models of analysis are presented and the implications for design are discussed.

Practical Approach to Modeling of Wood Truss Roof Assemblies

The main objective of this study was to investigate a practical approach to model trusses and roof truss systems using a commercially available software program, ETABS. The model developed in this study takes into account the system behavior from load sharing and composite action, the semirigidity of the metal-plate-connected joints, and joint eccentricity. The model was verified by comparing the predicted deflections, member internal forces, truss strengths, and load sharing of roof systems with the full-scale experimental results available in the literature.

Design of space truss structures

Space truss design usually involves two main assumptions: that truss members are pin-ended, and compression members possess brittle post-buckling characteristics. The validity of these assumptions in the design of a new group of space trusses with continuous chords and eccentric joints is questionable. With chord member continuity and the consequent improvement in compression member behaviour, current design practice might be too conservative. In this paper, it is shown that substantial improvements in overall truss strength have resulted when the true member end conditions are considered, thus indicating potential savings in truss weight with considerable magnitudes.

Design of single-angle compression members according to the Canadian standards: Discussion

In this note, the author has examined the design requirements according to the Canadian design standards (CSA 1989, 1994) and American standards (AISC 1986, 1994). The note is a direct follow up of the paper by Temple and Sakla (1996). It considers the design capacity of single-angle compression members attached by one leg as compared to the values obtained by experimental work and (or) by finite element analysis. The author has summarized the applicable code clauses in the form of a flowchart that should serve as a useful guide to a structural designer. However, in discussing the code clauses the author has drawn conclusions and made some statements which could be misleading. Although it is not stated clearly in the note or in Temple and Sakla (1996), the author is discussing only one type of application of single-angle equal-leg section with the end connection detail as shown in Fig. 1 of the note and Fig. 5 of Temple and Sakla (1996). The moments that are considered as affecting the design capacity of the single-angle compression brace are a result of eccentricity between the supporting gusset plate and the angle compression member. In practice, moments on single-angle compression members can also result from frame action when single angles are often used to form structural frames to support light equipment. This application is not included in the discussion. The following comments are therefore limited to the application discussed in the note or in Temple and Sakla (1996). 1. The author states that there are two design approaches, Canadian and American, and these two approaches yield different results. To summarize, the approaches stated by the author are (a) to ignore the end eccentricities and treat the members as concentrically loaded members (the simple column approach or Canadian approach), and (b) to consider end eccentricities by treating the angle as a beam-column (American or AISC approach). The author states that “The CISC Handbook of Steel Construction (1991) provides no guidance as to preferred design approach. It seems that the practice generally used in Canada is to ignore the end eccentricities and to design the angle as if it were concentrically loaded and as if it buckles about the z-axis, the minor principal axis of the angle cross section.” In fact, the Canadian Standard CSA S16.1-M89, Clause 21.1 (or CSA S16.1-M94, Clause 21.1) states that “axially loaded members that meet at a joint shall have their gravity axes intersect at a common point if practicable; otherwise, the results of bending due to the joint eccentricity shall be provided for.” While CSA S16.1 does not tell the designer how to account for the effects of eccentricity at joints, it nevertheless does not ask the designer to ignore it. Since there is no specific guidance given in the CISC handbook, the author interprets that it is a common design practice to ignore end eccentricities and design as a concentrically loaded member. In this regard, there is actually very little difference between the CSA Standard S16.1 and AISC specification. In the recent revision of the CSA S16.1 (CSA 1994), the beam-column formula may appear to be different from the AISC formula, owing to differences in the curve fitting expressions used by the AISC and CSA (see Fig. D1 of this discussion). However, the predicted failure loads by the CSA S16.1 differ very little from the calculated values using the AISC standard. The difference in the results becomes less significant when differences in the load factors and the resistance factors are taken into account. 2. The author states that “it is concluded that the simple column approach results in better estimates of the failure loads than does the AISC approach.” It is assumed this statement is based on the results presented in Fig. 8 of Temple and Sakla (1996). The author arrives at the above conclusion, since as per this figure, the failure loads predicted using the expressions for concentrically loaded column lie near or in between the two curves which show the experimental/finite element results, one corresponding to an angle section with pinned end condition and the other curve with fixed end condition. However, Fig. 8 of Temple and Sakla (1996) can be misleading. The figure shows the failure load versus slenderness parameter, λ, which the author appears to have defined without the effective length factor, K. As per the CSA S16.1 or the AISC specifications, the definition of slenderness parameter includes K (i.e., K = 1.0 for pinned end condition and 0.5 for fixed end condition). Therefore, to be consistent with the CSA S16.1 definition of the slenderness parameter, there should be Received October 25, 1996. Revised manuscript accepted January 15, 1997.

Strength of Bell‐and‐Spigot Joints

Although bell‐and‐spigot joints have been used successfully for many years in the construction of large‐diameter water‐pipe mains, questions sometimes arise concerning the efficiency of the joints under axial (longitudinal) forces. In this paper, a simple general‐yielding approach is used to develop an equation that can be used to directly determine the strength of such joints. The calculated joint efficiency, which depends on the ratio of the joint eccentricity to the pipe‐wall thickness, ranges from 0.41 for a zero gap to 0.24 for a gap equal to the pipe thickness. The equation is reasonable and conservative when compared to finite‐element results, to the ASME code, and to test results. Also, tests show that a single full‐thickness fillet weld can provide adequate strength. Such joints should be used wherever possible because they are much more economical than butt joints.

