Tubular Braces Research Articles

Buckling strength of slender circular tubular steel braces strengthened by CFRP

Although the application of carbon fibre reinforced polymer (CFRP) on strengthening of concrete structures have been reported widely, there are relatively few applications to steel structures, especially to tubular steel structures. In order to investigate the performance of circular tubular steel braces retrofitted with CFRP, long hot finished circular hollow steel braces (88.9mm×4mm) wrapped with CFRP sheets are tested to study the effects of CFRP on overall buckling behavior. Based on the experimental results, a numerical model has been developed to predict the axial load capacity, lateral and axial displacements accounting for the initial out-of-straightness, material and geometric nonlinearities. It is found that CFRP layers could increase both the strength and stiffness of the brace considerably.

Perdido Development Project - Spar and Moorings

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper OTC 20885, ’Perdido Development Project - Spar and Moorings,’ by Curtis Lohr and Keith Smith, Shell International Exploration and Production, prepared for the 2010 Offshore Technology Conference, Houston, 3-6 May. The paper has not been peer reviewed. A spar platform was selected for the Perdido development during system selection in 2005 to provide a stable platform for direct vertical-access drilling and producing operations in nearly 8,000 ft of water. A combination polyester/chain mooring system was selected to place less vertical load on the floating structure (compared to an all-steel mooring system). Introduction The Perdido spar was Shell’s first spar-host production platform, but had the benefits of being developed as a joint venture with Chevron and BP. Technip was selected as the spar and mooring contractor. The spar comprises a cylindrical upper section (hard tank); a trussed midsection (truss) formed from tubular legs and braces with horizontal stiffened-plate flats, or heave plates; and a bottom section (soft tank) used to carry solid ballast and provide flotation during installation. Some key characteristics of the Perdido spar are listed below: Overall length—555 ft, draft 505 ft Hard tank—118-ft outside diameter (OD)×264-ft length, with 214-ft hard-tank draft Hull weight—approximately 20,000 tons including structural steel, appurtenances, and hull systems Nine mooring lines Two heave plates Three strakes A 46×46-ft moonpool Six riser slots Four-leg topside and truss Octagonal soft tank Six hydraulic tensioners supporting five production risers and one drilling/completion riser The spar was designed to accept a single-lift topside by limiting the spar’s OD to 118 ft and by placing storage tanks for diesel fuel, methanol, dead oil, low-dosage hydrate inhibitor, and drill water inside the upper compartment of the spar rather than on the topside. The spar was fabricated in Finland. Fabrication began in November 2006, dry transport to the Gulf of Mexico began in May 2008, and the wet tow offshore occurred in August 2008. One challenge that was unique to the spar fabrication was installation of components for the hydraulic riser-tensioning system in the main hull-fabrication yard. Tolerance for this system was extremely tight, and installation requirements were discussed well in advance to plan the work. Jigs to ensure that the installation of components was within tolerance were designed, fabricated, and used. Mooring System The Perdido mooring system uses nine polyester/chain lines averaging more than 2 miles in length, with the vast majority of each line’s length being polyester. The nine taut lines are oriented in a 3×3 pattern and consist of 9.68-in. polyester-rope main sections and short lengths of 5.28-in. chain at each end. The spar has an “active” mooring system to reposition it over drilling locations within an approximately 350-ft-diameter area. Because polyester is more difficult to handle in a winch, the top of each mooring line consists of a short length of chain, called the platform chain, which is connected to the spar. The platform chain runs through a below-water fairlead mounted on the outside shell of the spar. A dedicated chain jack controls each platform chain.

Cumulative Cyclic Deformation Capacity of Circular Tubular Braces under Local Buckling

Tubular-section members are commonly used as seismic-resistant braces because they have a higher moment of inertia than open cross sections of equivalent area. However, the cumulative cyclic deformation capacity of tubular sections after buckling is smaller than that of open sections, such as H-shaped sections, because fracture is initiated by local buckling of circular tubular sections. To evaluate the seismic performance of such diagonal braces, it is essential to predict the cumulative cyclic deformation capacity of these braces before any fracturing. In this study, the cumulative cyclic deformation capacity of circular tube braces under local buckling was assessed by performing cyclic loading tests for a range of slenderness and diameter-to-thickness ratios. The mechanism of strain concentration in the tubular braces was studied in various types of analysis, and a method is proposed for assessing the cumulative deformation capacity before fracture based on the entire axial deformation of the braces.

