Overcoming Data Gaps in Proprietary Cold-Formed Steel Connectors

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At times, engineers may come across a situation in the design process where a cold formed steel (CFS) proprietary product is used in a project. When that happens, many non-conventional steps often are taken to understand the performance of the product/system. Such products are becoming more common in the design practice.

The main challenge in using proprietary products in general lies in obtaining the technical data or resistance values of these products. Therefore, the inclusion of such systems/products in design projects has added new responsibilities on design engineers. Engineers cannot analytically evaluate the design resistance, nor can they access a table to pick a safety factor or a phi factor for a specific limit state. Rational Engineering approach is typically used to assess the performance of such systems. Rational Engineering is defined in AISI S-100 as an analysis based on theory that is appropriate for the situation, backed with relevant test data, if available, and sound engineering judgement.

The focus of this article is to highlight the process of determining the resistance of CFS proprietary products utilizing two techniques:

A modular CFS structural system will be used as an example in this article for all further explanations. These bolted CFS systems are used in various industrial and commercial-type support applications.

AISI S100 2016 “Reaffirmed 2020” provides guidance in chapter K, Section 2 on how to evaluate structural performance of proprietary CFS system in accordance with Rational Engineering analysis with confirmatory testing. Before going into an example, the overall procedure can be summarized as follows:

The following test elaborates on this method of evaluating the structural performance using the connector shown in Figure 1 as the test specimen. The goal is to evaluate only the +Fx direction load capacity.

a. Test setup:

Five tests were conducted using the setup shown below. The Load Displacement curves were developed, and the results were tabulated for all five tests (Table 1).
b. A mean test value was calculated from Table 1.

c. The phi factor for LRFD was calculated using
Eq. K2.1.1-2.

[See Digital Edition for summary of all the statistical factors used in the Eq. K2.1.1-2]

In addition to physical testing, FEA provides an alternative methodology for evaluating the structural performance of proprietary CFS systems when analytical equations are not applicable and physical testing alone is insufficient. AISI S100-2016 Chapter K recognizes numerical modeling as a valid component of Rational Engineering Analysis, provided that the model is appropriately calibrated and validated.

For proprietary CFS systems, particularly those with non-standard geometries, built-up assemblies, or unique connection details, FEA allows engineers to capture localized behaviors, nonlinear load paths, and connection interactions that cannot be represented by classical solutions prescribed in standard design codes such as AISI S100. The following section summarizes the overall process of developing and validating an FEA model to determine the +Fy and +Fz load capacities for the proprietary connector (shown as the test specimen in Figure 1) introduced previously.

Due to the complex nature of the mathematical solutions involved in FEA simulations, this article only addresses a few key components of FEA to provide a high-level understanding of how it can be used to evaluate the capacity of proprietary CFS products.

Modeling Approach

A square girder was attached to the connector and this connector and girder assembly as shown in Figure 3 was used for the FEA simulations to obtain the +Fy and +Fz load capacities of the connector. Since the connector is geometrically symmetric in both the Fy and Fz orientations, the load capacities in these directions are identical. The main modeling considerations are included below:

Geometry:
Before any modeling begins, a 3D CAD file is first provided, typically by the development/manufacturing team of the proprietary product. The 3D CAD geometry is verified against 2D drawings to ensure all overall dimensions are accurate and cross-checked. Additionally, the assembly is backchecked against the manufacturer’s installation instructions to ensure the model represents an approved configuration. The geometry is then cleaned to improve mesh quality and computational efficiency (Fig. 3).

Element Selection and Connection Modeling:
The assembly was modeled using thin-shell elements capable of capturing local buckling, crippling, and distortional deformations typical of CFS behavior. Bolt shanks and bearing surfaces were represented using solid elements for simplified geometry. Anchors embedded in concrete were modeled as tension only springs with a defined stiffness to replicate interaction with the concrete substrate.

Material Properties:
For this simulation, a bilinear elastic stress-strain curve was assumed and calibrated against empirical data that reflect the nominal steel properties.

