Bracing Arrangements Research Articles

Seismic effects of bracing irregularity of light timber-framed buildings

Most residential buildings in New Zealand are low-rise light timber-framed (LTF) buildings, constructed according to a prescriptive standard – NZS 3604:2011 Timber-framed buildings. NZS 3604:2011 tabulates the seismic demand and also specifies the test procedure for evaluating the seismic resistance of proprietary LTF walls, which are often plasterboard walls. Designers need to ensure the provided total seismic bracing capacity is at least equal to the total seismic bracing demand, provided that bracing arrangements satisfy the specified irregularity limits. The irregularity limits of bracing arrangements in NZS3604:2011 were established based on engineering rules of thumb rather than rigorous scientific evidence. Earthquake damage observed in the 2010/11 Canterbury earthquake sequence demonstrated that simple regular LTF houses performed well while irregular houses often had significant damage that was uneconomical to repair. This suggested that the irregularity of LTF buildings was an important factor responsible for the exacerbated earthquake damage. To quantify seismic effects of permissible irregularities in NZS 3604:2011 and provide scientific evidence for elaborating irregularity limits in NZS 3604:2011, three single storey LTF buildings with varying degrees of permissible plan irregularities were designed and their seismic performance was studied by conducting three-dimensional non-linear push-over analyses. For the non-linear push-over analyses, the in-plane behaviour of LTF walls and ceiling diaphragms were modelled using the models, developed based on NZ practice, as reported in previous research. The study revealed that permissible irregular bracing arrangements in NZS 3604:2011 could amplify lateral deflections significantly, in comparison with the regular counterparts. As a result, irregular LTF buildings within the scope of NZS 3604:2011 could be susceptible to damage due to excessive deformations in earthquakes and this suggests that the irregularity limits in current NZS 3604:2011 be reviewed and tightened.

Evaluation of Criteria for Out-of-Plane Stability of Steel Arch Bridges in Major Design Codes by FE Analysis

The provisions for out-of-plane stability of steel arch bridges in three major design codes are presented in this paper. By employing an existing steel arch bridge as a model, the influence of bridge type, arch rib to lateral bracing stiffness ratio, rise-to-span ratio, arch rib spacing, and range of lateral bracing arrangements on the out-of-plane critical axial force of the arch rib is studied using FE analysis. The accuracy of the critical axial force provisions is then evaluated against the FE analysis. The results show that the influence of the rise-to-span ratio on critical axial force is generally small. The critical axial force decreases with increasing arch rib spacing when the stiffness ratio is relatively large. A smaller ratio of arch rib length provided with lateral bracing (γ-value) significantly reduces the critical axial force and normalized critical axial force decreases with increasing stiffness ratio. The critical axial force of half-through type arch bridges is lowest when the stiffness ratio is relatively small. A deck-type bridge has a larger critical axial force than a through-type bridge when the stiffness ratio is relatively large, while the results are the opposite when the ratio is small. The different assumptions made in the provisions result in the various parameters having different impacts on the out-of-plane critical axial force in each code, thus affecting code accuracy. Considering the influence of the rise-to-span ratio, ratio of lateral bracing, and arch rib spacing with different stiffness ratios, factors to improve the accuracy of the critical axial force obtained by the three codes are proposed for a practical design process.

Progressive collapse mechanisms of multi-story steel-framed modular structures under module removal scenarios

Due to different assembly requirements and novel integration strategies, the modular structures may have undesirable loading mechanisms and distinct structural behaviors. However, there are limited data on the progressive collapse performances of modular structures, which impedes their industrial applications. This study numerically investigates the structural robustness of multi-story steel-framed modular structures against progressive collapse by using pushdown analysis. A six-story structure with five modules per floor is designed and established. The applied load, failure process and load redistribution mechanisms are studied under module removal scenarios. Parametric analyses are also conducted to investigate influences of cross-sectional dimensions of frame members, number of stories and bays, arrangements of bracings, and module removal scenarios on the robustness of the modular structures. The results show that the prototype modular structure designed with Hong Kong design code can withstand collapse under module removal scenarios. The load redistribution mechanisms among columns are significantly influenced by the overturning action and bracing effects. The ceiling beam in the lower module and the floor beam in the upper module form a double beam system. The column section, beam section, arrangement of bracings and number of stories significantly influence the robustness of the modular structure. Finally, a design method based on buckling load equation for columns is proposed to estimate the progressive collapse resisting capacity of modular structures with good accuracy and efficiency.

