Rigidity – The missing design component in base plate design with fasteners.
Anchor fastener technologies have developed significantly over the last couple of years. As a result of new technologies (especially for post-installed anchor fasteners) that address construction challenges like safety and productivity, a large market for these fixings have developed. This is also true for South Africa. Unfortunately, South Africa lags behind other countries in providing national design standards to safely design these fixings. This has led to structural engineers relying more and more on suppliers and manufacturers of these fixings to assist with the design component.
Various guidelines exist that provide regulations for designing structural fasteners in concrete and other base materials. The American Concrete Institute (ACI) Standard ACI-318, Building code requirements for structural concrete, as well as Eurocode 2, Design of concrete structures - Part 4: Design of fastenings for use in concrete, are among the most well-known and widely used codes when it comes to fastener design in concrete. The South Africa Institute of Steel Construction (SAISC) also provides guidelines for designing structural fastenings in concrete in their structural connection design publication (SAISC, 2012). The SAISC guidelines are conservative assumptions predominantly derived from ACI 318.
All the design provisions detailed in the above-mentioned codes provide calculations for forces on the anchors based on a linear elastic theory. This assumes that the base plate is sufficiently rigid to transfer the internal forces to the fasteners in a linear elastic manner. In other words, the base plate remains flat (without deformation) when subjected to a bending moment as shown in figure 1. In contrast to this, a flexible base plate will have a shorter lever arm and higher anchor forces, as well as increased concrete compression below the base plate.
Figure 1a (Left) indicated the general assumption that the base plate behaves in a rigid manor compared to figure 1 (b) left that indicates the effect of a decreased lever arm because of a flexible base plate.
What is “Sufficiently Rigid”?
The guidelines mentioned above do not provide sufficient information on how a sufficiently rigid base plate should be achieved. Some research has been done to determine what design measures would provide sufficient rigidity, but in a recently published structural journal, the author reviewed this research and concluded that there is no uniform and binding guideline on how sufficient rigidity should be achieved (Fitz, et al., 2018). Within the local South African framework and guidelines, the responsibility falls on the “competent person” to ensure the validity of this “rigid base plate” assumption through a rational design method as outlined in the national building regulations (Keuter, 2008), as well as the application thereof (SABS, 2008).
Base plate design procedure.
The first step in base plate design is to define the area of the required base plate as a function of the axial load and the compressive strength of the concrete. The next step is to calculate internal bending moments of the base plate, the loads acting on the fasteners, as well as verifying the bearing capacity based on the size of the base pate. The required thickness of the base plate can then be determined as a function of the internal bending moment, forces acting on the fasteners, and the steel grade. It’s important to note that the above approach uses a linear analysis in the calculation that therefore assumes a rigid base plate. The missing component from this methodology is to determine whether the rigid assumption is valid. The process of how to do this, however, is not clearly defined.
Fastener design regulations
The above-mentioned guidelines, ACI-318 as well as Eurocode 2- Part 4 require verifications in different possible failure modes. The tension load verifications include the following: Steel Failure, Concrete Cone Failure, Pull-Out Failure, Combined Pull-out and Concrete Cone Failure, Concrete Spitting Failure, as well as Blow-out Failure. The shear load verifications require the following failure verifications: Steel Failure, Concrete Pry-out Failure, and Concrete Edge failure. There is also a requirement to verify the tension and shear interaction for in case tension and shear forces were to act on the connection simultaneously. As one can see, most of the possible failure modes verification requirements are based on the failure of the concrete and not of the steel fastener itself.
The Eurocode 2- Part 4 method is also sometimes referred to as the Concrete Capacity Method (CC Method). Both ACI-318 as well as Eurocode 2-Part 4 require these checks for cast-in as well as post-installed fasteners. In order to thoroughly verify the resistance of fasteners according to these guidelines, software packages are often preferred. Many software packages that perform these calculations are readily available and some of them include what is referred to as “base plate calculations”. It is very important to analyse what is performed by these software packages when it comes to base plate design, as well as base plate and fastener interaction. It’s important to note that anchor or fastener design software does not perform a base plate design in the context of a normal column base plate design. Most anchor design software packages utilise the dimensions of the base plate in combination with the moments and loads acting on the connection to determine the resultant tensile and shear force. The base plate is assumed to be rigid so that linear elastic analyses and load distribution can be done to check the forces acting on each fastener. The resistance of each fastener is then calculated in accordance with the failure modes verification in the above-mentioned regulations. No check is done to verify if the rigid base plate assumption is valid. Both regulations (ACI-318 and EC2-Part 4) only consider parameters relevant to a rigid fixture assumption. This is especially important to note since both regulations provide guidelines on how eccentricity modification factors should be applied. Tension eccentricity parameters are calculated if the resultant tension load is eccentric with respect to the anchors in tension. If the base plate ends up not being rigid, the loads calculated to act on each fastener might be unconservative.
