Cold-formed Steel Floor Research Articles

Vibration Behavior of Composite Cold Formed Steel Floors with Concrete Topping due to Heel-Drop Loading

Human-induced vibration is an important serviceability issue of modern structural designs, especially for light long-span structures. The common heel-drop impact is usually considered in evaluating the vibration of cold-formed steel (CFS) floors. This paper proposes a simplified equation for determining the peak accelerations under transient impacts, based on the Duhamel integral. The analytical results were validated with a comparison with the results from the heel-drop test results on a CFS floor of 3 900[Formula: see text]mm × 5 600[Formula: see text]mm (at both construction and completion stages). The dynamic responses of the floor, including peak acceleration, maximum transient vibration value (MTVV), and crest factor (a ratio of MTVV-to-peak acceleration) were analyzed in detail. The natural frequencies of the floor were obtained from the FFT and FRF analysis of heel-drop and hammering test results. The investigated on-site composite CFS floor with concrete topping was found to have a high fundamental frequency: 17[Formula: see text]Hz at the construction stage and 21[Formula: see text]Hz at the completion stage. In determining the fundamental frequency of the CFS floor, the hammering was thought to be more effective than the heel-drop owing to the phenomenon of human-structure interaction (HSI). Moreover, finite element analyses were performed to study the effects of profiled steel sheeting type (Types 28-100-800, 21-180-900, and 14-80-640) and concrete thickness (40, 50, 60, 70, 80, 90, and 100[Formula: see text]mm). With the SCSC condition (two opposite edges clamped and the other two edges simply-supported), the peak acceleration decreased by 50% when the concrete thickness increased from 40[Formula: see text]mm to 100[Formula: see text]mm.

Full-scale fire resistance tests of lightweight steel framed floor systems

As cold-formed steel profiles are increasingly used for load-bearing structural systems, their fire design requires specific attention due to their thin-walled nature and high slenderness. In some specific applications such as in storage hall floor systems, these structures are used without sheathing or thermal protection on the fire-exposed side. Yet, there is a lack of data from fire resistance tests on full-scale load-bearing thin-walled steel structures, especially with directly exposed steel. This article describes two standard fire resistance tests on full-scale light gauge steel frame floors made of cold-formed steel lipped channel girders and joists topped by chipboard panels. The experimental program was designed to investigate the fire resistance of the unprotected girders. A specificity of this program was that the girders were subjected to a low load level to probe the ability to achieve without passive fire protection a 30-min fire resistance rating typical for storage hall structures in the Czech Republic. The absence of protection resulted in differences in thermal gradients and bracing compared to common fire tests, as well as a very low degree of utilization leading to an expected failure temperature higher than 800 °C. The results showed that the two floors remained stable during the 30 min with limited deflections, but failed the deflection rate criteria after 24 and 22 min, respectively. Comparison is provided with the calculation methods from the Eurocodes. The presented results provide new data on the response of full-scale, unprotected cold-formed steel floor systems subjected to fire, which can be used to calibrate numerical models and fire design methods outside of the range currently covered by the codes.

Study on in-plane and out-of-plane bearing capacity of cold-formed steel composite floor with opening

In this study, the in-plane cyclic and out-of-plane flexural performances of cold-formed steel (CFS) floors were experimentally investigated. The experimental study provided significant test data for theoretical analysis. Subsequently, the in-plane and out-of-plane performances were modeled using ABAQUS software. The numerical results agreed well with the test results. Consequently, finite element models of the CFS floors with openings were constructed. The effects of opening size, opening reinforcement measures, screw spacing, opening eccentricity, and slab material on the bearing capacity of CFS floors with openings were examined. According to the findings, the measures for improving the bearing capacity of the floor with opening include reducing the opening size, incorporating opening reinforcement measures, minimizing the screw spacing around the opening, keeping the opening centered, and prioritizing a steel deck and GSU (concrete) combination for the composite slab. In addition, the main parameter affecting the in-plane bearing capacity of the floor with opening was the opening size, and the main parameters affecting the out-of-plane bearing capacity of the floor with opening were the opening size and the opening eccentricity. Based on that, combining the simplified model of wide flange deep beam and T-shaped composite beam, the calculation method for evaluating the in-plane and out-of-plane ultimate bearing capacities of CFS floors with openings were developed by data fitting of the key factors. The calculated results were agreed well with the FE results. This study provides a scientific basis for the design of composite floors for multistory CFS structures.