Seismic Response Analysis and Control of Frame Structures with Soft First Storey under Near‐Fault Ground Motions

This paper proposes two kinds of arrangements of buckling‐restrained brace dampers to strengthen soft‐first‐storey structures locally. Two types of near‐fault ground motion, with and without pulse, were selected for a study of the seismic response characteristics of soft‐first‐storey structures with and without buckling‐restrained brace dampers, and the effects of different bracing arrangements on improving the seismic performance of soft‐first‐storey structures were recognized. The results show that, compared with pulse‐free ground motion, near‐fault pulsed ground motion results in a more severe seismic response in soft‐first‐storey frame structures, leading to more serious and rapid destruction of the main structure. Buckling‐restrained brace dampers have an obvious energy dissipation effect, play a better role in protecting the main structure, and have good practicality. Compared with structures in which the buckling‐restrained brace dampers are arranged only on the bottommost layer, the bottom‐four‐layer‐support structure is more advantageous in terms of seismic performance.

Experimental studies on in-plane performance of plasterboard sheathed ceiling diaphragms

The ultimate goal of this study is to develop a model representing the in-plane behaviour of plasterboard ceiling diaphragms, as part of the efforts towards performance-based seismic engineering of low-rise light timber-framed (LTF) residential buildings in New Zealand (NZ).
 LTF residential buildings in NZ are constructed according to a prescriptive standard – NZS 3604 Timber-framed buildings [1]. With regards to seismic resisting systems, LTF buildings constructed to NZS3604 often have irregular bracing arrangements within a floor plan. A damage survey of LTF buildings after the Canterbury earthquake revealed that structural irregularity (irregular bracing arrangement within a plan) significantly exacerbated the earthquake damage to LTF buildings. When a building has irregular bracing arrangements, the building will have not only translational deflections but also a torsional response in earthquakes. How effectively the induced torsion can be resolved depends on the stiffness of the floors/roof diaphragms. Ceiling and floor diaphragms in LTF buildings in NZ have different construction details from the rest of the world and there appears to be no information available on timber diaphragms typical of NZ practice.
 This paper presents experimental studies undertaken on plasterboard ceiling diaphragms as typical of NZ residential practice. Based on the test results, a mathematical model simulating the in-plane stiffness of plasterboard ceiling diaphragms was developed, and the developed model has a similar format to that of plasterboard bracing wall elements presented in an accompany paper by Liu [2]. With these two models, three-dimensional non-linear push-over studies of LTF buildings can be undertaken to calculate seismic performance of irregular LTF buildings.

Impact modelling for progressive collapse assessment of selective rack systems

Storage rack structures are pre-engineered light weight steel structures with standard details which can be quickly assembled because all building components arrive on site pre-fabricated in line with the agreed design. Since they are made using thin-walled perforated steel sections, mostly cold-formed, with mechanical joints, generally partial-strength, semi-rigid, and with a low structural redundancy, these structures are vulnerable to any action capable to induce a local damage of a member or connection. When heavily loaded, the local damage may spread progressively, generating an overall collapse or disproportional damages. Difficulties in predicting the structural behavior of storage pallet racks are amplified by the specific geometry of the structural components: members made by high slenderness thin-walled and open-section profiles (hence prone to global, local and distortional buckling problems), flexible beam-to-upright and baseplate connections with a non-linear behavior. Due to their peculiarities, additional modelling and design rules are required for these non-traditional steel structures and reference cannot be made to usual structural design recommendations and standards. In case of Selective Pallet Rack (SPR) structures, except for the earthquakes, collision of forklift trucks or other moving equipment with front upright is considered as one of the most frequent causes of local failure, with potential to develop into a progressive collapse. In the study, the robustness of SPR structures under accidental loading situations involving collision with forklift truck is assessed using both notional upright removal and explicit forklift impact approaches. Structural configurations are varied to consider different connection properties (upright base and beam-to-upright) and brace arrangements (spine bracing, top plan bracing). SPR structures are susceptible to global failure, especially if the spine bracing is in just a few spans and the rigidity of connections is low. The explicit modelling of the forklift impact provides the most accurate results, as the effects associated with the forklift impact are not properly captured when the response is evaluated using the dynamic analysis and notional upright removal approach. Non-linear static pushdown analysis can provide satisfactory results at the least computational effort, but the dynamic increase factors may require corrections.

