Post-installed Fasteners for Non-structural Connections in Concrete

Post-installed fasteners are used for various structural and non-structural connections when a steel member is connected to concrete. The installation method of the fasteners depends on the fastener system (e.g., mechanical or chemical fasteners), but the main principles are the same: a hole is hammer drilled (or diamond cored) into the hardened concrete, the borehole is cleaned and the fastener is installed into the borehole in accordance with the fastener manufacturer’s installation instructions, and when the fastener has reached its full design capacity (that can be immediately after the installation or later) the connected steel member is fixed with steel base plates or other fixing devices (Fig. 1).

Fig. 1. Post-installed Fastner Systems

Post-installed fasteners are designed to resist different types of loads throughout the specified intended life of the building or the attached building element, i.e., permanent load from self-weight, imposed actions from variable sources, fatigue load by cyclic load repetitions from machinery or traffic, and accidental or environmental loads from extreme events like earthquake, fire, explosion, impact etc. Both the concrete and the fasteners react differently to these different types of loads; therefore, the selection and design of post-installed fasteners need the existence of 1) fastener selection and acceptance criteria, and 2) fastener design methods for different load types (Fig. 2).

Fig. 2. Interrelation of qualification guidelines, technical approvals and design codes [1]

This article gives a brief overview of these criteria and methods, with a special focus on the seismic performance of building elements. An important thing to clarify is…..

Does non-structural mean not safety-critical?

In short? No.

The New Zealand Building Code (NZBC) extends its objective, functional, and performance requirements to building elements. In this context, building elements include any structural and non-structural component or assembly incorporated into or associated with a building. Included are fixtures, services, drains, permanent mechanical installations for access, glazing, partitions, ceilings, and temporary supports. Safeguarding people from injury or loss of amenity and protecting other property from physical damage is the broad understanding of the objectives of safety-critical applications in the NZBC. The level of chosen safety is always related to the consequences of failure and lies within the responsibility of the designer. NZBC and the Standards referenced in it set a framework for the minimum level of safety. Non-structural connection design and installation are included in the cluster of building elements and as such, regulated by NZBC Clause B1 Structure.

Fig. 3. The concept of safety-critical applications for building elements in the framework of the New Zealand Building Code (NZBC)

What happens to connections in earthquakes?

Earthquake motions are characterized at the ground level by the time histories of acceleration, velocity and displacement of the movement, however, the demand on connections is different from these. This is due to the amplification of earthquake motions by buildings and their connected elements. If fixtures, equipment, installations, etc., inside buildings are fastened to resist earthquakes, it will not be the ground movement, but the movement of the structure where they are installed to be relevant for their design and construction. Fig. 4. indicates a schematic representation for the amplification of the earthquake motions for non-structural building elements. Such amplifications may be calculated by simplified assumptions given in the Standards or with specific design and analysis. It should be noted that the fundamental vibration period of non-structural elements is usually product-specific, and designers can find this information in the manufacturer’s technical data. Product-specific design software are also available.

Fig. 4. Amplification of seismic demand on non-structural building elements

Selection of suitable systems and design methods for fastenings

Provided by the framework of the NZBC and the Standards referenced in it, the designers have three levels of consideration in the selection process and seismic design of fastenings:

• When to perform seismic design; demand (refer to loading Standards, e.g., NZS 1170.5)

• How to perform seismic design; capacity (refer to material Standards, e.g., NZS 3101)

• What product to use; suitability (qualifications, assessments, certifications, listings, etc.)

In New Zealand, similarly to many other countries around the globe, the selection and design of post-installed fasteners in concrete is addressed in the relevant material Standard, NZS 3101 Concrete structures standard. Clause 17.5.5 of NZS 3101 gives clear guidance on the prequalification testing requirements and seismic design of post-installed fasteners in New Zealand (Fig. 5).

Fig. 5. Acceptance criteria and design method for post-installed anchors in accordance with NZS 3101

When to perform seismic design for non-structural connections?

Using the example of automatic fire sprinkler systems, it can be agreed that these are “stand-alone” systems, with no interaction with other systems/utilities and their operation must be ensured before, during, and after an earthquake. NZS 4541 delegates the seismic demand to NZS 1170.5 but allows the use of more conservative approaches too and cites NZS 4219 in the calculation of the design horizontal seismic acceleration demand on a component.

How to perform seismic design for non-structural connections?

Clause 17.5.5 of NZS 3101 states that post-installed mechanical anchors and post-installed adhesive anchors shall be designed in accordance with EOTA TR 045. This cited document was a design guideline published in 2013 by the European Organization for Technical Assessment and provided methods for the seismic design of post-installed fasteners in concrete. The document has been superseded by EN 1992-4 in 2018. In accordance with the interpretation of the reference documents in NZBC, currently EN 1992-4:2018 is the material Standard that shall be used in New Zealand for the seismic design of post-installed fasteners in concrete.

