Cold-formed Steel Buildings Research Articles

Experimental studies on screw connections between cold-formed steel framing and sandwich sheathing

Cold formed steel (CFS) framed wall panels with sheathing is one of the main lateral load resisting systems in light gauge structures. The nonlinear behavior of the CFS wall panel under in-plane shear loading is dictated by the nonlinear behavior of the screw connections between CFS framing and sheathing. Necessity for new and high performance CFS shear wall configurations is clearly felt by the engineering community in order to cater for the increased seismic demands in mid-rise CFS buildings. To this end the experimental studies on screw connections are essential to understand the behavior of full scale wall panels by phenomenological models. This paper presents the experimental studies on the screw connection between CFS framing and new sheathing configuration with screw subjected to shear parallel to the free edge of the sheathing. This setup mimics shear experienced by the screw as a component in full scale wall panel tests. The proposed new sheathing configuration consists of a brittle sheathing material (one of cement particle board, fiber cement board and gypsum wallboard) sandwiched between two thin steel sheets. The experiments are performed for both monotonic and cyclic loading protocols and the stiffness, ultimate strength and ductility showed a significant increase due to the sandwich steel sheets. Ultimately, the configuration with sheathing material which provides higher strength as well as ductility has been proposed for future study by full scale experiments.

Cold-formed steel sheathing connections at elevated temperature

The objective of this paper is to provide experimental results related to the elevated temperature performance of connections between cold-formed steel members and sheathing. Cold-formed steel building structures rely on sheathing for their mechanical benefits including bracing against member twist, global flexural and flexural-torsional buckling, and cross-section distortional buckling, as well as to supply lateral strength and energy dissipation in shear walls and diaphragms. Sheathing is also relied upon for non-structural benefits, including: fire, acoustic, and thermal performance. Predicting the degradation of the connection performance between cold-formed steel members and sheathing at elevated temperature is critical for any attempt to predict the structural performance of cold-formed steel buildings under fire demands. Steady-state connection tests were conducted under in-plane shear and pull-through at temperatures up to 400 °C for cold-formed steel members attached to gypsum board and oriented strand board. By combining the conducted tests with others in the literature retention factors for initial stiffness and ultimate strength of the connections are proposed.

Fire tests and thermal analyses of LSF walls insulated with silica aerogel fibreglass blanket

Load-bearing light gauge steel framed (LSF) wall is one of the main structural elements in cold-formed steel (CFS) buildings. When cavity insulation is used to enhance the energy performance of LSF walls, their structural adequacy based fire resistance levels (FRL) decrease considerably. Hence, new insulation materials and/or different wall configurations are considered. This research study investigated the use of silica aerogel fibreglass blanket as external insulation to enhance the FRL of LSF walls. Three fire tests were conducted to determine the time-temperature profiles of different wall components and the FRL of aerogel blanket insulated LSF walls. Suitable finite element (FE) models were then developed, validated and used to further optimise the LSF wall configuration. This study shows that silica aerogel fibreglass blanket, when used as a form of external insulation, enhances the fire resistance by delaying the stud temperature development by about 20 min and reducing the stud flange temperature difference by up to 200 °C when compared with cavity insulated LSF walls. However, the aerogel blanket accelerates the temperature development of fire protective gypsum boards, which leads to an early failure of these boards. This paper presents the details of the fire tests, finite element study and their findings.

Seismic responses and damage assessment of a mid-rise cold-formed steel building under far-fault and near-fault ground motions

The responses and damages of cold-formed steel (CFS) structures under near-fault ground motions are not clear currently, which are important for seismic design of CFS structures. This paper investigates the effects of near-fault ground motions on a 6-story CFS building. Three groups of records were chosen from the same events and different stations. The mean damage index of the first-story wall increases by 6.9∼18.7% and 34.3∼53.4% under the near-fault (no-pulse) and near-fault (pulse-like) records, respectively. The effects of near-fault earthquakes should be considered, and the Park-Ang seismic damage index is more appropriate for the seismic design of CFS buildings.

