Brace Tension Research Articles

Is valgus unloader bracing effective in normally aligned individuals: implications for post-surgical protocols following cartilage restoration procedures

Utilizing valgus unloader braces to reduce medial compartment loading in patients undergoing cartilage restoration procedures may be an alternative to non-weightbearing post-operative protocols in these patients. It was hypothesized that valgus unloader braces will reduce knee adduction moment during the stance phase in healthy subjects with normal knee alignment. Gait analysis was performed on twelve adult subjects with normal knee alignment and no history of knee pathology. Subjects were fitted with an off-the-shelf adjustable valgus unloader brace and tested under five conditions: one with no brace and four with increasing valgus force applied by the brace. Frontal and sagittal plane knee angles and external moments were calculated during stance via inverse dynamics. Analyses of variance were used to assess the effect of the brace conditions on frontal and sagittal plane joint angles and moments. With increasing tension in the brace, peak frontal plane knee angle during stance shifted from 1.6° ± 4.2° varus without the brace to 4.1° ± 3.6° valgus with maximum brace tension (P = 0.02 compared with the no brace condition). Peak knee adduction moment and knee adduction impulse decreased with increasing brace tension (main effect of brace, P < 0.001). Gait velocity and sagittal plane knee biomechanics were minimally affected. The use of these braces following a cartilage restoration procedure may provide adequate protection of the repair site without limiting the patient's mobility.

Experiments on Effective Widths and Angles of Gusset Plates Jointed to a Tension Brace with High-Strength Bolts

Experiments on Effective Widths and Angles of Gusset Plates Jointed to a Tension Brace with High-Strength Bolts

Hysteresis Model of Thin Infill Plate for Cyclic Nonlinear Analysis of Steel Plate Shear Walls

A hysteresis model for thin infill steel plates was developed to evaluate the nonlinear cyclic behavior of steel plate shear walls. Nonlinear finite-element analysis was performed for thin steel plates with a rigid boundary frame. Based on the analysis results, the hysteretic behavior of the infill steel plate was simplified as an equivalent uniaxial stress-strain relationship in the direction of tension-field action. The proposed hysteresis model was implemented in macroscopic analysis models for infill steel plates, i.e., the tension strip model and equivalent tension brace model. The proposed method was applied to existing test specimens with various design parameters and loading conditions. The prediction results were compared with the test results.

Impact of Diaphragm Behavior on the Seismic Design of Low-Rise Steel Buildings

Modern building codes allow engineers to use reduced seismic loads in design provided that the seismic load resisting system (SLRS) of the structure is adequately designed and detailed to withstand strong ground shaking through ductile inelastic response. This approach has been adopted by the North American model codes which typically include special provisions to achieve satisfactory inelastic seismic performance. Single-story buildings often incorporate a steel roof deck diaphragm that is relied on to transfer lateral loads to the vertical bracing bents. The vertical braces are usually selected as the energy dissipating fuse element, while the diaphragm and other elements in the SLRS should be designed such that their capacity exceeds the nominal resistance of the braces. Steel bracing members designed for compression inherently possess significant reserve strength when loaded in tension, which means that large brace tension loads must be considered in the design of the surrounding protected structural components. Capacity design seismic provisions have led to the need for much thicker roof deck panels and more closely spaced diaphragm connection patterns compared with past practice in Canada. This paper describes the current U.S. seismic design approach and provides examples as it is applied to single-story buildings and their diaphragms. An overview of the related aspects of an on-going research project on the flexibility and ductility of the roof diaphragm in low-rise steel buildings is also included.

Effect of tension bracing on the collapse mechanism of steel moment frames

The nonlinear response of Tension Braced Moment Resisting Frames (TBMRF) under lateral loading with regards to their sequential failure mechanisms and ductility is studied. Both distributed and lumped plasticity finite element approaches are considered and parametric studies are carried out to evaluate the sequence of element failures in various single and multi-storey TBMRFs. Three possible failure mechanisms representing different specific characteristics of TBMRF systems are presented and their differences are outlined. Also, several multi-storey dual-system TBMRFs are analyzed and the results are utilized to set up an analytical model which can appropriately differentiate and evaluate the load carrying contribution of brace and frame elements. Finally, an empirical formula and design curves to calculate frame-brace contribution ratios in different performance levels are proposed.