Study on the load-carrying capacity of an innovative cold-formed steel floor system

To account for the higher structural ductility requirement and higher load-bearing capacity and stability requirements of floor-to-wall connections of mid-rise cold-formed steel (CFS) structures, an innovative CFS floor system with gridded beams and filler blocks (BB-based CFS floor system, where BB stands for “beams” and “blocks”, respectively) was first proposed. Then, experimental and refined numerical simulation studies were performed to investigate the flexural behaviour of the floor system. Afterwards, a simplified calculation method of the load-carrying capacity of the floor system was deduced based on its mechanical mechanism. The results show that (1) the deformation of the floorboard-to-beam screw connections and the relative buckling of the compression beam flange and adjacent web are the primary reasons for the nonlinear load-deflection relationship and bending failure of the floor system. (2) Compared with the existing CFS composite floor systems, the combination effect of each component in the BB-based CFS floor system prevented the beams from unstable failure after losing the restraint of floorboards, thereby effectively improving the load-carrying capacity and nondeformability of the floor system. (3) After decomposing the BB-based CFS floor system into composite floor beams, the load-carrying capacities of the floor system can be determined simply according to the cumulative flexural bearing capacities of these composite floor beams based on plasticity theory, the relative errors between the deduced load-carrying capacities and the test results and simulated results of different BB-based CFS floor systems are within 10%, and the calculated results are relatively safe.

New predictive equations for tilting and bearing resistance of U-shaped shear connectors in composite cold-formed steel floor joists using neural networks and stepwise regression

Due to the low thickness of cold-formed steel (CFS) sections, welding of traditional studs on them is not recommended; and there is a need to use shear connectors suitable for this type of sections. In this paper, the behavior of U-shaped shear connectors as connectors compatible with CFS composite beams has been investigated, to offer practical relationships for predicting the ultimate strength of this type of connectors. Accordingly, after the development of finite element (FE) models with the ability to predict the ultimate strength of U-shaped connectors, their performance against experimental results was validated. Then, an extensive parametric study, consisting of 216 numerical samples, was accomplished to provide a reliable database. As the main and most important of this study, two methods of artificial neural networks and stepwise regression were developed, and practical formulations were proposed to predict the ultimate strength of U-shaped shear connectors in CFS composite beams. Finally, in addition to evaluating the accuracy of the proposed formulas, a comparison was made between them and the relationships proposed by AISI, AS/NZS and EC3 codes for CFS screw connections. The relationships presented in this paper can be used by practical engineers in the design process of CFS composite beams.

Vibration serviceability evaluation of lightweight cold-formed steel floor systems

Featured by lightweight and high strength-to-weight ratio, cold-formed steel framing has been increasingly used in floor systems of low-rise and mid-rise residential and commercial building constructions. The high stiffness-to-weight ratio and low damping of such floor systems may result in potential risk for annoying floor vibrations and then bring forth a serviceability issue, particularly in an attempt to achieve longer floor spans. Several design methods have been published to enable engineers to ensure human comfort on wood-framed and structural steel-framed floors. However, although significant advancement has been attained in cold-formed steel framing, a practical and efficient design method of evaluation vibration serviceability for cold-formed steel floor systems is yet available. This paper presents a simplified design method for vibration serviceability of lightweight cold-formed steel floor systems, in which a new acceptance criterion for vibration serviceability was proposed by using binary logistic regression to correlate human prediction on floor vibration performance with vibration characteristics of the floor systems based on available test data. The procedure and corresponding equations for computing design parameters which signify the vibration characteristics of the floor system are presented. Two examples are provided to demonstrate efficiency of the proposed design method.