Fire tests of cold-formed steel walls made of hollow section studs

Light gauge steel frame (LSF) walls as primary load-bearing components have been increasingly used in the construction of cold-formed steel (CFS) buildings. Their fire safety has thus become an important consideration for designers. Currently, LSF walls made of lipped channel section (LCS) studs are commonly used in low-rise buildings, but with the expansion of CFS applications to mid-rise buildings, cold-formed steel square and rectangular hollow section (SHS/RHS) studs are being used to meet the higher load capacity demands. However, the fire resistance of LSF walls made of SHS/RHS studs has not been investigated yet, so their fire resistance levels (FRLs) are unknown, restricting their use in mid-rise buildings. Four full-scale load-bearing standard fire tests were therefore conducted to investigate the performance of cold-formed steel SHS/RHS stud walls exposed to fire. The test results showed that CFS SHS/RHS stud walls achieved a higher FRL due to the superior elevated temperature mechanical properties of CFS SHS/RHS, while the temperature development exhibited good similarity with uninsulated LCS stud walls of the same wall configuration. However, a different thermal behaviour was observed when SHS/RHS studs were used in cavity insulated walls, as the hollow cavity reduced the temperature difference between the stud hot and cold flanges. This eliminated the fire resistance reduction caused by thermal bowing deformations and provided an improved fire performance than cavity insulated LCS stud walls. This paper presents the details of the fire tests and the results.

Innovative UHS Steel Material for Tension-Only Braced CFS Walls

Lightweight Steel (LWS) systems, made of Cold Formed Steel (CFS) profiles, are widespread in seismic areas and often preferred to traditional systems. Improving structural performances in order to have higher building is now the goal, which could be achieved thanks to the use of steel innovative material. In particular, to increase the seismic performances of CFS buildings, the University of Naples “Federico II”, in cooperation with the Italian company Lamieredil S.p.A., has developed an innovative Lateral Force Resisting System (LFRS). The LFRS mainly consists of CFS frame (studs and tracks) braced by “V” shaped pre-tensioned Ultra High Strength (UHS) steel bars. The bracing was designed to be able to limit global displacement of structure, working as elastic spring, and to dissipate seismic energy by yielding. In order to prove the effectiveness of the new system, an extensive experimental campaign has been conducted. This paper focuses on material and component tests, describing tension tests on traditional materials and UHS steel, creep tests on UHS steel and nut-bar assembly tests.

Numerical Modelling of CFS Three-Story Strap-Braced Building under Shaking-Table Excitations.

In recent research activities, shake-table tests were revealed to be useful to investigate the seismic behavior of cold-formed steel (CFS) buildings. However, testing full-scale buildings or reduced-scale prototypes is not always possible; indeed, predicting tools and numerical models could help designers to evaluate earthquake response. For this reason, numerical modelling of two strap-braced prototype buildings, recently tested on shake-table at University of Naples Federico II in cooperation with Lamieredil S.p.A. company, was developed. The models were validated trough the comparison between experimental and numerical results, in term of dynamic properties (fundamental period of vibration and modal shapes), peak roof drift ratios and peak inter-story drift ratios. Although dynamic properties of mock-ups were captured with accuracy by the developed models, the comparison highlighted the need to consider accumulation of damage and rocking phenomenon in the modelling to capture with good accuracy the seismic behavior of CFS strap-braced building, subjected to high intensity records.

STUDY ON THE MECHANICAL PROPERTIES OF COLD-FORMED STEEL FRAMED SHEAR WALL WITH SLITS

In order to study the mechanical behavior of the cold-formed steel framed shear wall with slits (CFS-WS) which is suitable for low-rise and multi-story cold-formed thin-walled steel buildings, pseudo-static tests of 1 ordinary CFS-WS and 3 buckling-restrained CFS-WS were conducted to gain the mechanical properties of CFS-WS, including the failure modes, load-displacement hysteresis curves, skeleton curves and energy dissipation capacities, and the design value of shear capacity was put forward. The test results indicates that the CFS-WS relies on the torsion-recovery-reverse torsion of steel plate between vertical slits and the deformation of the steel frame to resist horizontal loads and dissipate energy. The steel plate tears and the end of the hat column buckles when the CFS-WS is broken. The CFS-WS has good bearing capacity, plasticity, ductility and energy dissipation capacity; however, the rheostriction of its load-displacement hysteresis curve is rather severe. Compared with ordinary CFS-WS, the buckling-restrained CFS-WS has higher shear stiffness, bearing capacity and energy dissipation capacity, and the rheostriction of its load-displacement hysteresis curve is mitigated. Furthermore, the stiffeners and the cold-formed steel beams and columns are connected to form a steel frame through the stiffener connectors, which can effectively enhance the early shear stiffness, bearing capacity and energy dissipation capacity of CFS-WS, and greatly improve the seismic performance of the structure.