A STUDY ON APPLICATON OF VISCOELASTIC DAMPERS TO LIGHT-GAGE STEEL FRAMES HAVING TENSION BRACES

Passive control of light-gage steel frames is discussed. Disadvantage of the conventional frame having tension braces are explained and the viscoelastic damper is applied instead of the brace. The passive control frame is modeled as spring and damper connected in series. And it is important to increase the spring stiffness for the performance of the passive control. Procedure to decide the damper size is proposed and a frame having the viscoelatic damper is developed. The performance is validated through the dynamic loading test. Shaking table tests are also conducted to three specimens that have the damper and/or the tension brace. Response characteristics of specimens are evaluated using the seismic response spectrum and the time-history analysis.

Pushover Response of a Braced Frame with Suspended Zipper Struts

This paper presents the results from an experimental pushover test on a one-third-scale model of a special inverted-V-braced steel frame with zipper struts. Zipper elements are vertical elements added at the intersections of the braces above the first floor and designed to carry upward the unbalanced loads resulting from buckling of the braces. As far as the writers know, this is the first test on such a configuration that has been suggested as an alternate to conventional braced frames in the American Institute of Steel Construction seismic specification for many years. The model was pushed to a target roof drift ratio of +3.58%, which was achieved without strength degradation and followed a trilinear backbone curve. The load was then reversed until the bottom story brace in tension fractured at a drift of -0.78%. After this fracture, the frame still carried about 37% of the maximum base shear. The zipper elements demonstrated their ability to activate buckling in all stories except the top one, redistributing the loads in the structure and minimizing strength losses. Two-dimensional and one three-dimensional frame models were analyzed and found to reproduce satisfactorily the experimental results up to the beginning of tearing in the tension brace.

Earthquake testing and response analysis of concentrically-braced sub-frames

The results of shake table tests on three concentrically-braced sub-frames are compared with a series of correlative inelastic analyses. Both transient time-history and nonlinear static (pushover) analyses are considered. The lateral resistance of the test frames is provided by a pair of cold-formed tubular steel members. The brace cross-section is varied between tests to investigate the influence of brace slenderness on the stiffness, resistance and ductility displayed by the frame under strong earthquake loading. Response simulations using a two-dimensional analytical model of the test frame are compared with the experimental results. Brace strengths and elastic test frame properties established in complementary static tensile tests and low-level shake table tests, respectively, are used to determine model input data. The transient acceleration and displacement response of the frames calculated in nonlinear time-history analyses are compared with corresponding experimental measurements. The base shear–frame drift relationship is calculated using pushover analyses that consider either the resistance of both frame braces or the tension brace alone. These are compared with the experimental hysteretic response of the test frame. On the whole, the analytical results are observed to agree well with the experimental results, especially with respect to brace tension forces and frame base shear. Cases where the experimental and analytical responses deviate are identified and the implications for earthquake response assessment and design are discussed.

Inelastic performance of screw-connected cold-formed steel strap-braced walls

Cold-formed steel, screw-connected strap-braced walls (15 specimens, 2.44 m × 2.44 m each), designed following a capacity-based approach, were tested to evaluate their performance in the inelastic range of behaviour. Gross cross-section yielding of the tension braces was the specified failure mode in the design procedure. Extended tracks and additional shear anchors were installed, such that inelastic deformations would be limited to tension yielding of the braces. Walls without extended tracks were able to reach their yield level; however, damage to other frame elements occurred, thus reducing the ductility level that was attained. Cyclically loaded walls (0.5 Hz) with track extensions showed that wall performance is also dependent on the strain rate experienced by the braces. An Fu / Fy ratio greater than 1.2 is necessary to limit the possibility of brace fracture under seismic loading. Preliminary force modification factors, Rd = 2.0 and Ro = 1.3, are recommended for walls designed and detailed to achieve ductile performance.