An experimental study on structural performance of U-shaped shear connector in composite cold-formed steel floor joists

Since few studies have focused on the shear connectors in the composite cold-formed steel beams so far, this paper studies the behavior of the U-shaped shear connectors which can be used in the composite beams comprising cold-formed steel beam and concrete slab. Accordingly, ten full scale push-out tests were conducted to investigate the ultimate load, failure modes, ductility and stiffness of the U-shaped shear connectors. For this purpose, four types of shear connectors by different screw diameters and orientation of U-shaped shear connectors with respect to the direction of the load were considered; and utilized by two types of cold-formed steel beams with thickness of 1 and 2 mm. The results have shown that change of the screws’ diameter from 4.8 to 6.3 mm, leads to an increase of the ultimate load about 25% and 40% for 1 mm and 2 mm thick specimens, respectively. It is also concluded that transverse shear connectors have a better performance than longitudinal ones considering both ultimate load and ductility. The main observed failure mode of all specimens was tilting and bearing in shear connectors except for two specimens which was incorporated with pull-over mode. Moreover, ductility of the shear connectors was examined according to EC4 code. The results show that except for two specimens, the other U-shaped shear connectors used in this study can be accounted ductile.

Shear behaviour and design of doubly symmetric hollow flange beam with web openings

Cold-Formed Steel (CFS) sections are extensively used in structural engineering applications replacing the conventional hot-rolled sections due to their inherent advantages. Web openings are generally placed in CFS floor joists and bearers to accommodate the building services, which leads the reduction of floor height. The web openings significantly influence the shear behaviour and reduce the shear capacity. However, very limited research studies have been conducted on hollow flange CFS beams with web openings under shear load. Therefore, this study presents a detailed Finite Element (FE) investigation of the shear behaviour of CFS doubly symmetric Rectangular Hollow Flange Beams (RHFBs) with unreinforced circular web openings. Non-linear FE models were created and validated using the laboratory shear test results. A detailed parametric study was then carried out by extending the validated FE models for doubly symmetric RHFBs with circular web openings. In total,126 FE models were investigated with different geometric parameters including section depth, thickness, material strength and web opening sizes to observe their behaviour on shear strength. The results showed that the existing shear strength design equations are either unsafe or conservative for doubly symmetric RHFBs with unreinforced circular web openings. Therefore, new Direct Strength Method (DSM) based shear design rules are proposed to estimate the shear strength of doubly symmetric RHFBs with unreinforced circular web openings in conjunction with proposed shear reduction factor (qs).

Effects of connections on the behaviour of cold-formed unlipped channel section bearers

Cold-formed steel (CFS) unlipped channel sections are commonly used as bearers in floor systems while rectangular hollow sections (RHS) are used as joists. The transverse loads from the joists do not pass through the shear centre of channel bearers due to different joist to bearer and bearer to column connections. Hence, the channel bearers are subjected to torsion in addition to concentrated loads and bending. This study investigated the effects of two joist to bearer (bolted/welded) connections, two bearer to column (cap plate/angle cleat) connections and fly braces on the structural behaviour of channel bearers. Eight full-scale tests were performed using simplified floor arrangements, made of two unlipped channel bearers with eight and six RHS joists located at 450 mm spacing. Suitable finite element models of tested channel bearers including their connections were developed and validated in terms of ultimate failure moment, failure mode and deflections. A parametric study was then performed using the validated finite element models. The effects of bolted/welded joist to bearer connections and cap plate/angle cleat bearer to column connections on the behaviour of channel bearers were determined and discussed based on the experimental and numerical results. Suitable recommendations were then made to the design methods based on the observed failure modes, and the most suitable combination of connections for the CFS floor systems investigated in this study.