Neural networks for predicting shear strength of CFS channels with slotted webs

Cold-formed steel channels are made with staggered courses of slots for reduced thermal conductivity and improved energy efficiency of cold-formed steel buildings. The reduced shear strength of the slotted channels must be accurately evaluated in the design to ensure the safety of the buildings with such members. This paper proposes artificial neural networks for predicting the elastic shear buckling loads and the ultimate shear strengths of the channels with slotted webs. The neural networks were trained using a large dataset consisting of 3512 finite element simulation results and a ten-fold cross-validation method. Hyperparameter tuning with a grid search was performed to determine the optimal hyperparameters of the models. The effects of the channel properties on the elastic shear buckling loads and the ultimate shear strengths of the channels were evaluated using the SHAP method. The selected neural networks with optimal hyperparameters showed excellent agreements with the finite element simulation results and exceeded the accuracy of the available design equations.

Moment-rotation response of cold-formed steel joist-to-ledger connections with variable-finishes in ledger-framed construction

Ledger framing is the dominant framing system in North American cold-formed steel (CFS) construction. In this system, the floor, which is typically sheathed with oriented strand board, is framed into the top of the wall via a rim track (ledger) and clip angle connection. The diaphragm is essentially hung from the walls. Recently completed experimental work involving full-scale shake table tests on a ledger-framed CFS building exhibited the presence of alternate load paths that contributed to the lateral force resisting system. Within the goal of quantifying this contribution to the overall diaphragm response, experimental tests on joist-to-ledger connection have demonstrated to have a complex behavior involving limit states such as ledger flange buckling, stud web crippling, and fastener pull-out. Current design practices do not account for these limit states. To improve design recommendations and understanding of load transfer mechanisms, a robust finite element model for a joist-to-ledger connection in ABAQUS, validated via existing experimental tests, is presented herein. Experimental testing on the pull-out capacity of screw-fastened connections was conducted for implementation into the finite element environment. Results were then implemented directly in ABAQUS as connection behavior. Generally, the modeling approach accurately captures connection capacity and behavior. In addition, influence of other floor sheathing materials, fiber cement board and steel deck, on the moment-rotation stiffness and strength of this connection is explored. In steel deck sheathing, effects of deck thickness and fastener spacing are explored. This work enables predictions for both design and system-scale characterization.

Study on Seismic Behaviour of Cold Formed Steel Structure Based on Simplified Mechanical Model

Cold-formed thin-walled steel buildings adopt the process of layered assembly, which is easy to disassemble and assemble and conforms to the requirements of industrial construction. But there is no clear method of seismic design. Taking a cold-formed thin-walled light steel structure house in Dazhou city as an example, the Pivot connection unit that can characterize the structural damage and the finite element software SAP2000 are used to establish a model, then this model is used to analyse the basic dynamic characteristics of structures and to analyze the time-history response of structures under seismic action. The results show that the simplified modeling analysis of the structure has a strong feasibility, and the lateral stiffness of the structure can resist the action of multiple earthquakes. Under the action of rare earthquake, the displacement between the layers of the structure is lower than the limit of the code, and the hysteretic performance of the structure is good, which can ensure the safety and reliability of the structure.

On the general requirements for design of earthquake resistant buildings in the Turkish Building Seismic code of 2018

Turkish Building Seismic Code has been revised after eleven years once more and its application is compulsory since the beginning of 2019. The new code is published together with a new Earthquake Hazard Map and it consists of 17 chapters. Some of the chapters have been revised thoroughly updated, whereas some new chapters are added as well. In the revision of the code, developments in the earthquake engineering and in the building technology are taken into consideration. Requirements related to ground motion are given in a separate chapter by defining four levels of earthquake motion. The code contains the strength design of the buildings as it is accustomed in the previous codes. However, performance based design is highlighted for high-rise buildings and for socially important buildings located moderate and high seismicity regions. The code has new separate chapters for high-rise buildings, cold-formed steel buildings, timber buildings and seismically isolated buildings. Furthermore, it has a new chapter for non-structural elements as well. The chapters involving design of steel structures, seismic evaluation of the existing buildings, and structural and geotechnical design of foundations are revised significantly. The chapter on the design of reinforced concrete contains a limited modification as well. However, since most of the buildings are reinforced concrete, this chapter relatively is important. In the present paper, the general design and evaluation approach of the code presented in detail and comparatively by considering the previous Turkish Building Seismic Codes.