Elastic–plastic crack driving force for tubular K-joints with mismatched welds

This study examines the elastic–plastic driving force in shallow surface cracks located in welds near the crown point of the tension brace toe in a circular hollow section K-joint — with strength mismatch between the chord material and welds. The remote loading at the brace end imposes displacements acting along the brace axis. The 3-D finite element models couple a global, topologically continuous mesh and a separate, local crack-front model through mesh-tieing. The numerical solver computes the elastic–plastic crack driving force ( J -value) locally along the crack front through a domain-integral approach. The numerical analyses employ stress–strain curves for representative high-strength steels now used in offshore construction. The yield strength of the welds varies as σ y w = m σ y c , where m denotes the mismatch ratio and σ y c is the chord yield stress. The strain hardening property of the welds remains the same as that of the chord material. Unlike historical research on weld mismatch effects for simple, through-crack fracture specimens, the surface crack considered here in the tubular K-joint resides in the base metal (chord) adjacent to the weld toe of the hot-spot location rather than in the welds. The computed J -values demonstrate that the crack driving force increases with increased weld strength — thus a higher potential for initiation of ductile tearing. The numerical results show that a relatively larger elastic–plastic crack driving force exists for joints with a high brace to chord outer diameter ratio ( β ) or with a large brace to chord intersection angle ( θ ). For joints with m ≥ 0.8 , the welds are sufficiently strong to mobilize significant plastic deformation in the adjacent chord material near the crack surface, and thus prevent large-scale yielding in the welds.

Mode mixity for tubular K-joints with weld toe cracks

This paper examines the mode-mixity of stress-intensity factors for surface cracks located near weld toes of the crown point at the toe of the tension brace in circular hollow section K-joints under different chord/brace end constraints. The remote loading applies a uniform stress at the end of the brace along its axis. The 3-D finite element models employ mesh-tieing between a topologically continuous, global mesh and a separate, local crack-front mesh. The linear-elastic analyses compute the mixed-mode stress-intensity factors along the crack front using an interaction-integral approach. The analyses for three different crack locations along the brace–chord intersection demonstrate that the critical location for the surface flaw lies at the crown point of the brace toe. The numerical investigation of six different boundary conditions indicate that the unbalanced loading conditions cause significant bending stresses in the chord and generate a considerably larger mode I stress-intensity factor than found for the balanced loading case. The mixed-mode stress-intensity factors indicate that the crack front experiences predominantly mode I loading, with K III → 0 near the deepest point on the front ( ϕ = π/2). The non-dimensional, mode I crack driving force, described by F I = K I / σ ¯ br π a , reaches a maximum value at the deepest point of the crack for the crack aspect ratio a/ c = 0.25 considered here. The mode-mixity angle, ψ = tan −1( K II/ K I), at ϕ = π/2 is compared for a range of practical K-joint configurations. The present study demonstrates that the mode-mixity angle ψ becomes significantly larger in the balanced loading conditions than in the unbalanced loading condition. Values of the non-dimensional stress-intensity factors ( F I = K I / σ ¯ br π a and F II = K II / σ ¯ br π a ), however, decrease for the balanced loading condition. Variations in the brace-to-chord diameter ratio ( β) and chord radius to wall thickness ratio ( γ) generate significant changes in the mode mixity. Thin-walled joints ( γ = 20) generally experience a larger crack driving force with a higher mode-mixity angle.

Reserve capacity of fatigue damaged internally ring stiffened tubular joints

Offshore platforms have to serve in harsh environments and hence are likely to be damaged due to wave induced fatigue and environmental corrosion. Welded tubular joints in offshore platforms are most vulnerable to fatigue damage. Such damages endanger the integrity of the structure. Therefore it is all the more essential to assess the capacity of damaged structure from the point of view of its safety. Eight internally ring stiffened fatigue damaged tubular joints with nominal chord and brace diameter of 324 mm and 219 mm respectively and thickness 12 mm and 8 mm respectively were tested under axial brace compression loading to evaluate the reserve capacity of the joints. These joints had earlier been tested under fatigue loading under corrosive environments of synthetic sea water and hence they have been cracked. The extent of the damage varied from 35 to 50 per cent. One stiffened joint was also tested under axial brace tension loading. The residual strength of fatigue damaged stiffened joint tested under tension loading was observed to be less than one fourth of that tested under compression loading. It was observed in this experimental investigation that in the damaged condition, the joints possessed an in-built load-transfer mechanism. A bi-linear stress-strain model was developed in this investigation to predict the reserve capacity of the joint. This model considered the strain hardening effect. Close agreement was observed between the experimental and predicted results. The paper presents in detail the experimental investigation and the development of the analytical model to predict the reserve capacity of internally ring stiffened joints.