In-plane buckling of trusses

One of the main influences in the design of planar triangulated trusses is the choice of effective lengths for the various members. The article proposes an iterative method which converges to the exact buckling load for each of the members according to the bifurcation theory. Using the concept of equivalent modulus in compression, the method is then extended to the inelastic domain. Member resistances are compared with exact results published in Europe. It is shown that, for conventional trusses, good accuracy may be expected. The influence of joint flexibility and eccentricity in tubular trusses is examined to show that, while moderate eccentricities may be altogether disregarded, angular joint flexibilities tend to increase the buckling length of web members. Key words: buckling, eccentricity, effective, flexibility, joint, resistance, triangulated, truss, tubular.

On primary and secondary stresses in triangulated trusses

The importance of primary and secondary stresses on the strength of statically loaded triangulated trusses having rigid joints is reviewed. Support is given to the contention that accurate modelling of material properties and member end conditions is justified both for predicting overall failure loads and for assessing the sequencing through which member stiffness deteriorates to a consequent mode of failure. In this regard, the results of two large scale tests on welded steel Pratt trusses are presented. A comparison that is made between measured responses up to the ultimate load with that predicted by an elasto-plastic model that accounts for moment-axial force interaction shows excellent agreement. A parametric study on the influence on local and overall strength of a change in diagonal to chord angle and joint eccentricity ratio reveals wide ranging degrees of sensitivity. It was found, for example, that the truss ultimate load exhibited nearly flat response at small and very large values of eccentricity (both positive and negative) whereas significant changes in overall strength emerged at intermediate values. The failure load predicted by the simple pin-ended truss model is shown to be conservative for small eccentricities but would provide an unsafe estimate of resistance if primary moments caused by joint eccentricities were not taken into account.

Interaction of shear and tension in welded truss connections

In many truss joints consisting of double-angle web members welded to stems of inverted lower chord tee sections, adequate fillet weld lengths may be provided without the need for gusset plates. In such cases where a joint may also be subjected to high combinations of shear and tension, the possibility of overstressing of a tee stem exists. In the investigation of this problem, four analytical techniques, including two proposed herein, and experimental results of 30 full-scale specimen tests, 22 of which were conducted during the investigation presented herein, are presented and evaluated. Analytical and experimental results indicate that the interaction of shear and normal stresses is an important consideration in the design of such joints and that an interaction check is required.Of the many parameters investigated as part of the experimental investigation, the two that most significantly influence the magnitude of tension force that a lower chord can simultaneously carry are the magnitude of applied shear force averaged over the available web shear area and the joint configuration itself. The inclination of a diagonal web member as well as intentional joint eccentricities have little effect on the strength of these joints, although joint ductility appears to be affected by eccentricities.Although all four analytical techniques have similar overall accuracies of prediction, a method based on applying a von Mises criterion at ultimate to determine an equivalent shear area in a tee chord is recommended as being the most suitable. Based on this method, as verified and validated by comparison with extensive experimental results of tests on full-scale specimens, a simplified design curve and associated equations are developed and proposed for design office use. Key words: connection, interaction, shear, tension, welded, truss.

Recent research on eccentrically braced frames

Recent research developments on eccentrically braced steel frames for seismic design are reviewed. The emphasis is placed on the design of links, which are short sections of beams between columns and braces, and similar elements at eccentric joints. The review includes some highlights of the latest experiments with one-third scale models employing different eccentric bracing schemes, an updated classification of links, and special design requirements for different types of links. Some results are given on recent cyclic tests of full-size links.

Eccentrically Braced Frame Construction--A Case History

The building described in this paper is the proposed new Bank of America Regional Office and Branch Bank Building located in San Diego, California. The building is currently under construction. Completion is scheduled for December 1981. The building has a tower incorporating frames having joints with intentionally built-in eccentricities. The joint eccentricity is illustrated in Figs. 1 and 2. In Fig. 1, one sees an elevation of a frame having joints referred to in this presentation as "eccentric." In Fig. 2, a detail of an eccentric joint is shown. Providing this eccentricity is, of course, exactly what engineers were taught to avoid during their years of formal education. The motivation for providing the eccentricity is to achieve a framing configuration that will deform in a highly energy-absorbing manner when subjected to severe ground shaking of the type associated with great earthquakes. Concentrically braced frames do not have the ability to absorb large amounts of energy when subjected to seismically-induced forces. The means of achieving the large energy-absorbing capability consists of detailing the eccentric link in a manner that assures an inelastic moment capacity that is larger than the inelastic shear strength. For interesting and comprehensive descriptions of the technical aspects of this type of framing, how it performs under very high loads and the principles used in their design, the works of Roeder, Popov and Teal are recommended references. In a seismic structural design this is a relatively new and unique design concept. One might wonder why it would be used. The principal purpose of this paper is to describe why eccentrically braced frames were selected for the San Diego Bank of America building.

Influence of joint eccentricity and rigidity on the load capacity of a space truss sub-assemblage

Abstract Experimental testing of bolted and welded space truss sub-assemblages with large joint eccentricities has displayed satisfactory performance. Tests have been carried out on five sub-assemblages from a double layer (plate-like) space truss configuration. The distinctive features of the configuration employed were chord member continuity and the resulting large joint eccentricities. The particular layout adopted furnished orthotropic behaviour for stiffness and strength, so two sub-assemblage configurations were adopted. To provide control and comparisons a welded concentrically jointed sub-assemblage was tested in addition to the bolted and welded sub-assemblages. Although the large joint eccentricities reduced sub-assemblages stiffness and peak load capacity, the continuity of chords largely offset these reductions even when bolted joint details were used; for the welded joint cases peak load capacities were at least 50 per cent higher again. The stiffness of such systems is seen to be sensitive to member and joint imperfections, and joint slip.