Experimental and numerical analysis of cold-formed steel floor trusses with concrete filled compression chord

Floor trusses fabricated from cold-formed steel (CFS) sections offer several advantages over more conventional floor systems. The current paper reports the findings of an integrated experimental and numerical study focusing on the behavior of CFS floor trusses made of lipped channel sections with the compression chord member filled with concrete. This type of configuration occurs when CFS trusses are used together with a cast-in-place concrete slab. The experimental part of the study included load testing of seven full-scale CFS floor trusses, while numerical investigation included three-dimensional finite element modeling of these trusses. An analytical procedure based on a simple linear-elastic mechanics approach was also developed in an attempt to predict the response of steel-concrete composite CFS trusses without the need of detailed FE analysis or physical testing. Test results indicate that presence of concrete infill inside the compression chord member prevented the initiation of chord local/distortional buckling. In these specimens the failure mode was tension yielding of the bottom chord member. With the introduction of concrete infill within compression chord, significant improvement in truss response in terms of increase in stiffness and load capacity was obtained. The increase in load capacity was 41% and 46% depending on the orientation of the compression chord member. Presence of a concrete slab resulted in an additional 27% increase in load capacity. Truss response predicted by nonlinear FE analyses agrees well with the measured response, both in terms of global load-deflection behavior and chord strains.

Human-induced vibration of cold-formed steel floor systems: Parametric studies

Designing a lightweight floor to prevent annoying vibrations induced by human activities is still a challenge because numerous factors need to be considered. In consequence, there is still a lack of reliable models and adequate design guidelines pertinent to the vibration serviceability of cold-formed steel floor systems. To facilitate understanding the fundamental concepts of lightweight floor vibrations for serviceability design, a newly proposed damped plate-oscillator model was adopted in this research to predict dynamic responses of cold-formed steel floors induced by human walking. Three loading methods were developed based on this model. By using these loading methods, comprehensive parametric studies involving step frequencies, mass ratios, damping ratios, walking paths and end-support restraints were conducted. The present analytical studies show that the influence of moving or stationary occupants depends on the mass ratio of occupants to the floor and its significance could be negligible for small mass ratios. In addition, the boundary restraints at floor end-supports may not always reduce floor responses. On the contrary, relatively larger responses could be excited by human walking for the restrained floors having the fundamental frequency close to the multiple of the footstep frequency.

Experimental Investigation for Non and Partially Composite Cold-Formed Steel Floor Beams

In this study, six full-scaled models of RC floors supported by cold-form steel sections have been tested. Each model consists of RC 75mm thick slab supported on two parallel cold-formed steel beams with a span of 3m and spacing of 500mm. The slab has an overhang part of 250mm on each side. In the first and fourth models, the slab has been casted directly on the top flanges with no shear connector to simulate the effectiveness of friction in resisting of the lateral-torsional buckling. Shear studs have been drilled in the second and fifth models to ensure the composite action. Finally, the flanges have been embedded for the third and sixth models. A single channel beam is used in the first, second, and third models while a built-up beam is used in the fourth, fifth, and sixth models. Each model has been loaded up to failure under a pure bending with two-line loads located at the third points. Data for loads, deformations, and strains have been gathered. Except the fourth and the sixth models that failed in local buckling modes, all other models failed in global lateral-torsional buckling modes. For the single beam models; the load carrying capacity of the non-composite model is 82.9% less than the capacity of the composite models with shear studs and embedded flange. For the built-up models; the load carrying capacity of the non-composite model is 44.2 % less than the loads of the composite model with shear stud and 48.7% less than the model with the embedded flange.

Diaphragm performance of cold-formed steel floors with steel form-deck and gypsum-based self-leveling underlayment

Cold-formed steel composite floor systems are being widely used in cold-formed steel framing for mid-rise building construction, primarily because of their structural and economic efficiency. This study investigated the diaphragm performance of cold-formed steel composite floors through cyclic load testing and numerical simulation. The composite floors consist of cold-formed steel C-shaped joists and a subfloor with cold-formed steel form-deck and gypsum-based self-leveling underlayment. The test results indicated that the primary failure mode of the specimens was shear failure of the self-drilling screws that connected the steel form-deck and end joists. A finite element model was developed and validated with the test results in terms of hysteretic curves, failure modes, and seismic performance indices such as the ultimate load-bearing capacity and stiffness degradation. Moreover, extensive parametric studies were conducted using the proposed models. The results of analyses showed that the spacing of the screws connecting the steel form-deck to the joists at the floor perimeter and as well as the floor aspect ratio have significant impacts on the diaphragm performance. By contrast, the thickness of the gypsum-based self-leveling underlayment, joist depth, and web perforations exert only a minor influence on the diaphragm performance. Finally, a simplified method was proposed to calculate the central displacement of composite floors based on theoretical analysis. This study provides a useful design basis for the design and application of cold-formed steel composite floors in engineering practice.