Stability of pretwisted beams in cross bracings

New systems of pretwisted bars are suggested, and their stability under a conservative load is investigated. In this paper, closed‐form relationships are obtained for the direct evaluation of critical loads of pretwisted cross bracings with different end connections and an arbitrary ratio of the dimensionless parameters of tension and compression elements. The equation for the critical loads is derived for tension and compression braces with different section properties, lengths, and axial loading. It is found that the critical load of a twisted brace is higher than that of a nontwisted brace. Parametric solutions are graphically displayed to clarify the distinct behavior, including the boundary separating symmetric and antisymmetric modes of buckling.

Biomechanics and Functional Anatomy of Hollow Stemmed Sphenopsids: III. Equisetum hyemale

Abstract:As in Equisetum giganteum, the hollow stem of Equisetum hyemale owes the mechanical stability of the internodes to an outer ring of strengthening tissue (hypodermal sterome) which provides stiffness and strength in the longitudinal direction. In contrast to hollow‐stemmed grasses, the hypodermal sterome consists of living cells. The compound inner tissue of the overwintering aerial stem of Equisetum hyemale includes a continuous inner and outer endodermis layer of vital thick‐walled cells that have slightly lignified Casparian thickenings. The two endodermis layers provide an inner tension and compression bracing which lend resistance to local buckling. The stress‐strain relation in longitudinal tension is biphasic with remarkably high critical strains especially in the upper parts of the stem. Scraping off part of the epidermis with the built‐in silicate does not change this behaviour, except in the initial steep part of the curve where the Young's modulus is reduced by 20%. No contribution of the endodermis and the parenchyma could be detected in tension tests of longitudinally‐oriented strips. Relaxation experiments reveal viscoelastic behaviour. As with the biphasic stress‐strain relation and the critical strains, the viscoelastic properties have largely to be ascribed to the hypodermal sterome.

Use of steel bracing in reinforced concrete frames

In this paper the use of steel bracing in concrete-framed structures is investigated. The investigation is carried out through a series of tests conducted on a number of model frames. The object of the tests was to determine the degree of effectiveness of different diagonal bracing arrangements to increase the in-plane shear strength of the concrete frame and to observe the relative behaviour of tension and compression braces. The important question of the proper connections between the steel braces and the concrete frame is also considered. The test results indicate a considerable increase in the in-plane strength of the frame due to steel bracing. As an overall conclusion it is noted that, with proper connection between the brace and the frame, the steel bracing could be a viable alternative or supplement to shear walls in concrete framed buildings in seismic areas.

Shear Resistance of Gypsum-Sheathed Light-Gauge Steel Stud Walls

This paper presents detailed results on the shear behavior of 2.44 m by 2.44 m light-gauge steel stud walls for three different shear resisting systems: framed walls with 20 gauge flat strap X-bracing on the face—type A; framed walls with 12.5 mm (1/2 in.) single-ply gypsum wallboard on the back and 12.5 mm (1/2 in.) single-ply gypsum sheathing board on the face—type B; and framed walls with 12.5 mm (1/2 in.) single-ply gypsum wallboard on the back, 12.5 mm (1/2 in.) gypsum sheathing board on the face, and 20 gauge flat strap X-bracing on face—type C. The steel framing used in these tests is typical of framing used in residential construction. The behavior of the type A walls was governed by the yield strength of the straps with practically no resistance provided by flexure in the studs. In the type B and type C tests, the measured maximum load was controlled by the breaking of the wallboard along its edges. The failure mechanism was initiated by a rotation of the screws at the edges. This was followed by the partial pull-through of the screws at the edges of the gypsum board and simultaneous breaking of the board at the edges. Each 1.22 m by 2.44 m (4 ft by 8 ft) gypsum panel was observed to behave independently during loading. A comparison of the results from the type B and type C tests showed that the use of 50.8 mm, 20 gauge tension bracing prevented cracking of the boards at the perimeter, reduced the lateral displacement, and increased the maximum load capacity of the wall by as much as 28%.