What product to use?

Clause 17.5.5 of NZS 3101 also specifies that post-installed mechanical anchors and post-installed adhesive anchors shall pass the prequalification testing stipulated in ETAG 001, Annex E. Similarly to EOTA TR 045, the ETAG 001 was published by the same organization in 2013, providing methods for the seismic assessment of post-installed fasteners in concrete. In 2016 this cited document has been superseded by EOTA TR 049. Following the same interpretation for the reference documents in NZBC, the actual versions in force of all cited documents shall be used, consequently EOTA TR 049 for anchor prequalification testing.

The attention of the readers is called here to a hierarchical relationship between material Standards (e.g., concrete design, steel design, timber design, etc.) and application Standards (e.g., engineering systems, automatic fire sprinkler systems, fire hydrant systems, etc.) that is not always communicated formally, however, is an underlying basic assumption in the objectives of the NZBC and the related engineering design (Fig. 6). As a golden rule, if an application Standard gives recommendation or commentary in the scope of a material Standard, then it must be equally or more conservative than the provisions given in the material Standard. It is generally accepted that application Standards should not set a lower level of safety than that is set in the relevant material Standard for a safety-critical application. In case of doubt, users shall seek for guidance from the design engineer. It has utmost importance for, e.g., automatic fire sprinkler systems, where currently NZS 4541:2020 does not directly cite NZS 3101, and in this way leaves the sprinkler system certifier (SSC) and the end users of the Standard in doubt for the correct interpretation. Such confusion has led to Standards NZ issuing a formal interpretation to clarify what NZS 4541:2020 intended.

Fig. 6. Hierarchy of standards and legislative documents

How can fasteners be assessed?

Seismic design of fasteners in accordance with EN 1992-4:2018 requires the fasteners to be qualified for cracked concrete and seismic applications. EN 1992-4:2018 also requires a relevant European Technical Assessment (ETA) – approval document – for such fasteners. The concrete in the region of the fastener is assumed to be cracked unless it can be demonstrated that the concrete remains uncracked during an earthquake. It is noted here that concrete has a low tensile strength, and cracks are expected to form in service conditions too, without any earthquake motions. Experience shows that the service crack widths usually do not exceed 0.4 mm and wider cracks of up to 0.5 mm to 0.6 mm are to be expected only under the maximum permissible service loads [2]. The width and the movement of cracks have a profound negative influence on the fastener behavior.

Earthquake motions are expected to result in the cracking of concrete coinciding with the fastener locations. These cracks open and close during the earthquake motions (click here to see an animation of this phenomenon in this video: LINK). The fast variation of crack widths during an earthquake can accelerate the strength degradation of fasteners. Prediction of crack widths under seismic loads is very complex, however, there is consensus in the practice that for tensile and flexural cracking, 0.8 mm is generally acceptable as the upper-bound crack width to occur at the onset of yielding of the reinforcement just outside the plastic hinge zones [3]. Post-installed fasteners are not allowed to be designed into plastic hinge zones. Cracks do not only open but can also be closed during earthquakes which can significantly affect the performance of the fastener. This complex behaviour of the fasteners must be accounted for in simulated seismic tests during product assessment. EOTA TR 049 introduces two different simulated seismic test protocols for fastener assessment, out of which, only one fits to the above needs. The complexity of non-structural fastener behavior under earthquake loading is shown schematically in Fig. 7.

Fig. 7. Actions acting on a non-structural fastener under earthquake loading [4]

The two seismic performance categories for fasteners

Post-installed fasteners became very popular in the 1970s–1980s, but in the lack of Standards, the design was mainly done based on fastener manufacturers’ handbooks. The listed allowable loads were derived from static load tests performed in non-cracked concrete. Earthquake load was not considered during the selection and design. The situation changed dramatically after the 1994 Northridge earthquake in the San Fernando Valley region of California. This earthquake did not only result in one of the biggest damage repair costs due to a natural disaster in modern US history, but also in a two-year full ban of post-installed fasteners from seismic applications. After intensive research, the ACI 355 Committee published seismic assessment and acceptance criteria in 2001, in ACI 355.2. These protocols – nevertheless represented a huge leap in fastener assessment – did not meet the requirements for modern simulated seismic test protocols explained above, for multiple reasons:

• The load protocols did not reflect on the well-accepted, stepwise increasing cyclic loads for seismic simulations, but rather were based on a low cycle fatigue loading regimen [5]

• The maximum crack width was set to 0.5 mm that is not representative for crack widths expected during a large earthquake event

• No testing protocol was developed to assess the fastener behavior under cyclic opening and closing of the cracks

The fastener research in seismic applications gained considerable momentum in the 2000s. European and American universities, manufacturers and government agencies supported multiple research projects, e.g., the five years BNCS project (Building Non-Structural Components and Systems) at the University of California in San Diego (click here for a video: LINK). As a result, the European Organization for Technical Assessment published new seismic assessment and acceptance criteria in 2013, in ETAG 001 Annex E. The new protocol (called seismic C2 performance category) fully meets the requirements for modern simulated seismic test protocols:

• It utilizes stepwise increasing cyclic load protocols

• The maximum crack width has been increased to 0.8 mm

• Testing protocol is available to assess the fastener behavior under cyclic opening and closing of the cracks

To assist readers understanding the context, Fig. 8 indicates a timeline with the key dates and events in the development and implementation of seismic testing protocols worldwide. In the figure, ASPC denotes anchor seismic performance category in the parlance of ICC-ES (International Code Council Evaluation Service, United States).