Prediction of the in-plane mid-span displacement of cold-formed steel floor with steel form-deck and gypsum-based self-leveling underlayment

Cold-formed steel composite floor systems have been widely used in cold-formed steel framing for mid-rise building construction, primarily owing to its structural and economic efficiency. The composite floors consist of cold-formed steel C-shape joists and subfloor with cold-formed steel form-deck and gypsum-based self-leveling underlayment. This paper presents a theoretical method, an experimental investigation, and a simplified model analysis on the in-plane displacement of cold-formed steel composite floors under horizontal loads. Based on the design method of timber floors, the calculation methodology of the mid-span in-plane displacement of cold-formed steel floor with steel form-deck and gypsum-based self-leveling underlayment is proposed. Subsequently, cyclic shear tests of two full-scale floors with and without gypsum-based self-leveling underlayment were executed to investigate the failure mode, load bearing capacity, and in-plane displacement. The test results indicated that the primary failure mode of the specimens was the shear failure of self-drilling screws that connected the steel form-deck and end joists. Furthermore, the in-plane displacement of the test results agreed well with that of the proposed method. Finally, referring to the deformation results of floors under different loading stages, the composite floor analysis model was simplified to a nonlinear diagonal spring model according to the method of equivalent in-plane stiffness, and it was verified through ABAQUS finite element analysis. This study provides a useful design basis for the design and application of cold-formed steel composite floors.

Vibration of cold-formed steel floors with a steel form deck and gypsum-based self-leveling underlayment

The vibration response and static deflection of cold-formed steel floor systems with a form deck and gypsum-based self-leveling underlayment were investigated through an experimental study and a finite element analysis. The floor system was constructed with cold-formed steel joists as supports and a cold-formed steel form deck subfloor with gypsum-based self-leveling underlayment on the surface. Dynamic tests and 1 kN static tests were carried out on three specimens, and design specifications including shear resistance construction and floor width were varied to explore their effects on the fundamental frequency, damping ratio, and center deflection of floors. Then, finite element models were developed and verified with the experimental test results, and parametric studies were conducted to consider the effect of boundary conditions on the vibration performance of the same floor systems. Based on the result, a minimum limit of fundamental frequency of 10 Hz and a maximum center deflection limit under a 1 kN point load of 1 mm were recommended for cold-formed steel floor systems with a form deck and gypsum-based self-leveling underlayment. Finally, methods to calculate the fundamental frequency and center deflection of this floor systems were proposed.

A simplified method to evaluate the flexural capacity of lightweight cold-formed steel floor system with oriented strand board subfloor

Lightweight floors built with cold-formed steel C-shape joists and oriented strand board sheathing is one of the primary floor systems in cold-formed steel residential construction. In order to investigate the flexural capacity with accounting for the composite action of the floor system, two tests on the flexural capacity of the floor system were carried out. The test results indicated that the load-carrying capacity of the floor system was significantly affected by the spacing of the screws that connect the joist and oriented strand board sheathing. The primary failure mode of the floor system was the local buckling at the critical section of the C-shape joist although there was tendency of lateral-torsion buckling of the joist. The wavelength of the buckling mode was approximately equal to the screw spacing. A finite element mode was established to simulate the structural behaviour of the floors by commercial software ANSYS and it was found that the numerical results agreed well with the test data. Parametric studies were carried out based on numerical analysis to investigate how the flexural capacity of the floors was influenced by different factors such as screw spacing, joist spacing and sheathing thickness, etc. Finally, based on the test data and numerical analysis results, a simplified design method was proposed to predict the flexural capacity of the floors with accounting for the composite action. The results obtained from the proposed method were validated by both experimental and numerical investigations of two full-scale lightweight floors built with cold-formed steel C-shape joist and oriented strand board subfloor.