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.

Design of Imperfect Cross‐Bracings

In recent years, several analytical studies have been conducted on the behavior of cross‐bracing members subjected to out‐of‐plane buckling. Most of these studies do not consider boundary conditions such as the relative stiffness of the end connections and adjoining members and eccentricity in loading. In this paper closed‐form relationships are obtained for the direct evaluation of the critical loads of cross‐bracings with semirigid ends for any value of relative stiffness of the end connections and any ratio of the dimensionless parameters of tension and compression members. The criteria are formulated for the general case, i.e., tension and compression braces have different section properties, lengths, and axial loading. Parametric solutions are graphically displayed to clarify distinct behavior including the boundary separating symmetrical and antisymmetrical modes of buckling. Analytical expressions for eccentrically loaded cross bracings with simple end connections are further derived and studied to yield a result that the tensile force does not contribute substantially to the load‐carrying capacity of the bracing system.

Discussion: Seismic response of concentrically braced steel frames

The article discusses the nonlinear response of a number of braced frame structures designed in accordance with CAN/CSA-S16.1-M89 (CSA 1989) and the National Building Code of Canada (NRC 1990), using a time history analysis. The article considers designs based on the rules for ductile braced frames, nominally ductile braced frames, and frames with no provisions for ductility. Firstly, the writer found some aspects of the paper somewhat confusing, particularly in the authors' use of the terms and when referring to the braces acting in compression. From the information supplied it appears that the failure mechanism in is buckling, not yielding. The use of in reference to a buckling-type failure is unusual. Further, the writer does not see how the discussion of compression ductility is relevant to the performance of an X-braced system; the key element is the tension brace, not the brace. Secondly, from the article, it appears that most of the braces either did not yield or only yielded once in tension. While not conclusive, this may indicate that the approach taken in CSA (1989) and other codes for the design of braced frames is too conservative. This hypothesis is supported by the generally good behaviour of braced frame structures in prior earthquakes, despite the bracing being both substantially smaller in size and more slender than required by current standards. One possible explanation for this apparent discrepancy is the recent focus on the energy dissipation capabilities of ductile elements. Due to their pinched hysteresis loop, steel braces are not able to dissipate energy as well as plastic hinges in either steel or confined concrete. What appears to have been neglected, however, is the isolating effect once the braces have yielded in tension, which reduces the energy braces. The reduced energy input into the structure partially offsets the poorer dissipation capacity of braced frames when compared to structures with plastic hinges. If this explanation is correct, then there are only two ways to rationally design a braced frame structure: either elastically or as a ductile braced frame. For elastic design, it would be prudent to use an R value of less than 1 to account for potential peak responses, as essentially no provisions would be made for ductility. For a ductile braced frame, it is imperative that general yielding can occur in the braces, while the connections and other members must be capable of resisting the forces resulting from the yielding of the braces and the impact that occurs as the braces take up load. As many braces are field-bolted, there is a tendency for the braces to fail at the net section through the bolts before general yielding can occur (Ellingwood 1980). To preclude this type of failure, the writer recommends the area around bolt holes be reinforced, so that the net section at the bolts is greater than the gross area of the brace. This should permit yielding across the gross section of the brace, greatly increasing ductility. This line of reasoning, while different from the authors', results in many of the same conclusions expressed by them.

Stability of Nonsymmetric Cross‐Bracing Systems

When a lateral force acts on a cross‐bracing system, one brace member is subjected to a compressive force while the other is subjected to a tensile force. The tension brace provides a restraint to its compression counterpart at the point of intersection, if they are connected. This interaction of braces reduces the effective length of the compression brace member and should be taken into consideration to avoid overconservative designs. This paper presents a numerical method based on the energy principle for the determination of buckling load and effective length factor of the compression brace member. Typical design charts are given for a quick estimation of the effective length factors. Moreover, closed‐form solutions are presented for the special case of symmetric brace systems.