Numerical study on racking behavior of light steel frames with K-shaped bracing

PurposeThis paper aims to study the lateral behavior of cold-formed steel walls with K-shaped bracing by finite element modeling.Design/methodology/approachThe braces which have the same section as those for studs and tracks are connected to the frame by screw connections. By pushover analysis, lateral performance of two frame categories, with different dimensions and bracing arrangements, is examined, and the force-displacement diagram and the ultimate strength of walls are extracted. Probable failure modes during lateral loading including distortional buckling of studs, buckling in braces and failure of connections are simulated in the numerical model, and some strengthening suggestions would be offered to prevent brittle failures and, therefore, to increase the lateral strength of the walls.FindingsThe strengthened walls are examined, and their seismic behavior is compared with the original walls. Finally, a parametric study is carried out to evaluate the effect of factors such as thickness of frame members, frame height and yield tension of members on lateral behavior of the shear walls.Originality/valueIn the present research, lateral strength and failure modes of nine types of cold-formed steel shear walls with different arrangements of K-shaped bracing are examined by non-linear finite element analysis, and a parametric study is carried out to extract the effect of the wall frame characteristics on the lateral behavior. Shear walls are classified into two series.

Evaluating inelastic performance of mega-scale bracing systems in low- and medium-rise structures

In this paper, the various aspects of the seismic performance of different concentric bracing arrangements have been inelastically analyzed in a comprehensive manner. The results suggested that mega-scale bracing systems rendered more controlled performances during severe seismic events. These systems distributed the plastic hinges in much superior manners, allowing for the inelastic capacity to come into use. They reduced the target displacements due to the reduction in the natural periods of the structures. The mega-scale bracing systems also led to 25% reduction in the weight of the structures, 54% less drift ratios and 45% smaller uplift forces. The stiffness of the stories in these systems increased up to 100% and lateral displacements reduced by up to 32%.

Analysis of temporary steel grandstand with different bracing systems exposed to crowd load

Grandstands are structures which are regularly subjected to dynamic loads generated by crowd motions. It is a dangerous situation when spectators induce rhythmic jumping, dancing, stamping, etc. If the synchronized movement of spectators excites a natural frequency of the structure, resonant response might occur. To avoid such situations, temporary steel grandstands are commonly strengthened using additional elements that create bracing system which is selected depending on the size of the structure, type of the event, acting load, etc. It was proved that not only the use but also the arrangement of such structural members is crucial for the dynamic structural resistance. The aim of the present study is to determine the most effective arrangement of bracing system for a typical example of the temporary steel grandstand which is exposed to dynamic load induced by spectators. Three different arrangements of bracing systems have been analysed using five criteria recommended in the literature. The results of the study clearly show that the dynamic parameters of the grandstand are substantially different for various types of bracing systems. The largest improvement in the structural behaviour has been obtained for the grandstand equipped with the bracing system satisfying all proposed criteria. The peak accelerations for this case have been found to be nearly twice as low as for structures with other bracing arrangements. The application of such a system for the grandstand which is exposed to human-induced vibrations allows for safe use of the structure as well as improves comfort of spectators.

Seismic response of concentrically braced frames with staggered braces in split-x configurations

Special concentrically braced frames (SCBFs) are used in high-seismic regions to provide adequate lateral strength, stiffness, and ductility to a structural system. Although these frames help the structure to survive the lateral seismic actions, high uplifting/settling forces in the bracing columns may, sometimes, damage the structural integrity. This detrimental effect can be mitigated by distributing the concentrated uplifting forces among the columns using braces in staggered arrangement. Thus, the primary aim of this study is to understand the seismic response of SCBFs with staggered braces arranged in split-X configurations as compared to the conventional single-stack braced frame. Six different brace arrangements are considered for a five-story steel braced frame. Nonlinear static analyses are conducted to evaluate the lateral strength, ductility, and yielding mechanism of these study frames. In addition, nonlinear dynamic analyses have been carried out to assess the inter-story drift response and the occurrence of soft-story mechanisms in the braced frames. Analysis results highlighted the performance of staggered braced frames nearly at par with the conventional single stack arrangement. Axial force demand at the column bases are also remarkably reduced in case of staggered braces. Hence, to improve the performance of staggered braced frame a revised design method was implemented which resulted in improved lateral strength, ductility, and yield mechanism.

Assessment of eccentrically braced frames strength against progressive collapse

One of the most important and effective factors of structural strength against the risk of progressive collapse is the type of lateral load bearing system of a building. In this research, strength of dual steel moment frames equipped with a variety of eccentric bracings against progressive collapse was evaluated by using nonlinear static alternate path method. 6-floored building samples were designed with steel frame using a dual steel moment system together with 3 different types of bracing, including inverted eccentrically V-shaped bracing (chevron bracing), eccentrically V-shaped bracing and eccentrically X-shaped bracing, each with two different kinds of arrangement of bracings in the structural plan, in form of alternate and neighbor. The effects of sudden removal of columns on different floors of these buildings were examined. These studies showed that dual steel moment frames equipped with eccentric bracings generally exhibited desirable strength against progressive collapse. A change in the type of bracing resulted in significant changes in the system capacity in the progressive collapse. Among the different types of braces assessed, chevron type eccentrically brace showed higher strength against progressive collapse. Also, that alternate arrangement of bracings in structure plan demonstrated better performance than neighboring arrangement.