Effect of material properties on hollow section performance

Hollow structural sections for onshore steel construction are manufactured in diverse locations around the world, by either a hot-finishing or seamless process or – far more commonly – by cold-forming. The implications of using material by a particular process in various statically loaded or dynamically loaded applications are not fully appreciated. This paper hence reviews some of the key geometric and material properties that are inherent in several common hollow section production standards. Problems that have been encountered with certain cold-formed square and rectangular hollow sections, particularly with regard to hot-dip galvanising in North America and Asia, are elaborated. Besides galvanising, these same sections are now also subject to constraints for seismic applications. The difference in seismic performance between cold-formed, cold-formed and stress-relieved, and hot-finished hollow sections, when used as energy-dissipating members such as braces, has been a point of debate and speculation to date. An experimental project, in which the relative performance of each material type is evaluated by performing inelastic cyclic testing of large-scale tubular braces, is described, with comments on the suitability of various tube material types for concentrically braced steel frames under severe seismic loading. Considering all of these material and production issues, an industry-driven reappraisal of the cold-formed hollow structural section manufacturing specification in North America is being undertaken, which is also presented herein.

SEISMIC RESPONSE OF STEEL STRUCTURES INCLUDING FRACTURES AT TUBULAR BRACES USING COLLABORATIVE PSEUDO DYNAMIC TEST

This paper presents seismic response of steel structures including fractures at tubular braces. We apply pseudo dynamic test of substructure to collaborative structural analysis system in order to conduct high-accuracy analyses because it is very difficult to predict fracture life of steel members accurately. Analyses of three-story steel moment frame with loading tests of tubular braces are conducted using various earthquake intensities. The results show tubular braces are hard to fracture under severe earthquake even if the braces have low deformation capacity, and affects of the brace fracture after local buckling is limited because the strength is considerably degraded.

Experimental evaluation of the seismic performance of modular steel-braced frames

Several studies have shown that the lateral response of concentrically-braced frames is dominated by the inelastic behavior of the bracing members. However, the overall performance of the entire frame depends on the frame configuration including its connections. In this study, the hysteretic characteristics of modular steel-braced frames under reversed cyclic loading are evaluated. The design and construction of the test specimen accounted for the unique detailing requirements of these frames. A regular concentrically-braced frame with similar physical characteristics was also tested for comparison. Both test specimens consisted of a one-storey X-braced system with tubular brace cross-section. This paper describes the behavior characteristics and provides a detailed comparison of the two systems to assess the strength, stiffness, inelastic force and deformation, and energy dissipation characteristics of the modular system. An analytical model capable of capturing the effect of the system’s unique detailing requirements is proposed and validated using the test results.

Cast Steel Connectors for Circular Hollow Section Braces under Inelastic Cyclic Loading

Although concentrically braced frames are an efficient means of providing lateral support in steel structures, many such frames designed to resist seismic loading and featuring hollow structural section braces have performed poorly in recent earthquakes—primarily due to unexpected connection failures. To address this issue, the use of a cast steel connector that fits between a tubular brace and a gusset plate is presented as an alternative to the reinforced, fabricated connections that are commonly used in seismic load resisting braced frames. The resulting connector was shaped using solid modeling software, verified by finite-element analysis, and finally cast to ASTM A958 standards. Laboratory results from static and cyclic testing of concentrically loaded brace-connector assemblies showed that the use of a cast steel connector is a viable means of connecting to tubular brace members for seismic or even static applications. Further, as the connector was crafted to fit a range of tubular members and can be mass produced, the proposed solution is an economical alternative to conventional tube-to-gusset connections under seismic loading.

Thin circular hollow section-to-plate T-joints: Stress concentration factors and fatigue failure under in-plane bending

Welded thin-walled ( t<4 mm) tube-to-plate T-joints made up of cold-formed circular hollow sections welded onto a plate to form a moment resistant connection are used in the road transport and agricultural industry to manufacture equipment and other structural systems. Fatigue design of these joints is not available in current standards. An understanding of the stress concentrations and failure in these connections is therefore necessary as a step towards understanding the fatigue behaviour of these connections. Stress concentration factors (SCFs) of welded thin-walled ( t<4 mm) circular hollow section (CHS)-to-plate T-joints are determined at different locations along the weld toes on the tubular brace. The distribution of SCFs along the weld toes shows that the highest SCF occurs at the weld toes in the circular brace at the 0° line. The ratio of the end of test fatigue life ( N4) to the through-thickness fatigue life ( N3) in the thin CHS-plate T-joints is found to fall within the range of N4/ N3 found in previous research of both thick and thin-walled joints. Surface crack growth monitoring is used to obtain an approximation of the length of surface crack at the point of occurrence of a through-thickness crack. The relationship between surface crack length and the occurrence of a through-thickness crack is important in that it can be used as a measure of the criticality of a surface crack during structural health monitoring of equipment or structures.