Experimental study on fire resistance of cold-formed steel built-up box columns

Abstract Cold-formed steel built-up box columns are commonly used in mid-rise cold-formed steel buildings to resist applied loads. Numerous studies have been conducted to evaluate the load-bearing capacity of the columns at ambient temperature but there is very limited research on the fire resistance of the built-up columns. In this paper, a full-scale experimental investigation on fire responses of sixteen axially loaded cold-formed steel built-up box columns was conducted. As a benchmark, the load-bearing capacity of the columns at ambient temperature was tested first. The effects of heating rate, load ratio, and temperature distribution pattern on the fire responses of the columns were investigated in the fire tests. The test results indicated that increasing the load ratio led to a significant decrease in the member critical temperatures, which consequently reduced the fire resistance of the columns considerably. For those columns with an applied load ratio lower than 0.6, they were expected to have member critical temperatures higher than 500 °C. Moreover, with thin-walled steel and no fire protection the column temperatures increased quickly, leading to failure in no more than 12 min in a fast fire even with a low load ratio. It was also found that creep deformation associated with the low heating rate was one of the important factors that could be detrimental to the fire resistance and member critical temperature. As for the effect of temperature distribution patterns in the columns on the member critical temperature, it was found that the member critical temperatures for specimens with non-uniform temperature distribution were slightly higher than those with uniform temperature distribution.

Seismic performance of prefabricated sheathed cold-formed thin-walled steel buildings: Shake table test and numerical analyses

Cold-formed thin-walled steel (CFS) building systems are widely used in residential construction due to their good seismic behaviour, rapid construction efficiency and high material properties. Although significant work has been conducted on CFS members and subsystems, including studs, beams, shear walls and floors, few researches exist on the seismic performance of full-scale building systems. In order to investigate the seismic response of the CFS building system, shake table tests on a prefabricated full-scale one-story complete building were carried out. The fundamental frequency, damping ratio, displacement response and acceleration response were summarized in this paper. Then a simplified nonlinear finite element model was established with commercial software ABAQUS and validated against the test results. It was found that the developed model was suitable to accurately simulate the dynamic responses of the tested building. Hereafter, parametric studies were preformed based on the validated model to investigate the influence of key parameters on the seismic behaviour of the CFS systems, including sheathing of shear walls, diagonal brace, numbers of stories, aspect ratio as well as plan layout. Finally, based on the test data and numerical analysis results, improved seismic construction measures were proposed to provide more applicable objectives for CFS structures.

Collapse mechanism and shaking table test validation of a 3D mid-rise CFS composite shear wall building

Abstract Structural vulnerability theory (SVT) is a method that identifying the weakness and potential collapse modes of building structures in the finite element (FE) aspect, and the determinant of the global stiffness matrix |K|≤0 is proposed as the judgment of collapse of building structures. Due to the limitation of the SVT in identifying plastic hinge failure modes of the framed structures, this paper proposes the improved SVT (SVT), in which the transformation processes of the connections from rigid ones to pinned ones are considered. Based on the foundations of the ISVT using in planar framed structures, which was conducted by the authors, this paper develops the ISVT from 2-dimension to 3-dimension, and a 5-story cold-formed steel (CFS) building was tested to verify the ISVT. The results show that: the predicted collapse mode with maximum vulnerability index from the ISVT is same with the failure patterns of the test building, the first unzipped elements predicted by the ISVT are same with the most serious damaged components observed form the test. Such two aspects demonstrate the feasibility of the ISVT in revealing the collapse mechanism of the 3D CFS structures. Besides, this paper recommends the expected collapse mode for mid-rise CFS structures, and proposes “strong-frame weak-wall” concept to realize seismic design of the expected collapse mode. The findings can be used for collapse resistance design for mid-rise CFS buildings subjected to earthquakes.

Quantifying the effects of various uncertainties on seismic risk assessment of CFS structures

This paper systematically investigates the various uncertainties involved in seismic analysis of cold-formed steel (CFS) structures, and aims to provide information for seismic risk assessment and risk-based decision-making of CFS structures. Numerical model of a typical 6-story CFS building is established in this paper, the various uncertainties are analyzed and quantified. The seismic risk of the building in servicing life is calculated, including: (a) the uncertainty in defining the performance limit is proposed, and it is quantified by statistical analysis on cyclic test data; (b) the error on displacement response between the test and the prediction is selected to calculate the modelling uncertainty taking the basis of statistical analysis; (c) the logarithmic standard deviation of the structural uncertainty is recommended as 0.15–0.25 due to the calculating process is complicate and the value has certain regularity, and the engineers could select the final value according to the constructional conditions and environment of the buildings. The results show that the effects of the uncertainty in defining the performance limit and the record-to-record uncertainty on seismic risk of the CFS building are larger than the effect of the structural uncertainty; more efforts are desired in improving the modelling method; in contrast to the case considering the record-to-record uncertainty only, the collapse probability in 50 years of the case considering various uncertainty increases 133%. This paper recommends that the various uncertainties should be considered in seismic risk assessment of CFS structures, and the methods for calculating these uncertainties are proposed in this paper.