Improvement of progressive collapse resistance potential of semi-rigid jointed steel frames through bracings

Progressive collapse studies of both unbraced and braced semi-rigid jointed steel frames have been carried out to evaluate the contribution of bracings in improving progressive collapse resistance potential. Numerical models of 10-story frames with different types of semi-rigid connections have been developed using SAP2000. Progressive collapse potential of semi-rigid frames is first investigated without bracings. Bracings are then included in a systematic manner, and response of the braced frame is compared with that of unbraced frame to evaluate the contribution of bracings. Two different arrangements of bracings, that is, bay-wise and floor-wise arrangements, are considered to find out a preferred arrangement of bracings. Parametric studies include eight column removal conditions at center and corner locations of different floors. Development of catenary action has also been considered as it gives additional resistance, especially to braced frame. Apart from nonlinear static analysis, effects of bracings are evaluated also through nonlinear dynamic analysis and the responses of the frames in nonlinear dynamic analysis are compared with those of nonlinear static analysis. From the study, it is found that provision of bracings significantly improves the progressive collapse resistance potential of the semi-rigid frames under different column removal conditions. Floor-wise arrangement of bracings is much effective as compared to bay-wise arrangement.

Inelastic behavior of cold-formed braced walls under monotonic and cyclic loading

The ever-increasing need for housing generated the search for new and innovative building methods to increase speed and efficiency and enhance quality. One method is the use of light thin steel profiles as load-bearing elements having different solutions for interior and exterior cladding. Due to the increase in CFS construction in low-rise residential structures in the modern construction industry, there is an increased demand for performance inelastic analysis of CFS walls. In this study, the nonlinear behavior of cold-formed steel frames with various bracing arrangements including cross, chevron and k-shape straps was evaluated under cyclic and monotonic loading and using nonlinear finite element analysis methods. In total, 68 frames with different bracing arrangements and different ratios of dimensions were studied. Also, seismic parameters including resistance reduction factor, ductility and force reduction factor due to ductility were evaluated for all samples. On the other hand, the seismic response modification factor was calculated for these systems. It was concluded that the highest response modification factor would be obtained for walls with bilateral cross bracing systems with a value of 3.14. In all samples, on increasing the distance of straps from each other, shear strength increased and shear strength of the wall with bilateral bracing system was 60 % greater than that with lateral bracing system.

Effect of Bracing Systems on Fire-Induced Progressive Collapse of Steel Structures Using OpenSees

This paper investigates the effect of various bracing systems on the fire-induced progressive collapse resistance of steel-framed structures using OpenSees. Two types of bracing systems (vertical and hat bracing) and various fire scenarios (single and multi-compartment fires) are considered. Four collapse mechanisms of steel frames in fire are found through parametric studies. General collapse is characterized by the collapse of the heated bay followed by lateral drift of adjacent cool bays. Global collapse of a whole frame is due to the buckling of ground floor columns. Another two lateral collapse modes (local and global) are caused by catenary action developed in the heated beams under large deflections. Vertical bracing systems have positive effects on increasing the lateral restraint of the frame against local or global drift, while when arranged at edge bays of frames they negatively contribute to the spreading of a local damage to global collapse in the form of sequential buckling of adjacent columns through load-transfer mechanisms. For a more realistic arrangement of vertical bracings inside the frame, the bracing acts as a barrier to restrain the spread of local damage to the rest of the frame. Instead, using hat bracing can effectively optimize the load-transfer path through a more uniform redistribution of loads in columns and enhance the resistance of structures against progressive collapse. It is found that the application of vertical bracing systems alone on the steel frames to resist progressive collapse is proved to be unsafe and a combined vertical and hat bracing system is recommended in practical design.