局部座屈を伴う鋼管ブレースの累積繰返し変形性能

Tubular sections are commonly used for truss structures or diagonal braces, because they have higher moment-of-inertia comparing to open cross-sections with the same area. However, their cumulative cyclic deformation capacities after buckling are known as quite small comparing to H-sections, because strain concentration occurs with local elbow buckling. For evaluating seismic performances of such structures, cumulative cyclic deformation capacity of braces before fracture is essential. In this paper, cumulative cyclic deformation capacity of tubular braces with local buckling is researched by cyclic loading tests with various slenderness ratios and thickness ratios. Their strain concentration mechanism is researched thorough various analyses, followed by proposing easy evaluation method of cumulative deformation capacity.

Inelastic Postbuckling and Cyclic Behavior of Tubular Braces

A nonlinear fiber element for the simulation of buckling, post buckling, and hysteric responses of tubular struts is formulated and implemented in the nonlinear program DRAIN-3DX. In this element both material and geometric nonlinearities are considered. The element is applied to simulate post buckling and hysteric response of pinned and fixed tubular struts subjected to cyclic loading. The results are in good agreement with available results, i.e., experiment and other analytical models data in terms of buckling load, load-deformation curve, strength, and stiffness degradation.

Comparing hysteretic behavior of light-gauge steel plate shear walls and braced frames

Braced frames and steel plate shear walls (SPSWs) have both been shown to be useful in the seismic retrofit of buildings. While both these systems have merit, no guidance exists to help the engineer determine which of the two approaches is preferable in terms of providing stiffness, maximum displacement ductility, cumulative hysteretic energy dissipation, and energy dissipation per cycle for a given strength. In an attempt to provide some quantitative data and insight for this purpose, this paper describes and compares the results from cyclic testing of six frames: four concentrically braced frames (two with cold-formed steel studs for in-plane and out-of-plane restraint of the braces and two without), and two light-gauge steel plate shear walls (one with a flat infill plate and one with a corrugated infill). The largest initial stiffness was provided by a braced frame specimen with cold formed steel studs and the largest ductility was achieved with a steel plate shear wall with flat infill. After scaling the hysteretic results to the same design base shear, it was found that both the energy dissipated per cycle and the cumulative energy dissipation were similar for flat plate SPSW and braced frames with two tubular braces, up to a ductility of four. After that the tubular braces fractured while the SPSW with a flat infill reached a ductility of nine before the energy dissipation per cycle decreased.

Seismic evaluation and upgrading of chevron braced frames

Abstract Many Chevron type “ordinary” steel concentric braced frame (OCBF) structures have suffered extensive damage in recent earthquakes which raises concerns about their performance in future earthquakes. A building in the North Hollywood area, which suffered major damage in the 1994 Northridge earthquake, was selected for detailed study. Response spectrum, nonlinear static (pushover), and nonlinear dynamic (time history) analyses for a ground motion recorded at a nearby site compared well with the observed damage. The state-of-health of the damaged structure was assessed to determine the need and extent of repair. The seismic performance of non-ductile CBFs can be improved by delaying the fracture of braces, e.g., in the case of the tubular braces by filling with plain concrete. Changing the bracing configuration from chevron to 2-story X configuration can avoid the instability and plastic hinging of floor beams. Further improvement can be achieved by redesigning the brace and floor beams to a weak brace and strong beam system, as in Special CBFs. This full upgrading to SCBFs results in excellent hysteretic response and, with inelastic actions confined to ductile braces, exhibits reasonable distribution of damage over the height of the building.

Tests of Cold-Formed Circular Tubular Braces under Cyclic Axial Loading

This paper describes a series of tests to failure of fix-ended tubular braces subjected to cyclic concentric axial loading. The braces were made from cold-formed steel grade C350L0 ~350 MPa nominal yield stress! circular hollow sections ~CHS!. Nine different diameter-to-thickness ratios in the range of 19,D/t,56 that have a moderate member slenderness in the range of 25,KL/r,41 were tested. The effects of three loading protocols on the inelastic hysteresis behavior of the CHS braces were examined. The CHS braces exhibited stable hysteresis behavior up to local buckling, and then showed considerable degradation in strength and ductility depending on KL/r and D/t ratios. First-cycle buckling loads were compared with design loads predicted using a number of steel specifications. The effects of section and member slenderness on strength, ductility, and energy absorption capacity of the braces were examined. The structure response factor ~ductility index! was determined and used to derive new section slenderness limits suitable for seismic design. CE Database subject headings: Cold-formed Steel; Bracing; Tubes; Cyclic loads.