Fig. 8. Timeline of the development and implementation of anchor seismic testing protocols

How can fasteners be selected in New Zealand?

EN 1992-4:2018 acknowledges the two EOTA seismic performance categories C1/C2 and provides guidelines to designers for the category selection. It should be noted that the European Standards (EN) delegate certain topics to the EU member countries to be addressed in their own National Annex Documents (NAD), and the C1/C2 selection criteria is in this category. Therefore, besides the EN 1992-4:2018 guideline is normative, it can still be subject to changes in EU member countries.

Table 1. Recommended seismic performance categories for fasteners in accordance with EN 1992-4:2018 (reprint of Table C.1)

Since New Zealand is not an EU member country it is not regulated on how to address this situation, and in the lack of any guideline from Standards NZ, it can be agreed to follow the EN 1992-4:2018 method in interim, as normative – obligatory to be used for NZBC compliance.

The basis of the C1/C2 category selection in EN 1992-4:2018 is the seismicity level of the location of interest. Specific limits are defined (in accordance with EN 1998-1) for the spectral acceleration at T=0 period, which can be easily translated to NZS 1170.5 terms.

It can be demonstrated that the decisive parameter for the C1/C2 category selection in New Zealand is the elastic site hazard spectrum C(T) at T=0 period (see Clause 3 of NZS 1170.5). The limiting value for C2 seismic performance category is C(T=0) > 0.1g horizontal acceleration for class A & B rocks that results in the need of C2 seismic performance category everywhere in New Zealand, for both structural and non-structural applications in every IL2 to IL4 Importance Level building (in accordance with NZS 1170.5, the absolute minimum is C(T=0) = 0.189g for Northland and C(T=0) = 0.246g for the rest of New Zealand).

Table 2. Translation of EN 1992-4:2018 Table C.1 to NZS 1170.5:2004 and AS/NZS 1170.0:2002 equivalent

Conclusions

In this article, some key aspects of the fastener selection and acceptance criteria, and the seismic fastener design for non-structural connections were addressed. Based on these findings the following conclusions can be drawn:

• Non-structural connection design and installation can be safety-critical application that is regulated by NZBC Clause B1 Structure.

• Seismic demand on non-structural connections is determined by NZS 1170.5.

• NZS 3101 in clause 17.5.5 follows the international best practice, clearly stating the criteria required for the seismic assessment (EOTA TR 049) and design of post-installed fasteners in concrete (EN 1992-4).

• EOTA TR 049 provides two performance classes for seismic qualification, C1 and C2. Taking assumptions for locations around New Zealand, anchors in concrete should be designed using C2 seismic performance category, for both structural and non-structural applications.

• The EOTA TR 049 protocols are well-recognized worldwide and have already been adopted in the US too, in the ICC-ES acceptance criteria AC510.

Users of Standards follow the logic of the hierarchy of Standards: it is generally accepted that application Standards should not set lower level of safety than that is set in the relevant material Standard for a safety critical application. In case of doubt, users shall seek for guidance from the design engineer.

References

[1] Mahrenholtz, P., Wood, R.L. (2020) European Seismic Performance Categories C1 and C2 for Post-Installed Anchors, ACI Structural Journal, V. 117, No. 6, November 2020, pp. 31-44. https://doi.org/10.14359/51728071

[2] Eligehausen, R., Mallee, R., Silva, J.F. (2006) Anchorage in concrete construction, Ernst & Sohn, Berlin, 2006

[3] Mahrenholtz, P., Wood, R.L., Eligehausen, R., Hutchinson, T.C., Hoehler, M. (2017) Development and validation of European guidelines for seismic qualification of post-installed anchors, Engineering Structures, V.148, 2017, pp. 497-508. https://doi.org/10.1016/j.engstruct.2017.06.048

[4] Hoehler, M. (2006) Behavior and Testing of Fastenings to Concrete for use in Seismic Applications, PhD Thesis, Institut für Werkstoffe im Bauwesen der Universität Stuttgart, University of Stuttgart, 2006. https://doi.org/10.18419/opus-239

[5] Silva, J.F. (2001) Test methods for seismic qualification of post-installed anchors. International Symposium on Connections between Steel and Concrete, RILEM Publications, pp. 551-563.