Analytical Analysis of Seismic Behavior of Cold-Formed Steel Frames with Strap Brace and Sheathings Plates

Cold-formed steel frames (CFS) are popular all over the world. In this study, we have investigated 112 frames with different bracing arrangements and different dimensional ratios with different thicknesses of sheathing plates under cyclic and monotonic loading using Finite Element Nonlinear Analysis. We also evaluated seismic parameters including resistance reduction factor, ductility, and force reduction factor due to ductility for all specimens. On the other hand, we calculated the seismic response modification factor for these systems. The maximum modification factor among shear wall panels with sheathing plates related to GWB (gypsum wall board) specimen with thickness of 15 mm was 5.14; among bracing specimens in bilateral bracing mode related to B sample was 3.14. The maximum amount of resistance among the specimens with bilateral (2-side) bracing systems belongs to the specimen C (2-side double X-bracing) with the dimension ratio of 2 (4.8 m × 2.4 m) and resistance of 305.60 kN and also among the shear wall panels with sheathing plates, it belongs to DFP (douglas fir plywood) with a thickness of 20 mm and resistance of 371.34 kN.

Cyclic behavior studies on I-section inverted V-braces and their gusset plate connections

Inverted V-braces and their central gusset plate connections are popular patterns of brace arrangements for special concentrically braced frames (SCBF). To improve the understanding of their seismic performances and promote their applications in seismic designs, the hysteretic behavior of nine I-section inverted V-braces and their gusset plate connections subject to inelastic cyclic loading is examined through experiments and analytical simulations. It is found that the clearance at the brace end on the gusset plate, the locations of the intersection point of bracing members, and the ratio of the free edge length to the gusset plate thickness are the key parameters. The loading capacities of braced frames show no decrease before the brace low-cycle fatigue fracture, but a longer plateau at a lower load level exists in the hysteretic loops. Although specimens with a linear clearance exhibit better seismic behaviors, a negative clearance is also acceptable as long as the gusset plate does not fracture prior to the braces. A brace intersection point with moderate eccentricity is preferable for its better behavior and its economical dimension of the gusset plate, but the brace point location in the gusset plate could induce out-of-plane deformations in the gusset plate and cause the system ductility to deteriorate. Based upon test results, a suggested limitation of the ratio of the free edge length to thickness for the gusset plates is presented.

Collapse of Katowice Fair Building

The Katowice Fair Building in Poland collapsed on 28 January 2006. It was a steel building having dimensions 96.360 × 102.875 m in the general plan. The roof was constructed from truss elements, supported on 6 internal and 66 external columns situated along external walls. The results of engineering investigations on the reasons of total collapse of Katowice Fair Building structure has presented herein. Generally the collapse occurred as a consequences of: incorrect arrangement of the structure, unsufficient strength and rigidity of main structural elements, and finally of the overloading action of thick ice – snow layer. The following parameters have been estimated: the reliability model of the structure system, arrangement of bracings, construction of main columns, main girders and supporting, field connections of elements and shaping of the roof surface.

Two methods for the measurement of voluntary contraction torque in the biceps brachii muscle

Appropriate measurement of maximal voluntary contraction force of a single limb muscle or of a muscle group is important in clinical and research situations. To measure muscle force, one segment of an isometric measuring brace is fixed to a support and force is applied to the other. The output of this arrangement is affected by additional contributions such as pushing or pulling with the whole body. This paper quantifies the differences between measurements of torque produced by the biceps brachii with the brace fixed versus those produced when the elbow joint was isolated by suspending the brace from cables. No statistically significant differences were found between MVC values observed with the two methods within subjects. However, a statistically significant difference in EMG fatigue indices was observed and attributed to a different sharing of force production among different muscles in the two conditions. We conclude that different brace arrangements may lead to the same maximal force but to different rates of myoelectric manifestations of muscle fatigue since the effort may be shared differently among the muscles of synergic or stabilizing groups.

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.

不整形な架構を有する建家の弾塑性応答性状と耐震性 : 筋かい付鉄骨造発電所建家の耐震設計法に関する実証的研究(その3)

The purpose of this study is to establish a rational aseismic design method, through the experimental and analytical research, applicable to the large turbine building of a thermal power plant, which accomodates many important equipment like steam turbines, generators and incorporates aseismic elements composed of k-type braced frame. Since the turbine building has large void spaces and openings in floors for turbines, generators and etc., the floor cannot be treated as a rigid element, hence plane frames, under the effects of adjacent frames, behave with respective difference during earthquakes. And there is additional limitation on arrangement and number of bracings. Consequently it becomes very important in terms of seismic capacity beyond elastic range to take into account both three-dimensional effect and inelastic behavior including post-buckling phenomena of bracing during severe earthquakes. The method was developed for the dynamic response analysis of the whole structure, taking into account both three-dimensional effect and inelastic characteristics, in which plane braced frames were connected to each other by springs representing floor component. Using the dynamic analysis system originated, we carried out inelastic dynamic response analysis under severe earthquakes, and confirmed high seismic capacity of the turbine buildings.