Boundary Conditions and Loading:
Boundary conditions must accurately represent realistic support and loading scenarios. For this simulation, a free length is applied beyond the edge of the base connector, at least equal to the largest cross-sectional dimension. This free length helps simulate actual conditions and avoid unrealistic stiffness effects.

External loads are applied as displacement-controlled inputs, rather than direct force application. This approach enables the generation of force–deflection and moment–rotation curves, which are critical for evaluating connection behavior. In general, load application to the 3D analytical model should closely match the physical test loading protocol conditions in order to improve the correlation between simulation and experimental results.

Engineers are responsible for selecting appropriate validation and verification methods. The most common approach is comparison with physical test data. This ensures close alignment of load-displacement behavior and validates stiffness, assumptions, and load capacity. When validating the results by this method, engineers must follow AISI S100 Section K2.1.1(b), which states the correlation coefficient, Cc, between tested strength (Rt) and nominal strength (Rn) predicted from the FEA model must be greater than or equal to 0.80.

Another means of validating and verifying the numerical solution can be to reduce the connector to an idealized, simplified assembly to allow for the use of classical solutions prescribed in design codes such as AISI S100. If FEA or classical solutions using an idealized connector are performed, both which fall under Rational Engineering analysis, the following safety factors are applied, in accordance with AISI S100 Section A1.2(c). Per this specification, there are separate safety factors (and phi factors) for members and the connection. For members Ω_ASD = 2.0 and ф_LRFD = 0.80. Similarly for connections, Ω_ASD = 3.0 and ф_LRFD = 0.55.

Once geometry, boundary conditions, and model assumptions are verified, the connector capacity is determined. Figure 4 shows the 3D simulation force-deformation curve, which shows a peak nominal resistance of Rn = 106 kN (23.83 kip).

Per AISI S100 Section A1.2(c), a safety factor of 3.0 is applied for ASD.

Fy-z_ASD = 106 kN (23.83 kip) / 3 = 35.3 kN (7.94 kip)

Another critical check is to evaluate the bolts that connect the connector to the girder. Figure 5 shows an example of the applied load on the simulation and the location of bolt #1. Figure 6 provides a typical example of the bolt force curve for bolt #1.

The nominal shear strength of the single bolt is 28 kN (6.29 kip) and is based on physical testing of the individual bolt. Based on Figure 6, the maximum applied bolt shear load in the Y-direction is 3.57 kN (0.803 kip), and X-direction is 1.67 kN (0.375 kip). Therefore, the total resultant load is 3.94 kN (0.886 kip), well below the bolt’s nominal capacity.

Additional considerations would be the stress and deformation contour plots. Figure 7 provides an example of the deformation contour plot at an ASD level. The maximum deformation is 1.3867 mm (0.0546 in).

While physical testing remains the primary method for establishing product resistance, a calibrated FEA model provides several advantages:

When used in conjunction with confirmatory testing as outlined in AISI S100, FEA serves as a powerful tool for developing reliable technical data for proprietary cold-formed steel systems.

Without appropriate technical data, usually the resistance of proprietary CFS systems is estimated by engineers using a static model and hand calculations. This might be the least cost-effective method but tends to produce very conservative results. However, at times these results could also lead to unconservative and unsafe designs. Therefore, whether using physical testing or FEA, proper guidance as provided by AISI and highlighted in this article must be used as a practice for developing technical data for designing safe structures using proprietary steel products. ■

About the Authors

Arif Shahdin is a Technical Product Manager in Hilti. He has over 13 years of experience as a Hilti cold form/modular steel systems experts where he has worked on the development of technical data and design practices for Hilti’s modular support system product line, including developing an analysis software for the product. Shahdin is a degreed and practicing structural engineer and an adjunct faculty of structural engineering.

Jose Palao Jr, PE, is an Approvals Engineer at Hilti with two years of experience leading code approvals and technical compliance efforts for Hilti’s cold-formed steel modular support (MT) portfolio. He has over 10 years of structural engineering experience, including the design and evaluation of low-rise reinforced concrete and steel structures, waterfront and coastal resiliency projects, shallow and deep foundation systems, pump stations, building rehabilitation, temporary support systems, and permanent earth-retaining structures.