Multi-storey Modular Cold-Formed Steel Building in Hong Kong: Challenges & Opportunities

Modular building construction has become widely popular across the continents, such as in America, Europe (United Kingdom), Asia (Singapore), and more recently in Australia. Promoting sustainability in construction, the application of modular construction has been successfully implemented in very broad range building functionalities. These include student residences, private housings, hotels, commercial buildings, military accommodation, health facilities, as well as other services units attached to the main buildings. Several low-rise buildings (6 to 10 stories) as well as high-rise buildings (20 to 30 stories) have used this type prefabricated off-site building system as an alternative construction method. Among those existing projects, modular steel building brings significant advantage on the manufacturing process and erection speed. These will be beneficial for Hong Kong which has very high pressure on housing demands. Due to its excellence on strength and durability, the modular integrated construction (MiC) in Hong Kong is looking to the direction of using cold-formed steel as structural component. Some challenges and opportunities are exist and these will be discussed in this paper.

Estimation of Inelastic Interstorey Drift for OSB/Gypsum Sheathed Cold-Formed Steel Structures under Collapse Level Earthquakes

In order to estimate the inelastic interstorey drift of cold-formed steel (CFS) framed structures under collapse level earthquakes, the deflection amplification factor ηp is employed in this paper to compute the maximum interstorey drift ratio (IDRmax) from an elastic analysis. For this purpose, a series of CFS wall specimens were tested under cyclic horizontal loads, and then the hysteresis model of the walls was put forward by test results. In terms of the hysteresis model, a large quantity of elastic-plastic time-history analysis of CFS building structures was conducted based on the storey shear-type model. Furthermore, the deflection amplification factor ηp for estimating IDRmax and the parameters were analyzed. The results indicate that the deflection amplification factor ηp is highly dependent on yielding coefficient of storey shear force ξy, storey number N, period of structure T, and ground acceleration records GA. Eventually, an approximate ξy-N-ηp relationship for estimating the deflection amplification factor ηp is proposed in this paper, which can be used for seismic design in practices.

Efficient 3D lateral analysis of cold-formed steel buildings

Non-structural components contribute significantly to the lateral stiffness of a cold-formed steel (CFS) building structure, but are cumbersome to model explicitly in the structural analysis. They are therefore commonly ignored in a 3D structural analysis, and their benefits are lost to the design. This paper proposes an efficient modelling method that enables practical and accurate 3D elastic analysis of a multi-storey CFS building structure to study its lateral behaviour within the serviceability limit state. Each shear or gravity wall is represented by an equivalent shear modulus four-node orthotropic shell element, which incorporates the lateral stiffness (or flexibility) contributions of all components including sheathing, braces and fasteners as present in the wall. The equivalent shear modulus is determined from the experimental test of a representative wall panel, or from the analysis of a finely detailed finite element model of the panel. The resulting two-storey building model, which has much fewer degrees of freedom compared to conventional models, is verified against full-scale shake table test results with respect to the natural period, the peak storey drift and the peak floor acceleration at two different construction phases. This paper demonstrates that the proposed modelling method not only saves analysis time considerably through the drastic reduction of degrees of freedom, but also compares favourably against a published modelling method in terms of accuracy and modelling efforts.

Collapse behaviour of a fire engineering designed single-storey cold- formed steel building in severe fires

This paper describes a full-scale natural fire test to investigate the collapse behaviour of a single storey cold-formed steel (CFS) building, designed to behave in a specified way in a severe fire, with roof venting and partial wall collapse. The test building had a span of 8 m, height-to-eaves of 2.15 m, and length of 10 m. The walls of the CFS building were constructed from cantilever ‘stud & track’ panels, with stud spacing of 0.6 m. The roof of the building comprised CFS trusses pinned to the wall connection plates at the top. In this fire test, walls on two adjacent sides were lined internally with fire resistant lining to achieve a structural fire resistance of 30 min (R30) and the calculated fire load was provided to generate a structural fire severity of 30 min, taking into account roof venting. Thus the two protected walls were expected to remain vertical throughout the fire; the roof was expected to collapse first, followed by pulling in of the unprotected walls. The CFS cantilever wall/roof truss system collapsed with an inwards asymmetrical collapse mechanism at a truss temperature of 622.5 °C, with collapse being due to member buckling of the non-fire rated wall rather than failure of the screws or joints. A non-linear finite-element (FE) model is then described. The collapse temperature predicted using the FE was 628.2 °C, with a deformed shape similar to that observed in the fire test. The FE model has matched the experimental behaviour, thus making this model useful in understanding and predicting the behaviour of CFS cantilever wall/truss system in severe fire conditions.