Effect of insufficient rigidity
The question might be asked whether this theoretical problem highlighted occurs in practice and if it has practical implications. Figure 2 below shows an analysis done on a typical moment-resisting connection – the base connection of a structural warehouse. The analysis compared the effect of a rigid base plate (left) versus a flexible base plate (right). The analysis investigating the validity of what would have been the assumption if the above-mentioned procedure were followed (namely that the base plate is rigid), confirmed the rigid assumption to be invalid, suggesting that several components related to the design of this connection require closer consideration. Some of these components include large increases of forces on the fasteners, deformation of the base plate, as well as an increase in pressure under the base of the base plate.
Figure 2 – An analysis done in Hilti’s Profis Engineering Suite analyses the forces in a base plate that is assumed to act in a rigid manner compared to a realistic assumption of how the base plate would actually behave and what effect that would have on the anchor forces as well as the base plate deformation.
Alternative approach.
It is evident that an alternative, holistic approach is required when it comes to base plate and anchor fastener design. To accurately assess the rigidity of the base plate, all the various components that form part of the connections need to be considered, including the welds, the steel profile, the stiffeners, the fasteners, and the concrete. Realistic assumptions about load deformation behaviour of all these individual components are required, while also taking equilibrium and compatibility conditions into account. This is now possible with Hilti’s new Profis Engineering software (Hilti, 2019) that uses a component-based finite element analysis model, which individually verifies all the different elements of the structural connection. Anchor loads are first calculated according to a linear elastic load distribution that assumes the base plate to be rigid. Anchor loads are then calculated on the basis of realistic assumptions of the load displacements of the anchors through the finite element analysis module considering equilibrium and compatibility conditions. The loads calculated in each method are then compared and the actual rigidity of the base plate and the effect thereof can then be seen. The larger the difference in load values between the two methods, the more flexible the base plate.
The software then gives the designer various options that can be utilised to affect the rigidity and in turn, the forces on the fasteners. These include increasing the size of the profile to reduce the bending moment on the plate or adding welded stiffener between the profile and the plate. An anchor fastener with a decreased stiffness will also affect the rigidity of the base plate. This provides the designer with the option of checking a direct interaction effect between the base plate and the anchor and vice versa.
Summary.
Research has indicated that rigidity of the base plate can have a significant effect on a design and needs to be considered when designing a structural base plate connection, however no clear and uniform method of how rigidity of the base plate should be considered is available. This is especially true when designing fasteners in conjunction with the base plate. Hilti’s new Profis Engineering software addresses this problem with the new advanced base plate module. This will also increase the efficiency and safety aspect for the designer, since the anchor fasteners as well as the steel components of the connections will be designed simultaneously. To provide full transparency on this design approach, Hilti offers software licenses combined with accredited CPD training that will be available from the 1st of July 2019.
References
Fitz, M., Appl, J. & Geibig, O., 2018. Comprehensive base plate and anchor design based on realistic behavior – new design software based on realistic assumptions. Stahlbau, 87(December 2018), p. 10.
Hilti, 2019. Hilti Profis Engineering Software. [Online]
Available at: https://www.hilti.co.za/content/hilti/META/ZA/en/engineering/software/PROFISEngineeringSuite.html
[Accessed 10 June 2019].
Keuter, M., 2008. National Building Regulations and Building Standards Act. No. 103 of 1977 (as amended). 2008 ed. South Africa: Government Printer's Copyright Authority.
SABS, 2008. The application of the National Building Regulations Part B: Structural Design. Third Edition ed. Pretoria: South Africa Beurau of Standards.
SAISC, 2012. Structural Steel Connections: Limit State Design. Johannesburg: South African Institute of Steel Construction.