Determination of equivalent rigidities of cold-formed steel floor systems for vibration analysis, Part II: evaluation of the fundamental frequency

Structural properties of cold-formed steel (CFS) floor systems are essential for evaluating vibration performance of the floor systems. Comprehensive analytical and experimental studies on equivalent rigidities of CFS floor systems for vibration analysis based on orthotropic plate model are presented in this and the companion paper. In Part I of the companion papers, the equivalent rigidities were determined by using Rayleigh's method with considerations of the rotational restraints of joist ends and various configurations of transverse elements. Rotational fixity factors and restraint coefficients were introduced to characterize the effect of the rotational restraints. A joist contribution factor was defined to suitably consider the stiffness contribution of joists for a vibration mode. The Part II of the companion papers presents herein the development of design equations to predict the fundamental frequency of CFS floor systems for vibration serviceability evaluation. Simplified equations were proposed for evaluating the restraint coefficients. More importantly, rotational fixity factors for different CFS framings were investigated based on experimental data. Finally, the predicted results from the proposed method were compared with test results and other methods, and the applicability of the developed equivalent rigidities was assessed.

Determination of equivalent rigidities of cold-formed steel floor systems for vibration analysis, Part I: Theory

Cold-formed steel (CFS) floor systems have been increasingly used in residential and commercial buildings. The evaluation of vibration performance of these floors is a great challenge to be dealt with in the design stage. One of the reasons is that there is a lack of an applicable procedure to accurately determine the structural properties of the floors for vibration serviceability evaluation. Current design approaches for floor vibration serviceability are based on simplified equivalent models which lack the capability to account for effects of rotationally restrained floor edges and transverse elements. In this study, comprehensive analytical and experimental investigations have been carried out on the elastic rigidities of CFS floors for vibration analysis based on equivalent orthotropic plate model and results are reported in the present paper and its companion paper. In the present paper, the equivalent rigidities were determined by using Rayleigh's method with considerations of the rotational restraints on the joist ends and different transverse elements. Rotational fixity factors were introduced to represent the rotational restraints and restraint coefficients were designated for frequency calculation. A joist contribution factor was defined to suitably consider the stiffness contribution of joists for a vibration mode. Upon validating the proposed flexural rigidities for vibration analysis, a design equation was developed in the companion paper for predicting the fundamental frequency of the floor system for vibration serviceability evaluation. The results obtained from the equation were compared to those of tests. In addition, in the companion paper, rotational fixity factors for different type of CFS framings were discussed for evaluating the fundamental frequency based on the existing experimental data.

System Reliability Benefits of Repetitive Framing in Cold-Formed Steel Floor Systems

AbstractTypical cold-formed steel floor systems involve many repetitive joist members laid in parallel, yet the design specifications are based upon the reliability of individual members and fail t...

Design of Composite Cold-Formed Steel Flooring Systems

Recently conducted experimental and numerical investigations have shown that mobilisation of composite action within systems comprising cold-formed steel beams and wood-based floorboards is feasible and can lead to substantial improvements in structural performance. However, no design rules have yet been established for these systems in order to allow the beneficial effect of composite action to be exploited. In this paper, proposals for the design of such systems are devised and their theoretical basis is presented. At the core of the proposals is the calculation of the attained degree of partial shear connection and the shear bond coefficient for the composite members as a function of the geometric and material properties of their components and connectors. The accuracy of the devised design method for the prediction of moment capacity and flexural stiffness is demonstrated through comparisons with the results of 12 physical tests and about 80 numerical simulations reported in the literature. The proposals provide practical design rules for composite cold-formed steel floor beams, which are suitable for incorporation into future revisions of the Eurocodes.