Connections of CHS concrete-filled diagonals of X-bracings

Abstract The scope of the present paper is the numerical investigation of the behaviour of concrete-filled CHS bars used as X-bracings diagonals. The weak point for an X-brace is the point where the two diagonals intersect where, in order that the two bars remain co-planar, one of them is usually interrupted. In this case, a usual practice is to connect the interrupted bar to the continuous one by means of two gusset plates each one welded along a generatrix of the continuous bar. The result is, that the axial load is transferred as an inside-the-cross-section loading situation of the continuous bar thus causing a prohibitively excessive ovalisation of the cylindrical shell of the CHS. The in-plane transfer mechanism of the forces causes an increase of the axial flexibility of the interrupted bar by a factor of 50–300 thus making this type of connection too flexible and therefore unsuitable for use in X-braces. Filling the tubular braces with concrete is already known as a promising technique against local buckling: the subject of the present work is the assessment the effect of this technique to the axial flexibility of the interrupted bar. From the numerical simulation point-of-view, the problem is formulated as a unilateral contact problem between the concrete in-fill and the steel shell and a parametric analysis is made to elucidate flexibility and strength aspects of this connection for the most frequently used CHS profiles.

Grout Repair of Dent-Damaged Steel Marine Tubulars

A test program consisting of 13 steel tubular brace specimens with dent damage and various diameter-to-thickness ( D/t ) ratios was conducted to assess the residual strength and repair of damaged tubular braces used in offshore platforms. Specimens were inflicted with a dent having a depth of 0.10 D , with internal full grouting and grouted steel clamps, respectively, used to repair specimens. A baseline of unrepaired, damaged specimens subjected to concentric axial loading showed a significant reduction in residual strength due to the dent. A further reduction in axial load capacity was found in specimens subjected to combined loading. The use of grout to repair specimens was found to inhibit a growth in dent depth under axial loading and restore the capacity of the dented member to a level greater than the undamaged strength. A comparison with existing and modified analytical methods for strength prediction of unrepaired and grout repaired braces showed that these methods provided reasonable results.

Hysteretic modelling of tubular members and offshore platforms

The inelastic cyclic behaviour of tubular members in offshore platforms and the characteristics and limitations of the phenomenological, physical theory and finite element (FE) models for predicting the hysteretic behaviour of braces are reviewed. Then, based on the behavioural characteristics of tubular braces and legs, proper modelling procedures are proposed and conventional beam-column finite elements are used to simulate the responses of experimental leg and brace members under severe cyclic excursions. A test platform is also analysed under cyclic displacements simulating strong earthquake actions. Based on the comparisons between the FE and test results, optimum mesh sizes and modelling techniques which render realistic response simulations at reasonable computational effort are recommended for FE analyses of offshore tubular members and structures. The FE results for the test brace and platform are also compared with the results of a phenomenological model, and the merits of the proposed FE techniques are discussed.

Stability of Orthogonally Intersecting Equal-Leg Angle Cross-Bracing Systems*

When a sway force acts on a cross-bracing system, one of the braces goes into a tension state while the other goes into a compression state. The tension brace helps to stiffen the compression brace through a bolted intersecting point. This paper presents an energy method for determining the improved buckling capacity of such a compression brace. The braces considered consist of equal-leg angle members that intersect orthogonally. The present problem is more difficult to solve when compared to earlier studies made on rods and tubular brace members, because angle members may buckle in a complicated flexural torsional mode rather than in a simple flexural mode.

3-D Inelastic Behavior of Tubular Members Supported on Nonlinear Rotational Springs

This paper considers a theoretical analysis of the inelastic response of tubular brace members of offshore jackets which are supported on nonlinear rotational end springs and subjected to axial load and generally applied end bending moments. The present analysis is based on the concept of 3-D plastic hinge formation. The method assumes that plasticity is confined to discrete plastic hinges while the rest of the member remains elastic. This allows a rapid nonlinear analysis of the axial behavior of the member by a sequence of elastic solutions of the classical beam-column equations. It is shown that the model is able to simulate closely and efficiently the 3-D inelastic behavior of brace members under the foregoing support conditions.

Cyclic Load Behavior of Concrete‐Filled Tubular Braces

Cyclic load behavior of concrete‐filled rectangular steel tubular bracing members is investigated. Nine full‐scale specimens made from A500 Grade B cold‐formed steel tubes were tested under quasi‐static cyclic loading. The main prameters of the study are: 1) Presence of concrete; 2) strength of concrete; 3) effective slenderness ratio; and 4) width‐thickness ratio. General cyclic behavior and modes of failure of the specimens are presented. Based on the observations of the behavior of test specimens, two procedures are given to compute the first buckling load for such bracing members.

Local buckling and inelastic cyclic behavior of tubular members

Closed form expressions are obtained for the load-deflection behavior of the locally buckled circular tubular braces under the reversed loading conditions. These expressions are obtained by supplementing the deflected shape of the brace with closed form expressions for moment-curvature relationship of locally buckled circular tubular section. The developed closed form expressions are used to study the effects of the slenderness ratio and diameter-to-thickness ratio on the behavior of braces under reversed loading conditions.